High energy, high current neutral beam injector operation with single stage and two-stage multi-aperture extraction systems

Nuclear Instruments and Methods 203(1982) 389-402 North-Holland Publishing Company

389

HIGH ENERGY, HIGH CURRENT NEUTRALBEAM INJECTOR OPERATION WITH SINGLE STAGE ANDTWO-STAGE MULTI-APERTURE EXTRACTION SYSTEMS R. BECHERER, M. DESMONS, M. FUMELLI, P. RAIMBAULT and F.P.G . VALCKX Association E.,... -CEA sur la Fasion, Déparörrent de Rec/rerchec .cur la Faciou Cnntrrilie. C-Boîte 6, 9:260 Fontetru3-uux-Rorer, France

Received 31 March 1982 Neutral beam development for JETinjectors at FAR laboratory has led to the study of properties of a single stage (trirxlel and a two-stage (tetrode) multi-aperture extraction system at ion beam powers exceeding the megawatt level and up to 80 keV beam energy, The results of the experimental measurements andof anumerical study of the beam optical qualities andgrid power loadings of these systems are presented. Grid powerloading levels of less than 1 `.K of the high-voltage drain power were measured in both the triode and the tetrode accelerators. This would allow long pulse operation (10 s with water-cooling) as required for Jtïl' . The. scant divergence angle (a =0.7o) and the transmission characteristics were almost identical . At the same energy . higher current densities, at optimum perveance, were obtained with the triode at a lower electric field stress on the high-voltage gap. The triode offers the additional advantage of being simpler from the mechanical and electrical points of view . Operation 4 the injection line with an electrostatic beam dump associated with agrounded source is also demonstrated for a 25 A ion heam up to 60 keV,

1 . Introduction Neutral beam injection units for plasma heating its the JET fusion device will require highly collimated beams and total single pulse energies well in excess of those of existing injection systems. These units will utilize a multi-aperture ionbeam extraction system (with a close coupled neutralizing gas cell) and thus the energy handling capability of such a system and its optical quality are the critical features of their design . The final design goal for these injectors is 0.65 MW neutral beam power, in full energy particles, transferred to the plasma, for "r=10s',pulse length . This corresponds to a 60 A extracted ion beam current (J' - 200 mA/cm') at 80 keV energy, for hydrogen, and a beamtet divergence angle a<0.7° . To produce such energetic ion beams, several authors [1-3] proposed the use of two-stage extraction systems. Instead of using three grids as in thesingle stage acceldecel system, the two-stage system uses an additional grid, the gradient grid, to amouple the ion accelerating gap in two parts. The advantages claimed are that such an electrode provides an additional degree of freedom to optimize the beam optics and that it improves the high-voltage hold-off of the total accelerating gap. Nevertheless, a tetrode system is more complex from both the mechanical and electrical points of view . If the gradient grid cannot be very thin, as is the case for a large area electrode which must be water-cooled (as in JET injectors), the total accelerating gap length will be 0167-5087/82/0000-0000/$02 .75 (D 1982 North-Holland

increased, which is not desired because this reduces the extracted ion beam perveance and can increase the charge exchange and ionization losses of the beans. In addition, in most of the existing two-stage systems, the high voltage accelerating potential is not equally shared over the two gaps which sacrifies considerably the advantage of the HV hold-off. Because of these considerations we have developed a thr-;e-electrode extraction system, for 80 keV operation, which is based on the accel-decel configuration, but takes advantages of the additional room between the apertures necessary for watercooling, to give a pscudttPierce shape to the plasma electrode and thus produce beams with good optics . This system has been operated up to 40A extracted ion beam at 80 keV energy. We have also, after these experiments, operated the injector with a two-stage extraction system developed at the Culham laboratory for the JET injectors. Some significant results of the study of these two different extraction systems are presented and discussed in this paper. This work has been done in the context of u collaboration programme with JET and the Culham laboratory. 2. Experimental apparatus The ion source used was the large rectangular periplasmatron developed at FAR [4] . The ion beam was extracted from a rectangular surface of 12 x 38 cm=.

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R. Be, h- et al. / Neutnrl beum injc
The pulse length vat :ed between 0.1-0.2s. The neutralizer was a rectangular stainless-steel chamber 2 m in length and 16 X46 cm- in cr_- ,;=t;on-. the gascoming from thesource (12Torr " f/s, typical value) provided in this chamber, in the absence of the beam, a gas target thickness e, = 7 X lots moll/em=. Theinjector was operated in all the experimentswith the ion source at ground potential, the neutralizer being biased at the negative high-voltage potential corresponding to thedesired beam energy. In this electrical disposition, which was utilized some years ago in our laboratory for energy recovery experiments (by electrostatic deceleration) [5], the extraction from theneutralizer of beam plasma electrons, and their substNuent acceleration by theapplied FIV electric field, must be avoided. One of the methods developed in the past to create an electrostatic potential barrier for these electrons consisted in using acylindrical grid, negatively biased with respect to the neutralizer potential, which surrounded the beam at the exit of the neutralizer [6]. This god, called the suppressor grid, must allow the pumping of the neutral gas to reduce the cold plasma density . An essential feature of üs operation was that the energetic ions were deflected outward front the neutral beam by the field of the space charge sheath

