Microwave reflections from a vacuum ultraviolet laser produced plasma sheet

A pulsed, 193 nm excimer laser is utilized to photoionize the organic gas tetrakis- dimethylamino-ethylene (TMAE). The laser ionizes a plasma sheet with a width of 7.8 cm and an adjustable thickness of 0.7-1.4 cm. The axial scale length of the plasma density is a function of TMAE neutral pressure and is typically 50 cm. X-band (10 GHz) microwaves are incident on the plasma with the electric field polarized parallel to the laser beam axis. The power reflection coefficient and the phase of the reflected signal are studied as a function of time. A monostatic homodyne detection system with a response time of 10 ns is utilized to determine the amplitude and phase of the reflected wave. The peak plasma density is n_e≈4×10¹³cm^-3, sufficiently above the critical density (n_crit=1.2×10¹²cm^-3) to produce reflections comparable to a conducting sheet placed in the same position as the plasma. A computer model is developed to interpret and optimize the plasma conditions which provide the highest backscatter and phase-stable reflection coefficient for the longest period of time. The presence of axial density gradients causes the reflected wave to be scattered through a wide angle. As the gradients relax, the backscatter reflection coefficient increases to a value of nearly 100%. The plasma density and two-body recombination coefficient are measured by means of microwave backscatter plasma reflectivity and Langmuir probes.