One step close to fusion power

An automobile engine that fired one cylinder and then required hours to fire again wouldn’t take a car very far. Similarly, a fusion machine intended to provide humanity unlimited electrical energy from cheap, abundant seawater can’t fire once and quit for the day. It must deliver energy to fuse pellets of hydrogen a number of times per minute and keep that pace up for millions of shots — a kind of an internal combustion engine for nuclear fusion.

A new electrical circuit that should carry enough power to attain the long-sought goal of controlled high-yield nuclear fusion also meets the other, equally important requirement for sustained fusion energy: It can fire every 10.2 seconds in brief, powerful bursts.

“This is the most significant advance in primary power generation in many decades,” says Keith Matzen, director of Sandia’s pulsed power center. The circuit would replace the large capacitors used today in Sandia’s pulsed power Z machine, called Marx generators, that store electrical energy and release it in microseconds to Z’s center. There, pellets are compressed by X-rays generated by the electromagnetic forces.

Made in Russia

The new system, called a linear transformer driver (LTD), was created by researchers at the Institute of High Current Electronics in Tomsk, Russia, in collaboration with colleagues at Sandia. The circuit has undergone preliminary validation at Sandia.

The circuit is about the size of a shoebox. Tightly packed in groups of 20 in a circular container, and electrically connected in parallel, the aggregate of the circuits, called a cavity, can transmit a current of 0.5 megamperes at 100 kilovolts. A test cavity at Sandia has fired without flaw more than 13,000 times.

Because the cavities are modular, they can be stacked like donuts on a metal stalk. Arranged thus, they could generate 60 megamperes and six megavolts of electrical power, enough (theoretically) to generate high-yield nuclear fusion within the parameters necessary to run a power plant.

The next-generation cavity model, now being tested in Tomsk, transmits 1.0 megamperes at the same voltage and with the same rapidity. Five such units have been built; four have been purchased by Sandia and one by the University of Michigan. They, too, are performing without flaw.

New technology advantages

The new switch would eliminate the need for the hundreds of thousands of gallons of insulating water and oil in which the present Z structure is submerged. The switch would achieve this in part by eliminating Z’s huge plates and extensive wiring, all of which generate magnetic fields.

Sandia’s high current LTD laboratory

Because of the proximity of the working components, there are few magnetic fields to slow the passage of current. Thus, linking the components in parallel decreases resistance and adds voltages. This allows for a powerful machine to fire very rapidly, with only a thin layer of oil bathing the rings and rows of switches.

The LTD technology is 70 percent more efficient than current Z machine firings, in terms of the ratio of useful energy out to energy in, says Matzen.

Funding for Z historically has been for defense purposes. Its experiments have been used to generate data for high-energy physics simulations on supercomputers that help maintain the strength, effectiveness, and safety of U.S. nuclear weapons.*

A powerful LTD machine would better simulate conditions created by nuclear weapons, so that data from the laboratory-created explosion of a Z firing could be used with greater certainty in computer simulations. The U.S. has refrained from actual testing of nuclear weapons for 15 years.

Overshadowed approach

For electricity-generation purposes, the repetitively firing fusion approach at the Z machine, known as inertial confinement, historically has been overshadowed by the technique called magnetic confinement — using a magnetic field to enclose a continuous fusion reaction from which to draw power.

Artist's conception of a future-phase LTD test facility with ten 10 mega-amp lines.

But fired repeatedly, the revamped Z machine could well form the basis of a fusion energy plant. To confirm the new Z concept would cost $35 million over five to seven years using a test bed with 100 cavities. If successful, future generations of Z-like facilities would be constructed with LTDs.

Funding thus far has come from two U.S. congressional initiatives, Sandia’s program, and Sandia’s inertial confinement fusion program.

“We think we need 60 megamperes to make large fusion yields,” says Matzen. “But though our simulations show it can be done, we won’t know for certain until we actually build it.” The device was designed by Tomsk pulsedpower head Alexander Kim, with the switch developed by Boris Kovalchuk. The work was led at Sandia and Tomsk by Sandia’s Mike Mazarakis. Sandia has filed a U.S. patent application on a high-power pulsed-power accelerator invented by William Stygar that can use an LTD as the primary power generator to replace the conventional Marx generator.