A status report of the current U.S. program in magneto-inertial fusion (MIF) conducted by the Office of Fusion Energy Sciences (OFES) of the U.S. Department of Energy is given. Magneto-inertial fusion is an emerging concept for inertial fusion and a pathway to the study of dense plasmas in ultrahigh magnetic fields (magnetic fields in excess of 500 T). The presence of magnetic field in an inertial fusion target suppresses cross-field thermal transport and potentially could enable more attractive inertial fusion energy systems. The program is part of the OFES program in high energy density laboratory plasmas (HED-LP).
Magneto-Inertial Fusion Introduction
The essential ideas behind magneto-inertial fusion (MIF) have existed for a long time. The concept involves freezing magnetic flux in the hot spot of an inertial fusion target or embedding magnetic flux in a target plasma bounded by a conducting shell serving as a magnetic flux conserver.
In a manner similar to conventional inertial fusion, the hot spot or the conducting shell is imploded. As the shell or the hot spot implodes, the magnetic flux is compressed with it, thus the intensity of the magnetic field is increased. The intense magnetic field suppresses cross-field thermal diffusivity in the plasma during the compression, and thus facilitates the heating of the plasma to thermonuclear fusion temperatures. The extremely high magnetic field created in the hot spot or the target plasma also enhances alpha-particle energy deposition in the plasma when fusion reactions occur.
There are two main classes of MIF, the class of high-gain MIF and the class of low-to-intermediate gain MIF. Both attempt to make use of a strong magnetic field in the target to suppress electron thermal transport in the target and thus rely upon the same scientific knowledge base of the underlying plasma physics. However, their strategies for addressing the two challenges of IFE, suitable targets and drivers, are different.
In the U.S., magneto-inertial fusion is currently being pursued as a science-oriented research program in high energy density laboratory plasma (HED-LP) by the Office of Fusion Energy Sciences (OFES) of the U.S. Department of Energy (DOE). Dense plasma in ultrahigh magnetic field (> 500 T), or magnetized HEDLP, is one of the thrust areas of HEDLP. Magneto-inertial fusion (MIF) is a pathway to create and study dense plasmas in ultrahigh magnetic fields.
It was shown by Kirkpatrick et al. that ignition is possible with lower implosion velocity with magnetized targets. Magnetic fields from 1,000 T to 10,000 T are required for typical ICF scenarios, due to the high burn-time density in typical ICF targets. Research is required to develop the scientific knowledge base on the physics of dense plasmas in ultrahigh magnetic fields and the capabilities in creating and applying ultrahigh magnetic fields to facilitate ignition in conventional ICF with lower implosion velocity and driver energy.
Experiments are in progress at the University of Rochester to compress a seed magnetic field in surrogate ICF targets using the OMEGA laser facility. The apparatus for generating a seed magnetic field of the order of 10 – 15 T has been developed and tested successfully. The magnetic field is generated by a large current flowing in small external coils surrounding the target. Compression of the magnetic flux using a high-temperature conductive plasma will be attempted next.
If successful, the flux will be compressed to produce a plasma with an embeded magnetic field of several thousand Tesla. Another method to create a seed magnetic field in dense plasma is by laser driven current drive. Theoretical and computational research to explore the concept is underway at Princeton University
Low and intermediate gain MIF
Low-gain MIF trades fusion gain in favor of non-cryogenic gaseous targets and high-efficiency lowcost drivers, so that the very high gains and high costs traditionally associated with ICF may not be needed. Electromagnetic pulsed power has lower power density than lasers or particle beams, but it has much higher wall-plug efficiency and much lower cost per unit energy delivered. By using both a magnetic field in the target and a lower-density target plasma, the required compression and heating power density is reduced to such an extent as to allow direct compression of the target by electromagnetic pulsed power. With considerably higher wall-plug efficiency, target fusion gain needed for economic power generation can be much lower than for conventional laser driven ICF. For example, if the wall-plug efficiency of the driver is higher than 30%, a fusion gain as low as 30 may be acceptable for IFE purposes.
Solid and liquid shells (called liners) have been proposed for compressing various types of magnetized target plasma for low gain MIF in which fusion gain in the range of 10 – 30 is sought. Plasma liner might prove to be more attractive for energy applications eventually, and are being explored for its potential for achieving intermediate fusion gain up to about 50.
