Thomson-Scattering Techniques at the OMEGA Laser Facility OMEGA Workshop 2010
D. H. Froula Lawrence Livermore National Laboratory
April 2010
This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344
D. H. Froula, Thomson-Scattering on OMEGA OMEGA Workshop 2010 LLNL-PRES-429371 Thomson scattering is a core diagnostic in MHD facilities and should be adopted in the laser plasma community
2T α > i α>1 ZTe Electron Plasma Wave Feature Ion-Acoustic Feature
• Intro to Thomson scattering
−1/ 2 • Electron temperature ZTe α ≈ −1 3Ti measurements
• Electron Density α<1 • Driven plasma waves Electron Distribution Function • Ion temperature measurements
Complete Thermal Scattering Spectrum
D. H. Froula, Thomson-Scattering on OMEGA OMEGA Workshop 2010 Thomson scattering 2 “Elastic” scattering of electromagnetic-waves from free electrons (hν< Non-collective Thomson Scattering (λ < λD) Boltzmann distribution Optical Laser ΕS λ ∼ λ0 Ε Scattering from 0 free electrons v Te λ0 Plasma λD Wavelength Collective Thomson Scattering Collective Features Optical Laser ΕS vphase Scattering form thermal fluctuations Thermal fluctuations λ0 Plasma λD Wavelength D. H. Froula, Thomson-Scattering on OMEGA OMEGA Workshop 2010 Thomson scattering in laser produced plasmas is typically collective as a result of the relatively high densities (α~2-3) 2T α > i α>1 ZT Non-collective Regime (α < 1) e Electron Plasma Wave Feature Ion-Acoustic Feature 2λ 2T ∆λ ≅ probe sin(θ 2) e 1/ e c M −1/ 2 ZTe α ≈ −1 3Ti Collective Regime (α > 1) 1/ 2 2 n vth ∆λ ≈ 2λprobe + 3 2 α<1 ncr c Electron Distribution Function Collective Regime (α2 > 1/(ZTe/3Ti - 1)) ZT ∆λ ≅ 4λ sin(θ 2) e probe M Complete Thermal Scattering Spectrum D. H. Froula, Thomson-Scattering on OMEGA OMEGA Workshop 2010 At OMEGA, a 2ω or 4ω laser beam can be configured as the TS probe and scattered light is collected in TIM6 target J. S. Ross et al. Rev. Sci. Instr., 77 10E520 (2006) Top TIM4 chamber TS Alignment TIM 2 f/10 collection lens TIM6 TS Collection P9 2ω/4ω Bottom Beam D. H. Froula, Thomson-Scattering on OMEGA OMEGA Workshop 2010 LOCAL plasma parameters are measured with Thomson scattering – Light is only scattered from the regions of the plasma that over lap the probe beam – The slits on the diagnostics limit the region where light is collected Projection of the 200µm spectrometer slit into the plasma plane Probe Laser Projection of the 150µm streak camera slit into the plasma plane D. H. Froula, Thomson-Scattering on OMEGA OMEGA Workshop 2010 Section I Electron Temperature Measurements 1 0.8 ∆λ 0.6 0.4 0.2 0 ScatteredIntensity -3 -2 -1 0 1 2 3 Wavelength (A) The separation between the ion-acoustic features in the scattering spectrum provides a direct measure of ZTe ZT ∆λ ≅ 4λ sin(θ 2) e probe M Assuming ZTe >> 3Ti Typical Thomson scattering setups can measure the electron temperature to within 15% Au Plasma H2 Stalk 4 Thomson volume 4w probe 2 3 (60x100 um ) Time (ns) Time 2 -4 -2 0 2 4 ∆λ (A) 1 1 1.2 keV 0.8 keV 0.5 0.5 Normalized Normalized ScatteredPower 0 ScatteredPower -4 -2 0 2 4 ∆λ (A) 0 For high-Z plasmas, the ionization state can be -3 -2.5 -2 -1.5 -1 -0.5 0 determined using spectroscopy ∆λ (A) D. H. Froula, Thomson-Scattering on OMEGA OMEGA Workshop 2010 Aligning the spectral slit with the probe beam provides a scattering profile that can be used to