Investigation of parameters of plasma in nanosecond breakdown channels in alkali-halide single crystals

The work is devoted to investigation of plasma in breakdown channels initiated from the needle anode in alkali-halide single crystals by pulses of high voltage. Velocities of breakdown channel propagation were measured, currents of channel formation were defined, and some parameters of plasma in channel were estimated. Nanosecond pulses of high-voltage were provided by a generator that was a Tesla transformer built in coaxial forming line with an impedance of 50 Ω [1]. Working voltage of the generator was 140 k V, pulse duration was 8 ns, and rise times were less than 0.5 ns. Alkali-halide single crystals KCl and KBr have been chosen as samples alongside with single crystals of YAG. Behavior of YA G under high electrical stress is interesting for practical purposes. Discharge configuration was point-plane with a needle anode. Velocities of channel propagation were obtained using oscilloscope waveforms of current and voltage drop across the sample at the moment of breakdown. Velocities at these pulsed conditions were 4.6·10⁷ cm/s and 3.5·10⁷ cm/s for KCl and KBr, respectively. Currents of channel formation were defined using a method [2] which is based on experimentally obtained reduction of velocity of channel propagation in presence of a ballast resistor in needle anode circuit, and dependence of this velocity on applied voltage. Values of currents were 20 and 8 A for KCl and KBr. Theoretically, velocity of propagation might be linked with channel conductance [3], it gives a possibility to estimate these currents independently. An estimation is in order-of-magnitude agreement with experiment, resulting in values of several amperes for both materials. Additional tests were made without plane cathode. Instead, the corresponding crystal surface faced air. In relatively long samples of KCl (more than 4 cm) complex breakdown patterns with sequential channel constrictions were observed. Diameter of a constricted part was 10 - μm, whereas diameter of main channel was up to 100 μm. It might be suggested that it is a result of pulsed character of channel propagation. The pattern is new and never have been reported in the literature. From the geometry of the channel, breakdown delay time, pulsed voltage conditions, and value of channel formation current thermodynamic parameters of plasma might be estimated.