R. R. Whitlock

Laser‐plasma interaction and ablative acceleration of thin foils at 1012–1015 W/cm2

The interaction physics and hydrodynamic motion of thin‐foil targets irradiated by long, low‐flux Nd‐laser pulses (3 nsec, 1012–1015 W/cm2) are studied experimentally and compared with theoretical models. Laser light absorption is high (80%–90%) and thin‐foil targets are accelerated up to 107 cm/sec with good (20%) hydrodynamic efficiency in the 1012–1013 W/cm2 range. These results agree with a simple rocket ablation model. Details of thermal heat flow, both axially (related to ablation depth) and laterally (related to beam uniformity requirements), are also presented.

Ablative acceleration of planar targets to high velocities

Laser irradiated targets are ablatively accelerated to velocities near those required for fusion pellet implosions while remaining relatively cool and uniform. The target velocities and velocity profiles are measured using a double-foil method, which is described in detail. Also, the ablation plasma flow from the target surface is spatially resolved, and the scalings with absorbed irradiance of the ablation pressure, ablation velocity, and mass ablation rate are determined. Results are compared with hydrodynamic code calculations.

Laser‐produced‐plasma energy transport through plastic films

The transport of energy from a 1.06‐μm, 95‐psec laser pulse at an irradiance of 1015 W/cm2 through a thin layer of polystyrene into an Al substrate was studied by x‐ray, ion, and scattered‐light measurements. The intensities of the following quantities were measured as a function of polystyrene thickness: (1) x‐ray line radiation from the Al backing, (2) bremsstrahlung continuum from 3 to 88 keV, (3) ions of several keV energy, and (4) scattered laser light. The results indicate that a polystyrene thickness of no more than 0.5 μm is sufficient to inhibit substantial heating of the Al substrate.

X‐ray yields of plasmas heated by 8‐nsec neodymium laser pulses

36 elemental targets, beryllium through uranium, were irradiated with 8‐nsec FWHM Nd : glass laser pulses focused to ∼4×1013 W/cm2. X‐ray spectra were recorded with a convex‐crystal spectrograph and with a 22‐channel PIN‐diode spectrometer. Line and continuum spectra indicated temperatures of 36589 eV. Total x‐ray irradiances in the energy range 0.7–20 keV showed strong peaking as a function of atomic number with a maximum conversion efficiency of 9.6% relative to the incident laser energy. An indication of enhanced N‐shell x‐ray emission near U is reported here for the first time.

Direct measurements of compressive and tensile strain during shock breakout by use of subnanosecond x‐ray diffraction

Shock waves of order 100 kbar were launched into 50‐μm‐thick single crystals of silicon (111) by irradiation with nanosecond pulses of 1.05‐μm laser light at irradiances in the region of 2×1010 W cm−2. A separate laser beam, synchronous but delayed with respect to the shock‐driving beam, and containing approximately 25 J of 0.53‐μm laser light in a pulse of 1 ns (FWHM), was focused to a tight (<100 μm) spot on a separate titanium target to produce a plasma which was a prolific source of He‐like Ti x rays. The x rays were Bragg diffracted from the rear surface of the shocked crystal and the spectrum recorded on an x‐ray streak camera. Changes in interatomic spacings in a region within several microns of the surface were thus deduced from the resultant shift in Bragg angle with a temporal resolution of 50 ps. Shock waves with compressions of order 6% were observed. We observed the crystal in a state of dynamic tension as the two rarefaction waves met. The results are in good agreement with hydrocode simulations in conjunction with x‐ray diffraction calculations.

Density and temperature profiles within laser-produced plasmas in the classical-transport regime

Using planar polystyrene targets with embedded Al tracer dots, improved x‐ray spectroscopic measurements of density and temperature profiles in laser‐produced plasmas were obtained. Time‐integrated, spatially resolved spectra of the Al tracer emission were collected at laser intensities where the energy absorption and transport is expected to be classical. The plasma density and temperature profiles were determined by comparing the observed x‐ray line intensities with collisional‐radiative equilibrium calculations. Plasma profiles were obtained for the region blowoff plasma with densities between one‐tenth and twice critical for the 1.05 μ laser excitation. Near agreement is found between the measured density and temperature profiles and 2‐D (cylindrical) hydrodynamic calculations with energy absorption via classical inverse bremsstrahlung and energy transport via classical thermal conduction.

Spot Spectroscopy: Local Spectroscopic Measurements within Laser-Produced Plasmas.

Use of a locally embedded tracer in laser-irradiated solid targets yields a localized source of diagnostic x-ray line radiation in the blowoff plasma. This technique potentially eliminates problems of chord-integration over regions of varying density and temperature in an inhomogeneous plasma, and reduces complications due to plasma opacity effects in the interpretation of spectra. Spectra obtained in an experimental test of this new technique are of a quality superior to those obtained from standard laser-produced plasmas, and should provide the best tests to date of spectroscopic models for these plasma conditions.

Flash x radiography of laser‐accelerated targets

Flash x radiography of ablatively accelerated planar foils has provided quantitative measurements and qualitative observations regarding several parameters of critical interest to direct illumination laser fusion. A 1.05‐μ, 3.3‐ns driver beam was focused onto carbon foils in a large (0.7–1‐mm diameter) spot to reduce edge effects. From images produced by a backlighting x‐ray flash, we have measured overall coupling efficiency, smoothing of laser nonuniformities, target velocity, and ablation pressure. The high velocity targets maintain a localized, high density (≳3% of solid). In contrast to other workers’ recent measurement of pressure from x‐ray imaging, our x‐radiographic results, including pressure, are in general agreement with earlier NRL studies. Our results have also provided further insights into double foil interactions and planar target preheat measurements.

Novel measurements of high‐dynamic crystal strength by picosecond x‐ray diffraction

The fracture strength of many brittle materials is known to increase with the cubed root of the strain rate. At ultrahigh strain rates, the strengths should approach the ideal fracture strength. We have made direct measurements of the dynamic tension in laser‐shocked silicon wafers at strain rates in excess of 108 s−1 by the use of picosecond x‐ray diffraction. The elastic strains observed (3.4%±0.2%) correspond to tensile stresses of the order of 70 kbar, and are comparable to the highest fracture strengths observed in static measurements.

Flash X Radiography of Laser-Accelerated Foils.

The successful compression of laser‐driven pellets to thermonuclear ignition depends on the stability and uniformity of the motion with which dense shells can be imploded. The motion of planar foils accelerated by the Pharos II laser has been studied by two‐dimensional, flash x radiography employing pinhole imaging and slitted crystal imaging. The acceleration was driven by a 3–5‐ns duration, 1.05‐μm laser focused to 3–6×1012 W/cm2 in a millimeter diameter spot, while a second laser beam of shorter duration produced the x‐ray flash for imaging purposes. The x‐ray images obtained clearly show that the planar foil targets are ablatively accelerated to velocities of 3×106 cm/s while maintaining a density above 3% of solid. The axial extent of the accelerated, high‐density material has been observed to be as small as 25% of the distance traveled. The sides of accelerated portions of the foil connect smoothly to the stationary regions removed from the laser illumination. This connection apparently isolates the...