ECR Plasma in a High Power Millimeter-Wave Beam (Report II) : Time Variation of Electron Density(Welding Physics, Process & Instrument)

概要

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Using a quasi-steady high power 60 GHz Gyrotron a high density and high temperature Ar plasma is produced in a simple mirror field. Time variation of the electron density ne is studied in detail for various external parameters. The filling gas pressure before discharge is set around 1×10^<-1> Pa. Complicated variation of the line density with time is observed. Strong punping effect of the neutral gas by the plasma causes a rapid reduction of the gas pressure and a decrease of n_e after the fast build-up process of the plasma during the high power millimeter wave input. The line density reaches to a nearly constant value at t=20-30ms and an almost fully ionized plasma is sustained with n_e 4-5×10^<12>cm^<-3>. The efficient electron heating is also observed in this stage accompanying a rapid hot electron production. In the afterglow phase hot electrons continue to ionize the background neutral gas showing a long decay time of 30-50ms. The importance of a high power input to the plasma is verified to obtain a high density and high temperature state of the plasma.
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