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Particle accelerators for fusion research
Physica 104C (1981) 199-212 © North-Holland Publishing Company
PARTICLE ACCELERATORS FOR FUSION RESEARCH E. T H O M P S O N JET Joint Undertaking, Abingdon, Oxfordshire OX14 3EA, UK
Invited paper The next generation of controlled fusion experiments repuire the injection of intense beams of neutral atoms for plasma heating. Deuterium beams of several tens of megawatts at energies of up to 160 keV with pulse lengths of 1-10 s are presently under development for JET, the Joint European Torus. The basic physicsgoverning the design of these heating systems is presented. In addition, the various technologicalproblemsand their solution are describedwith emphasis on the ion accelerator and associated high voltage problems.
1. Introduction
major radius minor radius
Particle accelerators play an increasingly important role in controlled thermonuclear fusion research. There are essentially two fundamentally differing approaches to controlled fusion each of which employ different types of accelerator. In studies of inertial confinement in which small pellets of fuel are compressed to densities many times greater than that of normal solids, short pulse high intensity (kA), beams of either ions or electrons are used as "drivers" as an alternative to lasers. These beams are generated using the high voltage diode guns developed for flash X-ray systems. The other, and more extensively studied approach to fusion, is based upon the magnetic confinement of high temperature plasmas in which long pulse high power accelerators play an increasingly important and, in some cases, essential role. It is this latter type of accelerator which forms the subject of this paper. The most widely studied magnetic confinement system is the tokamak, in which the plasma is confined and stabilised in toroidal geometry by magnetic fields generated by external coils and by that produced by a current induced in the plasma by transformer action. The Joint European Torus (JET) apparatus [1] shown pictorially in fig. 1 and now under construction, is the largest experiment of this type and has the following parameters:
toroidal magnetic field plasma current
2.96 m 1.25 horizontal, 2.10 vertical 2.77 T (3.45) 3.8 M A (4.8)
The figures in parentheses refer to extended performance of the device which can be achieved by the addition of larger power supplies. Owing to the fundamental properties of a plasma, the heating effect of the plasma, the heating effect of the plasma current (ohmic heating) is, in general, insufficient to heat the plasma to the high temperature required to achieve thermonuclear ignition. Therefore alternative means of heating the plasma are required. The most widely applied method of supplying additional heating to tokamak plasmas, in order to extend plasma parameters beyond those achievable by ohmic heating alone, is the use of neutral injection (NI) [2-4]. High power beams of energetic neutral atoms which can freely cross the magnetic fields used to confine the plasma, are ionised by the confined plasma and become trapped. The resulting high energy ions circulate within the plasma as a group of suprathermal particles dissipating their excess energy via Coulomb collisions. Using 2.4 M W of NI ion temperatures of about 7 keV ( ~ 8 x 107K) have been obtained in the Princeton Large Tokamak Experiment for durations of the order of 50 ms [5]. The coming generation of tokamaks will use 199
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200
CRYOPUMP ~Radiation
SYSTEM s h i e l d and
upport joist ~ : 0 p ~ i n manifold Rotary
~ ~
]
Jet Torus vacuum vessel
n
Neutral Injection box -
JET TORUS VACUUM VESSEL AND NEUTRAL INJECTOR BOX Fig. 1. View on JET, the Joint European Torus. NI of power levels of up to a few tens of MW with pulse lengths in excess of 1 s. In addition to heating tokamaks, neutral beam injectors are also used to produce and sustain high temperature plasmas in magnetic mirror devices which did, in fact, provide the original impetus for the development of these powerful injection systems [6].
2. Basic design concepts In its simplest form a high power neutral injector consists of four basic components: (i) a plasma source which provides the primary ions; (ii) an accelerator structure to accelerate the ions to the required energy and produce a high quality ion beam;
(iii) a neutraliser which converts the ion beam to neutral atoms having the same energy and trajectories as the primary ion beam; and (iv) a beam disposal system to handle the fast ions which are not converted to neutral atoms. In addition to these basic components there are a variety of sub-systems: power supplies, pumping systems, beam targets, etc. each of which have to fulfil stringent requirements. A schematic diagram of an injector is shown in fig. 2 where the parameters of the various sub-systems relate to the injectors for the JET experiment. The choice of beam energy is the most important single parameter which determines the overall design of a neutral injector. The lower limit is determined by the requirement that the neutral beam penetrates the plasma to a
E. Thompson~Particle accelerators for fusion research
20i
Ho
PIo:sma Source
Ion Beam Accelerator
T
Neutraiiser
... T
T
5 MW 100 kW
Disposal System
30 tls
, -1
4 x 106 ls-1
electric % Y 8x
5 MW
Cryopumps
neutrol
J
Fig. 2. Schematic of neutral injector. Numbers refer to one J E T neutral injector beam line.
sufficient depth so as to deposit energy in the central region of the plasma rather than on its outer edge. The upper limit to the beam energy is set by the requirement that virtually all of the beam is trapped by the plasma. These requirements are met if the neutral beam traverses approximately two mean free parts for trapping in the plasma, where the total cross-section for trapping of the incident neutral beam by the plasma is the sum of the cross-sections for charge transfer and ionisation by both the plasma ions and electrons. At energies below 30 keV H ° (60 keV D°), the major process governing the attenuation and hence trapping of the neutral beam by the target plasma is charge exchange between the beam and the plasma ions. At higher energies, ionization becomes dominant and the net result is that the total trapping cross-section falls montonically with increasing injection energy. Hence, in order to obtain the longer trapping lengths (~1 m) dictated by the increased physical size of the next generation fusion experiments, it is necessary to operate at higher energies-typically in the range of 50-80 keV for I ~ and 100160 kV for D °. From the beam energy, the required equivalent current of neutral atoms is simply obtained from the total power requirement which depends upon the total number of particles to be heated and the expected loss rate. The
results of detailed calculations to predict the power necessary to achieve significant thermonuclear yield in the forthcoming large experiments, indicate that powers of up to 50 MW may be required with pulse lengths of several seconds The ion currents necessary to produce the required neutral current is obtained from the well known thick target yield F0~ for the conversion of ions to neutrals by electron capture in a gas cell (the neutralizer). F0= =
°'t° 0"01 + 0"10'
where try0 is the cross-section for electron capture and trot that for electron stripping. Owing to the rapid fall of o'~0 with increasing energy, ion currents considerably in excess of the neutral equivalent current are required at high energies. In view of uncertainties in the total power requirement and also in order to ease the many technological problems, injection systems now tend to be based upon a modular design with a unit size giving ~ 50-100 A of extracted ion current. 3. The major components of a neutral injector
Following the schematic diagram of fig. 2, we will discuss the major components of an injector with particular emphasis on the accelerator structure and associated problems.
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3.1 The ion accelerator 3.1.1. Ion optics The design of the ion accelerator for high powered injectors has been the subject of considerable analytic, computational and experimental work by many groups throughout the world [7]. Some of the aspects of the ion optics and beam formation are closely allied to the design of high perveance electron guns, the major difference being that in an ion gun the emitting surface is esentially a free boundary whose position and shape is determined by the density of the current arriving at the surface and the extraction field. In an electron gun the shape and position of the cathode are well defined. The extracted ion current density J+ is governed by the well-known Child-Langmuir equation: "Me-1/2 I/3/2
where K is a constant which includes the charge of the ion, Me and Mi the mass of the electron and ion, V is the applied voltage, and d the extraction gap width. For reasonable values of the electric field in the region of 100kV cm -1 we find that the extracted current density is of the order or less than 0.5 A cm -2 from which it immediately follows that the maximum total current that can be extracted from a circular aperture is less than a few amperes ( ~ 1 Acm -1 for slit systems) if the well-known "anode hole" problem is to be avoided. This arises when the extraction aperture is considerably greater than the extraction gap and it becomes difficult to maintain a uniform electric field over the extraction area. Consequently, in order to obtain the required tens of amperes of ion beam, many such systems are operated in parallel and the accelerator electrodes become essentially a set of grid structures containing carefully aligned circular or slit apertures, over an area of several hundred square centimeters. A schematic diagram of the three electrode accelerators universally used for present genera-
tions systems operating at energies up to ~50 kV is shown in fig. 3. Ideally the first, or beamforming electrode is shaped to the required Pierce profile necessary to ensure uniformly convergent space charge limited flow from the plasma boundary towards the second or negative electrode, the aperture of which acts as a diverging lens. This cancels the initial beam convergence, resulting in essentially a collimated beam. Space charge neutralization of the ion beam is maintained by the weak electric field between the negative and the third or ground electrode which prevents electrons formed in the beam by ionization of the background gas, being drained out of the beam by the accelerating field. Although the use of an approximate Pierce profile on the beam forming electrode has been shown to reduce aberration and the overall divergence of the ion beam [8], it does introduce problems in terms of manufacture and in reducing the overall transparency of the accelerator [9]. For beam energies higher than 80 keV, there is a strong tendency towards the use of four electrode accelerators (fig. 4). The original motivation [10] came from the restrictions placed by the vacuum breakdown on the expected value of current density from a three electrode system as the voltage was increased. By assuming that the maximum voltage that can be applied across a vacuum gap scales as either directly as the gap d, or as d 1/2, we see from the above equation for
II
/ t Source plasma
xtraction Gap
/
M V+
Sinqle
V(- o.w +)
Earth electrode
Extraction Gap_
Fig. 3. Schematic diagram of a three electrode extraction system.
