In order to investigate the plasma expansion behaviors and the electrical recovery process after the maximum implosion in our tin fueled laser-assisted discharge plasma (LDP) 13.5 nm EUV source, we developed and evaluated a cost-efficient spectroscopic method to determine the electron temperature T e and density n e simultaneously, by using Stark broadenings of two Sn II isolated lines (5s 2 4f 2 F 5/2-5s 2 5d 2 D 3/2 558.9 nm and 5s 2 6d 2 D 5/2-5s 2 6p 2 P 3/2 556.2 nm) spontaneously emitted from the plasma. The spatial-resolved evolutions of T e and n e of the expansion plasma over 50 to 900 ns after the maximum implosion were obtained using this modified Stark broadening method. According to the different n e decay characteristics along the Z-pinch axis, the expansion velocity of the electrons was estimated as $1.2 Â 10 4 ms À 1 from the plasma shell between the electrodes towards the cathode and the anode. The decay time constant of n e was measured as 183 6 24 ns. Based on the theories of plasma adiabatic expansion and electron-impact ionization, the minimum time-span that electrical recovery between the electrodes needs in order to guarantee the next succeeding regular EUV-emitting discharge was estimated to be 70.5 ls. Therefore, the maximum repetition rate of our LDP EUV source is $14 kHz, which enables the output to reach 125 W/(2psr). V
Hydrogen pellets are injected into the current free neutral beam injection (NBI) heated plasma in Heliotron E to effectively increase the plasma density and the internal energy. The chord averaged density of the target plasma is about 2 × 1019 m−3, and the density increase by pellet injection is (4 – 6) × 10l9 m−3. During this process the plasma remains stable. Under the present operational conditions, the pellet reaches the central axis: the penetration depth ranges from 27 cm to 35 cm (the magnetic axis corresponds to 30 cm). To optimize the plasma parameters obtainable by pellet injection, the NBI heating power is modulated. It is found that the mode of operation in which pellets are injected at relatively low NBI power followed by a higher-NBI-power phase is preferable to that in which pellets are injected during a constant high power phase. The main reason for this difference is the penetration depth of the pellet in the plasma which determines the deposition profile of particles in the plasma.
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