J. C. Glowienka

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.

Second stability in the ATF torsatron—Experiment and theory

Access to the magnetohydrodynamic (MHD) second stability regime has been achieved in the Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)]. Operation with a field error that reduced the plasma radius and edge rotational transform resulted in peaked pressure profiles and increased Shafranov shift that lowered the theoretical transition to ideal MHD second stability to β0≊1.3%; the experimental β values (β0≤3%) are well above this transition. The measured magnetic fluctuations decrease with increasing β, and the pressure profile broadens, consistent with the theoretical expectations for self‐stabilization of resistive interchange modes. Initial results from experiments with the field error removed show that the pressure profile is now broader. These later discharges are characterized by a transition to improved (×2–3) confinement and a marked change in the edge density fluctuation spectrum, but the causal relationship of these changes is not yet clear.

Finite-beta equilibrium magnetic field perturbations in stellarator plasmas

Magnetic field perturbations due to finite-beta operation in stellarators have been simulated by using the three-dimensional free-boundary equilibrium code VMEC to overcome the limitations imposed by averaged equilibrium and fixed-boundary methods. Results of these computations have been compared with analytic predictions for cylindrical stellarator models and confirm a linear relationship between the average beta and the plasma dipole moment. Only a weak sensitivity of the computations to details of the pressure profile is found. The distortion of the magnetic surfaces can be significant even at moderate beta, so that careful modelling is required when analysing the data.

Recent results from the ATF torsatron

Recent experiments in the Advanced Toroidal Facility (ATF) torsatron [Plasma Physics and Controlled Nuclear Fusion Research 1990 (IAEA, Vienna, in press)] have emphasized the role of magnetic configuration control in transport studies. Long‐pulse plasma operation up to 20 sec has been achieved with electron cyclotron heating (ECH). With neutral beam injection (NBI) power of ≥1 MW, global energy confinement times of 30 msec have been obtained with line‐average densities up to 1.3×1020 m−3. The energy confinement and the operational space in ATF are roughly the same as those in tokamaks of similar size and field. The empirical scaling observed is similar to gyro‐reduced Bohm scaling with favorable dependences on density and field offsetting an unfavorable power dependence. The toroidal current measured during ECH is identified as the bootstrap current. The observed currents agree well with predictions of neoclassical theory in magnitude and in parametric dependence. Variations of the magnetic configuration ...

ATF heavy ion beam probe: Installation and initial operation

Installation of the HIBP on ATF began in the summer of 1988. All of the major hardware components have now been installed. The initial operation of the diagnostic has begun amid the final stages of testing and control system integration. The existence of significant magnetic fields and gradients outside of the main plasma volume and fully three‐dimensional particle trajectories have raised several interesting issues during the design, assembly, alignment, and operation of the beamline and analyzer. The diagnostic must function in a challenging environment. It must perform satisfactorily despite electrical interference from several nearby sources, pressure excursions caused by gas puffing, and UV/plasma loading.

Fluctuation and modulation transport studies in the Advanced Toroidal Facility (ATF) torsatron

The Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)] has completed experiments focusing on microwave scattering measurements of density fluctuations and transport studies utilizing the modulation of dimensionless parameters. Microwave scattering measurements of electron density fluctuations in the core of low‐collisionality electron cyclotron heated (ECH) plasmas show features that might be evidence of trapped electron instabilities. Starting from gyro‐Bohm scaling, the additional dependence of confinement on the dimensionless parameters ν* and β (collisionality and beta) has been investigated by modulating each of these parameters separately, revealing the additional favorable dependence, τE∝τgBν*−0.18β+0.3.

In situ analyzer calibration methods for heavy ion beam probes installed on stellarator-like devices

An absolute calibration of an installed heavy ion beam probe (HIBP) energy analyzer is possible by detecting secondary ions that come from a source at a known location that is maintained at a known potential. It is also necessary that the magnetic field that exists during the calibration be the same magnetic field that exists when a plasma is present. These conditions can be met on stellarators, heliotrons, and torsatrons, which produce their magnetic configurations with external coil sets. Since no internal plasma current is required, suitable sources for producing secondary ions by interaction with the primary beam can be placed inside the vacuum vessels of these devices at known locations. Secondary ions can be produced by the interaction of the primary beam with thin films, gas box probes, electron beams, or neutral gas filling the vacuum vessel. Details of a spherical mesh probe that combines the advantages of several of these methods are given.

The ATF heavy ion beam probe (invited)

A heavy ion beam probe (HIBP) has been implemented on the ATF torsatron at Oak Ridge National Laboratory with the primary goal of providing direct measurements of the plasma potential radial profile and thus of the radial electric field. The complex ATF geometry and magnetic field structure presented a diagnostic environment more challenging than that found on previous beam probe systems. Particular attention has therefore been given to in situ system alignment and control capabilities. Measurements of electric potential profiles, electron density profiles, electron density fluctuations, and electric potential fluctuations have now been made with this system. Most of the data obtained were for ECH heated discharges, but we were also able to make measurements of a few NBI heated plasmas. In addition to our calibration techniques, we were able to establish a reasonable confidence level for the data obtained since we could identify the most important potential profile characteristics predicted by theory and ...

Radiative losses and improvement of plasma parameters after gettering in the advanced toroidal facility

The characteristics of plasmas in the Advanced Toroidal Facility (ATF) have proven to be strongly dependent on the type of wall conditioning employed. A succession of techniques, beginning with glow discharge cleaning and baking, and evolving to gettering with chromium and titanium, have led to progressive improvement of the plasma parameters. Gettering with titanium has reduced the low-Z impurity content by a factor of 3, lowered the radiated power by a factor of 2.5–3.5, and improved the control over the electron density. The maximum values achieved for stored energy, line averaged density and confinement times are 28 kJ, 1.2 × 1014cm−3 and 25 ms, respectively. These parameters are comparable to the best results achieved in the ISX-B tokamak which had the same average minor radius and one half the major radius of ATF. Quasi-steady operation for 200 ms of neutral beam injection (NBI) has been obtained in high density, titanium gettered plasmas without the collapses that were typical earlier periods of operation. Neon injection experiments have helped to delineate the limits on the global levels of radiation that can be maintained and have supported the conclusion that mechanisms other than radiative losses are important for initiating the collapses still observed in low density NBI plasmas.

Runaway electron studies in the ATF torsatron

Runaway electron formation and confinement occur readily in pulsed torsatrons and heliotrons because of the high loop voltages during initiation and termination of the helical and vertical fields (‘‘field ramping’’) and the inherently good containment of the electrons on the flux surfaces in the vacuum fields. This has been confirmed for the Advanced Toroidal Facility (AFT) [Fusion Technol. 10, 179 (1986)] and other stellarators by orbit calculations. Since runaway electrons can cause an unacceptable level of hard x rays near the machine, a runaway electron suppression system was incorporated in ATF. The main component of the system is a movable paddle, which is normally left in the center of the plasma chamber during the field ramps. This device, in conjunction with programmed vertical field ramping, which reduces the volume of good flux surfaces, has proved to be very effective in reducing the runaway electron population. Measurements of hard x rays from ATF have shown that the runaway electrons are pro...