jeremy m. hanson

Research: plasma physics, fusion energy

Dr. Hanson's research focuses on key challenges for fusion energy: understanding and controlling the stability of plasma discharges in tokamak devices. If developed, fusion could replace today's fossil fuel and fission power plants, providing safe, carbon-free base-load electricity generation.

Harnessing fusion requires the long term confinement of a high pressure plasma. Stars, including our sun, achieve this confinement using gravity. In laboratories here on earth, magnetic fields are used instead of gravity to confine fusion plasmas. Experiments with tokamak devices have attained record plasma pressures and confinement times, and the tokamak is the leading candidate for a fusion energy power plant.

Some fundamental aspects of the tokamak configuration lead to interesting research challenges. The tokamak is a toroidal (donut-shaped) device with a high degree of toroidal symmetry. For this reason, the toroidal angle coordinate can be ignored in many cases, allowing a two-dimensional, or axisymmetric, characterization of the plasma equilibrium. In addition, the tokamak's confining magnetic fields come from two sources: (a) currents in electromagnets surrounding the plasma, and (b) a secondary electrical current that is driven in the plasma itself. This secondary plasma current is a source of free energy that can drive symmetry-breaking magnetohydrodynamic (MHD) instabilities that cause sudden losses of plasma confinement. The plasma pressure gradient is an additional source of MHD instability drive that sets up a tension between increasing the pressure to maximize fusion power and avoiding instability boundaries.

A great deal of progress in understanding MHD instability boundaries and actively controlling the instabilities has accompanied the tokamak's record setting achievements. Ideal MHD theory can be used to predict the maximum pressure for a tokamak, above which an instability would be expected. However, experiments have demonstrated passively stable operation above the ideal MHD pressure limit, and attained still higher pressures using magnetic feedback control to help maintain axisymmetry. These results have pointed the way to interesting physical effects outside the ideal MHD model and have demonstrated the potential for optimizing the tokamak for energy production.

Nonetheless, important questions remain. One open research topic deals with unraveling the physical mechanisms that influence the damping of tokamak instabilities. The simplest theory, ideal MHD, treats the plasma as a perfectly conducting fluid. Dr. Hanson works on comparing instability damping measurements with non-ideal extensions of the theory. For example, some of the plasma ions can move in patterns that differ signficantly from the average fluid motion, sometimes traveling in banana-shaped orbits that do not fully transit the tokamak. These motions, treated by the theory of kinetic modifications to ideal MHD, can damp instabilities would change the magnetic flux enclosed by the orbit path. In addition, plasmas have a small amount of electrical resistivity that can act to weaken stability, and Dr. Hanson has recently made comparisons of instability damping with resistive MHD theory predictions.

One straightforward method of finding an instability boundary is to vary an aspect of the system, such as plasma pressure, until the boundary is crossed. However, this approach can be challenging in tokamak experiments because crossing the stability boundary is undesirable or because there may not be sufficient actuator power to fully explore the boundary. Instead, it can be more fruitful to learn about the system by perturbing it in such a way so as to excite a damped, stable response. In plasma experiments, this technique is called active MHD spectroscopy, and it involves applying small, symmetry-breaking perturbations that resonate with the plasma modes of interest. In addition to using active MHD spectroscopic damping measurements as an observable for the comparisons with theory described above, Dr. Hanson has pioneered a real-time version of this technique that can be used for plasma control. For example, the damping measurements can help warn of an approaching instability limit, or they can be coupled with a plasma pressure actuator to allow safe operation near, but below the limit.

Ultimately, it may be desirable for tokamaks to operate above the passive stability pressure limit, and developing symmetry feedback control strategies for stabilized high pressure operation is an ongoing research thrust. This type of feedback utilizes arrays of magnetic sensors to detect small departures from axisymmetry and arrays of discrete, picture-frame shaped coils to produce a compensating magnetic field. Both the plasma instability and feedback response induce electrical eddy currents in nearby passive conducting structures, such as the vessel wall, that must be accounted for. Dr. Hanson's Ph.D. dissertation dealt with the use of a state-space observer (Kalman filter) to mitigate sensor noise in tokamak feedback control. Subsequently, Dr. Hanson supervised graduate student research on extending the state-space approach to implement an optimal control algorithm and to include a more sophisticated, three-dimensional model of the vessel wall, coils, and sensors. This research demonstrated that control coils located outside the vessel wall could provide effective feedback. Ongoing graduate student research is investigating the use of state-space control to stabilize multiple plasma modes simultaneously and techniques for making active MHD spectroscopy measurements while control is being maintained.

Dr. Hanson's research is presently conducted at the DIII-D National Fusion Facility in San Diego, California. In addition to performing DIII-D experiments in the areas described above, Dr. Hanson is a co-leader of the DIII-D "3D & Stability Physics" topical area, a member of the physics operator team that helps pilot DIII-D experiments, and a member of the run coordination team that facilitates the planning, scheduling, and execution of DIII-D experimental campaigns.