E. Thompson~Particle accelerators for fusion research
f"r°"°°nn Vl++ V2-1"
v2"t-
V--
Earth electrode
Double Gap_: Extraction + Post Acceleration. Fig. 4. Schematic diagram of a four electrode extraction system. current density that it will fall either as V -lrz or V -5/:, respectively. Hence, in order to maintain a high extracted current density at higher beam energies, an extra electrode was incorporated into the accelerator in order to extract the beam at modest energy prior to acceleration. This potential gain in current density may of course be offset in real systems operating at voltages lower than 100 kV when the finite thickness of the extra electrode is taken into account. This is particularly significant for accelerators designed for long pulse operation which need space for the provision of water cooling or a reasonable volume of material to give sufficient thermal capacity-both factors also having an adverse effect on the overall grid transparancy. Nevertheless, the addition of the extra electrode introduces other beneficial aspects. In particular it allows considerable freedom in the optical design, since the aperture in the extraction electrode can be made to act either as a positive or as a negative lens, or be neutral, depending upon the strength of the electric fields on each side of the aperture. The negative electrode which plays the same role as in the three electrode system still behaves as a diverging lens. In the author's opinion the ion optical equivalent of a telescopic system with essentially parallel flow from the emitting surface is to be favoured. In this case the waist of the emergent parallel beam is maximised without sacrificing the
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overall transparency of the multi-aperture array as would be the case if the ion optics were arranged to give an expanded beam. The larger beam waist minimises the small but nevertheless significant space-charge expansion of the nominally space-charge neutralised beam. A further advantage is the degree of control exerted on the trajectories of secondary particles produced in the accelerators as will be discussed below. Using this type of accelerator, ion beams have been generated with a divergence smaller than -+0.5° which is limited only by the finite temperature of the extracted ions [11]. In many cases it is advantageous to focus the ion beam from the large area extraction system in order to reduce the cross-sectional area of the subsequent neutral beam to match the input geometry of the tokamak and to reduce the flow of neutral gas. This focusing is obtained either by curving the accelerator grids or by a programmed offset of the individual apertures in plasma grids [12]. Either method gives in principal a minimum spot size corresponding to that of a single beamlet at the focal distance of the array in which case neutral power densities in excess of 10 kW cm -2 are achievable. 3.1.2. Power loading
The pulse length of several seconds required for next generation injectors exceeds the thermal time constant of the accelerator grids. A detailed understanding of the mechanisms by which power is deposited in the various electrodes is essential in order that the power loading and hence cooling requirements can be minimised and the overall grid transparency maximised. Power loading arises from direct interception of aberrated ions (which can be minimised by good ion-optical design) and also from interception of ionization and charge-exchange products formed in the gaps by collisions between the beam ions and the neutral gas from the plasma source and/or the neutraliser. Extensive computer calculations [13-16] have been made of trajectories of these ionization products. Present designs incorporate features which both minimise the power loading due to direct inter-
204
E. Thompson~Particle acceleratorsfor fusion research
ception of these particles and, more importantly, attempt to ensure that secondary electrons generated by those ions which are intercepted by the accelerator are not accelerated to high energies. The four grid accelerator, operated as a collimating system, has the advantage that the majority of the ionization products are focused through the accelerator, Furthermore, secondary electrons generated at the negative electrode which are accelerated back through the structure are focused to pass through the extraction aperture, rather than be intercepted by the grid. The taper in the negative electrode (fig. 4) serves to shape the electrostatic field in this region so that secondary electrons, generated by impact of slow ions accelerated from the neutrafizer plasma, are returned to the neutralizer, rather than being accelerated to high energies. Power loadings which are considerably less than 0.5% of the transmitted beam power have been measured for each electrode of a four electrode accelerator [17]. Measurements on three grid systems indicate that this loading can be appreciably higher on the beam forming grid, but comparisons between various systems are complicated by differences in ion optical design, operating pressure, etc. In addition to the power loading associated with the ion beam, the first or beam forming electrode also receives power from the plasma source due to the direct impact of thermal ions or electrons, which is typically some tens of watts cm -2 plus a small contribution from the hot cathodes used in the plasma source. Actively water-cooled grid structures are presently being developed which utilize small bore water cooled molybdenum tubes which form the grid rails of an array of slots [18] and also arrays of water cooled circular apertures in which etched cooling channels are incorporated in a sandwich type of construction [19] or formed by electro-deposition [20]. 3.1.3. Electrode conditioning and breakdown
This is undoubtedly the most difficult area as regards the commissioning of a multi-megawatt injection system and also that which is least
understood. To our knowledge, we still do not know what are the major factors responsible for electrical breakdown between the accelerator grids and what are the physical phenomena taking place during conditioning of the accelerators. Typically the accelerator grids, which are cleaned (in some cases ultrasonically) and assembled under clean conditions, are conditioned in vacuum, using a high impedance power supply to, or even above their working voltage. It is invariably found, however, that in spite of normal conditioning and "spot knocking", the voltage hold-off in the presence of extracted beam is drastically reduced to ~25-50% of the vacuum conditioned level and the system has to be reconditioned with the beam present, i.e. the voltage is reduced until stable operation can be achieved, and then the beam voltage and current are increased in small steps with several breakdowns taking place at each new voltage level. This is continued until the working voltage is reached or in some cases exceeded at which point the system gradually becomes more stable as operation continues, provided that "reconditioning breakdowns" (see below) do not occur. After a considerable amount of operation (which is ill-defined), the accelerator usually becomes quite stable and tolerant to a considerable degree of perveance mismatch. Although the above procedure is usually followed, some workers in the field claim that equally good results can be obtained in shorter overall time by simply allowing the system to undergo repeated breakdowns ~50-100 during each beam pulse lasting typically 100 ms. It is suspected that molybdenum is the best material from the high voltage operation point of view, but again we do not have any quantitative data and copper structures appear to operate equally well. Molybdenum does of course have advantages as regards its mechanical properties and is used for some present generation 100-ms injectors in which the accelerator is uncooled. An equally important and unresolved problem regarding the high voltage behaviour of these accelerator structures is that of so-called deconditioning breakdown and the maximum unprotected stored energy that can be tolerated in
E. Thompson~Particleacceleratorsfor fusion research
the system. These are electrical breakdowns in the accelerator structure after which the accelerator will not operate, owing to frequent repetitive breakdowns. In general, stable operation can only be recovered by reducing the operating voltage and reconditioning the accelerator by repeated operation at successively higher voltages. In order to protect the grid structure from excessive damage during a breakdown, the high voltage power supplies used to drive the accelerator incorporate fast protective devices. The most common system is a high voltage tetrode in series with the output of the power supply which can be driven to cut-off in a few microseconds, thus interrupting the current and limiting the energy in the breakdown. Alternatively, fast shunt protection is provided in order to commutate the fault current out of the accelerator. However, these fast switching elements are ineffective in limiting the energy which can be dissipated in the breakdown which is stored in the stray capacity of the cables and bushings after the switch, and the capacity of the accelerator structure itself. In addition, there is a significant amount of stored energy associated with the power supplies required to drive the plasma source which supplies the ions to the accelerator. Measurements made at the Lawrence Berkeley Laboratory (LBL) [21] by adding extra capacitance to the accelerator indicate that the maximum unprotected stored energy necessary to avoid deconditioning breakdowns is ~5 J. A similar value was obtained by the DITE group at Culham Laboratory [22]. The group at Oak Ridge National Laboratory (ORNL) claim [23] that even with added capacitance corresponding to 50J of stored energy, they are able to achieve reliable operation following a deconditioning breakdown after careful reconditioning of the accelerator. The only apparent significant difference between the various systems being that ORNL are unique in their capability of operation at high repetition frequency (one lOOms shot per 20 s) and the unresolved question remains as to whether it would be possible to exceed the 5 J limit repor-
205
ted by LBL and DITE by much more extensive conditioning. It is clear that there are (at least) two types of grid breakdown [21]. Those which do not lead to deconditioning, in which the peak current is relatively low (loo's of amperes) and its rate of rise is relatively slow (100's of ns). The second class of breakdowns which does result in deconditioning is characterised by high (kA) oscillatory currents which have a rapid rise time. Apparently, either type can occur under a given set of operating conditions, but the group at LBL ensure that the discharge current from the major fraction of the stray capacity of the plasma source power supplies and the main power supply cable (after the modulator) is always limited in rise time and peak current by the use of snubbers [24]. These devices introduce a transient series impedance into the circuit by the use of a carefully designed set of ferromagnetic cores which are positioned around the bundle of input cables close to the plasma source and accelerator. The value of the maximum allowable stored energy will ultimately set a limit on the maximum area of the accelerator, due to its selfcapacity. Using the Child-Langmuir equation with the assumption of a constant electric field, Cooper [25] has estimated that the maximum module size for a 2OO-kV accelerator with 60% transparency would be ~35 A extracted current if the 5 J limit does in fact apply. Recent work by Bottiglioni and Bussac [26] on a simulated grid structure indicate that, in the absence of the beam, the non-deconditioning energy seems to increase with increasing accelerator gap spacing and could be in the range of 25-1OOJ for 20-mm gaps appropriate to a 2OO-kV hold-off voltage. This is a more encouraging result, but there is a clear need to gain more understanding and quantitative data on these topics in addition to resolving the rather wide discrepancies which exist at present. 3.2. The plasma source
The function of the plasma source is to
206
E. Thompson~Particle accelerators for fusion research
supply the required flux of ions to the accelerator. The plasma current density of ~--0.20.3 A cm -2 must be uniform and quiescent over the relatively large extraction area-typically ~400-1000 cm 2- from which the ions are extracted to ensure all apertures are perveance matched. In addition there is a high premium on the plasma source being able to produce a high yield (~> 80%) of atomic ions in order to ensure that most of the neutral beam will satisfy the penetration requirement. A high electrical and gas efficiency of the plasma source is also a desirable but somewhat secondary requirement. All plasma sources used to date are variants of the hot cathode low pressure arc discharge. Two types are under development in Europe, the so-called Periplasmatron at Fontenay-aux-Roses [27] and the Bucket Source [28, 29] at Culham Laboratory. These are shown in figs. 5 and 6, respectively. In the Periplasmatron a weak cusp field is used to inhibit the primary electrons emitted by the hot cathode from being collected directly by the anode. The primary electrons oscillate along the magnetic field thus increasing their path length in the arc chamber which operates typically at a filling pressure of approximately 5 mT. The Bucket Source uses a high order multipolar magnetic field to confine the primary electrons away from the anode. Owing to the rapid decrease in magnetic field with distance from the anode, the inner volume is essentially field-free resulting in a high degree of plasma uniformity. The primary electrons are confined within this field-free volume thus enabling the source to operate at filling pressures in the region of 25 mT.
