A THz interferometer for measurement of plasma density in high intensity discharge lamps

Abstract

Summary form only given. A THz interferometer has been constructed with the goal of directly measuring plasma electron densities in HID lamp plasmas. The system utilizes a 600 GHz source, capable of measuring plasma densities up to 4times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . Due to the short wavelength of 600 GHz radiation, the focal spot size can be less than a millimeter in diameter. These advantages will allow a THz interferometer to measure electron densities in plasmas previously unavailable to interferometric methods. In addition to the use of THz radiation for interferometry, a significant departure from typical microwave measurement will be made in the elimination of heterodyning from the system. Due to the high density and AC nature of HID lamp plasmas, the plasma is expected to recombine twice per cycle when the discharge current is equal to zero. The effect on the beam will change over time as the discharge current passes through its full AC range, allowing calculation of the density as a function of time with zero-points contained within the measurement itself. The only limitation of such a method is the bandwidth of the detector, in that the bandwidth must be higher than the frequency of the AC current in the plasma. Sensors with bandwidths of 1 kHz or greater are available and should prove sufficient for a self-heterodyning interferometer. This self-heterodyning system can eliminate the complexity associated with traditional heterodyne systems, while simultaneously providing time-dependent information on the behavior of electrons in HID plasmas, and eliminating the need for separate measurements of zero-density points for calibration.