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  • 1
    Electronic Resource
    Electronic Resource
    Modeling of active control of external magnetohydrodynamic instabilities : The 42nd annual meeting of the division of plasma physics of the American Physical Society and the 10th international congress on plasma physics (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 2170-2180 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A general circuit formulation of resistive wall mode (RWM) feedback stabilization developed by Boozer [Phys. Plasmas 5, 3350 (1998)] has been used as the basis for the VALEN computer code that calculates the performance of an active control system in arbitrary geometry. The code uses a finite element representation of a thin shell structure in an integral formulation to model arbitrary conducting walls. This is combined with a circuit representation of stable and unstable plasma modes. Benchmark comparisons of VALEN results with large aspect ratio analytic model of the current driven kink mode are in very good agreement. VALEN also models arbitrary sensors, control coils, and the feedback logic connecting these sensors and control coils to provide a complete simulation capability for feedback control of plasma instabilities. VALEN modeling is in good agreement with experimental results on DIII-D [Garofalo et al., Nucl. Fusion 40, 1491 (2000)] and HBT-EP [Cates et al., Phys. Plasmas 7, 3133 (2000)]. VALEN feedback simulations have also been used to evaluate and optimize the sensor/coil configurations for present and planned RWM experiments on DIII-D. These studies have shown a clear advantage for the use of local poloidal field sensors driving a "mode control" feedback logic control loop and configurations which minimize the control coil coupling to the stabilizing resistive wall. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1362532
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  • 2
    Electronic Resource
    Electronic Resource
    Measurement of large displacements of the toroidal current centroid using an external coil diagnostic (1988)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 59 (1988), S. 1057-1062 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Several sinusoidal and saddle coils have been used to measure the toroidal current centroid of discharges produced in a pulsed tokamak experiment. Careful analysis of the external coil measurements and elaborate calibrations made the measurements possible even for large displacements of the plasma current and the noncircular geometry of the diagnostic coils. The calibration techniques and the data analysis formula are described, and examples of the use of the external coils to measure m=1 kinks of unstable toroidal Z-pinch discharges are presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1139725
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  • 3
    Electronic Resource
    Electronic Resource
    Nonstationary signal analysis of magnetic islands in plasmas (1999)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 70 (1999), S. 4545-4551 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Rotating magnetic islands produce fluctuations on a variety of diagnostics in magnetic fusion energy plasmas. The analysis of these fluctuations requires the calculation of the amplitude, phase, and frequency of the oscillations. These three spectral quantities generally evolve in time, necessitating nonstationary signal analysis techniques. The Hilbert transform offers an efficient and accurate method of calculating these three quantities from one diagnostic signal. This feature allows the Hilbert transform to determine the success of the active rotation control of magnetic islands, and to calculate the profile of the diagnostic measurements in a frame of reference co-rotating with the magnetic island. Comparisons to quadrature and spectrogram techniques demonstrate the accuracy of the Hilbert transform method. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1150110
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  • 4
    Electronic Resource
    Electronic Resource
    Suppression of resistive wall instabilities with distributed, independently controlled, active feedback coils (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 3133-3136 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: External kink instabilities are suppressed in a tokamak experiment by either (1) energizing a distributed array of independently controlled active feedback coils mounted outside a segmented resistive wall or (2) inserting a second segmented wall having much higher electrical conductivity. When the active feedback coils are off and the highly conducting wall is withdrawn, kink instabilities excited by plasma current gradients grow at a rate comparable to the magnetic diffusion rate of the resistive wall. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.874223
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  • 5
    Electronic Resource
    Electronic Resource
    Active control of 2/1 magnetic islands in a tokamak : The 39th annual meeting of division of plasma physics of APS (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 1855-1863 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Closed and open loop control techniques were applied to growing m/n=2/1 rotating islands in wall-stabilized plasmas in the High Beta Tokamak-Extended Pulse (HBT-EP) [J. Fusion Energy 12, 303 (1993)]. HBT-EP combines an adjustable, segmented conducting wall (which slows the growth or stabilizes ideal external kinks) with a number of small (6° wide toroidally) driven saddle coils located between the gaps of the conducting wall. Two-phase driven magnetic island rotation control from 5 to 15 kHz has been demonstrated powered by two 10 MW linear amplifiers. The phase instability has been observed and is well modeled by the single-helicity predictions of nonlinear Rutherford island dynamics for 2/1 tearing modes including important effects of ion inertia and finite Larmor radius, which appear as a damping term in the model equations. The closed loop response of active feedback control of the 2/1 mode at moderate gain was observed to be in good agreement with the theory. Suppression of the 2/1 island growth has been demonstrated using an asynchronous frequency modulation drive which maintains the inertial flow damping of the island by application of rotating control fields with frequencies alternating above and below the natural mode frequency. This frequency modulation control technique was also able to prevent disruptions normally observed to follow giant sawtooth crashes in the plasma core. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872856
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  • 6
    Electronic Resource
    Electronic Resource
