PROGRESS TOWARD IGNITION AND BURN PROPAGATION IN INERTIAL CONFINEMENT FUSION To achieve efficient inertial confinement fusion one must produce a small hot spot within the imploding target from which thermonuclear burn can ignite. John D. Lindl, Robert L McCrory and E. Michael Campbell For the past four decades, scientists throughout the world to be excessive and recommended that DOE review them have pursued the dream of controlled thermonuclear and schedule further declassification of target physics. fusion. The attraction of this goal is the enormous energy DOE is continuing to review its classification policy. that is potentially available in fusion fuels and the view of For D-T fuel, both the magnetic and inertial ap- fusion as a safe, clean energy source. The fusion reaction proaches require a fuel temperature in excess of 100 with the highest cross section uses the deuterium and million K and a fuel particle density n and confinement tritium isotopes of hydrogen, and D-T would be the fuel of time r such that TIT = 1014-1015 sec/cm3. Magnetic con- choice for the first generation of fusion reactors. (See the finement fusion operates in a regime with r ~ l sec and article by J. Geoffrey Cordey, Robert J. Goldston and n=:1014 cm"3. For magnetic confinement fusion, the Ronald R. Parker, January, page 22.) density is limited by the maximum magnetic field that can Development of an economically viable fusion reactor be generated, which is determined by the strength of the would literally give us the energy equivalent of oceans of material of the confinement vessels. Inertial confinement oil. Because seawater contains about 40 g of deuterium fusion relies on the inertia of an imploding target to and 0.1 g of lithium per tonne, every barrel of seawater provide confinement.1 Confinement times are less than contains the energy equivalent of almost 30 barrels of oil 10 10 sec, and particle densities in the fuel are typically in deuterium fuel and about one-fifth of a barrel of oil in greater than 1025 cm 3. D-T fuel (where tritium is obtained from neutron reac- tions on lithium). A volume of seawater equal to the top Implosion ond burn of ICF forgets meter of the Earth's oceans would yield enough fuel to High-gain ICF targets have features similar to those supply D-T fusion reactors for thousands of years of shown in figure 1. These capsules consist of a spherical electricity production at today's rate of usage. shell filled with low-density (S1.0 mg/cm3) equimolar The two primary approaches to developing fusion are deuterium-tritium gas. The shell is composed of an magnetic confinement fusion, reviewed in the January ablator and an inner region of D-T, which forms the main 1 issue of PHYSICS TODAY, and inertial confinement fusion, fuel. Energy from a driver is rapidly delivered to the reviewed in this article and the article by William J. ablator, which heats up and expands. As the ablator Hogan, Roger Bangerter and Gerald L. Kulcinski on page expands outward, the rest of the shell is forced inward to 42. Significant elements of the work presented here were conserve momentum. The capsule behaves as a spherical, carried out under classified Department of Energy pro- ablation-driven rocket. grams and have been only recently declassified. In its 2 The fusion fuel is imploded with a typical efficiency of review of ICF carried out in 1990, the National Academy 5-15%. That is, 5-15% of the total absorbed energy goes of Sciences found the DOE classification guidelines for ICF into the fuel. In its final configuration, the fuel is nearly isobaric at pressures up to about 200 gigabars but consists of two effectively distinct regions: a central hot spot, John Lindl is the ICF target physics program leader at containing about 2-5% of the fuel, and a dense main fuel Lawrence Livermore National Laboratory, in Livermore, region (the "cold fuel pusher"). Fusion begins in the California. Robert McCrory is director of the Laboratory for central hot spot, and a thermonuclear burn front propa- Laser Energetics at the University of Rochester, in Rochester, gates rapidly outward into the main fuel, producing high New York. Michael Campbell is deputy associate director gain. and ICF program leader at Lawrence Livermore. The efficient arrangement of the fuel in this configu- 3 2 PHYSICS TODAY SEPTEMBER 1992 © 1992 Americon Insrirure of Physics t i l l 1 1 1 1 1 Hot spot Main fuel Temperature\ 1 \ Density 1 1 1 1 /''//I \ \\Y-\ Proposed ICF capsules are imploded by ablation of an outer shell. Ignition occurs in a central hot