Tokamaks are devices that use magnetic fields to confine plasma in a toroid shape. Recent advances in the efficiency and control of tokamaks have put us closer than ever to the goal of controlled thermonuclear fusion.
Three major tokamak innovations recently made their debuts and the announcements are particularly exciting.High-Bootstrap Current
In the high-bootstrap current scenario, self-generated (“bootstrap”) electrical current is enhanced to find an optimal tokamak configuration for fusion energy production. Recent investigation into this scenario has directly demonstrated the stabilizing effect of reducing the plasma-wall distance in tokamaks with high plasma pressure and large bootstrap current fraction.
The investigation was an attempt to resolve a performance-inhibiting wobbling effect known as “kink mode” instability. The proposed solution was to move the plasma closer to the reactor wall, a suggestion that was initially met with resistance.
"This is unlike any other regime," said Dr. Andrea Garofalo of General Atomics, who led the investigation. "It's very risky to move the plasma that close to the wall. The chief operator said 'You can't do that anymore, you're going to damage the machine,' so it was a struggle to prove our theory was correct."
External view of the DIII-D tokamak. (Image courtesy of Nick Balshaw.)
Fortunately for Garofalo and her colleagues, the risk paid off. Moving the plasma closer to the wall of the DIII-D tokamak suppressed kink mode and enabled higher pressure inside the device. The result is a “pressure-driven” plasma flow that maintains confinement quality even with lower external injection velocity. This is a significant advancement, given the difficulty and expense of driving rapid plasma flow externally.
Shattered Pellet Injection
Recent experiments have reduced hot spots in tokamaks by rapidly injecting frozen pellets directly into the plasma. A pellet is injected into the plasma “at the speed of a bullet” but shatters before entering the tokamak, inspiring the method’s name. The pellet fragments cause the plasma to radiate energy uniformly as light, reducing localized heating of the tokamak wall as well as the mechanical forces exerted on the machine.
Plasma disruptions occur when the plasma loses its thermal and magnetic energy in a few thousandths of a second. This results in very large heat loads and mechanical forces on the walls surrounding the plasma, which could damage the reactor.
Using pellets composed of a mixture of neon and deuterium allowed researchers at Oakridge National Laboratory to control various aspects of plasma disruptions. Varying the amount of neon resulted in significant differences in concentrated heat loads transferred to the machine wall. Other properties of disruption, such as plasma current decay and the flow of halo currents, also varied significantly depending on the pellet mixture.
Super-H Mode
Finally, a new state of tokamak plasma raises pressure at the plasma’s edge beyond what was previously thought possible. This has the potential to increase power production.
The research was motivated by theoretical predictions of a plasma state beyond H-mode, the current regime for high-level plasma performance. Using the EPED model, researchers at the Princeton Plasma Physics Laboratory (PPPL) predicted more than one type of edge region in tokamak plasmas, including the previously undiscovered Super-H.
These regions are called pedestals because they serve as ledges in H-mode plasmas from which the pressure drops off sharply. The higher and wider the pedestal, the greater the density and pressure of the plasma. The plasma’s density and pressure act together to contain it at over 100 million degrees Celsius.
Graph showing the relationship between plasma pressure (x-axis) and density (y-axis) in a tokamak reactor. Red signifies instability while blue is the quiescent region. (Image courtesy of General Atomics.)
Researchers were able to reach the Super-H mode regime by steadily increasing density in a quiescent state which naturally avoids pedestal collapses. This caused the plasma to follow the narrow path to the newly discovered Super-H mode.
"It's an important way that we can reach fusion conditions efficiently," said Philip Snyder, whose model predicted the new pedestal height corresponding to Super-H mode.
For more information on these projects, visit the website for the 57th Annual Meeting of the American Physical Society’s Division of Plasma Physics.