Experimental Studies on Switching Characteristics of a Field-Distortion Sparkgap Switch in Various SF6 Admixture

Pressurized sparkgap switches are widely used as fast closing switches in various pulsed-power systems. The stochastic closure time delay and jitter in these closing switches can lead to a significant impact on the characteristics of the output pulse. The experimental work has been carried out in various volumetric concentrations of SF
6
in SF
6
/N
2
, SF
6
/dry air and SF
6
/Ar in-order to study the effect of mixing ratio, and the rate of rise of the applied-trigger voltage (RRV) on stochastic behavior of the switch closure. The switching experiments have been carried out in indigenously developed co-axial sparkgap switch with corona discharge electrodes for a working coefficient of 0.8x SBV (Self-Breakdown Voltage) by using a trigger pulse with different dV/dt ~500 V/ns and 10 V/ns respectively. The intrinsic parameters of the switch such as breakdown delay
$(\mathrm{t}_{\mathrm{d}})$
and jitter were found to be strongly dependent on the nature of gas, % SF
6
and RRV. In SF
6
/N
2
, as %SF
6
increased from 5-50% the
$\mathrm{t}_{\mathrm{d}}$
increased from
$295\pm 27$
ns to
$458\pm 94$
ns and this increasing trend was observed even in SF
6
/dry air and SF
6
/Ar admixture. The
$\mathrm{t}_{\mathrm{d}}$
and jitter under fast rising trigger were significantly less than that of trigger pulse with lower dV/dt. To further investigate the stochasticity of streamer discharge in SF
6
admixture numerical studies has been carried out by using PIC/MCC model. The results indicate that SF
6
/Ar with relatively low SF
6
content may be considered as a suitable candidate to replace SF
6
for pulsed power switching, however consequential effects for e.g., repetitive operation, electrode erosion rate and gas decomposition/recombination issue etc., may require further investigations.