Measurements of Strain-Induced Refractive-Index Changes in LiF Using Direct-Drive Ramp Compression Apparent particle velocity (nm/ns) 16 12 8 4 1 GPa 8 GPa 2 4 6 8 1 12 14 True particle velocity (nm/ns) D. E. Fratanduono University of Rochester Laboratory for Laser Energetics 51st Annual Meeting of the American Physical Society Division of Plasma Physics Atlanta, GA 2 6 November 29
Summary The refractive index of quasi-isentropically compressed LiF has been measured to ~8 GPa The shock-compressed refractive index of LiF was previously studied to 1 GPa* LiF is observed to be transparent up to 8 GPa with quasi-isentropic compression remains transparent for single shocks < 16 GPa Ramp-compressed LiF refractive index is in agreement with existing data does not depend on loading technique (shock versus ramp compression) LiF refractive index scales linearly with density up to 8 GPa E18359 *J. L. Wise and L. C. Chhabildas, presented at the American Physical Society Topical Conference on Shock Waves in Condensed Matter, Spokane, WA, 22 July 1985.
Collaborators M. A. Barrios T. R. Boehly D. D. Meyerhofer University of Rochester Laboratory for Laser Energetics R. Smith J. H. Eggert D. G. Hicks P. M. Celliers G. W. Collins R. Rygg Lawrence Livermore National Laboratory
Changes in the refractive index affect VISAR measurements L Diamond U s n Reflecting shock Window VISAR VISAR measures the rate of change of the optical path length (OPL) OPL depends upon n and n s L OPL = # n ( x ) dx Diamond U s VISAR Corrections must be made for shocked materials (n s ) n s n Reflecting surface E18447
Transparency of shocks in LiF windows makes it possible for VISAR to probe the material interface LiF window Shock LiF Diamond U p U s VISAR n s n Reflecting surface Single shocks up to 16 GPa are transparent in LiF multi-shocks up to 5 GPa are transparent VISAR probes through compressed material, this alters its sensitivity For shock compression up to 1 GPa, the refractive index scales linearly with density:* n = a + bt E1846a *J. L. Wise and L. C. Chhabildas, Shock Waves in Condensed Matter 1985, DEAC4-DP789
Simultaneous measurement of free-surface and apparent particle velocities provide index correction Diamond OMEGA Free surface velocity VISAR Diamond isentrope* and LiF EOS are known Use impedance matching to infer true particle velocity Gold layer LiF Apparent particle velocity Derive correction from measured versus true particle velocity E1836 *D. K. Bradley et al., Phys. Rev. Lett. 12, 7553 (29).
Apparent U p is compared to true U p (derived from U fs ) to obtain correction Velocity (nm/ns) 4 3 2 Quasi-Isentropic Ramp Compression Diamond-free surface Apparent LiF particle True LiF particle Ratio: correction factor 1 3 4 5 Time (ns) Shot 54781 6 E18362
Quasi-isentropic ramp compression allows for continuous measurements to be made U p (nm/ns) * 5 4 3 2 1 Wise (1985) Lalone (28) Wise (Fit) Janus (28) Ramp comp. 2 du du * p p = dn n Z t d t Ramp experiment exhibited higher temperatures than predicted 1 2 3 4 U p (nm/ns) E18363
Glue layers compromised ramp measurements, but the final state can be used to obtain correction Diamond OMEGA VISAR 8 Final state Velocity (nm/ns) 2 1 2 54948 Diamondfree surface 4 LiF Glue Time (ns) LiF interface 6 Pressure (GPa) 6 4 2 1 Diamond isentrope LiF hugoniot (STP) LiF isentrope (HEL) LiF hugoniot (1) HEL 4 8 12 16 2 Particle velocity (nm/ns) E18364
LiF refractive index depends linearly on density to 8 GPa U p (nm/ns) * 16 12 8 4 Wise (1985) Lalone (28) Wise (Fit) Janus (28) Ramp comp. Ramp peak 1 GPa 8 GPa 2 4 6 8 1 12 14 U p (nm/ns) For compressed material: du du * p p This case = n s du* du dn Z t d t p p s = constant n s = constant + n t E18434
Summary/Conclusions The refractive index of quasi-isentropically compressed LiF has been measured to ~8 GPa The shock-compressed refractive index of LiF was previously studied to 1 GPa* LiF is observed to be transparent up to 8 GPa with quasi-isentropic compression remains transparent for single shocks < 16 GPa Ramp-compressed LiF refractive index is in agreement with existing data does not depend on loading technique (shock versus ramp compression) LiF refractive index scales linearly with density up to 8 GPa E18359 *J. L. Wise and L. C. Chhabildas, presented at the American Physical Society Topical Conference on Shock Waves in Condensed Matter, Spokane, WA, 22 July 1985.
The optical path length of linear window materials does not affect the particle velocity Diamond t(x) LiF VISAR Reflecting surface Free surface E1847a The apparent particle velocity [V*(t)] of the interface is dependent d xr upon the refractive index [V*(t)] = b # n^xl, thdx l dt x ( t ) If LiF follows Gladstone Dale (n = a + bt) d x [V*(t)] = ; s a x / t x/ t x, t dx dt ^ s h ^ h + b # t^ l h l E Mass conservation x ( t ) V * ^th True particle velocity [V(t)] is V^th = a No dependence upon the optical path length in LiF if the behavior follows Gladstone Dale
Determination of laser pulse design to maintain shockless compression Free-surface velocity Simulated Versus Measured Velocities 25 2 15 1 5 ASBO 1 ASBO 2 P51 The requirement for shockless compression over distance x (without reflection) dp dt C x 2 dp C < L L Pulse profile design is complicated with the inclusion of reflected waves correction for wave interactions* 2 3 4 Time (ns) 5 6 Predictions correlate well with experimental data E18361 *S. D. Rothman and J. Maw, J. Phys. IV France 134, 745 (26).
Methods to study the LiF refractive index Temperature (1 K) 8 7 6 5 4 3 2 1 LiF Boehler melt (1997) SESAME 7271 SESAME 7271 MD simulations Melt line Isentrope Hugoniot Two methods of study shock compression ramp compression Each method makes it possible to study different regions of phase space Study whether loading technique affects the index of refraction 5 1 15 2 Pressure (GPa) 25 3 Understand if melt causes LiF window blanking E1851a