SYstem for Microwave PLasma Experiments (SYMPLE) for Investigation of Microwave Absorption in Over-Dense Plasma

Abstract

A system called SYstem for Microwave PLasma Experiments (SYMPLEs) is set up in our laboratory to undertake experimental investigations on the interaction of high power microwave (HPM) with over-dense plasma with plasma frequency $f_{pe} \gt f_{\mu }$ , the wave frequency. While the plasma system of SYMPLE has been discussed earlier, present article focuses on how the HPM and coupling parameters have been chosen in a manner that ensures meeting critical requisites of the interacting wave. The frequency (3 GHz) and power (1–3 MW) of the pulsed ( $5~\mu s$ ) microwave are decided based on wave-plasma pressure balance requirements. A scheme is adopted to excite microwave in transverse magnetic (TM) mode so as to have the wave electric field $E_{\mu } \parallel \bigtriangledown n$ for wave launched along the axial ( $\hat {z}$ ) density gradient $\bigtriangledown n$ of the plasma column. With the help of different types of step transition structures (STSs), desired TM (i.e., TM01, TM02, TM03, etc.) mode can be launched to the plasma. For a given density gradient, the exact TM mode launched determines the location of the wave return, that is, the location where the relative plasma permittivity $\epsilon _{r}(z)$ corresponds to the cut-off value $\epsilon _{r,\text {ret}}$ . Drawing analogy with laser-plasma experiments having obliquely incident p-polarized laser, an “effective angle of incidence,” is defined for the present scheme of normally incident microwave, given by $\theta _{\mu, \text {eff}} = \sin ^{-1}((\epsilon _{r,\text {ret}})^{1/2})$ . The present article gives a detailed account of the HPM-plasma experimental scheme and performance of the indigenously developed coupling components covering mode converter, STSs, and dc-break. Simulation and experimental results confirm attainment of desired TM modes and thereby desired wave field pattern, value of $E_{\mu } \approx 200~$ kV/m (at 1 MW) satisfying wave-plasma pressure balance requisite and control over $\theta _{\mu, \text {eff}}$ . Power loss along the integrated coupling system is better than 1 dB, within permissible limit. How the integrated HPM and coupling unit is suitable for systematically addressing wave absorption in plasma is demonstrated by marking points in the theoretical wave absorption curve that can be cross verified with experiments.