Development of a low energy small electron gun to study electron transport in hydrogen negative ion source plasmas.

Abstract

We developed a small-size electron gun capable of producing electrons with kinetic energy less than few tens of eV to investigate the slowing down and transport mechanisms of electrons in hydrogen negative ion source plasmas. The maximum extractable beam current density reached 36 μA/cm2 for 1 eV beam energy in a preliminary experiment. Although the present electron current density is still insufficient compared with our target value, 1 mA/cm2, we have found some hints to realize larger beam current density from the electron gun through this study. The measured beam profile along the electron beam axis has shown that the electron beam could travel approximately 7 mm from the electron gun in vacuum. The Particle-In-Cell (PIC) simulation explained the measured beam profile well and indicated that the electron beam has an energy spread as small as 0.1 eV compared to the 1 eV mean energy. The PIC simulation showed a discrepancy from the measurement in the dependence of the electron beam current on the beam extraction voltage of the electron gun. It implies that we should introduce a more realistic filament structure inside the electron gun in the PIC simulation in order to study the transport of low energy electrons more precisely.