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L3 UWVRENCE UVERMORE tABORATORY Unc*rstiyctCamne/Uwnxre.C3ifom>a

CGWARISCN OF IASER-HEAIED FUSICW PLASMfc PROPOSALS

J. W. Shearer

March 10, 1973

-NOTICE- This report was prepared as an account of work sponsored by the United Slates Government. Neither the United States nor the United Stites Atomic Energy Commission, nor any of their employees, nor any of thek contractors, subcontractors, or their employees, makes eny warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, com­ pleteness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights.

To be submitted to the 6th European Conference on Controlled Fusion and Plasm Physics, to be held an Moscow (USSR) July 30 - August 3, 1973

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CCMPARISON OF -HEATED FUSION PLASMA PROPOSALS * J. W. Shearer In recent years, high power have become an important potential tool for manipulating fusion plasmas. Several fusion plasma proposals invol­ ving laser radiation are briefly reviewed here.

The classical collisional absorption mean free path La for laser light in a hydrogen plasma can be written:

where T. is the electron temperature (eV), X, is the laser light wavelength e u in microns, N is the electron density, and N is the cutoff density; 2 N - »/rX where r is the classical electron radius. The condition for propagation of light is N < N ; therefore, the optimum value of density for strong absorption is near, but not greater than, N . For neodyniun lasers,

19 \ - 1.06u, and Nc - 10 ; for carbon dioxide lasers, A « 10.6)1, and Nc - 10 . These plasmas are dense compared to most fusion plasna proposals; the

7 corresponding pressures (P • 2NkT, assuming Te - T-), are "- 3 x 10 atm and

•* 3 x 10 atm, respectively. Such pressures cannot be contained in stationary apparatus; this difficulty aust be circumvented. The laser-pellet proposal uses the inertia of the reacting plasma itself as containment. The plasma is proposed to be formed at extremely high densities 27 (fie " 10 ) by a symmetrical implosion driven by the absorption of laser energy at the surface of a tiny solid - (BT) sphere suspended in a

vacuum chamber. The outer layers of the sphere are ablated,2 forming a high teaperature plume which acts like a rocket to push the inner layers together. It has been shown that the Lawson criterion (for DT at a temperature of

10 keV), NT £ 5 x 10 3a (where o is the desired energy multiplication) can

•Work performed under the auspices of the U.S. Atomic Energy Commission. -2- be satisfied if the laser energy input E, to the pellet is, approximately: Ej^ = 10 o /en joules where e is the overall efficiency of conversion of laser light energy into the final compressed sphere, and where n is the ratio

Ne/Ng, where Ns is the density of solid heavy hydrogen. The thermonuclear reactions occur before the sphere can come apart; the vacuim chamber is large enough to contain the resultant 'feicroexplosion". The time scale for the nuclear reaction would be 10 - 10 sec; the laser pulse width would be

10"10 - 10"9 sec.

A nine-beam laser system is being used at the Lebedev Institute for preliminary research into the unsolved problems of this pellet proposal; single beam lasers are in use at many other laboratories. There are, of course, no problems of magnetic field containment; however, several new effects such as parametric instabilities, high energy electrons, and self-focusing are under study. At present these appear to be serious problems, but the research is just beginning, and the laser pellet proposal has the merit of being very different'from other fusion research.

A more conventional proposal is to heat a plasma in a magnetic field with a long wavelength laser at low enough densities to permit plasma containment 8 17 with stationary apparatus. For example, at a density of N= 3 x 10 and 4 a temperature of 10 eV, the collisional absorption mean free path (equa. 1) is 10 cm, which is about the length of a linear theta pinch that can satisfy the Lawson criterion, before the plasma streams out the ends. Such a 1 km long mac' ine is large, but may not be unreasonable for a plant. The corresponding pressure is i» 5,000 atmospheres, which may be achievable in a stationary pulsed system. The high temperature and pressure would last about 1 millisecond. Detailed estimates of the required laser energy indicate o that a few mega joules would be needed; however, the laser pulse length can -3- be * 10" - 10" sec instead of the much shorter pulses required in the pellet approach. Megagauss magnetic fields have been proposed to contain laser-heated plasmas. ' However, these proposals have the disadvantage that the Magnetic field pressure is too great to be contained by stationary apparatus. One possible laser-augmented, pinch is considered in detail -- laser heating to temperature of about 1 keV roller- • :-r pinch compression and confinement to Teach the thermonuclear rea. -: - '"Vaierature of 10 fceV. In this example the absorption mean free path for laser light (equa. 1) at T - 1 keV 18 and N • 10 is 300 cm, implying that much shorter machines are possible. Axial losses would be reduced by using end plugs — a procedure that is especially effective at high densities tfiere thermal conduction losses are less serious. The laser-heated plasma bubble would be compressed to N * 10 - 10 by a cold, dense plasma sheath driven by a large magnetic pinch. This cold sheath would be thermally insulated from the hot plasma by the magnetic field. The major external energy source would be a conventional capacitor bank or large inductance to supply the pinch current; the energy of the laser should be rather modest in comparison. The unique capability provided by the laser is its ability to provide localized heating at a prescribed position and time within a plasma. No other heating mechanism can presently provide the required temperature profile for this proposed reactor. Further evaluation is in progress. -4- REFERENCES 1. John Nuckolls, Lowell Wood, Albert Thiesson, and George Zi—rrunn, Nature 239, 139-142 (1972). 2. O.N. Krokhin, "High. Temperature and Plasma Phenomena Induced by Laser Radiation" in Physics of High Energy Density. P. Caldirola and H. Knoepfel, Eds. (Academic Press, New York, 1971), pp. 278-305. 3. R.E. Kidder, Some Aspects of Controlled Fusion by Use of Lasers, Lawrence Livermore Laboratory Report UCRL-73500 (1971), to be published in tile Proceedings of the Esfahan Syaposiun on Fundamental and Applied Laser Physics, 1971 (J. Wiley 5 Sons, New York).

4. N.G. Basov, O.N. Krokhin, G.V. Sklizkov, S.I. Fedotov, and A.S. Shikanov, ZhETF 62. 203 (1972) [English translation Spy. Phys. JETP 35, 109 (1972)].

5. Marshall N. Rosenbluth, Phys. Rev. Lett. 29. 565 (1972). 6. R.E. Kidder, J.W. Zink, Nuclear Fusion 12. 325 (1972). 7. J.W. Shearer and J.L. Eddleaan, Laser Light Forces and Self-Focusing in Fully Ionized Plas»as. Lawrence Livermore Laboratory Report UCRL-73969 (1972), submitted to Physics of Fluids. 8. John M. Dawson, Abraham Hertzberg, George C. Vlases, Harlow G. Ahlstrom, Loren C. Steinhauer, Ray E. Kidder, and N.L. Kruer, "Controlled Fusion Using Long Wavelength Laser Heating with Magnetic Confinement", to be published in the Proceedings of the Esfahan Symposium on Fundamental and Applied Laser Physics, 1971 (J. Wiley S Sons, New York).

9. P.P. Pashinin and A.M. Prokhorov, ZhEIF 62, 189 (1972) [English translation Sov. Phys. JETP 35, 101 (1972)]. 10. J.L. Bobin, D. Coloabant, G. Tonon, Nuclear Fusion 12, 445 (1972).