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Temperature measurements at an implosion focus

Spectroscopic temperature measurements were made at the focal point of imploding shock waves in the UTIAS implosion chamber, which has a 20 cm diameter hemispherical cavity. The chamber was filled with a stoichiometric H2-O2 gas mixture at different initial pressures (1.4-6.9 MPa). The mixture was ignited at the origin by an exploding wire generating an outgoing detonation wave, which reflected at the chamber wall as an imploding shock wave (gas runs). Experiments with an explosive shell of pentaerythritol tetranitrate (PETN: C5H8N4O12) placed at the hemispherical wall were also conducted. The shell was detonated by the impact of the reflected gaseous detonation wave at its surface, an intense implosion wave being generated thereby (explosive run). The temperatures were measured at the implosion focus by using a medium quartz Hilger spectrograph with an eight-photocell polychromator attachment over the visible wavelength range. The measured radiation intensity distributions were fitted to black-body curves. The temperatures were 10000-13000 K, for gas runs, and 15000-17000 K, for explosive runs. The continuous spectra from photographic film and the measured emissivities, which were very close to unity, confirmed that the plasma was a black body. Numerical studies with the random choice method and classical strong-shock theory were used to analyse the flows in the entire range of the implosion process. They provided much information on the entire implosion process within the restriction of a perfect-gas assumption, which was found to be reasonable as a first step in this kind of analysis. The experimental data were compared with the analytical results. For both gas and explosive runs, the temperatures were lower than the calculated values and reasonable explanations are given for this deviation.