Ion-Acoustic-Wave Instability from Laser-Driven Return Currents

Transcription

1 Ion-Acoustic-Wave Instability from Laser-Driven Return Currents 3.0 3~ beam 2.5 4~ TS beam nm TS volume Thomsonscattered light Wavelength shift (Å) D. H. Froula University of Rochester Laboratory for Laser Energetics 52nd Annual Meeting of the American Physical Society Division of Plasma Physics Chicago, IL 8 12 November 2010

2 Summary An ion-acoustic-wave instability is observed for large ZT e /T i (i.e., weak ion Landau damping) The instability is enhanced when the target is cooling The instability saturates with signatures of trapping Weakly ion damped systems (ZT e /T i > 30) are susceptible to enhanced ion fluctuations This instability has implications for laser plasma instabilities (enhanced T i ) and laser-beam absorption (turbulence) The return current instability produces lower LPI thresholds and higher laser-beam absorption. E19464

3 Collaborators V. N. Goncharov, S. X. Hu, and J. F. Myatt University of Rochester Laboratory for Laser Energetics J. S. Ross, L. Divol, and S. H. Glenzer Lawrence Livermore National Laboratory Livermore, California

4 Instability Ion-acoustic waves become unstable when the drift velocity exceeds the sound speed Heat is carried by fast electrons The relative drift between the electrons and ions maintains a quasi-neutral plasma When the drift exceeds the ion-acoustic phase velocity, electrons enhance the wave (electron Landau growth) If the Landau growth rate is larger than the ion Landau damping, the waves are unstable v d > c s Heat flux Electron Landau growth E ~/k ~ c s v d v

5 Ion-Acoustic-Wave Amplitude The amplitude of the scattered light is a function of the electron and ion Landau damping v th T i ~ k Electron Landau damping Ion Landau damping ZT e V Normalized damping Electron Landau damping Ion Landau damping ZT e /T i The ion Landau damping is negligible for ZT e /T i > 30, and the ion-wave amplitude is governed primarily by the electron-distribution function. E19466

6 Experimental Setup The ion-wave damping was varied by changing the target material (CH, V, Ag, Au) The targets are heated by 1-, 2-, or 3-ns-long laser pulses The 4~ beam and the collection direction are in the plane of the foil 3~ beam I ~ W cm 2 Solid target k 0 4~ TS beam nm TS volume Thomsonscattered light 4~ beam k k s k s k 0 = k k = k 0 63º k 0 TS collection lens Thomson-scattering is measured 400 nm from the target surface The system probes waves that are propagating radially E19467

7 Plasma Characterization Thomson-scattering measurements provide a direct measure of ZT e /T i and the amplitude of the ion-acoutic waves Scattering from the ion-acoustic waves provides a measure of ZT e, T i Intensity (arbitrary units) E T e = 1.8 kev T i = 0.4 kev t = 900 ps ZT e 15 T i Wavelength shift (Å) The amplitude of the scattered power is determined by Landau damping Ion Landau damping dominates Normalized power scattered CH V Electron Landau damping dominates ZT e /T i Au

8 Instability The ion-acoustic waves propagating to the center of the plasma are measured to be unstable in high-z (Au) plasmas CH Au Au Total scattered power Total scattered power Total scattered power ZT e ZT 15 e > 40 T i T i ZT e 70 T i 2.5 No instability Instability ns Instability 2.5 ns Wavelength shift (Å) Wavelength shift (Å) Wavelength shift (Å) Laser heating Laser heating Laser heating E19469 The instability is enhanced when the plasma is cooling.

9 Instability For ZT e /T i < 40, the ion-acoustic waves are damped sufficiently by the ions to remain stable Total scattered power V ZT e 35 T i Au ZT e 70 T i Wavelength shift (Å) Laser heating Ion-wave amplitude CH V Au ZT e /T i Landau damping E19470

10 Instability This instability is likely driven by the return current (return-current instability) E19471 Heat flux v d > c s Electron Landau growth 0 ~/k ~ c s v d v When the return current shifts the peak of the electron-distribution function beyond the sound speed, the electrons drive the wave. v d > c s Dm (Å) Unstable ion waves

11 Summary/Conclusions An ion-acoustic-wave instability is observed for large ZT e /T i (i.e., weak ion Landau damping) The instability is enhanced when the target is cooling The instability saturates with signatures of trapping Weakly ion damped systems (ZT e /T i > 30) are susceptible to enhanced ion fluctuations This instability has implications for laser plasma instabilities (enhanced T i ) and laser-beam absorption (turbulence) The return current instability produces lower LPI thresholds and higher laser-beam absorption. E19464

12 The frequency shift in the driven ion-acoustic wave is consistent with trapping Total scattered power Au ZT e 70 T i Instability 2.5 ns Wavelength shift (Å) Laser heating Dm/m (10 3 ) P driven /P thermal E19472 The ion temperature and laser-beam coupling are enhanced by the unstable ion-acoustic waves.

13 At stable conditions, the drift velocity can be measured and compared with fluid simulations q = b T e n e [v d + o T m e adln(n e )] Electron temperature (kev) 2.5 fl = v/c s Plasma flow Drift velocity E19486