• Strongly Coupled Plasmas -- DeSilva
• Magnetic Fusion
  • Edge Plasmas -- Griem, Elton
  • Innovative Fusion Experiments -- Ellis, Hassam, DeSilva
• Inertial Confinement Fusion -- Elton, Griem
• Picosecond Laser-Produced Plasmas -- Griem, Iglesias, Elton
• 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.

MAGNETIC FUSION

Edge Plasmas (Griem, Elton)

A thorough understanding of the edge regions of current and future magnetic fusion devices is essential for preventing both the hot central plasma from reaching the wall and the cold wall material from entering the plasma. Experiments are being performed on the Alcator C-Mod tokamak at MIT in an effort to understand these regions, and scientists from Maryland (IREAP) are involved with these experiments. (A photo of the plasma appears below.) Line widths and shifts of visible and ultraviolet radiation from impurity species are being used to study the temperature, density, and electric/magnetic fields in the plasma, as well as the plasma flows.

This research is supported by the Department of Energy.

Innovative Fusion Experiments
(Ellis, Hassam, DeSilva)

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.

INERTIAL CONFINEMENT FUSION
Elton, Griem

In inertial confinement fusion, small pellets of hydrogen isotopes are inertially compressed following rapid surface heating by a high power laser and subsequent ablation to form extremely high density and temperature plasmas. Internal dynamic conditions are studied using penetrating x-rays. Self-emission is measured spectroscopically to determine plasma densities and temperatures, as well as the localized electric fields associated with the intense laser pulse. Instrumentation and techniques are developed at the University using small-scale laser-produced plasmas and ultimately taken to large national laboratories such as the Naval Research Laboratory (NRL), the University of Rochester Laboratory for Laser Energetics (LLE) and the Los Alamos National Laboratory (LANL) for actual experiments. Previous successes have included spectral line broadening and merging measurements from which plasma densities have been deduced, and early time prcursor emisssion. The figure shows IREAP scientists operating their x-ray spectrograph to diagnose early-time plasma characteristics at the OMEGA inertial confinement laser-fusion facility at LLE. This device utilizes 60 laser beams with a total energy of 30 kJ focused onto a 1-mm diameter pellet.

This research is supported by the Department of Energy.

PICOSECOND LASER-PRODUCED PLASMAS
Griem, Iglesias, Elton

We are mainly interested in the atomic physics of dense plasmas produced by focusing the light from a homemade, low maintenance, high power 10 ps KrF discharge laser system onto targets of various materials. With laser powers on the order of 10¹⁵Watt cm² we achieved extremely dense, 10²² electrons/cm³, high temperature equal to or approximately 200 eV, plasmas that we used as the object of our studies to develop spectroscopic methods for analysis of plasma properties under such extreme conditions. So far we have made measurements of the emitted light that range from soft x-rays to the near ultraviolet range of the electromagnetic spectrum, and successfully observed such elusive effects as the plasma polarization shift on members of the Lyman series of C VI. Ultraviolet laser interferometry is used to determine the electron density by comparison with numerical modeling, shown in the figure. We are also studying the generation of magnetic fields in these plasmas, and we are looking for evidence of an anomalous signature in the recombination spectrum, the "transparency window" effect.

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.