The Laser Plasma Branch of the Naval Research Laboratory (NRL) has participated in laser fusion research since the program's beginnings in the early 1970s. NRL's early research helped pioneer the Nd:glass laser technology that is now used in the great majority of laser fusion facilities around the world. Our lab was also the first to propose the use of combined spatial and temporal incoherence to smooth laser beams and the first to demonstrate the many advantages of laser beam smoothing.
By the mid-1980s, NRL scientists invented an improved optical smoothing technique that was naturally matched to a KrF gas laser. It now appeared that KrF lasers would be superior to Nd:glass lasers for direct-drive laser fusion. On the downside, the technologies of KrF lasers and of the new beam-smoothing technique were relatively undeveloped and generally viewed as risky.
In the late 1980s, we began development of a new KrF laser called Nike. This laser is now completed and ready for laser-target experiments, and it meets the original expectation: the beam uniformity of Nike is about a factor of 10 better than the best existing short-wavelength glass lasers. Our target design calculations suggest that direct-drive pellets, using a few-MJ KrF laser, may produce thermonuclear energy gains greater than 100, This gain would meet the original promise of fusion.
We outline below the basic ideas of laser fusion, the Nike laser-smoothing concept, and our proposed laser-target experiments. We also outline an overall plan to proceed with laser fusion based upon the uniform illumination that is now available with the KrF laser, with its potential for reaching high thermonuclear energy gains.
NRL's Nike - named after the Greek goddess of victory - is a recently completed krypton-fluoride (KrF) gas laser that now produces 4000-5000 J of UV light out of the large amplifier in a 4-ns pulse. Nike's unique feature is its excellent beam uniformity. By using a KrF laser with induced spatial incoherence (ISI) optical smoothing, the modulations in the laser focal profile are only 1% in one beam and <0.3% with a 44-beam overlap. These modulations are a factor of 10 less than what has been achieved with the best short-wavelength glass laser.
An example of the beam's uniformity is illustrated by the cover figure above. It is an experimentally measured focal profile of one of the Nike KrF laser beams. The modulations in the laser intensity are only 1% rms, over a diameter that is 50% of the full-width half maximum. Nike overlaps 44 of these beams on a target, reducing the modulations to <0.3%. The laser energy in all of the beams was 3900 J, and the pulse duration was 4 ns.
This laser program is funded by the Department of Energy's (DOE) Defense Programs to evaluate the potential of direct-drive laser fusion. Because KrF can produce uniform laser illumination, the fusion energy gain is predicted to be more than 100. This high gain, if proven feasible, would be attractive not only for military applications but also for a fusion reactor. Nike will be used now in laser-target experiments to determine if this laser uniformity is necessary and sufficient to accelerate a target under fusionlike conditions, without excessive hydrodynamic distortion or fuel preheat.
Early research with Nike will emphasize the science of laser fusion. The laser also has several other possible applications:
Direct-drive laser fusion has two basic advantages over other inertial confinement fusion concepts: Simplicity. The target is geometrically simple; the fewer physics phenomena that can occur, the less risk of failure. Efficiency. The coupling efficiency of laser light to the pellet is relatively high: ~85% absorption and ~11% rocket efficiency. The total efficiency of ~9% is about six times higher than the mainline indirect drive targets. The major remaining uncertainties in the direct-drive concept have been the laser beam quality and its impact on the stability and symmetry of the implosion. With the excellent beam uniformity of a KrF laser using ISI optical smoothing, there is now the possibility that the direct-drive pellet concept could meet the highest potential of laser fusion achieving energy gains above 100.
The basic concepts of laser fusion: (a) laser beams symmetrically heat the outside of a pellet to a temperature ~2 keV, generating pressures of ~50 MBar. The inner part of the pellet shell is kept on a low isentrope at a few eV. The hot corona drives the cold shell inward like a rocket; (b) the cold fuel is compressed into a small, high-density shell of ~500 g/cc surrounding a central hot-spot "ignitor" fuel at ~50 g/cc; the ignitor is self-heated by the alpha particles; and (c) a burn wave then propagates through the remaining cold fuel.