which develops between the grid internal surface and the beam plasma . In the experiments reported here we have used an "electrostatic beam dump" (EBD)system based on the suppressor grid operation principle . This system allowed to stop the beam plasma electrons and, at the same time, to produce a pure neutral beam by removing the fast ion component. It consisted (sec fig. l) of a rectangular cage, 120m in length and 16 cm X 75 cm internal cross section, placed at the exit of the neutralizer . Like the suppressor grid it was negatively biased (the "suppressor voltage") with respect to the neutralizer. The two ends of this cage were open to allow the beam to pass; the lateral walls, adjacent to the beam, consisted of 66 copper bars - I cm thick, 5 cm wide, 120 cm long andappropriately inclined and spaced - which operated as baffles for neutral gas pumping . The charged component of the beam was deflected by theplasma sheath and fell, with grazing incident, on the baffle elements where its energy was dissipated. In addition, a difference of potential (the baffle voltage) was applied to each pair of adjacent baffle elements to create a local electric field which prevented the secondary electrons, emitted by ion impact, to be injected into thebeam plasma . At theexterior, around the EBD cage, a 90% transparent, negatively biased metal grid

Qt

RECTANGULAR PERIPLASMATRON NEUTRALIZER

Fig. 1 . View of theinjector with the electrostatic beam dump .

ELECTROSTATIC BEAM DUMP

R. Bechere, et al. / Neutral heran injector operation was mounted, which stopped any electron emission from thewalls of the system . Both the triode and the tetrode systems were operated with this system up to an accelerated ion beam of 25 A, 60 kcV. Beyond this energy, the EBD was not used because theelectrical powerrequired for its operatic-n exceeded the installed auxiliary power supply ei, pacity . In several experiments made with the triode extract on system the grounded ion source disposition was %till used but theelectron acceleration was prevented by placing, close to the exit of the neutralizer and electriM,ally connected to it, awater-cooled calorimeter. A 2 cm gap was left between the neutralizer and this calorimeter to pump, with an appropriate baffle system, the gas coming from the source. 3. Diagnostic tools When the injector was operated with the EBD the same diagnostic tools were used for both the triode and the tetrode system studies. In the triode experiments without the EBD, somewhat different diagnostics were used . 3.1 . Beam pou"er tratisntirsion The transmitted beam power was measured calorimetrically. For the experiments without the EBD a calorimeter placed at the end of the neutralizer was used. It was a thick water-cooled copper plate which intercepted the beam on a 16 x 46 cm= surface at tin from theion source ; the beam wasa mixture of fast ions and neutrals. The transmitted energy in a pulse was deduced from the cooling-water temperature rise taken as a function of time at a known water flow . In experiments with the EBD system we measured the transmitted neutral beam power at 5m from the ion source. The calorimeter was a 33 x 60 cm2, 1cm thick copper plate, biased at ground potential. The energy dissipated by the beam was deduced from the adiabatic increase of its temperature measured with three Cr-Al thermocouples. 3.2. Beam profile When the EBDwas nl!. used, the measurements were made at 2m fro,., the source . Three apertures in the calorimeter, 12 mm 0, were used to analyse the beam at three locations along the small dimension : on the midplane and on both sides at 5 and 7cm from the midplane. A secondary emission fast particle detector wasplaced behind each aperture . The beam profile was obtained by changing the direction of the beam in the horizontal plane, by the rotation of the extraction grids

391

support. With this method. the whole transversal beam profile could be scanned in a few shots. The same method was used to scan thewhole neutral beam profile at 5 m from the source when the EBD was used . On the calorimeter, an array of 56 small holes was drilled with a secondary electron emission detector behind each aperture. With these detectors it was also possible to measure electrically the transmitted neutral beam profile when thebeam axis direction was fixed and centered for the best transmission . A fraction of thebeam, in this case, was intercepted by the neutralizer and the b:atn dump . 3.3. Beatrt dicevgencc The beamlet "equivalent" divergence of each extraetion system aperture was inferred from the profile and transmission measurements . Another diagnostic which was used in all the experimental situations consisted o, two circular concentric secondary emission lieteclors, o 1 and 3.5 cm, placed behind a Icm diameter aperture at the center of the calorimeter. The system was aligned with the beam axis. The beatulet divergence angle was deduced by comparing the measured ratio of the concentric collector currents with the calculated one by assuming a parallel Gaussian beamlet emission from the extraction system apertures . 3.4. Grid loaditkq arul hackstreanùngedecvrcnt The power loading of the grids in presence of the beam was deduced from the adiabatic increase of the temperature at the center of the grids measured with Cr-Al thermocouples. Tbc backstreaming electron power was dissipated in a water-cooled copper plate placed at the back of the -urce at 18 cm from the extraction surface. The power was deduced from the cooling-water temperature variation . 4. Triode system 4.1 . Description The system consisted of three flat copper grids, each with 232 circular apertures in a hexagonal array. No focalization of the beam was provided. Thegeometry of one set of apertures of the triode is shown in fig . 2. The diameter of the extraction apertures, in the plasma grid, was 10 mm and the effective total extraction surface S= 182 cm = . The plasma grid apertures were shaped in order to produce the correct curvature of the equipotential surfaces near the beam edge and thus avoid the creation of aberrated trajectories [7,81. As it can be seen in fig. 2, this leaves sufficient space for water-cooling

