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Author(s):
A. Taniike
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Title:
Energy Loss Mechanism of a Gold Ion Beam on a Tandem Acceleration System
Date of publication:
May 1995
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Key words:
plasma sputter type ion source, energy width, energy broadening, negative gold ion beam, positive gold ion beam, tandem acceleration, electron stripping, elastic energy loss process, inelastic energy loss process, energy straggling, heavy ion beam probe
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Abstract:
A heavy ion beam probe (HIBP) is used as a reliable method to measure plasma potential and its fluctuation in a magnetically confined fusion plasma. A singly charged positive ion beam of mass about 200 amu with energy of 6 MeV is required in order to apply the HIBP to a large and strong magnetic field device such as the Large Helical Device (LHD) which is under construction at National Institute for Fusion Science, Japan. A primary beam with a small energy spread and of a current larger than 10 muA is required to measure the plasma potential as small as a few keV. The origins of the energy spread on a tandem acceleration system are: 1. the fluctuation of acceleration voltage, 2.the energy spread of negative ions produced in an ion source, 3. the energy broadening caused in a charge stripping gas cell. In the present work, I have carried out experimental and theoretical studies mainly on the second and the third problems. In order to study energy loss mechanism on a tandem acceleration system, a tandem acceleration test stand has been constructed, and the charge state fraction, the beam profiles and the beam energy spectrum of an Au^+ beam have been measured. The test stand consists of a negative gold ion source, a tandem acceleration system, a movable Faraday cup and an energy analyzer. The energy spectrum of an Au^- beam extracted from the ion source was measured, The energy width was less than 6 eV when Cs vapor was introduced to reduce the surface work function of the gold target. The dependence of the energy width on the work function was well explained by the theory of the surface production of a
ion. The energy shift between the primary negative ion beam and a positive ion beam converted in a gas cell at small gas thickness was measured and found to be about 12 eV. The shift is caused by the two-electron loss process, and amounts to the sum of the electron affinity and the first ionization energy of the projectile. Energy loss spectra of Au^+ ions produced from Au^- ions by electron stripping in He, Ar, Kr and Xe have been measured in the impact energy range from 24 to 44 keV, under the condition that the charge stripping gas thickness is thin enough so that the two-electron stripping process (Au^ rightarrow Au^+) is dominant and the multiple collision processes are negligible. The full width at half maximum (FWHM) is typically 20 to 80 eV, and it increases with the impact energy. In general, a broader width is observed with a target of a lower mass number. The energy broadening of the Au^+ beam is caused by elastic and /or inelastic loss processes. The amount of energy loss due to the elastic and the inelastic processes depends upon the impact parameter. The range of the impact parameter, which contributes to the energy loss spectrum is determined by the apparatus geometry (b_min) and by the threshold energy for charge stripping(b_max). The sum of elastic and inelastic energy losses causes the energy broadening of an Au^+ beam. There is no significant contribution of the elastic energy loss to the energy broadening in the energy range in the present experiment (24keV~44keV). A simple model is proposed using the semi-classical internal energy transfer function of Firsov's and the scattering by the unified potential of Ziegler's. the theoretical prediction of the present model reproduces the energy and mass dependence of the broadening. However, the absolute values of the theoretically predicted width are much smaller than the measured widths. The present model predicts that the energy spectrum in the higher energy region saturates with a FWHM of less than 10eV, and the target mass dependence disappears. In the target density region that the multiple collision is not negligible, the energy straggling of a beam is generally explained by the L.S.S. (J. Lindhard, M. Scharff and H.E. Schiott) theory. In this theory, the squared energy straggling is given by an integral of the squared energy loss function over the impact parameter from zero to infinity. However, when an energy loss spectrum of a singly-charged component of a beam is measured at forward angle, the energy loss function in a small impact parameter region does not contribute to the straggling. Therefore, a lower limit of integral region is taken into account. It is determined by the minimum impact parameter, b_min, or the impact parameter which corresponds to the inelastic energy transfer to produce an Au^++. Energy straggling of the present calculation is closer to the measured value than that calculated by the original L.S.S. theory. The energy broadening of an Au^+ beam produced by a tandem system can be estimated by the present theoretical prediction. The gas cell of the 3 MV tandem accelerator of LHD will have the same geometry as that of the present test stand. The energy broadening due to the electron stripping can be calculated to be 7 eV. and it does not depend upon the target mass. The calculation of the charge fraction predicts that the optimum gas thickness for Au^+ beam production is about 6x10^14 cm^-2. Together with the energy broadening due to the multiple collision at this target thickness, the total energy width of the Au^+ beam produced by a tandem system might be less than several tens eV. The voltage ripple of the tandem power supply is also about several tens eV. therefore, the total energy spread of an Au^+ beam is about a hundred eV. It will be small enough for a HIBP diagnostics on LHD where the plasma potential is a few keV.
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