H-mode pedestal, ELM and power threshold studies in NSTX

Maingi, R.; Bush, C. E.; Fredrickson, E. D.; Gates, D.; Kaye, S. M.; LeBlanc, B.; Ménard, J.; Meyer, H.; Mueller, D.; Nishino, Norikazu; Roquemore, A. L.; Sabbagh, S.A.; Tritz, K.; Zweben, S. J.; Bell, M.G.; Bell, R.E.; Biewer, T.; Boedo, J.A.; Johnson, D.; Kaita, R.; Kugel, H.; Maqueda, R. J.; Munsat, T.; Raman, R.; Soukhanovskii, V.; Stevenson, T. N.; Stutman, D.

doi:10.1088/0029-5515/45/9/006

Cited by 74 publications

(71 citation statements)

References 50 publications

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“…2). A comparison to small ELMs in NSTX and Alcator C-MOD [12] shows that these ELMs are distinct from the small type-V ELMs observed on NSTX [13], because of the different mode structure and rotation direction. In SN only type-I ELMs have been observed [10], although the edge stability is not changed by the configuration [9].…”

Section: Elms and Pedestalmentioning

confidence: 78%

Overview of physics results from MAST

Raman¹,

Ahn²,

Allain³

et al. 2011

Nucl. Fusion

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Several improvements to the MAST plant and diagnostics have facilitated new studies advancing the physics basis for ITER and DEMO, as well as for future spherical tokamaks. Using the increased heating capabilities P NBI ≤ 3.8 MW H-mode at I p = 1.2 MA was accessed showing that the energy confinement on MAST scales more weakly with I p and more strongly with B t than in the ITER IPB98(y,2) scaling. Measurements of the fuel retention of shallow pellets extrapolate to an ITER particle throughput of 70% of its original design value. The anomalous momentum diffusion, χ φ , is linked to the ion diffusion, χ i , with a Prandtl number close to P φ ≈ χ φ /χ i ≈ 1, although χ i approaches neoclassical values. New high spatially resolved measurements of the edge radial electric field, E r , show that the position of steepest gradients in electron pressure and E r are coincident, but their magnitudes are not linked. The T e pedestal width on MAST scales with the β pol rather than ρ pol . The ELM frequency for type-IV ELMs, new in MAST, was almost doubled using n = 2 resonant magnetic perturbations from a set of 4 external coils (n = 1, 2). A new internal 12 coil set (n ≤ 3) has been commissioned. The filaments in the inter-ELM and L-mode phase are different from ELM filaments, and the characteristics in L-mode agree well with turbulence calculations. A variety of fast particle driven instabilities were studied from 10 kHz saturated fishbone like activity up to 3.8 MHz compressional Alfvén eigenmodes (CAE). The damping rate of ellipticityinduced AE was measured to be 4% using the new internal coils as antennae. Fast particle instabilities also affect the off-axis NBI current drive and lead to fast ion diffusion of the order of 0.5 m 2 /s and reduce the driven current fraction from 40% to 30%. EBW current drive start-up is demonstrated for the first time in a spherical tokamak generating plasma currents up to 55 kA. Many of these studies contributed to the physics basis of a planned upgrade to MAST. Introduction: MAST [1]is one of the two leading tight aspect ratio (A = ε −1 = R/a = 0.85 m/0.65 m ∼ 1.3, I p ≤ 1.5 MA) tokamaks in the world. The hot T ≤ 3 keV, dense n e = (0.1 − 1) × 10 20 m −3 and highly shaped (δ ≤ 0.5, 1.6 ≤ κ ≤ 2.5) plasmas are accessed at moderate toroidal field B t (R = 0.7 m) ≤ 0.62 T and show many similarities to conventional aspect ratio tokamaks. Detailed physics studies using the extensive array of state of the art diagnostics and access to different physics regimes help to consolidate the physics basis for ITER and DEMO [2,3], and explore the viability of future devices based on the spherical tokamak (ST) concept such as a component test facility (CTF) [4] or an advanced power plant [5]. The challenge for today's experiments is to find an integrated scenario that extrapolates to these future devices, in particular to develop plasmas with reduced power load on plasma facing components, notably from edge localised modes (ELM), but high confinement facilitated by internal or edge transport ba...

