Optimizing C4+ and C5+ beams of the Kei2 electron cyclotron resonance ion source using a special gas-mixing technique

Abstract

With the prototype electron cyclotron resonance ion source for the next carbon therapy facility in Japan a series of measurements has been performed in order ͑a͒ to find the best condition for producing high beam currents of C 4+ ions, and ͑b͒ to study the effect of "special" gas mixing by using a chemical compound as a feed gas. The effect would then appear as an increase in high charge state production in this case of C 5+ ions. In "regular" gas-mixing experiments it is well known that an isotopic phenomenon occurs: a heavier isotope of the mixing gas is increasing the production of high charge states of the beam gas ions. A similar isotopic effect has been found in the present experiment: with deuterated methane ͑CD 4 gas͒ the C 5+ beam currents are about 10% higher than with regular methane ͑CH 4 gas͒. The "mixing-gas" ratio D ͑or H͒ to C can be decreased by choosing, e.g., butane gas; in this case the isotopic effect for C 5+ production is even stronger ͑Ͼ15% ͒. For production of C 4+ ions the isotopic effect appears to be absent. Clearly this is related to the much easier production. It turns out that the relative amount of carbon is much more important: butane gives about 10% higher C 4+ -ion currents than methane.

Multiply charged carbon-ion production for medical application

“…8 An investigation of the gas selection suitable for highly charge state is in progress. 9 In order to influence the charged-particle fluxes to the wall of the plasma chamber, it is a popular technique to apply a special dielectric material on the wall, a so-called "wall coating." For a dominating axial field, the electron flux dominates the axial losses, whereas the ionic flux determines the radial diffusion.…”