•  Characterize and optimize High-Harmonic Fast Wave (HHFW) coupling in deuterium H-Mode plasmas using the new double-feed antenna configuration and lithium edge density control.
•  Utilize HHFW heating and current drive for non-inductive plasma current ramp-up and sustainment (Milestone R10-2)

FY2010 Research Milestone (R10-2):  Characterize HHFW heating, current drive, and current ramp-up in deuterium H-mode plasmas.

HHFW/ICRF auxiliary heating is expected to be important in next-step STs such as NHTX and ST-CTF as a means of supplementing NBI heating for plasma ramp-up and sustainment. Building on the improved understanding and mitigation of parasitic surface-wave excitation in 2006-2007, the HHFW system on NSTX will be tested as an efficient bulk heating and central current profile control tool in deuterium H-mode plasmas. HHFW has previously produced bootstrap fractions as high as 85% in low-IP H-mode ramp-up plasmas, but was limited by antenna voltage constraints and the deleterious effects of ELMs on RF wave coupling. Antenna upgrades to increase the coupled power and to provide ELM resilience will be implemented to develop bootstrap current overdrive ramp-up of an ST plasma for the first time. The same antenna upgrades will also enable improved electron heating in reduced-density sustained H-mode discharges which could enhance the NBICD (and HHFW-CD) efficiency. Power deposition and current-drive calculations will be performed, and these results will be utilized by the TRANSP code to characterize the heating efficiency, current drive, and transport modifications induced by HHFW. The interaction of HHFW with fast-ions from NBI will be modeled to assess the electron heating efficiency in advanced scenarios with strong NBI heating. Finally, simulations of the plasma current ramp-up will be utilized to understand and optimize current ramp-up with HHFW. Understanding and improving the performance of HHFW/ICRF coupling and heating could significantly reduce the risk of extrapolating RF technologies to next-step STs and ITER.

Key Research Topics

High Harmonic Fast Wave Research
Research will primarily be directed towards optimizing HHFW heating and current drive operation with NBI in the H-mode regime with the upgraded antenna. We will use a larger plasma-antenna gap to permit greater stability at higher power (more voltage standoff and greater power for the same voltage). An even larger gap will probably be required for operation with all three high-power NBI sources in the H-mode regime. Results from these experiments will be important for benchmarking the 3-D AORSA and TORIC codes. Experiments will also continue to develop HHFW-assisted startup and plasma current ramp-up.

Energetic Particle Research
The highest priority will be to complete validation of numerical models for predicting fast ion transport as measured in studies of the TAE avalanches. Experiments will address the scaling of the onset thresholds with q-profile, fast ion distribution (pitch angle, energy) and Vfast/VAlfvén through density and toroidal field scans. The neutral beam voltage and source dependence of TAE avalanches, a high priority in terms of impact on fast ion transport, will be investigated.  An experiment will be dedicated to producing TAE modes and a search for continuum damping via coupling to KAW will be made using the high-k scattering diagnostic.  Experiments will include a careful documentation of the current profile evolution.

The study and modeling of the impact of EPMs on beam driven currents will begin with FIDA and BES diagnostic capability. The parametric dependencies of these modes on q-profile and beta will be investigated. Mode structure measurements will be used to validate the NOVA and PEST linear ideal codes as well as the M3D-K non-linear code. Predictions of fast ion transport with ORBIT and M3D-K will be compared with experimental data.

Documentation of the Alfvén Cascade eigenmode evolution to TAE modes will be completed.  Density, toroidal field, and beam voltage scans will help to identify the parametric scaling of the GAM frequency from which the effective specific heat for the fast ion distribution can be determined.  The data will provide important validation of the NOVA and M3D codes for plasmas with reversed core magnetic shear. We also plan to complete documentation of internal mode structure and begin scaling of threshold parameters of the beta-induced Alfvén acoustic mode (BAAE), utilizing the high-k scattering and BES diagnostics.

ITPA Participation

• EP-1 Measurement of damping rate of intermediate toroidal mode number Alfvén Eigenmodes
• EP-2 Fast ion losses and redistribution from localized Alfvén Eigenmodes

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