The Contact Surface in an Electromagnetic Shock Tube

Abstract

An investigation of the luminosity structure, electron density, temperature, and arrival time of electrode material in the shock heated region of the plasma produced in a T-tube has disclosed that for the conditions studied the luminosity from this region originates primarily from gases that have passed through the contact surface. A plausible explanation for this mixing of discharge gases with shock heated gas is discussed. The proposed inertial instability of the contact front is an alternative explanation rather than radiation absorption in the gas ahead of the luminous plasma for the high values of temperature and anomalous values of electron density that have been measured in electromagnetic shock tubes by numerous workers.For high MACH numbers and an initial pressure of the order of 1 mm Hg CLOUPEAU 1 has shown that most of the luminosity in an electromagnetic Hshaped shock tube originates from the plasma produced by the driving current and not from gases that are shock heated. This conclusion is in agreement with the work of JEANMAIRE, KLINGENBERG and REI-CHENBACH 2 and of BARNARD and CORMACK 3 . CHANG 4 has shown that the distance required for dissociative equilibrium to be reached is long, typically 0.4 to 4 cm for a M25 shock passing into un-preexcited hydrogen at an initial pressure of 0.2 mm Hg.MCLEAN, FANEUFF, KOLB and GRIEM 5 have explained their much shorter observed times to reach equilibrium as being due to the absorption of radiation in the gas in front of the shock. BREDERLOW 6 has determined the position of the contact front and has observed luminosity emanating from the shockheated region in hydrogen in a T-tube. The object of the present work is to extend BREDERLOW'S investigations by making a spectroscopic study of the plasma produced under transition conditions when the gas in the shock heated region is becoming luminous. Spectroscopic methods described by GRIEM 7 and the method of determination of the source of the plasma, based on the arrival time of certain impuri-1 M.

Flow modes in shock tubes with electromagnetic driving sections

“…The mixing of driver and driven gases has since been amply confirmed in several cases (Cormack 1964, Barach and Mayes 1968, Thornton and Cambel 1964, but the cause of the instability is still obscure although several suggestions have been put forward. The front velocity in an electromagnetically driven shock tube attenuates very rapidly owing to the small mass of the ejected plasma from the discharge.…”

“…The density jump across the shock front itself (but not across the contact surface) is in the correct direction to support Rayleigh-Taylor instability; however, even if the disturbances are propagated through the flowing gas to the shock, it has been shown (Freeman 1955) that the shock front is in fact inherently stable to small disturbances. Cormack (1964) suggested that heavy ions from the driver discharge may cause an instability as they will require several mean free paths to decelerate owing to their large mass. However, allowing for a mass ratio of 100 between the relatively few heavy impurity ions and the filling gas atoms, the penetration of the heavy ions across the contact surface should be only about 100 mean free paths, or about 10-4 cm under typical conditions, which is much less than the estimated shock thickness and is of the same order of magnitude as the disturbance of a classical contact surface due to diffusion and thermal conduction (Hall 1954).…”

“…A characteristic feature of published streak and framing photographs of many flows in which the system fails to establish true shock flow is the high degree of turbulence associated with the flow (McLean et al 1960, Cormack 1964. Photographs of the stable shock system however, do not show this highly developed turbulence; a good example of this transition can be seen in the Mach-Zehnder interferograms of Brinkshulte and Muntenbruch (1965).…”