Experimental and analytical investigations of low pressure wall-stabilized segmented arc heaters

Development of thermal protection systems for new reusable launch vehicles and other reentry capsules demands further development of material test facilities. Several types of aerospace plasma heating devices are used for material tests including wall-stabilized multi-segment arc heaters, Huels or vortex-stabilized heaters, magnetically-stabilized heaters, induction-coupled devices, and magnetoplasmadynamic devices. A survey of facilities that utilize these devices is presented. Trends in the modernization and further development of these facilities is discussed. Industrial or commercial heaters and applications are also briefly discussed with emphasis given to environmental remediation techniques using plasma systems. An explanation of required enthalpy for aerospace heaters based on sample surface temperatures, emissivity and catalycity is presented. Pressure and mass flowrate requirements are developed based on the sonic flow relationship. With enthalpy and pressure requirements determined, governing equations, boundary conditions and relationships that are used in developing arc heaters are explained. Relevant plasma system thermo-physical properties are discussed in this section as well. A summary of arc heater numerical codes is presented. The primary focus of this work has been experimental and analytical investigations of low pressure wall-stabilized multi-segment arc heaters. Research topics include electrode erosion causes and effects, internal arc heater heat flux distribution and effects of gas vorticity and distribution on heater performance. Experimental work was performed primarily at the Deutsche Forschungsanstalt fur Luft-und Raumfahrt (DLR) L3K arc facility near Cologne. Germany. Experimental results demonstrate that a plasma arc can be divided between cathodes at arc chamber pressures up to 17 bar. Previously, arc division between electrodes had been limited to 10 bar. Sensitivities of stable current division between electrodes to changes in arc current, ballast resistor settings, and gas distribution are observed and reported. In addition the results show that electrode erosion is strongly affected by backside cooling water velocities and distribution. Electrode copper loss is shown to be a linear function of the incident current squared and time. Experimental results are presented for off-nominal water flows to individual segments. These results show that under some off-nominal water flow conditions, cusped segments can overheat at heat flux levels less than 20 MW/m^{2>/sup>. Other experimental data have shown that cusped segments can withstand much higher heat flux when water flowrates are nominal. A possible failure process is postulated. Experimental results show the value of voltmeters between segments and photocells as diagnostic aids during heater system development. Diagnostic techniques and operating guidelines were developed to limit arc heater damage during anomalous operations.}