Measurement of Long-Scale-Length Plasma Density Profiles for Two-Plasmon Decay Studies

Transcription

1 Measurement of Long-Scale-Length Plasma Density Profiles for Two-Plasmon Decay Studies Plasma density scale length at cm 3 (nm) Flat foil Target diameter (mm) f hot Plasma density scale length at cm 3 (nm) D. Haberberger University of Rochester Laboratory for Laser Energetics 43rd Annual Anomalous Absorption Conference Stevenson, WA 7 12 July 2013

2 Summary The dependence of two-plasmon decay (TPD) on the plasma scale length is isolated by using targets of varying radii Angular filter refractometry (AFR) was developed to measure high-density, long-scale-length plasmas AFR measures the refractive contour map of the probe beam from which the density is calculated up to ~10 21 cm 3 (n e = 0.1 n cr ) The scale length is measured to increase from 150 to 300 nm as the target diameter is increased from 0.4 to 8 mm The hot-electron production is found to increase rapidly with the measured plasma density scale length E22181

3 Collaborators D. H. Edgell, S. X. Hu, S. Ivancic, B. Yaakobi, R. Boni, and D. H. Froula University of Rochester Laboratory for Laser Energetics

4 The dependence of TPD on the plasma scale length is isolated by using targets of varying radii on OMEGA EP The TPD gain is proportional to IL n /T e (at n cr /4) I: laser intensity L n : plasma density scale length T e : thermal electron temperature Simulations show that I/T e is approximately constant for these experiments By decreasing the radius of a spherical target, L n decreases as the plasma expansion becomes more divergent E22182

5 The experiment measures the plasma density scale length and the hot-electron production Four UV laser beams 4~ probe beam ŷ ẑ xˆ CH foil CH Mo f/4 collection to access high densities 5-nm resolution mm field of view Hard x-ray detector (T hot ) X-ray spectrometer (k a yield) E22055a The plasma density scale length is measured with a new diagnostic based on refractometry.

6 Angular filter refractometry (AFR) maps the refraction of the probe beam at target chamber center (TCC) to contours in the image plane TCC object plane Fourier plane Foil target Image plane Foil target E22129b

7 Angular filter refractometry (AFR) maps the refraction of the probe beam at target chamber center (TCC) to contours in the image plane TCC object plane Fourier plane Foil target Image plane Foil target x, y, i (x, y) x, y i (r) The edges of the rings map a certain refraction angle to the spatial location in the object plane. E22129c

8 The diagnostic is calibrated using a negative lens, which has a well-defined i (x, y) Negative spherical lens at TCC Round beam shape at Fourier plane Image plane using a spherical lens xˆ ŷ ẑ i y direction (mm) i 6 i 4 i 2 i 1 i 3 i x direction (mm) The association of these angles with the specific angular spectral filter bands can be applied to a plasma to measure its refraction profile. E21941

9 The experimental AFR images are analyzed using the calibration angles Experimental AFR image y direction (mm) Original target (diam = 1.9 mm) x direction (mm) 1 E22187 Processing of the experimental AFR images creates a contour map of the refraction angle.

10 The plasma density profile can be deduced from the refractive contour map i r Experimental ASF image 2r z = ir dr m # 2r ne z = c 2n md, m # Phase 3 3 Density cr y-direction (mm) Original target diam = 1.9 mm 1 0 x direction (mm) x direction (mm) x direction (mm) (rad) (cm 3 ) E22188 The 2-D density map has an error of ±15%.

11 The AFR images show refractive contours moving away from the target surface as the target diameter increases Laser: 2 ns square pulse, probe timing: 1.5 ns 2.5 D = 0.4 mm D = 0.8 mm D = 1.9 mm D = 3.0 mm D = 8.0 mm 2.0 y direction (mm) x direction (mm) E22183

12 Analysis of the AFR images shows the plasma expansion is confined as the target diameter is increased Laser: 2 ns square pulse, probe timing: 1.5 ns Plasma density (cm 3 ) mm 0.8 mm 1.9 mm 3.0 mm 8.0 mm Flat foil n e /n cr L n (nm) at cm Flat foil Space (mm) Target diameter (mm) E22184

13 Two-dimensional flux-limited (f = 0.06) hydrodynamic simulations show that the density scale length at n cr /4 saturates after about 1 ns Plasma-density scale length (nm) I/T e ( W/cm 2 /kev) Time (ns) Shell diameter (mm) At 1.5 ns, the I/T e varies little over the range of targets tested. E22241

14 Comparisons of the experimental data to DRACO hydrodynamic simulations show significant differences Plasma density (cm 3 ) mm experiment 1.9 mm simulations n e /n cr L n (nm) at cm Flat foil Space (mm) Target diameter (mm) Hydrodynamic simulations overestimate the density and scale length compared to measurements at n cr /10 at 1.5 ns. E22228

15 The hot-electron fraction is measured to increase rapidly with the plasma density scale length 10 2 f hot L n (nm) at cm 3 (0.1 n cr ) The fraction of hot electrons increases from ~0.005% to 1% while increasing L n from 150 to 300 nm. E22185

16 Summary/Conclusions The dependence of two-plasmon decay (TPD) on the plasma scale length is isolated by using targets of varying radii Angular filter refractometry (AFR) was developed to measure high-density, long-scale-length plasmas AFR measures the refractive contour map of the probe beam from which the density is calculated up to ~10 21 cm 3 (n e = 0.1 n cr ) The scale length is measured to increase from 150 to 300 nm as the target diameter is increased from 0.4 to 8 mm The hot-electron production is found to increase rapidly with the measured plasma density scale length E22181

17 The 4~ probe laser system delivers a 3.3-mm spot to target chamber center (TCC) with picosecond timing accuracy TCC illumination f/25 diverging beam Diameter = 3.3 mm Diagnostic table 5-nm resolution mm field of view Space (mm) Timing ±25-ps accuracy ±5-ps post-shot measurement Space (mm) Nd:glass laser 4~ (263 nm) ~ 20 mj Pulse width = 10 ps E22054