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R. Bee:"erer e: al. / Neutral hen- injc<7nr apenaion 75mm

2 .5mm

-SmB_

1=325 mAltm' (proton equrvateN) Fig, 2. Configuration of oneset of apertures of the triode system andcomputed ion trajectories.

channels between the apertures . The resulting transparency of the system is 40%. The attenuation of the plasma ion flux depends strongly on the shape of the plasma electrode behind the extraction surface (on the plasma side). This shape was not optimized in that respect, in order to allow forthe maximumtransparency in presence of water-cooling. The accelerating gapwidth (d= 10 mm) was seen to be adequate for 80 keV operation . 3.2. Experimental results 4.2.1 . Extracted fat be.., pereeance In the range of extracted ion current densities from 30 to 200 mA/cm2 , the total ion beam perveanceforthe best optics wasobserved to increase from 1 .6 X 10 --6 to 1.9 X 10 --6 A/V' . .2 corresponding to the measured 14] decrease of the molecular species fraction of the ion beam. Themaximum performance was a 40 A extracted fort beam at 80 keV energy for 100 ms. The corresponding current density of 220 mA/cm2 was slightly below the valueat optimumperveance (240 mA/cm2 ). 4.2.2. B-det divergence This was deduced from the previously described diagnostics; Taking into account the discrepancy between the results of the different diagnostics and the dispersion of the experimental points, we can conclude that thequantity is known within =15% as indicated in fig. 3. Themeasurements show that the divergence angle decreases with an E- I,` law as expected for an emittance dominated beam. An example of the transversal beam profile as measured at 2m from the source (ions +neutrals) is given in fig . 4.whereit is compared with a few theoretical profiles calculated by assuming a uniform Gaussian beamlet emission from the extraction

surface. A divergence angle of =0 .7° can fit the data. Forcomparison we report in fig. 5 the measured neutral beam profile at 5m from the source. We deduce from this profile a central divergence of 0.8 ° and a large tail on the beam edge which could explain the 0.9-I° angle of divergence measured with the concentric collectors. The measured profile for helium is also shown in the same figure, no tail can be detected and a minimum divergence of 0.6° is obtained . In this case the same divergence angle of 0.6° was measured with the concentric collectors. This last result seems to rule out

Zo V

lo

m m

c m

0,8o Q6° 0,4°~,

ö 0zoa

10

20

30

40

50 60 708090100

Energy ( keV.)

Fig . 3. Beamtet equivalent divergence versus beam energy, with the triode system, as deduced from different diagnostics.

R. B,,herer et aL / Neutrat haam My--p-d-

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o FAR TRIODE '< CULRAM TETRODE

ï

i

S

Ep Eô05 dx z 01

Fig. 4.

Source.

Triode beam profile (ions+neutrals) at 2 ni from the

extraction system defects or large angles introduced by the beam dump. 4.2.3. Beam transmission The transmitted beam (ion +neutral) power on the calorimeter placed at the end of the neutralizer corresponded to about 83% of theHV drain power at 50 keV and at a typical value of the source filling pressure of 5 mTorr . It was observed that this power decreased roughly linearly with the ion source filling pressure. The extrapolation at zero pressure of the measured dependance was close to a 951$ transmission indicating that in normal operating conditions of pressure about 12% of the drain power was lost by ionization and charge exchange process on the neutral gas of the accelerating gap. The residual 5% losses would result from other unidentified mechanisms . In the experiments made with the EBD the neutral beam power transmitted to the large calorimeter at 5 m

1". POSITION ON TARGET AT Sm

Fig 5. Triode neutral beam transversal profiles measured at 5 m from the source .