3.3. The neutralizer This is a simple tube surrounding the ion beam. In present generation injectors operating at ~30 kV the thermal gas operating from the plasma source supplies sufficient gas to give the required gas target necessary for a conversion of ions to atoms which is close to the theoretically predicted thick target yield. Injectors operating at higher energies per nucleon will require an
additional gas feed which will also enable the neutral gas density profile to be optimised in terms of reducing the power loading in the accelerator. However, the higher target thickness ( ~ 300 mT cms) necessary at higher energies, leads to a higher overall gas flow into the injector.
3.4. Charged beam disposal The fraction of the original ion beam which is converted to neutral atoms is a decreasing function of the beam energy (fig. 7). The remaining fast ions must be removed from the neutral beam in order to prevent them being deflected by the plasma confinement magnetic fields, giving rise to uncontrolled and high power loading and gas generation close to the confinement system. This usually is achieved using a magnetic field perpendicular to the beam axis which directs the unwanted ion beam onto a cooled beam dump. For short pulse injectors (less than 1 s), the beam dump is a simple copper plate with sufficient thermal capacity to absorb the total energy which is removed between pulses. The plates are suitably inclined with respect to the incident beam in order to reduce the specific power density and hence surface temperature and thermal stress. Next generation long pulse injectors will require actively cooled beam dumps capable of dissipating total powers in the region of a few megawatts with a surface power density ~1 kW cm -2. This can be accomplished using either very high Reynolds number water flow or, as is proposed for the JET injector, the hypervapotron principle developed by Thomson CSF for anode cooling of high power vacuum tubes. An interesting alternative under development at KFA Jfilich is that of a rotating beam dump [30] based on the technology developed for high intensity X-ray targets.
4. Auxiliary systems We can only briefly mention here two of the major auxiliary systems which are necessary for the operation of complete injection systems [31].
E. Thompson~Particleacceleratorsfor fusion research
207
/
/J
/ ....
,,~ / / . ~ . .
~.
~
5
®
lOcln
Fig. 5. Periplasmatron developed at Fontenay-aux-Roses [27]. 1-Anode; 2-intermediate electrode; 3-cathode; 4-3 electrode extraction system.
4.1. Pumping
The neutral injectors must of course operate in a high vacuum environment to prevent a large influx of thermal gas into the confinement device. A more restrictive condition is placed on
the operating pressure by the need to prevent re-ionization of the neutral beam during its flight path to the plasma. Apart from the adverse effect on total power transmission, the resultant energetic ions are deflected by the stray fields
E. Thompson~Particle accelerators for fusion research
208
INSULATOR
MULTIPLE APERTURE EXTRACTION GRID
SCALE lOcm
_ WATER COOLING
Fig. 6. Bucket Source developed at Culham Laboratory [28, 29].
1.0 0.8 D-
0.6 0.4
o.2I 0 0
100
200
300
Energy per nucleon
400
500
(keY)
Fig. 7. Efficiency ~ for atom production from D ÷ and D- ion beams as a function of its energy per nucleon (deuterium energy is twice this value).
from the confinement system and can be deposited in the input duct. Unless the system is correctly designed the gas generated in this duct can lead to a higher pressure in this region, resulting in more re-ionization and hence even higher gas pressure, etc. [32].
In order to pump the considerable quantities of gas introduced into the system- mainly from the neutraliser and beam dumps [~100's of Torr l s-1] -large condensation cryopumps have been developed [33] which operate at liquid helium temperature. Installed pumping speeds larger
E. Thompson~Particleacceleratorsfor fusion research
than I x 1061 s-1 using such pumps are a common feature of all present and next generation injectors. In one or two cases titanium gettering is used for injectors having pulse lengths of 100 ms.
4.2. Power supplies Prior to the need for long pulse lengths, capacitor banks, some of which incorporate a series tetrode for regulation and fast switching, were used. Larger installations use multimegawatt transformer rectifier sets with either series or shunt protection and/or regulation which in themselves are a considerable "tour de force" [ The few hundred kilowatts needed to drive the plasma source are provided by the transformer rectifier sets isolated to the accelerator potential. Special care is taken to reduce the stray capacity to the ground of these units by, for example, using SF6 rather than oil to provide the insulation.
5. Advanced systems 5.1. Negative ions As is obvious from fig. 7, the power conversion efficiency for the production of neutral atoms by electron capture by positive ions falls to unacceptably low values at energies larger than 100keV per nucleon. The production of fast neutrals by electron stripping of negative ions is clearly more efficient. However, injectors based on negative ions still require considerable development [34]. There are essentially two main options for the production of the primary beam of negative ions. (a) Sequential double electron capture in which a low energy beam of positive ions ( ~ few keV) is converted to negative ions of the same energy by electron capture in an alkaline vapour cell. For instance, caesium exhibits a conversion of positive to negative ion of approximately; 20% at ~1 keV per nucleon. Sodium is somewhat less efficient but the peak cross-section is at higher energy, which alleviates the problem of formation and transport of the low energy ion beam [35-37]. The negative ions are electrostatically
209
separated from the electrons generated in the conversation cell prior to acceleration to the required energy. They are then converted to fast neutral atoms by electron stripping using a gas or, for greater efficiency, a plasma target. Any residual ions (positive and negative) must then be removed as in conventional injectors. As is perhaps obvious from the above brief description, such an injector would be considerably more technologically complex than the present systems. (b) Direct extraction is, in the author's opinion, a technologically more attractive option. In this system the plasma source is designed to produce a high yield of negative ions which can be extracted and accelerated in a similar manner to that already used for positive ion systems. .There is of course the additional complication of electron suppression of the plasma source to prevent the extraction and acceleration of a large electron current along with the negative ions, which would have a disastrous effect on the efficiency. Multi-ampere beams of negative ions have in fact been generated for ms pulse lengths following the original work of Dimov [38, 39] in which a controlled amount of Cs is introduced onto the cathode of the plasma source. Fractional monolayer coverage of Cs on tungsten or molybdenum has the effect of reducing the work function of the metal to a sufficiently low value to allow the formation of H- from low low energy H + impacting on the surface. A high degree of electron suppression is obtained by the magnetic field which plays an essential role in these magnetron sources. Other schemes (as yet unpublished) utilizing caesiated surfaces in other types of plasma source are being studied in LBL and ORNL, but to the author's knowledge there is as yet no negative injector being designed for use on present or the coming generation of confinement experiments.
5.2. Energy recovery The final topic we wish to outline is one which could have a favourable, albeit limited, impact on injector efficiency and could also alleviate some of the other technological problems
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210
encountered in a positive ion injector. The basic idea is to recover the energy of the fast ions emerging from the neutralizer directly, by retarding them in an electrostatic field. This has been studied in some detail at Fontenay-auxRoses (FAR) [40], Lawrence Livermore Laboratory (LLL) [41], and the Oak Ridge National Laboratory (ORNL) [42]. One of the major problems in this field of energy recovery is that of suppressing the electrons generated in the neutralizer and preventing them from being accelerated by the electric field used to decelerate the fast ions. Experiments at FAR were the first to demonstrate that efficient energy recovery can be obtained using a series of high transparency grids at the neutralizer exit. This relatively simple system cannot be extrapolated to long pulse high energy systems owing to the power loading on the grids which intercept the beam. It has been proposed that a "separated beam" system, in which the energy recovery system is virtually identical to an accelerator grid system working in reverse, could overcome this difficulty, but this has not been demonstrated. Computed trajectories of the fast ions in the FAR structure are shown in fig. 8. The work at LLL/LBL also used electrostatic electron suppression using large electrodes which surround the total beam. Problems were
Neutrolizer r°.
