Scaled Final-Focus Experiment
To ignite a HIF target, the ion beams must be focused onto a very small region. The typical indirect-drive HIF target is a small metal cylinder surrounding an even smaller sphere of fusion fuel. The ends and, for some designs, the walls of the cylinder are made of a materials that convert the ion beam energy into x-ray energy to heat and compress the target. For present target designs, these "converters" have a diameter of approximately 1-2 cm, which sets the size of the "bulls eye" that beams with thousands of Amperes of current must hit.
A straightforward approach to the final-focus problem is to use series of quadrupole lenses to direct the beam onto the target. Ballistic focusing, where the ion beam is mostly un-neutralized as it approaches the fusion target, is considered a baseline method. A series of specially designed quadrupole magnets is placed at the end of the accelerator. These magnets produce a conically converging beam with a cone half angle of about 1°, causing the beam diameter to shrink to the size of the target as it reaches the center of the reactor chamber. This convergence angle must overcome the repulsive space-charge force of the beam ions, as well as the small random spread in their transverse velocities (emittance). Such a system was part of the a report detailing a complete HIF power plant design. It was this system that was chosen to be the subject of experimental study at LBNL. To obtain a relevant result, the scaled experiment should duplicate many of the physical properties in the design appropriate to focusing the beam. In particular, the 100 meter long system used in was modeled at one-tenth dimensional scale with magnets that have a 5 centimeter bore diameter inside a 10 meter vacuum system. Fig. 1 shows a photograph of the beam line. The experimentally focused beam fit inside a 1mm diameter circle, as seen in Fig. 2.
Some driver designs call for beams which have so much force from the space charge that ballistic focusing by itself will not get the beam small enough to match the size of the target. In this case, one could imagine using electrons within the beam to shield the forces of one ion against its neighbors; this is known as neutralizing the beam. We explored this technique experimentally by increasing the beam current by a factor of four, and then adding electrons to the beam by passing it near a hot tungsten wire, as seen in Fig. 4. Figure 3 shows the difference between the un-neutralized and neutralized focal spots.
H. Wollnik et al., "HIBALL-II: An Improved Conceptual Heavy Ion Beam Driven Fusion Reactor Study," KfK-3840/UWFDM-625, July 1985.
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.