The periplasmatron, An ion source for intense neutral beams

NUCLEAR

INSTRUMENTS

AND

METHODS

i35 (I976) 203-209; ©

NORTH-HOLLAND

PUBLISHING

CO.

THE P E R 1 P L A S M A T R O N , AN I O N S O U R C E FOR INTENSE NEUTRAL BEAMS M

F U M E L L I and F. P. G. V A L C K X

Assoctatton Euratom-CEA sur la Fuston, Ddpartement de Phystque du Plasma et de la Fuston ContrOlde, Centre d'Etudes Nacldatres, B.P no 6. 92260 Fontenay-aux-Roses, France Received 5 March 1976 A n intense fast hydr ogen a t o m b e a m o f 300 k W power has been o b t a i n e d with a new type of ion source, the p e n p l a s m a t r o n . Source and neutral beam o p e r a u o n p a r a m e t e r s are given

1. Introduction A new type of ion source is being developed at our laboratory for the production of intense neutral hydrogen atom beams for application in controlled fusion experiments. In this source, which was originally called annular duopigatron 1) is used a particular method of creating a homogeneous plasma over large surfaces, which consists of injecting energetic electrons emitted by a ring cathode from the periphery toward the axis of a hydrogen gas filled cylindrical chamber from which the beam is extracted axially. This injection takes place in the median plane of a cusp-like magnetic field configuration where the energetic electrons are temporarily trapped: this effect increases their ionlsation efficiency. By a combined system of electrodes held at a floating potential the electrons are forced to cross the magnetic field lines in order to reach the anode like in a Penning discharge. In order to emphasize the characteristics of this source and to avoid confusion with the Oak Ridge duopigatron 2) its name was recently changed into periplasmatron. A new version of this source was constructed which permits to illuminate a circular extraction surface of about 20 cm diameter. We give here the results obtained with a multihole three electrode extraction system of 14.5 cm diameter. The source is operated at earth potentiala). The neutralizer (a hydrogen gas cell adjacent to the extraction system where the accelerated ions are partially converted into fast atoms by charge exchange and dissociation) is biased at the negative potential corresponding to the desired beam energy. At the exit of the neutralizer there exists in this disposition a strong electric field which decelerates the energetic ions remaining in the beam. This field removes the nonneutralized fraction from the beam and allows even-

tually to provide for the energy recovery of this fraction4).

2. Experimental system 2.1. THE PLASMASOURCE A schematic view of the ion source and the injection system is given in fig. 1. An axasymmetric cusped magnetic field configuration is created by the coils shown in the figure. The zero magnetic field point is at the center of the chamber. The field lines concentrate at the exit of the annular Intermediate electrode made of mild steel. The annular cathode is placed in the magnetic field-free region behind these electrodes. The primary electrons are injected radially following the magnetic field lines toward the extraction chamber. Here the first electrode of the extraction system and a a stainless steel electrode on the opposite side act as reflectors for the electrons. In order to reach the anode the electrons must diffuse across the magnetic field lines. In previous experiments 1) the cathode was a circular oxide-coated mckel ribbon of 28 cm diameter. This cathode is now changed. Instead of the ribbon 12 aligned spiral filaments of ~ 1 mm tungsten wire are used. They are ac heated, each in counter phase to its neighbours. Each filament is connected through a 1 resistor to the arc power supply. The total emitting surface of the cathode is 70 cm 2. The anode is grounded, the reflector electrodes and the intermediate electrodes are connected to the anode through resistors of 20 Q and 100 £2 respectively. The hydrogen gas inlet is on the extraction chamber 2.2. THE ~EAMFORMINGSYSTEM -- The ion beam is formed with a multi-aperture three electrodes system. The electrodes are 0.3 cm thick copper disks with a 14.5 cm diameter free region

25

!

50¢m

iI

Zig 1. Schematic view of the perlplasmatron and injector hne (1) anode, (2) intermediate electrode, (3) cathode, (4) multi-aperture electrodes, (5) neutralizer, (6) grid ystem, (7) floating wall.

t

/

-)

a.

