Off-axis current drive and real-time control of current profile in JT-60U

Abstract

Aiming at optimization of current profile in high-β plasmas for higher confinement and stability, a real-time control system of the minimum of the safety factor (q min) using the off-axis current drive has been developed. The off-axis current drive can raise the safety factor in the centre and help to avoid instability that limits the performance of the plasma. The system controls the injection power of lower-hybrid waves, and hence its off-axis driven current in order to control q min. The real-time control of q min is demonstrated in a high-β plasma, where q min follows the temporally changing reference q min,ref from 1.3 to 1.7. Applying the control to another high-β discharge (βN = 1.7, βp = 1.5) with m/n = 2/1 neo-classical tearing mode (NTM), q min was raised above 2 and the NTM was suppressed. The stored energy increased by 16% with the NTM suppressed, since the resonant rational surface was eliminated. For the future use for current profile control, current density profile for off-axis neutral beam current drive (NBCD) is for the first time measured, using the motional Stark effect diagnostic. Spatially localized NBCD profile was clearly observed at the normalized minor radius ρ of about 0.6–0.8. The location was also confirmed by multi-chordal neutron emission profile measurement. The total amount of the measured beam driven current was consistent with the theoretical calculation using the ACCOME code. The CD location in the calculation was inward shifted than the measurement.

21st IAEA Fusion Energy Conference: summary of sessions EX/D, EX/S and EX/W

“…Analysis of far off-axis CD in JT-60U [27], presented in the right part of Fig. 6, clearly showed that current can be driven as far off-axis as ρ=0.8, but again the profile of the driven current was not consistent with the theoretical prediction that in this case showed more inward shifted profiles.…”

Evidence for Fast-Ion Transport by Microturbulence

“…Other authors stress that the Kubo number (ratio of decorrelation time to fast-ion time of flight) is a crucial parameter and that phase averaging may not occur at all [5]. A compilation of data [6] in the large energy regime (E=T ) 10) confirms the conventional wisdom, but some anomalies at smaller E=T have been reported [7,8].…”

Interaction of energetic particles with large and small scale instabilities

“…In most of these experiments the beam driven current was located on-axis, including those with N-NBI [16,17]. The only off-axis current drive experiments (with 2 MW NBI power) -apart from ASDEX Upgrade -have been reported by JT-60U [15,19].…”