Some Remarks on Electric-Discharge Shock-Tube Diagnostics

An investigation was made to determine whether there is a basic version of discharge tube which can serve to produce a quasi-uniform block of plasma. Operating conditions are described for a diaphragm type device with a Mach-25 shock proceeding into room-temperature argon at 1-mm pressure. The basic difference between the diaphragm type tube and the discharge type tube is that the latter makes use of a current flow as part of the energy addition process, which permits magnetic forces to aid in the acceleration of the gas. No simple conclusions can be drawn from previous results, but the results may be related to a basic requirement for a general-purpose electricdischarge shock tube, namely, the uniform-flow regime behind the advancing shock. Possible mechanisms in the rather constant wave and flow speeds are: energy-transfer time, vaporization of material in the electrode region, and electrode geometry. The uniformity of the plasma behind the shock is limited in any strict sense but may be adequate for the order of microseconds for aerodynamic and other applications. The use of a maintained piston-like drive all along the tube may be superior to controlled energy feed-in. (B.O.G.)