CORDIS—an improved high-current ion source for gases

Vacuum/volume34/numbers1-2/pages31 to 35/1984 Printed in Great Britain

0042-207X/8453.00+.00 Pergamon Press Ltd

C O R D I S - - a n improved high-current ion source for gases R Keller, F NShmayer, P Sp~dtke, GSI-Geselischaft f6r Schwerionenforschung, Darmstadt, W Germany and M - H Sch6nenberg, Fachhochschule Wiesbaden, W Germany

The multipole/reflex discharge ion source ELSIRE delivered high-current heavy ion beams (20-50 mA) of low emittance, but with poor power- and gas-efficiencies. Therefore, a new version of the source, CORDIS, was constructed, based on the same discharge principle. The diameter of the discharge chamber was doubled, compared to ELSIRE, thus allowing enlarged extraction areas. At the same time, a modifed magnetic multipole field leads to enhanced electron confinement, while a shift of the extraction plane increases the effective plasma density. The measurements show a reduction of discharge power and gas pressure by a factor of ten. Thus reliability and life-time are much improved and dc operation has also become possible. With 1.3 kW dc discharge power, the source life-time exceeds 80 h. Several test runs with single- and multi-aperture extraction systems will be discussed. The most prominent result amounts to 71 mA for xenon ions or 814 (mA) normalized current (proton equivalent).

1. Introduction The high-current ion source for gases, ELSIRE 1, has been used at GS] for several years on a test stand for studies of extraction systems a'2, on the heavy ion accelerator U N I L A C for beam transport studies at injection velocity 3, and recently on the 300 kV injector prototype, again for transport studies 4. Though this source proved to be reliable enough for routine operation it is still affected by some disadvantageous features. Power and gas efficieneies are very poor, with the consequence that the source can only be operated in the pulsed mode at about 10~ duty factor for most of the applications. Also, the maximum usable plasma diameter for large area extraction systems does not exceed 15 ram, limiting the normalized currents I,, defined as 1~~ I x ~

(A: a t o m i c mass number),

for high-brightness beams to I, < 300 (mA). It was the aim of this study to develop an improved source of the ELSlRE type, avoiding the mentioned draw-backs and extending the ion beam intensity range to I, = 600 (mA) normalized current.

2. Source design Like ELSIRE the new source CORDIS (Cold Reflex Discharge Ion Source), see Figure I, relies on a reflex discharge, radially confined by a magnetic multipole. The cathode is now placed within the main discharge chamber and usually consists of six tantalum filaments, suspended on a tantalum disc. The inner anode diameter has been increased from 40 to 96 mm and 18 cobalt/samarium permanent magnets of 2 kG pole strength are put around it on a 100 mm wide cylinder surface.

The arrangement of the magnets is of fundamental importance for this type of source. It is known from elementary plasma physics that the electron confinement depends on the field compression value, i.e. the ratio between the radial field components within the source and at the wall. Thus the anode wall should be placed at the radius where the field is highest, exactly on the magnet pole faces. On the other hand, confinement by a magnetic multipole implies that all the discharge power flowing to the anode reaches it just where the magnetic field lines cross the surface, directly under the pole faces, ff the magnets are radially oriented. Therefore, the anode wall of ELSIRE had been designed quite thick to allow for the insertion of cooling channels under all the magnets. ConsequenOy, the plasma confinement was far from optimum and much power was needed to sustain a discharge of the required density. A simple rotation of the magnet orientation by 90 °, however, eliminates this problem because then the line where the magnetic field lines cross the anode wall is shifted to the mid-plane between two adjacent magnets while the general field pattern remains unchanged, according to the easy-axis principle s (see Figure 2). This configuration offers two advantages. First, the heat load is now directed away from the magnets and efficient cooling ofthem is no longer necessary. Second, one can give the anode a corrugated profile, as shown in Figure 3. This positions its inner surface exactly at the radius where the magnetic flux density is maximum, while near the magnets the anode is somewhat narrower (see Figure 3) allowing the magnets to be kept a certain distance from the hot anode and outside the vacuum. For technical reasons, in the current CORDIS model the magnets are still directly cooled and the anode is designed as a 31

R Keller et ah An improved high-current ion source for gases

t

100ram

4

Figure 1. Sectional view of CORDIS. A: anode. C: cathode. EX: extraction plates, here: accel/decel-system.PM: permanent magnets. Rl : reflector electrode on cathode side. R2: reflector on extraction side, bearing the outlet electrode.

