PIG ion source with end extraction for multiply charged ions

NUCLEAR INSTRUMENTS AND METHODS I22 0974) 517-525;

© NORTH-HOLLAND PUBLISHING CO.

P I G 1ON S O U R C E W I T H E N D E X T R A C T I O N F O R M U L T I P L Y C H A R G E D I O N S * H. BAUMANN* and K. BETHGE? 11. Physikalisches lnstitut, UniversitfJt Heidelberg, Heidelberg, Germany

Received 16 September 1974 A PIG discharge ion source with end extraction of the ions has been developed for a continuous operation of the discharge with voltages up to 6 kV. The axial magnetic field is produced by a solenoid. The total extracted beam current was thoroughly investigated concerning its dependence on the source parameters which yielded data on the optimal operating conditions. Depending on ion mass total beam currents between 1 and 4 mA have been extracted with an average power consumption (without

magnetic field) of about 20 W/mA. The fractions of multiply-charged ions of the ion beam have been measured for a source operation with noble gases and several gaseous compounds. A comparison with cross sections for the ionization of neutral atoms by single electron impact suggests that this process is dominant in producing the multiplycharged ions.

1. Introduction

view in fig. 1. The modifications are the following: The magnetic field is produced by a solenoid. It is insulated against the cathode flanges by means of two overlapping discs of Frialit* and Vespelt and is kept at anode potential. Therefore the anode cylinder could be held directly by the core of the coil so that a discharge is only possible inside the anode cylinder. All parts of the source can easily be replaced, in particular the anode cylinder and the cathode discs. The experiments were performed with an anode cylinder (molybdenum) of 8 m m internal diameter (23 m m length) and a cathode separation o f 27 ram. The cathode discs were made of tantalum. The volume of the discharge chamber was reduced by this design of the source which resulted in a remarkable decrease of the outgassing rate. That is important, because outgassing (normally air and water vapor) influences the plasma behaviour and the source operating conditions. It is furthermore the main cause for "impurities" of the beam which makes more difficult the identification of multiply-charged heavy ions in the mass range of 1 to 40 of an ion mass spectrum. The complete experimental arrangement is shown by a schematic diagram in fig. 2. Excepting the 60°-analyzing magnet all beam handling components are electrostatic. Thus the focussing of the ions becomes independent of charge state and mass of the ions at given extraction voltage. Ions are extracted from the source through an aperture (3 mm diameter) in the cathode with 25 kV extraction voltage (end extraction). The ion beam is focussed by two "einzellenses" into an entrance slit o f the analyzing magnet. In front of the analyzing magnet the total beam current Ii,,o, can be measured

The growing field of heavy ion physics demands beams of multiply charged positive ions of almost all elements o f the periodic system. For solid state research work, e.g. ion implantation as well as for all basic investigations in atomic and nuclear physics these ions are used for further acceleration over a wide range of energies (electrostatic, linear and cyclic acceleration). F o r these applications the following properties of an ion source are required: high particle output, high yield o f multiply charged ions, low power and gas consumption, low emittance of the extracted beam, long lifetime and small size o f the source. P I G sources for multiply charged ions have been reviewed recently1). Most o f the mentioned sources exhibited a high yield of highly ionized ions but also a short lifetime because they are operated in a high current low-voltage discharge mode with large power o f the discharge. In the present paper we desclibe the performance and operating conditions of a P I G - t y p e source with end extraction, low power consumption and small size. The source has been investigated under continuous operation in a low current-high voltage discharge mode which favours inner shell ionization. The yield of multiply charged ions will be presented for operation with noble gases and some gaseous compounds. 2. 1on source and experimental arrangement The ion source which has been developed with reference to an earlier design 2) is shown by a sectioned * Supported by the GSI, Darmstadt, Germany. t Permanent address: Institut far Kernphysik, Frankfurt, Frankfurt am Main 90, Germany.

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* Aluminium oxid, Friedrichsfeld GmbH. t Polyimid, Dupont.

