A new interferometer to measure preheat in laser targets

Many interferometer designs exist to measure the distance change through a fringe displacement. Different interferometers were used to measure the mov ement of the backside of a heated foil in laser plasma interactions. Howeve r, when a laser heated target sits in the middle of a target chamber with n oise from pumps and other sources of vibrations, the fringes become difficu lt to observe and align. We have built a differential interferometer where the light paths in each arm of the interferometer retrace each other, hence making the fringes much less sensitive to vibrations than a regular interf erometer. This allows the main part of the interferometer to sit outside th e vacuum chamber, where it is easy to align. The final focusing element res ides close to the target and collects a significant amount of the scattered light. In this investigation we will use the interferometer to measure the expansion of the backside of a preheated target in the TRIDENT chamber. Us ing the coefficient of linear expansion, the heat capacity of the material, and the expansion distance, we estimate a time dependent preheat temperatu re of the target. [S0034-6748(99)74801-X].