Electrostatic-Instabilities as a Source of Picosecond Termination of Runaway-Electrons Beam in High-Voltage Gas-Filled Ultra-Fast Diode

Abstract

The fast disruption of the runaway electrons-beam in a high-voltage picosecond atmospheric-discharge [G. A. Mesyats and M. I. Yalandin, IEEE Trans. Plas. Sci. 37 785 ( 2009)] is considered. The plasma electrostatic instabilities are proposed as a mechanism of such disruption. Strong over-voltage (more than 1 MV/cm at gas pressure 1 atm) provides an intense electrons-acceleration in the runaway regime. It is shown that neither collisions nor a static potential profile could deliberately prevent this runaway regime. Whereas, the cathodeanode bridging impossible within tens of pisoseconds. It is obtained that the characteristic times of a simplest electrostatic-instabilities build-up are consistent with the observed runaway-electrons beam-duration at the certain plasma density, which was considered as a parameter. The agreement our estimation results and the measurements [G. A.

The critical avalanche of runaway electrons

“…The estimated CREA parameters for the air pressure p = 0.25 are l a ≈ 29 m, τ cr ≈ 4.4 μs, N cr es ≈ 6.2 • 10 17 , Δ ≈ 23 m, and Δ ⊥ ≈ 43 m; these parameters for p = 0.5 atm are l a ≈ 40 m, τ cr ≈ 6.2 μs, N cr es ≈ 1.3 • 10 18 , Δ ≈ 24 m, and Δ ⊥ ≈ 65 m. Using these estimates, we obtain I cr ≈ 6.5 • 10 5 A and B cr ≈ 30 Gs for p = 0.25 atm and I cr ≈ 1.3 • 10 6 A and B cr ≈ 40 Gs for p = 0.5 atm. The density of electrons can be estimated as n = N cr es / 4 3 πl 3 a , which is ≈10 12 1/m 3 for both cases. We see that for this case (at a fixed E), a decrease in air pressure results in an increase in avalanche current, which is due to a lower contribution of elastic electron scattering at a lower gas pressure.…”