TURBULENCE AND TRANSPORT

IN TOKAMAK PLASMAS

The FRC offers its students the opportunity to study plasma turbulence and its impact on confinement in state-of-the-art fusion devices. The research is conducted at the University of Texas at Austin and at other labs in the U. S. where the FRC has permanent on-site staff. Even when doing research away from the university, students will have the advantages of using the most advanced diagnostic techniques under the supervision of senior FRC research staff and of strong engineering and computational support from the FRC. Close links with the university faculty are maintained via video-conferencing and the Internet.

OVERVIEW OF THE RESEARCH PROGRAMS

Hot magnetically confined plasmas are turbulent. The turbulence limits the degree to which the plasma can be confined. Since we want to know the cause of the plasma turbulence and the link between turbulence and confinement, we are conducting experiments on the two major magnetic confinement devices in the U.S.: Alcator C-Mod and DIII-D. Both were constructed to investigate the physics of magnetically confining a plasma and then heating it to produce thermonuclear fusion in a controllable environment.

What do we really need to know about turbulence to understand its effect on plasma confinement? This question led us to adopt several different approaches in our research. In the simplest, we are guided by our best theorists who will propose a source of the turbulence and then work out its expected hallmarks. We then look for the hallmarks in the confined plasmas. One example is a current research project based on the hypothesis that the free energy source for the turbulence is contained in the gradient of the ion temperature profile. If we confirm the speculations about the ubiquity and virulence of the turbulence thus generated, then not only would we have achieved a very interesting physics result, but we would have made it possible to assess the performance of planned devices and perhaps suggest improvements. On the other hand, if the turbulence is not as predicted, then we will still make progress in the physics by pointing out that the prediction was faulty.

Perhaps we do not need to know the source of the turbulence to make significant progress in understanding the effect of turbulence on current plasmas and, more importantly, allowing us to predict the behavior of plasmas in devices not yet conceived. The concept of marginal stability appears frequently in descriptions of transport in tokamaks. In essence, it is the idea that when a local gradient exceeds a critical value set by a stability criterion, the fluctuation driven flux increases rapidly to drive the gradient back toward the critical value. If this is an accurate representation of the physics, this concept would be valuable because it provides a means for predicting the effects of transport on turbulence without actually knowing the source of the turbulence. We feel that a demonstration of marginal stability in a particular case would go far in securing the foundation for this concept. Though apparently amorphous, this hypothesis leads to testable statements concerning the turbulence. We are testing those now.

Whether one or the other of these approaches will give us the greatest progress, we still need to measure the turbulence, document the plasma profiles, and look for phenomenological correlations between them. For example, in measuring the turbulence, we must measure the frequency spectrum, spatial and temporal distances over which the fluctuations are correlated, and the degree to which the confining fields introduce spatial asymmetry into the correlations. This involves fielding standard diagnostic techniques, sometimes developing new techniques, investigating and applying new analysis tools, and staying well attuned to the direction of contemporary theoretical research.

OPPORTUNITIES FOR STUDENTS

Graduate and even undergraduate students can become involved in all of these areas. Those interested in joining our research as graduate students would complete the bulk of their course work at the University of Texas and then do their thesis work on Alcator C-Mod or DIII-D. While taking courses at Texas, the student would sharpen laboratory skills by conducting experiments on small devices, assisting in construction of apparatus for use on the larger machines, and analyzing data from those devices. Exposure to the larger facilities would be gained through video-conferencing with on-site FRC staff, real-time participation in experiments via the Internet, and possibly by spending summers on-site working under the supervision of an FRC staff member. After completion of course work in the case of masters candidates or after admission to candidacy in the case of Ph.D. students, the student would be eligible (and expected) to spend substantial time on-site conducting thesis experiments.