When a cold, higher density fluid is accelerated by a hotter, lower density fluid, the system is hydrodynamically unstable.The amplitude of any initial perturbation can grow exponentially, eventually breaking the pellet shell. These computer simulations show three times during the acceleration: (a) initially; (b) during the exponential growth phase; and (c) after nonlinear saturation. These perturbations can be limited by minimizing the laser nonuniformities and the pellet fabrication nonuniformities and by the choice of materials used in the pellet design.
Advanced computer models developed at NRL allow studies of perturbations into the nonlinear regime. The perturbation wavelengths (or mode number Q) can be divided into three categories: (a) short wavelengths that saturate before breaking through the pellet shell; (b) intermediate wavelengths that can distort the pellet shell and break it; and (c) long wavelengths that can distort the overall shape of the pellet and lead to a mix of the cold fuel with the hot ignitor fuel. The NRL computer simulations displayed here predict that the long and short wavelengths will not be very harmful. The greatest uncertainty, and the greatest need for experimental evaluation, is in the intermediate wavelengths that are on the order of the shell thickness. The initial growth of these intermediate wavelengths can be evaluated with planar targets.
The 1-D spherical target designs at NRL predict target gains of 100-300 for a few-MJ laser. This gain curve is an upper bound on possible target performance. A gain of at least 100 is required for fusion-reactor applications. There have been enough 2-D and 3-D calculations at NRL for cautious optimism that a gain above 100 is possible. Nike laser-target experiments and improved target calculations should substantially reduce the uncertainties over the next few years.
The large laser systems used in laser fusion programs have numerous optical components. In conventional laser systems, the net phase error from all of the optical surfaces produces a nonuniform focal profile that is unsuitable for direct-drive laser fusion. The solution to this problem was based on the observation that the most uniform light is not a laser but incoherent light, such as sunlight or a light bulb. It is the coherency of laser light that causes the problem.
Typically, laser light can be focused to tens of microns while a high-gain laser-fusion target is several thousand microns in diameter. Thus a laser is more coherent and focusable than necessary. One can then trade-off the focusability of laser light for a more uniform focal profile. This is the principle of optical smoothing using ISI.
There are several laser requirements for this application:
KrF lasers are an ideal match to these requirements. The wavelength is in the UV, 0.248 gum. The practical bandwidth is 1-3 THz. Since KrF is a gas laser, the optical path in glass is minimized, and there is less nonlinear distortion. It was in the late 1980s, therefore, that the NRL laser fusion program shifted its effort from glass lasers to the development of a new KrF laser named Nike.
The Propagation Bay is a 155-ft long insulated room. The temperature throughout can be held uniform to within a half degree Fahrenheit so that a diffraction-limited beam can propagate back and forth without distortion. Charcoal filters eliminate the UV absorbing gases. At each mirror array, all 56 beams can be simultaneously aligned in a few seconds by an automatic alignment system.
The 60 x 60 cm2 KrF amplifier cell is pumped from two sides by identical electron beams generated from Marx banks. The large black magnetic field coils of 2-4 kG are used to guide the electron bearns through the gas cell.
ISI is a fine optical system that images the uniform aperture onto the target. If the optical distortion of the system is small compared to the size of the focal spot image, then the profile shape will be only slightly distorted, when averaged over relevant hydrodynamic time scales.
For a random fluctuation, the nonuniformity scales as the square root of the laser's coherence time divided by the laser averaging time. Because of its inherently broader bandwidth, Nike has less nonuniformity than existing glass lasers at 1/3 micron. With Nike's 44-beam overlap, the nonuniformity is reduced by as much as the square root of 44, excluding the very short wavelength interference between beams.
