Long-Wavelength, High-Powered Lasers for Controlled Thermonuclear Fusion

This paper discusses the possibilities of applying relatively long-wavelength lasers to the problems of obtaining controlled thermonuclear reactions. In particular, the very attractive possibilities opened up by the recent development of high-powered, high-efficiency 10.6-μm N2 -CO2 lasers are examined. These lasers can effectively heat plasmas in the density range of 1017 -1019 particles/cm3 to thermonuclear temperatures. Absorption lengths for 10-μm radiation range from a few tens of centrimetres to a few kilometres, depending on density and temperature; calculations show that the laser beam will be self-focusing and thus confined to the plasma. Such plasmas can be confined by available magnetic fields; for the lower densities, fields of 200-300 kG are required, while at the higher densities fields of 2-3 MG are needed. A number of reactor possibilities are considered. The simplest is a straight device operated in a pulse mode, similar to the β-pinch configurations considered by Ribe. However, the laser method has the advantage that the magnetic field is not used to produce and heat the plasma. The coils can thus be designed for optimum production of the field with respect to field strength, magnetic energy required, and energy dissipation in the coils. The size of such a device depends on the strength of the magnetic field that can be used and on one's ability to inhibit plasma and heat loss from the ends. For fields of 300 kG, it appears that a working reactor of roughly 500 m is possible; for higher fields shorter devices will suffice, with the length scaling like 1/B2. Calculations of the parameters for such devices will be given. Small experiments, using available pulsed hi h-field magnetics and CO2 lasers, should be able to answer many of the questions related to building such a device. They should also be able to provide hot plasma in a number of different confinement configurations for conducting important experiments. These might include wave propagation, instability, and plasma confinement studies in finite, but adjustable-β, plasmas. (author)