The Plasma Fusion Center is recognized as the leading university laboratory in developing the scientific and engineering aspects of magnetic confinement fusion and related plasma science and technology. Its research programs continue to produce significant results on several fronts: (a) experimental confinement research on the Alcator C-Mod tokamak (investigations of the stability, heating, and transport properties of compact high magnetic field, diverted plasmas), (b) the basic physics of energetic plasmas (plasma theory, theoretical support of ITER and IGNITOR, new confinement concepts, nonneutral plasmas, coherent EM wave generation, development of high-temperature plasma diagnostics, basic laboratory and ionospheric plasma physics experiments, and novel diagnostic of inertial fusion experiments), (c) a broad program of fusion technology and engineering development that addresses problems in several areas (e.g., magnetic systems, superconducting materials, fusion environmental and safety studies, advanced millimeter-wave sources, system studies of fusion reactors, including operational and technological requirements), and (d) a significant activity of environmental waste treatment using plasmas.
Approximately 30 percent of the Center's activities are associated with the Alcator C-Mod tokamak experiment, 40 percent with the research and development on superconducting magnet system components for future fusion devices (including substantial industrial subcontracts), and the remaining 30 percent with the many other R&D activities.
The Plasma Fusion Center R&D programs are supported principally by the Department of Energy's Office of Fusion Energy Sciences. There are approximately 250 personnel associated with PFC research activities. These include: 18 faculty and senior academic staff, 49 graduate students and 15 undergraduate students, with participating faculty and students from Electrical Engineering and Computer Science, Materials Science and Engineering, Mechanical Engineering, Nuclear Engineering, and Physics; 77 research scientists and engineers and 31 visiting scientists; 34 technical support personnel; and 26 administrative and support staff.
This was a turbulent year. As a result of major DOE-OFE budget cuts (33%) by Congress, the total funding of the Plasma Fusion Center decreased by about 25% and total staff and number of students were reduced from 305 to 250. These cuts threatened to terminate the Center's high-field tokamak, Alcator C-Mod. However, the project was ultimately saved and continued operation at a reduced budget (approximately a 38% reduction from the year before). A 15% reduction in staffing was necessary, but no graduate students or RAs were terminated. Because of prior year investments, the experiment operated with great success. Another project that did not fair well was our collaboration on the Tokamak Physics Experiment (TPX), to be built at Princeton. This project was terminated by Congress in 1995. Nearly all the termination costs were covered by DOE-OFES.
The PFC programs also support ITER in critical technology areas, including superconducting magnets and development of millimeter wave RF sources suitable for heating and driving current near the electron gyro-frequency. In the magnetics area, the PFC leads the US ITER-Home Team effort in an extensive, internationally coordinated program of superconducting magnet development leading to construction of magnets on a scale and at a performance level well beyond that of present-day experience. The funding of the magnet program remained strong in FY'96. Noteworthy in FY'96 was the startup of the Pulsed Test-Facility (or PTF), a $2M superconducting magnet test facility built in the Nabisco Laboratory. In the area of millimeter wave RF source development, the gyrotron program was in danger of termination, but ultimately the MIT research activity was saved. The Varian/CPI (California) development program was cut significantly and may be terminated next year. MIT is acting as the DOE-OFES oversight monitor for this activity.
In the area of plasma treatment of contaminated soil and waste, the PFC has enjoyed considerable media success. There were numerous television broadcasts and newspaper articles on our results, including the visit of Jack Williams of WBZ (Channel 4) TV. We were pleased to learn that Drs. Daniel Cohn and Paul Woskov won another R&D 100 award for their invention of a microwave plasma emissions monitoring system.
Our theory program enjoyed considerable scientific success and a modest increase in funding was obtained this year. The basic experimental plasma physics program lost OFES funding, but it was countered with new funding from the inertial fusion research activity through collaboration with Lawrence Livermore National Laboratory (LLNL) and the University of Rochester Institute of Laser Energetics (ILE).
The Alcator Division, led by Prof. Ian Hutchinson, carries out experimental research on Alcator C-Mod, a compact, high-performance, high magnetic field divertor tokamak devoted to investigating the physics of high temperature magnetically confined fusion grade plasmas. The total staff of the Alcator Project is about 95, including 16 full-time research physicists, 2 faculty members and 20 graduate students. In the past year a sustained campaign of experimentation was conducted during November 95 through March 96, during which a variety of important results were obtained, especially in the areas of divertor physics, radiofrequency heating, confinement, and disruption studies. The capabilities of the tokamak were further enhanced and its operating space was broadened. Details are outlined in the succeeding sections. These results were accomplished despite a significant funding reduction in FY'96 from $16.2 M to $10 M, which necessitated significant staff reductions, as well as reduced operations relative to the prior year.
Alcator C-Mod is now established as one of the three major U.S. tokamak facilities, along with DIII-D at General Atomics, San Diego, and TFTR at Princeton. It is also recognized as one of the five key divertor tokamaks in the world. Alcator C-Mod is the only diverted high-field compact experiment in operation, and therefore it plays a unique role in providing critical tests of confinement scaling and theory at high power density. Alcator C-Mod is thus extremely favorably placed to contribute vital information to fusion science research, and to do so in a highly cost effective way because of its compact approach. Because of its high power density, C-Mod will contribute uniquely to ways of achieving ignition at low cost, and of exploring advanced tokamak physics regimes. Its vertical plate divertor geometry has been adopted as the reference design for ITER, and its shape is essentially the same as that envisioned for ITER. It also is presently unique in having high-Z metallic plasma facing components. Such components are planned for ITER.
