MST Frequently Asked Questions

* What is MST?

MST is physics research device (not a reactor!) in which an intense magnetic field is used to confine an electrically conducting gas (called a plasma), heated by electric currents to temperatures of several million degrees celsius.

* What do the letters stand for?

Madison Symmetric Torus. A torus is a geometric shape of which a doughnut is perhaps the most familiar example. Symmetric means that any slice through a cross-section of the doughnut looks like any other, in this case, a circle. Madison, Wisconsin is the home of the MST.

* How does it work?

The torus is evacuated to a pressure of a billionth of an atmosphere. A puff of hydrogen gas is admitted and quickly fills the torus. A voltage of about 100 volts is produced with a 50-ton iron transformer core that links the torus and is driven by a bank of capacitors. This voltage creates an electrical discharge much like a bolt of lightning with an electric current approaching one million amperes. This current heats the gas to several million degrees whereupon it turns into a plasma. The current also creates a magnetic field that helps confine the plasma away from the walls of the torus which would cause it to cool. The whole process takes place in less than a tenth of a second and is repeated about every three minutes.

* Why study plasmas?

Plasma is the fourth state of matter. Like solids, liquids and gases, it has unique properties. Just as any substance becomes a solid if cooled sufficiently, any substance will become a plasma if heated enough. In a plasma the electrons are stripped from the atoms, creating a substance that resembles a gas but that conducts electricity. Over 99% of the universe is a plasma. The stars, in particular, are giant balls of plasma. The earth is an unusual place where matter exists in the other three forms. Plasmas occur on the earth in electrical discharges, fluorescent lamps, the upper atmosphere (ionosphere) and the aurora borealis (northern lights). Plasmas represent a fundamental field of study, with many astrophysical and terrestrial applications. One important application is the use of plasmas to produce electricity from magnetic fusion energy (sometimes called controlled nuclear fusion).

* What is fusion energy?

Fusion is a nuclear process, not to be confused with the fission process that powers all present nuclear reactors. Fusion is quite different from fission. Fission uses as its fuel heavy elements like uranium which are split into lighter elements with the production of undesirable radioactivity. By contrast, fusion combines light elements like isotopes of hydrogen to form heavier elements, producing relatively little radioactivity. Fusion fuel is abundant and inexpensive. Each gallon of water has about an eighth gram of deuterium, which if burned in a fusion reactor, would produce as much energy as 300 gallons of gasoline. A fusion reactor is also much safer than a fission reactor because very little fuel is in the reactor at a time. Although the stars are fusion reactors, the wide-scale production of controlled fusion energy on earth has not yet been achieved, despite some forty years of worldwide effort.

* Why is controlled fusion so difficult?

Unlike fission, which is easy to achieve, fusion occurs only at very high temperatures--at least one hundred million degrees celsius. The nuclei of the atoms have electric charge and naturally repel one another. Thus they can be made to collide and fuse only when moving very rapidly. Not only is it difficult to heat the fuel to such high temperatures, but a special kind of bottle is required to confine the hot plasma. One would think that the plasma would melt the walls of the bottle, but the amount of fuel is too small even for this. Instead, the plasma has to be prevented from touching the walls lest it cool down and stop reacting. In MST, this is accomplished with a magnetic field.

* How does a magnetic field confine a plasma?

Unlike an ordinary gas whose molecules are electrically neutral, the constituents of a plasma are positively charged nuclei and negatively charged electrons. A magnetic field exerts a force on a moving charged particle, causing it to circulate in a tight orbit around the magnetic field line. On the other hand, the particles are free to move along the direction of the magnetic field, like beads on a string. By wrapping the field into the shape of a doughnut, this motion along the direction of the field does not result in loss of the plasma. Another way to think of it is to consider the plasma as a current-carrying electrical conductor on which a magnetic field can exert a force, as in an electric motor.

* How does MST differ from other plasma confinement devices?

