Cathode Spot Development on a Bulk Cathode in a Vacuum Arc

A transient model of spot on a bulk cathode is developed, considering the initial adjacent plasma generated during arc triggering. A self-consistent approach is described and a closed mathematical solution is presented to understand the transient cathode phenomena and the time-dependent cathode potential drop (CPD), considering the kinetics and gas dynamics of the cathode plasma flow. The time-dependent spot development is calculated by considering different existing lifetimes
$\tau$
of an initial plasma adjacent to the cathode for Cu, Cr, and W and 10-A spot current. The lifetime
$\tau$
is in the range of 2–100 ns. The solution shows that for Cu, the cathode temperature increased from 3500 to 4300 K with spot time. The CPD decreased with spot time from initial values 100–45 V (depending on
$\tau$
) to 14–15 V at steady state. The solution for a refractory W cathode is obtained using a previously developed virtual cathode model. Calculation shows that a spot current density of
${\sim}{\rm 10}^{7}~{\rm A}/{\rm cm}^{2}$
can support the spot initiation in a time of 2 ns considering W cathode vaporization with plasma generation by atom ionization. When
$\tau$
increased from 2 ns to the 2
$\mu{\rm s}$
range, the W cathode temperature decreased from
${\sim}{\rm 10000}~{\rm K}$
to a relatively low level of 7500 K.