FOR the past decade, most research in magnetic fusion energy has centered on the doughnut-shaped tokamak approach to generating fusion reactions. Tokamak work continues in the United States and abroad, but Department of Energy fusion energy scientists are also revisiting the spheromak, an alternative concept for attaining magnetic fusion.|
Much of the renewed interest in spheromaks is focused on a research effort at Lawrence Livermore called the Sustained Spheromak Physics Experiment (SSPX). The SSPX was dedicated on January 14, 1999, in a ceremony attended by representatives from DOE and collaborating scientists from the Sandia and Los Alamos national laboratories. SSPX is a series of experiments designed to better determine the spheromak's potential to efficiently contain hot plasmas of fusion fuel, in this case, the hydrogen isotope deuterium.
According to SSPX leader David Hill, the tokamak concept is considered the leading contender to generate sustained fusion reactions by heating plasmas to more than 100 million degrees Celsius (much hotter than the core of the sun) and confining them with magnetic fields. However, the tokamak's magnetic fields are generated by large, external magnetic coils surrounding the doughnut-shaped reactor. These large coils would increase the cost and complexity of generating electricity.
Spheromaks, however, confine hot plasma in a simple and compact magnetic field system that uses only a small set of external stabilizing coils. The necessary strong magnetic fields are generated inside the plasma by what's known as a magnetic dynamo. In this regime, the plasma-fast-moving, superhot ions and electrons-produces its own confining magnetic fields. The magnetic fields pass through the flowing plasma and generate more plasma current, which in turn reinforces the magnetic fields.
The powerful internal currents and magnetic fields become aligned so that they are nearly parallel to each other. Together, they form what Hill describes as something akin to a very hot smoke ring made of electrical currents.
Simple Design, Complex Behavior
Reanalysis Revived the Concept|
Interest in reviving the spheromak concept was triggered by a review of data from key Los Alamos experiments conducted more than 10 years ago. A thorough reanalysis led by Ken Fowler and Bick Hooper, former associate director and the assistant associate director, respectively, for Magnetic Fusion Energy at Livermore suggested that the plasma's energy confinement was up to 10 times better than originally calculated.
The analysis also showed that plasma confinement improved as the temperature increased. The thinking, says Hill, is that as temperatures increase in the spheromak, electrical resistance in the plasma decreases, so fusion reactions can occur more easily.
In light of the reanalysis, the scientific community and DOE managers considered it worthwhile to pick up where the Los Alamos experiments left off some 10 years earlier. Hill notes that the experiment is one of several alternative concepts being supported by DOE's Office of Fusion Energy Sciences, concurrent with its funding of tokamak research. (The Lawrence Livermore spheromak research is also supported by the Laboratory Directed Research and Development program.)
The overall goal of SSPX is to better understand spheromak physics by studying how magnetic fluctuations affect confinement. The experiments are designed to reach plasma temperatures of about 4 million degrees Celsius, similar to what the Los Alamos experiments obtained. Although this temperature is some 25 times cooler than that necessary to achieve fusion, it is sufficiently hot that energy is lost by processes similar to those that would occur in a fusion reactor. The experimental team will also attempt to keep the dynamo maintained in a hot plasma for 2 milliseconds instead of the 0.5 milliseconds achieved at Los Alamos.
The team is in the early phases of the project and is performing activities such as learning how to form the deuterium plasmas, achieving vacuum conditions, removing plasma impurities, and debugging diagnostic instruments.
Measuring Hot, Moving Currents|
The experiments involve injecting deuterium plasma into a reactor's 1-meter-diameter by 0.5-meter-high vacuum vessel. A 10-kilovolt, 0.5-megajoule startup capacitor bank supplies a voltage across two electrodes to form the deuterium plasma. A separate 5-kilovolt, 1.5-megajoule power system sustains the plasma for 2 milliseconds. During this brief moment, the plasma balloons down into the vessel, forming a hot, moving circular current of ions that creates magnetic fields, which in turn induce more current within the plasma.
Improving the understanding of spheromak physics requires accurate measurements of plasma density, temperature, turbulence, and magnetic field fluctuations. Because probes inserted into the plasma would disrupt the experiments, the researchers must rely on remote measurements taken through a slot located around the spheromak's center.
One measurement instrument, called a reflectometer, was designed by scientists at Lawrence Livermore and the University of California at Davis. It yields profiles of the magnetic field strength by injecting waves of varying polarized light into the plasma. The reflected waves carry information about the changing plasma density and the magnetic fields. Another measurement instrument injects a glass pellet across the plasma; a laser views the pellet and determines the magnetic field from the reflected light.
Hill notes that although situated at Livermore, the SSPX work benefits from contributions from colleagues at Los Alamos, Sandia, General Atomics, California Institute of Technology, University of California at Berkeley and at Davis, University of Wisconsin, University of Washington, and Swarthmore College. He adds that SSPX also benefits from the wealth of information about plasmas that has been gained from the past decade of tokamak studies.
In light of the extensive collaborations with researchers from other institutions, the experiment control room is equipped with video cameras that permit collaborators to view experiments remotely from their computers. The video cameras are part of a system developed by Livermore researchers to link magnetic fusion experimental sites nationwide.
If the results from SSPX are sufficiently promising, the research team will develop a larger, follow-up experiment. This experiment would aim at achieving much hotter, longer lasting plasmas.
Clearly, many experts are speculating that the method nature chooses to confine plasmas in space may well be the process scientists should mimic in designing a fusion reactor to generate electricity on Earth.
Key Words: CORSICA, deuterium, dynamo, magnetic fusion energy, plasma, spheromak, Sustained Spheromak Physics Experiment (SSPX), tokamak.
For further information contact David Hill (925) 423-0170 (firstname.lastname@example.org).