Plasma processes on a high power X-ray gas attenuator

The high power delivered by modern synchrotron and XFEL sources often requires attenuators to absorb the parts of the X-ray spectrum not used in the experiments, reducing the heat load in monochromators and other optical elements. Compared to solid attenuators, gas attenuators have the advantages of a tunable absorption ratio, reduced cooling requirements and absence of themomechanical stresses. However, the ionization of the gas by the Xray beam triggers a cascade of processes that affects the X-ray absorption. In particular, a high ionization degree leads to the formation of plasma. The understanding of the different energy transfer mechanisms within the plasma is necessary to predict the heating of the gas, which determines its density and ultimately the X-ray absorption.

In this work we have carried out an experimental and theoretical study of the plasma formed in the attenuator. In the experimental study, Optical Emission Spectroscopy (OES) and Tunable Laser Absorption Spectroscopy (TLAS) were used to obtain a spatial profile of the excited states of the gas, usually argon or krypton, together with the X-ray absorption by the gas attenuator. The theoretical study consisted on the development of a hybrid model combining Monte Carlo and fluid modeling techniques, each of them adapted to processes with different characteristic energy. The results of the model were compared to those obtained in the experimental study, showing a good agreement between them. This enables us to use the model for the design and operation of existing and future gas attenuators, and as a starting point for further studies in gas attenuators used in new X-ray facilities.