Transport with reversed shear in the National Spherical Torus Experiment

Levinton, F. M.; Yuh, H.; Bell, M.G.; Bell, R.E.; Delgado‐Aparicio, L.; Finkenthal, M.; Fredrickson, E.; Gates, D.; Kaye, S. M.; LeBlanc, B.P.; Maingi, R.; Ménard, J.; Mikkelsen, D. R.; Mueller, D.; Raman, R.; Rewoldt, G.; Sabbagh, S. A.; Stutman, D.; Tritz, K.; Wang, W.

doi:10.1063/1.2734124

Cited by 44 publications

(50 citation statements)

References 45 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Within this region, there is a factor of 3 to 10 difference in both χ e and χ i between the two discharges. 24 Perturbative studies using Lithium pellets and the Soft X-ray array (SXR) to diagnose fast changes in the electron temperature also showed the influence of the q-profile on plasma temperature gradients and transport. Fig.…”

Section: Local Transport Resultsmentioning

confidence: 99%

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

et al. 2007

View full text Add to dashboard Cite

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.

show abstract

Section: Local Transport Resultsmentioning

confidence: 99%

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

et al. 2007

View full text Add to dashboard Cite

show abstract

“…3d). The qprofiles were reconstructed using Motional Stark Effect measurements of the magnetic field pitch angle [54] and the LRDFIT equilibrium code [55].…”

Section: Introductionmentioning

confidence: 99%

Fast-ion energy loss during TAE avalanches in the National Spherical Torus Experiment

et al. 2012

Self Cite

View full text Add to dashboard Cite

Strong TAE avalanches on NSTX, the National Spherical Torus Experiment [M. Ono, et al., Nucl. Fusion 40 (2000) 557] are typically correlated with drops in the neutron rate in the range of 5% -15%. In previous studies of avalanches in L-mode plasmas, these neutron drops were found to be consistent with modeled losses of fast ions. Here we expand the study to TAE avalanches in NSTX H-mode plasmas with improved analysis techniques. At the measured TAE mode amplitudes, simulations with the ORBIT code predict that fast ion losses are negligible. However, the simulations predict that the TAE scatter the fast ions in energy, resulting in a small (≈ 6%) drop in fast ion β. The net decrease in energy of the fast ions is sufficient to account for the bulk of the drop in neutron rate, even in the absence of fast ion losses. This loss of energy from the fast ion population is comparable to the estimated energy lost by damping from the Alfvén wave during the burst. The previously studied TAE avalanches in L-mode are re-evaluated using an improved calculation of the potential fluctuations in the ORBIT code.

show abstract

“…T e r is very flat near the magnetic axis [ = a 1=2 0:3] where microtearing modes are stable; e there is very large due to other mechanisms not yet identified. Should microtearing modes be the dominant electron transport mechanism, e will be significantly reduced when these modes are stable, which is the case in a plasma with reversed central magnetic shear [18]. Figure 4(a) shows the growth rate at = a 1=2 0:3 for such a shot (no.…”

mentioning

confidence: 99%