The unique capabilities of Alcator C-Mod were recognized by the Fusion Energy Sciences Advisory Committee (FESAC) in its report on Restructuring the U.S. Fusion Program, which recommended increased support to bring the C-MOD facility closer to "full, maximally productive utilization." According to this report, the program should maintain "Continued full utilization of DIII-D and C-Mod at least through 2001, including some upgrades, as user facilities to pursue the rich science to be gained." Although the level of support expected for C-MOD in the next funding cycle falls somewhat short of that consistent with the FESAC recommendation (the present Presidential funding projection is $13.5 M at MIT), we are proceeding, to the extent possible, to exploit the scientific promise of this unique facility mostly through collaborations with the University of Texas (an additional $1.1 M ) and the Princeton Plasma Physics Laboratory (an additional $0.6M ). In essence, we are moving toward becoming a national resource for fusion science research.
The operations and engineering section, led by Dr. James Irby, is comprised of more than 50 engineers, supervisors, and technicians. This group is responsible for operation and maintenance of the tokamak facility and also for most facility upgrades. We continue to make changes to the machine and power systems required to achieve higher plasma performance. During the last year, a boronization system was implemented which greatly improved the stored energy and impurity levels in our discharges. After an extensive modeling effort, changes were made to the outer divertor so that plasma currents of up to 1.5 MA at toroidal fields of 8 Tesla can be obtained during the next run campaign. We are now in the process of installing a prototype divertor cryopump that will provide better plasma density control. New tunable radio-frequency sources (40-80 MHz) are being brought on line to increase the versatility of our RF heating capability power levels. Depending on available funding, in FY'97 a diagnostic neutral beam will be installed to provide a new means of measuring, among other things, ion temperature, density fluctuations, and plasma current profiles.
Our alternator has undergone a complete inspection during the last year. This system provides approximately 250 MW of pulsed power to the magnets during a plasma discharge. Very detailed ultrasonic measurements of the flywheel, rotor, and bearings were made. In addition, high voltage tests of the windings were done to assure safe operation at 13.8 kV. The alternator is now back in operation and is being brought back up to full speed.
This section, led by Prof. Miklos Porkolab and Dr. Yuichi Takase, implements and analyzes plasma heating using radio frequency (RF) power, including investigation of advanced tokamak physics (ATP) scenarios.
Operational improvements implemented during the past year have increased the reliability of high power operation (up to 3.5 MW into the plasma) of the 4.0 MW (source power) 80 MHz RF heating system, which is essential for carrying out various elements of the Alcator C-Mod experimental program. Reliable coupling of RF power into a variety of plasmas including pellet injection, was achieved. High power heating of H-mode ("high-confinement") plasmas in the H minority regime (5.3T) produced impressive plasma parameters, approaching the MHD stability limit. Heating efficiency in low single-pass absorption schemes, including the 3He minority heating scheme at 8T, was improved substantially by controlling the high-Z impurity influx using boronization of the first wall. Off-axis electron heating by ion Bernstein waves produced by mode conversion was demonstrated in D-3He plasmas at 8T. This mode of operation will provide the current profile control capability necessary for future advanced tokamak experiments. Particularly impressive advanced tokamak scenarios with high confinement and high beta have been predicted by numerical modeling. In collaboration with the Princeton Plasma Physics Laboratory, current drive studies are scheduled to start in 1997-98 using the new 40 MHz power (currently being installed).
The Plasma Section, led by Dr. Stephen Wolfe, is involved in advancing the understanding of the tokamak configuration in the areas of transport (coordinated by Dr. Martin Greenwald) and MHD physics (Dr. Robert Granetz), as well as for developing and implementing optimized control procedures for tokamak operation.
Improved H-mode confinement results were obtained after implementation of boronization for effective wall-conditioning, which reduced the level of core radiation during high-power ICRF heating. H-factors (the ratio of the energy confinement time relative to standard L-mode scaling) above two were obtained over a wide range of plasma parameters. This result is particularly significant because extrapolations from larger experiments had predicted much smaller H-mode confinement enhancements on C-Mod.
Experiments on the threshold conditions for the L-H transition have continued, leading to the observation that the threshold corresponds to a necessary condition on the magnitude of the edge electron temperature. For typical C-Mod conditions, with B~ 5 Tesla, the transition occurs when the temperature at the 95% flux surface exceeds 150eV, and a reverse transition back to the L-mode happens when the temperature again falls below this value. Further experiments are planned to determine the underlying physics of H-Mode transition.
Analysis of the MHD stability properties of the high performance H-mode discharges indicates that the edge pressure gradients increase to approximately the so-called ideal "ballooning limit." The total "normalized" pressure reaches bN ~ 1.5, about a factor of two below the expected b (b is the ratio of plasma pressure to magnetic field pressure, bN=b/(I/aB) where I is the plasma current, a is the minor radius, and B is the toroidal magnetic field) limit with optimized profile shape, but in the range where pressure-driven MHD modes may appear.
Studies of disruptions have continued, with special attention to halo currents, which flow partly in the plasma and partly through the conducting vessel. The ratio of halo current to plasma current has been shown to be proportional to 1/q (with q = (BT/Bp)(a/R), the "safety" factor). Toroidal asymmetries and rotation of the halo currents have been measured, and a correlation between halo currents and the occurrence of integral values of an effective rotational transform, including the current path through the wall, has been observed. Mitigation of disruption effects has been demonstrated using "killer pellets", or silver-doped plastic pellets which can dissipate most of the plasma kinetic and magnetic energy in a millisecond by means of radiation.
The experiments section, headed by Dr. Earl Marmar, is responsible for edge/divertor physics studies under the leadership of Dr. Bruce Lipschultz, and for the development of new plasma diagnostics. In the last 12 months, excellent progress has been made, both in terms of experimental capabilities and in divertor physics research. The achievement of high-power RF-heated H-mode plasmas with good energy confinement (reduced transport) has greatly expanded the operational space for edge studies. This type of operation has lead to narrower profiles of parallel heat flow in the edge plasma with accompanying parallel heat flows which are reactor-like (500 MW/m2, higher than achieved elsewhere). The research has concentrated on reducing power flow to the divertor plates through volumetric loss processes (radiation and charge-exchange by gas puffing), while maintaining good core confinement and plasma purity. These experiments have been very successful, with reductions of the power flow to the material surfaces by about a factor of 10 as the plasma is "detached" from the divertor plates.
