Investigating the electron density of multi-MeV X-ray-induced air plasmas at low pressures based on electromagnetic resonant cavity analysis

We investigate air plasmas generated by multi-MeV pulsed X-rays at pressures ranging from 10−5 to 10−1 mbar. The experimental approach used for these studies is based on measurements of resonant frequencies damping and shift for different electromagnetic modes within a cylindrical cavity. Time-integrated electron densities in X-ray-induced air plasmas are inferred from the damping rate of the measured magnetic fields and their corresponding frequency shifts. In the present study, electron densities ranging from 108 to 109 cm−3 at pressures ranging from 10−3 to 10−1 mbar have been measured. Experimental results were confronted to 3D Maxwell-Vlasov Particle-In-Cell simulations incorporating a radiation-induced electric conductivity model. The method used in this work enables determining microscopic and macroscopic physical quantities within low pressure air plasmas generated by pulsed X-ray.