Experimental research on time-resolved evolution of cathode plasma expansion velocity in a long pulsed magnetically insulated coaxial diode

Abstract

Unlike planar diodes, separate research of the axial and radial plasma expansion velocities is difficult for magnetically insulated coaxial diodes. Time-resolved electrical diagnostic which is based on the voltage-ampere characteristics has been employed to study the temporal evolution of the axial and radial cathode plasma expansion velocities in a long pulsed magnetically insulated coaxial diode. Different from a planar diode with a “U” shaped profile of temporal velocity evolution, the temporal evolution trend of the axial expansion velocity is proved to be a “V” shaped profile. Apart from the suppression on the radial expansion velocity, the strong magnetic field is also conducive to slowing down the axial expansion velocity. Compared with the ordinary graphite cathode, the carbon velvet and graphite composite cathode showed superior characteristics as judged by the low plasma expansion velocity and long-term electrical stability as a promising result for applications where long-pulsed and reliable operation at high power is required.

Investigating emission characteristics and plasma properties in a long-pulse magnetically insulated coaxial diode with different cathode materials

“…Figure 9 gives the temporal evolution of axial and radial plasma expansion velocity components of the edged aluminum cathode with different diode voltage. The axial and radial expansion velocities both reduce with the larger diode voltage especially for the pulse plateau, which agrees with reference [15]. As the plasma expansion velocities in MICDs are counteracted in some degree by their opposite electric fields [13], the increase of electric field with the larger diode voltage is responsible for the smaller plasma expansion velocity in figure 9.…”

“…Unlike the radial expansion velocity, which is directly constrained by the guiding magnetic field, the axial expansion velocity decreases by the smaller ratios of density grads to density due to the stronger guiding magnetic field [13]. The evolution trend of the cathode plasma expansion velocity has been discussed in [14,15], which is almost accordance with this trend in figure 8. Figure 9 gives the temporal evolution of axial and radial plasma expansion velocity components of the edged aluminum cathode with different diode voltage.…”

“…For the pulse plateau, with the smaller edge angle, the axial expansion velocity reduces, whereas the radial expansion velocity increases. This is because that the axial electric field becomes larger with the smaller edge angle, while the radial electric field goes smaller near the cathode when the outer diameter and length of the cathode remain unchanged as shown in figure 2(d), since the plasma expansion velocities are counteracted by their opposite electric fields [13,15].…”