The study of edge characteristics has also included a thorough investigation of the transport of the majority species (deuterium) in the edge. The results of this study shows that the edge perpendicular thermal diffusivity increases with increasing distance from the core plasma, and that the diffusivity drops by an order of magnitude when central plasma confinement increases after transition into H-Mode.
Experiments have also been performed to study impurity transport utilizing puffing of trace gases at different points around the plasma edge. These experiments have shown that for recycling gases, the impurity source location affects the probability of its penetration into the core plasma (as opposed to convecting into the divertor). These techniques have also been used to determine that impurities penetrate into the core plasma more easily after the plasma detaches from the divertor plates.
Headed by Prof. Miklos Porkolab, this Division seeks to develop a theoretical and experimental understanding of plasma physics and fusion science. Experimental work is carried out on smaller devices with more modest plasma parameters, or focuses on developing novel diagnostics for exploring new physics in large scale fusion devices. This Division is also a base for developing new confinement concepts, including "Proof of Principle" experiments, exploring inertial fusion energy, space plasma physics, and new physics applications of plasmas. In the past year the funding of this Division has remained relatively steady.
The MIT Divertor Task Force (Dr. Dieter Sigmar and coworkers) through its leadership of the national tokamak plasma edge physics and divertor initiative, has focused on analytic and numerical edge plasma physics investigations. Observations from the Alcator C-Mod tokamak and other devices are used to develop an improved understanding of basic plasma physics phenomena. The goal of this effort is to find ways to divert the severe heat flux from the first wall while maintaining good plasma purity in present day experimental fusion devices, and ultimately in ITER. This group has developed the only neutral-plasma fluid code capable of modeling current tokamaks and fusion reactor relevant high density divertor regimes. During the past year, the code has successfully modeled the C-Mod experiment and highlights the crucial role of charged particle recombination in dense plasmas. In addition, a fully kinetic edge plasma code has been developed which is used to interpret more accurately probe measurements in present day experiments.
For most operating regimes of interest there is a wide variation in the neutral particle mean free path length. Current approaches using either Monte Carlo or Navier-Stokes numerical techniques cannot yet deliver an entirely satisfactory description of neutral transport in the entire edge plasma domain. Recently, the MIT edge physics group has assumed leadership in a national effort to build a plasma-neutral hybrid code that will allow all regimes of neutral collisionality (arbitrary mean free path) to be modeled. When finished, these new codes, as well as existing ones, will be employed to develop scaling laws capable of predicting fusion reactor performance based on present experimental results.
The RF Plasma Theory Group under the direction of Prof. Abraham Bers and Dr. Abhay K. Ram has been extending its studies on the mode conversion of fast Alfvén waves to ion-Bernstein waves to include the effect of poloidal mode numbers and of magnetic shear. These studies show that, in addition to the ion-ion hybrid resonance, there exists another resonance where the fast Alfvén wave loses some of its power. Detailed analytical and numerical analysis is ongoing to quantify the effect of this resonance on the mode conversion process. The same group has also continued its studies on the propagation and damping of ion-Bernstein waves in tokamaks. It has been found that the ion-Bernstein waves can be used to drive currents in plasmas provided that the magnitude of the parallel (to the magnetic field) wave numbers at mode conversion are below a critical value. This critical value has been determined numerically. Finally, the group has been investigating a mode conversion scenario for electron cyclotron heating and current drive in spherical tokamaks. Here the extraordinary mode of the electron cyclotron wave can couple to the electron- Bernstein wave at the upper-hybrid resonance. The previous expertise on mode conversion analysis is being extended to this high frequency regime.
The research program of the Physics of High Energy Plasmas (PHEP), under the leadership of Prof. Bruno Coppi, is concerned with the theoretical study of magnetically confined plasmas in regimes of relevance to present day advanced experiments, as well as the proposal and planning of new experiments directed towards the study of fusion burning plasmas. The recent redirection of the U.S. fusion program toward developing the basic science of fusion plasmas, as well as innovative concepts, brings the activities of this group (PHEP) directly into line with the mainstream goals of the new national program.
The PHEP group has been the first to undertake the proposal and the study of magnetically confined ignition experiments, taking a leading role in the development of both their physics and engineering. The Ignitor experiment was the first proposed (originally in 1975) ignition experiment in the world. It has become increasingly clear that the most suitable and cost effective type of experiment to pursue the goal of ignition is the line of machines that operate at high magnetic field, employing cryogenic normal conducting magnets, which this group has pioneered, such as the Alcator series of experiments at MIT, the FT machines at Frascati, Italy, and the ignition experiment "Ignitor" in Italy.
This group has also maintained a pioneering role in developing the theory of high temperature plasmas. Of note are the region of second stability for finite pressure plasmas, the principle of profile consistency of the electron temperature, the degradation of energy confinement by ion temperature gradient driven modes, the isotopic effect on the confinement time, the existence of impurity-driven modes localized at the plasma edge, the stabilization of sawteeth by energetic particles, and the time dependent path to ignition in magnetically confined plasmas.
The ideas concerning transport processes in high temperature plasmas and the fact that it can be nonlocal in nature, as indicated for example by the principle of profile (temperature) consistency, have been confirmed by different experiments around the world. These concepts now form the basis of several widely adopted approaches to the theory of plasma transport. Many of the ideas that the PHEP group has proposed in the 1970's, including the toroidal ion temperature gradient driven (or ITG) modes, have been incorporated recently in sophisticated codes that have been used successfully to interpret present experiments.
