• Investigate the mechanisms governing electron transport
• L-H transition physics
• Study relationship between particle/impurity transport and momentum pinch
• Beta scaling of confinement using dimensionless parameter scans in ELMy/ELM-free discharges
• Aspect ratio dependence of confinement scaling (DIII-D similarity, ITPA)

Research Milestone R(11-1) Study turbulence regimes responsible for ion and electron energy transport.

Results from 2006-2008 indicate that the scalings of electron and ion energy transport with magnetic field and plasma current differ in the ST, and also differ from high-aspect-ratio tokamak scalings. Understanding electron transport is particularly important as the electron channel is the dominant energy loss channel in NSTX plasmas, while ion transport commonly approaches neoclassical levels. High-k scattering measurements from 2007-2008 indicate that ETG turbulence is a leading candidate for anomalous electron energy transport. However, low-k fluctuations may also contribute to electron transport. The low-k portion of the turbulent density fluctuation spectrum will be measured with a Beam Emission Spectroscopy (BES) diagnostic, and low-k magnetic-field fluctuations will be measured using MSE and/or MSE-LIF diagnostics (if technically ready). Experiments will be performed to vary plasma parameters such as collisionality, ExB shear, magnetic shear, plasma current, and magnetic field to span the instability drive space of candidate micro-instabilities (ITG, CTEM, micro-tearing, and ETG) thought to possibly be responsible for anomalous energy transport. The measured k spectrum of the turbulence will be measured as function of plasma parameters and correlated with energy diffusivities inferred from power balance analysis, and these results will be compared with linear and non-linear instability calculations to identify, where possible, the micro-instabilities responsible for the observed transport. Improved understanding of electron and ion energy transport in the ST is highly desirable to reduce the uncertainty of extrapolation to next-step STs. This research also contributes broadly to a fundamental understanding of transport.

ITPA Participation

• TC-1 Confinement scaling in ELMy H-modes: beta degradation
• TC-2  Power ratio - hysteresis and access to H-mode with H~1
• TC-3  Scaling of the Low-Density Limit of the H-mode Threshold
• TC-4  H-mode transition and confinement dependence on ionic species
• TC-9  Scaling of intrinsic plasma rotation with no external momentum input
• TC-10  Experimental identification of ITG, TEM and ETG turbulence and comparison with codes
• TC-12  H-mode transport and confinement at low aspect ratio
• TC-15  Dependence of momentum and particle pinch on collisionality

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