Magnetized Target Fusion

R. Seimon
Los Alamos National Laboratory

STATUS OF RESEARCH AND ACHIEVEMENT:

Magnetized Target Fusion is a broad class of ideas that involves pulsed compression of a magnetized plasma. Like Inertial Confinement Fusion, systems with higher density than conventional MFE are envisioned with pulsed release of fusion energy. Unlike ICF, magnetic fields improve energy confinement in the compressed state, which offers the possibility of lower compression and a less powerful driver. Thus MTF can be described as intermediate in parameters between MFE and ICF. At Los Alamos we think the most promising systems involve > 1 pressure equilibrium between material walls and an ignited plasma. Magnetic fields embedded parallel to the material wall suppress thermal conduction. In addition the magnetic field might contain alpha particled and permit a burn to occur in a compressed deuterium-tritium fuel layer.

Although wall confinement and magnetic insulation were discussed early on by scientists such as Fermi, Budker, and Sakharov, relatively little work has been done in this arena. Published work includes: e.g. Gross et. al. at Columbia, Robson et. al. at NRL, Hartman and Hammer at LLNL, Lindemuth et. al. at LANL, and Russians such as Alikhanov, Mokhov, Vekshtein, and Ryutov.

The largest experimental effort at present involves a LANL-VNIIEF collaboration on a configuration called MAGO. Recently, a group at LLNL, including Russian scientist D. Ryutov, proposed that wall-confined Field-Reversed Configurations be compressed in a liner to achieve Q1. Also, related experiments are underway by Wessel et. al. at UC Irvine, funded by the innovative-concept program of MFES.

We argue here that this general thrust deserves attention, and do not promote any particular configuration. Even without a specific configuration in mind, some characteristics of an energy-producing MTF system can be anticipated:

The kopeck problem alludes to the fact that pulses of energy have little value when converted to electricity and sold at 5 cents per KW-hour. (At 5000 rubles per dollar, the kopeck is not worth much either.) For many ideas discussed in the past it was difficult to see how the revenue generated per pulse could pay for the costs of producing the pulse (amortized capital cost of a pulsed energy source and materials processing to prepare the target material consumed).

A major advance from the ICF program to help with the kopeck problem is called hotspot ignition -- compress DT fuel but heat only a small fraction to ignition conditions, after which the remaining fuel burns and releases a relatively large amount of energy compared to the input. We suspect that MTF systems will also require high gain, and that the magnetically insulated plasma should serve as the hot spot. The expectation for large pulses of energy follows from high gain and current estimates for the threshold energy needed to ignite a liner-driven magnetized-target system.

APPROPRIATE LEVEL OF MTF RESEARCH:

Because DOE Defense Programs are funding work in pulsed power, including the MAGO collaborations, a very effective program could be funded by OFES for relatively little money. The emphasis of OFES-funded work should be to study transport properties and then optimize a wall-confined plasma system. If successfully developed, the optimized configuration could then be imploded using DP facilities in jointly-funded experiments.

Given the limited past study of the physics of wall-confined plasmas, small experiments with supporting theory and modeling would be desirable at the present time. Many basic questions are unresolved: choice of magnetic filed topology, choice of wall material ( e.g., high Z or low), effects of turbulence (Ohm transport), etc.

International activities are not presently coordinated. France and Russia have small MTF efforts.

In the near term an OFES-funded program of a few million per year would allow the US to advance the subject rapidly and establish leadership in this unexplored approach to fusion.

RESEARCH IMPACT ON PLASMA AND FUSION SCIENCE

The physics of high-beta magnetized plasma in pressure equilibrium with a wall has been studied somewhat theoretically but very little experimentally. A major milestone for the near term would be a small-scale laboratory demonstration of such a configuration and documentation of its thermal confinement properties. The understanding gained by such studies may lead to new ideas for divertors or provide help in understanding current divertor operation. The physics may also lead to new ideas in plasma processing which generally involves plasma-wall interactions.

With respect to fusion science, MTF represents a broad thrust, comparable in scope and complexity to ICF. Like ICF, MTF is a true alternative to MFE that will succeed or fail for qualitatively different reasons. Thus, it fits well in a balanced portfolio of research efforts on alternative concepts for fusion energy.