G. R. Haste

Plasma properties in the ELMO bumpy torus

Experiments with 200 kW of applied electron cyclotron heating (ECH) power have demonstrated electron temperatures of about 1 keV in the ELMO Bumpy Torus-Scale (EBT-S) device. Electron densities are in the range of (0.5-1.5)*1018 m-3 and increase as the square root of the applied ECH power. A potential well is present, and its depth in V closely follows the electron temperature expressed in electron volts. Only low charge states of impurities are found, and Zeff approximately=1.0. Data from the electrons can be compared with simple scaling laws when scale lengths are held constant. These comparisons indicate that electron densities, temperatures, and confinement times scale according to neoclassical expectations.

Observation of hot electron ring instabilities in ELMO Bumpy Torus

A high‐frequency hot electron instability is observed in ELMO Bumpy Torus (EBT) plasmas when the hot electron‐to‐ion density ratio exceeds 0.4. Both the real frequency and the imaginary frequency are larger than the ion cyclotron frequency. The azimuthal mode number (m) is 7, and the instability rotates in the hot electron curvature drift direction. This instability is identified as a curvature‐driven mode. When it is strongly excited, the equilibrium of the hot electron annuli and confined plasmas are destroyed (disruption). Ion heating and neutron bursts are associated with this instability.

Electron confinement studies on the EBT-S Bumpy Torus Experiment using soft x-ray techniques

Soft x‐ray bremsstrahlung measurements have been performed on the ELMO Bumpy Torus (EBT‐S) plasma to determine the electron temperature Te and electron density density ne using a calibrated Si(Li) detector over a wide range of operating conditions. The purpose of this paper is to outline the necessary assumptions and essential x‐ray techniques that are inherent in soft x‐ray measurements in order to investigate the electron heating and confinement properties of EBT‐S. In addition, by utilizing the electron density as determined by the soft x‐ray measurements, the previous EBT‐S confinement analyses have been extended. The steady‐state plasma of EBT‐S is heated by microwaves using a continuous wave (cw) gyrotron that can operate up to power levels of 200 kW. From the soft x‐ray measurements, both the electron temperature and density are found to increase at higher microwave power levels. For operation at microwave power levels of 200 kW, Te approaches 1 keV while ne approaches 1.2×1012 cm−3. In general, co...

Microinstability Limitations of the DCX‐1 Energetic Plasma

A steady state plasma with an energetic (≈ 300 keV) proton component is formed by dissociation of part of a 600‐keV H2+ beam that makes a single pass in the midplane of a 2: 1 magnetic mirror field with a central field value of 10 kG. Both gas dissociation and Lorentz dissociation have been employed. The pontentially accessible density (density expected in the absence of instabilities) with Lorentz dissociation is the larger by more than an order of magnitude. The plasma is stable against flute instabilities, but microinstabilities (ion‐electron or negative mass instabilities) are evidenced by radio frequency signals at ω ≳ ωci. The lowest density threshold for observation of these signals is 4 × 105 fast protons cm−3. With Lorentz dissociation the microinstabilities eventually eject protons, as observed by detection of these direct losses correlated with microinstability signals and by failure of charge‐exchange accountability. The losses are first observed at densities of 5−10 × 107 cm−3; they amount to...

Energy distributions of protons in DCX

A particle spectrometer has been used to measure the energy distributions of neutral hydrogen atoms escaping from the 300 keV proton storage ring in DCX as the result of electron capture collisions between trapped protons and background gas molecules. A portion of the atoms were converted to protons by passage through an argon-filled gas cell, and the proton beam was then electrostatically analyzed. Energy distributions of the circulating protons were obtained by transformations applied to the measured distributions.For both gas and carbon arc dissociation the energy distributions were strong functions of the injected H2+ current and the location of the region of sampling relative to the median plane. A number of curves are shown illustrating these dependences.With arc dissociation, the circulating protons lost energy at a rate of about 20 keV/ms with 0.1 mA injected current, and at a rate twice this value when the current was increased to 2.3 mA. Most of the 20 keV/ms loss rate is believed to be due to coulomb collisions of the circulating protons with electrons in the dissociating arc. This loss rate is within a factor of two of that calculated on the basis of loss to electrons of a Maxwellian distribution, well within the accuracy of the arc parameters used in the calculation. Several mechanisms that might account for the additional energy loss rate at higher currents are suggested, but details of the origin of this loss are as yet unclear. The additional energy loss had the consequence of decreasing the mean storage time of the circulating protons.Measurements with gas dissociation also showed an increase in the rate of energy loss with injected current.With either arc or gas dissociation, the response of the energy distributions to changes in injected H2+ current indicated the presence of a non-collisional dispersing mechanism which increases in importance with increases in injected current. The nature of this mechanism is not clear.