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in a Faraday-cup. The separated beam components, e.g. 1;+ are again focussed by an electrostatic quadrupole lense into a vertically-slit-shaped aperture of an additional Faraday-cup. The resolution AM/M obtained with this arrangement was about 100 at a transmission rate of 50 to 90% depending on the beam intensity.

by primary electrons produced by ions of the discharge hitting the cathodes. Operating data of the source were:

3. Measurements The total extracted beam current and the yield of multiply-charged ions were measured for continuous operation of the source in the cold cathode discharge mode. High discharge voltage, low discharge current and low pressure in the discharge region are the characteristic features of that mode. The steady-state of the discharge is solely maintained

The total beam current (Ii.,ot) extracted from a P I G source is determined by the ion density (hi) and electron temperature (T e) of the discharge. These quantities can be influenced by the discharge voltage (Ud), the gas flow (F) and the magnetic field (H). Therefore the dependence of the total ion current on these source parameters was investigated for the determination of optimal operating conditions. The measurements were

Ua:I - 6 kV; Id: 1--20 mA; H: 0.5-1.5 kG; p ~ 1 0 .3 torr. 3.1. TOTAL BEAM CURRENT AND OPTIMAL OPERATING CONDITIONS

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performed in two steps: For given discharge voltage a maximal total beam current (optimal operating conditions) was obtained by adjusting the magnetic field strength and the gas flow rate. These values are termed with Hop t and Fopt. In the second step only one of the quantities Ua, F, H was varied. Fig. 3 shows the results of these measurements for Hop,, Fopt at Ud = 3.2 kV and argon as source gas. The total ion current increases with Ud up to a first maximum at 3.2 kV and beyond that value a kind of levelling off or saturation is observed (fig. 3a). A similar feature has also been observed for the discharge current (Id)' The ratio of Id/Ii,to t decreases with in, creasing discharge voltage and reaches a value of about 3 in the saturation region. The fluctuations disappear if the magnetic field strength is optimized at each value of the discharge voltage (fig. 3b). The variation of the total ion cunent with magnetic field is shown in fig. 3c. The discharge ignites at about 0.5 kG. The ion current increases rapidly and reaches again saturation showing several maxima. The magnetic field strength in the second maximum is the value of Hopt. The dependence of the total ion current on gas flow (fig. 3d) shows two distinct maxima. The value of the gas flow in the first maximum is Fopt. In the transition region between both ion current maxima a change in the discharge mode is observed. The discharge current increases rapidly and the cathodes start to glow. The behaviour of the total ion current as shown in fig. 3 does not change qualitatively if corresponding measurements are carried out for optimal operating

conditions at other values of Ud in the investigated voltage range. However, the values of Hopt and Fopt have to become larger with increasing discharge voltage in order to obtain the optimal discharge conditions related to Ua. Thereby the total ion current shows a linear increase with discharge voltage. These results are demonstrated in fig. 4. The lineai increase of /i,tot was also observed for operation with other noble gases (He, Ne, Kr, Xe) (fig. 5), but with a slope inversely proportional to the square root of the ion mass (m0 (fig. 5) which can be expressed as /i,

C tot =

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Ud.

(1)

x/mi The mass dependence of Ii,to t is due to the fact that the ion flow to the cathode (anticathode) depends on the ion current density Ji,c at the plasma boundary given by the relation 3) Jl, c = n l e ( k T e / m l )

~:.

(2)

The cathodes being kept on a strong negative potential ( ~ Ud) with respect to the plasma column act like a negative biased probe. If the optimal operating conditions have been obtained the ion current towards the cathode I~,c =jj,¢.S (S is the emissive surface of the plasma boundary at the cathode) is a saturation current, the intensity of which is only determined by ion density and electron temperature of the discharge (eq. (2)). An increase of these plasma parameters and thus also of If,tot can be achieved mainly with an increase of Ud and F as fig. 4 is showing.

