R. W. Short

Improved laser‐beam uniformity using the angular dispersion of frequency‐modulated light

A new technique is presented for obtaining highly smooth focused laser beams. This approach is consistent with the constraints on frequency tripling the light, and it will not produce any significant high‐intensity spikes within the laser chain, making the technique attractive for the high‐power glass lasers used in current fusion experiments. Smoothing is obtained by imposing a frequency‐modulated bandwidth on the laser beam using an electro‐optic crystal. A pair of gratings is used to disperse the frequencies across the beam, without distorting the temporal pulse shape. The beam is broken up into beamlets, using a phase plate, such that the beamlet diffraction‐limited focal spot is the size of the target. The time‐averaged interference between beamlets is greatly reduced because of the frequency differences between the beamlets, and the result is a relatively smooth diffraction‐limited intensity pattern on target.

Direct‐drive laser‐fusion experiments with the OMEGA, 60‐beam, >40 kJ, ultraviolet laser system

OMEGA, a 60‐beam, 351 nm, Nd:glass laser with an on‐target energy capability of more than 40 kJ, is a flexible facility that can be used for both direct‐ and indirect‐drive targets and is designed to ultimately achieve irradiation uniformity of 1% on direct‐drive capsules with shaped laser pulses (dynamic range ≳400:1). The OMEGA program for the next five years includes plasma physics experiments to investigate laser–matter interaction physics at temperatures, densities, and scale lengths approaching those of direct‐drive capsules designed for the 1.8 MJ National Ignition Facility (NIF); experiments to characterize and mitigate the deleterious effects of hydrodynamic instabilities; and implosion experiments with capsules that are hydrodynamically equivalent to high‐gain, direct‐drive capsules. Details are presented of the OMEGA direct‐drive experimental program and initial data from direct‐drive implosion experiments that have achieved the highest thermonuclear yield (1014 DT neutrons) and yield efficienc...

On the inhomogeneous two‐plasmon instability

The two‐plasmon instability in warm inhomogeneous plasma for a normally incident pump is considered. The complex eigenfrequencies of the absolute instability are obtained by reducing the linearized fluid equations to a Schrodinger equation in wavenumber space. These eigenvalues are obtained in several ways. One is by combining a perturbation expansion in powers of the reciprocal scale length with WKB theory. The resulting algebraic equations are solved by three analytical approximations and by direct numerical solution. A second way is by analysis of the Schrodinger equation using an interactive WKB computer code. A third way is by the use of a shooting code. These methods are all used and compared for threshold curves and growth rates above threshold. Some eigenfunction forms are also obtained. The threshold is near (v0/ve)2k0 L =3, and varies weakly with β≂v4e/v20c2, rising from near 2 to about 4 over six decades of variation of β. The corresponding critical value of (ky/k0)2 is near 0.2/β over this ran...

Direct-drive inertial confinement fusion: A review

The direct-drive, laser-based approach to inertial confinement fusion (ICF) is reviewed from its inception following the demonstration of the first laser to its implementation on the present generation of high-power lasers. The review focuses on the evolution of scientific understanding gained from target-physics experiments in many areas, identifying problems that were demonstrated and the solutions implemented. The review starts with the basic understanding of laser–plasma interactions that was obtained before the declassification of laser-induced compression in the early 1970s and continues with the compression experiments using infrared lasers in the late 1970s that produced thermonuclear neutrons. The problem of suprathermal electrons and the target preheat that they caused, associated with the infrared laser wavelength, led to lasers being built after 1980 to operate at shorter wavelengths, especially 0.35 μm—the third harmonic of the Nd:glass laser—and 0.248 μm (the KrF gas laser). The main physics areas relevant to direct drive are reviewed. The primary absorption mechanism at short wavelengths is classical inverse bremsstrahlung. Nonuniformities imprinted on the target by laser irradiation have been addressed by the development of a number of beam-smoothing techniques and imprint-mitigation strategies. The effects of hydrodynamic instabilities are mitigated by a combination of imprint reduction and target designs that minimize the instability growth rates. Several coronal plasma physics processes are reviewed. The two-plasmon–decay instability, stimulated Brillouin scattering (together with cross-beam energy transfer), and (possibly) stimulated Raman scattering are identified as potential concerns, placing constraints on the laser intensities used in target designs, while other processes (self-focusing and filamentation, the parametric decay instability, and magnetic fields), once considered important, are now of lesser concern for mainline direct-drive target concepts. Filamentation is largely suppressed by beam smoothing. Thermal transport modeling, important to the interpretation of experiments and to target design, has been found to be nonlocal in nature. Advances in shock timing and equation-of-state measurements relevant to direct-drive ICF are reported. Room-temperature implosions have provided an increased understanding of the importance of stability and uniformity. The evolution of cryogenic implosion capabilities, leading to an extensive series carried out on the 60-beam OMEGA laser [Boehly et al., Opt. Commun. 133, 495 (1997)], is reviewed together with major advances in cryogenic target formation. A polar-drive concept has been developed that will enable direct-drive–ignition experiments to be performed on the National Ignition Facility [Haynam et al., Appl. Opt. 46(16), 3276 (2007)]. The advantages offered by the alternative approaches of fast ignition and shock ignition and the issues associated with these concepts are described. The lessons learned from target-physics and implosion experiments are taken into account in ignition and high-gain target designs for laser wavelengths of 1/3 μm and 1/4 μm. Substantial advances in direct-drive inertial fusion reactor concepts are reviewed. Overall, the progress in scientific understanding over the past five decades has been enormous, to the point that inertial fusion energy using direct drive shows significant promise as a future environmentally attractive energy source.