_ [
Suppressor
Grid
- 96 kV
Plasmo SurfQce
experienced with gas emission from surfaces which were heated by particle bombardment giving ionization and a serious reduction in overall efficiency. These too, to large extent, were overcome, but nevertheless there does not appear to be any intention of using these systems in the near future. An alternative and attractive method of providing the electron suppression is that under study at ORNL in which a transverse magnetic field is used at the exit of the neutralizer and E x B drift motion of the electrons prevents them from being accelerated to high energy. The use of a successful energy recovery scheme would only have a favourable impact on power efficiency if either the source has a high proton yield and/or the energy of lower energy ions resulting from molecular ion break-up is recovered. Nevertheless, the use of energy recovery offers the advantages of essentially eliminating the problems associated with the beam dump (section 3.4) and also it allows the plasma source and its power supplies to be operated at ground potential with the neutralizer at high negative potential. Apart from the consequent simplification of the power supplies and control, the stray capacity and the resultant problems of stored energy discussed in section 3.1.3. could also be reduced.
.~ ~
X ~
~Recovery -~
PIQte
-80 kV Equipotential Surface
Fig. 8. Direct energy recovery system developed at Fontenay-aux-Roses [40]. Shown are the computed ion beam trajectories. Beam ions are gradually peeled off the main beam by the suppressor grid potential (a few k V negative with respect to the neutraliser) and collected at the recovery plate. Electrons are stopped at the plasma boundary impressed on the neutraliser plasma by the suppressor grid.
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6. Conclusion High power neutral injection has been extremely successful in producing high temperature plasmas for controlled thermonuclear fusion research. The coming generation of injectors operating at higher voltages and extended pulse lengths pose many technological problems for which workable solutions appear to be available. Nevertheless, considerable research and development remains to be carded out to meet the needs of higher efficiency at higher beam energy.
[15] [t6] [17]
[18]
[19]
[20]
Acknowledgements [21]
It is with much pleasure that I acknowledge the invaluable help and many discussions with colleagues throughout the world on all topics related to neutral injection and in particular those at Culham, FAR, LBL and ORNE. Particular thanks are due to Dr. A.P.H. Goede for his assistance in the preparation of this paper.
[22] [23] [24] [25] [26] [27] [28]
References [1] The JET Project-EUR 5791e (1977). [2] Proc. 8th I A E A Conf. Plasma Phys. & Control. Nucl. Fus. Res., Brussels (1980). [3] Proc. Joint Varenna--Crrenoble Syrup. on Heating in "For. Plasmas, Grenoble (1978). [4] T.S. Green, 10th Symp. on Fusion Technology, Padova (1978). [5] H. Eubank et al., Phys. Rev. Lett. 43 (1979)270; and Prec, 7th I A E A Conf. lnnsbruck, vol. I (1978), p. 167. [6] W.C. Turner et al., NucL Fusion 19 (1979) 1011; and Prec. 7th I A E A Conf. Innsbruck, vol. 2 (1978) p. 389. [7] Proc. 2nd Syrup. on Ion Sources and Formation of Ion Beams LBL-3399, Berkeley (1974). Also see [4] and the references therein. [8] E. Thompson, Prec. 2nd Syrup. on Ion Sources and Formation of Ion Beams LBL-3399, Berkeley (1974), p.ll-3. [9] L.R. Grisham, C. C. Tsai, J.H. Whealton and W.L. Stirling, Rev. Sci. Instrum. 48 (1977) 1037. [10] E. Thompson, Prec. 2nd Symp. Ion Sources and Formation of Ion Beams LBL-3399, Berkeley (1974), p.II-7. [11] A.J.T. Holmes, Phys. Rev. A19 (1979) 389. [12] J.R. Coupland and T.S. Green, Nucl. Instrum. Methods 125 (1975) 197. [13] A.C. Wager, E. Thompson and A.J.T. Holmes, Prec. Joint Varenna-Grenoble Symp., Grenoble (1978), p.89. [14] W.S. Cooper, K. Halbach and S.B. Magyary, 2nd Syrup.
[29]
[30] [31]
[32]
[33]
[34]
[35] [36]
211
on Ion Sources and Formation of Ion Beams, LBL-3399, Berkeley (1974), p.II-1. Y. Okumura, Y. Ohara and T. Ohga, JAERI-M7696 (1978). M.N. Semashko, I A E A Workshop on Neutral Injection, Culham (1978), to be published. A.J.T. Holmes and T.S. Green, 2nd Conference on Low Energy Ion Beams, Bath (1980) to be published by the Inst. of Physics, London. J.A. Paterson, G.W. Koehlen, R.P. Wells and L A . Biagi, 8th Symp. on Engineering Problems of Fusion Research, San Fransisco (1979), p. 20. A.J.T. Holmes, C.E. Brookes, A. DeVere, D.P. Hammond and T.S. Green, 8th Syrup. on Eng. Problems of Fusion Research, San Fransisco (1979), p. 20. H. Hemmerich and P. Kapschus, private communciation, KFA Jiilich. K.H. Berkner and A. Lidsky, LBL workshop on neutral injection, private communication (1979). R.S. Hemsworth, provate communication. H.H. Haselton and M. Mennon, LBL workshop on neutral injection (1979). J. Fink, W.R. Baker and H.M. Owren, IEEE Trans. Plasma Sci. 88 (1980) no. 1. W.S. Cooper, private communication. F. Bottiglioni and J.P. Bussac, EUR-CEA-FC no. 1052. M. Fumelli and F.P.G. Valckx, Nucl. Instrum. Methods 135 (1976) 203. R.S. Hemsworth et al., Proc. Joint Varenna-Crrenoble Symposium on Heating in Toroidal Plasmas, Grenoble (1978), p. 83; and 9th Eur. Conf. on Controlled Fusion and Plasma Phys., Oxford (1979), p. 17. A.P.H. Goede et al., Prec. Joint Varenna-Grenoble Symposium on Heating in Toroidal Plasmas, Grenoble (1978), p. 77; and 8th Syrup. on Engineering Problems of Fusion Research, San Fransisco (1979), p.12. H. Hemmerich and P. Kupschus, private communication, KFA Jiilich. See proc. of the Symposia on Fusion Technology, 8th SOFT, Noordwykerhout (1974), 9th SOFT Garmisch Patterkirchen (1976), and 10th SOFT Padova (1978). L D . Stewart, Report PPPL-1469, Princeton University; also A.C. Riviere and J. Sheffield, Nucl. Fusion 15 (1975) 944. J.R. Coupland, D.P. Hammond and E. Thompson, 7th Syrup. on Engineering Problems of Fusion Research, Knoxville (1977) 1976 E.B. Hooper Jr., Joint Varenna--Grenoble Syrup. on Heating in Toroidal Plasmas, vol. 2, (1978), p, 71; see also Symposium on the Production and Neutralisation of Negative Hydrogen Ions and Beams, Brookhaven (1977). T.S. Green, Workshop on negative ions, Cuiham Laboratory 1977). M.N, Semashko, V.V. Kuznetsov and A.I. Krylov, Symposium on Production and Neutralisation of Negative Hydrogen Ions and Beams (1977) Brookhaven National Lab.
212
E. Thompson~Particle accelerators for fusion research
[37] B.A. D'Yachkov, A.I. Drylov, V.V. Kuznetsov, B.P. Maksimenko and M.N. Semashko, Kurchatov Institute of Atomic Energy, Report 1AE-2523 (1975). [38] Yu. I. Bel'chenko, G.I. Dimov and V.G. Dudnikov, Sov. Phys Techn. Phys 20 (1975) 40. [39] K. Prelec and T. Sluyters, Proc. 2rid Symp. on ion Sources and Formation of Ion Beams, Berkley (1976), p. VIII-6; LBL Report no. 3399. [40] Ph. Raimbault and M. Fumelli, Proc. Joint VarennaGrenoble Int. Symp. on Heating in Toroidal Plasmas, vol. 1, (1978) p. 71.
[41] W.L. Barr, J.N. Doggett, G.W. Hamilton, J.D. Kinney and R.W. Moir, 7th Syrup. on Eng. Problems of Fusion Research Knoxville (1977), p. 308; and W.L. Barr, R.W. Moir, G.W. Hamilton and A.F. Lietzke, 8th Syrup. on Eng. Problems of Fusion Research, San Fransisco (1979), p. 20. [42] W.L. Stirling, J. Kim, H.H. Haselton, G.C. Barber, R.C. Davis, W.K. Dagenhart, W.L. Gardner, N.S. Ponte, C.C. Tsai, J.M. Whealton and R.E. Wright, Appl. Phys. Lett. 35 (1979) 104.
