STUDIES TO REDUCE MATERIAL EROSION IN ELECTROTHERMAL LAUNCHERS

During the exposure of launcher components to high heat fluxes a vapor shield (plasma boundary layer) is formed which absorbs a fraction of the incoming energy, and thus naturally reduces the surface erosion. Computer simulation has shown that a strong externally applied magnetic field parallel to the surface may reduce the surface erosion, since the energy transport through the vapor shield will be reduced due to decreased turbulence. The experimental electrothermal launcher device "SIRENS" has been operated to measure the erosion of material surfaces subjected to high heat flux from a high density low temperature plasma (1-3 eV) with a strong applied magnetic field. The plasma is produced by the ablation of the insulator (Lexan), with currents up to 100 kA over a pulse length of 100 mu-s, and flow through a cylindrical barrel which serves as the material sample. Ablation and erosion for both the insulator and sample surfaces are caused by convection and radiation emitted from the plasma. The ablated thickness of the Lexan insulator compares favorably with predicted values. The key parameter is "f", the fraction of total incident energy that is transmitted to the eroding surface which is flux and material dependent but in the range 5-20%. Different materials are tested and analyzed, showing strong axial erosion dependence for metallic surfaces but uniform ablation for insulators. Surface conditioning through multiple exposures showed a decrease in the incremental ablation depth for a Lexan insulator (inside the source and as a test sample), and an increasing erosion rate for metallic surfaces. An externally applied magnetic field parallel to the surface indicates a threshold for the onset of the magnetic vapor shielding effect. Above the threshold the ablation depth decreases. The optical spectra received end-on through a fiber optical cable connected to an optical multichannel analyzer showed a decrease in the relative intensity with an increase in the applied magnetic field, which may be indicative of the reduction of the ablated material.