A practical nonlocal model for electron heat transport in laser plasmas

A brief review of nonlocal heat‐flow models is presented. Numerical difficulties associated with their implementation, as recently demonstrated by Prasad and Kershaw [Phys. Fluids B 1, 2430 (1989)], are discussed and a simple solution is proposed. A new nonlocal heat‐flow formula is developed, based on numerical simulations of the decay of linearized thermal waves, using the electron Fokker–Planck code spark. The formula is tested by modeling the full implosion of a CH shell driven by 351 nm laser irradiation. Results are shown to be in good agreement with spark simulations.A brief review of nonlocal heat‐flow models is presented. Numerical difficulties associated with their implementation, as recently demonstrated by Prasad and Kershaw [Phys. Fluids B 1, 2430 (1989)], are discussed and a simple solution is proposed. A new nonlocal heat‐flow formula is developed, based on numerical simulations of the decay of linearized thermal waves, using the electron Fokker–Planck code spark. The formula is tested by modeling the full implosion of a CH shell driven by 351 nm laser irradiation. Results are shown to be in good agreement with spark simulations.

Improving the hot-spot pressure and demonstrating ignition hydrodynamic equivalence in cryogenic deuterium–tritium implosions on OMEGAa)

Reaching ignition in direct-drive (DD) inertial confinement fusion implosions requires achieving central pressures in excess of 100 Gbar. The OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] is used to study the physics of implosions that are hydrodynamically equivalent to the ignition designs on the National Ignition Facility (NIF) [J. A. Paisner et al., Laser Focus World 30, 75 (1994)]. It is shown that the highest hot-spot pressures (up to 40 Gbar) are achieved in target designs with a fuel adiabat of α ≃ 4, an implosion velocity of 3.8 × 107 cm/s, and a laser intensity of ∼1015 W/cm2. These moderate-adiabat implosions are well understood using two-dimensional hydrocode simulations. The performance of lower-adiabat implosions is significantly degraded relative to code predictions, a common feature between DD implosions on OMEGA and indirect-drive cryogenic implosions on the NIF. Simplified theoretical models are developed to gain physical understanding of the implosion dynamics th...

Convective stimulated Raman scattering instability in UV laser plasmas

Time‐resolved stimulated Raman scattering (SRS) spectra from UV laser‐produced plasmas are reported. In analyzing these spectra the theory for SRS in inhomogeneous plasma is extended. This theory is applied to obtain temporally resolved electron temperatures in the plasma corona. Typical coronal temperatures for plastic targets irradiated at 1015 W/cm2 range up to 1.8 keV. A comparison is made with the predictions of hydrodynamic simulations.

Nonlocal heat transport effects on the filamentation of light in plasmas

The nonlocal nature of electron heat transport in hot coronal plasmas is investigated. Its effect on the linear theory of laser filamentation is reviewed. The nonlinear evolution of filaments is modeled with the two‐dimensional Fokker–Planck code spark, for the conditions of recent filamentation experiments. Simulation results agree with experimental data and imply the dominance of thermal over ponderomotive filamentation. The increased importance of the thermal filamentation mechanism is attributed to heat flux inhibition arising from nonlocal electron transport. The potential implications of filamentation for inertial‐confinement‐fusion target designs are discussed.

Diagnostic value of odd-integer half-harmonic emission from laser-produced plasmas

The diagostic value of odd‐integer half‐harmonic emission from laser‐produced plasmas is evaluated in the light of recent spectral measurements on UV irradiation experiments. It has been found that under certain conditions a sharp, slightly red‐shifted feature is observed in the ω/2 spectra. This feature has been identified with a particular mode of the 2ωp decay instability for which one of the plasmon wave vectors vanishes. This feature is eminently well suited for coronal electron temperature measurements. Another half‐harmonic (blue‐shifted) feature is more easily observed and may serve as a secondary—though less accurate—temperature diagnostic. In contrast, the spectral splitting of the (3)/(2) harmonic emission proved to be ill suited for temperature diagnostics because of its sensitivity to irradiation and observation geometry. Either ω/2 or 3ω/2 emission is, however, a good qualitative indicator for the presence of the 2ωp decay instability although quantitative inferences on the level of the 2ωp ...

Two-plasmon-decay instability in direct-drive inertial confinement fusion experiments

The two-plasmon-decay (TPD) instability in direct-drive irradiation OMEGA [J. M. Soures, R. L. McCrory, C. P. Verdon, et al., Phys. Plasmas 3, 2108 (1996)] experiments is seen in the half-integer harmonic emission. Experimental time-resolved ω/2 and 3ω/2 spectra indicate that the linear theory for the absolute TPD instability reasonably predicts TPD thresholds. The plasma wave spectra do not, however, agree at all with the predictions of the linear theory. This is most likely a consequence of the nonlinear evolution of this instability once it is above threshold. This is demonstrated with spectral data obtained from spherical implosion experiments as well as planar target experiments. In the latter, Thomson scattering shows the importance of the Landau cutoff. For the TPD instability, the Landau cutoff is found to be respected in all spherical and planar target experiments. In addition, the maximum plasma wave amplitudes appear to occur near the Landau cutoff.