Microwave reflections from a vacuum ultraviolet laser produced plasma sheet

A pulsed, 193 nm excimer laser is utilized to photoionize the organic gas t etrakis-dimethylamino-ethylene (TMAE). The laser ionizes a plasma sheet wit h a width of 7.8 cm and an adjustable thickness of 0.7-1.4 cm. The axial sc ale 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 powe r reflection coefficient and the phase of the reflected signal are studied as a function of time. A monostatic homodyne detection system with a respon se time of 10 ns is utilized to determine the amplitude and phase of the re flected wave. The peak plasma density is n(e) approximate to X 10(13) cm(-3 ), sufficiently above the critical density (n(crit) = 1.2 X 10(12) cm(-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 opti mize 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 throug h a wide angle. As the gradients relax, the backscatter reflection coeffici ent increases to a value of nearly 100%. The plasma density and two-body re combination coefficient are measured by means of microwave backscatter plas ma reflectivity and Langmuir probes. (C) 1999 American Institute of Physics . [S0021-8979(99)08501-1].