During the last 30 years or so, the U.S. Department of Energy (DoE) has supported many approaches to resolve these concerns. Renewable energy sources provide little benefit, due to their very large size. Fusion is a promising option, yet considerable scientific and technological hurdles remain that make this resource impractical for the next half century. Although dramatic advances have been made in fusion research, the most pressing obstacle is to demonstrate unity gain (i.e., g = energy produced / energy input ~ 1) in a laboratory experiment. Reactor scenarios could then be visualized that provide reliable, cost-effective, and safe operation.
The two most prominent fusion energy concepts are magnetic fusion and inertial fusion. Magnetic fusion involves strong magnetic fields to confine the ionized fuels in a linear or toroidal geometry, akin to confining a mass of hot liquid by a surface boundary of stretchable, and massless strings. Inertial fusion involves spherically convergent implosions of 100-micron diameter glass micro-balloons, driven by 1 megajoule bursts of energy delivered in 1 nanosecond (i.e., power levels of order 1015 Watts). It is no surprise that the driver technology for inertial fusion involves enormous laser-beam or particle-beam facilities.
Recently the DoE has expanded their fusion investigations to include alternate energy concepts that promise significant fusion energy gain or provide scientific understanding related to fusion. Scientists in the UCI Department of Physics and Astronomy are studying the Staged Z-Pinch as one means to contribute to the technical breadth of the fusion energy program and to provide new insights in reactor design. The principal investigators of this project are Frank J. Wessel, Norman Rostoker, and Hafiz Ur Rahman (from the University of California, Riverside).
A schematic illustration of the Staged Z-Pinch discharge (load) region
is provided in the figure. In this concept an annular plasma liner is
accelerated toward the axis of symmetry and collapsed onto a co-axial
deuterium-tritium (DT) fuel. The liner could be initiated from an
annular-gas shell, a metal foil, or a wire array, and the DT fuel
could be supplied by a cryogenically-extruded fiber or a gun-injected
plasma, or by both.
A sequence of precisely-timed events comprises the staging process. First the liner is energized by an external circuit that generates a multi-megampere current in less than a microsecond. The large current ionizes the liner material and generates an azimuthal, self-magnetic field. As the magnetic field increases the plasma liner accelerates toward its axis (z-pinch). Near the axis the liner attains a final radial velocity of the order of 0.5 x 108 m/sec, with a kinetic energy approaching 40 % of the driver energy.
The DT fuel would be initially magnetized by axial- and azimuthal-magnetic fields using a pre-pulse circuit, separate from the main-power supply. With such high radial velocities and conductivity the plasma liner would compress the internal magnetic fields to peak intensities in the range of 1-10 kiloTesla in a few nanoseconds. The net effect would be to heat the fuel inductively, in a dynamically-changing magnetic environment that enhances stability and confines fusion-reaction products, which is called staging. When ignited the fuel will surpass the required density-temperature-time parameter needed for fusion gain, nTt > 1018keV-sec/cm3.
To date our theoretical and computational accomplishments include: an improved formulation of current amplification and heating of the target, benchmarked 1-D code calculations, confirmed by a standard radiation hydro-dynamic code at Livermore National Laboratories, and 2-D modeling capability that includes the key physics content for the Staged Z-Pinch. For a DT fuel and the parameters of the UCI laboratory device, our 2-D calculations predict thermonuclear yields of 1015 neutrons/pulse, temperatures of 15 keV, and densities of 7 x 1023/cm3. At these levels the thermonuclear gain is of the order of 20%. These results suggest that the Staged Z-Pinch could achieve near break-even fusion in a 10-50 kiloJoule device, i.e., orders of magnitude less driver energy than for other fusion concepts.
For the experiment our specific accomplishments include: assembly of a 60 kiloJoule, 1.5 megaAmpere, 0.1 microsecond class pulsed-power driver, development and installation of high-bandwidth diagnostics, extrusion of 100-micron diameter, D2 cryogenic-fibers, and the demonstration of stabilized Z-pinch implosions onto a target plasma. Progress related to this work is periodically updated on the project web page (http://mainpinch.ps.uci.edu).
In summary, the Staged Z-Pinch may provide a near-term means to produce high-gain thermonuclear plasmas while addressing key physics problems of long-standing concern. These include plasma stability in a dynamically-changing and high-magnetic shear, magnetic diffusion in a high-density plasma, plasma-dynamic energy transfer, transport of fusion products in a high-magnetic field, material equation-of-state at extreme-energy density, and radiation transport in a burning plasma. Since Z pinches are under active investigation elsewhere for fusion applications, a host of advanced technology issues are also beneficially impacted. Finally, if the plasma-dynamic energy-transfer mechanism in this concept works, it would be a fundamentally new pulsed power technique that achieves high-specific-energy and -power density, with exceptionally short-timescales.