Part 2 of 2 Parts (Please read Part 1 first)
Biswas’s advisor Paul Bonoli has a great deal of experience with traditional “ray-tracing” models. These models evaluate a wave trajectory by dividing it into a series of rays. Bonoli has used this model for decades in his own research to understanding plasma behavior. Unfortunately, these models have serious limitations. Bonoli says that he is pleased that “the research results in Bodhi's doctoral thesis have refocused attention on the profound effect that edge turbulence can have on the propagation and absorption of radio-frequency power.”
Ray-tracing modeling of plasma scattering do not fully capture all the wave physics. A “full-wave” model of the old type would be prohibitively expensive. In order to solve the problem of tracking scattering economically, Biswas splits his analysis into two parts. The first part utilizes ray tracing to model the trajectory of the wave in the tokamak assuming that there is no turbulence present. The second part modifies this ray-trajectory with the new scattering model that takes turbulent plasma filaments into account.
Biswas says that “This scattering model is a full-wave model, but computed over a small region and in a simplified geometry so that it is very quick to do. The result is a ray-tracing model that, for the first time, accounts for full-wave scattering physics." Biswas points out that this model bridges the gap between simple scattering models that fail to match experimental data and full-wave models that are much too expensive. The result is that reasonable accuracy is achieved at a low cost. Biswas adds that “Our results suggest scattering is an important effect, and that it must be taken into account when designing future RF antennas. The low cost of our scattering model makes this very doable.”
Syun'ichi Shiraiwa is a staff research physicist at the Princeton Plasma Physics Laboratory. He said that "This is exciting progress. I believe that Bodhi's work provides a clear path to the end of a long tunnel we have been in. His work not only demonstrates that the wave scattering, once accurately accounted for, can explain the experimental results, but also answers a puzzling question: why previous scattering models were incomplete, and their results unsatisfying."
Work is now progressing to apply this new modeling method to more plasmas from the Alcator C-Mod and other tokamaks. Biswas believes that this new model will be especially applicable to high-density plasmas. The standard ray-tracing model has been noticeably inaccurate when attempting to deal with this type of tokamak. He is also excited that the model could be validated by the DIII-D National Fusion Facility. They are carrying out experiments in collaboration with PSFC. He said, “The DIII-D tokamak will soon be capable of launching lower hybrid waves and measuring its electric field in the scrape-off layer. These measurements could provide direct evidence of the asymmetric scattering effect predicted by our model.”
Tokamaks are used in many laboratories for fusion research. Any improvement in understanding how they operate will have a big impact on global fusion research.