Electron Density and Conductivity Measurements of MHD Plasma by an FIR Laser

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To estimate the performance of an MHD generator, it is necessary to know the behavior of the plasma parameters. The electron density and conductivity are the most important parameters governing the generator characteristics. These have been measured by electrostatic probes and/or rf probes. The insertion of probes into plasma, however, inevitably disturb the flow field inside an MHD channel, and the obtained values are thought to have large errors.In the present research, the electron density and conductivity are measured with the HCN far-infrared(FIR)laser. The target plasma is generated by the shock tunnel. The driver and driven sections of the shock tunnel are filled with helium and argon, respectively. The shock Mach number is varied by changing the gas pressure, and the plasma generated in the stagnation section upstream of the channel is used as a target. The Michelson interferometer is used to measure the fringe shift due to the plasma, from that the electron density is calculated. Also, the transmissivity of the FIR laser beam through the plasma is measured, and the conductivity is determined from the measured electron density and transmissivity. Major improvement of this FIR diagnostic technique is achieved by applying a Schottky barrier diode for the signal detection. This diode has very fast response time, an order of a few nano seconds, and with this capability, it gas become possible to clarify the temporal variation of the plasma with high accuracy.