• Strongly Coupled Plasmas -- DeSilva
• Magnetic Fusion
  • Innovative Fusion Experiments -- Ellis, Hassam, DeSilva
• Electron Cyclotron Emission from Fusion Plasmas -- Ellis

STRONGLY COUPLED PLASMAS
DeSilva

A plasma is said to be `strongly coupled' when the mean Coulomb energy of the ions is comparable to or less than the mean kinetic energy. This occurs in very cool dense plasmas, such as those found in the gaseous planetary interiors, dwarf stars, and in laser-produced plasmas. Transport properties of these plasmas are important in understanding of the structure of stars and gaseous planets, and in laser fusion. We create strongly coupled plasmas in the laboratory by rapid vaporization of thin metal wires, by means of a rapidly rising electrical current. The resulting plasma exists for a few microseconds, and its electrical conductivity may be studied as a function of the plasma density as the plasma expands, and the measured conductivity may be benchmarked against transport theory.

This research is supported by the National Science Foundation.

Innovative Fusion Experiments
(Ellis, Hassam)

Centrifugal forces from supersonic plasma rotation can be used to augment the usual magnetic confinement of plasmas. When optimized, this "knob" results in a device that features several advantages over conventional approaches.

The idea rests on two prongs: first, centrifugal forces can be used to contain plasmas to desired regions of appropriately shaped magnetic fields; second, the accompanying large velocity shear can stabilize even MHD instabilities. If these ideas are workable, the resulting coil configuration is simple and there are no substantial plasma currents.

As far as transport goes, the velocity shear can also quell microturbulence, leading to fully classical confinement as there are no neoclassical effects. Classical parallel electron transport then determines the confinement time. These losses are minimized by a large Pastukav factor resulting from the deep centrifugal potential well. At Mach 4-5, the Lawson Criterion is accessible.

An experiment to test these ideas (Maryland Centrifugal Experiment, MCX) has been funded and is under construction at IREAP. The central goal of the MCX experiment will be to obtain MHD stability from velocity shear. Specifically, it will be determined how much, if any, toroidal field is necessary to suppress residual wobbles and convection from the interchange. Previous experiments were probably MHD convection limited and did not have a toroidal field. In addition, the MCX experiment will feature a plasma of elongation 6-8, which should reduce the interchange growth rate and so reduce Mach number requirements. The CAD picture below shows the MCX with toroidal field coils.

This research is supported by the Department of Energy.

ELECTRON CYCLOTRON EMISSION FROM FUSION PLASMAS
Ellis

In a magnetic field B, electrons spiralling about field lines emit radiation at the electron cyclotron frequency and its harmonics. This radiation has proven to be an excellent diagnostic on tokamak devices for measuring the electron temperature and its radial variation. More recently, efforts have been made to measure the parameters of non-Maxwellian electron distribution functions by using the same electron cyclotron emission. In this case, the radiation is from high energy electrons (100 keV to a few MeV) which, because of their low collision frequency, can maintain a non-Maxwellian distribution.

We are currently operating a large Michelson interferometer on the DIII-D tokamak to measure the emission spectra from thermal and nonthermal electrons, using both a horizontal and vertical view. This instrument has become a baseline diagnostic for measuring electron temperature profiles.

This research is supported by the Department of Energy.