I

t---

--4---_ I

"-'"l

'

._..-----4I

Figure 2. Computer plot of the magnetic field pattern inside the source. The magnets are tangentially oriented, in alternating directions.

smooth cylinder. The latter is justified by a closer inspection of the field pattern, which shows that over the last few millimetres near the inner magnet radius the radial field strength does not vary much. Therefore, a somewhat narrower anode wall does not cause a confinement deterioration of any importance. The source insulators are well hidden from the discharge by the anode itself. The source is held together by stud bolts clamping the flanges that bear the two reflector electrodes. The first reflector, near the cathode holder, just closes the discharge chamber on this side. The other one bears the source outlet-plate. A great variety of outlet-apertures may be used: one or several round holes; straight or curved, rectangular or oval slits. Each of these may have a profile which is expected to yield high beam brightness under the chosen conditions. Details of the extraction 32

Figure 3. One section of the magnetic field pattern from Figure 2, with the optimum (broken line) and the actually chosen (full line) anode wall geometry. systems used are g~ven below. An important point is that the outlet-plate can be directly aligned to the extraction electrodes while the source body is still removed. An alignment accuracy of about 0.02 mm is necessary and can easily be reached. Experiments with ELSIRE had already shown that the power consumption of that source could be considerably reduced by

R Keller et al: A n i m p r o v e d

high-current

ion source for gases

placing the outlet-plane nearer to the magnet edges in the axial direction. The reason for this effect can be seen in the axial decay of plasma density, according to the weakening of the confining multipole field. Therefore, the axial distance between magnet edges and outlet-plane is now chosen as 12 ram, compared to 30 mm with the original ELSIRE design. Probe measurements of the plasma density have not yet been undertaken with CORDIS, but the largest employed extraction pattern was 35 mm wide and did not show any effect of radially inhomogeneous plasma density.

3. Source operation CORDIS can be operated in the de mode without difficulties, since the power-efficiency is increased by a factor of I0 compared to ELSIRE by the two changes in the design discussed above. If desired, pulsed operation is also possible. Usually we work at 50 Hz, 10% duty factor (2 ms pulse length), and even 1 kHz, 10% pulsing does not cause any problems. The maximum tolerated mean discharge power is about 4 kW. The electrical circuit is shown in Figure 4. If the connection between the two reflector electrodes is broken, by insulating the damping bolts, different potentials can be applied to both. In this way the outlet-plate can be given a potential which best suits the extraction conditions and the other reflector a potential which provides the best discharge conditions. Connection of this latter electrode to the cathode, for example, results in very constant discharge pulses and consequently in rectangular ion beam pulses, though the ion current is somewhat lower than with both reflectors on an equal floating potential. aEa [r,D

RE~

I'

4. Extraction systems The standard extraction system used with CORDIS is a conventional accel--decd system, as shown in Figure 1, and consists of three 1.5 mm thick, perforated molybdenum plates on high positive, weak negative and ground potential. Most experiments are performed with single- or seven-aperture systems, but also two, three or thirteen holes have been used occasionally. For a special application an accel/decel-system with three curves slits was installed ~, which delivered a hollow argon beam orS0 mA ion current at 33 kV. The most important parameter of any extraction system is the extraction field strength because the extracted beam current I generally varies s as locU3/Z/d 2, where U and d are voltage and width of the gap. A theoretical law for the minimum gap widths, frequently referred to in accelerator design, was published 9 in the implicit form WE 2 e x p ( - 1.7 × lOS~E)= 1.8 x 10 l+, where W is the beam 'energy' (V) and E is the field strength (V cm- l ). This Kilpatriek law indeed marks a close lower limit to extraction gap widths for voltages between 17 and 120 kV, but it appears a little too optimistic at the higher voltages in this range. Published gap data together with our own experiences rather suggest that for practical purposes one should respect a doc U 3/2 gap width scaling, with 5 mm for 50 kV (see Figure 5). dmi.(mm) = 0.014[ U(kV)] 3/2.

/

X+

/

h

/

_1

100~. --)-"-HVj

URC

•

(-~.kv)

/

,.

//

10o~ HVz

I, mF

1~5 ~ ' 1

MY1 (SKY) VH

-

1o:.

