I have posted a number of essays about research into nuclear fusion. If the reaction that powers the Sun and stars could be harnessed, it would provide abundant energy. The popular donut shaped tokamaks have serious problems confining the super-heated plasma to the center of the cavity.Super powerful magnets are necessary to create a container of magnetic fields. If the plasma breaks confinement and touches the walls of the cavity, it can quench the fusion reaction and cause damage to the walls. So it is very important to be able to correct any errors in the shape of the confining magnetic fields.
       Scientists at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) led a team of researchers drawn from the PPPL and, Sandia National Laboratory, General Atomics and Oak Ridge National Laboratory. The team found clear evidence of error fields in the first ten week test run of the National Spherical Torus Experiment—Upgrade (NSTX-U). The NSTX-U is considered to be the “flagship” fusion device at the laboratory. The exhaustive detection method used by the researchers could provide important information about such error detection in future fusion devices such as the ITER which is a huge international fusion research reactor currently being constructed in France.
       At the PPPL, the researchers have integrated experimental data, detailed measurements of the positions of the confining magnets and computer models of plasma response in an effort to pin down the source of the error fields in the NSTX-U. Their analysis revealed that there were many small error fields. This is thought to be an unavoidable result of the impossibility of making a perfectly symmetrical tokamak. In most cases, these small error fields can be easily corrected. However, there was one find in their study. If the magnetic coils that run down the center of the tokamak are slightly out of alignment, this can produce fields that wrap around the inside of the donut shaped tokamak plasma cavity.
       It turned out that this misalignment was exactly what the researchers were looking for. Nate Ferraro is the first author of the research report in the journal Nuclear Fusion. He said, “What we found was a small misalignment of the center-stack coils with the casing that encloses them.” The slight misalignment caused errors that resonated with the plasma and resulted in changes in the behavior of the plasma. One of the effects was what was referred to as a “braking and locking effect.” This kept the edge of the plasma from rotating. It also increased local heating on components that faced the plasma inside the tokamak.
       The PPPL researchers discovered the misalignment after the tokamak had been shut down for repair due to the failure of one of the coils. The researchers are working on creating new tighter tolerance requirements for the NSTX-U as it is being reconstructed. The alignment of the center stack of coils and the casing around the coils must be more precise. It is hoped that the tighter tolerances will reduce the deviation from optimal alignment of the two components to less than two one-hundredths of an inch.
       The authors of the research report hope that the new alignment will prevent localized heating and reduce the magnetic braking and locking. These improvements would improve the stability of the plasma. Ferraro said, “Every tokamak is concerned about error fields. What we are trying to do is optimize the NSTX-U.” 
       Amitava Bhattacharjee is the head of the PPPL Theory Department. He said that “This is an excellent example of the NSTX-U-Theory Partnership program that has been beneficial for both the NSTX-U and Theory Departments at PPPL, and which continues even when NSTX-U is in recovery.” The DOE Office of Science provided funding for this work. Members of the research team included scientists from PPPL, Sandia National Laboratory, General Atomics and Oak Ridge National Laboratory.