measure temperature gradients • We use an intensified CCD camera coupled to a spectrometer • The slit of the spectrometer is aligned parallel to the probe beam • Heating a hohlraum from one side provides a significant temperature profile that is used to test our nonlocal hydrodynamic simulations 4ω Thomson Probe Thomson Scattering (t=800ps) Flux Limit Diffussion (f=0.05) Nonlocal Model 3.5 1 3 2.5 0.5 2 0 1.5 1 -0.5 0.5 -1 0 -1.5 -1 -0.5 0 0.5 1 1.5 -6 -4 -2 0 2 4 6 Position (mm) Wavelength Shift (A) Spectral Slit J. S. Ross et al. Rev. Sci. Instr., 77 10E520 (2006) D. H. Froula, Thomson-Scattering on OMEGA OMEGA Workshop 2010 Last month S. Ross was able to measure the first high-frequency collective features from 4ω Thomson scattering ne Te=1.1 keV ω 20 -3 • 4 scattering provide access to high ne=1.2x10 cm densities . units) • The wavelength shift (EPW) is a measure arb of the density: 1/ 2 T 2 e n vth ∆λ ≈ λ + ( Intensity EPW 2 probe 3 2 ncr c 225 232 240 Wavelength (nm) • The width (EPW) is a measure of the Z=23 electron temperature ZTe • The IAW provides a measure of Z: . units) arb ZTe ∆λIAW ≅ 4λprobe sin(θ 2) M ( Intensity -4 -2 0 2 4 Wavelength (nm) Scattering from the EPW is weak; high phase velocities low number of particles D. H. Froula, Thomson-Scattering on OMEGA OMEGA Workshop 2010 Section II Local Electron Density Measurements using IAWs 1 0.8 ∆λ ∝ ωa 0.6 0.4 0.2 Frequency Scattered Intensity Scattered 0 Ion Acoustic Wave -3 -2 -1 0 1 2 3 0 1 2 3 4 5 Wavelength (A) kλDe Few experiments have successfully measured the local electron density in a laser produced plasma A calibrated Thomson scattering system is challenging on single shot laser facilities and is unrealistic at large facilities like Omega, NIF, LILL Collective Thomson scattering from electron plasma waves has been demonstrated, but requires significant probe energy Multiple Thomson-scattering diagnostics can be used to expose the sensitivity of ion-acoustic waves to Debye shielding and provide an accurate measure of the density Froula et al., Phys. Rev. Lett. 95, 195005 (2005) Ion-acoustic waves are dispersive; the sound speed is a function wavelength, density, and temperature Using two TS diagnostics to probe significantly different k-vectors, we have a closed system for (Te, Ne) ZKT 1 ω = e ZKTe 1 k1 2 2 kλDe<< 1 ω1 ≅ k1 M 1+ k1 λD M ZKT 1 ZKTe 1 ω = k e kλ > 1 ω = k 2 2 2 2 De 2 2 + 2λ2 M 1+ k2 λD M 1 k2 D k λ ∝ Te a De ne Assuming: ZTe >> Ti • One diagnostic is chosen to be insensitive to Ne, therefore measures Te • Second diagnostic is chosen to be sensitive to Ne • The combination is a closed system for (Te, Ne) Froula et al., Phys. Rev. Lett., 95, 195005 (2005) D. H. Froula, Thomson-Scattering on OMEGA OMEGA Workshop 2010 Thomson scattering measurements in half-hohlruams shot at OMEGA show a decrease in Te from 10.7 keV to 2.6 keV over 600ps. Temperature (keV) Density (ne/nc @ 3w) Temperature and Density 12 0.14 • 2ω and 4ω configurations are used 0.12 to probe significantly different IAW 10 @ (ne/nc 3w) Density vectors 0.1 8 0.08 • The uncertainty in Te is better than 10% 6 0.06 0.04 • The uncertainty in n is better than Temperature (keV) e 4 heater 20% 0.02 beams 2 0 800 1000 1200 1400 Time (ps) Ross and Froula (2008) D. H. Froula, Thomson-Scattering on OMEGA OMEGA Workshop 2010 Section III Measuring the