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The saturation current to the cathode also explains why we observe a levelling off for the total extracted ion current (fig. 3) proposing that all ions emitted from the plasma boundary layer towards the cathode are extracted through the hole in the cathode (li,~Ij,,o~). A photograph of the cathode and anticathode (fig. 6) demonstrates that this assumption is justified: The

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crater-shaped erosion by ion bombardment observed at the anticathode after a longer operating period looks like an image of the extraction hole whereas the cathode

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itself seems very little affected by ion sputtering. This result is important for long periods of operation of the source, therefore the operating conditions will stay constant. With the results (fig. 4) mentioned above the optimal operating conditions of the source can be described as follows: the optimal gas flow rate and the optimal magnetic field strength have to be increased proportional to the discharge voltage. The gas flow decreases with increasing mass of the source gas whereas the optimal magnetic field does not depend on it. The latter result means that the ionization state of the discharge and in t h a t way IL,ot is primarily influenced by the magnetically guided motion of the electrons in the discharge especially of the primary and fast oscillating electrons. Certainly an optimal oscillating factor can only be achieved for these electrons if their radius of gyration is sufficiently small compared to the internal diameter of the anode cylinder. On the other hand, with increasing U a not only the axial but also the radial component of the electron velocity increases. Thus the value of Hop, has also to be increased in order to keep the optimal discharge conditions.

C H A R G E S T A T E DISTRIBUTION

In gas discharge ion sources two production mechanisms for multiply-charged ions are of importance: single impact ionization (d+e--*A ~+ + ( ( + l)e) and successive impact ionization (A (;- 1)+ + e--~A ;+ + 2e). The electron density and the ion confinement time determine which process dominates4'5). In both cases the ion loss mechanism is given by wall recombination ~,6). The measurements of charge state distributions were performed at optimal discharge conditions as described in sect. 3.1. In fig. 7 the relative ion current of multiplycharged argon ions is shown as a function of the discharge voltage. Fig. 8 shows for all investigated noble gases the particle current (normalized to charge state 1 + ) obtained at U a = 3 kV and for comparison the relative cross sections for single impact ionization 7"~) measured at 3 keV electron energy. The increasing yield of the multiply-charged argon ions as well as the similar behaviour of particle current and cross section in fig, 8 suggest that single impact ionization dominates under the mentioned operation conditions.

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Ion production by successive impact ionization cart be almost excluded because of the electron density (from measurements with Langmuir probes numbers in order of 101° cm -3 were obtained) and too small confinement time of the ions. Taking thermic ion motion into consideration most of the ions can cross the plasma column (25 mm length) in the order of 1 0 - 4 S proposed an ion temperature of 10 2 to 10 3 K. On the other hand, at the measured electron density the ions must be confined in the plasma for about 10 to 10 2 s e c 9, to) which shows that the successive impact process is less probable. The charge state distribution could not be influenced by change of the magnetic field strength in the range of 0.5 to 1.5 kG. Changes of the gas flow in a small range around the optimal value also does not affect the charge state distribution. Measurements at an ion source with radial ion extraction which had the same geometrical size did not yield a different charge state distribution but two orders of magnitude smaller ion currents. This result suggests the conclusion that no specific region inside the discharge is favoured for production of multiply-charged ions. 3.3. [ON CURRENTS OF DIFFERENT ELEMENTS

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The ion production was also investigated for a number of gaseous compounds of different elements. Multiply-charged ions of uranium were obtained by operating the source with UF6 gas as well as by using uranium cathode discs in a noble gas discharge. In table 1 the measured currents of the multiply-charged ions are summarized with the source operating data. Figs. 9-12 show typically ion spectra for the different kind of source gascs used. From noble gas discharges ion spectra were obtained only with lines due to the different charge states of the noble gas ions only. The intensity of singly-charged ions was always strongest. The intensities of the ions with higher charge states decreased almost exponentially which seems to correspond to the bebavioul of the cross sections of single impact ionization (fig. 8). If the source is operated with molecular gases the spectra are more complex because of the formation of molecular ions. Fig. 10 shows the ion spectrum obtained for nitrogen as source gas and fig. 11 the sulfur ion spectrum of a H 2S-discharge (as an example for source operation with diatomic and hydrogen compound gases). The number of possible molecular ions increases with the number of atoms per molecule. In some cases e.g. UF6-discharge also multiply-charged molecular ions were observed (fig. 12 and table 2). At operation with gaseous compounds the fraction of molecular ions of the total beam current is almost

