fusion energy foundation

cesses take place at plasma densities below that of n, r , andthese processes are nonlinear and complex.In the case of the soft X-ray radiation, the primary formof the initial energy output from a fission atom bomb, thecorresponding electron density that gives a plasma frequencyin the same range would be 10" to 10 25 per cubiccentimeter. This is very close to the atomic number densitywe find with most solid materials. Therefore, for thte caseof soft X-rays it could be expected that the radiationwould readily penetrate the exhaust plasma to a pointvery close to the solid surface. This produces a veryefficient ablative implosion.On the other hand, if much shorter wavelength, "hard"X-rays were used to drive the implosion, the electromagneticradiation would have a frequency in excess 6f thatpossessed by a solidlike-density plasma, and the X-rayswould readily penetrate toward the interior of the target.This would heat the inner fuel before compression, preventingthe achievement of efficient isentropic compression.This effect is called preheating.Figure 2 is a general schematic giving the various regionsof radiation-plasma interaction that would be encounteredfor radiation incident on a flat slab target. Thesection on the right represents the solid surface of thetarget. The middle section is made up of a high-densityablation plasma ranging from critical density to soliddensity. The radiation incident from the left does notpenetrate this region. On the left is a region mad^ up ofablating plasma with densities less than the critical density.Why Short Wavelengths?The original specification U.S. fusion researchers atLawrence Livermore Laboratory made in 1963 to achievelaser fusion target gains greater than 1 called for 100,000joules of laser light with a wavelength of 0.69 micron andan intensity of 500 trillion watts per square centimeter.This meant that the laser would have to attain a poweroutput of up to several hundred trillion watts with a pulselength of about 1 billionth of a second. 4 This 1963 observationis very close to present-day estimates that areinformed by more than a decade of laser-matter experiments.The crucial question identified at this early point in thedevelopment of the U.S. laser fusion program was whateffect the laser light wavelength would have on both theefficiency and effectiveness of laser-matter interaction fordriving ablative implosions. Pragmatically, one would callfor duplicating the existing, successful inertial system, theH-bomb, by choosing wavelengths that would replicatethose of soft X-rays, about 0.01 micron.From a theoretical standpoint, however, use of shortwavelengthradiation in the driver also makes sense. Asnoted previously, soft X-rays have the ideal wavelengthfor penetrating the ablating plasma and depositing thedriver energy where it is needed at the surface of thetarget. Furthermore, the higher plasma densities at whichthe soft X-rays are absorbed are less conducive to nonlinearinteractions. This is because plasmas at higher densitiesare more "collisional," exhibiting far fewer of the collec-Figure 2LASER IRRADIATION OF SLAB TARGETThis schematic of a slab target being irradiated withlaser light shows three regions: first, an outer layerwith density less than or equal to the critical densitythat is directly heated by the laser light—the underdenseregion; a high-density region that is heatedindirectly by energy conduction; and the solid densityregion.tive interactions that dominate less dense plasmas. As aresult, short-wavelength radiation produces the most efficientand effective ablative-driven implosion.Even if soft X-ray lasers were a technological reality (andthey are not at the present time), handling electromagneticradiation below 0.2 to 0.1 micron is quite difficult,and all of the advantages of coherent laser light, such asease of transport and focusing of the light beam by usingmirrors, lenses, and other optical devices, are lost. Opticaltechnology gives a lower limit between 0.1 and 0.2 micronfor the wavelength of the driver radiation; the kryptonfluoride laser has a wavelength of 0.25 micron; and thatof the existing high-power, neodymium-doped glass lasersis 1.06 microns.As theoretically predicted, recent experiments withshort-wavelength laser light have indicated that absorptionefficiency and quality greatly improve with decreasingwavelength. 5 The original laser fusion experiments withlaser light of only 1.06 microns have shown a tendency52 FUSION September 1980