In the research and development of a nuclear fusion reactor, one of the major problems is keeping the super-hot plasma in a stable configuration that does not touch the interior wall of the vacuum chamber that contains the plasma. If the plasma touches the wall, it can lose shape and temperature, quenching the reactions that produce fusion.
Copper coils have fast reaction time to reshape the magnetic fields confining the plasma but they require huge amounts of energy. Superconducting coils are much more efficient and require a great deal less power to maintain magnetic fields but they are slower to adjust their configuration. One aspect of the configuration of the plasma that is especially important is the vertical height that the plasma can reach and still be stably maintained.
The Korea Superconducting Tokamak Advanced Research (KSTAR) device is one of the biggest superconducting tokamaks in the world. It uses niobium and tin in its superconductors. This is the same type of alloy that will be used to construct the superconductors that will be used on the ITER fusion reactor being built in France by an international consortium.
U.S. researchers from the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL), General Atomics in San Diego and Korean researchers from the National Fusion Research Institute (NFRI) in South Korea formed a team to work on the stable containment problem. The team was led by Dennis Mueller of the DoE PPPL. They have radically improved the ability of the KSTAR to maintain the stability of the vertical dimension of the plasma cloud. The new control method was demonstrated this summer. It managed to control the vertical stability in the KSTAR where, previously, the plasma bounced up and down in the eleven foot tall device.
Mueller gave a presentation to 59th annual meeting of the American Physical Society Division of Plasma Physics in October. He said "As the plasma got taller it moved away from stable operation, the new correction method stops the plasma from bouncing up and down by stabilizing the vertical center of the plasma. Control of the vertical instability has allowed for taller plasmas in KSTAR than the original design specifications."
The key to managing the plasma turned out to be modifying the electronics for the sensors that monitored the configuration of the magnetic field in the plasma as well as the motion and position of the plasma. The sensors were reconfigured to speed up the control signal that informed the system of the vertical position of the plasma. There is a vertical control coil inside the vacuum chamber that contains the plasma. This control coil is able to push back against the plasma and prevent it from contacting the walls of the vacuum chamber.
Nicholas Eidietis of General Atomics devised a control system that can tell the difference between fast and slow changes in the sensor signals. The new control system can use different coils to modify plasma movement to maintain stability on different time scales. The final result of this work is the development of a control system that responds quickly to specific movements of the plasma which allows the reactor to operate with taller plasmas than the KSTAR was originally designed to handle.
Korean Superconducting Tokamak Advanced Research (KSTAR) device:
