Dual-pulse generation from a velvet cathode with a new inductive voltage adder for x-ray flash radiography applications

Abstract

The evolution of two intense pulsed electron beams for an x-ray flash radiography application is characterized in a coupling of experimental and simulation studies. For this purpose, experiments were performed at Commissariat \`a l'Energie Atomique et aux Energies Alternatives with a new dual-pulse inductive voltage adder named ``Mi2.'' This experimental setup has been designed and built to produce two intense pulsed electron beams of 2 kA, 700 keV, and 80 ns full width at half maximum from a velvet cold cathode. The delay between the pulses ranges from tens of nanoseconds to a few microseconds. This study quantifies the effect of the plasma dynamics on the beam current of the second pulse, which represents an important parameter from the beam transport point of view. A multidimensional particle-in-cell simulation model is used to provide a numerical analysis in order to determine the influence of velvet plasma dynamics on the second pulse. Analysis of the experimental results indicates that the motion of the electron extraction zone within the expanding plasma remains lower than 1 mm for $3\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$ delay, inducing a minor impact on the beam current extracted from the velvet cathode. Based on the developed model, this plasma expansion induces an increase of 5% for the current extracted from the second pulse with a delay of $1\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$ for two pulses having the same voltage, which is consistent with linear induction accelerator injector parameters for multiple pulse use.