Search for Spectral Gaps in HEG

Abstract

Spontaneous emission from the shock heated gas (luminosity) in the high enthalpy shock tunnel at condition I (nominally 50 MPa burst pressure with specific enthalpy 21 MJ/kg) has been examined spectrally in order to search for spectral windows where LIF measurements could be corried out. Comparison with NO fluorescence spectra showed, that the fluorescence radiation following D(0)<--X(1)excitation only partially overlaps the lower wavelength regions of the intensity distribution of the luminosity. Two strategies to reduce the influence of this spontaneous emission have been developed: (1) for 1d-spatial resolution, spectral filtering using a spectrometer is appropriate: (2) for planar LIF-imaging narrowband reflection filters had to be developed. In spite of these filtering methods the (P)LIF method has still constraints imposed by the powerful radiation behind strong shocks.