Temperature and Density Measurements of the Collision of Two Plasmas

An important example of interstellar shock-cloud interactions occurs when the shocks are generated by the collision of two interstellar clouds. It has been suggested that the cloud-cloud interaction may result in new star formation with some recent examples found in W49N leading to enhanced O star formation. In an attempt to study aspects of the collision in the laboratory, we have initiated a series of experiments designed to investigate the hydrodynamic motion of high-temperature plasmas produced by volumetric heating of thin aluminum foils. The foils were heated by gold M-band X-rays (2-4 keV) produced by laser radiation from the Nova laser impinging on 1500 Å thick Au foils. In this way we were able to avoid nonuniformities which would exist with lower energy heating radiation. We studied single and double foils, and were able to measure the temperature and density of the plasma with accuracies sufficient for comparisons with radiation hydrodynamic simulations. The densities of the aluminum plasmas produced were inferred by measuring the transmission of the plasma to radiation produced by a tantalum backlighter. Time and spatial information were obtained using a gated X-ray framing camera. For the temperature measurements, we used a samarium backlight and X-ray absorption spectra were taken using a crystal spectrometer and photographic film. The backlight produced a 200 ps wide spectral snapshot. The temperature of each plasma was obtained by determining the ion balance in the plasma from the absorption spectrum of the aluminum 1-2 lines through the plasma. The temperature was extracted from the best fit in a series of transmission calculations at the measured density. The densities and temperatures the plasmas produced in these experiments were determined to be in the range 0.005-0.05 g cm^-3 and 20-40 eV. The temperature and density measurements were sufficiently accurate to make detailed comparisons to radiation hydrodynamic simulations and demonstrated some limitations of the simulations.