Abstract Details
Laser-induced magnetic fields in ICF capsules
Author: Erick L. Lindman
Requested Type: Oral Only
Submitted: 2009-04-21 12:40:46
Co-authors:
Contact Info:
Otowi Technical Services
101 Navajo Rd.
Los Alamos, NM 87544-2
USA
Abstract Text:
The performance of an inertial-confinement-fusion (ICF) capsule can be improved by inserting a magnetic field into it before compressing it [Lindemuth and Kirkpatrick, Nucl. Fus. 23, 263 (1983)]. 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 Megagauss (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.
We have used computer-simulation techniques to address a number of issues that are relevant to the use of this mechanism to insert fields into ICF capsules. We have reproduced the published simulations of this effect and extended the results to relativistic hot electrons. We have shown that the magnetic field will indeed go through a gap in a capsule wall and set up a magnetic field in the interior. And, we have identified resistive losses in the surface return current as a serious issue that must be addressed in future work. In addition to these issues, we will 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. Work supported by the U.S. Department of Energy, Grant No. DE-FG02-08ER85128.
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