The Gigabit network project was established to demonstrate ATM technology in a realistic metropolitan environment running realistic applications, which would stretch its capacity. Despite considerable obstacles, both technical and logistical, the Gigabit Network project succeeded in establishing a network infrastructure that has served the Oak Ridge complex well during the last two years, and will continue to serve it in the future. The project did not, however, succeed in demonstrating the showcase applications on an ATM network. Development and delivery of a working ATM switch ultimately became the pacing item in the project, and after a number of delays, the project was terminated without placing a switch in service.

At the time of the start of this project, a battle was being fought between the computer networking technologies and telephone networking technologies. The telecommunications industry wanted to standardize on Asynchronous Transfer Mode (ATM) as the technology of choice for carrying all cross-country traffic. The computer industry wanted to use Packet Transfer Mode (PTM). The project had several goals, some unspoken. At the highest, most obvious level, the project goals were to test the high-speed components being developed by the computer technology industry. However, in addition, both industrial partners were having trouble finding markets for the high-speed networking technology they were developing and deploying. Thus, a part of the project was to demonstrate applications developed at Oak Ridge which would stretch the limits of the network, and thus demonstrate the utility of high-speed networks. Finally, an unspoken goal of the computer technology industry was to convince the telecommunications industry that packet switching was superior to cell switching. Conversely, the telecommunications industry hoped to see the computer technology industry`s packet switch fail to perform in a real-world test. Project was terminated early due to failure of one of the CRADA partners to deliver needed component.

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 nu* and beta (collisionality and beta) has been investigated by modulating each of these parameters separately, revealing the additional favorable dependence, tau(E) is-proportional-to tau(gB)nu*-0.18beta+0.3.

The results of dimensionless-parameter modulation and configuration control experiments in the Advanced Toroidal Facility (ATF) are presented. The global energy confinement time fits gyro-Bohm scaling better than Bohm-like scaling. An additional dependence was detected by modulation of single dimensionless parameters (collisionality {nu}* and beta {beta}), yielding {tau}{sub E}/{tau}{sub gB}{proportional_to}{nu}* {sup {alpha}v}{beta}{sup {alpha}{beta}}, where {alpha}{sub v} = 0. 1 8 {plus_minus} 0.03 and {alpha}{sub {beta}} = 0.3 {plus_minus} 0. 1. Application of this formula to NBI and ECH data significantly improves the fit, implying that improved confinement will result from increasing the heating power. Little change in confinement occurred for wide variations in the confined trapped particle fraction at constant magnetic well radius and shear. This may be explained by reduction of the helically trapped panicle loss region due to the radial electric field. Configuration modulation experiments showed that the energy confinement time improves as the magnetic well radius expands, the result expected from some stabilization of resistive interchange instabilities. This is consistent with fluctuation measurements and may explain the positive dependence of confinement on {beta}.

Electron cyclotron heating (ECH) experiments in the Advanced Toroidal Facility (ATF) torsatron exploit unique capabilities for external control of the magnetic configuration and long‐pulse operation. The ECH power deposition profile is determined from the change of the electron temperature profiles during ECH power turn‐off and from power modulation. The measured dependence of absorbed power on the magnetic field agrees well with that obtained from ray tracing calculations for first‐pass and multiple‐bounce absorption. In ATF, parameters of basic physics interest (magnetic shear, magnetic well/hill, and contained trapped particle fraction) have been varied dynamically during a single long‐pulse (up to 20‐s) discharge (dynamic configuration control). Varying a single physics parameter while keeping others fixed elucidates the parameter’s influence on plasma behavior. Modulation of the magnetic well significantly affects the plasma stored energy. This may be related to the observed electron density fluctuations, which are consistent with theoretical predictions of pressure‐gradient‐driven resistive interchange turbulence. Measurements of the electric potential with a heavy ion beam probe show positive potential in the core of typical ECH discharges and the presence of a velocity shear layer near the edge.

