Vertical Stability Improved for S. Korean Tokamak

Engineering 360 S. Himmelstein | 19 January 2018

Cross-section of the KSTAR tokamak showing select hardware components of the vertical control system: new magnetic flux loops (magenta circles) used to infer the vertical position of the plasma and vertical magnetic field coils (red squares) that control the position. A new algorithm sustained a stable plasma discharge #18380 (magenta) that was significantly taller than discharges such as #18602 (black) that used an earlier algorithm and suffered vertical oscillations. Image courtesy of Nick Eidietis, General Atomics

The Korean Superconducting Tokamak Advanced Research (KSTAR) device, the largest fusion system in South Korea, has operated successfully since 2008. However, maintaining the vertical position of the ultra-hot plasma, which is required for precise positioning of the plasma boundary, has proven difficult. A new control algorithm recently developed by U.S. Department of Energy Princeton Plasma Physics Laboratory researchers has improved the ability to control the vertical position and resulted in record tall plasmas that exceed the KSTAR design specifications.

The new scheme will allow researchers to study plasma conditions very similar to those that will be created in the International Thermonuclear Experimental Reactor (ITER) tokamak, which is being built in France as the world’s largest magnetic confinement plasma physics experiment.

The shape of the plasma boundary in fusion energy experiments, such as KSTAR and ITER, must be carefully controlled to achieve the plasma temperatures and densities required to access and sustain fusion burn. As plasma shapes become taller, or more “elongated,” larger plasma currents can be sustained and support increased fusion power output, but the requirements for stable control of the vertical position become more stringent.

Compared to conventional tokamaks that use magnetic field coils made from copper and located close to the plasma surface, the magnetic field coils in superconducting tokamaks are fewer in number and are located further away to accommodate coil cooling and radiation shielding systems. This coil configuration tends to couple plasma control loops that are largely decoupled in conventional tokamaks. The new digital control algorithm developed in the KSTAR plasma control system integrates multiple control schemes to effectively decouple the vertical position control from other control loops used to maintain the plasma current, plasma shape, and radial position.

Improved plasma positioning control will help the KSTAR team devise techniques for successful steady-state physics operation of ITER. The new capability also supports the main mission of the KSTAR project: to establish the scientific and technological bases for an attractive fusion reactor as a future energy source.