Investigation on recovery of gap insulation strength and EUV radiation in the post-discharge stage of a laser-triggered discharge produced tin plasma EUV source

This paper presents an investigation on the recovery of gap insulation strength and extreme ultraviolet (EUV) radiation in the post-discharge stage of a laser-triggered discharge produced tin plasma (LTDPP) EUV source. Gap insulation strength recovers more quickly under a shorter gap length. The gap breakdown voltage recovered to 19.4 kV with a breakdown probability of 25% at 80 µs after an 8 J discharge under a 2.5 mm gap. The same discharge energy under a 5 mm gap required 140 µs for equal recovery. The EUV radiation recovery process was measured utilizing a double laser triggered discharge system. EUV radiation recovery lagged behind the gap insulation strength recovery for both the 2.5 and 5 mm gap. In the case of the 8 J input pulse energy, EUV radiation recovered to the same level as the first EUV pulse at about 600 and 160 µs for the 5 and 2.5 mm gaps, corresponding to a 1.67 kHz and 6.25 kHz repetition rate, respectively. In the case of the 2.5 mm gap and 4 J input pulse energy, our EUV source could work with a repetition rate of upto 23 kHz, enabling the EUV output to reach 345 W/2πsr which was 4.5 times as the value of the 5 mm gap and 8 J input energy. To discuss the slower EUV radiation process, the initial laser trigger phase and plasma pinch stage during the first and second laser-triggered discharge were visualized by means of high speed visible light imaging and laser Schlieren imaging. Comparative results suggest that tin droplets influenced the initial laser-produced plasma plume, consequently affecting the Z-pinch plasma dynamics and its EUV radiation which resulted in the delayed EUV recovery process.