"/~////

÷

Figure 4. Electrical circuit with pcntode extraction system. 2-

Computer simulations indicate that there exists an optimum potential of the outlet-plate for the extraction of high brightness ion beams 6. These effects will be further studied in the near future. The source life-time is limited by cathode breaking when the filaments are sputtered too much. Under typical dc conditions (38 A x 40 V discharge, 150 A u 9 V filament heating, and 3.7 Pa argon pressure at the gas inlet-tube) the life-time was 83 h. It is important to note that with CORDIS one can always find very quiet discharge conditions (current modulations far below 1%) by properly tuning the operation parameters, over a very large range of discharge current values between about 2 and 200 A. This feature enables the user to apply arbitrary extraction systems, while providing the matching plasma density in every case and allowing the beam always to be fully space charge compensated.

// 1 .... 10

-3

' 2'" ~ ~ I I O0

i

Z tJ / kV

~ i I 1000

Figure 5. Minimum extraction gap widths, according to different authors. K: Kilpatrick law9. q: quadratic laws. h linear law ~0. U~],: empirical law, giving the closest lower limit to gap data listed in ref 11 or obtained during this study. In addition to the gap width, the outlet-aperture contour is an important feature. In first-order treatments s, the shape of the plasma meniscus is assumed to be spherical. In reality, however, the meniscus curvature is not uniform and near the aperture edge the plasma protrudes excessively into the gap, as computer simulations show 6. The resulting aberration effect can partially be 33

R Keller et al: An improved high-current ion source for gases

corrected by properly shaping the contour of the outlet aperture. A simple but already very useful correction is obtained by cutting a 45 ° angle from the outer edge of the outlet-aperture, penetrating of the total plate thickness ~2. More sophisticated recesses ~3.1, still improve the beam brightness, but are somewhat critical to exact matching of the discharge plasma density. We use the computer code AXCEI.A3SI t s, which is a modified version of the original AXCEL code 6, to find optimized extraction geometries. For a given configuration, we always vary the ion current density until the maximum transported beam current within a predetermined acceptance angle of typically 25 mrad is obtained. The result of such an optimization is shown as an example in Figure 6. This extraction system incorporates several features of different known systems. (1) The extraction gap is divided by a 'puller' electrode t e which mostly serves as an additional electrical correction for the above mentioned aberrations. (2) The electron screening electrode is made quite thick and its bore a fiat cone, opening in the downstream direction j 7, to direct secondary particles away from the source. (3) The screening electrode is enclosed between two grounded electrodes t s. This results in a better decoupling of the negative HV supply from the main HV supply and thus keeps the screening action effective even during a breakdown over the main gap. We tried two outlet contours on this system, as shown in Figure 7. Though the SKS contour resulted as the best from an extensive series of computer simulations, in reality the Ohara contour proved to be superior, as it yielded 3.5% more transported ion current and moreover suffered much less from a slight plasma density decay in the source during the discharge pulse.

\ \ \ Is

,5=

!

T O

I m

I

,t

b

C

Figure 7. Applied outlet-aperture contours, a: Ohara contour for pentode. b: SKS (Sch~nenberg/Keller/Spiidtke) contour for pentode, c" recessed contour for seven-aperture accel-decel-system (see Table 1). One interesting secondary result of these pentode tests was found by calculations, as well as experimentally, when we examined configurations with relatively wide puller aperture: the more this opening increases, the more must the puller potential approach that of the outlet electrode. Already for a puller aperture 1.7 times wider than the outlet aperture, both potentials must be equal. The system then actually resembles a conventional system with a thick, recessed outlet plate. The transported ion current in this case amounts to 8 5 ~ of the current with optimum puller aperture and potential. This means that a puller electrode is of use only if one looks for the extreme optimum of beam current.

E E v

Figure 6. Single-aperture pentode extraction system for CORDIS, yielding high brightness argon beams. Note that the radial and axial distances are drawn in different scales. OE: outlet electrode, + 50 kV. PE: puller electrode, +45 kV. G1, G2: first and second ground electrodes. SE: electron screening electrode, - 4 kV.

