G. T. Schappert

The interaction of a high irradiance, subpicosecond laser pulse with aluminum: The effects of the prepulse on x‐ray production

The conversion efficiency into kilovolt line radiation for 248‐nm light at 1017 W/cm2 on an aluminum target is measured. The x‐ray yield is found to increase with the scale length of the target plasma. The interaction is modeled as resonance absorption, and the plasma scale length is determined from the prelase energy and irradiance.

X‐ray generation by high irradiance subpicosecond lasers

We have studied the interaction of 290‐fs, 308‐nm laser pulses with aluminum targets at irradiances exceeding 5×1018 W/cm2. The x‐ray spectrum is dominated by the H‐ and He‐like lines from aluminum, with the brightest lines radiating 0.8% of the incident laser light energy. This fraction is close to that measured at 50 times less irradiance, but occurs at a slightly higher ionization stage. The x rays are emitted from a region of subcritical electron density at 3–6×1021 W cm−3. The radiance of the 1.73‐keV Lα line is measured to be 4×1012 W/cm2/sr.

Prepulse damage to targets and alignment verification

We measured the damage threshold of 10.6‐μm light incident on glass microballoon laser fusion targets. The threshold is several dozen microjoules, depending on target size and laser pulse width, and the damage mechanism appears to be thermal heating and rupture. Perforating glass microballoons proves to be a useful alignment verification technique.

Long term instability growth of radiatively driven thin planar shells

The Rayleigh–Taylor instability of radiatively driven thin copper foils is studied under pure ablation, as well as with beryllium buffers to provide additional pressure drive, in support of the target design for Inertial Confinement Fusion. Modeling was done with the RAGE adaptive mesh refinement code [R. M. Baltrusaitis, M. L. Gittings, R. P. Weaver, R. F. Benjamin, and J. M. Budzinski, Phys. Fluids 8, 2471 (1996)] of experiments done on the OMEGA [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1997)] laser. The copper foils were typically 11.5 μm thick with 0.45 μm amplitude and 45 μm wavelength cosine surface perturbations. The beryllium layer was 5 μm thick. The drive was a “PS26”-like [J. D. Lindl, Phys. Plasmas 2, 3933 (1995)] laser pulse delivering peak 160–185 eV radiation temperatures. Good agreement between experiment and simulation has been obtained out to 4.5 ns. Mechanisms for late time agreement are discussed.

Thin foil x-ray converters pumped by subpicosecond excimer lasers

We have measured the conversion efficiency, as well as the spectral and temporal x‐ray output of H‐ and He‐like carbon line emission, from a thin foil of polypropylene that has been irradiated with subpicosecond KrF laser pulses. The effect of amplified spontaneous emission prepulse on the x‐ray conversion process, as well as the application of these x‐ray ‘‘flashlamps’’ toward pumping soft x‐ray lasers, is discussed.

X-ray streak camera diagnostics of picosecond laser-plasma interactions

An x‐ray streak camera is used to diagnose a laser‐produced Al plasma with time resolution of ∼10 ps. A streak record of filtered emission and a time‐integrated transmission grating spectrum reveal that the plasma radiation is dominated by emission from He‐ and H‐like resonance lines.

Short‐pulse, high‐intensity lasers at Los Alamos

We describe two laser systems for high intensity laser interaction experiments: The first is a terawatt system based on amplification of femtosecond pulses in XeCl which yields 250 mJ in 275 fs and routinely produces intensities on target in excess of 1018 W/cm2. The second system is based on chirped pulse amplification of 100‐fs pulses in Ti:sapphire.

X‐ray production with subpicosecond laser pulses

The interaction of intense, subpicosecond laser pulses with solid targets produces intense picosecond x‐ray pulses. With focused laser pulses of several 1018 W/cm2, He‐like and H‐like line radiation from targets such as aluminum and silicon has been produced. The energy conversion efficiency from the laser pulse energy to the 1–2 keV line x‐rays is nearly one percent. The duration of the line x‐ray radiation is of the order of ten picoseconds, although this may be an upper estimate because of the temporal resolution of the x‐ray streak camera. The spatial extent of the x‐ray source region is only slightly larger than the laser focal spot, or about 10 μm in diameter. Given these characteristics, such x‐ray sources emit an intensity of nearly 1014 W/cm2. Experiments and modeling which led to the above conclusions will be discussed.

Atomic processes in plasmas under ultra‐intense laser irradiation

Lasers delivering subpicosecond pulses with energies of a fraction of a Joule have made it possible to generate irradiance levels approaching 1020 W/cm2. We presently operate two such systems, a KrF based excimer laser capable of producing a few 1017 W/cm2 at 248 nm with a repetition rate of 3–5 Hz and a XeCl based excimer laser capable of producing mid 1019 W/cm2 at 308 nm and 1 Hz. We will discuss some experimental results and the theory and modeling of the interaction of such intense laser pulses with aluminum. Because of a small ASE prepulse the high intensity interaction is not at the solid surface but rather at the ne=2×1022 cm−3 (KrF) laser critical density of the blowoff plasma generated by the ASE. The transient behavior of the plasma following the energy deposition by the intense subpicosecond pulse can be viewed as the energy‐impulse response of the plasma. Experimental results and modeling of the x‐ray emission from this plasma are presented.

Rayleigh–Taylor spike evaporation

Laser-based experiments have shown that Rayleigh–Taylor (RT) growth in thin, perturbed copper foils leads to a phase dominated by narrow spikes between thin bubbles. These experiments were well modeled and diagnosed until this “ spike” phase, but not into this spike phase. Experiments were designed, modeled, and performed on the OMEGA laser [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1997)] to study the late-time spike phase. To simulate the conditions and evolution of late time RT, a copper target was fabricated consisting of a series of thin ridges (spikes in cross section) 150 μm apart on a thin flat copper backing. The target was placed on the side of a scale-1.2 hohlraum with the ridges pointing into the hohlraum, which was heated to 190 eV. Side-on radiography imaged the evolution of the ridges and flat copper backing into the typical RT bubble and spike structure including the “ mushroom-like feet” on the tips of the spikes. RAGE computer models [R. M. Baltrusaitis, M. ...