PARTICLE ACCELERATORS FOR FUSION RESEARCH E. T H O M P S O N JET Joint Undertaking, Abingdon, Oxfordshire OX14 3EA, UK
Invited paper The next generation of controlled fusion experiments repuire the injection of intense beams of neutral atoms for plasma heating. Deuterium beams of several tens of megawatts at energies of up to 160 keV with pulse lengths of 1-10 s are presently under development for JET, the Joint European Torus. The basic physicsgoverning the design of these heating systems is presented. In addition, the various technologicalproblemsand their solution are describedwith emphasis on the ion accelerator and associated high voltage problems.
1. Introduction
major radius minor radius
Particle accelerators play an increasingly important role in controlled thermonuclear fusion research. There are essentially two fundamentally differing approaches to controlled fusion each of which employ different types of accelerator. In studies of inertial confinement in which small pellets of fuel are compressed to densities many times greater than that of normal solids, short pulse high intensity (kA), beams of either ions or electrons are used as "drivers" as an alternative to lasers. These beams are generated using the high voltage diode guns developed for flash X-ray systems. The other, and more extensively studied approach to fusion, is based upon the magnetic confinement of high temperature plasmas in which long pulse high power accelerators play an increasingly important and, in some cases, essential role. It is this latter type of accelerator which forms the subject of this paper. The most widely studied magnetic confinement system is the tokamak, in which the plasma is confined and stabilised in toroidal geometry by magnetic fields generated by external coils and by that produced by a current induced in the plasma by transformer action. The Joint European Torus (JET) apparatus [1] shown pictorially in fig. 1 and now under construction, is the largest experiment of this type and has the following parameters:
toroidal magnetic field plasma current
2.96 m 1.25 horizontal, 2.10 vertical 2.77 T (3.45) 3.8 M A (4.8)
The figures in parentheses refer to extended performance of the device which can be achieved by the addition of larger power supplies. Owing to the fundamental properties of a plasma, the heating effect of the plasma, the heating effect of the plasma current (ohmic heating) is, in general, insufficient to heat the plasma to the high temperature required to achieve thermonuclear ignition. Therefore alternative means of heating the plasma are required. The most widely applied method of supplying additional heating to tokamak plasmas, in order to extend plasma parameters beyond those achievable by ohmic heating alone, is the use of neutral injection (NI) [2-4]. High power beams of energetic neutral atoms which can freely cross the magnetic fields used to confine the plasma, are ionised by the confined plasma and become trapped. The resulting high energy ions circulate within the plasma as a group of suprathermal particles dissipating their excess energy via Coulomb collisions. Using 2.4 M W of NI ion temperatures of about 7 keV ( ~ 8 x 107K) have been obtained in the Princeton Large Tokamak Experiment for durations of the order of 50 ms [5]. The coming generation of tokamaks will use 199
E. Thompson~Particle accelerators for fusion research
200
CRYOPUMP ~Radiation
SYSTEM s h i e l d and
upport joist ~ : 0 p ~ i n manifold Rotary
~ ~
]
Jet Torus vacuum vessel
n
Neutral Injection box -
JET TORUS VACUUM VESSEL AND NEUTRAL INJECTOR BOX Fig. 1. View on JET, the Joint European Torus. NI of power levels of up to a few tens of MW with pulse lengths in excess of 1 s. In addition to heating tokamaks, neutral beam injectors are also used to produce and sustain high temperature plasmas in magnetic mirror devices which did, in fact, provide the original impetus for the development of these powerful injection systems [6].
2. Basic design concepts In its simplest form a high power neutral injector consists of four basic components: (i) a plasma source which provides the primary ions; (ii) an accelerator structure to accelerate the ions to the required energy and produce a high quality ion beam;
(iii) a neutraliser which converts the ion beam to neutral atoms having the same energy and trajectories as the primary ion beam; and (iv) a beam disposal system to handle the fast ions which are not converted to neutral atoms. In addition to these basic components there are a variety of sub-systems: power supplies, pumping systems, beam targets, etc. each of which have to fulfil stringent requirements. A schematic diagram of an injector is shown in fig. 2 where the parameters of the various sub-systems relate to the injectors for the JET experiment. The choice of beam energy is the most important single parameter which determines the overall design of a neutral injector. The lower limit is determined by the requirement that the neutral beam penetrates the plasma to a
E. Thompson~Particle accelerators for fusion research
20i
Ho
PIo:sma Source
Ion Beam Accelerator
T
Neutraiiser
... T
T
5 MW 100 kW
Disposal System
30 tls
, -1
4 x 106 ls-1
electric % Y 8x
5 MW
Cryopumps
neutrol
J
Fig. 2. Schematic of neutral injector. Numbers refer to one J E T neutral injector beam line.
sufficient depth so as to deposit energy in the central region of the plasma rather than on its outer edge. The upper limit to the beam energy is set by the requirement that virtually all of the beam is trapped by the plasma. These requirements are met if the neutral beam traverses approximately two mean free parts for trapping in the plasma, where the total cross-section for trapping of the incident neutral beam by the plasma is the sum of the cross-sections for charge transfer and ionisation by both the plasma ions and electrons. At energies below 30 keV H ° (60 keV D°), the major process governing the attenuation and hence trapping of the neutral beam by the target plasma is charge exchange between the beam and the plasma ions. At higher energies, ionization becomes dominant and the net result is that the total trapping cross-section falls montonically with increasing injection energy. Hence, in order to obtain the longer trapping lengths (~1 m) dictated by the increased physical size of the next generation fusion experiments, it is necessary to operate at higher energies-typically in the range of 50-80 keV for I ~ and 100160 kV for D °. From the beam energy, the required equivalent current of neutral atoms is simply obtained from the total power requirement which depends upon the total number of particles to be heated and the expected loss rate. The
results of detailed calculations to predict the power necessary to achieve significant thermonuclear yield in the forthcoming large experiments, indicate that powers of up to 50 MW may be required with pulse lengths of several seconds The ion currents necessary to produce the required neutral current is obtained from the well known thick target yield F0~ for the conversion of ions to neutrals by electron capture in a gas cell (the neutralizer). F0= =
°'t° 0"01 + 0"10'
where try0 is the cross-section for electron capture and trot that for electron stripping. Owing to the rapid fall of o'~0 with increasing energy, ion currents considerably in excess of the neutral equivalent current are required at high energies. In view of uncertainties in the total power requirement and also in order to ease the many technological problems, injection systems now tend to be based upon a modular design with a unit size giving ~ 50-100 A of extracted ion current. 3. The major components of a neutral injector
Following the schematic diagram of fig. 2, we will discuss the major components of an injector with particular emphasis on the accelerator structure and associated problems.
202
E. Thompson~Particleacceleratorsfor fusion research
3.1 The ion accelerator 3.1.1. Ion optics The design of the ion accelerator for high powered injectors has been the subject of considerable analytic, computational and experimental work by many groups throughout the world [7]. Some of the aspects of the ion optics and beam formation are closely allied to the design of high perveance electron guns, the major difference being that in an ion gun the emitting surface is esentially a free boundary whose position and shape is determined by the density of the current arriving at the surface and the extraction field. In an electron gun the shape and position of the cathode are well defined. The extracted ion current density J+ is governed by the well-known Child-Langmuir equation: "Me-1/2 I/3/2
where K is a constant which includes the charge of the ion, Me and Mi the mass of the electron and ion, V is the applied voltage, and d the extraction gap width. For reasonable values of the electric field in the region of 100kV cm -1 we find that the extracted current density is of the order or less than 0.5 A cm -2 from which it immediately follows that the maximum total current that can be extracted from a circular aperture is less than a few amperes ( ~ 1 Acm -1 for slit systems) if the well-known "anode hole" problem is to be avoided. This arises when the extraction aperture is considerably greater than the extraction gap and it becomes difficult to maintain a uniform electric field over the extraction area. Consequently, in order to obtain the required tens of amperes of ion beam, many such systems are operated in parallel and the accelerator electrodes become essentially a set of grid structures containing carefully aligned circular or slit apertures, over an area of several hundred square centimeters. A schematic diagram of the three electrode accelerators universally used for present genera-
tions systems operating at energies up to ~50 kV is shown in fig. 3. Ideally the first, or beamforming electrode is shaped to the required Pierce profile necessary to ensure uniformly convergent space charge limited flow from the plasma boundary towards the second or negative electrode, the aperture of which acts as a diverging lens. This cancels the initial beam convergence, resulting in essentially a collimated beam. Space charge neutralization of the ion beam is maintained by the weak electric field between the negative and the third or ground electrode which prevents electrons formed in the beam by ionization of the background gas, being drained out of the beam by the accelerating field. Although the use of an approximate Pierce profile on the beam forming electrode has been shown to reduce aberration and the overall divergence of the ion beam [8], it does introduce problems in terms of manufacture and in reducing the overall transparency of the accelerator [9]. For beam energies higher than 80 keV, there is a strong tendency towards the use of four electrode accelerators (fig. 4). The original motivation [10] came from the restrictions placed by the vacuum breakdown on the expected value of current density from a three electrode system as the voltage was increased. By assuming that the maximum voltage that can be applied across a vacuum gap scales as either directly as the gap d, or as d 1/2, we see from the above equation for
II
/ t Source plasma
xtraction Gap
/
M V+
Sinqle
V(- o.w +)
Earth electrode
Extraction Gap_
Fig. 3. Schematic diagram of a three electrode extraction system.
