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  • 11
    Electronic Resource
    Electronic Resource
    Stabilization of the external kink and control of the resistive wall mode in tokamaks : The 40th annual meeting of the division of plasma physics of the american physical society (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 1893-1898 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: One promising approach to maintaining stability of high beta tokamak plasmas is the use of a conducting wall near the plasma to stabilize low-n ideal magnetohydrodynamic instabilities. However, with a resistive wall, either plasma rotation or active feedback control is required to stabilize the more slowly growing resistive wall modes (RWMs). Previous experiments have demonstrated that plasmas with a nearby conducting wall can remain stable to the n=1 ideal external kink above the beta limit predicted with the wall at infinity. Recently, extension of the wall stabilized lifetime τL to more than 30 times the resistive wall time constant τw and detailed, reproducible observation of the n=1 RWM have been possible in DIII-D [Plasma Physics and Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1986), p. 159] plasmas above the no-wall beta limit. The DIII-D measurements confirm characteristics common to several RWM theories. The mode is destabilized as the plasma rotation at the q=3 surface decreases below a critical frequency of 1–7 kHz (∼1% of the toroidal Alfvén frequency). The measured mode growth times of 2–8 ms agree with measurements and numerical calculations of the dominant DIII-D vessel eigenmode time constant τw. From its onset, the RWM has little or no toroidal rotation (ωmode≤τw−1(very-much-less-than)ωplasma), and rapidly reduces the plasma rotation to zero. These slowly growing RWMs can in principle be destabilized using external coils controlled by a feedback loop. In this paper, the encouraging results from the first open loop experimental tests of active control of the RWM, conducted in DIII-D, are reported. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873495
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  • 12
    Electronic Resource
    Electronic Resource
    Transport and performance in DIII-D discharges with weak or negative central magnetic shear : The 38th annual meeting of the Division of Plasma Physics (DPP) of the American Physical Society (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 1596-1604 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Discharges exhibiting the highest plasma energy and fusion reactivity yet realized in the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] have been produced by combining the benefits of a hollow or weakly sheared central current profile [Phys. Plasmas 3, 1983 (1996)] with a high confinement (H mode) edge. In these discharges, low-power neutral beam injection heats the electrons during the initial current ramp, and "freezes in" a hollow or flat central current profile. When the neutral beam power is increased, formation of a region of reduced transport and highly peaked profiles in the core often results. Shortly before these plasmas would otherwise disrupt, a transition is triggered from the low (L mode) to high (H mode) confinement regimes, thereby broadening the pressure profile and avoiding the disruption. These plasmas continue to evolve until the high-performance phase is terminated nondisruptively at much higher βT (ratio of plasma pressure to toroidal magnetic field pressure) than would be attainable with peaked profiles and an L-mode edge. Transport analysis indicates that in this phase, the ion diffusivity is equivalent to that predicted by Chang–Hinton neoclassical theory over the entire plasma volume. This result is consistent with suppression of turbulence by locally enhanced E×B flow shear, and is supported by observations of reduced fluctuations in the plasma. Calculations of performance in these discharges extrapolated to a deuterium–tritium (DT) fuel mixture indicates that such plasmas could produce a DT fusion gain QDT=0.32. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872360
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  • 13
    Electronic Resource
    Electronic Resource
    Stability of negative central magnetic shear discharges in the DIII-D tokamak : The 38th annual meeting of the Division of Plasma Physics (DPP) of the American Physical Society (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 1783-1791 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Discharges with negative central magnetic shear (NCS) hold the promise of enhanced fusion performance in advanced tokamaks. However, stability to long wavelength magnetohydrodynamic modes is needed to take advantage of the improved confinement found in NCS discharges. The stability limits seen in DIII-D [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] experiments depend on the pressure and current density profiles and are in good agreement with stability calculations. Discharges with a strongly peaked pressure profile reach a disruptive limit at low beta, βN=β(I/aB)−1≤2.5 (% m T/MA), caused by an n=1 ideal internal kink mode or a global resistive instability close to the ideal stability limit. Discharges with a broad pressure profile reach a soft beta limit at significantly higher beta, βN=4 to 5, usually caused by instabilities with n〉1 and usually driven near the edge of the plasma. With broad pressure profiles, the experimental stability limit is independent of the magnitude of negative shear but improves with the internal inductance, corresponding to lower current density near the edge of the plasma. Understanding of the stability limits in NCS discharges has led to record DIII-D fusion performance in discharges with a broad pressure profile and low edge current density. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872366
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  • 14
    Electronic Resource
    Electronic Resource
    Wall stabilization of high beta plasmas in DIII-D (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 2390-2396 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Detailed analysis of recent high beta discharges in the DIII-D [Plasma Physics Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] tokamak demonstrates that the resistive vacuum vessel can provide stabilization of low n magnetohydrodynamic (MHD) modes. The experimental beta values reaching up to βT=12.6% are more than 30% larger than the maximum stable beta calculated with no wall stabilization. Plasma rotation is essential for stabilization. When the plasma rotation slows sufficiently, unstable modes with the characteristics of the predicted "resistive wall'' mode are observed. Through slowing of the plasma rotation between the q=2 and q=3 surfaces with the application of a nonaxisymmetric field, it has been determined that the rotation at the outer rational surfaces is most important, and that the critical rotation frequency is of the order of Ω/2π=1 kHz. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871262
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  • 15
    Electronic Resource
    Electronic Resource
    Experimentally determined profiles of fast wave current drive in a tokamak (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2846-2848 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Profiles of the noninductive current, driven by direct electron absorption of fast waves in the ion cyclotron range of frequencies, have been determined for DIII-D tokamak discharges [Luxon et al., Plasma Physics and Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159]. The results clearly indicate the presence of centrally peaked driven current and validate theoretical models of fast wave current drive. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871643
