DEFENSE PROGRAMS The Nova laser fusion research Jacility, currently under construc­ The Nova tion at LINL, will provide researchers with powerful new tools Jor the study oj nuclear weapons physics and inertial confinement fusion. laser fusion The Nova laser system consists oj 20 large (74-cm diam) beams, facility Jocused and aligned precisely so that their combined energy is brought to bear Jor a fraction oj a second on a tiny target containing thermonuclear fuel (deuterium and tritium). The ultimate goal oj the LINL inertial confinement fusion program is to produce fusion microexplosions that release several hundred times the energy that the laser delivers to the target. Such an achievement would make in­ ertial confinement fusion attractive Jor military and civilian applica­ tions. The Department oj Energy (DOE) has approved construction oj 10 Nova laser beams and the associated laboratory buildings (Phase 1). For further information contact Phase 2, which includes the remaining 10 beams and harmonic­ William W. Simmons (422-0681). conversion crystal arrays, is presently under consideration by DOE. By the mid- to late 1980s, Nova should demonstrate that we can produce the extremes oj heat and pressure required to achieve igni­ tion oj the thermonuclear fuel. Additional developments in the area oj high-efficiency drivers and reactor systems may make inertial con­ finement fusion attractive Jor commercial power production. One technical approach to the times liquid 0-T density), ignition problem of controlling thermo­ of thermonuclear bum, scientific nuclear fusion reactions is to bring breakeven (in which the energy in­ small deuterium-tritium (0-T) fuel cident on the target equals the ther­ pellets to very high temperatures monuclear energy released), and and densities in such a short time significant energy gain in an ICF ex­ that the thermonuclear fuel will periment. These achievements are ignite and bum before the com­ necessary precursors to the suc­ pressed core disassembles. This ap­ cessful realization of either the proach, known as inertial confine­ military or the energy objectives of ment, relies upon a driver (e.g., a the program. From a technical laser) to deliver the extremely high­ point of view, the ignition milestone power, short-duration burst of is the most important, and its at­ energy required. tainment will provide a strong LLNL has been involved in the signal to move forward national inertial confinement fusion aggressively. (ICF) program since the early We chose solid-state 1960s. The objectives of the neodymium-glass lasers for our ex­ program have been twofold: in the perimental driver systems because short term to develop the military applications of ICF, and in the long term to evaluate ICF as an energy source. 1,2 Our immediate scientific objectives are the demonstration of high compression (100 to 1000 1 that particular technology was most review by DOE, an additional 10 we can double or triple the fre­ advanced at the time the decision Nova beams will be installed in the quency of the basic 1.05-/-Lm light was made. In separate programs, Shiva building. The full Nova com­ from high-power Nd-glass lasers we have continued to develop ad­ plement of 20 beams will be with efficiencies exceeding 70%. vanced laser concepts with the goal brought to an integrated target Since shorter wavelengths are of reducing system costs and chamber in two opposed clusters of much more favorable for ICF laser providing high-efficiency, high­ 10 beams each. Other target con­ target physics, we plan to imple­ repetition-rate drivers. Over the figurations are possible if required. ment this option in the Nova past several years, we have built Like its predecessors, Nova will facility. (This enhancement of the and operated a series of in­ be a neodymium-glass system Nova facility is presently under creasingly powerful and energetic with a working wavelength near review by DOE and will be deter­ laser systems to study the physics of 1.05 /-Lm. However, in recent ex­ mined shortly.) We would thus be ICF targets and laser-plasma in­ periments on the Argus laser able to focus approximately 200 kJ teractions. Nova, the latest in this system, we have demonstrated that of green (O.53/-Lm) or blue series, is the successor to the Argus by using the nonlinear optical (O.35/-Lm) light onto laser fusion and Shiva lasers. The Nova laser properties of potassium dihy­ targets. The total cost of the Nova will consist of 20 beams, capable of drogen phosphate (KDP) crystals, project will be $195 million if we concentrating 200 to 300 kJ of energy (in 3 ns) and 200 to 3001W of power (in 100 ps) on experimental targets by the mid- 1980s. Its purpose is to demon­ strate the ignition of thermonuclear burn. The first phase of the Nova proj­ ect involves