,3

r~

r5

THE PERIPLASMATRON containing 450 holes of 0.55 cm diameter (107 cm 2 extraction area). The transparency is 0.65. The electrode spacings are 0.6 cm for the accelerating gap and 0.3 cm for the decelerating gap. These electrodes have a spherical shape with a radius of 160 cm and the apertures are drilled radially in order to make a converging beam. - The principal electrical connections of the grounded ton source system are the following: The neutralizer tube and the third extraction electrode are at the same potential. They are connected, by means of a variable resistor R, with the second extraction electrode which is connected to the hv generator, its potential being - V . When the beam is on, a positive current I, (due to the slow ions resulting from charge-exchange collisions of the ion beam with the neutral gas) polarizes the neutralizer and the third electrode at the "decel" potential:

205

meters of 5, 10, 15 and 20 cm diameter respectively. These calorimeters are molybdenum plates, thermally insulated. The temperature rise during the pulse is measured with Cr-AI thermocouples. The calorimeters can be swept out of the beam. In this case the neutral beam travels over about 3 m in the test chamber where it is partially reionised by collisions on the neutral gas. During the pulse, the pressure in the test chamber rises to about 6 × 10 - s torr. The composition of these fast ions is measured at the far end of the test chamber with a magnetic analyzer. From these measurements, corrected for the neutralion conversion coefficient of the test chamber gas target thickness one can deduce the relative abundance of H o fast atoms in the neutral beam of e, ½e and ½e energy, resulting respectively from the H +, H + and H~accelerated ion species.

3. Experimental results Va = R I . ~ 0 . 1 V ;

this potential is controlled by varying R. The energy of the ton beam in the neutralizer is e ~ 0.9 V (eV). - The neutralizer is a stainless-steel tube of 15 cm interior diameter, and 70 cm in length. The gas flow from the source gives a pressure gradient along the neutrahzer. The hydrogen gas-target thickness is 0.16 t o r r . c m which is, at 30 kV, sufficient to obtain the equilibrium between the ion and neutral components of the beam. At the exit of the neutralizer is mounted a supressor grid made of parallel tungsten wires of 0.6 m m diam., spacmg between wires 8 mm, transparency 93%. This grid, which is at the same potential as the second electrode of the extraction system, acts as a reflector for the plasma electrons of the neutralizer. It prevents these electrons to leave the neutralizer and to be accelerated. A screen grid is placed at 5 cm from the suppressor grid. This grid, made of parallel tungsten wires of 0.6ram diam., 16ram spacing and 96% transparency, is grounded and serves as electrostatic screen.

3.1. DISCHARGEMECHANISM In all experiments reported here the discharge was run at a hydrogen gas pressure of 10-2 torr. Because of the large ratio of the neutral gas conductance of the intermediate electrodes to the conductance of the extractton grids, a negligible pressure drop exists between the cathode region, enclosed by the intermediate electrodes, and the extraction chamber. The ionisation mean free path of the electrons is of the order of 25 cm and very few ionisations are produced in the cathode region; thus the plasma is mainly produced in the extraction chamber by the primary electrons which are temporarily captured in the magnetic trap of the cusp magnetic field before being lost on the reflectors or on the anode electrodes. Plasma electrons oscillate between the intermediate and the i

i

/a) .~

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~ so Q

2.3. TEST C H A M B E R

AND NEUTRAL

BEAM MEASURING

EQUIPMENT

The properties of the neutral beam are studied in a large chamber adjacent to the neutralizer. This chamber is pumped at a speed of about 105 1/s (hydrogen gas) by two oll diffusion pumps. The energy and the profile of the neutral beam were measured at 1.7 m from the extraction electrodes with four concentric callori-

I 100

I

200 DISCHARGE

I 300 CURRENT (A)

Fig. 2 Voltage-current characteristics of the d~scharge at a constant gas flow rate and magnetic field strength and for three values of the cathode filaments heating-current: (a) 53 A, (b) 55 A, (c) 58 A.

