Measuring electron densities in highly collisional plasmas using a 110 GHz interferometer

Summary form only given, as follows. A 110 GHz interferometer is used to measure electron densities in a highly collisional plasma. The plasma is formed and maintained with a helical coil driven at 13.56 MHz. In this regime, the background pressure approaches atmospheric pressure, and the collisionality is of the order /spl nu//sub e-n//spl ap/10/sub 9/-10/sup 13/. Probe measurements in this pressure range are inaccurate and unreliable due to complicated effects that electron-neutral collisions have in the sheath. It is necessary to operate an interferometer at a frequency high enough that the dominant characteristic frequency is the operating frequency (/omega>/spl nu//sub e-n/) yet low enough that small phase changes can be detected as a wave passes through the plasma. In this experiment, plasma dimensions are of the order 5-10 cm. In addition, the inelastic nature of collisions in this experiment make it necessary to interpret small phase changes with great care. A model has been developed help in that interpretation. Typical operating parameters are pressures of 1-700 Torr, applied magnetic field of 0-1000 Gauss, operating gasses of nitrogen, oxygen, argon, and air, and input RF power levels of 100-3000 Watts.