E. Thompson~Particle accelerators for fusion research
f"r°"°°nn Vl++ V2-1"
v2"t-
V--
Earth electrode
Double Gap_: Extraction + Post Acceleration. Fig. 4. Schematic diagram of a four electrode extraction system. current density that it will fall either as V -lrz or V -5/:, respectively. Hence, in order to maintain a high extracted current density at higher beam energies, an extra electrode was incorporated into the accelerator in order to extract the beam at modest energy prior to acceleration. This potential gain in current density may of course be offset in real systems operating at voltages lower than 100 kV when the finite thickness of the extra electrode is taken into account. This is particularly significant for accelerators designed for long pulse operation which need space for the provision of water cooling or a reasonable volume of material to give sufficient thermal capacity-both factors also having an adverse effect on the overall grid transparancy. Nevertheless, the addition of the extra electrode introduces other beneficial aspects. In particular it allows considerable freedom in the optical design, since the aperture in the extraction electrode can be made to act either as a positive or as a negative lens, or be neutral, depending upon the strength of the electric fields on each side of the aperture. The negative electrode which plays the same role as in the three electrode system still behaves as a diverging lens. In the author's opinion the ion optical equivalent of a telescopic system with essentially parallel flow from the emitting surface is to be favoured. In this case the waist of the emergent parallel beam is maximised without sacrificing the
203
overall transparency of the multi-aperture array as would be the case if the ion optics were arranged to give an expanded beam. The larger beam waist minimises the small but nevertheless significant space-charge expansion of the nominally space-charge neutralised beam. A further advantage is the degree of control exerted on the trajectories of secondary particles produced in the accelerators as will be discussed below. Using this type of accelerator, ion beams have been generated with a divergence smaller than -+0.5° which is limited only by the finite temperature of the extracted ions [11]. In many cases it is advantageous to focus the ion beam from the large area extraction system in order to reduce the cross-sectional area of the subsequent neutral beam to match the input geometry of the tokamak and to reduce the flow of neutral gas. This focusing is obtained either by curving the accelerator grids or by a programmed offset of the individual apertures in plasma grids [12]. Either method gives in principal a minimum spot size corresponding to that of a single beamlet at the focal distance of the array in which case neutral power densities in excess of 10 kW cm -2 are achievable. 3.1.2. Power loading
The pulse length of several seconds required for next generation injectors exceeds the thermal time constant of the accelerator grids. A detailed understanding of the mechanisms by which power is deposited in the various electrodes is essential in order that the power loading and hence cooling requirements can be minimised and the overall grid transparency maximised. Power loading arises from direct interception of aberrated ions (which can be minimised by good ion-optical design) and also from interception of ionization and charge-exchange products formed in the gaps by collisions between the beam ions and the neutral gas from the plasma source and/or the neutraliser. Extensive computer calculations [13-16] have been made of trajectories of these ionization products. Present designs incorporate features which both minimise the power loading due to direct inter-
204
E. Thompson~Particle acceleratorsfor fusion research
ception of these particles and, more importantly, attempt to ensure that secondary electrons generated by those ions which are intercepted by the accelerator are not accelerated to high energies. The four grid accelerator, operated as a collimating system, has the advantage that the majority of the ionization products are focused through the accelerator, Furthermore, secondary electrons generated at the negative electrode which are accelerated back through the structure are focused to pass through the extraction aperture, rather than be intercepted by the grid. The taper in the negative electrode (fig. 4) serves to shape the electrostatic field in this region so that secondary electrons, generated by impact of slow ions accelerated from the neutrafizer plasma, are returned to the neutralizer, rather than being accelerated to high energies. Power loadings which are considerably less than 0.5% of the transmitted beam power have been measured for each electrode of a four electrode accelerator [17]. Measurements on three grid systems indicate that this loading can be appreciably higher on the beam forming grid, but comparisons between various systems are complicated by differences in ion optical design, operating pressure, etc. In addition to the power loading associated with the ion beam, the first or beam forming electrode also receives power from the plasma source due to the direct impact of thermal ions or electrons, which is typically some tens of watts cm -2 plus a small contribution from the hot cathodes used in the plasma source. Actively water-cooled grid structures are presently being developed which utilize small bore water cooled molybdenum tubes which form the grid rails of an array of slots [18] and also arrays of water cooled circular apertures in which etched cooling channels are incorporated in a sandwich type of construction [19] or formed by electro-deposition [20]. 3.1.3. Electrode conditioning and breakdown
This is undoubtedly the most difficult area as regards the commissioning of a multi-megawatt injection system and also that which is least
understood. To our knowledge, we still do not know what are the major factors responsible for electrical breakdown between the accelerator grids and what are the physical phenomena taking place during conditioning of the accelerators. Typically the accelerator grids, which are cleaned (in some cases ultrasonically) and assembled under clean conditions, are conditioned in vacuum, using a high impedance power supply to, or even above their working voltage. It is invariably found, however, that in spite of normal conditioning and "spot knocking", the voltage hold-off in the presence of extracted beam is drastically reduced to ~25-50% of the vacuum conditioned level and the system has to be reconditioned with the beam present, i.e. the voltage is reduced until stable operation can be achieved, and then the beam voltage and current are increased in small steps with several breakdowns taking place at each new voltage level. This is continued until the working voltage is reached or in some cases exceeded at which point the system gradually becomes more stable as operation continues, provided that "reconditioning breakdowns" (see below) do not occur. After a considerable amount of operation (which is ill-defined), the accelerator usually becomes quite stable and tolerant to a considerable degree of perveance mismatch. Although the above procedure is usually followed, some workers in the field claim that equally good results can be obtained in shorter overall time by simply allowing the system to undergo repeated breakdowns ~50-100 during each beam pulse lasting typically 100 ms. It is suspected that molybdenum is the best material from the high voltage operation point of view, but again we do not have any quantitative data and copper structures appear to operate equally well. Molybdenum does of course have advantages as regards its mechanical properties and is used for some present generation 100-ms injectors in which the accelerator is uncooled. An equally important and unresolved problem regarding the high voltage behaviour of these accelerator structures is that of so-called deconditioning breakdown and the maximum unprotected stored energy that can be tolerated in
E. Thompson~Particleacceleratorsfor fusion research
the system. These are electrical breakdowns in the accelerator structure after which the accelerator will not operate, owing to frequent repetitive breakdowns. In general, stable operation can only be recovered by reducing the operating voltage and reconditioning the accelerator by repeated operation at successively higher voltages. In order to protect the grid structure from excessive damage during a breakdown, the high voltage power supplies used to drive the accelerator incorporate fast protective devices. The most common system is a high voltage tetrode in series with the output of the power supply which can be driven to cut-off in a few microseconds, thus interrupting the current and limiting the energy in the breakdown. Alternatively, fast shunt protection is provided in order to commutate the fault current out of the accelerator. However, these fast switching elements are ineffective in limiting the energy which can be dissipated in the breakdown which is stored in the stray capacity of the cables and bushings after the switch, and the capacity of the accelerator structure itself. In addition, there is a significant amount of stored energy associated with the power supplies required to drive the plasma source which supplies the ions to the accelerator. Measurements made at the Lawrence Berkeley Laboratory (LBL) [21] by adding extra capacitance to the accelerator indicate that the maximum unprotected stored energy necessary to avoid deconditioning breakdowns is ~5 J. A similar value was obtained by the DITE group at Culham Laboratory [22]. The group at Oak Ridge National Laboratory (ORNL) claim [23] that even with added capacitance corresponding to 50J of stored energy, they are able to achieve reliable operation following a deconditioning breakdown after careful reconditioning of the accelerator. The only apparent significant difference between the various systems being that ORNL are unique in their capability of operation at high repetition frequency (one lOOms shot per 20 s) and the unresolved question remains as to whether it would be possible to exceed the 5 J limit repor-
205
ted by LBL and DITE by much more extensive conditioning. It is clear that there are (at least) two types of grid breakdown [21]. Those which do not lead to deconditioning, in which the peak current is relatively low (loo's of amperes) and its rate of rise is relatively slow (100's of ns). The second class of breakdowns which does result in deconditioning is characterised by high (kA) oscillatory currents which have a rapid rise time. Apparently, either type can occur under a given set of operating conditions, but the group at LBL ensure that the discharge current from the major fraction of the stray capacity of the plasma source power supplies and the main power supply cable (after the modulator) is always limited in rise time and peak current by the use of snubbers [24]. These devices introduce a transient series impedance into the circuit by the use of a carefully designed set of ferromagnetic cores which are positioned around the bundle of input cables close to the plasma source and accelerator. The value of the maximum allowable stored energy will ultimately set a limit on the maximum area of the accelerator, due to its selfcapacity. Using the Child-Langmuir equation with the assumption of a constant electric field, Cooper [25] has estimated that the maximum module size for a 2OO-kV accelerator with 60% transparency would be ~35 A extracted current if the 5 J limit does in fact apply. Recent work by Bottiglioni and Bussac [26] on a simulated grid structure indicate that, in the absence of the beam, the non-deconditioning energy seems to increase with increasing accelerator gap spacing and could be in the range of 25-1OOJ for 20-mm gaps appropriate to a 2OO-kV hold-off voltage. This is a more encouraging result, but there is a clear need to gain more understanding and quantitative data on these topics in addition to resolving the rather wide discrepancies which exist at present. 3.2. The plasma source
The function of the plasma source is to
206
E. Thompson~Particle accelerators for fusion research
supply the required flux of ions to the accelerator. The plasma current density of ~--0.20.3 A cm -2 must be uniform and quiescent over the relatively large extraction area-typically ~400-1000 cm 2- from which the ions are extracted to ensure all apertures are perveance matched. In addition there is a high premium on the plasma source being able to produce a high yield (~> 80%) of atomic ions in order to ensure that most of the neutral beam will satisfy the penetration requirement. A high electrical and gas efficiency of the plasma source is also a desirable but somewhat secondary requirement. All plasma sources used to date are variants of the hot cathode low pressure arc discharge. Two types are under development in Europe, the so-called Periplasmatron at Fontenay-aux-Roses [27] and the Bucket Source [28, 29] at Culham Laboratory. These are shown in figs. 5 and 6, respectively. In the Periplasmatron a weak cusp field is used to inhibit the primary electrons emitted by the hot cathode from being collected directly by the anode. The primary electrons oscillate along the magnetic field thus increasing their path length in the arc chamber which operates typically at a filling pressure of approximately 5 mT. The Bucket Source uses a high order multipolar magnetic field to confine the primary electrons away from the anode. Owing to the rapid decrease in magnetic field with distance from the anode, the inner volume is essentially field-free resulting in a high degree of plasma uniformity. The primary electrons are confined within this field-free volume thus enabling the source to operate at filling pressures in the region of 25 mT.