L~ _

l.._i. ..0b_4__60 1 l.. .J 70 80 O ._20 30 4 50 Beam energy (keV)

Fig . 6. Neutral bearn power transmission ef6oieoay measured al 5 m from the source with the triode and Ilto tetrode sN:slclas.

from the source was measured as a function of the beam energy. The results are given in fig. 6. The beam was transversally collimated by the I:BD aperture. The fraction of the beam intercepted by this system was evaluated by subtracting from the whole neutral beam profile, as obtained by rotating the beam axis, the collimated profile taken in a fixed and centered beam position . For instance, about 10% of the beam was intercepted by the EBD at 55 keV beam energy: from this, an equivalent beamlet divergence angle a = 0.75° can be deduced. The measured 30% neutral beam power transmission efficiency (see fig.6) at this energy was slightly inferior to that calculated (35%) from the estimated neutralization efficiency in the neutralizer gas target (n 1 =4.5 x 10 15 mot/em`) and by taking into account the difference identified dissipation mechanisms. d.-ß .4. Grid potter loading andbackstreaminq eletvr~ats The grid loading increased with the source filling pressure. It was observed, in addition, that the power dissipated on the first grid by the beam was a function of the "decel" voltage as shown in fig. 7; it can be seen that for a 35 A, 78 keV beam, the minimum value was about 0.7% of the HV drain power, for a 3 kV decel voltage and at 6.5 mTorr source filling pressure . The power dissipated on the first grid by the source discharge, which in this case amounted to about 0.25% of the drain power, was not included in the curve of fig . 7. The grid 2 and 3 power loading measurements were rather approximate because at that time it was not possible to transmit the electrical signals of the thermocouples (which were at the HV potential) to the ground . Nevertheless, the peak grid temperatures (as read on a dial galvanometer) indicate that the power loading of these two grids did not exceed 1% of the HV drain power. The measured backstreaming electron power for a 78 keV beam is also illustrated in fig. 7.

R. B,,-I,- er al.

394

/

Neuwal beam oy-oroper
`d loadng drum

Fig. 8. Ion trajectory angle a at the exit of the triode system versus its initial radial position r for several counterbore radii of the plasma electrode.

.ACS 3

1

5

Decel voltage

6

kV

Trickle system : 78 keV, 35 A beam, 6 .5 mTorr. Power loading oat grid I and on the back of the source (B .S.) by -oudarv electrons. Fig. 7 .

4.3. ,\'unteriawl smear

of the triode system

4.3 l. Optimi-tion of the system The ientrajectories obtained in the triode accelerator were sim=,slated with the Stanford SLAC trajectory code [10,111 . The optimization of the pseudo-Pierce shaping of the plasma grid, performed with the code, is illustrated in fig. 8. where the final angle of each trajectory (at optimum perveance) is plotted versus its initial radial position at the emissive surface, for several values of the counterboreradius of theaperture. It can be seen that in irder to produce a beam without aberration it is necessary to use solution 3 which corresponds to a counterbore diameter equal to 14 mm . Fig . 2 shows the ion beam trajectories obtained from the code with solution 3 which was chosen for the design. 43,2, Beam opties andperceance The calculated divergence angle at optimum perveance is equal to a,=0.4° . This angle is the r .m.s . value of the ion trajectory angles at the neutralizer electrode aperture level . The divergence angle measured at the exit of the neutralizer is larger than the calculated value . This can be dueto several phenomena like ion temperature in the source, dissociation energy of molecular species, residual spacecharge effects in the neutralizer and scattering due to collisions on ions and neutrals. Thecalculated optimumcurrent density at 80 keV is 325 mA/cm=. The proton equivalent current density, obtained from theexperimental data when corrected for the molecular species composition, is slightly inferior (10%) to this value.

4.3.3. Grid power loading acrd bean: losses from pressure effects Apart from the powerdissipated on the grids by the ion sourccr operation, the electrode power loading is mainly due to energetic primary particles and accelerated secondary particles produced in the system . These secondary particles are, mainly, 1) secondary ions produced on the residual neutral gas by ionization or charge exchange collisions, 2) secondary electrons produced by ionization ; 3) charge exchange neutrals; 4) secondary electron produced on the electrode walls by the bombardment of fast particles. The trajectories of these particles were computed with the SLAC code [101. First, the trajectories of the primary ions were determined by solving the equation,,. of motion self-consistently with Poisson's equation . Analysis of thesecondary particle trajectories were then made by originating ions or electrons along the beam path or on electrode walls and computingtheir trajectories in the potential distribution obtained in the previous calculation. We notice from fig. 2 that the primary ion beam is well compressed and far from theelectrodes . Its contribution to the grid power loading should be negligible. The computer simulation shows that the charge exchange neutrals and the secondary electrons produced in the beam do not intercept any electrode. They enter directly the neutralizer or the ion source. The trajectories of thesecondary ions are shown in fig. 9. These ions are intercepted by the negative electrode or the neutralizer electrode or are directly accelerated to the neutralizer electrode aperture. Wall secondary electrons can only be accelerated if they are produced on the negative electrode. As shown in fig. 10, these electrons can be accelerated either to the ion source, to the plasma electrode, to the neutralizer or to the neutralizer electrode. The corresponding regions

R. Berl,erer et al. / Neo1ral bean, (ryeetar operation

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Pig. 9. Trajectories of secondary ions. Region (1): ions areaccelerated to the neutralizer electrode: Region (2) : ions are accelerated to the negative electrode: Region (3) secondary electrons are accelerated to the plasma electrode.

