Time-resolved observation of laser-assisted discharge plasmas for EUV sources

In this paper a birdcage discharge head with a plane tin cathode and a stainless steel ball anode was fabricated which enables us to access to the plasma easily. A pulsed laser light with a fluence of 10¹⁰ W/cm² was irradiated at a tin cathode surface to deliver tin vapor to the 5 mm electrode gap, where the high voltage was applied. High density hot plasmas were produced by the electromagnetic compression and the ohmic heating owing to the pulsed high current (20 kA, 150 ns) after the gaseous breakdown. The plasma compression process depends on the delay time dt from the laser irradiation to the breakdown because the laser produced vapor expands quickly, resulting in the change of the gas distribution. The EUV emission intensity was maximum when dt was 300 ns, while the emission region was minimum. The breakdown did not occur for dt smaller than 280 ns because the gas density might not be sufficiently large for the breakdown. The time-resolved imaging of the EUV emission using a gated pinhole EUV camera showed that the hot plasma was produced at first near the laser spot at the cathode and migrated toward the anode quickly. The migration of the hot plasma results in the enlargement of EUV emission region, which is unfavorable for a light source. The observation implies two mechanisms for the migration; one is the pressure wave propagation, and, the other is the collisional ionization of tin ions with electrons accelerated by the induced electric field, which is on the order of 1 MV/cm.