Recent computer simulations have suggested a novel method for launching the plasma without using solenoids. The simulations at the National Spherical Torus Experiment (NSTX), the major fusion facility at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) shows the formation of distinct, current carrying magnetic structures called plasmoids that can initiate the plasma and complete the complex magnetic field.
Left: Plasmoid formation in simulation of NSTX plasma during startup without solenoid. Right: Fast-camera image of NSTX plasma shows two discrete plasmoid-like structures. Image Credit: NSTX. Click image for the largest view.
The tokamak is an experimental chamber that holds a gas of energetic charged particles, plasma, for developing energy production from nuclear fusion. Most large tokamaks create the plasma with solenoids – large magnetic coils that wind down the center of the vessels and inject the current that starts the plasma and completes the magnetic field that holds the superhot gas in place. But future tokamaks must do without the solenoids, which run in short pulses – rather than for weeks or months at a time as commercial fusion power plants will have to do.
The device starts with magnetic field lines, or loops, that rise through an opening in the floor of the tokamak. As the field lines are electrically forced to expand into the vessel, a thin layer, or sheet, of electrical current can form. Through a process called magnetic reconnection, the sheet can break and form a series of ring-shaped plasmoids that are the magnetic equivalent to the bubble rings created by dolphins.
The computationally predicted plasmoids have been confirmed with fast-camera images inside at NSTX, the major fusion facility at PPPL where the facility has since been upgraded. The plasmoids merge to form a large ring carrying up to 400,000 amperes of current, creating a plasma start-up phase inside the tokamak.
This advanced modeling of plasmoids also led to another major finding: the conditions under which a large volume of field line closure and maximum start-up current can be achieved by the upgrade of the National Spherical Torus Experiment.
Plasmoid-like structures are also observed in nature such as during eruptive solar events. The global plasmoid formation observed in the tokamak sheds new light on the magnetic reconnection process and the trigger mechanism of solar flares. These findings also reveal that the same plasmoid-mediated reconnection that occurs in space has a leading role to play in closing magnetic field lines and starting up plasma in National Spherical Torus Experiment Upgrade.
There appears to have been quite a lot of ground gained in the past year for the Tokamak researchers. Today’s news starts to answer the major problem observers have had with the Tokamak for decades, the short repeating pulse in a massive device that itself consumes huge amounts of energy.