Amplitude Plasma Waves 1.2 1 0.8 0.6 Number 2T v ≈ i 0.4 th Ion A mp Time 1ns Probe Beam (175 (175 J) Beam Probe 1ns 0.2 0 1 T − e 1 Te 2 m 0 2 m Wavelength Shift p p 1ns Heater Beam (490 (490 J) Beam Heater 1ns Velocity The scattered power is a function of the Landau damping The Landau damping is a function of the ZTe/Ti through the slope and number of particles in the distribution function Section III Measuring the Amplitude Plasma Waves Electron Ion equilibration 1.2 1 Late Time 0.8 0.6 Number Early Time 0.4 Ion Time 1ns Probe Beam (175 (175 J) Beam Probe 1ns 0.2 0 T 1 T 1 T − e − e e Te m 2 m 0 2 m Wavelength Shift p p m 1ns Heater Beam (490 (490 J) Beam Heater 1ns Velocity The scattered power is a function of the Landau damping The Landau damping is a function of the ZTe/Ti through the slope and number of particles in the distribution function Ion vanadium, the electron and ion Landau damping have similar contributions to the total ion-wave damping An asymmetry is measured in the The ion damping in vanadium is on ion-acoustic features for mid-Z the order of the electron damping (Z~22) vanadium targets 1.2 Electron 1 Distribution 0.8 0.6 0.4 2T ≈ i Time vth Particle Number A mp 1ns Probe Beam (175 (175 J) Beam Probe 1ns 0.2 0 1 Te 1 T (1.5 kJ ) (1.5 Beam ns Heater − e 2 m 0 3 2 m Wavelength Shift p p Velocity The asymmetry changes directions mid-way Z Te through the probe beam; the heater beams vφ ≈ are on throughout the probe beam A mp Ion vanadium, the electron and ion Landau damping have similar contributions to the total ion-wave damping An asymmetry is measured in the Introducing a drift between the ion-acoustic features for mid-Z electrons and ions (via return current) (Z~22) vanadium targets changes the LD Early Time 1.2 vd 1 0.8 0.6 0.4 2T ≈ i Time vth Particle Number A mp 1ns Probe Beam (175 (175 J) Beam Probe 1ns 0.2 0 1 Te 1 T (1.5 kJ ) (1.5 Beam ns Heater − e 2 m 0 3 2 m Wavelength Shift p p Velocity The asymmetry changes directions mid-way The electron Landau damping is through the probe beam; the heater beams reduced/increase leading to an asymmetric are on throughout the probe beam scattering spectrum Ion vanadium, the electron and ion Landau damping have similar contributions to the total ion-wave damping An asymmetry is measured in the Introducing a drift between the ion-acoustic features for mid-Z electrons and ions (via return current) (Z~22) vanadium targets changes the LD Late Time 1.2 vd 1 0.8 0.6 0.4 2T ≈ i Time vth Particle Number A mp 1ns Probe Beam (175 (175 J) Beam Probe 1ns 0.2 0 1 Te 1 T (1.5 kJ ) (1.5 Beam ns Heater − e 2 m 0 3 2 m Wavelength Shift p p Velocity The asymmetry changes directions mid-way The electron Landau damping is through the probe beam; the heater beams reduced/increase leading to an asymmetric are on throughout the probe beam scattering spectrum Increasing the charge state (Z) eliminates the ion Landau damping and the ion wave becomes unstable When the return current changes In Au, there are no ions at the phase directions, the ion-acoustic wave velocity; the ion-damping is zero becomes unstable 1.2 vd 1 0.8 0.6 0.4 Time Particle Number 1ns Probe Beam (175 (175 J) Beam Probe 1ns 0.2 0 1 Te 1 T (1.5 kJ ) (1.5 Beam ns Heater − e 2 m 0 3 2 m Wavelength Shift p p Velocity When the drift velocity exceeds the ion-acoustic phase velocity, the