In this effort, under the leadership of Drs. Paul Bonoli, Jay Kesner, Jesus Ramos, and Profs. Jeffrey Freidberg and Miklos Porkolab, a state of the art simulation code has been developed to compute self-consistent MHD equilibria in the presence of non-inductively driven currents. Such studies are of great importance to the C-Mod program since they offer a means to improve tokamak performance, ultimately leading to a more attractive steady state tokamak reactor. This code has been coupled to an MHD stability code at Princeton in collaboration with Dr. C. Kessel. This combined model has been used to demonstrate the feasibility of achieving MHD stable operating modes in Alcator C-Mod near the beta limit. These so-called advanced tokamak operating modes are characterized by relatively high fractions of non-inductive bootstrap current (approximately 75%) and non-monotonic ("reversed shear") profiles of the safety factor. Such profiles are believed to improve tokamak stability and particle and energy confinement. The current profile control required to maintain these equilibria may be achieved through a combination of on-axis fast wave current drive and off-axis lower hybrid current drive. A reverse shear mode of operation at high bootstrap current fraction and moderate plasma current for the ITER device was also developed. Again the necessary current profile control is accomplished through a combination of off axis lower hybrid current drive and on-axis fast wave drive (or neutral beam injection).
Some of these codes were also used to analyze ICRF heating experiments in Alcator C-Mod in which off-axis electron heating via mode converted ion Bernstein waves (IBW) have been observed. Detailed comparisons have been made between the model predictions for RF electron power dissipation and the RF power densities inferred experimentally via RF power modulation technique. The implications of the off-axis electron heating for current drive via the mode converted IBW were also examined. Preliminary estimates indicate that significant RF current (up to 200 kA) can be generated in Alcator C-Mod, thus providing another means to achieve current profile control and advanced tokamak operation.
Tokamaks operating in a mode in which the current is mostly sustained by bootstrap current tend to naturally form reversed shear profiles. For a fixed heating profile the improved core confinement will cause the pressure profile to peak, which effects the equilibrium through the bootstrap current term. It has been shown that steady state equilibria do not, in general, exist for tokamaks with a high bootstrap current fraction and enhanced confinement in the reversed shear region, unless the following conditions are satisfied: (1) an additional stabilizing mechanism is utilized (for example flow shear) to create a region of reduced transport which can exceed the size of the region of reversed shear and, (2) the bootstrap fraction is reduced and non-inductive current drive is utilized to maintain a reversed shear current profile of substantial size.
Professor Jeffrey Freidberg and Dr. Ali Shajii have explored the possibility of applying plasma theory techniques to a class of fluid flow problems in superconducting technology. As a result of these studies, the following discoveries were made: (a) A new class of low Mach number discontinuous fluid flows was developed. The flows are similar to "standard" contact discontinuities except that they are strongly modified by the presence of a metalic wall. The wall not only introduces new velocity jumps into the flow, but produces a thermal drag that can stabilize the flow against Rayleigh-Taylor instabilities which always destroy the structure of the "standard" contact discontinuity. This phenomena is important in understanding the propagation of a quench in large, superconducting, fusion magnets. (b) A new theoretical model describing low Mach number compressible flow was developed. Such flows occur in long superconducting cables used for fusion magnets. A surprising result is that when sufficient external heat is supplied to the fluid, the flow direction actual reverses against the direction of the pumping pressure. This phenomena has been observed experimentally and cannot occur in an incompressible fluid. (c) A theoretical explanation of the phenomenon, "thermal hydraulic quenchback," was developed.
The PFC has collaborated for the past several years with General Atomics, in San Diego, California, on turbulence studies in the DIII-D tokamak. Mr. Stefano Coda, a graduate student in the Physics Department under the direction of Professor Porkolab, carried out his Ph.D. thesis research on location at General Atomics. The project consisted of the development and operation of a novel diagnostic apparatus, a CO2-laser phase-contrast imaging (PCI) system, which provides detailed measurements of the density fluctuations at the tokamak edge with excellent sensitivity and fine spatial and temporal resolution. The chief motivation for this undertaking has been the need for a more accurate characterization of the turbulence spectrum in the edge region, especially in the long (several-cm) wavelength range which is believed to have a strong influence on energy transport. A similar PCI system is now being developed at the PFC and will be installed on the Alcator C-Mod tokamak, where it will be employed both to investigate turbulence and to carry out a novel study of externally launched radio-frequency waves in the plasma.
Thorough studies of the properties of edge turbulence in various heating and confinement regimes have been carried out in the course of this project. In the past year, particular attention was devoted to the fine-scale correlation characteristics of the fluctuations, especially to the rapid change in the correlation length that accompanies the transition from the low (L) to the high (H) confinement mode. The reduced correlation length was shown to be correlated with the reduction in transport, resulting in improved confinement. In addition, the leading theory of the H-mode, which relates the modification in the turbulence spectrum to an increased shear in the plasma rotation velocity, has been confirmed quantitatively for the first time. The novel observation of the existence of radially propagating spectra has also been found to be in agreement with recent analytical and numerical work on the global structure of a class of plasma instabilities (ITG modes) that are considered to be a dominant component of transport-enhancing turbulence.
The PCI system has also been successfully employed to investigate the nature of semi-periodic edge instabilities known as ELMs (edge localized modes). These modes, if properly controlled, will be essential to the operation of a future reactor, as they serve the dual purpose of ejecting deleterious impurities from the plasma and of regulating the plasma energy content in a stable steady state. In spite of their importance, the experimental and theoretical understanding of ELMs is still in its infancy. We have been able to shed new light on these phenomena by characterizing their spectral content and temporal evolution, and by determining systematic differences between some of the known subclasses of ELMs.
