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Nuclear fusion for energy generation is an area of intense research today. The ability to harness the process that keeps the sun burning in the sky would be a major leap in the search for cheap and clean energy. Many companies are working on the development of commercial nuclear fusion reactors.
Researchers at the Princeton Plasma Physics Laboratory (PPPL) have carried out an analysis which proves that a new non-standard way of initiating plasma in nuclear fusion reactors. The new technique is referred to as “transient coaxial helical injection” (CHI). This new technique eliminates the central magnet that is currently used to create the plasma inside a tokamak. Tokamaks are the most common type of fusion reactors under development.
The elimination of the central magnet may facilitate steady state fusion reactions. In addition, the removal of the central magnet will free up space inside the center of compact spherical tokamaks, Most tokamaks are shaped like a donut. The plasma is confined to a ring around the inside of the tokamak. Spherical tokamaks are shaped more like an apple which has the core removed. This design leaves less room in the center of the spherical tokamak than is found inside the donut shaped tokamaks and it would benefit from gaining additional space in the center.
This increased space in the center of the tokamak could have a variety of uses. Additional magnets could be installed that would help confine the plasma and improve the performance of the reactor. The new space could also simplify the design of compact spherical tokamaks.
In a conventional donut-shaped tokamak, magnets in the center of the donut induce a current in the neutral gas that has been injected into the donut-shaped chamber. The current separates the electrons from the atoms of the gas which creates the charged gas known as a plasma. The current also creates a magnetic field which joins the fields generated by the magnets wound around the donut. These fields confine and heat the plasma to allow the fusion reaction.
In the new CHI, electrodes are placed at the top or bottom of the central cavity. These electrodes produce the electrical currents needed to create the plasma. Kenneth Hammond is a physicist at the Max Planck Institute of Plasma Physics. He is the lead author of the paper reporting this research that was published in the Physics of Plasmas journal. He said, “What we primarily focused on was the beginning stage of forming the plasma.”
The new plasma initiation process is called “transient CHI” because it is only turned on briefly at the beginning of and experiment and does not run constantly. This new technique was developed using the small Helicity Injection Torus at the University of Washington and the bigger National Spherical Torus Experiment (NSTX) at PPPL. The process was also modeled on computers at the PPPL.
One of the problems with scaling up the new technique for larger spherical tokamaks lies in the placement of the electrodes. It took some work to find the correct placement of the electrodes necessary to use the CHI in bigger reactors. The CHI scaling simulations were carried out in a special computer modeling language called the Tokamak Simulation Code which was developed at PPPL. A new test is scheduled in which there will be two different sets of electrodes to see if that will improve performance.