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

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 (a) to find the best condition for producing high beam currents of C4+ ions, and (b) 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 C5+ 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 (CD4 gas) the C5+ beam currents are about 10% higher than with regular methane (CH4 gas). The 'mixing-gas' ratio D (or H) to C can be decreased by choosing, e.g., butane gas; in this case the isotopic effect for C5+ production is even stronger (>15%). For production of C4+ 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 C4+-ion currents than methane