The interaction of intense, ultra-short microwave beams with the plasma generated by gas ionization

Abstract

Results of the non-linear interaction of an extremely short (0.6 ns) high power (∼500 MW) X-band focused microwave beam with the plasma generated by gas ionization are presented. Within certain gas pressure ranges, specific to the gas type, the plasma density is considerably lower around the microwave beam axis than at its periphery, thus forming guiding channel through which the beam self-focuses. Outside these pressure ranges, either diffuse or streamer-like plasma is observed. We also observe high energy electrons (∼15 keV), accelerated by the very high-power microwaves. A simplified analytical model of this complicated dynamical system and particle-in-cell numerical simulations confirm the experimental results.

Experiments Designed to Study the Non-Linear Transition of High-Power Microwaves through Plasmas and Gases

“…A matrix of Neon lamps placed on plane perpendicular to the microwave beam axis inside or outside the chamber, is also used to produce a two-dimensional spatial integrated distribution of the electromagnetic beam power. All our experiments so far have been performed in this chamber [35,[42][43][44].…”

“…In the left panel of Figure 5 a side view of the integrated light emission from the interaction chamber during and after the transition of a pulse of the These irregularities in transmitted microwave power raised the suspicion that plasma is formed during the HPM neutral gas interaction which was indeed validated in the images of the light emission which appeared in the chamber. In the left panel of Figure 5 a side view of the integrated light emission from the interaction chamber during and after the transition of a pulse of the microwave beam for three values of He gas pressure are presented [42]. In the right panel of Figure 5 a smaller region near the dielectric lens is depicted by a fast framing camera [43] for the same He pressure as in frame (b) on the left panel.…”

“…Plasma 2019, 2 FOR PEER REVIEW 6 microwave beam for three values of He gas pressure are presented [42]. In the right panel of Figure 5 a smaller region near the dielectric lens is depicted by a fast framing camera [43] for the same He pressure as in frame (b) on the left panel.…”

“…We also observed that the trace of the beam on a planar matrix of Ne lamps placed~10 cm from the focal plane of the beam is hollow for certain gas pressures ( Figure 6 left panel, frames c and d) or of a smaller diameter ( Figure 6 left panel, frames b and e). microwave beam for three values of He gas pressure are presented [42]. In the right panel of Figure 5 a smaller region near the dielectric lens is depicted by a fast framing camera [43] for the same He pressure as in frame (b) on the left panel.…”

“…We think that the mechanism responsible for this behavior is that the MW beam accelerates background electrons to high energies which ionize the gas to form a plasma channel through which it propagates as predicted by Bogomolov et al, 1987 [34]. We developed a simple 1D analytical model [42,43] which explains this process of self-channeling and is the result of the non-monotonic behavior of the electron impact ionization cross-section which for He has a maximum at~150 eV and decreases by more than an order of magnitude for electron energies above 10 keV [54]. If we assume a background electron density n 0 ≤ 10 5 cm −3 , then for a microwave beam of Gaussian distribution around r, we obtain an ionization electron density by ln[n e (r, t)/n 0 ] = n g t 0 v(r, t )σ i [w(r, t )]dt , where n g is the gas number density, v the electron velocity and w the electron energy.…”