Laser-induced magnetic fields in ICF capsules, Final Report, DE-FG02-08ER85128, Phase 1

[摘要] Laser-induced magnetic fields in ICF capsules Final Report, DE-FG02-08ER85128, Phase 1 E. L. LINDMAN, Otowi Technical Services, Los Alamos, NM. The performance of an inertial-confinement-fusion (ICF) capsule can be improved by inserting a magnetic field into it before compressing it [Kirkpatrick, et al., Fusion Technol. 27, 205 (1995)]. To obtain standoff in an ICF power generator, a method of inserting the field without the use of low-inductance leads attached to the capsule is desired. A mechanism for generating such a field using a laser was discovered in Japan [Sakagami, et al., Phys. Rev. Lett. 42, 839 (1979), Kolodner and Yablonovitch, Phys. Rev. Lett. 43, 1402 (1979)] and studied at Los Alamos in the 1980s [M. A. Yates, et al., Phys. Rev. Lett. 49, 1702 (1982); Forslund and Brackbill, Phys. Rev. Lett. 48, 1614 (1982)]. In this mechanism, a p-polarized laser beam strikes a solid target producing hot electrons that are accelerated away from the target surface by resonant absorption. An electric field is created that returns the hot electrons to the target. But, they do not return to the target along the same trajectory on which they left. The resulting current produces a toroidal magnetic field that was observed to spread over a region outside the hot spot with a radius of a millimeter. No experimental measurements of the magnetic field strength were performed. Estimates from computer simulation suggest that field strengths in the range of 1 to 10 Mega gauss (100 to 1000 Tesla) were obtained outside of the laser spot. To use this mechanism to insert a magnetic field into an ICF capsule, the capsule must be redesigned. In one approach, a central conductor is added, a toroidal gap is cut in the outer wall and the DT fuel is frozen on the inner surface of the capsule. The capsule is dropped into the reaction chamber and struck first with the laser that generates the magnetic field. The laser hot spot is positioned at the center of the toroidal gap. As the magnetic field spreads from the hot spot over the surface that contains the toroidal gap, it will propagate through the gap and set up a steady state in the capsule. The main compression is then initiated. First, it closes the gap and crow-bars the field, then it compresses the fuel to ignition. In addition to this application, we discuss the use of this mechanism to induce Mega-gauss fields in laboratory apparatus for measurements of the effects of large magnetic fields on material samples. A preliminary target design for this purpose is presented. It is made of high-density material with no hydrogen surface contamination to minimize fast ion losses and to minimize x-ray preheat of the sample (the material, whose magnetic properties are to be measured). In it, the gap is designed to allow the magnetic field to move into the interior of the target while minimizing the flow of hot electrons into the interior. By adjusting the size of the gap as well as its configuration, the hot electron effects can be minimized. Since the strength of the magnetic field depends on the radial distance to the sample from the center of the conductor carrying the return current, the sample is located at a point of minimum conductor radius. This location also minimizes the effects on the measurement of any hot electrons that flow into the interior. Useful experiments can be accomplished with the “seed” field alone in this geometry. Compressing the capsule after the insertion of the seed field may allow experiments with even larger magnetic fields. We have used computer-simulation techniques to address a number of issues. Our conclusions include: • This magnetic-field generating mechanism is a viable method for generating magnetic fields in ICF targets and for laboratory experiments. • Useful experiments on material samples can be done with the seed field of 1 to10 Mega gauss (100 to 1000 Tesla) and higher magnetic fields can be obtained by subsequently compressing the capsule. • The results reported here can be studied experimentally with a modest CO2 laser that emits 4.5 J of energy in a 1-ns pulse focused into a 60-Îźm-diameter spot. • We have reproduced the published calculation of Forslund and Brackbill and extended the results to relativistic hot electrons. The difference in our peak magnetic field (1.25 Mg) versus that in the published calculation (1.3 Mg) is negligible. • We have shown that hot electrons can jump a gap while the magnetic field penetrates through the gap into an internal region. • We find that background plasma or a plasma gradient near the accelerating region is essential to the formation of the magnetic-field structures. • And, we have identified resistive losses in the surface return current as a serious issue that must be addressed in future work.

[发布日期] 2009-11-05

[学科分类] 原子、分子光学和等离子物理

[关键词] Resonant absorption;Hot electrons;Mega-gauss magnetic fields;Magneto-inertial fusion