Study of Laser-Hole Boring into Overdense Plasmas

Reflectivity and spectral shift from laser plasmas generated by high-contrast, high-intensity KrF laser pulses

The energy and spectrum of the reflected 248 nm radiation are studied from solid targets up to 1.15 × 1018 W cm-2 intensity. The experiments used the 700 fs directly amplified pulses of the KrF system which was cleaned from prepulses with the new Fourier-filtering method providing 12 orders of magnitude temporal contrast. Increasing the intensity from 1015 W cm-2 results in increasing absorption up to more than 90% above 1018 W cm-2. This is accompanied by increasing x-ray conversion exhibiting a less steep power law dependence for low-Z matter than for gold. Strong blue shift of the reflected radiation from the backward propagating plasma was observed. It is shown that in the case of KrF laser pulses of highest contrast, vacuum heating can be one of the dominant absorption mechanisms. This article is part of a discussion meeting issue 'Prospects for high gain inertial fusion energy (part 1)'.

Plasma dynamics near critical density inferred from direct measurements of laser hole boring

We have used multiframe picosecond optical interferometry to make direct measurements of the hole boring velocity, v_{HB}, of the density cavity pushed forward by a train of CO_{2} laser pulses in a near critical density helium plasma. As the pulse train intensity rises, the increasing radiation pressure of each pulse pushes the density cavity forward and the plasma electrons are strongly heated. After the peak laser intensity, the plasma pressure exerted by the heated electrons strongly impedes the hole boring process and the v_{HB} falls rapidly as the laser pulse intensity falls at the back of the laser pulse train. A heuristic theory is presented that allows the estimation of the plasma electron temperature from the measurements of the hole boring velocity. The measured values of v_{HB}, and the estimated values of the heated electron temperature as a function of laser intensity are in reasonable agreement with those obtained from two-dimensional numerical simulations.

Dynamics of relativistic laser-plasma interaction on solid targets

A novel time-resolved diagnostic is used to record the critical surface motion during picosecond-scale relativistic laser interaction with a solid target. Single-shot measurements of the specular light show a redshift decreasing with time during the interaction, corresponding to a slowing-down of the hole boring process into overdense plasma. On-shot full characterization of the laser pulse enables simulations of the experiment without any free parameters. Two-dimensional particle-in-cell simulations yield redshifts that agree with the data, and support a simple explanation of the slowing-down of the critical surface based on momentum conservation between ions and reflected laser light.

Single-shot microscopic electron imaging of intense femtosecond laser-produced plasmas

A simple technique for single-shot microscopic electron imaging was demonstrated for the study of intense femtosecond laser-produced plasmas. Passed through a permanent magnet lens designed for 110-keV electrons, hot electrons emitted from the plasma produced by a single laser pulse of 0.8 mJ with intensity of 3 × 10(16) W/cm(2) were successfully imaged. Analyzing this image, we found that electrons were emitted from an area of 3 μm in diameter. At higher laser intensity of 10(18) W/cm(2), distinct structures were observed in and near the focal spot of the laser; that is, the electrons were emitted from several separate spots. These results show that laser-plasma electron imaging is promising for studying the interactions of femtosecond lasers with high-density plasmas.

Absorption of short laser pulses on solid targets in the ultrarelativistic regime

We report the first direct measurements of total absorption of short laser pulses on solid targets in the ultrarelativistic regime. The data show an enhanced absorption at intensities above 10(20) W/cm(2), reaching 60% for near-normal incidence and 80%-90% for 45 degrees incidence. Two-dimensional particle-in-cell simulations demonstrate that such high absorption is consistent with both interaction with preplasma and hole boring by the intense laser pulse. A large redshift in the second harmonic indicates a surface recession velocity of 0.035c.

Enhancement of energetic electrons and protons by cone guiding of laser light

Energetic electrons and protons are observed when a target consisting of a reentrant cone with a disk at the tip is irradiated by a petawatt (PW) laser at an intensity of approximately 10(19) W cm(-2). The angular distribution of the electrons and protons, dependent on the open angle of the reentrant cone, is found to differ from that in the case when a target with planar geometry is used. Two jet beams are observed, in directions parallel to the cone axis and normal to the cone-shaped wall. The number and cutoff energies of the generated protons are also related to the open angle of the cone. The efficiency of the generation of energetic electrons from the cone target is 2-3 times higher than that from a simple plane target. These results indicate a guiding of the PW laser beam in the cone geometry.

