Pressure-yield characteristics of rf ion-sources

NUCLEAR INSTRUMENTS AND METHODS I22 (I974) 303-306;

© NORTH-HOLLAND PUBLISHING CO.

PRESSURE-YIELD CHARACTERISTICS OF RF ION-SOURCES

S.N. MISRA and S. K. GUPTA Van de Graaff Laboratory, Nuclear Physics Division, Bhabha Atomic Research Centre, Bombay 400 085, India

Received 9 July 1974 A system which analyses the components of the charged particle beam current just after the rf ion-source has been used to measure the yields of the components as a function of the pressure at which the gas is fed into the ion-source. The yield of an HH + beam is more than that of an H+ beam at a lower hydrogen gas

feed and the situation reverses at higher gas feed. With 3He the fraction of aHe++ beam is obtained to be only ~0.4% under optimum conditions. A qualitative explanation for the behaviour of ion-beam both with hydrogen and helium gases has been suggested.

1. Introduction

B - A H - T U - 7 6 types of rf ion-sources. In the last section we suggest an e x p l a n a t i o n for the pressure yield curve obtained both with hydrogen a n d helium gases.

R f ion-sources have been in use with the electrostatic accelerators for a long t i m C ) . I n spite of this, the yields of the individual c o m p o n e n t s of the b e a m as a f u n c t i o n of i n p u t pressure have not been reported in the literature t h o u g h the total yield of a n ion-source as a function of pressure has been already reported by G a n g u l y and BakhruZ). In this paper we describe a fast b e a m - a n a l y z i n g system which is essentially a m i n o r modification of W i e n ' s velocity filter and the results o b t a i n e d with this system using hydrogen a n d 3He gases in the Ortec model 320 and HVEC model

- - % 5 KVPROBE ION SOURCE

2. Description of the apparatus and results The beam from the ion-source enters an einzel lens and a velocity filter consisting of crossed electric a n d magnetic fields. This system closely follows the design of a similar system described by Taylor and Weil3). Instead of applying dc voltage to the deflecting plates in the crossed field analyzer we apply 50 Hz sinusoidal ac voltage. In fig. 1 we have shown a sketch of the apparatus which has already been described in detail in ref.

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Fig. 2. Effect of applying ac voltage-to Wien's filter. 303

304

S. N. M I S R A A N D S. K. G U P T A

4. With the ac voltage applied to the deflector, the condition for the ions to come undeflected to the Faraday cup is satisfied twice in a cycle. This is shown in fig. 2. The ion with a velocity v comes undeflected when z, = E / H ,

(l)

where E is the instantaneous value of the ac electric field applied to the deflecting plates and H is the fixed magnetic field of 750 G in the crossed field analyzer. The velocity of an ion depends on the extraction

Fig. 3. I o n - s p e c t r u m obtained on the oscilloscope with 3He gas feed. T h e strong peak is due to the :3He+ and the small peak is due to N ] + CO ~.

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voltage Ve applied to the ion-source and is given by

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where Z and m are the charge and the mass of the ion. The beam collected on the Faraday cup is connected to the oscilloscope where 1 MQ resistance at the input of the oscilloscope drains away the charge. The beam current, when displayed on the oscilloscope, shows the ion-spectrum shown in fig. 3. The various peaks can be identified by varying the magnitude of the ac voltage applied to the deflector. The advantage of seeing the spectrum on the oscilloscope is cofisiderable because any change occurring in the system can immediately be seen. This helps in optimizing wtrious parameters of the ion source. In the gas feed line a thermocouple gauge was installed. The gas was fed through two needle valves. The gauge was calibrated for air and its readings should be corrected lbr the gas used using the accommodation coefficients given by LeckS). The pressure-versus-yield characteristics of H +, H H + and H H H ÷ have been shown in figs. 4 and 5 for the Ortec source. The actual pressure inside the ionsource has to be further corrected for the pressure gradient up to the ion-source from the gauge. Taking the results published by Ganguly and Bakhru2), the measured pressure by the gauge in our system is ~ 8