206

M. F U M E L L I

AND

reflector electrodes (along the magnetic field lines) like in a Penning discharge. The current of the primary electrons represents the dominant fraction ( ~ 9 5 % ) of the total discharge current. The voltage-current characteristics of the discharge for three differents values of the filament heating current are shown In fig 2. The calculated values of the thermo-iomc emission current are for these cases respectively 100, 160, and 300 A. It can be observed that the electrical impedance of the discharge increases rapidly when passing from a space-charge hmited emission mode to a thermoionic emission limited mode. In the experiments reported here the filaments beating current was ~ 58 A (curve C of fig. 2). These voltage-current characteristics were taken at a fixed value of the magnetic field current Im=2.5 A, the mean value of the magnetic field in the chamber being about 20 G. When lm lS increased above a crittcal value of 2.7 A a sudden decrease of the dtscharge current is observed and correspondingly an increase of both the discharge voltage and the (negative) potential of the reflector electrodes (see fig. 3). This new mode appears roughly when the Larmor radius of the primary electrons becomes of the order Arc voltage 0 Arc current

200 C~

150

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3.2.

-,'~---

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50 I

MAGNETIZING CURRENT(A) ,

,

,

,

,

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The discharge can run at zero magnetic field strength, however, in this case, the ionlsation efficiency ~/= I +/I~, i.e. the number of plasmaions produced per one primary electron is very low" ~/~0.3. The lomsatlon efficiency is evaluated from the experimental data, assuming plasma homogeneity, by the relation: J+Z

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FLASMA PRODUCTION

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100 U

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of half the distance between the two anode edges, i.e p ~ l cm (see fig. 1). Under these conditions the transverse m o u o n of these electrons would be strongly hindered (~oo%>>1) and a negative space charge shear will develop near the anodes giving rtse to a potential drop, so that a higher voltage must be supplied in order to sustain the discharge. The observed reduction of the primary electron emisston (the filament heating current was not changed) is explained by a virtual cathode near the filament surface. The discharge, immediately beyond the transition point, is still quiescent but by a slight increase of lm, or sometimes spontaneously, the discharge falls Into a new mode characterised by a high noise level, higher discharge current and lower applied voltage. For Ira>4 A only this unstable state exists. Clearly the electron conduction across the magnetic field lines has been re-established by an instability, in a weakly lonised plasma the transverse drift of the plasma across a magnetic field can result from charge separauon provided by the difference In drag forces on ions and electrons colliding with the neutral gas. An instabihty of this type which is driven by the application of a potential difference across magnetic field lines and which may have direct relevance to our discharge, is described by SimonS). However the study of this instability was beyond the purpose of the experiments

200

100

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F. P. G . V A L C K X

~ ~ .AG.E~Z'NG C~R~NTIAI "

F~g 3. Effect o f t h e m a g n e t i z i n g c u r r e n t lm o n t h e s o u r c e p a r a m eters. (a) V a n a U o n o f t h e d m c h a r g e v o l t a g e a n d c u r r e n t , (b) E l e c t r i c a l p o w e r in t h e d i s c h a r g e f o r t h e p r o d u c U o n o f I A o f extracted mn beam and reflector electrode potenttal The e x t r a c t e d i o n b e a m c u r r e n t v a n e s f r o m I + = 5 A a t I m = 0 to 1+ =115Aatlm=25A

Id_J+ S,

(1)

where J + , Io, 22, S are respectively the extracted ion beam current density, the discharge current, the total surface of the chamber which collects plasma ions (anodes are excluded), the total cathode surface. By increasing the magnetic field strength q grows rapidly. Its value is of the order of 1 at the transition region, decreases in the unstable mode but it starts to increase again at higher magnetic field strengths. The electrical power spent m the discharge to produce 1 A of extracted ion beam has been taken as a crlterium for the source efficiency. This power, given In fig. 3b as a function of the magnetizing current, shows roughly the same behaviour as the iomsatlon

