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 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.
Jeffrey P. Freidberg Professor of Nuclear Engineering; Associate Director, Plasma Science and Fusion Center.
BEE ‘61, MS ‘62, PhD ‘64 (electro-physics) Polytechnic Institute of Brooklyn.
Theoretical plasma physics; Fusion systems engineering.
Magnetohydrodynamics (MHD) deals with the behavior of an electrically conducting gas, i.e. a plasma in a magnetic field. The subject is of fundamental importance to all forms of magnetic confinement fusion research, providing a link between the physics behavior of the plasma and the engineering characteristics of the confinement device. At Alcator C-Mod, our group is concerned with analyzing, controlling, and predicting the plasma equilibrium, as well as studying the stability properties of C-Mod plasmas with respect to macroscopic and microscopic modes. Tools include loops that measure the magnetic fields outside the plasma, soft-X-ray and electron-cyclotron-emission diagnostics sensitive to fluctuations in the hot core of the plasma, and a variety of computer programs for interpreting the data.
Our graduate students are actively involved in MHD research, discovering new ways to improve tokamak performance, participating in the design and installation of diagnostics, planning and carrying out experiments, developing new computational tools, and analyzing and interpreting results. Past and ongoing projects include analysis of the stability of the plasma to gross axisymmetric displacements, and development and testing of active feedback control schemes to counter these instabilities; studies of disruptions (plasma terminations due to uncontrolled MHD instabilities); analysis of ideal and resistive modes observed during high performance operation; and interpretation of x-ray emissivity profiles to provide information about the internal magnetic geometry of the plasma.
Future opportunities for student projects include studying the stability properties associated with large pressure gradients observed in high-performance plasmas; actively driving electromagnetic waves in the 100-800 kHz range, characteristic of the Alfvén eigenmodes that interact with energetic particles and may be important in future burning plasma experiments; using new diagnostics to measure fields inside the plasma for inclusion in equilibrium and stability computations; and analyzing forces on the vessel in support of designs for new divertor structures.
There are also new opportunities to investigate the macroscopic stability properties of the levitated dipole configuration, the spherical torus, and the compact Stellarator. These studies are motivated by the need to obtain a wide understanding of the basic plasma physics associated with the long term goals of fusion power.
Kim Molvig Associate Professor of Nuclear Engineering
BS ‘70 Cornell; PhD ‘75 (physics) University of California.
Magnetic confinement fusion has the promise of becoming the ultimate nuclear power plant. Its fuel supply is virtually limitless. It converts mass to energy very efficiently, has no air pollution or contribution to global warming, is without the risk of uncontrolled energy release or the threat of proliferation of nuclear weapons material. Relative to conventional fission plants, it has dramatically reduced high level nuclear waste.
Physics and scientific feasibility of magnetic fusion are nearly established but not yet certain. One of the key physics issues remains the understanding and quantitative prediction of energy transport from the hot fusion core to the magnetic bottle periphery. My theoretical work is focused on this problem in a variety of areas, ranging from the purely classical collisional (so-called neoclassical) transport in a quiescent plasma to the turbulent transport due to elevated collective fluctuations.
Tokamak plasmas exhibit several exotic but important classical phenomena that merit study. These arise because the free paths of charged particles are so long that global machine transits occur before collisions take place. This allows interesting particle orbit shapes to contribute profoundly to transport. One phenomenon is the so-called ‘bootstrap current,’ whereby radial losses of heat and particles drive the toroidal electric current needed to sustain the magnetic geometry. This could be key to sustaining steady state of the confining magnetic bottle. Another classical phenomenon is the ‘pinching’ effect whereby inward particle flows are driven by the toroidally oriented forces that drive the electric current. This is important in controlling fueling profiles and inducing the so-called “high confinement” regime so necessary for practical tokamak operation.
Losses of heat will be determined by turbulent processes, whereby collective electromagnetic plasma fluctuations grow to elevated levels, producing flow eddies that transport energy rapidly in much the same way that roils in boiling water move heat. Much of this study is done with numerical simulation codes, but the physics and analytical theory of the processes remain critical. Our work combines both simulation and analytic approaches.
Ronald R. Parker Professor of Electrical and Nuclear Engineering
BSEE ’60 Tufts, SM ’63, ScD ’67 MIT
From 1992-1998, I was Associate Director of ITER (International Thermonuclear Experimental Reactor) and headed the Design Center in Garching, Germany, which was responsible for the design of the in-vessel components of the ITER reactor. Since returning from the ITER Project to MIT, I have concentrated part of my research effort on preparing a 3 MW, 4.6 GHz RF system whose purpose is to drive a substantial fraction of the toroidal current in Alcator C-Mod (MIT’s major tokamak facility) by means of RF waves. Most of the toroidal current in a conventional tokamak is induced by induction; in effect, the plasma ring is the one-turn secondary of a transformer whose primary is a powerful solenoid. In Alcator C-Mod, the toroidal current is typically about 1 MA and the RF system now nearing completion is potentially capable of driving up to about half of this current. When operated at high pressure it turns out that the plasma itself generates a substantial toroidal current, the so-called bootstrap current. By combining the RF driven current with this bootstrap current it should be possible to completely eliminate the role of the transformer in operating a tokamak, thereby establishing the basis for a tokamak to become a steady-state fusion reactor. Further, by optimizing the location of the current in the plasma, it is possible at the same time to minimize the flow of heat out of the plasma and to optimize the overall plasma performance. Elucidating the physics of RF current drive in Alcator C-Mod and optimizing its performance will be a major goal of this experiment during the coming five to ten years.
In addition to leading this investigation of using RF current drive to optimize the performance of Alcator C-Mod, I have interests in developing diagnostics to detect and understand the behavior of supra-thermal particles generated by plasma interactions with RF waves. In the case of the RF current drive outlined above, the current is the result of resonant interactions between waves propagating in the toroidal direction and the plasma electrons. This process creates a population of supra thermal electrons with energies in the range of a few hundred keV (the plasma temperature is typically a few keV). When these electrons collide with ions, they emit photons (Bremsstrahlung) which can be detected by a specially designed imaging X-ray spectrometer. By unfolding the data obtained by this instrument important information can be learned about the location and intensity of the RF waves and the RF-driven current.
While the waves responsible for current drive are at a relatively high frequency and interact with electrons, lower frequency RF waves are also used in Alcator C-mod to interact with ions at their Larmor frequency, thereby heating them. An important issue is then to understand the physics of this interaction and its effect on the distribution of fast ions in energy and in space. When a neutral beam is injected into the plasma, fast ions will exchange charge with the neutrals in the beam and then escape the confining magnetic field, allowing them to be detected at the plasma periphery. Solid state detectors are being used in Alcator C-Mod to detect the flux and energy of fast neutrals produced by this charge exchange process. The data are being analyzed and compared with theoretical models to more fully understand the physics of heating plasmas by intense RF waves.
The imaging X-ray spectrometer and the charge-exchange neutral particle analyzers just described form the basis for PhD research for two of my students. Other topics include characterization of MHD waves driven unstable by a fast particle population; the physics of heating and current drive by waves excited near the ion Larmor frequency; and the physics of coupling of the RF waves used for current drive to the plasma. As the current drive experiment becomes operational, additional topics for exciting PhD research will become available at the frontier of fusion research.
