The use of intense laser light to bring about thermonuclear reactions in a plasma is of considerable current interest. We present detailed analytical and computational studies which show the feasibility of laser-driven fusion. The required laser technology and the presently anticipated practical difficulties are discussed in outline.

Laser-driven fusion

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.

Direct-drive inertial confinement fusion: A review

Data on the emission of energetic ions produced in laser–matter interactions have been analyzed for a wide variety of laser wavelengths, energies, and pulse lengths. Strong correlation has been found between the bulk energy per AMU for fast ions measured by charge cups and the x‐ray‐determined hot electron temperature. Five theoretical models have been used to explain this correlation. The models include (1) a steady‐state spherically symmetric fluid model with classical electron heat conduction, (2) a steady‐state spherically symmetric fluid model with flux limited electron heat conduction, (3) a simple analytic model of an isothermal rarefaction followed by a free expansion, (4) the lasnex hydrodynamics code [Comments Plasma Phys. Controlled Fusion 2, 85 (1975)], calculations employing a spherical expansion and simple initial conditions, and (5) the lasnex code with its full array of absorption, transport, and emission physics. The results obtained with these models are in good agreement with the experiments and indicate that the detailed shape of the correlation curve between mean fast ion energy and hot electron temperature is due to target surface impurities at the higher temperatures (higher laser intensities) and to the expansion of bulk target material at the lower temperatures (lower laser intensities).

Fast ions and hot electrons in the laser–plasma interaction

The initial attraction of polysilicon was the ability to deposit semiconductor layers on a wide range of substrates. This leads to the development of polysilicon gate MOS, polysilicon emitters and a range of passive devices. In the field of sensors and actuators, the strain sensors, based on piezoresistive effect, was one of the first successful applications. However, it was probably the development of surface micromachining, which received the most attention. The ability to deposit polysilicon on oxide meant that free-standing structures could be fabricated and this has lead to a wide range of successful devices. Polysilicon has a structure comprising small single-crystal grains with many orientations separated by thin grain boundaries. The size of these grains is highly dependent upon processing and therefore a wide range of electrical and mechanical properties can be achieved. This yields greater flexibility but this sensitivity to process parameters can lead to problems in obtaining a stable process. This paper will look back at the development of polysilicon, its structure, fabrication and both mechanical and electrical properties.

Polysilicon: a versatile material for microsystems

When focusing an Nd glass laser (frequency ωo) onto a cryogenic target (D2 or H2) quite intense lines with frequencies 2ωo, 3ω0/2, ω0/2 are found in the reflected light in both backward and 45° directions. The lines at 2ω0 and 3ω0/2 are broadened on the low frequency side only. The occurrence and broadening of such lines may be related to parametric excitation of waves.

Harmonic generation and parametric excitation of waves in a laser-created plasma

As lasers have an ability to deliver a large amount of energy very rapidly to matter, one can produce a plasma of thermonuclear temperature by laser bombardment of matter. We observed a neutron yield from a solid deuterium target irradiated by the beam of a glass laser, which had a power of 20 GW for 2 nsec. The theoretically estimated threshold laser power for anomalous heating owing to the parametric instability was in agreement with the experimental result. Above this threshold, an increase in the electron temperature, the appearance of a fast-ion group, and an anomaly in the reflection of light from the plasma were observed. These phenomena were closely correlated with the neutron yield. At the high-temperature region above a few hundred electron volts, the anomalous heating plays an essential role in reaching the fusion-reaction temperature.

Anomalous Heating of a Plasma by a Laser

The properties of plasmas produced by high power laser were investigated. The laser beam was irradiated on the various targets of beryllium Be, carbon C and polyethylene (CH 2 ) n of high z number. The influence of the charge number z to the heating of the plasma was discussed. The electron temperatures of plasmas from different targets were estimated by the measurement of soft X-ray. The time variations of the reflected laser light from plasma were also investigated. From the time of flight measurements with a Faraday cage and an electrostatic energy analyser, the different charged ions could be separated and ion temperatures and drift velocities were obtained from their velocity profiles. No influence of laser pulse duration to the temperature was observed in the cases of durations 2 n s , 4 n s and 10 n s . The experimental results for fast rising laser pulse were discussed and compared with those for slow rising one.

Heating of High z Plasma by Laser

Properties of Plasma Produced by High Power Laser

Ion Temperature of Laser-Produced Plasma

The neutron yield of thermonuclear fusion from a solid deuterium target irradiated by the beam of glass laser, the power of which was 20 GW at 2 ns, was observed. The anomalous heating was introduced by the nonlinear parametric interaction of laser radiation and plasma.

M. Waki

Papers

Anomalous Heating of a Plasma by a Laser

Heating of High z Plasma by Laser

Properties of Plasma Produced by High Power Laser

Ion Temperature of Laser-Produced Plasma

Nonlinear interaction of laser radiation and plasma