3.3. The neutralizer This is a simple tube surrounding the ion beam. In present generation injectors operating at ~30 kV the thermal gas operating from the plasma source supplies sufficient gas to give the required gas target necessary for a conversion of ions to atoms which is close to the theoretically predicted thick target yield. Injectors operating at higher energies per nucleon will require an
additional gas feed which will also enable the neutral gas density profile to be optimised in terms of reducing the power loading in the accelerator. However, the higher target thickness ( ~ 300 mT cms) necessary at higher energies, leads to a higher overall gas flow into the injector.
3.4. Charged beam disposal The fraction of the original ion beam which is converted to neutral atoms is a decreasing function of the beam energy (fig. 7). The remaining fast ions must be removed from the neutral beam in order to prevent them being deflected by the plasma confinement magnetic fields, giving rise to uncontrolled and high power loading and gas generation close to the confinement system. This usually is achieved using a magnetic field perpendicular to the beam axis which directs the unwanted ion beam onto a cooled beam dump. For short pulse injectors (less than 1 s), the beam dump is a simple copper plate with sufficient thermal capacity to absorb the total energy which is removed between pulses. The plates are suitably inclined with respect to the incident beam in order to reduce the specific power density and hence surface temperature and thermal stress. Next generation long pulse injectors will require actively cooled beam dumps capable of dissipating total powers in the region of a few megawatts with a surface power density ~1 kW cm -2. This can be accomplished using either very high Reynolds number water flow or, as is proposed for the JET injector, the hypervapotron principle developed by Thomson CSF for anode cooling of high power vacuum tubes. An interesting alternative under development at KFA Jfilich is that of a rotating beam dump [30] based on the technology developed for high intensity X-ray targets.
4. Auxiliary systems We can only briefly mention here two of the major auxiliary systems which are necessary for the operation of complete injection systems [31].
E. Thompson~Particleacceleratorsfor fusion research
207
/
/J
/ ....
,,~ / / . ~ . .
~.
~
5
®
lOcln
Fig. 5. Periplasmatron developed at Fontenay-aux-Roses [27]. 1-Anode; 2-intermediate electrode; 3-cathode; 4-3 electrode extraction system.
4.1. Pumping
The neutral injectors must of course operate in a high vacuum environment to prevent a large influx of thermal gas into the confinement device. A more restrictive condition is placed on
the operating pressure by the need to prevent re-ionization of the neutral beam during its flight path to the plasma. Apart from the adverse effect on total power transmission, the resultant energetic ions are deflected by the stray fields
E. Thompson~Particle accelerators for fusion research
208
INSULATOR
MULTIPLE APERTURE EXTRACTION GRID
SCALE lOcm
_ WATER COOLING
Fig. 6. Bucket Source developed at Culham Laboratory [28, 29].
1.0 0.8 D-
0.6 0.4
o.2I 0 0
100
200
300
Energy per nucleon
400
500
(keY)
Fig. 7. Efficiency ~ for atom production from D ÷ and D- ion beams as a function of its energy per nucleon (deuterium energy is twice this value).
from the confinement system and can be deposited in the input duct. Unless the system is correctly designed the gas generated in this duct can lead to a higher pressure in this region, resulting in more re-ionization and hence even higher gas pressure, etc. [32].
In order to pump the considerable quantities of gas introduced into the system- mainly from the neutraliser and beam dumps [~100's of Torr l s-1] -large condensation cryopumps have been developed [33] which operate at liquid helium temperature. Installed pumping speeds larger
E. Thompson~Particleacceleratorsfor fusion research
than I x 1061 s-1 using such pumps are a common feature of all present and next generation injectors. In one or two cases titanium gettering is used for injectors having pulse lengths of 100 ms.
4.2. Power supplies Prior to the need for long pulse lengths, capacitor banks, some of which incorporate a series tetrode for regulation and fast switching, were used. Larger installations use multimegawatt transformer rectifier sets with either series or shunt protection and/or regulation which in themselves are a considerable "tour de force" [ The few hundred kilowatts needed to drive the plasma source are provided by the transformer rectifier sets isolated to the accelerator potential. Special care is taken to reduce the stray capacity to the ground of these units by, for example, using SF6 rather than oil to provide the insulation.
5. Advanced systems 5.1. Negative ions As is obvious from fig. 7, the power conversion efficiency for the production of neutral atoms by electron capture by positive ions falls to unacceptably low values at energies larger than 100keV per nucleon. The production of fast neutrals by electron stripping of negative ions is clearly more efficient. However, injectors based on negative ions still require considerable development [34]. There are essentially two main options for the production of the primary beam of negative ions. (a) Sequential double electron capture in which a low energy beam of positive ions ( ~ few keV) is converted to negative ions of the same energy by electron capture in an alkaline vapour cell. For instance, caesium exhibits a conversion of positive to negative ion of approximately; 20% at ~1 keV per nucleon. Sodium is somewhat less efficient but the peak cross-section is at higher energy, which alleviates the problem of formation and transport of the low energy ion beam [35-37]. The negative ions are electrostatically
209
separated from the electrons generated in the conversation cell prior to acceleration to the required energy. They are then converted to fast neutral atoms by electron stripping using a gas or, for greater efficiency, a plasma target. Any residual ions (positive and negative) must then be removed as in conventional injectors. As is perhaps obvious from the above brief description, such an injector would be considerably more technologically complex than the present systems. (b) Direct extraction is, in the author's opinion, a technologically more attractive option. In this system the plasma source is designed to produce a high yield of negative ions which can be extracted and accelerated in a similar manner to that already used for positive ion systems. .There is of course the additional complication of electron suppression of the plasma source to prevent the extraction and acceleration of a large electron current along with the negative ions, which would have a disastrous effect on the efficiency. Multi-ampere beams of negative ions have in fact been generated for ms pulse lengths following the original work of Dimov [38, 39] in which a controlled amount of Cs is introduced onto the cathode of the plasma source. Fractional monolayer coverage of Cs on tungsten or molybdenum has the effect of reducing the work function of the metal to a sufficiently low value to allow the formation of H- from low low energy H + impacting on the surface. A high degree of electron suppression is obtained by the magnetic field which plays an essential role in these magnetron sources. Other schemes (as yet unpublished) utilizing caesiated surfaces in other types of plasma source are being studied in LBL and ORNL, but to the author's knowledge there is as yet no negative injector being designed for use on present or the coming generation of confinement experiments.
5.2. Energy recovery The final topic we wish to outline is one which could have a favourable, albeit limited, impact on injector efficiency and could also alleviate some of the other technological problems
E. Thompson~Particle accelerators for fusion research
210
encountered in a positive ion injector. The basic idea is to recover the energy of the fast ions emerging from the neutralizer directly, by retarding them in an electrostatic field. This has been studied in some detail at Fontenay-auxRoses (FAR) [40], Lawrence Livermore Laboratory (LLL) [41], and the Oak Ridge National Laboratory (ORNL) [42]. One of the major problems in this field of energy recovery is that of suppressing the electrons generated in the neutralizer and preventing them from being accelerated by the electric field used to decelerate the fast ions. Experiments at FAR were the first to demonstrate that efficient energy recovery can be obtained using a series of high transparency grids at the neutralizer exit. This relatively simple system cannot be extrapolated to long pulse high energy systems owing to the power loading on the grids which intercept the beam. It has been proposed that a "separated beam" system, in which the energy recovery system is virtually identical to an accelerator grid system working in reverse, could overcome this difficulty, but this has not been demonstrated. Computed trajectories of the fast ions in the FAR structure are shown in fig. 8. The work at LLL/LBL also used electrostatic electron suppression using large electrodes which surround the total beam. Problems were
Neutrolizer r°.
