MIT Department of Nuclear Science and Engineering
MIT Department of Nuclear Science & Engineering (NSE)
Plasma physics and fusion research in our Department is comprised of analytical, numerical, and experimental investigations. Large projects such as the Alcator C-Mod experiment are complemented by smaller innovative experiments such as the Levitated Dipole Experiment. Engineering research activities are also undertaken in such areas as superconductivity, materials, system design and optimization, and millimeter wave generation.
As fusion technology refocuses on more basic engineering science issues, the Department is perfectly positioned to play an important role in research on superconducting magnets, high power millimeter wave generation, and advanced materials.
Professor and Department Head, Nuclear Science & Engineering; Co-Principal, Alcator Project, PSFC.
BA ‘72 Cambridge University; PhD ‘76 (plasma physics) Australian National University.
Experimental plasma physics; controlled fusion.
The magnetic confinement approach to fusion energy has been most successful to date in a configuration called the tokamak. MIT’s Alcator C-Mod tokamak has the world’s highest magnetic field and produces the highest density tokamak plasmas. It thus has a unique role in international fusion research. MIT graduate students are an integral part of our team, and have a unique opportunity to participate at the forefront of plasma research. Many graduate students are engaged in the development of diagnostic techniques and subsequent deployment of instrumentation. These efforts, combined with the planning and execution of associated experiments, form the most frequent path to successful completion of PhD theses.
To make use of fusion as a practical energy source, we must learn how to confine hot plasmas with magnetic fields. Plasma confinement is an interesting problem and has turned out to be much more difficult than anyone could have guessed. Plasmas in magnetic confinement experiments exhibit strong turbulence: chaotic behavior driven by the free energy of the system which degrades confinement. The study of turbulence is one of the greatest challenges in physics, and on Alcator C-Mod we conduct experiments with the aim of characterizing and improving confinement and comparing it to existing theories of turbulence. Students are involved in the design of experiments, in the development of analytical and numerical models, and in the analysis of data in order to understand small and large fluctuations of the plasma.
The use of electromagnetic waves for heating and current drive in plasmas has been a major part of the fusion program since its inception. Externally launched electromagnetic waves can be used as probes to study fundamental physics phenomena such as transport processes in plasmas and the confinement of energetic particles. Successful development of these methods depends upon understanding the physics of wave-particle interactions, wave propagation, power partitioning among particles, heating and current drive efficiency, and edge interactions. Students make substantial contributions to these physics topics, and various topics are available for thesis research.
Magnetic confinement of the hot plasma is not perfect. Inevitably, the plasma leaks out and contacts the internal surfaces of the tokamak vessel, creating problems for both the plasma and the surface of the vessel. The high temperature plasma may erode surfaces, causing impurities released from the vessel to enter the plasma region and dilute and cool the hot plasma core.
We study these processes with a large array of experiments, including diagnostics to determine the temperature and density of the plasma in contact with the vessel and a large variety of spectrometers to determine the level of impurities in the plasma, as well as surface heat flux measurements at critical locations. In addition, we are developing means of ameliorating the resulting problems. The primary method we are presently investigating is use of a magnetic “divertor” to direct the particles leaving the plasma into a region remote from the hot plasma core. Impurities thus produced find it difficult to find their way back into the core. We further optimize the divertor operation by seeding the divertor region with gaseous impurities. This enhances the radiative energy loss rate there, thus cooling the divertor plasma in contact with the vessel, yet leaving the hot core unaffected.
The Levitated Dipole Experiment, also called LDX, is a new concept exploration experiment headed by Dr. Jay Kesner of MIT’s Plasma Science and Fusion Center and funded by the DOE as a joint project with Columbia University. The levitated dipole approach was inspired by observations of high-pressure plasmas confined by planetary dipole fields. Planetary magnetospheres provide the best examples of the natural occurrence of magnetic confinement. The dipole configuration is axisymmetric and is the simplest magnetic configuration. Using super conductors, the dipole configuration is intrinsically steady state. Compared with the traditional fusion approaches, the levitated dipole may permit the confinement of higher pressure plasmas with reduced cross-field transport.
