Plasma characterization on carbon fiber cathode by spectroscopic diagnostics

This paper mainly investigates plasma characterization on carbonfiber cathodes with and without cesium iodide (CsI) coating poweredby a $sim300$~ns, $sim 200$~kV accelerating pulse. It was foundthat the CsI layers can not only improve the diode voltage, but alsomaintain a stable perveance. This indicates a slowly changed diodegap or a low cathode plasma expansion velocity. By spectroscopicdiagnostics, in the vicinity of the cathode surface the averageplasma density and temperature were found to be $sim 3times$10$^{14}$~cm$^{ - 3}$ and $sim 5$~eV, respectively, for anelectron current density of $sim 40$~A/cm$^{2}$. Furthermore, thereexists a multicomponent plasma expansion toward the anode. Theplasma expansion velocity, corresponding to the carbon and hydrogenions, is estimated to be $sim 1.5$~cm/$mu $s. Most notably, Csspectroscopic line was obtained only at the distance $ le 0.5$~mmfrom the cathode surface. Carbon and hydrogen ions are obtained upto the distance of 2.5~mm from the cathode surface. Cs ions almostremain at the vicinity of the cathode surface. These results showthat the addition of CsI enables a slow cathode plasma expansiontoward the anode, providing a positive prospect for developinglong-pulse electron beam sources.