_ [
Suppressor
Grid
- 96 kV
Plasmo SurfQce
experienced with gas emission from surfaces which were heated by particle bombardment giving ionization and a serious reduction in overall efficiency. These too, to large extent, were overcome, but nevertheless there does not appear to be any intention of using these systems in the near future. An alternative and attractive method of providing the electron suppression is that under study at ORNL in which a transverse magnetic field is used at the exit of the neutralizer and E x B drift motion of the electrons prevents them from being accelerated to high energy. The use of a successful energy recovery scheme would only have a favourable impact on power efficiency if either the source has a high proton yield and/or the energy of lower energy ions resulting from molecular ion break-up is recovered. Nevertheless, the use of energy recovery offers the advantages of essentially eliminating the problems associated with the beam dump (section 3.4) and also it allows the plasma source and its power supplies to be operated at ground potential with the neutralizer at high negative potential. Apart from the consequent simplification of the power supplies and control, the stray capacity and the resultant problems of stored energy discussed in section 3.1.3. could also be reduced.
.~ ~
X ~
~Recovery -~
PIQte
-80 kV Equipotential Surface
Fig. 8. Direct energy recovery system developed at Fontenay-aux-Roses [40]. Shown are the computed ion beam trajectories. Beam ions are gradually peeled off the main beam by the suppressor grid potential (a few k V negative with respect to the neutraliser) and collected at the recovery plate. Electrons are stopped at the plasma boundary impressed on the neutraliser plasma by the suppressor grid.
E. Thompson~Particle acceleratorsfor fusion research
6. Conclusion High power neutral injection has been extremely successful in producing high temperature plasmas for controlled thermonuclear fusion research. The coming generation of injectors operating at higher voltages and extended pulse lengths pose many technological problems for which workable solutions appear to be available. Nevertheless, considerable research and development remains to be carded out to meet the needs of higher efficiency at higher beam energy.
[15] [t6] [17]
[18]
[19]
[20]
Acknowledgements [21]
It is with much pleasure that I acknowledge the invaluable help and many discussions with colleagues throughout the world on all topics related to neutral injection and in particular those at Culham, FAR, LBL and ORNE. Particular thanks are due to Dr. A.P.H. Goede for his assistance in the preparation of this paper.
[22] [23] [24] [25] [26] [27] [28]
References [1] The JET Project-EUR 5791e (1977). [2] Proc. 8th I A E A Conf. Plasma Phys. & Control. Nucl. Fus. Res., Brussels (1980). [3] Proc. Joint Varenna--Crrenoble Syrup. on Heating in "For. Plasmas, Grenoble (1978). [4] T.S. Green, 10th Symp. on Fusion Technology, Padova (1978). [5] H. Eubank et al., Phys. Rev. Lett. 43 (1979)270; and Prec, 7th I A E A Conf. lnnsbruck, vol. I (1978), p. 167. [6] W.C. Turner et al., NucL Fusion 19 (1979) 1011; and Prec. 7th I A E A Conf. Innsbruck, vol. 2 (1978) p. 389. [7] Proc. 2nd Syrup. on Ion Sources and Formation of Ion Beams LBL-3399, Berkeley (1974). Also see [4] and the references therein. [8] E. Thompson, Prec. 2nd Syrup. on Ion Sources and Formation of Ion Beams LBL-3399, Berkeley (1974), p.ll-3. [9] L.R. Grisham, C. C. Tsai, J.H. Whealton and W.L. Stirling, Rev. Sci. Instrum. 48 (1977) 1037. [10] E. Thompson, Prec. 2nd Symp. Ion Sources and Formation of Ion Beams LBL-3399, Berkeley (1974), p.II-7. [11] A.J.T. Holmes, Phys. Rev. A19 (1979) 389. [12] J.R. Coupland and T.S. Green, Nucl. Instrum. Methods 125 (1975) 197. [13] A.C. Wager, E. Thompson and A.J.T. Holmes, Prec. Joint Varenna-Grenoble Symp., Grenoble (1978), p.89. [14] W.S. Cooper, K. Halbach and S.B. Magyary, 2nd Syrup.
[29]
[30] [31]
[32]
[33]
[34]
[35] [36]
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on Ion Sources and Formation of Ion Beams, LBL-3399, Berkeley (1974), p.II-1. Y. Okumura, Y. Ohara and T. Ohga, JAERI-M7696 (1978). M.N. Semashko, I A E A Workshop on Neutral Injection, Culham (1978), to be published. A.J.T. Holmes and T.S. Green, 2nd Conference on Low Energy Ion Beams, Bath (1980) to be published by the Inst. of Physics, London. J.A. Paterson, G.W. Koehlen, R.P. Wells and L A . Biagi, 8th Symp. on Engineering Problems of Fusion Research, San Fransisco (1979), p. 20. A.J.T. Holmes, C.E. Brookes, A. DeVere, D.P. Hammond and T.S. Green, 8th Syrup. on Eng. Problems of Fusion Research, San Fransisco (1979), p. 20. H. Hemmerich and P. Kapschus, private communciation, KFA Jiilich. K.H. Berkner and A. Lidsky, LBL workshop on neutral injection, private communication (1979). R.S. Hemsworth, provate communication. H.H. Haselton and M. Mennon, LBL workshop on neutral injection (1979). J. Fink, W.R. Baker and H.M. Owren, IEEE Trans. Plasma Sci. 88 (1980) no. 1. W.S. Cooper, private communication. F. Bottiglioni and J.P. Bussac, EUR-CEA-FC no. 1052. M. Fumelli and F.P.G. Valckx, Nucl. Instrum. Methods 135 (1976) 203. R.S. Hemsworth et al., Proc. Joint Varenna-Crrenoble Symposium on Heating in Toroidal Plasmas, Grenoble (1978), p. 83; and 9th Eur. Conf. on Controlled Fusion and Plasma Phys., Oxford (1979), p. 17. A.P.H. Goede et al., Prec. Joint Varenna-Grenoble Symposium on Heating in Toroidal Plasmas, Grenoble (1978), p. 77; and 8th Syrup. on Engineering Problems of Fusion Research, San Fransisco (1979), p.12. H. Hemmerich and P. Kupschus, private communication, KFA Jiilich. See proc. of the Symposia on Fusion Technology, 8th SOFT, Noordwykerhout (1974), 9th SOFT Garmisch Patterkirchen (1976), and 10th SOFT Padova (1978). L D . Stewart, Report PPPL-1469, Princeton University; also A.C. Riviere and J. Sheffield, Nucl. Fusion 15 (1975) 944. J.R. Coupland, D.P. Hammond and E. Thompson, 7th Syrup. on Engineering Problems of Fusion Research, Knoxville (1977) 1976 E.B. Hooper Jr., Joint Varenna--Grenoble Syrup. on Heating in Toroidal Plasmas, vol. 2, (1978), p, 71; see also Symposium on the Production and Neutralisation of Negative Hydrogen Ions and Beams, Brookhaven (1977). T.S. Green, Workshop on negative ions, Cuiham Laboratory 1977). M.N, Semashko, V.V. Kuznetsov and A.I. Krylov, Symposium on Production and Neutralisation of Negative Hydrogen Ions and Beams (1977) Brookhaven National Lab.
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[37] B.A. D'Yachkov, A.I. Drylov, V.V. Kuznetsov, B.P. Maksimenko and M.N. Semashko, Kurchatov Institute of Atomic Energy, Report 1AE-2523 (1975). [38] Yu. I. Bel'chenko, G.I. Dimov and V.G. Dudnikov, Sov. Phys Techn. Phys 20 (1975) 40. [39] K. Prelec and T. Sluyters, Proc. 2rid Symp. on ion Sources and Formation of Ion Beams, Berkley (1976), p. VIII-6; LBL Report no. 3399. [40] Ph. Raimbault and M. Fumelli, Proc. Joint VarennaGrenoble Int. Symp. on Heating in Toroidal Plasmas, vol. 1, (1978) p. 71.
[41] W.L. Barr, J.N. Doggett, G.W. Hamilton, J.D. Kinney and R.W. Moir, 7th Syrup. on Eng. Problems of Fusion Research Knoxville (1977), p. 308; and W.L. Barr, R.W. Moir, G.W. Hamilton and A.F. Lietzke, 8th Syrup. on Eng. Problems of Fusion Research, San Fransisco (1979), p. 20. [42] W.L. Stirling, J. Kim, H.H. Haselton, G.C. Barber, R.C. Davis, W.K. Dagenhart, W.L. Gardner, N.S. Ponte, C.C. Tsai, J.M. Whealton and R.E. Wright, Appl. Phys. Lett. 35 (1979) 104.