This program, under the leadership of Dr. Richard Petrasso, is the key element of our participation in inertial fusion energy (IFE) research. The National Ignition Facility (NIF), currently under design, would be the center of inertial fusion research in the U.S. in the next several decades. With colleagues at the Lawrence Livermore National Laboratory and at the University of Rochester, our group at the PFC has written an article on the "Science On The NIF," a document which explores the unique opportunities for the exploration of new science on NIF. For example, in the NIF experiments, we expect to achieve densities and temperatures of 1000g/cm3 and 100,000,000o K. At these conditions we can, for the first time, study fusion energy reactions under conditions similar to that found in the center of stars. Equally important and interesting, the pressures we will achieve are some 1,000 billion atmospheres. To put this in some perspective, the pressure at ocean bottom is about 1000 atmospheres; at earth's center, a few million; and at the center of the sun, 100 billion atmospheres.
A second major project has focused on the development of novel diagnostics for the NIF. Because of the extraordinary densities, current diagnostics will be ineffective. We recently submitted to Physical Review Letters a scientific paper which proposes the utilization of energetic 31 MeV protons, generated within the NIF capsule, to diagnose both the implosion symmetry and core capsule conditions.
A third major effort involves the building of novel charged-particle spectrometers for both the NOVA and OMEGA laser fusion facilities at Lawrence Livermore and the University of Rochester, respectively. These spectrometers exploit the properties of CCDs to measure very accurately the energy of protons, deuterons, and tritons that are emitted during the implosion process. From the flux and energy of these charged particles, important physics information can be gleaned about the dynamics of the implosion process. This will be the first time such spectrometers have been fielded on inertial fusion experiments. This proposal is based on the favorable experimental results we obtained in our laboratory at MIT during the past year (see last year's President's Report).
The PFC Ionospheric Plasma Research Group (Prof. Min-Chang Lee, Visiting Professor and students) has been conducting experiments on RF (radio-frequency) wave excitation and interaction with magnetized plasmas on the Versatile Toroidal Facility (VTF). They have successfully reproduced the intriguing spectra of radio wave-induced Langmuir waves observed at Arecibo, Puerto Rico. The results of VTF experiments support a theory developed by Prof. Min-Chang Lee and coworkers that the frequency-downshifted (cascading) spectrum of Langmuir waves is produced by the parametric decay instability (PDI), while the frequency-upshifted spectrum results from the nonlinear interactions of PDI-excited Langmuir waves with pre-existing lower hybrid waves. In collaboration with the Air Force Phillips Laboratory, Arecibo Observatory/Cornell University and Stanford Research Institute (SRI), the PFC's Arecibo experiments using the NSF's newly upgraded radar and HF heater have been scheduled in September, 1996 and January, 1997. These experiments are aimed at investigating ionospheric plasma turbulence and its effects on radio communications and space weather. A Ph.D. student, Dan Moriarty, recently completed his Ph.D. dissertation entitled Laboratory Studies of Ionospheric Plasma Processes with the Versatile Toroidal Facility (VTF). Following Dan Moriarty's work, several graduate students and UROP students will continue the research on laboratory simulation of RF suppression or enhancement of ionospheric plasma turbulence.
Dr. Jay Kesner and Prof. Miklos Porkolab have submitted a proposal to DOE to test a new tokamak confinement approach: a cross-section shaping that features an oblate plasma with negative triangularity which leads to a comet shaped cross-section. It has been suggested [R. Miller, M.S. Chu, R. Dominguez, T. Ohkawa, Comments Plasma Phys. Controlled Fusion, 12, 125 (1989)] that this shaping would reduce or reverse the curvature driven precessional drift of ions and electrons and is therefore expected to improve confinement with respect to these modes. This proposal was favorably reviewed but due to lack of funds was not funded. Our group will continue to study theoretically the feasibility of a COMET shaped tokamak as a means to improve tokamak confinement.
It was suggested in 1987 by Hasegawa [Comm. Pl. Phys. & Cont. Fus, 1, (1987) 147.] that a levitated dipole might provide a considerably simpler confinement scheme for fusing plasmas. Dipole confinement is observed in nature (in the earth's and the jovian magnetospheres) and in fact this suggestion was based on an increasing understanding of magnetospheric confinement. Satellite measurements have shown that in magnetospheric plasmas the ratio of plasma pressure to magnetic field pressure (or beta) can exceed unity and the plasma appears to be relatively quiescent. Ultimately, the levitated dipole configuration would most likely be utilized at high betas for advanced fuels (D-D or D-3He). Conceptually, a dipole offers several potential improvements vis-a-vis a tokamak as a fusion reactor. It is inherently steady state and free of disruptions. Since the plasma is located outside of the coil the flux can be greatly expanded so as to reduce the divertor heat load. Current drive is not required as there are no non-inductive currents flowing. There are no particle drifts off from the flux surfaces and there are theoretical reasons to believe that confinement may be good, possibly close to irreducible minimum value (termed "classical"). This may be the "ultimate" alternate confinement concept. Potential problems include shielding of the floating superconducting ring from plasma and heat bombardment.
In the laboratory, the dipole confinement concept may be realized by levitating a super conducting coil in a large vacuum chamber, and forming a plasma in the magnetic configuration by RF (microwave) heating. Dr. Jay Kesner in collaboration with Columbia University scientists (Prof. Michael Maul) is exploring such a concept. A proposal may be prepared and submitted this fall to DOE-OFES, for building such a facility in the Nabisco Laboratory. Such a project may carry an annual budget of $2 M, equally shared between MIT and Columbia University. This project will be oriented towards basic plasma physics and it is expected to include strong student involvement.
In the past year (July 1995 - June 1996) the Princeton - MIT team (Profs. Suckewer, Porkolab and coworkers) has performed several experiments on the Versator II Tokamak. The main goal of the experiments was to measure (a) the radiation near 13 nm and (b) the total radiation versus the injected gases like Ar, Kr and Se, as well as versus the influx of C from the graphite rod introduced to a plasma. Initial measurements using a time integrated bolometer indicated, as expected, an increase of the emitted radiation with increasing density of low- and high-Z atoms in the hydrogen plasma. However, a deduction of quantitative values was difficult to make, owing to decreasing of the plasma duration time with increasing density of the highly radiating ions.
