Improving Fast-Ion Confinement by Reducing Alfvén Eigenmodes in the qmin>2 Steady-State Tokamak Scenario
Abstract
Experiments in the DIII-D tokamak show that a broadened fast-ion pressure profile enables better control of Alfvén Eigenmodes (AEs), improves fast-ion confinement, and allows access to new regimes with 15% higher normalized plasma beta (βN) than previously achieved in high-field, steady-state scenarios with negative central shear and qmin>2. Reversed Shear Alfvén Eigenmodes (RSAEs) were reduced in the current ramp by increasing the off-axis neutral beam power fraction, resulting in ∼24% higher ratio of measured neutrons to calculated classical neutrons. The neutron fraction was further improved using Electron Cyclotron Current Drive aimed on-axis, which suppressed RSAEs by moving the qmin location inward toward reduced beam pressure gradient and higher plasma pressure, resulting in a ∼36% higher neutron ratio than the reference shot. In flattop, fast-ion confinement improved by ∼25% after reducing beam pressure gradient (thus AE drive) by increasing the off-axis beam power fraction from 30% to 70%. Record parameters were achieved by increasing the relative density, reaching βN ∼ 3.1 and H89 ∼ 2.3 at BT = 2.0 T and q95 = 6.0. These experiments mark significant progress in understanding potential optimized regimes for steady-state advanced tokamaks that can avoid AE-induced fast-ion redistribution, loss, reduced heating efficiency, and limits to the achievable βN.
Work supported by US DOE contracts DE-FC02-04ER54698, DE-FG03-94ER54271, and DE-AC02-09CH11466.- Publication:
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APS Division of Plasma Physics Meeting Abstracts
- Pub Date:
- 2020
- Bibcode:
- 2020APS..DPPP06011C