spot, and fusion burn propagates out through the main mass of D-T fuel. Figure 1 ration places stringent requirements on the details of the Ignition occurs when the energy deposited by products driver coupling, including the time history of the irra- of the thermonuclear burn during one confinement time diance and the hydrodynamics of the implosion.3 Several equals the energy required to heat the plasma to features of the implosion process are important. One is thermonuclear burn temperatures. The specific energy the in-flight aspect ratio RlLR, where R is the shell radius (in J/g) required to heat a D-T plasma with equal electron and Aif its thickness, both taken during the implosion. and ion temperatures T (in keV) is The shell thickness Ai? is less than the initial thickness be- 9 9 cause the shell is compressed as it implodes. Hydrody- Eheat = 0.1152 xlO T= 2.3 xlO J/g at 20 keV (2) namic instabilities4 similar to the classical Rayleigh- In general only the charged-particle reaction products Taylor fluid instability impose limits on R/hR, which are available to heat the fuel because most of the neutrons result in a minimum required pressure or absorbed driver escape the plasma. For D-T fusion irradiance. For in-flight aspect ratios of 25-35, the peak values of pressure and irradiance during a pulse must D + T - n(14.06 MeV) + a(3.52 MeV) (3) 15 2 reach about 100 megabars and 10 W/cm , respectively. and er = 3.54x10" J/g. If we assume that all of the The required pressure depends on the required implosion alphas are deposited, then the energy per gram deposited velocity, which is determined by the capsule size. Mini- in the fuel is mum velocities are in the range of 3-4 x 107 cm/sec. Another important parameter is the convergence 6.68x10" (4) ratio, denned as the initial outer radius of the ablator RA pr + 6 g/cm divided by the final compressed radius of the hot spot rHS. Equating £heat and Eu from equations 2 and 4 shows Typical values are 30-40. To maintain a near-spherical that at 20 keV, ignition occurs for pr>0.21 g/cm2 implosion, implosion velocities must be uniform to about (nr > 1.7 X1014 sec/cm3), corresponding to a burn efficiency 1%. Control of the implosion symmetry and minimization of about 3.4%. Ignition is an adequate achievement for a of mixing of the fuel by Rayleigh-Taylor-induced instabili- magnetic fusion plasma if the energy required to maintain ties are crucial to the successful formation of the central confinement is much less than the energy needed to heat hot spot. Mixing and departures from spherical symmetry the plasma. However, ignition alone is insufficient for ICF can prevent the conditions required for ignition during the power production, which must overcome a factor of 10-20 final stages of the capsule implosion. due to implosion efficiency and a factor of 3-20 due to The fusion yield EF from an ICF capsule is given by driver efficiency. EF = ef<f>M, where ef is the specific energy of fusion One of these factors of 10 can be recovered by (energy per unit mass), <f> is the burn efficiency, and M is increasing the burn efficiency. Compression of the D-T the imploded fuel mass. For D-T fusion at a burn fuel mass makes it feasible, in the laboratory, to achieve temperature between 20 and 40 keV, the burn efficiency is the pr = 3 g/cm2 necessary for (/>=%. For a sphere, approximately M = (4Tr/3){prf/p2. Hence the mass (and driver energy at fixed coupling efficiency) required for a given pr scales as 2 ;! 2 (1) 1/p . For example, at a density of 400 g/cm (plasma pr + 6 g/cm density of 102fi cm"3), a spherical shell with a thickness of where p and r are the compressed fuel density and radius, r/2 and radius r would have pr = 3 g/cm2 with a mass of 5 respectively. Because the inertial confinement time is mg. This mass would be imploded in about 10 nsec, would proportional to r, the fuel areal density pr is equivalent to burn in about 10"'" sec and would have a yield of about the density-confinement time product nr that is used to 6 X 10* J. This yield is readily contained, and at 5-6 pulses describe progress in magnetic confinement fusion. We can per second such targets could drive a 1-gigawatt electric use the burn efficiency given by equation 1 to compare the reactor for power production. requirements of magnetic confinement fusion with those The fuel compression is energetically efficient if the of ICF. D-T remains nearly Fermi degenerate.' Only 6.5 X104 J is PHYSICS TODAY SEPTEMBER, 1992 33 required to compress 5 mg of fuel to 400 g/cm3 in this case, and indirect drive.