207

THE P E R I P L A S M A T R O N

efficiency but it has a well defined minimum just before the transition point, at 2.5 A. Because of the high ionisation and power efficiency and the very good plasma stability, this value of magnetizing current has been chosen for normal operatmn. In the range of the working points used, the power needed for 1 A mn beam varies between 0.7 kW/A and 0 9 kW/A. The mnisation efficiency q varies between 0.6 (3 A mn beam) to 0.9 (20 A ion beam) depending on the discharge voltage. At the cathode the ratio of the emitted electron current density over the plasma ions current density varies in the same range from Je/J + =28 to Je/J + = 17 (20 A ion beam). These values satisfy the Langmmr sheet stability criterium 3.3. THE INJECTOR PROPERTIES The new cathode allows the discharge to run m dc state. When the beam ~s on, the thermal loads on the extraction electrodes and on the grid system hmit the pulse length. For a 600 kW ion beam power, the grid system limits the allowed pulse length to about 50 ms. The measured gas flow from the injector is 7 1. torr/s. The neutral gas pressure at the exit of the neutraliser is about 3x 10 -4 torr. The neutralizer gas target thickness is 0.16 torr.cm. The extracted ion beam current is shown in fig. 4 as a function of the arc current. The error bars in the

20 tlJ

figure represent the dispersion of the experimental points observed over a period of about six months. The beam was extracted at approximately constant perveance value: I + = k V 3/2, with k = 3.5 x 10- 6 A V - 3 / 2 . The total neutral beam power measured within the 20 cm diam. calorimeter at 1.70 m from the source, represents roughly 50% of the total drain hv electrical power. The neutral beam power measured within the four concentric calorimeters is shown in fig. 5 as a function of the radius and for three values of the ion power: 600 kW (20 A, 30 kV), 400 kW (15 A, 27 kV), and 200 kW (10 A, 20 kV). It can be seen that the 300 kW neutral beam power is distributed as follows: 60 kW within 5 cm diam., 130kW within 10cm diam. and 2 4 0 k W within 15 cm diam. By extrapolating this curve it can be estimated that about 40 kW neutrals would be at a radius larger than 10cm. Taking into account that 11% of the beam is intercepted by the grid system and assuming a neutralisation efficiency of 75% it is clear that about 15% of the beam power is lost by interception of the beam by the neutralizer and the extraction electrodes. The neutral beam current density (A eqmvalent) as a function of the radius is given in fig. 6. The neutral beam equivalent current ]s taken as /~q = W/0.9 tV, where W a n d t are respectively the energy transferred to the calorimeter and the pulse length. Experiments at lower extracted beam current show that the beam optics is improved by working at a lower perveance value ( k ~ 2 . 3 x 10-6AV-3/2). Unfortunately, due to the actual hv system limitations it was not possible to reach this value at high extracted beam current.

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£ 200 t3d rn 0£

u3 100 Z

,3°

23° DISCHARGE CURRENT (A)

Fig 4 Ion beam intensity (dram current) as a function of the dJscharge current

5

10

CALORIMETER RADIUS[cm)

Fig. 5 Total neutral beam power as a function o f the target radms (a) 2 0 A ]on beam, 30kV, (b) 1 5 A ion beam, 27kV, (c) 10 A ion beam, 20 kV.

208

M. F U M E L L I A N D F. P. G. V A L C K X

3.4. G R I D SYSTEM The suppressor grid produces a potential barrier for the neutralizer plasma electron by acting as a negative probe and imposing a positive space charge sheet. One expects that if the thickness 2 of such a sheet is of the size of the grid opening, the potential barrier will be formed i.e. if:

L the neutralizer length and r the radius. P(x) and n(x) can be approximated by:

/2 ( X ) =

(f/max - - g/min) X

?/mm "~

=

An

D'mm -~ - -

L 2 1> c¢d,

(6)

(2)

where ~ is a factor which depends on the particular grid system which is used. The thickness of such a sheet can be estimated from the Child law:

where nmax and n~,,, are respectively the neutral gas pressure at the entry and at the exit of the neutralizer. By mtegrating eq. (4) it follows that:

J+

9

=

l+ a' { /'/mm 2 nr z

['L + r -- 4 ( L 2 +

r2)]

+

J~+ +-2L

where V~ is the grid potential (negative in respect to the neutralizer potential) and J~+ is the cold ion current density at the exit of the neutralizer. This current density can be expressed as:

j+

X,

L

= ~rcr2 fLo I+ n(x) a, P(x) dx,

(4)

where n (x) is the neutral gas density in the neutralizer, ai the total cross section for slow ions production, I + the beam intensity, P(x) the probability for a slow ion produced at a distance x from the exit to reach the exit, .