Experimental measurements of deep directional columnar heating by laser-generated relativistic electrons at near-solid density

In our experiments, we irradiated solid CH targets with a 400 J, 5 ps, 3 x 10(19) W/cm(2) laser, and we used x-ray imaging and spectroscopic diagnostics to monitor the keV x-ray emission from thin Al or Au tracer layers buried within the targets. The experiments were designed to quantify the spatial distribution of the thermal electron temperature and density as a function of buried layer depth; these data provide insights into the behavior of relativistic electron currents which flow within the solid target and are directly and indirectly responsible for the heating. We measured approximately 200-350 eV temperatures and near-solid densities at depths ranging from 5 to 100 microm beneath the target surface. Time-resolved x-ray spectra from Al tracers indicate that the tracers emit thermal x rays and cool slowly compared to the time scale of the laser pulse. Most intriguingly, we consistently observe annular x-ray images in all buried tracer-layer experiments, and these data show that the temperature distribution is columnar, with enhanced heating along the edges of the column. The ring diameters are much greater than the laser focal spot diameter and do not vary significantly with the depth of the tracer layer for depths greater than 30 microm. The local temperatures are 200-350 eV for all tracer depths. We discuss recent simulations of the evolution of electron currents deep within solid targets irradiated by ultra-high-intensity lasers, and we discuss how modeling and analytical results suggest that the annular patterns we observe may be related to locally strong growth of the Weibel instability. We also suggest avenues for future research in order to further illuminate the complex physics of relativistic electron transport and energy deposition inside ultra-high-intensity laser-irradiated solid targets.

Direct observation of the ponderomotive force effects in short-scale-length laser plasmas by frequency-domain interferometry

We report on single-shot frequency-domain interferometric measurements showing space- and time-resolved ponderomotive electron density profile steepening of a short-scale-length ultraintense laser-produced plasma. The density gradient scale length is varied by applying a time-delayed laser prepulse. The measured absolute position of the critical density surface is found to be in agreement with one-dimensional hydrodynamic simulations for the range of scale lengths studied.

Parametric instabilities of intense lasers from interaction with relativistic hot plasmas

The stimulated Raman scattering (SRS) and relativistic modulational instability (RMI) of intense lasers in relativistic hot plasmas are studied, using the waterbag distribution in momentum space. Two cases are considered: a single hot electron distribution and a cold plasma attached with a hot electron tail. It is shown that both high temperatures and hot electron tails can significantly reduce the instability growth rates, shift the unstable regions in the wave-vector space, and change the Raman scattering frequency spectra from the cold plasma theory. In particular, at low-light intensities, there exists a cutoff thermal velocity beyond which SRS is completely suppressed. At high-light intensities, no clear cutoff thermal velocity is found owing to the merging of SRS with RMI.

Laser-hole boring into overdense plasmas measured with soft X-Ray laser probing

A laser self-focused channel formation into overdense plasmas was observed using a soft x-ray laser probe system with a grid image refractometry (GIR) technique. 1.053 &mgr;m laser light with a 100 ps pulse duration was focused onto a preformed plasma at an intensity of 2x10(17) W/cm (2). Cross sections of the channel were obtained which show a 30 &mgr;m diameter in overdense plasmas. The channel width in the overdense region was kept narrow as a result of self-focusing. Conically diverging density ridges were also observed along the channel, indicating a Mach cone created by a shock wave due to the supersonic propagation of the channel front.

Long-scale jet formation with specularly reflected light in ultraintense laser-plasma interactions

Long-scale jetlike x-ray emission was observed in a 100-TW laser-plasma interaction. The jet was well collimated with a divergence of 30-40 mrad and continued from the target surface into underdense regions for a distance over 4 mm in the specular direction of the laser light. A two-dimensional particle-in-cell simulation shows an electron acceleration with the specularly reflected laser light and collimation of the electron stream by a self-generated magnetic field, resulting in the electron jet to the direction of the specularly reflected light.

Performance comparison of self-focusing with 1053- and 351-nm laser pulses

Hole boring characteristics of laser beams are studied using two different laser wavelengths in preformed plasmas with overdense regions. We have shown that a whole beam self-focusing is created in plasma with a considerable density scale length using a 1 microm wavelength laser. The whole beam self-focusing of this type could be used for guiding the ultrahigh intense laser pulse to a highly compressed core for studying the feasibility of a fast ignitor. There is a clear difference in the hole-boring characteristics between two laser wavelengths at 1053 and 351 nm, both in the experiment and the simulation. Using the third-harmonic laser, a whole beam self-focusing is never created. The 351-nm laser beam broke up into filaments resulting in plasma jets observed in our interferogram.