Axial distribution for a hot electron plasma

Bremsstrahlung measurements have been used to determine the axial distribution of density and temperature in an electron cyclotron heating experiment. The results show two heating mechanisms. Low energy electrons turn at the resonance position and are resonantly heated. High energy electrons turn well inside their resonance position and are nonresonantly heated.

Results of Folded Waveguide Tests on RFTF

Experiments with the 80‐MHz prototype folded waveguide on the Radio‐Frequency Test Facility (RFTF) at Oak Ridge National Laboratory have achieved substantially higher power levels than any previous tests on comparably sized loop antennas. This result, combined with a superior wave spectrum, suggests that the folded waveguide should be capable of coupling several times the power flux of a loop antenna into a tokamak plasma.

SUMMARY OF ELMO BUMPY TORUS EXPERIMENTS FROM 1982 TO 1984

Experiments were conducted in the ELMO Bumpy Torus (EBT) from 1973 until 1984. A number of papers have been published concerning various aspects of experiments during the final two years of operation. The paper summarizes the final experimental conclusions and discusses those issues which remain unresolved. The main conclusions are as follows: (1) measurements of the width of the hot electron rings showed them to be wider than previously suspected, diluting their diamagnetism and negating their ability to locally reverse the magnetic field gradient; (2) two-dimensional plots of the plasma potential revealed open potential contours in the C-mode and closed potential contours near the plasma centre in the T-mode; (3) an in–out asymmetry in the plasma potential was always observed, giving rise to electric fields which drove convective plasma loss.

ICH antenna development on the ORNL RF Test Facility

A compact resonant loop antenna is installed on the ORNL Radio Frequency Test Facility (RFTF). Facility characteristics include a steady-state magnetic field of {approx} 0.5 T at the antenna, microwave-generated plasmas with n{sub e} {approx} 10{sup 12} cm{sup {minus}3} and T{sub e} {approx} 8 eV, and 100 kW of 25-MHz rf power. The antenna is tunable from {approximately}22--75 MHz, is designed to handle {ge}1 MW of rf power, and can be moved 5 cm with respect to the port flange. Antenna characteristics reported and discussed include the effect of magnetic field on rf voltage breakdown at the capacitor, the effects of magnetic field and plasma on rf voltage breakdown between the radiating element and the Faraday shield, the effects of graphite on Faraday shield losses, and the efficiency of coupling to the plasma. 2 refs., 4 figs.

Multiple‐frequency electron‐cyclotron heating of hot‐electron rings in a bumpy torus

The use of multiple‐frequency microwave power for electron‐cyclotron heating significantly increased the ring stored energy in the SM‐1 simple mirror device. Multiple‐frequency electron‐cyclotron heating (MFECH) was used on the ELMO Bumpy torus (EBT) in an effort to increase its hot‐electron beta. No substantial improvement in the ring parameters was observed in a series of two‐frequency ECH experiments, with frequency separations up to 90 MHz, in contrast to the dramatic improvement found in the axisymmetric SM‐1 experiment. The toroidal canting of the EBT mirror sectors introduces asymmetries that destroy the superadiabatic behavior of the energetic electrons, reduce microwave heating efficiency, and produce additional ring losses. These effects qualitatively explain the different multiple‐frequency heating results obtained in EBT and SM‐1.