VUV absorption spectroscopy measurements of the role of fast neutral atoms in a high-power gap breakdown

Abstract

The maximum power achieved in a wide variety of high-power devices, including electron and ion diodes, z pinches, and microwave generators, is presently limited by anode-cathode gap breakdown. A fiequentlydiscussed hypothesis for this effect is ionization of fast neutral atoms injected throughout the anode-cathode gap during the power pulse. We describe a newly-developed diagnostic tool that provides the first .@rect test of this hypothesis. Time-resolved vacuumukaviolet absorption spectroscopy is used to directly probe fast neutral atoms with 1 mm spatial resolution in the 10 mm anode-cathode gap of the SABRE 5 MV, 1 TW applied-B ion diode.Absorption spectra collected during Ar RF glow dischtiges and with C02 gas fills confirm the reliability of the diagnostic technique. Throughout the 50-100 ns ion diode pulses no measurable neutral absorption is seen, setting upper limits of 0.12-1.5x10'4 cm-3 for ground state fast neutral atom densities of H,C~,O#. The absence of molecular absorption bands also sets upper limits of 0.16-1.2x1015 cm-3 for common simple molecules. These limits are low enough to rule out ionization throughout the gap as a breakdown mechanism. This technique can now be applied to quantifi the role of neutral atoms in other high-power de-vices...

A passive measurement of dissociated atom densities in atmospheric pressure air discharge plasmas using vacuum ultraviolet self-absorption spectroscopy

“…Despite this, VUV irradiation has demonstrated 8 an increased ability to generate free electrons in N 2 environments, and recent studies 9 have investigated the earliest VUV production from N atoms in the streamer and spark phases of breakdown. As a result, it is desired to passively measure the dynamics of atom production from molecular dissociation in air discharge plasmas, without using a potentially invasive optical absorption diagnostic technique, such as two-photon laser induced fluorescence (TALIF) spectroscopy 10 or broadband absorption spectroscopy, 11 which can artificially increase the dissociation degree.…”