The most common and extensively studied plasma confinement device is the tokamak, developed first by the Soviets, but now used worldwide. Like MST, the tokamak is a symmetric torus. Unfortunately, the tokamak requires a very large magnetic field to work properly. The large field increases mechanical stresses and necessitates the use of expensive superconducting magnets. MST belongs to a class of device called a reversed field pinch (RFP). An RFP requires a much smaller magnetic field that goes one direction around the torus inside the plasma and the opposite direction outside (hence the term "reversed"). "Pinch" means that the magnetic field produced by the plasma current squeezes the plasma and minimizes its interaction with the wall. The lower field in an RFP means that reactors based on this concept would be more compact and less expensive.

* What is MST used for?

MST is used to investigate the cause and consequence of plasma instabilities and turbulence. A fascinating feature of magnetically-confined plasmas is that they spontaneously generate growing waves, which are unique to this state of matter. With many such instabilities present the plasma becomes a chaotic, turbulent medium. The understanding of turbulence and the distinction between order and chaos is one of the major persistent problems of twentieth century physics, with application to most scientific disciplines. Plasmas provide a vivid realization of such phenomena. The high level of turbulence in the RFP has both bad and good consequences. The turbulence causes enhanced loss of plasma energy to the wall. This loss is one of the key unsolved and limiting problems in fusion research. On the other hand, turbulence in the RFP is believed to cause the plasma to regenerate its own magnetic field. This "dynamo" effect may be similar to the dynamo that generates fields around stars and galaxies. In MST, these processes are being investigated. In addition, investigation of plasma confinement in a machine as large as MST is providing new insight into the plasma loss processes.

* Are there disadvantages to reversed field pinches?

The RFP is believed to require a shell with high electrical conductivity very close to the boundary of the plasma. This requirement is an unfortunate complication in a reactor. MST was designed to test this assumption and to learn how good the conductor must be and how close to the plasma it must be placed. In addition, the plasma confinement in the best RFP's is only about 1% as good as in the best tokamaks. On reason for this is that all existing RFP's are relatively small. MST is larger than any previous RFP device, and thus it can test this important size issue.

* Just how large is MST?

The major radius of the torus is 1.5 meters, and the minor radius is 52 centimeters. The outside diameter of the vacuum vessel is about fourteen feet. The volume of plasma is about eight cubic meters, approximately eight times greater than previous RFP devices. The maximum plasma current is about one million amperes, twice as large as in any previous RFP. Despite its large size, MST was constructed to very high precision to reduce magnetic field errors. Most dimensions are accurate to a few hundredths of an inch.

* How many other RFP's are there?

Several. In this country, RFP research was pioneered by the Los Alamos National Laboratory in New Mexico, but they no longer work in this area. There are RFP devices in Italy, Japan, and Sweden. The Italian device (RFX) is about the same size as MST but is capable of higher current operation.

* Who designed MST?

The MST device was designed by a scientific team consisting of University of Wisconsin-Madison Physics Professors Richard Dexter, Stewart Prager and Clint Sprott. Dexter was the Project Manager, responsible for the construction of the facility. Thomas Lovell was Engineering Manager and supervised all facets of the engineering design with the assistance of personnel from the University Physical Sciences Laboratory (PSL) in Stoughton, Wisconsin under the direction of Design Engineer Farshid Feyzi. Emeritus Professor Donald Kerst (deceased) was a consultant, and contributed substantially to the conceptual design.

* Who built it?

The construction was coordinated by PSL. The major component, the large toroidal vacuum vessel, was made under subcontract by an Italian fabricator, DePretto Escher-Wyss through competitive bids. The final assembly was coordinated by Dexter, Lovell, Feyzi and Chief Technician John Laufenberg with the help of a large team of graduate and undergraduate students.

* Who uses it?

The device is used by six faculty and scientists, about eight graduate students, several post-doctoral research associates, and visiting scientists from other labs. Modifications and maintenance are directed by Laufenberg employing a team of about a dozen part-time technical assistants.

* Who paid for it?

The construction and operation are funded by a grant from the U.S. Department of Energy.

* How much did it cost?

Funding for the official three-year construction period which ended March 15, 1988 totalled $3.6 million. Some of these funds were used for supporting experiments during the design and early construction phases.

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