WIR October 31, 2016
U.S. physicists have proposed a new plasma heating mechanism that prevents interior fusion reactor damage while maximizing energy transfer.
Utilizing one of the most important methods for heating plasma, the proposed strategy promises to help prevent damage to the interior surfaces of the reactor, while also keeping a tighter control of the amount of energy injected into the system.
The General Atomics team, which works on the DIII-D tokomak at the center’s San Diego lab, was responsible for tackling difficulties with the device’s neutral beam injection system.
According to the physicists, neutral field injection works by accelerating positively-charged ions through an intense (90,000V) electric field and sending them into a chamber of dense gas, where they pick up electrons and lose their positive charge. The acceleration field gives them so much momentum that they fly through the neutralizing gas and straight into the tokamak.
However, in the past this has caused problems. Injecting these fast particles into the plasma can cause or amplify electromagnetic waves that kick the particles straight out of the plasma and into the tokamak walls, causing damage to the wall surface and losing the energy that the particle was supposed to transfer to the plasma.
Now the General Atomics team has successfully devised a method for tuning the accelerating field so that the velocity of the neutral particles as they enter the plasma differs. This responds to changes in the behavior of electromagnetic waves in the plasma as it heats up, which in turn changes the way that neutral particles interact with it.
The new system varies the velocity of the neutral particles to minimize their interaction with the electromagnetic waves; this keeps the particles in the plasma while also maximizing the input heating power.
This was a major undertaking, in part because of the sheer size of the neutral beam injection equipment. DIII-D is a relatively small tokomak, 1.67 m in diameter, but its eight neutral beam injectors are housed within four truck-sized units.
“This project involved two years of engineers and physicists working hard to create something new, and it’s wonderful to see it working successfully on DIII-D,” said David Pace, a physicist who led the project for the GA Energy Group, “Now we get to focus on the next exciting step, which is demonstrating all the ways these variable voltage beams can improve magnetic fusion in machines across the world.”