April 15-17

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Author: Cornwall H Lau
Requested Type: Poster
Submitted: 2019-02-21 11:42:08

Co-authors: N. Bertelli, E.H. Martin, R.I. Pinsker, S. Shiraiwa, G.M. Wallace, M. Brookman and the RF SCIDAC team

Contact Info:
ORNL
Building 5600, Room M401-16, M
Oak Ridge, TN   37830
United States

Abstract Text:
Lower hybrid (LH) and helicon waves drive off-axis current in tokamaks to enable steady state advanced scenarios. Previous experimental, computational and theoretical investigations have indicated the importance of scrape-off-layer (SOL) density fluctuations on the propagation and absorption waves of LH and helicon waves. This paper presents a summary of recent computational results demonstrating the impact of synthetic turbulent density profiles using a cold plasma full wave model for LH waves on Alcator C-Mod and helicon waves on DIII-D:
• SOL density fluctuations at large amplitudes (50-100%) and with wavelengths close to the LH wavelength consistent with ballooning mode turbulence are necessary to explain experimentally measured LH polarization modification on Alcator C-Mod
• Scattering, interference, and large LH electric fields can occur in the SOL, increasing SOL collisional losses for fluctuation lengths near the LH wavelengths
• Strong SOL electric field structures and collisional absorption for helicon waves on DIII-D can form at high SOL densities
• SOL density fluctuations at large amplitudes and specific wavelengths can create large SOL electric fields and losses for helicon waves on DIII-D at both low and high SOL densities

These physics highlights are established from a computational study of 2-D axisymmetric full wave models for a wide range of plasma and antenna parameters such as density profile, density fluctuation amplitude, density fluctuation poloidal wavelength, collision frequency, antenna toroidal mode number, and realistic vacuum vessel geometry. These results show the impact of SOL density and density fluctuations on LH and helicon wave SOL propagation, increased SOL absorption and reduced core coupling, and the importance of full wave models in capturing strong SOL electric fields that cannot be simulated by standard ray tracing models.
1. E.H Martin et al, submitted to NF 2018
2. C. Lau et al., Nuclear Fusion, 2018

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