THE MACHINES

Alcator C-Mod

The FRC is collaborating with the MIT Plasma Science and Fusion Center (PSFC) to investigate the links between turbulence and transport in tokamaks operating at extreme magnetic fields and particle densities. An FRC staff member and an FRC postdoctoral fellow are permanently on-site and have primary responsibility for the development of a diagnostic neutral beam and associated diagnostics. Most of the other FRC staff contribute their skills in designing, conducting, and analyzing turbulence experiments either through periodically visiting the PSFC or participating in experiments and data analysis via remote access and video-conferencing over the Internet. In this collaboration we apply our full repertoire of techniques -- neutral beam diagnostics, ECE radiometers, electrostatic probes optimized for turbulence studies, novel optical diagnostics for turbulence and phase contrast interferometry -- and further call upon our theory colleagues in the Institute for Fusion Studies to supply us with new ideas to test. This work is conducted under the sponsorship of the Department of Energy.

At present, these are the diagnostic systems that we are developing for Alcator C-Mod:

• A diagnostic neutral beam. This device produces a high energy, collimated beam of neutral atoms. Though the beam has little effect on the confined plasma, its interaction with plasma ions and impurities results in the generation of visible light which can be analyzed to make the following measurements:
  - ion density fluctuations via beam-emission spectroscopy (BES),
  - profiles of impurity density, temperature, and rotation velocities via charge-exchange recombination spectroscopy (CXRS),
  - toroidal current-density profiles via the motional Stark effect (MSE).

• An electron-cyclotron-emission (ECE) heterodyne radiometer. Confined plasma electrons orbit the magnetic field lines and emit radiation in the gigahertz range because of the acceleration required to keep them in a near circular orbit. This radiometer is used to measure the frequency and intensity of the radiation from which profiles of electron temperature (including fluctuations) can be derived.

• Electrostatic probes optimized for turbulence measurements in the plasma edge. Tungsten wires inserted into the low temperature regions of the confined plasmas can be biased to draw small currents from the plasma. From the relationship between bias voltage and current can be derived temperature and density of the plasma. For properly designed probes, the fluctuations in these quantities can be measured as well.

• An optical diagnostic to measure fluctuations in otherwise inaccessible regions of the edge plasma. Turbulence in a plasma parameter such as electron density can result in predictable fluctuations in the light emission by plasma ions, neutrals and impurities. It is often possible to see a plasma region, but not access it with physical probes. In these cases, optical diagnostics can provide information in otherwise inaccessible plasma regions.
  

DIII-D

The FRC is also participating in turbulence and transport studies on the DIII-D tokamak at General Atomics Co. (GA) in San Diego, CA. These involve

i) collaborating with UCLA on ECE fluctuation measurements and analysis,

ii) collaborating with the University of Wisconsin, Madison (UWM) on BES measurements and analysis, and

iii) proposing and carrying out experiments aimed at understanding the role of turbulence in tokamak transport.

We also are involved in high-resolution (both space and time) ECE measurements of electron temperature. An FRC staff member, primarily involved in the ECE measurements, is on site at GA full time. As with Alcator C-Mod, other FRC staff either periodically visit GA during experiments, or participate via remote access and video-conferencing.

Though smaller than at Alcator C-Mod, our collaborative program at DIII-D is crucial because it allows us to be involved in the same types of measurements (BES and ECE) on the two remaining U.S. tokamaks. The two machines are radically different: DIII-D is much larger and operates at lower particle densities with lower confining magnetic fields. It is heated by neutral beams in such a way as to spin the plasma to the achieve rotation velocities as high as any observed in tokamaks. Alcator C-Mod is heated with ion-cyclotron resonance heating which does not result in such high rotation velocities. The opportunity to do experiments on both machines puts us at the very center of machine-to-machine comparison studies. This is certainly one of the most exciting areas today because comparing and contrasting results in disparate plasmas will likely lead to insight into the fundamental mechanisms of turbulence and transport.

Students interested in any of these areas of research opportunities can contact Dr. William Rowan. Video-conferencing facilities are available for further one on one communication.