Compressed-shell integrity measurements in spherical implosion experiments

Abstract

The shell integrity near peak compression of spherical implosions using the 60-beam, 30-kJ UV OMEGA laser system [Opt. Commun. 133, 495 (1997)] has been measured. Hot core emission backlights a shell with a thin titanium-doped layer that is imaged at x-ray photon energies above and below the titanium K edge. The x-ray intensity ratio between the two images is related to perturbations in the cold, or absorbing, part of the shell. The measured cold-shell areal-density modulations, integrated over the time of peak compression, are of the order of 25% to 50% with nonuniformity spectra peaked at spatial wavelengths of 30 to 50 μm and with the smallest detectable nonuniformity features extending down to spatial wavelengths of 12 to 15 μm. Hot-shell areal-density modulations of the emitting part of the shell (inner edge) are of the order of 13% to 20%. The measured shell modulations are in agreement with the results of two-dimensional simulations that include initial shell perturbations, imprinted shell modulations due to nonuniformities in a single laser beam, and a beam-to-beam energy imbalance in the laser drive.

Direct-drive inertial confinement fusion: A review

“…Extensive analysis of the differential-imaging technique is given in Refs. 293 The modulations are highest at the inner surface, which was unstable during the deceleration phase. These modulations decrease in the bulk of the shell and then increase near the outer surface (in the 9-lm layer), which was unstable during the acceleration phase.…”

Evolution of Shell Nonuniformities near Peak Compression of a Spherical Implosion

“…This emission can be used as a backlighter to probe the outer, colder shell [12]. The first shell-integrity measurements, based on this method, were time integrated over the duration of peak compression (ϳ200 to 300 ps) of the implosion [12,13]. They used shells with titanium-doped layers and imaging at photon energies above and below the titanium K edge.…”

“…Core images at photon energies below the K edge (not absorbed by the shell) provide the spatial shape of the backlighter, while core images at photon energies above the K edge (highly absorbed by the shell's titanium) contain information about the structure of shell-areal-density modulations in the titanium-doped layer. The experiment described in this Letter, based on the techniques developed in timeintegrated experiments [12,13], is the first measurement of the evolution of shell nonuniformities near peak compression of a spherical-target implosion using targets with titanium-doped layers. The growth of shell modulations in the deceleration phase is measured for the first time in ICF implosion experiments.…”

“…The measured spectra (shown in Fig. 1) contain information about (i) evolution of Hea, Ha, and Heb line emission of titanium ions mixed with the core fuel, (ii) 1s-2p absorption lines (near ϳ4.6 keV) of warm titanium with temperatures T ϳ 500 and 700 eV in the shell, (iii) absorption above the K edge (at 4.966 keV) of cold titanium (T , 500 eV) in the shell [12,13], and (iv) hot core continuum emission. The effective electron temperature in the emission region T e and average cold titanium areal density has been calculated by fitting the function I͑E͒ I 0 e ͕2E͞T e 2m Ti ͑E͒?͓rd͔ Ti ͖ to the measured spectra (outside of the absorption area of warm titanium near ϳ4.6 keV, and the shifting K edge), where E is the photon energy, m Ti ͑E͒ and ͓rd͔ Ti are the cold titanium mass absorption coefficient and average areal density, respectively, and I 0 is the constant.…”

“…where m .K 0.37 6 0.02 cm 2 ͞mg and m ,K 0.11 6 0.02 cm 2 ͞mg are the spectrally weighed mass absorption coefficients of cold titanium at photon energies above and below the K edge, respectively [13]. The absorption coefficients have been calculated for each time t using the measured x-ray spectra shown in Fig.…”