Ionization-Induced Self-Channeling of an Ultrahigh-Power Subnanosecond Microwave Beam in a Neutral Gas

Abstract

Ionization-induced self-channeling of a ≤500 MW, 9.6 GHz, <1 ns microwave beam injected into air at ∼4.5×10^{3} Pa or He at ∼10^{3} Pa is experimentally demonstrated for the first time. The plasma, generated by the impact ionization of the gas driven by the microwave beam, has a radial density distribution reducing towards the beam axis, where the microwave field is highest, because the ionization rate is a decreasing function of the microwave amplitude. This forms a plasma channel which prevents the divergence of the microwave beam. The experimental data obtained using various diagnostic methods are in good agreement with the results of analytical calculations, as well as particle in cell Monte Carlo collisional modeling.

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

“…It also shows that a threshold field amplitude exists for this process to occur, E th (V/cm) ≈ (4 − 5) × 10 5 /λ, where for λ =3 cm, the wavelength associated to the 9.6 GHz microwave beam E th = 150 kV/cm. The 1D Particle in Cell (PIC) simulations [43] confirm that high energy electrons are indeed produced during the process (Figure 7). Plasma 2019, 2 FOR PEER REVIEW 7 beam (Figure 6c).…”

“…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 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. As the He pressure increases up to ~10 kPa, the beam power transmitted downstream decreases and then it increases as the pressure increases above 10 kPa (Figure 5a).…”

“…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.…”