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  • 16
    Electronic Resource
    Electronic Resource
    Nonlinear three-dimensional self-consistent simulations of negative central shear discharges in the DIII-D tokamak (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 3605-3619 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Nonlinear simulations of experimentally observed magnetohydrodynamic (MHD) bursts in DIII-D [J. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] L-mode negative central magnetic shear (NCS) discharges were performed with a full three-dimensional nonlinear MHD code. The effects of plasma rotation in the presence of resistivity and viscosity are included and an effectively implicit numerical scheme allows the transport profile to evolve self-consistently with the nonlinear MHD instabilities and externally applied sources and sinks. The simulations follow the MHD bursts and disruptions through the linear and nonlinear phases and identify the connections between the early MHD bursts and the ultimate disruption phase. Specific predictions of the growth and saturation of the modes are directly compared with experimental diagnostic measurements in DIII-D. The simulations show that the bursts observed in experiments are triggered by MHD instability of a resistive interchange mode and a resistive kink mode that are excited for critical plasma profiles. The critical profiles are determined by the balance between inductive and noninductive sources of current density. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1380235
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  • 17
    Electronic Resource
    Electronic Resource
    Motional Stark effect upgrades on DIII-D : Proceedings of the tenth topical conference on high temperature plasma diagnostics (1995)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 66 (1995), S. 373-375 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The measurement and control of the plasma current density profile (or q profile) is critical to the advanced tokamak program on DIII-D. A complete understanding of the stability and transport properties of advanced operating regimes requires detail poloidal field measurements over the entire plasma radius from the core to the edge. In support of this effort, we have recently completed an upgrade of the existing motional Stark effect (MSE) diagnostic, increasing the number of channels from 8 to 16. A new viewing geometry has been added to the outer edge of the plasma which improves the radial resolution in this region from 13 to ∼2 cm. This view requires the use of a reflector that has been designed to minimize polarization effects. Vacuum-compatible polarizers have also been added to the instrument for in situ calibration. Future use of the MSE diagnostic for feedback control of the q profile will also be discussed. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1146416
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  • 18
    Electronic Resource
    Electronic Resource
    X-ray diagnostic for measurement of hot-electron power loss : Eighth topical conference on high−temperature plasma diagnostics (1990)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 61 (1990), S. 2777-2779 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A significant fraction of the power lost from a confined plasma can be carried by energetic electrons. When such electrons are incident on a probe or limiting surface, they produce thick-target x rays. It is shown that a properly filtered measurement of this x-ray flux is proportional to the electron-power loss, with little dependence on the electron loss mechanism and velocity-space distribution. The diagnostic was successfully implemented to study the radial loss of hot electrons in the Tandem Mirror Experiment-Upgrade. The method provides a very fast measurement of electron loss processes, and could therefore be useful in other areas of plasma research such as magnetic confinement in tokamaks, and inertial-confinement fusion.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1141828
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  • 19
    Electronic Resource
    Electronic Resource
    Simultaneous measurement of q and Er profiles using the motional Stark effect in high-performance DIII-D plasmas (invited) : Proceedings of the 12th topical conference on high temperature plasma diagnostics (1999)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 70 (1999), S. 815-820 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The motional Stark effect (MSE) diagnostic was developed to provide a measurement of the magnetic pitch angle or q profile in tokamaks. The technique relies upon polarization measurements of Stark broadened Dα emission to determine the pitch angle of the Lorentz vb×B electric field, where vb is the injected neutral beam particle velocity and B is the total magnetic field. However, in many advanced confinement regimes, large values of the plasma radial electric field, Er, are observed and can affect the interpretation of MSE measurements. Viewing fixed locations in the plasma from two different viewing angles allows one to separate the Er field from the vb×B field, thus providing simultaneous measurement of the Er and q profiles. To achieve this measurement, the DIII–D MSE diagnostic was recently upgraded from 16 to 35 channels with three independent viewing angles. The new instrument provides an Er resolution of 5–10 kV/m with a time response of 1 ms. Measurement results from the VH mode, reverse shear, and H mode plasmas are presented. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1149317
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  • 20
    Electronic Resource
    Electronic Resource
    Design of the divertor Thomson scattering system on DIII-D : Proceedings of the tenth topical conference on high temperature plasma diagnostics (1995)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 66 (1995), S. 493-495 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Local measurements of ne and Te in the divertor region are necessary for a more complete understanding of divertor physics. We have designed an extension to the existing multipulse Thomson scattering system [Carlstrom et al., Rev. Sci. Instrum. 63, 4901 (1992)] to measure ne in the range 5×1018–5×1020 m−3 and Te in the range 5–500 eV with 1 cm resolution from 1 to 21 cm above the floor of the DIII-D vessel (eight spatial channels) in the region of the X point for lower single-null diverted plasmas. One of the existing, 20 Hz, Nd:YAG lasers will be redirected to a separate vertical port and viewed radially with a specially designed f/6.8 lens. Fiber optics carry the light to polychromators whose interference filters have been optimized for low Te measurements. Other aspects of the system, including the beam path to the vessel, polychromator design, real-time data acquisition, laser control, calibration facility, and DIII-D timing and data acquisition interface, will be shared with the existing multipulse Thomson system. An in situ laser alignment monitor will provide alignment information for each laser pulse. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1146534
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