Simulation of surface x-ray emission from the ASTERICS ECR ion source

The bremsstrahlung x-ray emission induced by the impact of plasma electrons de-confined on the chamber wall of the ASTERICS electron cyclotron resonance ion source is investigated through a suite of two simulation codes. The electron high energy temperature distribution tail at the wall is found to be anisotropic and increases with Bmin. The electrons impinge the walls with broad angular distribution peaking at angles ranging between 5-25° with respect to the surface, which has consequences on the x-ray emission directionality and on the yield of electrons bouncing back toward the plasma, reaching up to 50%. The x-ray dose is mapped inside and around the ion source for Bmin = 0.8 T and an electron temperature artificially increased to 120 keV to dimension with margin the cave shielding. The dose without shielding reaches 100 $μ$Sv/h per kW of impacting electrons at 5 m. A set of internal and external shielding is presented to attenuate this dose and reduce it to less than 1 $μ$Sv/h per kW of electrons. A parametric electron distribution temperature study with Fluka indicates that the deposition of 1 W of heat in the superconducting cold mass per kW of plasma electrons, as reported experimentally, is obtained when the temperature is set to 380 keV. Such a result is compatible with previous experiments achieved on several ion sources showing an x-ray spectral temperature 3 to 4 times higher radially.