We have employed a soft x-ray Mach-Zehnder interferometer, using a Ne-like Y x-ray laser at 155 8, as the probe source, to study large-scale-length, high-density colliding plasmas and exploding foils. The measured density profile of counter-streaming high-density colliding plasmas falls in between the calculated profiles using collisionless and fluid approximations with the radiation hydrodynamic code LASNEX. We have also performed simultaneous measured the local gain and electron density of Y

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... -ray laser amplifier. Measured gains in the amplifier were found to be between 10 and 20 cm-l, similar to predictions and indicating that refraction is the major cause of signal loss in long line focus lasers. Images showed that high gain was produced in spots with dimensions of -10 pm, which we believe is caused by intensity variations in the optical drive laser. Measured density variations were smooth on the 10-pm scale so that temperature variations were likely the cause of the localized gain regions. We are now using the interferometry technique as a mechanism to validate and benchmark our numerical codes used for the design and analysis of high-energy-density physics experiments.