10 kHz repetition rate operation of an ArF excimer laser for microlithography is studied by focusing on the spatial separation between the discharge and the gas density depletion that is created by the previous discharge and that moves out of the discharge area due to the enforced laser gas circulation. The widths of discharge and the gas-density depletion were measured in detail by Mach-Zehnder interferometry. As a result, the minimum required distance between the center of the discharge and the center of the gas-density depletion area was found to be 1.4 times larger than the width of the discharge and is 12 % larger than the center-to-center distance at which the discharge area is just detached from the gas-density depletion area. This minimum required distance is 4.9 mm and 2 times as large as the distance estimated only from the absolute value of the gas density depletion.
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