In order to perform more precise measurements, highly sensitive bolometers were introduced, which provided information about the time evolution of emitted radiation. Measurements of total radiation and radiation in a quite narrow spectral band (~1 nm) in the vicinity of 13 nm were performed using a multilayer mirror with maximum reflectivity at 13 nm and a A l-filter for a cut-off flux of visible and VUV radiation to a detector. The plasma was viewed at a different height from the midplane. The radiation profile and its time evolution were in quite good agreement with the expectation, however the absolute value of radiation intensity at 13 nm was too high in comparison with the intensity of the total radiation. We suspect that some of the VUV radiation was not sufficiently absorbed by the A l-filter. Therefore we have designed two new composed filters, which will be installed in the front of the multilayer mirror and the bolometer in a new series of experiments, the results of which will be a part of the proposal for design and construction of a small tokamak as a new source of radiation.
This Division is headed by Dr. Daniel Cohn with Associate Division Head Dr. Paul Woskov. The main activity of this Division is to develop new plasma technologies which provides major improvements over existing technology. Particular emphasis is given to the development of new environmental technology.
Capability for sensitive real-time monitoring of plutonium in an air stream was demonstrated using a microwave plasma spectrometer system. Laboratory tests were conducted at the Battelle Pacific Northwest National Laboratory at the Department of Energy (DOE) Hanford site. This capability is important for the DOE objectives of using thermal technologies to meet environmental remediation goals with acceptable environmental impact.
A plasmatron device for conversion of hydrocarbon fuels into hydrogen-rich gas was constructed, and initial tests were made at the PFC. The initial tests have shown that both methane and gasoline can be converted into hydrogen in compact devices without soot production.
The calibration and range of application of the microwave plasma air stream metals emission monitoring system have been significantly improved. The monitoring system is being prepared for use on an engineering-scale furnace which will be operated in a radioactive environment at Battelle Pacific Northwest National Laboratory. This furnace and the related monitoring technology is regarded as one of the leading efforts in the DOE program to treat mixed (radioactive and chemical) waste.
An evaluation of practical commercial application of the electron-beam generated cold plasma technology developed at the Plasma Fusion Center was prepared for the Department of Energy.
In the area of fusion plasma diagnostic development, alpha particle gyrotron scattering diagnostic measurements were carried out in the TFTR tokamak at the Princeton Plasma Physics Laboratory. The interpretation of these low-power experiments were hindered by non-thermal lower-hybrid wave emission. This experiment will be continued at high scattering power, using a gyrotron source at the Joint European Tokamak (JET). Personnel have already been transferred to Europe to prepare these experiments.
Future plans include participation in plasma vitrification tests at the Battelle Pacific Northwest National Laboratory facility which will use radioactive material. The Plasma Fusion Center will also implement the microwave plasma metals emission monitoring system and a millimeter wave pyrometer system for temperature measurements at the Battelle Pacific Northwest National Laboratory Facility.
At the PFC, new environmental diagnostics will be developed including a refractory material deterioration monitoring system. Meanwhile, more detailed experimental and theoretical studies will be made of plasma manufacturing of hydrogen.
An R&D 100 Award for the microwave plasma metals emission monitoring system was received in September 1995 at an Award Ceremony in Chicago. Paul P. Woskov and Daniel R. Cohn from the Plasma Fusion Center attended, along with collaborators Jeffrey E. Surma of Battelle Pacific Northwest National Laboratory and Charles H. Titus of T & R Associates. This is the second consecutive year that the Plasma Technology and Systems Division has won and R&D 100 Award.
The microwave plasma metals emission monitoring system was also a finalist in the Discover Magazine Innovation Award competition. Paul P. Woskov, representing the Plasma Fusion Center group, attended the Award Ceremony in Orlando, Florida. Paul was one of seven finalists in the environmental area out of a starting field of about 500.
Three plasma-based environmental technologies developed at the Fusion Center were identified by Energy Secretary Hazel O'Leary as outstanding fusion spin-off applications at a Fusion Forum luncheon on Capitol Hill in May 1996.
The Technology and Engineering Division is headed by Drs. Joseph Minervini and Bruce Montgomery, and comprises 35 engineers, scientists and administrative and support staff. It supports graduate and undergraduate students in the Nuclear Engineering, Mechanical Engineering, Electrical Engineering and Computer Science, and Materials Science and Engineering departments. Funding for this Division remained relatively steady this year.
This year most of the Division's work continued to focus on magnetics R&D for the Department of Energy, Office of Fusion Energy Sciences supported next step tokamak project, the International Thermonuclear Experimental Reactor (ITER). Work on the U.S. Tokamak Physics Experiment (TPX) continued through September 30, 1995 at which point the project was canceled by the U.S. Congress. Some project termination costs ($400k) allowed completion of reporting and documentation activities through December 1995, and paid for the severance pay of most of the engineers associated with TPX.
In-house research for ITER concentrates on superconductor development, subscale testing, and magnet design and analysis. Significant results have been obtained in understanding the stability limitations of fast ramping the superconducting coils, and an understanding of current distribution and crossover from strand to strand within a conductor is beginning. A new area of developing fiber optic instrumentation for superconducting coil diagnostics has continued with the highly successful tests of fiber optic sensors located in a subscale magnet coil and tested at the SULTAN facility at EPFL-CRPP in Switzerland. The test, called QUELL (quench in long lengths) was part of the ITER program, and all ITER parties contributed sensors. The MIT fiber optic sensors were the most successful and performed better than the conventional sensors also tested. The Pulse Test Facility (PTF), a new moderate-sized, test facility has been completed in the Nabisco Laboratories at a cost of $2M, and will start testing large size superconductors and joints for ITER.
