Since the beam power and intensity of present accelerators for heavy-ion fusion are orders of magnitude below values required to ignite a capsule, target-design research in the US is aimed at improving the target performance in numerical simulations. In an indirect-drive target, ion beams are absorbed by a cylindrical metal shell, called a Hohlraum, surrounding the fuel capsule, and the energy is reradiated as X rays. These X rays would then heat and compress the capsule, producing a hot region at the center that would initiate the fusion reaction. The X-ray intensity around the capsule must be very symmetrical to avoid hydrodynamic instabilities during compression. Earlier designs had radiators at the the ends of the Holhraum, but it proved difficult to maintain adequate X-ray symmetry with this geometry. During the last several years, target physicists have developed a new family of designs in which the ion beams are absorbed in the Hohlraum walls, so that X rays are radiated from a large fraction of the solid angle surrounding the capsule. With a judicious choice of absorbing materials, this arrangement, referred to as a "distributed-radiator" target, gives better X-ray symmetry and target gain in simulations than earlier designs. A typical distributed-radiator target is shown in the two sketches here.
The earliest distributed-radiator targets had three principal shortcomings: the rather high energy input (about 6 MJ), the small beam spot size (about 3 mm), and the need for "foot" pulses at a lower ion energy to heat the Hohlraum before the arrival of the main pulses. Subsequent work has made important progress in the first two of these problems. The "close-coupled" target, first reported in 1999, reduced total beam energy by 40%, to 3.5 MJ, by reducing the size of the Hohlraum while driving the same capsule. Work on a "hybrid" target, which combines aspects of the end-radiator and distributed-radiator designs, has been underway since 2000 and, if successful, might allow a spot size of nearly 5 mm. The principal difficulty in this work is developing a design that allows beams to approach the target from acceptably large angles. The problem of an energy difference between the "foot" and main pulses is more intractable. The energy difference is needed because the range of ions in a material decreases as the material is heated, so higher-energy ions are needed later in the heating sequence for the beams to deposit their energy at the same depth as earlier pulses. Delivering pulses at two energies is nonetheless possible and at worst introduces some added complexity near the end of the accelerator.
Current target simulations have several goals, all related to improving the match between target requirement and the parameters of feasible driver beams:
Although target design is not directly supported by the HIF-VNL, the work is crucial to the heavy-ion fusion program because of its "high leverage." Any improvements in target performance or reductions in the required beam energy or quality affect all parts of a fusion driven and can lead to dramatic decreases in the size, complexity, and cost of a HIF power plant.
- Reducing or eliminating the difference in ion energy between the "foot" pulses and the main pulses.
- Increasing the allowable spot size of beams on the target.
- Modifying targets so that beams can approach at larger angles to the target axis.
- Reducing the sensitivity of targets to beam non-idealities, such as transverse offset, energy and current variations, and density non-uniformities.
J. Lindl, "Development of the Indirect-Drive Approach to Inertial Confinement Fusion and the Target Physics Basis for Ignition and Gain," Phys. Plasmas 2, 3933 (1995).
D. A. Callahan-Miller and M. Tabak, "A Distributed Radiator, Heavy Ion Target Driven by Gaussian Beams in a Multibeam Illumination Geometry," Nuclear Fusion 39, 883 (1999).
D. A. Callahan-Miller and M. Tabak, "Increasing the Coupling Efficiency in a Heavy Ion, Inertial Confinement Fusion Target," Nuclear Fusion 39, 1547 (1999).
For comments or questions contact WMSharp@lbl.gov or DPGrote@lbl.gov. Work described here was supported by the Office of Fusion Energy at the US Department of Energy under contracts DE-AC03-76SF00098 and W-7405-ENG-48. This document was last revised June, 2002.