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Section: Elms and Pedestalmentioning

confidence: 78%

Overview of physics results from MAST

Raman¹,

Ahn²,

Allain³

et al. 2011

Nucl. Fusion

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“…D α mid responds to the ELM density as opposed to the loss of core energy which drives the rise of D α div over the longer period of ~ 1.5 ms. The emidf signal shows the MHD oscillation characteristic of the ELM [22,23] that accompanies the large rise of the ELM on the XHD14 signal. Precursors are evident on all three signals during the 0.2 ms time frame of the arc (P RF is sampled here at 5 kHz).…”

Section: Elm Characteristicsmentioning

confidence: 97%

Recent Fast Wave Coupling and Heating Studies on NSTX, with Possible Implications for ITER

Hosea¹,

Bell²,

Feibush³

et al. 2009

AIP Conference Proceedings

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Abstract. The goal of the high harmonic fast wave (HHFW) research on NSTX is to maximize the coupling of RF power to the core of the plasma by minimizing the coupling of RF power to edge loss processes. HHFW core plasma heating efficiency in helium and deuterium L-mode discharges is found to improve markedly on NSTX when the density 2 cm in front of the antenna is reduced below that for the onset of perpendicular wave propagation (n onset ∝ B*k || 2 /ω). In NSTX, the observed RF power losses in the plasma edge are driven in the vicinity of the antenna as opposed to resulting from multi-pass edge damping. PDI surface losses through ion-electron collisions are estimated to be significant. Recent spectroscopic measurements suggest that additional PDI losses could be caused by the loss of energetic edge ions on direct loss orbits and perhaps result in the observed clamping of the edge rotation. Initial deuterium H-mode heating studies reveal that core heating is degraded at lower k φ (-8 m -1 relative to 13 m -1 ) as for the Lmode case at elevated edge density. Fast visible camera images clearly indicate that a major edge loss process is occurring from the plasma scrape off layer (SOL) in the vicinity of the antenna and along the magnetic field lines to the lower outer divertor plate. Large type I ELMs, which are observed at both k φ values, appear after antenna arcs caused by precursor blobs, low level ELMs, or dust. For large ELMs without arcs, the source reflection coefficients rise on a 0.1 ms time scale, which indicates that the time derivative of the reflection coefficient can be used to discriminate between arcs and ELMs.

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“…21͒ and ASDEX-U ͑Ref. 22͒ groups, various small ELM regimes at JT-60U, 23,24 ASDEX-U, 25 NSTX, 26,27 JET, 28 C-Mod, 29 JFT-2M, 30,31 Compass-D ͑Ref. 32͒ and DIII-D, 33 and ELM-free operation in the QH-mode.…”

Section: Introductionmentioning

confidence: 99%

Effect of island overlap on edge localized mode suppression by resonant magnetic perturbations in DIII-D

Fenstermacher¹,

Evans²,

Osborne³

et al. 2008

Physics of Plasmas

152

230

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Recent DIII-D [J. L. Luxon et al., Nucl. Fusion 43, 1813 (2003)] experiments show a correlation between the extent of overlap of magnetic islands induced in the edge plasma by perturbation coils and complete suppression of Type-I edge localized modes (ELMs) in plasmas with ITER-like electron pedestal collisionality νe*∼0.1, flux surface shape and low edge safety factor (q95≈3.6). With fixed amplitude n=3 resonant magnetic perturbation (RMP), ELM suppression is obtained only in a finite window in the edge safety factor (q95) consistent with maximizing the resonant component of the applied helical field. ELM suppression is obtained over an increasing range of q95 by either increasing the n=3 RMP strength, or by adding n=1 perturbations to “fill in” gaps between islands across the edge plasma. The suppression of Type-I ELMs correlates with a minimum width of the edge region having magnetic islands with Chirikov parameter >1.0, based on vacuum calculations of RMP mode components excluding the plasma response or rotational shielding. The fraction of vacuum magnetic field lines that are lost from the plasma, with connection length to the divertor targets comparable to an electron-ion collisional mean free path, increases throughout the island overlap region in the ELM suppressed case compared with the ELMing case.

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H-mode pedestal, ELM and power threshold studies in NSTX

Cited by 74 publications

References 50 publications

Overview of physics results from MAST

Overview of physics results from MAST

Recent Fast Wave Coupling and Heating Studies on NSTX, with Possible Implications for ITER

Effect of island overlap on edge localized mode suppression by resonant magnetic perturbations in DIII-D

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