Confinement studies of auxiliary heated NSTX plasmas

LeBlanc, B.P.; Bell, R.E.; Kaye, S. M.; Stutman, D.; Bell, M.G.; Bitter, M.; Bourdelle, C.; Gates, D.; Maingi, R.; Medley, S. S.; Ménard, J.; Mueller, D.; Paul, S. F.; Roquemore, A. L.; Rosenberg, A.; Sabbagh, S. A.; Soukhanovskii, V.; Synakowski, E. J.; Wilson, J. R.

doi:10.1088/0029-5515/44/4/005

Cited by 48 publications

(44 citation statements)

References 31 publications

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“…Thus, as for high β T discharges in NSTX, low-n mode induced confinement degradation and rotation decay appear to be important factors in the disruption dynamics of high β P discharges. Prior to the onset of the abrupt drops in β P in Figure 13, the global energy confinement time for this 6MW discharge is 40-50ms with H-factor of 2-2.5 relative to ITER-89P scaling [31]. This confinement estimate ignores the 1 to 2MW loss of NBI heating power to prompt loss and charge exchange due to operating at reduced plasma current and low internal inductance.…”

Section: High β P Dischargesmentioning

confidence: 91%

Beta-limiting MHD Instabilities in Improved-performance NSTX Spherical Torus Plasmas

Menard¹,

Bell²,

Bell³

et al. 2003

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Abstract. Global magnetohydrodynamic stability limits in the National Spherical Torus Experiment (NSTX) have increased significantly recently due to a combination of device and operational improvements. First, more routine H-mode operation with broadened pressure profiles allows access to higher normalized beta and lower internal inductance. Second, the correction of a poloidal field coil induced error-field has largely eliminated locked tearing modes during normal operation and increased the maximum achievable beta. As a result of these improvements, peak beta values have reached (not simultaneously) β t = 35%, β N = 6.4, β N = 4.5, β N /l i = 10, and β P = 1.4. High β P operation with reduced tearing activity has allowed a doubling of discharge pulse-length to just over 1 second with sustained periods of β N ≈ 6 above the ideal no-wall limit and near the with-wall limit. Details of the β limit scalings and β-limiting instabilities in various operating regimes are described.

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Section: High β P Dischargesmentioning

confidence: 91%

Beta-limiting MHD Instabilities in Improved-performance NSTX Spherical Torus Plasmas

Menard¹,

Bell²,

Bell³

et al. 2003

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“…As seen in Figure 6, the improvement in global electron confinement with increasing lithium deposition is reflected in a reduction in the electron thermal diffusivity calculated by TRANSP [18] using the measured profiles of the temperatures and density and assuming classical thermalization of the neutral beam heating.…”

Section: Lithium Coating Techniques and Their Effects In Nstxmentioning

confidence: 97%

Plasma response to lithium-coated plasma-facing components in the National Spherical Torus Experiment

Bell

Kugel

Kaita

et al. 2009

Plasma Phys. Control. Fusion

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Experiments in the National Spherical Torus Experiment (NSTX) have shown beneficial effects on the performance of divertor plasmas as a result of applying lithium coatings on the graphite and carbonfiber-composite plasma-facing components. These coatings have mostly been applied by a pair of lithium evaporators mounted at the top of the vacuum vessel which inject collimated streams of lithium vapor towards the lower divertor. In NBI-heated, deuterium H-mode plasmas run immediately after the application of lithium, performance modifications included decreases in the plasma density, particularly in the edge, and inductive flux consumption, and increases in the electron and ion temperatures and the energy confinement time. Reductions in the number and amplitude of ELMs were observed, including complete ELM suppression for periods up to 1.2 s, apparently as a result of altering the stability of the edge. However, in the plasmas where ELMs were suppressed, there was a significant secular increase in the effective ion charge Z eff and the radiated power as a result of increases in the carbon and medium-Z metallic impurities, although not of lithium itself which remained at a very low level in the plasma core, <0.1%. The impurity buildup could be inhibited by repetitively triggering ELMs with the application of brief pulses of an n = 3 radial field perturbation. The reduction in the edge density by lithium also inhibited parasitic losses through the scrape-off layer of ICRF power coupled to the plasma, enabling the waves to heat electrons in the core of H-mode plasmas produced by NBI. Lithium has also been introduced by injecting a stream of chemically stabilized, fine lithium powder directly into the scrape-off layer of NBI-heated plasmas. The lithium was ionized in the SOL and appeared to flow along the magnetic field to the divertor plates. This method of coating produced similar effects to the evaporated lithium but at lower amounts.

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“…6 Local transport analyses of the MAST discharges indicated that the ion thermal transport was close to the neoclassical level, 7 consistent with early results on NSTX as well. 8 Microinstability analyses with linear gyrokinetic codes indicated that the ExB shearing rates in MAST were sufficiently high as to suppress long-wavelength Ion Temperature Gradient (ITG) turbulence. The source of the anomalous electron transport was believed to be either microtearing modes in the core, or radial streamers associated with saturated Electron Temperature Gradient (ETG) turbulence 7,9 .…”

Section: Introductionmentioning

confidence: 99%

Confinement and local transport in the National Spherical Torus Experiment (NSTX)

et al. 2007

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NSTX operates at low aspect ratio (R/a∼1.3) and high beta (up to 40%), allowing tests of global confinement and local transport properties that have been established from higher aspect ratio devices. NSTX plasmas are heated by up to 7 MW of deuterium neutral beams with preferential electron heating as expected for ITER. Confinement scaling studies indicate a strong B T dependence, with a current dependence that is weaker than that observed at higher aspect ratio. Dimensionless scaling experiments indicate a strong increase of confinement with decreasing collisionality and a weak degradation with beta. The increase of confinement with B T is due to reduced transport in the electron channel, while the improvement with plasma current is due to reduced transport in the ion channel related to the decrease in the neoclassical transport level. Improved electron confinement has been observed in plasmas with strong reversed magnetic shear, showing the existence of an electron internal transport barrier (eITB). The development of the eITB may be associated with a reduction in the growth of microtearing modes in the plasma core. Perturbative studies show that while L-mode plasmas with reversed magnetic shear and an eITB exhibit slow changes of L T e across the profile after the pellet injection, H-mode plasmas with a monotonic q-profile and no eITB show no change in this parameter after pellet injection, indicating the existence of a critical gradient that may be related to the q-profile. Both linear and non-linear simulations indicate the potential importance of ETG modes at the lowest B T . Localized measurements of high-k fluctuations exhibit a sharp decrease in signal amplitude levels across the L-H transition, associated with a decrease in both ion and electron transport, and a decrease in calculated linear microinstability growth rates across a wide k-range, from the ITG/TEM regime up to the ETG regime.

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Confinement studies of auxiliary heated NSTX plasmas

Cited by 48 publications

References 31 publications

Beta-limiting MHD Instabilities in Improved-performance NSTX Spherical Torus Plasmas

Beta-limiting MHD Instabilities in Improved-performance NSTX Spherical Torus Plasmas

Plasma response to lithium-coated plasma-facing components in the National Spherical Torus Experiment

Confinement and local transport in the National Spherical Torus Experiment (NSTX)

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