The Electra KrF Laser Program

J.D. Sethian,1 M. Myers,1 M. Friedman,1 R. Lehmberg,1 J. Giuliani,1 S. Obenschain,1 F. Hegeler,2 S. Swanekamp,3 and D. Weidenheimer4
1 Plasma Physics Division
2Commonwealth Technology, Inc.
3Jaycor, Inc.
4Titan-Pulse Sciences Division

Introduction: In thermonuclear fusion, two light nuclei are combined to produce energy. Fusion is the power source of the Sun. If fusion could be harnessed on Earth, the power plant would have unlimited fuel (the ingredients are deuterium (a hydrogen isotope) and lithium (a plentiful element)), no chemical by-products, and no long-term radioactive waste. The payoffs are so large that numerous scientific institutions worldwide have been working on this problem. However, after almost 50 years the solution is still elusive and challenging. Recently, NRL has spearheaded an approach that appears to be very promising: An array of intense krypton fluoride (KrF) lasers are used to directly compress and heat a small pellet of fuel to the conditions needed for fusion reactions. Experiments and computations at the Naval Research Laboratory show that this approach is scientifically viable and should provide sufficient energy release for a fusion reactor.1-3 However, the present highpower laser used in this research fires twice every hour and requires periodic maintenance. In contrast, a laser for a fusion power plant must fire five times per second for several years and meet stringent cost and efficiency requirements. The Electra Laser Program at NRL will develop a laser that can meet these requirements. Electra will run at 5 Hz with a laser output of 400 to 700 Joules. This will be large enough to develop technologies that can be scalable to the 50 to 150 kJ needed for a fusion power plant beam line.

Fig1 Image

Key components of an electron beam pumped KrF laser.

Components of a KrF Laser: In a KrF laser, electron beams are used to excite the krypton and fluorine. The fundamental laser wavelength is in the ultraviolet at 248 nm. In the mid 1990s, NRL built the Nike laser that demonstrated this process.4 Nike can produce more than 5000 J of laser light, with a beam spatial nonuniformity of less than a few tenths of a percent. It is now being routinely used for laser fusion experiments. This outstanding beam uniformity, which is necessary to achieve uniform pellet implosions, plus the relatively low cost and scalability to power-plant size systems, makes KrF lasers promising for fusion. Figure 1 shows the fundamental laser components. Applying voltage from a pulsed power system to a field emission vacuum diode creates the electron beams. The beams pass through a thin foil that isolates the diode from the high-pressure laser gas. The foil support structure, known as a "hibachi" because of its grill-like shape, is one of our technical challenges. It needs to be highly transparent to the electron beam, yet survive the hostile environment of the laser cell (hydrostatic shock, ultraviolet light, X rays, electrons, fluorine, and HF). The laser needs to have windows with highly transparent antireflective coatings that can also survive this environment and a recirculator to make the gas quiescent before the next shot. Our plan is to perform the research needed to develop these components and then combine them into an integrated system.

Fig2 Image
The Electra Laser Facility.

Progress in Laser Development: Electra is installed in a newly refurbished 7000 square foot laboratory. Figure 2 shows the new, first-generation, pulsed power system that we have built explicitly for this task. This system uses an array of capacitors that pulse charge a pair of water dielectric electrical transmission lines to 1.2 MV in 3.5 ms through a 12.1 step-up transformer. The lines are discharged through gas switches into the electron beam diode. We have two such systems. Each produces a 500 kV, 100 kA, 100-ns long electrical pulse five times a second (25 MW), and each can run for up to 100,000 shots before requiring minor (2-hour) refurbishment of the gas switches. This 5-hour duration is unprecedented for a pulsed power system of this size and is more than adequate to develop the initial laser components. We are also developing a more advanced pulsed power system that can meet the ultimate requirements for durability and efficiency. The key component is a new solid-state, four-junction, silicon switch triggered by an integral diode laser. We recently demonstrated this concept with a prototype device. It will eventually replace the existing gas switch technology.

Fig3 Image

Hibachi concept. The louvers are rotated between shots to deflect the gas flow onto the pressure foil.

We have designed a hibachi (Fig. 3) for high efficiency and long life. The efficiency is achieved by using an advanced design that eliminates the conventional anode foil and by patterning the beam to miss the hibachi ribs. The latter is more difficult than one would expect because the beam rotates as it propagates from the electron beam emitter to the hibachi. Nevertheless, we have modeled this with three-dimensional particle-in-cell codes and, more importantly, have demonstrated that we can "miss the ribs" experimentally. The same model accurately predicts the electron beam energy deposition in the laser gas. Cooling the hibachi should be achievable by momentarily rotating louvers to deflect the laser gas flow to the foil. Our modeling shows that the louvers can be retracted in time to allow the gas to return to a quiescent state before the next shot.

Fig4 Image

Comparison of KrF kinetics code with experiments at Keio University (left) and NRL Nike (right).

In the arena of KrF physics, we have developed a KrF physics model that features an automated chemistry solver that tracks 24 species, 20 excited KrF states, and 122 reactions. It includes a three-dimensional model of the amplified spontaneous emission. Such sophistication is needed because previous models have been found to be valid over only a limited range of conditions. In contrast, as shown in Fig. 4, our new model can predict the performance of KrF lasers operating under a wide range of conditions.

Summary: Electra is a multifaceted research and development program to develop a KrF laser for fusion energy. The program makes full use of the multidisciplinary technical expertise that is available at NRL. The first-generation pulsed power system has given us a platform to develop the laser components, and we have already made significant advances in the fields of electron beam physics, the hibachi structure, and KrF kinetics. The compact advanced solid-state switch that we have demonstrated has the potential to meet not only the Electra requirements, but also to enable a wide range of Navy applications. We anticipate that we will start operating Electra as a laser sometime in 2002.

[Sponsored by DOE]

1 S.E. Bodner et al., "High Gain Direct Drive Target Design for Laser Fusion," Phys. Plasmas 7, 2298-2301 (2000).
2S.E. Bodner et al, "New High Gain Target Design for a Laser Fusion Power Plant," Proc. IAEA Technical Meeting on Inertial Fusion Energy and Chambers, June 7, 2000, Madrid, Spain, pp. 363-372.
3 D.G. Colombant et al., "Effects of Radiation on Direct Drive Laser Fusion Targets," Phys. Plasmas 7, 2046-2054 (2000).
4 S.P. Obenschain et al., "The Nike KrF Laser Facility: Performance and Initial Target Experiments," Phys. Plasmas, 3, 2098-2107 (1996).

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