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  • 1
    Electronic Resource
    Electronic Resource
    Heavy-ion beam probe for the Advanced Toroidal Facility (1988)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 59 (1988), S. 1670-1672 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Neoclassical theory predicts that radial electric fields develop in plasmas confined in nonaxisymmetric tori and that they strongly influence the confinement in such devices. A heavy-ion beam probe has been planned for 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 present a diagnostic environment more challenging than that found on previous beam probe systems which have operated on tokamaks, mirrors, and bumpy tori. Particular attention has been given to system alignment and control capabilities. Since the probing ion trajectories remain essentially the same with or without the plasma, an in situ calibration scheme using suitable ionization targets is possible. Installation of the complete diagnostic system is planned for 1988.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1140128
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  • 2
    Electronic Resource
    Electronic Resource
    Overview of results from the ATF torsatron (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 1347-1352 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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 n¯e≤9×1019 m−3 have been obtained, with global energy confinement times τ@B|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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859554
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  • 3
    Electronic Resource
    Electronic Resource
    Effects of magnetic geometry, fluctuations, and electric fields on confinement in the Advanced Toroidal Facility (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 2104-2110 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860018
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  • 4
    Electronic Resource
    Electronic Resource
    Second stability in the ATF torsatron—Experiment and theory (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 1353-1358 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859555
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  • 5
    Electronic Resource
    Electronic Resource
    Fluctuation and modulation transport studies in the Advanced Toroidal Facility (ATF) torsatronf| (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 2513-2518 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860737
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  • 6
    Electronic Resource
    Electronic Resource
    Recent results from the ATF torsatron (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 2261-2269 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859645
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  • 7
    Electronic Resource
    Electronic Resource
    Runaway electron studies in the ATF torsatron (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 1671-1686 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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 produced primarily during the field ramping but that there is usually also a small steady-state runaway electron component during the"flat-top'' portion of the fields. The paddle is the main source of the hard x radiation (thick-target bremsstrahlung). There is evidence that some of the runaway electrons may be confined to islands. The maximum x-ray energy found by pulse height analysis is ∼12–15 MeV. The mean energy appears to be a few million electron volts. There is a noticeable forward peaking of the bremsstrahlung from the paddle. The limiters do not appear to be major sources of x rays.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859687
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  • 8
    Electronic Resource
    Electronic Resource
    Experimental investigation of a coherent flute instability using a heavy ion beam probe (1988)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 31 (1988), S. 2704-2709 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A coherent, low-frequency instability found in a cylindrical, hollow cathode arc plasma has been investigated by using a heavy ion beam probe (HIBP). The energy density of the plasma was high enough to render it inaccessible to Langmuir probes, but the HIBP was able to provide measurements throughout the plasma cross section. The data clearly show that azimuthal symmetry does not exist. Radial profiles of steady-state density and space potential and of simultaneous ñ, φ˜ amplitude and phase were obtained to allow detailed comparison between theory and experiment. Predictions from a cylindrically symmetric, small-perturbation theoretical model provide reasonably conclusive identification of the instability as a Kelvin–Helmholtz flute driven by and localized in a region of fluid shear. The most serious discrepancy was with regard to the oscillation frequency, which was consistently predicted to be three to four times lower than that observed experimentally. The reason for the discrepancy is not understood, but it is probably related to inadequacies in the theory caused by assumptions of azimuthal symmetry and of small linear perturbations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.866549
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  • 9
    Electronic Resource
    Electronic Resource
    Effect of electric fields and fluctuations on confinement in a bumpy torus (1987)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 30 (1987), S. 847-856 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In order to understand the relationships between confinement and space potential (electric field) and between confinement and density fluctuations, plasma parameters in the ELMO Bumpy Torus Scale (EBT-S)[in Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Tokyo, 1974), Vol. 2, p. 141; Plasma Phys. 25, 597 (1983)] have been measured systematically for a wide range of operating conditions. Present EBT plasma parameters do not show a strong dependence on the potential profile, but rather exhibit a correlation with the fluctuations. The plasma pressure profile is found to be consistent with the profile anticipated on the basis of the flute stability criterion for a marginally stable plasma. For a heating power of 100 kW, the stored energy density is found to be restricted to the range between 4.5×1013 eV-cm−3 and 7×1013 eV-cm−3. The lower limit remains constant regardless of heating power and pertains to plasmas lacking an equilibrium and/or stability. The upper limit increases with heating power and is found to result from the onset of instabilities. In between the two limits is a plasma that is in an equilibrium state and is marginally stable. Operational trajectories exist that take the EBT plasma from one limit to the other.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.866337
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  • 10
    Electronic Resource
    Electronic Resource
    ATF heavy ion beam probe: Installation and initial operation : Eighth topical conference on high−temperature plasma diagnostics (1990)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 61 (1990), S. 2961-2963 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1141739
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