Prof. Ron Ballinger's Materials Science and Technology Group continues a new ITER task for detailed mechanical characterization of the superalloy Incoloy 908 which was initially developed in his laboratory for superconducting magnet applications. Results to date have removed many of the concerns of our ITER allies regarding the use of this material in the conduits for the ITER cable-in-conduit conductor.
Extensive collaboration with U.S. industries continued under the ITER program for fabrication of the U.S. contribution to the model coil program, including sub-contracts with Lockheed Martin, INCO Alloys International, Teledyne Wah Chang, and Intermagnetics General Corp., among others. As a cost-saving measure, a site has been leased in Hingham, MA where PFC engineers and technicians will perform a significant part of the coil fabrication, with the balance being subcontracted to several industries.
Recent congressional actions on the fusion energy budget for next fiscal year indicate that a second reduction is likely. Although, at this time, the main ITER program funding appears secure, the Technology and Engineering Division has begun actively seeking new programs outside the Department of Energy supported fusion program. New initiatives have resulted in funding through INEL for a large scale, electromagnetic seismic simulator platform, and from the U.S. Navy for design and analysis of conductor types for a Superconducting Magnetic Energy Storage magnet system. The Basic Energy Sciences Department of South Korea has expressed interest in MIT, assisting them in the development of a new superconducting tokamak called StarX. Funding for this program will come through Princeton Plasma Physics Laboratory and a small contract ($40 K) this year. Significantly greater funding may be available next year. This work takes place in conjunction with the Department of Energy under a recently signed U.S. - Korea bilateral research agreement. As StarX will be similar to TPX in scope and design, this is a good match to our relevant experience from the TPX and ITER programs. The Division also has active proposals in several other areas of magnet technology which are likely to result in near level personnel support into the next fiscal year.
In a new collaborative research project with Applied Science and Technology (ASTEX) of Woburn, MA, (Dr. Jay Kesner and Professor Kevin Wenzel at the PFC, and Larry Bourget and Xing Chen at ASTEX) graduate research assistant Khash Shadman is studying high-density plasma vapor deposition of metal oxide materials and metals. The goal of the metal oxide deposition is to develop and understand novel methods for fabricating high-temperature supeconducting materials. The films will be deposited at laboratory facilities at ASTEX, and their electrical properties will be determined at the PFC.
The Waves and Beams Division, headed by Dr. Richard Temkin, conducts research on novel sources of electromagnetic radiation and on the generation and acceleration of particle beams. This effort has experienced substantial budget cuts by OFES, mostly through a cut of the funding of the CPI/Varian effort (a subcontract through PFC, including monitoring responsibility). The MIT research effort was saved, including the salaries of the research staff.
The gyrotron is a novel source of microwave, millimeter wave and submillimeter wave radiation. It uses a helical electron beam in a high magnetic field to generate radiation by stimulated emission at the electron cyclotron frequency. Gyrotrons are under development for electron cyclotron heating (ECH) of present day and future plasmas as well as for high frequency radar. These applications require tubes operating at frequencies in the range 100-300 GHz at steady-state power levels approaching 1 MW. The gyrotron research group is led by Dr. Kenneth Kreischer.
Research has concentrated on investigating the physics issues which affect the efficiency of operation of high power, high frequency gyrotrons. Efficiency is a critical issue because it determines the recirculating power needed to sustain a practical fusion reactor and also greatly impacts the reliability and cost of plasma heating systems. We have completed the first phase of a program of research to demonstrate a high power, high frequency gyrotron suitable for application to the International Thermonuclear Experimental Reactor (ITER). A prototype experiment at MIT has been built and has met its first milestone: demonstration of a 1 MW power level gyrotron at a frequency of 170 GHz with an efficiency of over 35%. This was the first successful demonstration of a prototype 1 MW, 170 GHz gyrotron by any of the four parties in the ITER program. This work was carried out in collaboration with Communication and Power Industries Inc. (CPI, formerly a part of Varian), General Atomics, Univ. Wisconsin, Univ. Maryland and Lawrence Livermore National Lab. The MIT gyrotron group has the lead role in this effort. In the next phase of this work, we intend to demonstrate power output in a Gaussian free space mode. We also plan to study a depressed collector which can increase the gyrotron efficiency to over 70%.
A program of research is also underway to demonstrate a coaxial cavity gyrotron. This experiment is at 140 GHz in collaboration with Dr. Michael Read of Physical Sciences, Inc. of Alexandria, Virginia. The coaxial cavity gyrotron may be capable of higher power than conventional cavity gyrotrons, up to 3 MW. In 1995-6, this experiment was successfully operated at power levels of up to 1 MW. The power was limited by the focusing of the electron beam. Improvements to the gyrotron electron gun are planned to improve the focusing.
The High Gradient Accelerator Group is preparing a novel, 17 GHz microwave driven, photocathode electron injector. This device, sometimes called an RF gun, can generate a 2 ps beam of 2-3 MeV, 50-500 A electrons at high repetition rate. A 17 GHz klystron power source will drive the electron gun. This electron beam can be directly applied to microwave generation experiments or it can be used as an injector into a 17 GHz, high gradient accelerator. This research supports the program to build new electron accelerators which can reach the TeV range of energies.
The RF gun experiment has been operated with a microsecond pulse length klystron source at power levels of 5 to 10 MW at 17.145 GHz. The power coupled into the electron gun was monitored using the forward and reflected microwave power. A stored field equivalent to an on-axis accelerating gradient as high as 150 MeV/m was obtained, a record high value. Work is also progressing on generating the required laser pulse for the photocathode. Laser operation has been significantly improved in the last year making the picosecond laser system reasonably reliable and useful. The laser pulse must be timed to an accuracy of 1 ps in order to coincide with the 17 GHz accelerator field at a phase accurate to within 6 degrees.
