부산대학교 도서관

At the EPURE center, in France, a high power electron diode is used to generate an intense pulsed electron beam (2.6 kA, 4 Me V) in order to produce a bremsstrahlung pulse for flash radiography. The plasma created by the surface flashover of the velvet cathode provides a zero work function emitter used to produce the space-charge-limited electron beam [1]. In order to optimize the design of diodes and to better understand the plasma dynamics, a OD Collisional-Radiative Model (CRM) is developed in order to simulate the time-dependent evolution of the mean electron energy and the density of charged, neutrals and excited plasma species. The plasma composition is assumed be a mixture of hydrogen, oxygen and carbon, coming from gas desorption and eroded material from the velvet surface flashover [2]. Nonequilibrium EEDFs are estimated from the solution of the kinetic Boltzmann equation solved in the framework of multi-term approximation [3]. EEDFs with cross sections of electron impact process are used to determine the reaction coefficients needed to calculate time-dependent plasma species densities. Visible emission spectroscopy measurements were performed in order to investigate the plasma composition, to measure the $\mathrm{H}\alpha/\mathrm{H}\beta$ intensity ratio and to estimate the electron density. The OD CRM results are compared to the experimental results in terms of electronic density and $\mathrm{H}\alpha/\mathrm{H}\beta$ intensity ratio. Based on the main processes involved in the evolution of the electron density, a reduced model is built with a minimum number of reactions and species. The reduced set of reactions and species will allow us to investigate the plasma effects within the diode using particle-in-cell simulations.