524

H. BAUMANN AND K. BETHGE Tame 2 Uranium ion currents and source operating data.

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larger than of the atomic ions. The currents of the singly and multiply charged atomic ions are much smaller compared to the noble gas discharges. This is due to the fact that production of highly ionized ions starts from the neutral atom as mentioned above because of a too small confinement time for the ions in the discharge. On the other hand, in dissociation processes by electron impact only a relative slnall fraction of neutral atoms is formedll). Thus for UF 6 U 4+ could be observed as highest charge state of uranium whereas in case of the source operation with uranium cathodes U s + has been identified (fig. 13). For both charge states the extracted particle current was of the same order (10 ~~) (table 2). 3.4. ADDITIONAL PROPERTIES OF THE SOURCE The energy spread o f the extracted total ion b e a m

3.5 7.5 0.5

6+

0.45

0.1

0.2 1

was measured by a retarding field methodJ2'13). Values of about 60 to 80 eV were found for argon at different discharge voltages and optimal source conditions. The measurements showed that the cathode potential drop is always 95% of Ud. First measurements of the emittance of the total ion beam carried out with a two slits method TM) gave the result that the emittance of the beam after the first "einzellense" is of about 25 cm mrad for ion currents of I mA and extraction voltage of 20 kV in a distance of 80 cm after the first "einzellense". Power and gas consumption of the source mainly depend on kind as well as mass of the operating gas used. The power consumption obtained with the data of table I varies from 13 or 15 (for hydrogen compounds respectively noble gases) to about 30 W (for diatomic gases) per

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PIG ION

1 mA extracted ion current which gives an average value of about 20 W/mA. The gas consumption decreases with increasing mass of the operating gas as shown by values for the noble gas discharges: He 4%, Ne 12%, Ar 29%, Kr 37%, Xe 49%. These data were calculated for measured ion currents and gas flow rates at Ud = 3 kV and optimal source operation.

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(~ = 3 to 6) of heavy ions. Compared with PIG sources using side extraction the yields of highly ionized ions are lower since the confinement time of the ions in the continuous cold cathode discharge is not sufficient for production of multiply-charged ions by successive impact ionization. The chance of improving the charge state distribution with pulsed source operation 15) as well as with use of higher magnetic fields (up to I0 kG) has yet to be proved. The small size and the very low power consumption presents the source to application for instance in single stage Van de Graaff accelerators.

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We thank Dr E. Heinicke for helpful advice in the early stage of that work and Mr G. Klein for help during the emittance measurements.

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Fig. 11. Spectrum o f sulfur ions.

References 100 • UB"

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300

1) I. R. I. Bennett, IEEE Trans. Nucl. Sci. NS-19 (1972) 48. 2) K. Bethge and E. Heinicke, Nucl. Instr. and Meth. 30 (1964) 283. a) D. Bohm, in: Characteristics o f electrical discharges in magnetic fields, Eds. A. Guthric and R. W. Wakerling (McGraw-Hill, New York, 1949). 4) A. Septler, 1EEE Trans. Nucl. Sci. NS-19 (1972) 22. a) A. van der Woude, ibid, 187. 6) W. B. Kunkel, in: Plasma physics in theory and application (McGraw-Hill, New York, 1966) Ch. 10. 7) B. L. Schramm, A. J. H. Boerboom and J. Kistemaker, Physica 32 (1966) 185. 8) B. L. Schramm, Physica 32 (1966) 197. 9) N. J. Peacock and R. S. Pease, Brit. J. Appl. Phys. 2 (1969) 1705. 10) G. F. Tonon, IEEE Trans. Nucl. Sci. NS-19 (1972) 173. 11) j. H. Freeman and G. Sidenius, Nucl. Instr. and Meth. 107 (1973) 477. a2) H. Boersch, Z. Physik 139 (1954) 115. 13) H. Baumann, Thesis (Heidelberg, 1973) p. 35. 14) A. van Steenbergen, Nucl. Instr. and Meth. 51 (1967) 245. 15) K. Prelec and M, Isaila, Nucl. Instr. and Meth. 92 (1971) 1.