Scientists Evaluate the Effects of Fusion on Plasma

Columbia University Record, Vol. 19, No. 15

Photo: Prof. Gerald Navratil (left) and Prof. Michael Mauel (right)

Columbia researchers will begin a series of experiments in February or March designed to show that close to break-even fusion reactions can be sustained using reduced levels of electric current to confine the extremely hot plasma in which controlled fusion takes place. If successful, the experiments would significantly advance the goal of producing economical fusion power.

The experiments will be conducted at Princeton's Tokamak Fusion Test Reactor (TFTR), where researchers last month produced a record six million watts of fusion power using a new fuel mixture of deuterium and tritium, heavy isotopes of hydrogen. The addition of tritium to the tokamak's extremely hot plasma, or collection of charged particles, facilitates the fusion of hydrogen nuclei and releases huge quantities of energy.

Princeton scientists are working to achieve outputs as high as 10 million watts in the coming year, about half the energy required to produce the reaction. Fusion experimenters expect the Joint European Torus (JET) in Oxfordshire, England, the world's largest tokamak, to produce the same amount of energy as required to sustain the reaction--break-even--before the year 2000.

EXPERIMENTS SINCE 1990

In experiments since 1990 at the Princeton facility and at Columbia's own smaller tokamak, Columbia researchers have developed a high-pressure operating regime that requires less current to heat and confine the plasma, about one million amperes rather than TFTR's normal two million amperes.

In these experiments, Columbia scientists hope to increase the tokamak's pressure at the lower current, extend its stable operating time and produce results comparable to those Princeton researchers expect, in the range of five to seven million watts.

NAVRATIL AND MAUEL

The Columbia researchers, Gerald A. Navratil, professor and chairman of the department of applied physics, Michael Mauel, associate professor of applied physics, and Steven Sabbagh, research scientist, expect their work at TFTR to lead to a reduction in size and cost of future fusion power plants.

Columbia's fusion research has focused for nearly 30 years on creating a high beta, or pressure, since the energy output of a fusion power plant is a function of the square of its internal pressure.

The goal of Columbia's experimental fusion program is to understand and control instabilities that can occur in the 100-million-degree plasmas required for controlled nuclear fusion.

The Columbia project includes work at its own High Beta Tokamak-Extended Pulse (HBT-EP), located on the Morningside Heights campus and at the Princeton tokamak. The tokamak design, in which magnets confine the plasma in a doughnut-shaped chamber, was developed in the Soviet Union in the 1960s. "Tokamak" is a Russian acronym for "toroidal magnetic chamber."

A key scientific question to be investigated in the deuterium- tritium fusion experiments Columbia researchers will undertake is the effect of fusion reaction products on plasma stability. Instabilities can result from the high-energy behavior of helium nuclei, or alpha particles, that are a product of fusing tritium and deuterium.

About 20 percent of fusion energy generated in deuterium-tritium fusion reactions is transferred to alpha particles. Columbia scientists don't know--but hope to find out--whether the energetic alpha particles promote fusion by increasing the plasma pressure limit or impede it by destabilizing the plasma.

"The Columbia team is anxious to proceed with the proposed experiments, which will show us whether the alpha particles spoil the performance of this low-current mode of operation," Navratil said.

IMPROVING PLASMA STABILITY

To improve the plasma stability at the Princeton tokamak under the low-current regime, the Columbia researchers have changed the spatial distribution of the current flowing in the plasma.

In experiments now underway at Columbia's HBT-EP, an experimental team, led by Navratil and Mauel and consisting of Tom Ivers and Viju Sankar and eight graduate students, are using new methods, unavailable at TFTR, to increase further the tokamak's pressure limits and better control instabilities.

Among those techniques are wall stabilization, using HBT-EP's half-inch-thick internal aluminum shell as a conducting surface to slow instabilities, and active feedback control, measuring the temperature, shape and density of the plasma and attempting to correct instabilities by manipulating the magnetic field surrounding the plasma.

$600 MILLION EXPERIMENT

Columbia's fusion research has played an important part in the design of a new, $600 million Tokamak Physics Experiment, on which construction is to begin at Princeton late next year. Researchers hope that experiment, which will use superconducting magnets to confine the plasma, will attain continuous operation.