mode becomes unstable Increasing the charge state (Z) eliminates the ion Landau damping and the ion wave becomes unstable When the return current changes In Au, there are no ions at the phase directions, the ion-acoustic wave velocity; the ion-damping is zero becomes unstable Return Current 1.2 Instability vd 1 0.8 0.6 0.4 Time Particle Number 1ns Probe Beam (175 (175 J) Beam Probe 1ns 0.2 0 1 Te 1 Te (1.5 kJ ) (1.5 Beam ns Heater − 2 mp 0 2 m Wavelength Shift 3 p Velocity When the drift velocity exceeds the ion-acoustic phase velocity, the mode becomes unstable Section VI Ion Temperature Measurements H-Mode H-Mode 1.2 C-Mode C-Mode 1 0.8 0.6 2Ti vth ≈ 0.4 A mp Particle Number 0.2 ScatteredPower 0 T 1 T 1 T − e − e e Te m 2 m 0 2 m Wavelength Shift Z T p p m ≈ e vφ Velocity A mp Theoretically the ion temperature can be determined from the width of the ion-acoustic features — due to gradients within the TS volume make this measurement uncertain and therefore, unreliable in laser produced plasmas Multi-ion species plasmas allow the accurate measurement of ion temperature from the relative amplitude of the ion-acoustic modes Multi-ion CH plasmas allow an accurate measure of both the electron and ion temperature 400ps 600ps 800ps 1000ps 1100ps Te=1.7keV Te=1.85keV Te=2.1keV Te=2.7keV Te=2.2keV Ti=0.15keV Ti=0.23keV Ti=0.31keV Ti=0.59keV Ti=0.7keV 1 0.8 8 0.6 6 0.4 4 0.2 2 0 0 -10 -5 0 5 1010 -5 0 5 10 -5 0 5 -1010 -5 0 5 -1010 -5 0 5 10 Wavelength (A) Wavelength (A) Wavelength (A) Wavelength (A) Wavelength (A) H C Wavelength 200 400 600 800 1000 1200 Time (ps) These results were used to validate hydrodynamic simulations which provides a foundation for our Laser-plasma interaction studies 33 frequency tripled (355nm) laser beams Varying the energy in the 33 Heater beams are used to heat the target scales the temperature in the gas fill HYDRA (f=0.05) TS Data HYDRA (nonlocal) Along the axis of the hohlraum a uniform hot A peak temperature of 3.5 keV is plasma is produced measured for a ne/ncr=6% D. H. Froula et al. Phys. Plasmas, 13 052704 (2006) Conclusions • Thomson scattering is a proven diagnostic to measure local plasma parameters in laser produced plasma experiments • We have shown a new technique for measuring the LOCAL electron density using the ion-acoustic features • Currently we are investigating the effects of heat flow in high-Z plasmas where we have recently observed a large asymmetry (100:1) in the ion- acoustic spectrum •We have been successful in measuring TS spectra using 4 laser energies between 0.5J and 200J. 3.5 –Energy requirement is determined by background generated by 3 heater beams: 2.5 •Small single beam facilities require ~1J 2 •Large multi-beam facilities (>10 beams) require 10-100J Time (ns) Time 1.5 1 0.5 -4 -2 0 2 4 ∆λ (Ang.) D. H. Froula, Thomson-Scattering on OMEGA OMEGA Workshop 2010 Thomson scattering in laser produced plasmas is typically collective as a result of the relatively high densities (α~2-3) Collective Thomson Scattering Typical Setup Optical Laser f/7 probe lens ΕS ion-acoustic wave λD k a = k probe − k s f/10 collection θ lens ks kprobe ka f/10 focusing lens k = 2k sin(θ 2) a probe θ=102o at OMEGA Scattering from defined wave vector D. H. Froula et al. Rev. Sci. Instr,, 77 10E522 (2006) D. H. Froula, Thomson-Scattering on OMEGA OMEGA Workshop 2010