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.
The results of the characterization of large-scale RF plasma for studying nonlinear interaction with a high-power (∼400 MW) short duration (∼0.8 ns) microwave (∼10 GHz) beam are presented. The plasma was generated inside a Pyrex tube 80 cm in length and 25 cm in diameter filled by either Ar or He gas at a pressure in the range 1.3-13 Pa using a 2 MHz RF generator with a matching system and a quadruple antenna. Good matching was obtained between the plasma parameters, which were determined using different methods including a movable Langmuir probe, microwave cut-off, interferometry, and optical emission spectroscopy. It was shown that, depending on the gas pressure and RF power delivered to the antenna, the plasma density and electron temperature can be controlled in the range 1×10 10 -5×10 12 cm −3 and 1-3.5 eV, respectively. The plasma density was found to be uniform in terms of axial (∼60 cm) and radial (∼10 cm) dimensions. Further, it was also shown that the application of the quadruple antenna, with resonating capacitors inserted in its arms, decreases the capacitive coupling of the antenna and the plasma as well as the RF power loss along the antenna. These features make this plasma source suitable for microwave plasma wake field experiments.
The feasibility of an experiment which is being set up in our plasma laboratory to study the effect of a wakefield formed by an ultra-short (10 À9 s) high-power ($1 GW) microwave (10 GHz) pulse propagating in a cylindrical waveguide filled with an under-dense [(2-5) Â 10 10 cm À3 ] plasma is modeled theoretically and simulated by a particle in cell code. It is shown that the radial ponderomotive force plays a circular key role in the wakefield formation by the TM mode waveguide. The model and the simulations show that powerful microwave pulses produce a wakefield at lower plasma density and electric field gradients but larger space and time scales compared to the laser produced wakefield in plasmas, thus providing a more accessible platform for the experimental study.
The results of the generation of a high-power microwave ($550 MW, 0.5 ns, $9.6 GHz) beam and feasibility of wakefield-excitation with this beam in under-dense plasma are presented. The microwave beam is generated by a backward wave oscillator (BWO) operating in the superradiance regime. The BWO is driven by a high-current electron beam ($250 keV, $1.5 kA, $5 ns) propagating through a slow-wave structure in a guiding magnetic field of 2.5 T. The microwave beam is focused at the desired location by a dielectric lens. Experimentally obtained parameters of the microwave beam at its waist are used for numerical simulations, the results of which demonstrate the formation of a bubble in the plasma that has almost 100% electron density modulation and longitudinal and transverse electric fields of several kV/cm.
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.
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