Tests of EXAFS on OMEGA: Feasibility for Shock-Heating Measurements

92 LLE Review, Volume 86 Introduction As part of its participation in the Stockpile Stewardship Program (SSP), LLE is studying the feasibility of using extended x-ray absorption fine structure (EXAFS)1 to characterize the properties of solid materials shocked at moderately high pressures (up to a few megabars). In particular, there is an interest in examining material properties since they affect the growth of hydrodynamic instabilities. Thus, a shocked material that retains its strength (or shear) will be more resistant to such instabilities than a molten solid. EXAFS is sensitive to the short-range order in crystals (as opposed to diffraction, which depends on longer-range order). It can be seen in amorphous materials2,3 as well as liquids,4–6 but the reduction in shortrange order due to melting is then evident in the EXAFS spectrum. EXAFS can measure the density and temperature of the shocked solid in addition to indicating melting. The main limitation in shock-heating studies is the reduction of the EXAFS modulation amplitude with increasing temperature. This problem is discussed in detail in this article. EXAFS can possibly indicate whether the shock compression of the solid is oneor three-dimensional: the distances to the nearest atomic neighbors (which is measured by EXAFS) are different in the two cases. Initial tests show very high contrast EXAFS modulations when a thick, undriven Ti foil is backlit by the x-ray radiation from an imploded CH shell. The high contrast achieved in these tests is due to three factors: (a) using an imploded target as a backlighter, (b) using a very thick Ti absorber, and (c) using a CID array for detection. In preparation for future shocked-Ti experiments, we scope out the range of shock strengths where significant EXAFS modulation can be expected. Briefly, although a higher temperature reduces the amplitude of EXAFS modulations, a higher density mitigates this reduction (by raising the Debye temperature). For varying shock strengths we determine the temperature and density of the Ti metal and then the expected EXAFS visibility. EXAFS has been previously seen in laser-produced plasma experiments,7 and at LLE it has been studied8 in imploded spherical targets (where the absorber was also Ti). Tests of EXAFS on OMEGA: Feasibility for Shock-Heating Measurements