Research is continuing on a high power, 17 GHz klystron in collaboration with Haimson Research Corp. of Palo Alto, CA. The klystron electron gun and the klystron cavities were built by Haimson Research. The klystron has previously demonstrated power levels of up to 26 MW in 1 us pulsed operation using a 560 kV, 95 A beam. These are record power levels for a relativistic klystron operating at such a high frequency in pulse lengths in the us range. An efficiency as high as 51% was achieved. Work is continuing on optimizing the klystron performance and applying it to high gradient acceleration experiments. The klystron has been recently rebuilt by Haimson Research to eliminate parasitic microwave emission. Testing of the rebuilt source is now underway.
This effort lead by Dr. Chiping Chen, has contributed very significantly to our understanding of coherent radiation generation and particle acceleration. Topics covered include coherent radiation sources (CARM, FEL, gyrotron, relativistic klystron, relativistic TWT), intense beam transport and beam halo formation, beam-beam interactions, cyclotron resonance accelerators, two-beam accelerators, photocathode design, and other topics. Multimode codes have successfully predicted new phenomena in both the cyclotron autoresonance maser and free electron laser amplifier.
A new research program has been initiated by Dr. Chiping Chen on the topic of theoretical and computational investigation of periodically focused intense charged particle beams. This research will support the U. S. program to construct advanced accelerators for such applications as nuclear waste treatment, heavy ion fusion and free electron lasers. Research will explore self-field-induced nonlinear resonant and chaotic phenomena in intense charged particle beams.
The Plasma Fusion Center has established an educational outreach program primarily focused on heightening the interest of K-12 students in scientific and technical subjects. The Mr. Magnet Program, headed by Technical Supervisor Paul Thomas, has been particularly successful. Mr. Magnet, with the help of a graduate student, brings a traveling demonstration on magnetism into local elementary schools, inspiring and exciting students with the chance to take part in hands-on experiments with magnets. He stresses that science is a valid pursuit for boys and girls. Over the past year he has worked with over 15,000 students. The PFC also seeks to educate students and the general public by conducting general tours of experiments being done here. Special "Outreach Days" are held twice a year, encouraging high school and middle school students from around Massachusetts to visit the PFC for a day of hands-on demonstrations and tours.
Given the success of "Mr. Magnet," we are exploring ways to raise additional funding for Mr. Paul Thomas. Until now he has been funded by Dr. Cohn's Division. Beginning this fall, we will fund him from other Divisions up to 25% of his time. However, given that he is booked up to 50% of his time with school visits, we need to find other sources of support (from schools, local industry, and possibly DOE), or his activities will have to be reduced from 50% to 25% of his time.
The PFC has also been involved in the Contemporary Physics Education Project (CPEP), a collaborative effort of fusion facilities around the U.S. The goal of this group is to create a fusion-oriented curriculum, along with supporting hands-on experiments and graphics, for use in high schools around the country. Mr. Paul Rivenberg has worked on the "Chart Committee" of this project, which has created a wall chart that will aid in the understanding of fusion.
The Fusion Forum, held each year on Capitol Hill, is a community-wide effort to show Congress the goals of the national fusion program and its gains over the past year. Fusion fundamentals are also outlined to educate new Congressmen and staff members. In May 1996 Miklos Porkolab, Bruce Montgomery, Dan Cohn, Albe Dawson and Paul Rivenberg, together with Tobin Smith of the MIT Washington office participated in the Forum. An exhibit was brought to Washington to show 1) our K-12 educational outreach programs, 2) Alcator C-Mod and ITER magnetics accomplishments and 3) the PFC's plasma-science-based spin-off technologies, including hazardous waste remediation, microchip manufacture, and cutting tool plasma-spray coatings to increase surface hardness and tool life up to 100-fold at a very small cost increase. Videotapes, showing plasmas in the C-Mod vacuum chamber and internet control of the Alcator C-Mod tokamak, received considerable attention. The PFC also contributed to a special exhibit on the topic of superconductivity.
During the past year, there have been several important appointments and promotions in Plasma Fusion Center program areas:
Appointments include: Jennifer Crockett (Consortium of Financing Higher Education) appointed Assistant Fiscal Officer in the Fiscal Office.
Internal promotions in the Plasma Fusion Center during the past year include: Joanna Iwanow, promoted to Assistant Fiscal Office in the Fiscal Office; Sangkwon Jeong, promoted to Research Engineer-Experimental in the Fusion Technology and Engineering Division; Stephen Kochan, promoted to First Wall Engineer in the Alcator Division; Sergeui Kracheninnikov, promoted to Research Scientist-Theoretical, in the Physics Research Division; Joseph Minervini, promoted to Division Head in the Fusion Technology and Engineering Division; and Christopher Reddy, promoted to Magnet Cooling and Structural Engineer, in the Alcator Division.
The Plasma Fusion Center has also hosted 68 Visiting Scientists, Engineers and Scholars during the past year.
During the past year, the following students graduated with theses in plasma fusion and related areas: Phillip Borchard, M.S., Mechanical Engineering; Darren Garnier, Ph.D., Physics; James Gilmore, Ph.D. Nuclear Engineering; Ruxandra Golinescu, Ph.D., Nuclear Engineering; David Jablonski, Ph.D., Nuclear Engineering; Boris Lekakh, Ph.D., Nuclear Engineering; Hunwook Lim, Ph.D., Mechanical Engineering; Daniel Moriarty, Ph.D., Nuclear Engineering; Suzanne Murphy, M.S., Electrical Engineering and Computer Science; David Nelson, M.S., Mechanical Engineering; Artur Niemczewski, Ph.D., Nuclear Engineering; Todd Rider, Ph.D., Electrical Engineering and Computer Science; Ying Wang, Ph.D., Physics; and Mamoon Yunus, M.S., Mechanical Engineering. We take this opportunity to wish these graduates success in their future professional endeavors.
MIT Reports to the President 1995-96