of emission can be seen in the figure . The regions which emit electrons to the plasma electrode have been indicated in fig. 9 as well. We notice that at 3 kV suppression voltage for80 keV beam energy, no secondary ions fall in these regions (region 3). The minimum distance 8 between region 3 and the closest impact point for ions coming from the neutralizer cell is shown in fig. 1 I as a function of suppression voltage. The position of this closest impact point is directly related to the energy these ions gain with the decel voltage. At high decel voltages, these ions have enough energy to get close to the source side of the negative elecirode . In fig. 11 the experimentally nie., sured variation of the plasma grid loading with decel voltage is drawn as well. The increase of this power loading with decel voltage can thus be explained qualitatively. However, the absolute value of the minimum and the raise below the critical decel voltage is not understood. The simulation indicates in this case that electrons coming from the neutralizer cell or the neutralizer electrode which can now be accelerated by the high voltage are all trapped by the beam space charge and are accelerated into the ion source. Quantitative estimation of the power loading on the different electrodes are made by evaluating the production of secondaryparticles in the different regions which

are involved . These values are obtained as fractions of the extracted beam current or power . At each inereniental production zone, a calculated ion energy, and thus a cross section value, is assigned . The local neutral density, and the difference of potential between this zone and the bombarded electrodes, allow to compute Ilte desired quantities . To compare with experimental values, the total beam energy is taken as 78 keV and the pressure is supposed to vary litrcarly with distance between 5 mTorr in the source and 2 mTorr in the neutralizer. Theassumed beam composition in 11' , H: "rid H; is respectively 60°i, 20% and 20`°0 of the beam current . The results are presented in table 1 . Reasonable agreement between calculated and experimental values can be observed . 5. Tetrode system Th ..y stem was based on the Culham design 1121 shown in figs. 12 and 13. As in the triode system the grids were flat and the axis of individual apertures were parallel . The first grid transparency was increased with respect to the transparency of the previously described triode system, in order to compensate for the lower

Pig. 10. Trajectories of secondary electrons accelerated from the negative grid: Region (3): electrons are accelerated to the plasma electrode .

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Table 1 Comparison of computer simulation results and experimental data for the FAR triode s-crn E=78 kcV, P_,, =S rnTorr, Pr, =2 mTorr, H' =60%. H_ =20%, H,' =20"* Calculated

Measured

Backstreaming electron power from ionization in high-voltage gap

3 .1% P

2% P.,e

Total secondary ion power from production in HV gap

8.1 , P.

Sceondary ion power to neutralizer andbeam line from production in liv gap

6.451

Secondary ionpower to neutralizer grid from HV gaps

1 .4% P 

Secondary ion powerto negative grid from HV gaps

0.3% P 

Total secondary part. power from production in HV gaps

14.7% Ps,

Secondary ion current to mrotralizer and be..line from HV gaps

7 .9%, 1,

Secondary ion current wneutralinr grid from liv gaps

3,4% l,

P

Total current to negative grid Secondary ion current to negative grid from production in HV gaps Average energy of seconddary ions produced in HV gaps incident on the negative grid

+secondary electron power I %. P.

+ion power from gas cell I % P..,

(pressure dependant

1

12% P s,

transmission losses)

=fluty+lmu,XI+y'1=12% 1_

l o,= 3 .4%1e

(V)-8% E

1,0 =extracted current ; E=beam energy . P  =extracted power; 1 =secondary ion current to the negative grid coming from the neutralizer plasma ; y'=secondary electron emission coefficient from ion impact on the negative grid .

value of the extracted ion current density of the tetrode for the same beam energy. The total useful extraction area was 221.7 cmz (196 holes of diameter 1 .2 cm), corresponding to a transparency T=49%. Such transparency is inadequate for water cooling and cannot be utilized for JET injectors . Grid loading measurements were made more extensively compared to the triode system and, concerning the grids 2, 3 and 4(which are at the HV potential) with an improved equipment for the transmission of the electrical signals to ground.

5.1 . Experimental results 5.1 .1 . Beam petroeance The extracted ion beam perveance for the best optics increased with T (the ratio of the second to the first accelerating gap applied potential). The maximum allowed value of T for reliable operation was =9andin this case the extracted ion current density was - 20% lower than that of thetriode system so that the total ion beam perveance was roughly the same as for the triode .

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R. llechercr er al / Netaral be-, myceror op-i-

397

source is shown in fig. 15 . It can be seen that it is almost identical to that obtained with the triode. Similarly, thetransmitted neutral beam power on the calorimeter at 5m from the source was the same for both systems as shown in fig . 6. The results of the calculated ion trajectories for the two systems also indicate that the beams must have the same optical quality . Nevertheless, the measured divergence in both cases is by afactor of 2 to 3 higher than the calculated one. This can be due to several factors like break-up energy of particles from molecular ions, beam plasma potential, thermal energy, electrodes misalignement, etc. Fig. 11 . Distance 8from the closest impact point of neutralizer ions on the negative grid to region 3, as a function of decel voltage. The measured power loading on the plasma grid and the calculated p-mial barrier 0 for electrons is shown as well.

