• Number 316  |
• July 19, 2010

Cranking up the voltage in electrostatic accelerators

Electrostatic accelerators have been around for a while in everything from scientific instruments to television sets and inkjet printers.

But none of these have required the electrical voltages or currents demanded by accelerators in present and future plasma and nuclear fusion machines.

Scientist Larry Grisham from DOE's Princeton Plasma Physics Laboratory has proposed an idea for enhancing the voltage capacity of accelerators that produce beams of charged particles to heat and drive current in plasmas.

“It’s a natural fit to what we do in fusion,” Grisham said. “We have very extreme conditions, and the breakdown problems are much more severe.”

The voltage in an accelerator is limited by the conditions of spontaneous electrical breakdown, in which electrons leave the surface of one electrode and travel to the surface of one at a higher voltage. The result is often electrical sparking in the gap between the electrodes.

As the voltage increases, the distance between electrodes at which breakdown occurs also increases.
The high voltages in accelerators in plasma and fusion reactors motivate the development of ideas for increasing the voltage capacity for electrodes with smaller spacing.

Grisham’s idea, which he said is based on straightforward electromagnetic physics, is to run electrical current through the stages of the accelerator to produce a magnetic field that will discourage electrons from leaving an electrode surface. This would allow accelerators to be smaller but still contain high voltages.

Grisham said the concept will be tested at PPPL later this year or early next year.  Since the idea has potential for commercial application, a patent application has been filed.

The applications will depend on whether experiments show that moderate or very strong magnetic fields are required to enhance voltage capacity. But it will almost certainly be of interest for plasma and fusion research.

“Making the gaps in the accelerator very large has big drawbacks,” Grisham said. “We have a lot of incentive to try to make the gaps smaller.”

If the experiments go very well, Grisham said there is a chance the concept could be applied to future upgrades of the particle-injection system of ITER, the world’s first burning-plasma experiment that is scheduled to begin operating later this decade, or to future plasma and fusion reactors.

[Patti Wieser, 609.243.2757,
pwieser@pppl.gov]