Fast wave heating experiments in the ion cyclotron range of frequencies (ICRF) were performed on target plasmas produced by 350 kW of electron cyclotron heating at 53 GHz and also by neutral beam injection in the Advanced Toroidal Facility (ATF). Various heating regimes were investigated in the frequency range between 9.2 MHz and 28.8 MHz with magnetic fields of 0.95 T and 1.9 T on axis. The nominal pulse lengths of up to 200 kW RF power were in the range between 100 and 400 ms. Data from spectroscopy, loading measurements, and edge RF and Langmuir probes were used to characterize the RF induced effects on the ATF plasma. In the hydrogen minority regime at low plasma density, large suprathermal ion tails were observed with a neutral particle analyser. At high density (ne ⩾ 5.0 × 1013 cm-3) substantial increases in antenna loading were observed, but ICRF power was insufficient to produce definitive heating results. A two-dimensional RF heating code, ORION, and a Fokker-Planck code, RFTRANS, were used to simulate these experiments. A simulation of future high power, higher density experiments in ATF indicates improved bulk heating results due to the improved loading and more efficient thermalization of the minority tail

Recent experiments in the Advanced Toroidal Facility (ATF) [Fusion Technol. 10, 179 (1986)] have been directed toward investigations of the basic physics mechanisms that control confinement in this device. Measurements of the density fluctuations throughout the plasma volume have provided indications for the existence of theoretically predicted dissipative trapped electron and resistive interchange instabilities. These identifications are supported by results of dynamic configuration scans of the magnetic fields during which the magnetic well volume, shear, and fraction of confined trapped particles are changed continuously. The influence of magnetic islands on the global confinement has been studied by deliberately applying error fields which strongly perturb the nested flux-surface geometry, and the effects of electric fields have been investigated by means of biased limiter experiments.

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 greater-than-or-equal-to 1 MW, global energy confinement times of 30 msec have been obtained with line-average densities up to 1.3 x 10(20) 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 in discharges heated by ECH alone and by NBI change plasma transport and plasma profiles. Magnetic fluctuations respond to the concomitant pressure profile variations. Comparative studies of edge fluctuations in the Texas Experimental Tokamak (TEXT) [Plasma Physics and Controlled Nuclear Fusion Research 1990 (IAEA, Vienna, in press)] and the ATF stellarator showed remarkable similarity in the levels of fluctuations and the existence of a velocity shear layer.

Recent biasing experiments on tokamaks have been very successful in improving the global particle (H-mode-like) confinement resulting from the setup of an outward-pointing radial electric field at the edge. These experiments have been extended to the current-free Advanced Toroidal Facility (ATF), and initial biasing experiments have been carried out in electron-cyclotron-heated (ECH) plasmas. ATF has a torsatron configuration with {ell} = 2, 12 field periods (M = 12), a major radius R{sub 0} = 2.1 m, and an average plasma radius a = 0.27 m. The current-free magnetic configuration of ATF that is produced by external means has moderate shear; the rotational transform ({chi} = 1/q, where q is the safety factor) at the last closed flux surface (LCFS) is {chi} {approx} 1, which is about a factor of 3 higher than the central value. ECH plasmas are created at a magnetic field B = 0.95 T using a 53-GHz gyrotron source with heating power up to P {approximately} 400 kW. In these ECH plasmas, a representative line-averaged plasma density is {bar n}{sub e} {approximately} 5 {times} 10{sup 12} cm{sup {minus}3}, and the plasma stored energy W{sub p} {approx} 2 kJ. A pair of rail limiters, which are normally floating, more » one at the top and one at the bottom of the device, are biased at positive and negative potentials with respect to the vessel. 7 refs., 5 figs. « less

Measurements of poloidal magnetic field fluctuations outside currentless, finite-beta ( less-than-or-equal-to 0.5%, beta-0 less-than-or-equal-to 3%) plasmas with peaked pressure profiles in the Advanced Toroidal Facility (ATF) torsatron reveal bands of small (B approximately-theta/B-phi approximately 10(-5), coherent fluctuations in the frequency range 5-60 kHz. The geometrical structure of these fluctuations shows n = 1 toroidal mode symmetry, with inferred poloidal mode numbers of m = 2 and 3. The coherent B approximately-theta amplitudes increase with for less-than-or-similar-to 0.3%, and then saturate and begin to decrease with for greater-than-or-similar-to 0.3%.

