X-ray gas attenuators are used in high-energy synchrotron beamlines as high-pass filters to reduce the incident power on downstream optical elements. The absorption of the X-ray beam ionizes and heats up the gas, creating plasma around the beam path and hence temperature and density gradients between the center and the walls of the attenuator vessel. The objective of this work is to demonstrate experimentally the generation of plasma by the X-ray beam and to investigate its spatial distribution by measuring some of its parameters, simultaneously with the X-ray power absorption. The gases used in this study were argon and krypton between 13 and 530 mbar. The distribution of the 2p excited states of both gases was measured using optical emission spectroscopy, and the density of argon metastable atoms in the 1s state was deduced using tunable laser absorption spectroscopy. The amount of power absorbed was measured using calorimetry and X-ray transmission. The results showed a plasma confined around the X-ray beam path, its size determined mainly by the spatial dimensions of the X-ray beam and not by the absorbed power or the gas pressure. In addition, the X-ray absorption showed a hot central region at a temperature varying between 400 and 1100 K, depending on the incident beam power and on the gas used. The results show that the plasma generated by the X-ray beam plays an essential role in the X-ray absorption. Therefore, plasma processes must be taken into account in the design and modeling of gas attenuators.

We present experimental results on the characterization of ID-Hall, a double stage Hall thruster with an inductively coupled magnetized ionization stage. This first experimental prototype of ID-Hall operates in a low power regime, typically below 400 W. The purpose of this work is not only to study the properties of ID-Hall and provide directions for optimization but also, and more generally, to study the relevance of the double stage concept with respect to important physical issues such as current oscillations and anomalous electron transport. We present experimental measurements of the current–voltage characteristics, extracted ion beam current (Faraday cup measurements), and ion energy and angular distribution functions (RPA measurements) in single and double stage regimes of operation, as a function of DC voltage and RF power. The results demonstrate that ions can be extracted from the inductive RF ionization stage even under conditions where the applied DC voltage is not sufficient to ionize the gas flow, i.e., ID-Hall can indeed operate in a double stage regime. At intermediate voltages, the overshoot observed in the current–voltage characteristics of single stage thrusters disappears in double stage operation, indicating a decrease in anomalous electron transport. Time-resolved and time-averaged ion energy distribution measurements show a coupling between ion energy, breathing mode oscillations, and the magnetic configuration of ID-Hall.