Plasma Measurement by Feedback-Stabilized Dual Beam Laser Interferometer

摘要

The plasma density in a dynamic magneto arcjet was measured by a stabilized dual beam laser interferometer. The fringe shift for a 0.63 micrometer beam of He-Ne laser is used to stabilize the interferometer against the effect of mechanical vibration by means of a feedback controlled speaker coil, while the other beam of 3.39 micrometer for which the effect of mechanical vibrations is excluded, is used to measure plasma density. Stability of approximately 1/500 of one fringe for 0.63 micrometer is obtained during a long period for frequencies lower than a few Hartz. Stability for higher frequencies is limited to approximately 1/30 of one fringe for 0.63 micrometer which corresponds to approximately 1/200 of one fringe and a line electron density of approximately 1.5 x 10 to the 14th power/sq cm by acoustic noise picked up by the speaker coil. The response of the interferometer extends down to zero frequency. Since the effect of the neutral gas background is practically reduced by zero, the inferometer can be used to diagnose the plasma produced in high pressure gases.