    Observation of wall stabilization and active control of low-n magnetohydrodynamic instabilities in a tokamak (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1926-1934 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The High Beta Tokamak-Extended Pulse (HBT-EP) experiment [J. Fusion Energy 12, 303 (1993)] combines an internal, movable conducting wall with a high-power, modular saddle coil system to provide passive and active control of long wavelength magnetohydrodynamic (MHD) instabilities. Systematic adjustment of the radial position, b, of the conducting wall elements in relation to the surface of the plasma (minor radius a) resulted in the suppression of β-limiting disruptions for discharges in which b/a〈1.2 and a positive plasma current ramp was maintained. Conducting wall stabilization of kink instabilities was observed in discharges with strong current ramps and in plasmas with β values near the Troyon stability boundary. The frequency of slowly growing modes that persisted in wall-stabilized discharges was controlled by applying oscillating m=2, n=1 resonant magnetic perturbations. A compact, single-phase saddle coil system permitted modulation of the rotation velocity of internal m/n=2/1 instabilities by a factor of 2. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871988
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  • 7
    Electronic Resource
    Electronic Resource
    Rotational and magnetic shear stabilization of magnetohydrodynamic modes and turbulence in DIII-D high performance discharges (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1951-1958 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The confinement and the stability properties of the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159] high-performance discharges are evaluated in terms of rotational and magnetic shear, with an emphasis on the recent experimental results obtained from the negative central magnetic shear (NCS) experiments. In NCS discharges, a core transport barrier is often observed to form inside the NCS region accompanied by a reduction in core fluctuation amplitudes. Increasing negative magnetic shear contributes to the formation of this core transport barrier, but by itself is not sufficient to fully stabilize the toroidal drift mode (trapped-electron-ηi mode) to explain this formation. Comparison of the Doppler shift shear rate to the growth rate of the ηi mode suggests that the large core E×B flow shear can stabilize this mode and broaden the region of reduced core transport. Ideal and resistive stability analysis indicates the performance of NCS discharges with strongly peaked pressure profiles is limited by the resistive interchange mode to low βN≤2.3. This mode is insensitive to the details of the rotational and the magnetic shear profiles. A new class of discharges, which has a broad region of weak or slightly negative magnetic shear (WNS), is described. The WNS discharges have broader pressure profiles and higher β values than the NCS discharges, together with high confinement and high fusion reactivity. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871991
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  • 8
    Electronic Resource
    Electronic Resource
    Demonstration of high-performance negative central magnetic shear discharges in the DIII-D tokamak (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1983-1991 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Reliable operation of discharges with negative central magnetic shear has led to significant increases in plasma performance and reactivity in both low confinement, L-mode, and high confinement, H-mode, regimes in the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159]. Using neutral beam injection early in the initial current ramp, a large range of negative shear discharges have been produced with durations lasting up to 3.2 s. The total noninductive current (beam plus bootstrap) ranges from 50% to 80% in these discharges. In the region of shear reversal, significant peaking of the toroidal rotation [fφ(0)∼30–60 kHz] and ion temperature [Ti(0)∼15–22 keV] profiles are observed. In high-power discharges with an L-mode edge, peaked density profiles are also observed. Confinement enhancement factors up to H≡τE/τITER-89P∼2.5 with an L-mode edge, and H∼3.3 in an edge localized mode (ELM)-free H mode, are obtained. Transport analysis shows both ion thermal diffusivity and particle diffusivity to be near or below standard neoclassical values in the core. Large pressure peaking in the L mode leads to high disruptivity with βN≡βT/(I/aB)≤2.3, while broader pressure profiles in the H mode gives low disruptivity with βN≤4.2. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871994
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  • 9
    Electronic Resource
    Electronic Resource
    Deuterium–tritium high confinement (H-mode) studies in the Tokamak Fusion Test Reactor (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 2366-2374 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: High or enhanced confinement (H-mode) plasmas have been obtained for the first time with nearly equal concentrations of deuterium and tritium in high-temperature, high poloidal beta plasmas in the Tokamak Fusion Test Reactor (TFTR) [McGuire, Phys. Plasmas 2, 2176 (1995)]. Tritium fueling was provided mainly through high-power neutral beam injection (NBI) with powers up to 31 MW and beam energies of 90–110 keV. A transition to a circular limiter H-mode configuration has been obtained, following a programmed rapid decrease of the plasma current. Isotope effects, due to the presence of tritium, led to different behavior for deuterium–deuterium (DD) and deuterium–tritium (DT) H-modes relative to confinement, edge localized magnetohydrodynamic modes (ELMs), and ELM effects on fusion products. However, the threshold power for the H-mode transition was the same in DD and DT. Some of the highest values of the global energy confinement time, τE, have been achieved on TFTR during the ELM-free phase of DT H-mode plasmas. Enhancements of τE greater than four times the L-mode have been attained. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871490
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  • 10
    Electronic Resource
    Electronic Resource
    Magnetohydrodynamic stability in a levitated dipole (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 3431-3434 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Plasma confined by a magnetic dipole is stabilized, at low beta, by magnetic compressibility. The ideal magnetohydrodynamic (MHD) requirements for stability against interchange and high-n ballooning modes are derived at arbitrary beta for a fusion grade laboratory plasma confined by a levitated dipole. A high beta MHD equilibrium is found numerically with a pressure profile near marginal stability for interchange modes, a peak local beta of β∼10, and volume averaged beta of β¯∼0.5. This equilibrium is demonstrated to be ballooning stable on all field lines. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873601
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