Laser heating of noble gas droplet sprays: EUV source efficiency considerations

Transcription

1 Laser heating of noble gas droplet sprays: EUV source efficiency considerations S.J. McNaught, J. Fan, E. Parra and H.M. Milchberg Institute for Physical Science and Technology University of Maryland College Park, MD

2 Abstract Our recent experimental studies of intense laser pulse interactions with noble gas droplets have shown that for fixed pulse energy, pulse duration is an important parameter for EUV source efficiency [1]. In general, one must first identify the desired emission feature. This feature will usually be associated with a particular ion state that should be present in high relative concentration for the full duration of the laser pulse. We have found that if this spectral feature is associated with an excitation process, the optimal laser pulse duration is set mainly by droplet size. In that case, it is desirable that the laser heating occur when the droplet is at or above the critical electron density. If the spectral feature is associated with recombination, this condition is relaxed, and the most EUV efficient laser pulse duration is generally longer. For that case, it is determined by achieving at least several recombination cycles. [1] S.J. McNaught, J. Fan, E. Parra, and H.M. Milchberg, Appl. Phys. Lett. 79, 4100 (2001)

3 Experimental Set-up Sample EUV spectra Nozzle Signal (AU) Filter transmission Wavelength (nm) Signal (AU) Wavelength (nm) To EUV grazing incidence monochromator (λ = 2-44 nm) Amplification circuit Laser Nd:YAG (λ = 1064 nm) t = Hz Cluster, Droplet Beam X-ray diode Silicon photodiode with 150 nm Al coating

4 Pump Probe Experimental Setup LASER Nd:YAG (1064 nm) Pulsewidth : 100 ps Energy: Hz Peak intensity: W/cm 2 Probe delay line (τ delay = 0 14 ns) X-ray detector (>1.5 kev) Side scatter CCD Focal spot CCD Nd:YAG Laser Probe 200 mj TFP Pump 200 mj TFP f = 150 mm 15 cm Droplet jet Paraboloidal condenser τ delay Grazing incidence spectrometer (λ = 2-44 nm)

5 Sample EUV Spectra Kr 9+ Signal (AU) Kr 7+ Kr 6+ Kr 5+ KRYPTON Wavelength (nm) Ar 7+ Ar 7+ Ar 6+ Ar 9+ Ar 5+ Signal (AU) Ar 8+ Ar 6+ Ar 4+ Ar 4+ ARGON Wavelength (nm)

6 Droplet Size Distributions Argon droplets (600 psi, 138 K) Droplet size histogram d mean = 4.8 µm σ = 0.9 µm Small argon droplets Droplet diameter (µm) 3 mm Krypton droplets (200 psi, 143 K) 300 µm Droplet size histogram d mean = 10.2 µm σ = 2.1 µm Large krypton droplets Droplet diameter (µm)

7 Argon Droplet Plasma Emission Droplet plasma EUV emission (Argon droplets, d mean = 4.8 µm) EUV emission at 26.0 nm 2p 6 4f 2p 6 3d in Ar 7+ ions Recombination from Ar E i (Ar 7+ ) = 143 ev 100 Ar 8+ emission lines observed Signal (a.u.) τ decay Ar 9+ EUV (pump) Ar 9+ EUV (probe) Ar 7+ EUV (probe) Ar 9+ decay time = 190±60 ps Ar 7+ decay time = 2.1±0.3 ns EUV emission at 16.6 nm 2s2p 6 2s 2 p 5 in Ar 9+ ions 20 Electron collisional excitation E i (Ar 9+ ) = 479 ev Delay (ns) Ar 10+ lines not observed

8 Krypton Droplet Plasma Emission EUV emission from probe (Krypton droplets, λ = 10.0 nm) Signal (a.u.) µm droplets 7-µm droplets Decay time = 2.5±0.4 ns Decay time = 1.7±0.2 ns Delay (ns) EUV emission at 10.0 nm 3p 6 3d 8 4p 3p 6 3d 9 in Kr 9+ ions Recombination from Kr 10+ E i (Kr 9+ ) = 275 ev Poor indicator of laser coupling

9 Droplet Plasma X-Ray Emission X-ray emission from probe (Argon, E > 1.5 kev, d mean = 4.8 µm) X-ray emission from probe (Krypton, E > 1.5 kev) µm droplets 7-µm droplets Signal (a.u.) Decay time = 160±30 ps Signal (a.u.) Decay time = 295±50 ps Decay time = 210±20 ps Delay (ns) Delay (ns)

10 Green Probe Scattering Images Argon droplets (4.8 µm diameter) 0 ns 125 ps Green scattering signal (integrated in center channel) ps 500 ps 80 Signal (a.u.) Decay time = 265±60 ps 3 ns 10 ns Delay (ns)

11 Disassembly Time Scales Droplet disassembly time Laser-droplet plasma disassembly τ 1/ 3 d N 0 2 = e crit cs Ncrit Initial electron density: N e cm -3 Plasma critical density: N crit cm -3 Plasma sound speed: c s 10 7 cm/s Disassembly time (ps) Ar Ar Kr Kr t = τ crit (c s = 0.9 x 10 7 cm/s) 16.6-nm EUV (Ar 9+ ) x-rays (>1.5 kev) Droplet diameter (µm)

12 The Laser Droplet Interaction t = 0 time N e = N e0 N e = N crit N e < N crit N e << N crit Case #1: Short probe delay Pump Probe X-rays or excitation emission Recombination emission Case #2: Long probe delay Pump Probe

13 Conclusions Characterization of the nozzle in the droplet flow regimes is important for the proper interpretation of EUV / x-ray emission data. Laser coupling efficiency is highly dependent on the target size and the duration of the laser pulse. Optimum pulse width for best radiation generation efficiency is dependent on the type of emission: (1) EUV recombination emission t crit < t laser < t recomb (2) EUV excitation or x-ray emission t laser t crit

14 Preliminary Xenon Results: Size histograms Pressure (psi) P T phase space of gas jet Frequency psi, 240 K <d> = 5.4 ± 1.3 µm 100 psi, 190 K <d> = 7.4 ± 1.8 µm Temperature (K) FWHM Diameter (µm)

15 Preliminary Xenon Results: EUV spectra 2.0 Signal (AU) spectra at 100 PSI, 190 K Wavelength (nm)