At the Shiva Star pulsed power facility at AFRL-Kirtland, we have successfully demonstrated the implosion of an aluminum liner of the required geometry (30 cm long, nominally 10 cm in diameter and 1.1 mm thick) for compressing an FRC in 24 μs, achieving a velocity of 0.5 cm/μs, a kinetic energy of 1.5 MJ from stored capacitor energy of 4.4 MJ, and a radial convergence of 16 without observable Rayleigh-Taylor instability.
A dedicated experimental facility (FRX-L) for developing high-density, compact FRC as targets for MIF, including the translation and capture of the FRC by a metallic liner, has been developed at the Los Alamos National Laboratory. The FRC is formed by a field-reversed theta pinch in a quartz tube about 0.5 m long and 10 cm in diameter. Experiments at FRX-L have produced FRC with densities of about 3 x 10^16 cm-3, temperature () of about 300 eV corresponding to pressure of about 30 bar with a lifetime of about 10 μs. It has also developed a considerable database for the FRC behavior for various combinations of bank voltages, trigger timing and pre-fill gas pressure.
The experiment is now ready to combine the FRC generation technique developed at Los Alamos with the Shiva Star facility to perform an integrated liner-on-plasma implosion experiment. The integrated experiment will advance our predictive understanding on the compression heating of the FRC to multi-keV temperatures and 10^19 cm-3 plasma densities. This experiment will be performed over the next few years.
Helion and Related
Plasma liner formed by plasma jets provides an avenue to address three major issues of low-to intermediate gain magneto-inertial fusion: (1) standoff delivery of the imploding momentum, (2) repetitive operation, and (3) liner fabrication and cost. If the plasma liner is used to compress the magnetized plasma directly, very high Mach number (> 15) is required of the plasma liner in order to reach fusion conditions.
At UC-Davis, the acceleration of compact toroids is being studied in the CTIX facility. CTIX is a switch-less accelerator with the repetitive rate currently limited only by the gas injector. Magnetized plasma with a density of 10^16 per cm3 has been accelerated to 150 km/sec in the 1.5m long accelerator at a repetition rate of 1 Hz. Up to a thousand plasmas per day may be formed without the need to refurbish machine parts. At Caltech, an experimental facility is available for addressing the fundamental science issues governing magnetic reconnections, MHD-driven jets and spheromak formation. The inter-shot time is 2 minutes, and a large number of shots can be taken without hardware damage.
The plan for the next 3 years is to demonstrate acceleration of plasma to form jets with velocity exceeding 200 km/s and Mach number greater than 10 and to conduct experiments to explore the physics of merging jets. Concurrently standoff methods to produce seed magnetic fields will be explored conceptually.
At MSNW Inc. and the University of Washington in Seatle, WA, an experimental facility is being established to generate a database on plasma-liner compression of a magnetized plasma. Two inductive plasma accelerators (IPA) have been constructed and tested forming a stable, hot (400 eV – 800 eV) target FRC with density 5 x 10^14 cm-3 for compression. 2D cylindrical imploding plasma shell will be created by theta pinch and will be available in the near future for experimental campaigns to compress the FRC. If successful, research will continue in the next five years to create high-density (> 10^17 per cm3) and keV magnetized plasmas.
More Details on magnetized high energy density laboratory plasmas
At Caltech, an experimental facility is available for addressing the fundamental science issues governing magnetic reconnections, MHD-driven jets and spheromak formation. The research emphasizes experimental reproducibility, diagnostics, and achieving agreement between observations and first-principles theoretical models. The inter-shot time is 2 minutes, and a large number of shots can be taken without hardware damage.
The plan for the next 3 years is to demonstrate acceleration of plasma to form jets with velocity exceeding 200 km/s and Mach number greater than 10 and to conduct experiments to explore the physics of merging jets. Concurrently standoff methods to produce seed magnetic fields will be explored conceptually. If successful, research will continue in the next 5 years to increase the Mach number to 20 and to develop a user experimental facility with an array of plasma jets to form plasma liners for a variety of research. The research will include creating high energy density matter and compressing magnetized plasmas to reach keV temperatures and high magnetic fields.