34

R Keller et al: An improved high-current ion source for gases

5. Results All experiments were performed on the GSI high-current test stand, equipped with magnetic X/Y-steerer and an 80 mm wide magnetic quadrupole triplet. The beam current was recorded behind the triplet in a 45 mm wide and 250 mm long, magnetically and electrostatically screened Faraday cup. Emittances were measured in the same plane, using a pepper pot plate to produce the beamlets and a Kapton foil to detect their sizes and positions. Three data sets are selected here, out of many experiments, to demonstrate the source performance with either single- or sevenaperture extraction systems (see Tables I and 2). The extraction voltage is always 50 kV. The listed beam and source parameters are pulsed values (2 ms long, 50 Hz), except for the dc cathode heating and the gas pressure. Table 1. Measured and derived ion beam parameters. El: dement. 1: transported ion current, mA. I,: normalized ion current, mA; I, EIx/'A; A: atomic mass. e: absolute emittance (area divided by it). mm tared. B,: normalized brightness, A/(mm mrad)2; B,,ffil/t~; e,,=~gy~, no: number of apertures. Fo: total aperture area, cm'. drc: distance of Faraday cup or emittance measuring device from source, m NO El

I

1 2 3

41.5 262 120 759 71 814

Ar Ar Xe

I.

t

B.

n~

Fo

drc

System

52 200 270

5.7 1.1 1.2

1 7 7

1.33 1.98 1.98

2.7 1.6 1.6

Pentode Standard Standard

Table 2. Source operation conditions under which the beams of Table 1 were obtained. 1~: discharge current, A. U~R: anode-reflector voltage, V. Uac: reflector-cathode voltage, V. le Us: cathode heating current and voltage, A and V. p: gas pressure at inlet tube, Pa. Note that the filaments were already thinned out during experiment no 1

No

8

UA~

URc

~

U,

p

1

18 35 27

95 70 65

I0 -15 -20

139 158 151

8.2 8.0 7.3

2.7 8.0 3.0

2 3

The beam current values given in Table 1 are not the absolute limits for CORDIS. With a pentode extraction system, more current could be obtained by increasing aperture diameters and main gap width together and applying higher extraction voltages, according to the breakdown limits discussed above. With multiaperture systems such as in numbers 2 and 3, the current may be nearly doubled by adding a second ring of six holes around the existing pattern, The experiments showed that a cross section of at least 35 mm die offers constant plasma density, while the largest actual system needs less than 23 ram.

6. Acknowledgements The authors would like to thank F Sch&q'er and R Vrtal for technical assistance with the experiments.

References J R Keller, Nucl lnstrum Meth 189, 97 (1981). : R Keller, P Spidtke and K Hofmarm, Proc 4th lnt Conj"on Ion Implantation: Equipment and Techniques, Berchtesgaden (1982). 3 j Klabunde, N Angen, R Keller, P Sp-~idtke, J Struckmeier and H Trautman, IEEE Trans Nucl Sci NS-28, 3452 (1981). 4 p Sp~idtke and B H Wolf, Vacuum 34, 73 (1984). s K Halbach, Nucl lnstrum Meth 169, 1 (1980). e j H Wheahon and J C Whitson, Particle Accelerators 10, 235 (1980). P Krejcik and R Keller, Vacuum 34, 11 (1984). s j R Coupland, T S Green, D P Hammond and A C Riviere, Rev Sci lnstrum 44, 1258 (1973). 9 W D KilDatriek.R ~ Sci Insr~__um_~lg,_~.(.1957 1o T S Green, Proc 2nd Low Energy Ion Beams Conf. (Bath 1980),lnst Phys Conf Ser 54, 271 (1980). 11 R Keller, Proc Syrup on Accelerator Aspects of Heavy Ion Fusion, GS1-82-8, 87 (1982). x2 y Ohara, S Matsuda, H Shirakata and S Tanaka, 3ap J Appl Phys 17, 423 (1978). 13 W S Cooper, K H Berkner and R V Pyle, Nucl Fusion 12, 163 (1972). 14 M R Shubaly, Chalk River Nucl Lab AECL-6578 (1979). is p Sp~dtke,, GSI Internal Report, to be published. 1s E Thompson, Proc 1st Low Energy Ion Beams Conf. (Salford 1977),Inst Phys Conf Ser 38, 236 (1978). x~ A J T Holmes and T S Green, Proc 2nd Low Energy Ion Beams Conf. (Bath 1980), lnst Phys ConfSer 54, 163 (1980). is E A Meyer, D D Armstrong and J D Schneider, IEEE Trans Nucl Sci NS-28, 2687 (1981).

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