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.

The toroidal current observed during electron cyclotron heating in the Advanced Toroidal Facility torsatron is identified as bootstrap current. The observed currents, ranging between +4 and -2 kA, agree well with predictions of neoclassical theory in magnitude and parametric dependence, as determined by systematic scans of quadrupole (shaping) and dipole (magnetic axis shift) moments of the poloidal magnetic field. It has been shown that the bootstrap current in a stellarator can be externally controlled, and zero-current operation can be achieved.

Power and particle balance studies on the Advanced Toroidal Facility (ATF) torsatron are carried out using a rail limiter system. Both top and bottom limiters are made of graphite tile arrays, and these tiles are instrumented with thermocouples and Langmuir probes for calorimetric and particle flux measurements. Initial experimental results indicate that the limiter power loss accounts for about 12% of the total and the radiation loss for about 30% of the total; the rest of the plasma heating power appears to be going to the vessel wall. The particle flux to the limiters is also about 18%. The fractions of power and particle flux to the limiters are relatively lower than in Tokamaks because of the low edge safety factor, q approximately 1 rather than q approximately 3 as in a typical Tokamak, at the natural boundary of the ATF plasma (which results from the magnetic stellarator configuration of this currentless device). Therefore, for limiters of the same size, these fractions are about a factor of q lower in ATF than in a comparable Tokamak device.

The diamagnetic diagnostic on ATF consists of two systems. The first uses a single-turn diamagnetic loop in a stainless-steel reentrant tube inside the vacuum vessel. Compensation signals are derived from Rogowski coils mounted on the main helical and vertical coil buses. This arrangement provides maximum sensitivity and the fastest time response, but results in signals which are dominated by noise created by the large ATF SCR power supplies. The nonlinear nature of these supplies, and their coupling, requires the use of hybrid noise reduction processing. The analog compensation loops remove the low-frequency components and digital post-processing removes the high-frequency ones. The second diamagnetic signal is derived from a set of saddle loops which respond directly to the Pfirsch-Schlüter current in the plasma. Typical results are presented. This research was sponsored by the Office of Fusion Energy, U.S. Department of Energy, under contract DE-AC05-84OR21400 with Martin Marietta Energy Systems, Inc.

Experiments involving plasma improvement, confinement scaling, bootstrap currents, and edge fluctuations have been carried out in the Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)]. Average densities ne≤9×1019 m−3 have been obtained, with global energy confinement times τ*E≤20 msec. Confinement times generally follow the stellarator/torsatron empirical scaling law, τSL =0.17×P−0.58n0.69eB0.84a2R0.75 (with τSL in seconds, power P in megawatts, density ne in 1020 m−3, and plasma radius a and major radius R in meters). Gas injection during neutral beam injection (NBI) causes increases in ne, so that τ*E does not decrease during NBI. Edge plasma fluctuations are found to exhibit a mode change near the peak of the energy confinement time. Plasma currents observed during electron cyclotron heating have been identified as bootstrap currents.

An overview of recent AFT (Advanced Toroidal Facility) experimental results and program plans is presented, with emphasis on the role of magnetic configuration controls in transport studies. The ATF operating space is bounded by a density limit that effectively sets a limit on the energy confinement time τE. Although this limit is not solely due to impurities, it has recently been raised by improved cleanliness following titanium gettering. This has led to collapse-free neutral beam injection discharges with global τE ≈16 ms. Preliminary experiments show that stored energy and bootstrap current are sensitive to details of the magnetic configuration