construction of a 10 684-m 2 ( 115,000-ft2) laboratory building and installation of a 10-beam neodymium-glass laser system adjacent to the existing Shiva facility (see Fig. 1). With Phase 2 funding, presently under lFD(Qlo II The Nova laserfusion research facility, currently underconstruc- c:2) tion at LLNL, will provide researchers with powerful new toolsfor the study of nuclear weapons physics and inertial confinement fusion. Its 20 large (74-cm diam) beams, focused, timed, and aligned precisely, will concen­ trate 200 to 300 kJ of energy for a few billionths of a second on a tiny pellet con­ taining thermonuclear fuel. Nova is being built to demonstrate the ignition of advanced thermonuclear-fuel targets in the mid-1980s. 2 DEFENSE PROGRAMS are limited to the 1.05-,um the fuel to twice the no-bum tem­ into 20 beams. After traversing an wavelength and close to perature. If our experiments are adjustable optical delay path (used $238 million if we develop the successful, it is also possible that to synchronize the arrival of the green or blue light capability. the Nova facility will achieve scien­ various beams at the target), the The light from Nova, particularly tific breakeven (in which the energy pulse enters the amplifier chain, if it is of the green or blue type, released by the thermonuclear where (1) disk amplifiers increase should be capable of producing the reaction equals the laser energy the pulse power and energy, (2) extremes of temperature and delivered to the target). spatial filters maintain the spatial pressure required to ignite a small smoothness of the beam profile pellet of thermonuclear fuel. That Laser design and while expanding its diameter, and is, it should create conditions such performance (3) isolators prevent the entire laser that the energy released from alpha The Nova laser system has from breaking spontaneously into particles in the central core of the master-oscillator-power-amplifier oscillations that could drain its compressed fuel (accounting for (MOPA) architecture. As shown in stored energy and damage the 3.5 MeV of the 14.6-MeV total Fig. 2, a laser pulse of requisite target prematurely. (For a discus­ energy released by the 0-T reac­ temporal shape is generated by the sion of spatial filtering and isolation tion) is trapped and used to heat oscillator, preamplified, and split processes, see the box on pg. 4.) Output sensor . .. Spatial filters package • Amplifiers +.L Isolation D Space for added amplifiers 46.0-cm diam '\. / Turning mirrors cm diam equal ization 15.0-cm diam 20.8- cm diam [pO 0 ~ Schematic diagram showing the major op- in cm) is expanded in increments by spatial filters judiciously lJ@o tical components through which each of 20 spaced in the chain. The large-output sections of the chain Nova laser beams will pass on its way from the oscillator to (31.5, 46.0, and 74.0- cm in diameter) will require significant the target. The overall Nova system architecture is similar extensions of (and some innovation from) Shiva optical to that of the Argus and Shiva solid-state lasers (currently technology. The plasma shutter, which isolates the laser operational at LLNL). A collimated laser pulse traverses from potentially damaging reflections from the target, will progressively larger amplifier sections between the os­ be used for the first time. cillator and the target. The beam diameter (expressed above 3 Spatial filtering and isolation A spatial filter consists of a pair of lenses plasma). The plasma then expands to fill the hole (positioned so that the distance between them is through which the pulse is passing. We refer to this equal to the sum of their focal lengths) and a small phenomenon as "self-closure." No closure occurs if pinhole at their common focus. Spatial filters are the laser pulse is short compared to the pinhole used in the laser chain to increase the diameter of closure time. However, for long pulses, the intensity the beam and to remove intensity variations (Le., of the beam focal spot at the pinhole edge must be lighter or darker regions of the beam produced by less than the threshold for plasma formation, or the optical imperfections and self-focusing effects). The pinhole must be much larger than the product of the input lens of a spatial filter focuses the main beam pulse duration and plasma velocity. For Nova, f/20 (all parallel light) to a tiny spot that can pass through spatial filters satiSfy all imaging, filtering, and pinhole a correctly positioned pinhole without being clipped self-closure criteria. (The f number describes the on the edges. The nonparallel light, which is respon­ ratio of the focal length to the beam diameter.) sible for intensity variations, hits the edges of the Isolators act as optical diodes in the laser chain, pinhole and is stopped.