As previously observed, the extracted ion beam perveance increased when the ion current density was increased, because of the higher proton yield . In these experiments the injector was operated only with the EBD; themaximum extracted ion beam power was25 A at 60 keV. 5.1.2. Beans divergence

The beam divergence was almost independent of l' in the range =6 to I --- 9. The results of the measure-nts were very similar to those obtained with the triode. This is illustrated by figs . 6, 14 and 15, where some results of different beam diagnostics aregiven. In fig. 14 is shownthe variation of the beamlet divergence angle with the perveance, as deduced from concentric detector measurements at 50 keV. The triode results are also given. The best optics corresponded in both cases to about =0 .7° divergence angle. The neutral beam profile taken at 5 m from the

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5.1 .3. Grid pmrer loading and /xukstreumfag clrclrrurs

The grid power loading and the power from back. streaming electrons were roughly proportional to the ion source filling pressure indicating that the interaction with the neutral gas and the associated secondary eltrtron emission from electrodes are the dominant factors (fig. 16). As with the triode, the grid 1 power loading given in fig. 17 increased when the decel voltage was increased but with some difference probably related to thedifferent depth and tapering of the grid 3 apertures. It was also observed that the minimum rower loading for grid I, and to a lesser extent for grid 2, occurred for an extracted ion beam perveance about 35% lower than that required for the best optics. This effect is shown in fig . 18 for a 50 keV ion beam at = 9. By decreasing I', the effect was still observed but the grid loading was reduced. For instance at I'=6 the grid 1 loadingwas, at the best optics, 0.35%. The powerdissipated on grids 2 and 4 for different values of the perveance is given in fig. 18 as well.

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5.'. "', é'leclric"alinsrahilirvnJgrfd : :When a high impendance resistive divider was used to bias this grid, we observed, at high I' and/or at high perveance values, an electrical instability characterized by a sudden transition to a state characterized by a very

r=94 J=270 mA (proton equivalent) m2 Fig. 12. Primary ion beam trajectories in the 80 keV tetrode system at 1'=9 .4.

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R. Ber*- ec aL / Neutra! henni Inje"
Fig . 13. ton trajectories at r=7.3 .

low grid voltage and relatively large (positive) current. The phenomena is explained as follows : When some extracted ions areintercepted by grid 2, this grid (if not electrically stabilized) becomes more positive leading to an increase of which in turn increases the flux of the intercepted ions. the process continues until a value l' - x is reached . Theslope of the electrical volt-ampere characteristics of this grid is, in these conditions, negative. When a 70 kß resistive divider was used, the allowed perveance value for reliable operation at = 9 was inferior to that corresponding to the best optics. reliable operation was possible at the best optics for I'=6. In general, we used for biasing grid 2 a separate electrical powersupply.

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5.2. Numerical wucit- of thetetrode rrstem Tetrode systems can be operated at different values of l'. With increasing the final diameter of the accelerated beam increases and theoptimumposition of theplasma emitting surface movestoward the high-voltage side of theplasma electrode as shown in figs. 12 and 13. At very low l' values (1'S 6) the plasma is far

r,

ôs -

15 PIN t Fig. 14. Variation of the beamlet divergence with perveance for the triode and the tetrode at 50 keV beam energy (concentric detectors).

recessed inside the electrode aperture and this produces aberrated rays at the beam periphery . On the other hand, high operation is risky since the beam trajectories are close to the electrode aperture edges. This can increase grid power loadings and can, furthermore. produce the electrical instability of the gradient grid as observed experimentally (section 5.1) .

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?.2.1. Analysis ofprirrtarv -d .--day particle trajecvories We notice in fig. 12 that direct interception of the gradient grid by primary ions can effectively occur at 1'=9,4 and will thus prcduce the already mcmionned electrical instability if the net current on this grid is positive . No direct interception of the negative or the neutralizer electrode can however be observed as shown in figs. 12 and 13 . The grid loading resulting from charge exchange neutrals was seen to be negligible. The electrons resulting from the ionization of the neutral gas produce a negative current on the gradient grid. This cu rent can compensate the positive current due to ion interception and thus, depending on the neutral pressure . change the conditions for the onset of the electrical instability observed on this grid .

Pe51TpN GN TARGET AT 5m

Fig. 15. Neutral beam profiles for the triode and the tetrode at 5 m from the source .

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wW1 dram °ns [%1 3-

m

s

a+ àa

Ô

0,5% 5 1o Source Idling pressure Im1-l

Fig. 16. Telrode system : Power loading versus pressure for 50 keV, 19 A at l'=9.

ion

source filling

The trajectories of the secondary ions arc shown in fig . 19 for t=9.4. These ions are the dominant factor for the power loadingon the negative and the neutralizer electrode. The trajectories of the secondary electrons emitted from the negative electrode are shown in fig. 20. As in the triode system they are accelerated to the ion source, to the plasma electrode (region 4), to the neutralizer lr to the neutralizer electrode . Thecorresponding emission regions can be seen in the figure. Region 4 is shown in fig. 19 as well . We notice that. for T=9.4 and optimum perveance, some secondary ions produced in the accelerating gap hit the negative electrode in a region which produces secondary electron emission to theplasma electrode. This situation wasnot so critical iu the triode system because the beam was more compressed.

