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

TL;DR

This work addresses x-ray emission from bremsstrahlung produced by plasma electrons deconfined at the ASTERICS ECR ion source wall. It combines a Monte-Carlo electron-dynamics model for wall impacts with Fluka radiation transport to map dose and evaluate shielding. The study finds highly anisotropic wall EEDFs that increase with B_min, with incidence angles dominated by large values and substantial back-bounce toward the plasma; shielding can suppress cave doses to safe levels, though high radial temperatures are implied by the 1 W/kW heating requirement at Te ~ 380 keV. The results inform safety design and contribute to understanding plasma-wall interactions and x-ray spectral-temperature anisotropy in ECRIS, while outlining future work to refine the model with standing-wave effects and full-dose mapping.

Abstract

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.

Figures (9)

• Figure 1: Cutaway view of the ASTERICS ion source design.
• Figure 2: EEDF of the electrons hitting the plasma chamber wall for (a)$B_{min}$ = 0.3 T and (b)$B_{min}$ = 0.8 T. The black, blue and red plots are respectively recorded on the injection (z=$z_{inj}$), radial ( r=$r_{wall}$) and extraction surfaces (z=$z_{ext}$). The cyan EEDF plots corresponds to the electron still confined in the plasma chamber volume after 1 ms.
• Figure 3: Distribution of the angle of incidence of electron impacting the plasma chamber walls ($\theta=(\widehat{\vec{v},\vec{n}}$)), $\vec{n}$ normal to the surface) (a) for $B_{min}$ = 0.3T and (b) for $B_{min}$ = 0.8T. The black, blue and red curves correspond to the injection, radial and extraction surfaces respectively.
• Figure 4: Electron density distribution at the injection ((a) and (b)), radial ((c), (d), (e) and (f)) and extraction ((g) and (h)) surface of the plasma chamber wall for $B_{min}$ = 0.3T (top plots) and $B_{min}$ = 0.8T (bottom plots). The dimension scale of images between the top and the bottom is conserved.
• Figure 5: Sectional view of the ion source geometry modeled with Fluka. Detail of the materials thickness used to shield the x-ray emission from the source are provided. See text for details.