The Nike laser system uses both discharge preamplifiers and E-beam pumped amplifiers. Because the E-beam amplifiers have a long pulse duration, the laser beams are "multiplexed" into 56 separate beams that pass through the amplifier successively and are then recombined onto the target. Forty-four of the beams are used for target acceleration and 12 to produce a backlighter for target diagnostics.
NRL computer simulations indicate that the Nike laser uniformity may suffice to accelerate a high-gain fusion target. Laser-target experiments will now be used to test those computer predictions.
There are two ways to scale down from a multi-MJ fusion design to a nearer-term and less-costly facility. One can maintain the spherical geometry and reduce all lengths and times by some factor e. The laser energy is then reduced by the factor e cubed. The other approach is to maintain the length and time scales but reduce the solid angle of the pellet; in other words, use a flat foil. There are advantages and disadvantages to both scalings, and neither by itself is fully satisfactory. Smaller pellets can evaluate spherical implosion geometry, but some of the physics does not scale properly. For example, it is impossible to reduce the coherence time of the laser by the same factor e; laser-beam smoothing is therefore less effective. With flat foils, the temporal scaling is better, and it is easier to diagnose the target, but one cannot evaluate convergence effects and lower-mode perturbations. The finite size of the foil also limits the distance that one can accelerate a target. The fusion program therefore uses both types of scaled experiments.
Nike will use planar foil targets. This geometry is better suited to evaluate the imprinting by laser nonuniformities and to diagnose the growth of the more dangerous Rayleigh-Taylor instability modes. The experiments will use target thicknesses, target materials, laser pulse durations, and laser pulse shaping that match as closely as possible the parameters of a high gain target.
The other Nike experimental programs would use different targets. For example, to generate multi-Mbar pressures at low temperatures for new material research, one would place the test material on top of a layer of relatively incompressible material, such as diamond, and below a thin coating of high density material, such as gold or lead, with the laser hitting the gold side of the sandwich.
Nike laser-target experiments will begin with flat, plastic targets. In 1996, the program will shift to cryogenic deuterium targets. This is the preliminary design of such a target, consisting of liquid D2 wicked into a CH (plastic) foam at 0.05 g/cc, with an equilibrium D2 gas fill, an outer cladding on the foam, and a rear window. The cladding naturally bows to match the curvature of a high gain target.
The long-term goal of the ICF program has been to build a Laboratory Microfusion Facility (LMF) with an energy gain of more than 100 and a yield of 200-1000 MJ. This LMF would be more useful for various military applications, and it would demonstrate the target performance necessary for a fusion reactor. The LMF may still be possible if one uses direct-drive pellets with a KrF laser. Several program elements would have to be resolved before the U.S. could proceed with this facility.
High-mode pellet perturbations can be evaluated using flat foils, for example with the Nike laser. Lower mode spherical perturbations are better studied in the near term with glass-laser implosion facilities. Several labs are designing high gain targets, and several are developing cryogenic pellets. NRL has a small effort in the design of the MJ KrF laser system.
A sixth program element will be needed: development of a larger KrF amplifier module, with an energy of 30-50 kJ. This module would be the basic building block for the MY laser. There are relatively conservative amplifier designs that could be scaled from Nike, but there are also more adventurous and untested amplifier design concepts with a more attractive cost.
Assuming success in these six program elements, the Department of Energy could proceed with an LMF with modest technical risk. If the long-term goal of ICF included a power reactor, it would be advisable to add a seventh element: develop and demonstrate (in the near term) an E-beam amplifier with a repetition rate of at least 5 Hz, an overall efficiency of at least 5%, a reliability of hundreds of thousands of shots, and an energy output-per-shot of at least several hundred joules.
As a side development, a 200 kJ KrF laser facility could be useful for scaled-up military experiments. Because of its uniform illumination, KrF could produce a large-area uniform shock for the study of nuclear weapons science. Because KrF has a shorter laser wavelength than glass, it would couple better inside closed cavities. Also, in an open geometry, this laser energy would produce several kcal of useful multi-keV X rays for weapons effects testing.