0

o

asL i t is Perveanc.AN k, Pig. 18. 'lltrode system : 50 keV, l' _-.9. Grid power loading and hcandet divergence angle versus perveance. The ion current which is involved in this process is directly related to the dimension of the beam waist at the negative electrode aperture . This dimension is a strong function of l' and perveance. Fig. 21 shows this calculated current as a function of these twoparameters for a 50 keV beam and a pressure variation between 4 and 2 naTorr between source and neutralizer The experimental data of the fractional power loading on the plasma electrode is shown in the same figure. We observe clearly the same dependance. As expected, in the experiment this loading was seen to be directly proptxtioualto thegas pressure. In fig. 22 theexperimental dependance of the plasma electrode power loading with suppression voltage is compared with the distance from the impact point of the ions produced in the ueutralizer to the region of the negative electrode which is involved in the plasma elt-ctrode loading. We notice that the flat part of the experimental curve corresponds to the region where no neutralizer ions are involved . This domain covers a larger range for the suppression voltage than the triode system because of the thicker negative electrode and the reduced beam space charge potential.

v

l7

5. ?.?. Comparison data

0

1

2

3

4

5

6

7

Decel voltage (kV) Fig. 17. Tetrode system: Grid 1 andgrid 4 loading versus decel potential.

of

simulation results

atd expev:.

rental

The resulting calculated power loading% on the different electrodes are shown in tablet and compared to experimental data . These numbers were obtained for a 50 keV beam, for a pressure variation between 4 ntTorr in the source to 2 rnTorr in the neutralizer. We notice slightly lower loadings in the experiment than in the calculation which may be due to a slightly lower neutral density between the grids.

400

R . Beclierer et al. / A --I b,.,,,

Fig. 19. Trajectories of secondary ions . Region (1) : ions am accelerated to grid 4; Region (2) : ions are accelerated to grid (3) : ions am accelerated to region 4; Region (4) : secondary electrons am accelerated to grid I .

Fig. 20. Trajectories of secondary electrons accelerated from grid 3. Region (4): electrons am accelerated to grid 6. Electrostatic beam dump The eletrostatic beam dump operated reliably and without electron losses . Its potential advantages with respect to the conventional ion beam magnetic deflection systems can be that the ion beam power is better distributed over very large surfaces and in a more

Region

1.

compact disposition by the electrostatic deflection at both lateral sides of the beam . In addition, it simplifies the operation of the ion source which is at grour I potential . A negative aspect (at the present stage of development) is the relatively large electrical power needed for its operation . The "suppressor voltage" which wasgenerally created by biasing the neutralizer by means of a series resistor was about 5 kV for50 keV operation and it would be of the order of 10 kV for an 80 keV, 40A pylir_-s) âext. ri.

à(^1 0 _1 t' 15 (kV)

051,

3;

- Calculated _ ___ Eapenmenl

i n

t5%

i

19. 0

Fig. 21 . Calculated secondary ion current J6Nt to region 4 or grid 3(r =6. 7.3, 9.4) as a function of perveance, compared to the measured power loading on grid I (r=6, 9).

OS'l.

0;

01

2

3

Fig. 22 . Same as fig .

4 I1

5

6

7

for the tetrode system.

8 VS(kb

401

R. Bechere, et al. / Neutral heaat injector operation

Table 2 Comparison of computer simulation results andexperimental data for the Culham telrode system E=50 kcV, P_=4Torr, P ,,=2 Torr, H' =60%, Hi =20%, H ; =205(. Calculated

Measured

Back streaming electron power to the ion source (electrons from ioni zation)

2.5% P..

Secondary electron current to the gradient grid (from ionization) (l'=9.4)

1 .3Sç P_

0.4% l_

Secondary electron power to the gradient grid (from ionization) (l'=9.4)

0 .04% P.,

Total secondary ion power from production in HV gaps

8.3%

P w,

Secondary ion power to neutralizer and beam line from HV gaps

7.4%

P

Secondary ion power to negative grid from HV gaps

0.6%. 0.6%.

Secondary ion power to neutralizer grid from HV gaps

P P

0.33%

Total secondary part . powerfrom HV gaps

17% P. .,

Secondary ion current to negative grid from HV gaps Secondary ion current to neutralizer grid from HV gaps

P

Average secondary electron emission coefficient deduced from (2), (4) and (5)

-0.23f- P.

(1)

rsecondary electron power 0.4 P.  pressure dependant transmission losses - 12'G

1 .3:6 1_ I

=(l,nc +

(V )=8.7% E

Secondary ion current from neutralizer to negative grid

Secondary ioncurrent to negative grid from production in HV gaps deduced frorn(1)and(3)

l' -= l,~.(")

tan =6.9C; 1.., .