,,

,,

x

L2 - L x / ( L 2 + r 2 ) - r 2 1 n x

1}

L + x/~L,Z+r 2) '

.

(7)

More exact calculations of the sheet thickness have been made taking into account also the unneutralized fraction of the beam by integrating the Poisson equation (neglecting the electron density): 1

vZ q~ = - -- (Pcold+ Pf, s,)"

(8)

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< E

tu lOO nt

~

75

7 Q <

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25

0,5 -10

-5

I

0

.5 10 RADIUS (cm}

Fig. 6. Beam density as a function o f the radtus for a 300 kW neutral beam.

0,75

I

1,'25

1,S X/d

Fig. 7. Efficiency of the suppressor grid in retaining the neutrahzer plasma electrons. The ratio o f accelerated electron beam over the ion b e a m I d I + as a function of the ratio of the sheath thickness over the size of the grid opening J,ld.

THE PERIPLASMATRON

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OA 0 Z

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5- H~at O, D.

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50 Z 0

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100

200

209

tracted ion beam density). The results are given in fig. 8. It can be seen that for a 250 A discharge current (extracted ion beam density of the order of 0.2 A/cm 2) the neutral beam is composed essentially of Ho atoms with energies e and ½e at almost equal concentrations. From these results the composition of the extracted ion beam can be deduced. At the highest current density the H + fraction represents 70% of the total ion beam.

300

D I S C H A R G E C U R R E N T (A)

Fig. 8. Neutral beam composztlon for different values o f the dJscharge current.

However for the actual beam energy (10-30 keV) the neutrahzation efficiency is high and the beam space charge gives only a small correction to the sheet thickness as calculated from eqs. (3) and (7) ( ~ 10%) By working at a constant perveance value of the extracted ion beam and at a fixed ratio R = V/~, where V is the ion beam accelerating potential, the sheet thickness remain almost constant as can be seen from eq. (3) and taking into account that for a fixed gas flow rate: J~+ ocI +. In our case 2 was almost constant (2-- 1.1 cm). The sheet thickness can be varied by changing the grid potential Vg, thls permits to determine the value of m eq. (2). The result is shown in fig. 7. F r o m these measurements a value of ~ 1 is obtained. 3.5. NEUTRAL BEAM COMPOSITION The composihon of the neutral beam was measured for different values of the discharge current (or ex-

4. Conclusions The perlplasmatron is a reliable ion source having a homogeneous, quiescent plasma. The performance of this source is in constant progress.* Neutral beams of 300 kW are readily obtained. This power is limited mainly by the actuel hv and protection system. The pulse length is actually limited by the thermal load on the grid system at the exit of the neutralizer. We wish to thank Messrs S. Dimarco, M. Block and J. Godaert for their technical assistance. This work was partly supported by JET contract B-FC-74. References 1) M F u m e l h , R. Becherer, L. Bergsten a n d F.P G. Valckx, Proc 2nd Int. Conf. on Ion sources, Vienna (•972) p 289. 2) W L StNrhng et a l , Proc Syrup. on Ion sources and formation o f ton beams, B r o o k h a v e n N.L. (1971) p. 167. 3) M F u m e l h , Nucl Instr. and Meth. 118 (1974) 337. 4) M. F u m e l h and F P G Valckx, Proc. 2nd Syrup. on 1on sources and formation o f ton beams, Berkeley (I 974) p. VI-6-1. 5) A Simon, Phys. F l m d s 6 (1963) 382. * Indeed, recent m e a s u r e m e n t s at 4 0 0 A arc current (100 V apphed voltage) show that a n 1on b e a m o f 40 A could be extracted f r o m the source.