A.C. Plasma Anemometer for Hypersonic Mach Number Experiments

A working prototype of a miniature 2 MHz a.c. driven, weakly-ionized plasma anemometer for measurements at hypersonic Mach numbers has been developed. This device uses a plasma discharge between two encapsulated electrodes as the primary sensing element. The discharge is driven by a low power (less than 5 Watt) a.c. source at voltages on the order of 350 Vr.m.s.at atmospheric pressure with electrode gaps of 0.076 mm (0.003 in.). The discharge has demonstrated sensitivity to mean and dynamic mass-flux variations at Mach numbers up to 5.0 and at frequencies of 200 kHz. In principle, a frequency response in excess of 1 MHz is possible. The advantages of the plasma anemometer are that it requires no frequency compensation up to its a.c. carrier frequency, has an amplitude-modulated output that has excellent common-mode rejection with a signal-to-noise ratio that is much better than a hot-wire, is robust with no sensor element to break, can have a small spatial volume, and is insensitive to temperature variations making calibration easier than thermal-based sensors. This sensor has applications for measurements in gas-turbine machinery, shock tubes, shock-boundary layer experiments, high-enthalpy hypersonic flows, and in plasma tunnels.