Establishment of quasi-steady operation in a pulsed MPD arcjet

Abstract

XPERIMENTAL studies were made on the properties of the plasma stream emanating from a pulsed MPD arcjet, expecially with respect to the quasi-steady operation.14 From diagnostic measurements, a large fraction of the discharge current was found to extend in the plasma downstream of the electrodes. It has been questioned whether this extending current is desirable in the operation of an MPD arcjet. The purpose of the present work is to answer this question with respect to the establishment of the quasi-steady operation. The quasi-steadiness of the operation should be inferred from the properties of the plasma stream as well as the current and voltage of the arc discharge. Contents The pulsed MPD arcjet used in the present experiment is similar to that described in Ref. 1. The maximum discharge current is 12.5 ka and the maximum power is about 2 Mw. The discharge current and voltage are steady and last for more than 800 jusec. An annular coil is mounted concentric to the arcjet system with its center plane at the front surface of the anode. The maximum strength of the magnetic field is 20 kgauss at the center of the coil. The working gases are hydrogen, helium and argon. The mass-flow rate m is variable—for instance, from 50 mg/sec to 300 mg/sec for hydrogen. The MPD arcjet is installed on an end flange of a vacuum tank which is 1.5 m in diameter and 2.8 m in length. The metal surface inside the tank is completely covered with Mylar sheet in order to prevent the discharge current from flowing in the tank wall. The tank pressure before each run is about 10 ~5 torr. The working gas injection phenomena was studied to determine the trigger time of the arc discharge so that the amount of the neutral gas injected before triggering the arc discharge could be minimized, and also to find how a quasi-steady injection of the working gas is established in the discharge region. To this end, the pressure was measured in the jet of neutral gas by a newly developed fast ionization gauge.5 From the gauge signal, the gas pressure was found to rise in about 600 jusec after opening the gas ports and reach a constant value. A steady state lasts for 3 ~ 5 msec. Figure 1 demonstrates the time variation of the pressure distribution along the center line in the jet of neutral argon. The origin of time in this figure is taken arbitrarily and the normal trigger time is 1 msec in this case. As the time increases, the experimental curve tends to follow a Z~2 law. The coordinate Z is taken along the center line and measured from the anode surface. The pressure dependence on Z~2 law is predicted from the spherically expanding steady flow at frozen