Laser Transient Systems Karl R. Umstadter for Team Center for Energy Research University of California San Diego, USA February 11, 2009
Overview Introduction Use of Laser Heat Pulses PA Short Pulse Update PA Long Pulse Initial Results PB Long Pulse Install
Introduction When an ELM occurs in tokamaks, up to 30% of the pedestal energy can be deposited on the plasma facing boundary Result is heating & material loss due to sublimation, evaporation and melt splashing of plasma facing components and Expansion of the ejected material into the plasma
IFE/MFE Transients & Experiments Q LP 5.E-9 J. Linke, F. Escourbiac, I.V. Mazul, R. Nygren, M. Rodig, J. SchlosseR, S. Suzuki, J Nucl. Mat. 367 370 (2007) 1422 1431
A Γ~10 17 10 19 D + /cm 2 s -1 n ~ 10 12 D + /cm 3 Te ~ 5-10eV V bias ~ up to 250V R p ~ 4cm A & Beam Delivery Laser Path
Introduction Calculations indicate that a pulsed laser system can be used to simulate the surface effects of the heat pulse of ELMs Surface temperature during an ELM is a function of the energy density of deposition and thermal conduction to the bulk during and following the deposition Lasers of varied pulsewidth can be utilized to mock transients such as ELMs and IFE wall impacts.
Temperature (C) 3500 3000 2500 2000 1500 1000 500 0 1.0E-08 Q-Switch Nd:YAG as ELM Mimic ELM (300usec) ELM+60usec YAG (5nsec) YAG+1nsec YAG Pulse Simulates Surface Heating Well 1.0E-07 1.0E-06 Longer duration results in deeper heat transport May effect retention 1.0E-05 Depth in Target (m) 1.0E-04 1.0E-03 Surface temperature during an ELM is a function of the energy density of deposition and thermal conduction to the bulk during and following the deposition Best expressed as an Energy Impact Value with units MJ/m 2 -s 1/2
Mimic of ELM Transient on W Laser Exposure of W at 200C for 60min Laser Parameters 5nsec 4mm spot 166mJ per Shot ~10 8 W/cm 2 1200 Pulses Absorbed Energy Impact ~58 MJ/m 2 s 1/2 R W (λ=1064nm) ~ 70% ELM Equivalent 1MJ/m 2 @ 0.3msec Plasma Parameters Total Fluence ~ 10 26 D + /m 2 Ion Energy ~100eV
SEM Surface Analysis
Plasma + Heat Pulse Enhancement Absorbed Energy Impact ~45 MJ/m 2 s 1/2 + F~ 10 26 D + /m 2 T surf ~ 50ºC LASER SPOT LASER ONLY LASER + PLASMA
Mass Loss (mg) 1.4 1.2 1 0.8 0.6 0.4 Erosion of W PFC under Simulated ELM Transients at High Repetition Rate (low fluence between ELMs) Laser + Plasma Laser Only Plasma Only 3000 Transients with Absorbed Energy Impact ~45 MJ/m 2 s 1/2 F~ 10 26 D + /m 2 T surf ~ 50ºC 0.2 0 0 30 60 90 120 150 Ion Energy [ev] (Vbias-Vplasma)
Effects of D Loading on Damage SAMPLE F = 5 x 10 22 /m 2 F = 5 x 10 23 /m 2 F = 2 x 10 24 /m 2 Fluence to surface between heating transients V bias =125V Γ=2x10 22 /m 2 -sec T e =11eV n e =2x10 24 /m 3
Summary of Observations Fluence to -A targets between transients greater than operating tokamaks - approaching ITER Neutral and Ionized W found in range of mm to several cm in front of surface allows study of transport & redeposition Synergistic effect between thermal transients & plasma exposure leads to enhanced material removal Synergistic effect depends upon D fluence between transients
Long Pulse Laser New Laser System 0.3-20msec pulse E pulse = 1.5-50 J Installation on A Compare results to short-pulse results Test diagnostic systems Installation on B for Mixed Materials Studies Alloying of Materials Effect on Surface Nanostructures
Long Pulse Nd:YAG as ELM Mimic 5000 4000 0.3msec 1msec 5msec 10msec Temp (C) 3000 2000 1000 0 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 depth (m) Maximum system parameters at each pulse duration
PA Long Pulse Output beam collimated (divergence reduced) Laser safety windows installed Beam entirely enclosed outside vacuum No goggle operation Heat exchanger repaired Aux air cooling added (AC) Triggering system for remote/camera operation
Initial Experiments 25J 5msec - 1200 Laser Pulses @1/3 Hz 5 kw 50MJ/m 2 s 1/2 75V Bias Total Fluence ~10 26 D + /m 2 Room Temperature >500ºC
Surface Profiles 70E+3 RT 550C Height (nm) 35E+3 000E+0-35E+3-70E+3-500 0 500 1000 1500 2000 2500 3000 3500 4000 Position (um)
PB Implementation Update Spectroscopy Window with Laser Glass Shutter (Interlocked) Enclosure Pyrometer Laser Optics Laser-Safe Plastic Operators Window
Questions Karl R. Umstadter karl@ucsd.edu Team M.Baldwin, R.Doerner, J. Hanna, E. Hollmann, D. Nishijima, G.R.Tynan, and J. Yu