Total current to negative grid Average energy of secondary ions produced in HV gaps

+-primary fun power 11.2

+ y")-- 11 .34' 1 .,"

(2) (3)

1

l_=PV)=2 .64%-l,. 

3.3% I_

(4)

(51

y°=0 .9

coming from the 1. =extracted current: E =beam energy; P, =extracted power ; 1~. , =secondary ion current to the negcuivc grid neutralizer plasma.

R. Becherer et al. /

40 2

Neutra! beam h jertnr operation

10 .V

Beam energy

Fig. 23 . Electrical properties of the EBD. (a) Total electrical power needed to operate the EBD. (h) Measured and calculated -suppressor" voltage . (c) Baffle voltage . beam, the corresponding current being of the order of the drain current (85%). In addition, the grazing incidenceof thefast ions on thebaffle elements produced a high current of secondaryemission electrons which must be supplied by the auxiliary baffle voltage generator to prevent its release in the beam plasma. The measured baffle voltage for good operation is given in fig . 23 (curve c); in the same figure are also reported the measured and calculated "suppression voltage" (curve h) 161 and, finally, the total electrical power(normalized to the HV drain power) needed to operate the system (curve a) . An approximate formula giving the value of the suppressor voltage V" in termsof thebeam accelerating voltage Vis the following: V' V d

h L P k F A

__ _d' _Pror 24hL k ~1 -F')n'

transversal dimension of the beam dump, height of the beam, length of thebeam dump, total perveance of the extracted ion beam, 5.45 x 10 - " CZ/M, neutralkred fraction of the beam, fraction of the total beam entering the beam dump .

7, Conchlcioa Twotypesof extraction systems on a neutral injector unit have been studied . The triode and the tetrode produced a beam with identical optical qualities, appr.)priate for the JET fusion device. At 80 keV, the optimum perveance corresponds to extracted current

densities equal to 240 mA/cmZ for the triode and 190 m.A/cmZ for the tetrode at r = 9. At this energy the electrical stress on the high-voltage gap is respectively 83 kV/cm for thetriode and 100 kV/cm for the tetrode. Under these conditions, the power loading levels on individual grids, in both systems, were comparable and less than l8b/ of the extracted power which would permit water-cooling for DC operation. Lower grid power loadings were measured on the tetrode when operated at 1 =6; however, in this condition the optimum ion current density was reduced to 170 mA/cmZ at 80 keV. Computer simulations of ion trajectories and calculations of grid loadings are generally in good agreement with theexperimental results . In particular, it was possible to explain, for the tetrode, the increase of grid loading with the value of r and perveance as well as the grid 2 instability . Grid loadings may be enhanced if, as required by JET, beam steering by aperture displacements is used . The operation with an electrostatic beam dump was also demonstrated and allowed to produce a neutral beam at the level of 60 kV, 25 A with a grounded source . References

III E. Thompson, Pate. 2nd Symp. on Ion sources and forma-

tion of ion beams, Berkeley, 11-7 (1974) . [21 J . Kim, J.H. Whealton and G. Schilling, J. Appl . Phys . 49 (1978) 517. [31 K.H . Berkner, W.S. Cooper, K.W. Ehlers and R.V . Pyle, 7th Symp. on Eng. problems of fusion research, Kno ville, 1977 (Inst. Electr. and Electron. Eng. Inc., New York) p. 1405. [41 R. Becherer, M. Fumelli and F.P.G. Valckx, ibid, p. 237. [51 M. Fumelli and F.P.G . Valckx, Proc . 2nd Symp. on Ion sources and formation of ion beams, Berkeley, VI-6 (1974) . [61 M. Fumell' and P. Raimbault, Proc . of the Joint Varenna- Grenoble Int . Symp. on Heating in toroidal plasmas, Grenoble (1978) . [71 E. Thompson, Proc. 2ndSymp . on ton sources and formation of ionbeams, Berkeley, 11-3 ()974). [8) L.R. Grisham, C.C. Tsai, J.H. Whealton and W.L . Stirling, Rev. Sei. Instr. 48 (1977) 1037 . 191 A. Bariaud, R. Becherer, M. Desmons, M. Fumelli, P. Raimbault, F.P .G . Valckx, Proc. of the 2nd Joint Grenoble-Varenna Int. Symp . on Heating in toroidal plasmas, Como, 1980 (Commissionof European Communities, Directorale-General X11 Fusion Programme, Brussels) p. 927. [ 101 W.B . Hermannsfeldt, Stanford Linear Accelerator Center, SLAC Report 166 (1973). [11) Ph. Raimbault, M. Fortieth and R. Becherer, ref. 9, p. 935. [121 A.J.T. Holmes et al ., 8th Symp. on Eng. problems of fusion research, San Francisco, 1979 (Inst. Electr. and Electron. Eng. Inc., NewYork)p. 1060.