Microwave breakdown of low pressure N2 gas in microgaps

Abstract

The pressure dependence of the breakdown of nitrogen gas by a microwave field in microgaps down to 13 μm has been measured. Fits to a phenomenological function suggest that microwave breakdown in small gaps requires a higher collision frequency, νc. If the effective electric field on an electron in a microgap is interpreted as Eeff=Eoνc2/(νc2+ω2) and then as the gap size is reduced, we find that the collision frequency needed to produce a minimum breakdown electric field shifts from νc = ω for large gaps to νc = 2ω for small gaps.

Collision and diffusion in microwave breakdown of nitrogen gas in and around microgaps

“…Equation (1) fits these breakdown curves well with reduced chi square,  r 2 , ranging from 1 to 3. However, because of the high values of E bd for the smaller gaps, the breakdowns in Reference [14], all measured below a pressure of 30 Torr, were hypothesized to occur outside the microgaps, which is confirmed in this paper. Figure 2 shows E bd from eight different gap sizes with fits to Equation (1).…”

“…The effective threshold field E eff,bd N m depends on the number density N of neutrals and a power law m, which measures the extent to which the ion buildup is dominated by collisions 14 .…”

“…Low pressure microwave breakdown in gaps was previously measured by the current authors using a re-entrant resonant cavity similar to the cavity used here, benchmarked against historical data, and modeled as a collisional process 13 . These experiments have since ventured into microgaps as small as 13 m 14 . In large gaps, the threshold breakdown electric field E bd in the vicinity of 1 GHz has a minimum around 1 Torr.…”

Rescaling of microwave breakdown theory for monatomic gases by particle-in-cell/Monte Carlo simulations

“…[1][2][3][4][5][6][7][8][9][10][11] The breakdown threshold 5-11 plays a key role for the microwave gas discharge. There are many models developed for microwave breakdown, but most of them are empirical or semiempirical and are valid only in a limit range.…”

“…There are many models developed for microwave breakdown, but most of them are empirical or semiempirical and are valid only in a limit range. 8,9,[11][12][13][14][15] Fortunately, for monatomic gases, Vyskrebentsev and Raizer develop a theory model (Raizer model), which originates from optical radiation and is valid for a wide range of pressures and all frequencies from constant field to optical. 10 However, Raizer model assumes that the electron energy distribution function (EEDF) is dropped to zero when the electron energy is slightly over the ionization threshold.…”