A plasma source for high power microwave interaction studies

Summary form only given. It is well established that an unmagnetized plasma does not support propagation of low frequency ((ω≪ω_p) electromagnetic (e.m) waves with amplitude such that eE_e.m/mω ≪ c. However, the high amplitude waves, with their relativistic parameter ν = eE_em/mωc ~ 1 have been shown theoretically to penetrate as well as get absorbed inside a plasma. Experimental investigation of issues related to the transfer of energy from the wave to plasma and subsequent generation and propagation of fast electrons in the plasma have implications in fast ignitor fusion research. In this regard, the major challenges in carrying out experiments with lasers are related to the diagnostic access. The plasma that should remain over-dense to the laser frequencies is nearly solid-dense (n_e 10²⁷-10²⁸/m³), leading to fast time scales (femto seconds) and small scale lengths (~ a few μm) of instabilities, difficult to diagonise. The recent advent of high power (~ a few GW) microwave (HPM) sources opens up possibility of performing experiments on the said issues. For plasma-HPM interaction studies, resolution of appropriate length and time scales are not as stringent as in the case of lasers.Experimental studies are taken up by us on HPM-plasma interaction, using a pulsed (~50 ns) HPM source (~ 2-5 GHz) designed for 1-3GW. Desirable plasma parameters are estimated for Brunel absorption of the wave, generation and propagation of fast electrons in the plasma resulting in beam instabilities. The estimations show that the plasma (nB ~ (110)x10¹⁸/m³) should have a steep axial ne gradient (Lη ~ λHPM) at the wave interaction regime, followed by a uniform axial extent of ~ lm and a radial extent ~ 10 cm. We have developed a plasma system for HPM-plasma interaction studies. The requirement of the sharp gradien- at the regime of wave interaction has necessitated the choice of a washer-gun based, pulsed, moving plasma. Here, Lη / plasma velocity ≫ t_HPM so that the plasma front appears static to the incoming wave. Density ~ 1x10¹⁸/m³ is attained with ~ 20 MW input to the gun using a pulse forming network. By optimizing parameters like gas throughput, magnetic field and pressure, a parametric regime is identified in the post pulse regime where the required axial uniformity and the gradient is attained. A multi-gun configuration is adopted to attain the radial density extent. The present paper discusses the development and parameter optimization of the plasma source for the HPM-plasma interaction experiments.