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PRESSURE-YIELD

CHARACTERISTICS

times the pressure inside the ion-source. In the figures we have given the pressure measured by the gauge without any correction. As the pressure increases, first the H H + peak dominates and at higher pressure H + peak starts dominating. At higher pressure 85% of the beam is H +. There are always some heavy ions present as contaminants. The H H H + beam is negligible at all pressures. These curves have been obtained with four settings of the extraction voltage of the ionsource. These results repeat fairly well at different times and with different ion-sources but they should be considered only qualitative. The results for 3He gas fed into the ion-source are also shown in figs. 4 and 5. Our results for the 3He+ + yield as a function of pressure for the Ortec source are similar to those obtained recently by Tykesson6). The results for the HVEC source are shown in fig. 6. These are similar to those of the Ortec source. The HHH + yield is more for this ionsource.

3. Discussion and conclusion The results for hydrogen gas feed can be explained as follows: the mean electron temperature inside the ionsource decreases with the increase in the gas pressureT). HVEC ION SOURCEMODELB-AH-TU-76 it 0 KVPROBE

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305

At lower gas pressure, the mean electron temperature is high and therefore the reactionS). H 2

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15.4eV

(3)

takes place more easily. As the gas pressure increases and the mean electron temperature comes down enhancing the dissociation of the hydrogen molecule and then the ionization of the hydrogen atom takes place according to 8) H2+e-

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(4)

and H+e-

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(5)

The process (4) takes place prominently at the electron energy of ~ 9 eV. To obtain more of H + we have to go through two steps, while to obtain more of HH + we have to go through the reaction (3) only. The percentage of H + cannot increase until the process (4) takes place predominantly which occurs only around 9 eV, at lower electron temperatures. This therefore explains the pressure-yield curve obtained qualitatively. For 3He the removal of two electrons is a two-step process and the plasma temperature is never very high to produce a higher percentage of 3He + +. For getting more 3He+ + first 3He + has to be formed and then the electron temperature should be high enough to remove the second electron. At very low pressures not enough of 3He + ions are produced to give rise to more 3He+ + ions though the mean electron temperature is high. At higher pressures the electron temperature falls down and more of 3He ++ cannot be produced. Thus the yield of 3He++ ions goes through a maximum as a function of pressure. In these measurements we have used 3He gas because it is not possible to distinguish between 4He++ and HH + because their Z / m are approximately equal. The utility of these pressure-versus-yield characteristics is obvious. Depending on the requirement of an ion, the gas pressure inside the ion-source can be adjusted to have the maximum yield of that particular ion. By adjusting a lower pressure one can obtain more of H H + beam which can be useful to obtain higher molecular hydrogen beam current from the accelerators. The pressure-yield characteristics will also depend on the processes followed in making the ion-source as well as the age of the ion-source. This system can also be used for a routine bench test of the ion-sources. We are thankful to Dr M. K. Mehta and Dr A. S. Divatia for useful discussions and their continued interest in this work.

306

S. N. MISRA AND S. K. G U P T A

References 1) C. D. Moak, H. Reese, Jr. and W. M. Good, Nucleonics 9, no. 3 (1951) 18. e) A. K. Ganguly and H. Bakhru, Nucl. Instr. and Meth. 21 (1963) 56. 3) I. J. Tayler and J'. L. Wiel, Nucl. Instr. and Meth. 34 (1965) 197. 4) S. N. Misra and S. K. Gupta, Van de Graaff Lab. Progress Report, BARC-692 (1973) p. 38.

5) j. H. Leck, Pressure measurement in vacuum systems (Physics in Industry Series; Chapman and Hall, London, 1957) Ch. 2, p. 33. 6) p. Tykesson, Nucl. Instr. and Meth. 98 (1972) 185. 7) F. Gordon, lonisation phenomena in gases (Butterworths, London, 1960) p. 42. 8) p. K. McTaggart, Plasma chemistry in electrical discharges, Elsevier, Amsterdam, 1967)p. 16.