Fast-ion transport in qmin>2, high-β steady-state scenarios on DIII-D

Abstract

Results from experiments on DIII-D [J. L. Luxon, Fusion Sci. Technol. 48, 828 (2005)] aimed at developing high b steady-state operating scenarios with high-q min confirm that fast-ion transport is a critical issue for advanced tokamak development using neutral beam injection current drive. In DIII-D, greater than 11 MW of neutral beam heating power is applied with the intent of maximizing b N and the noninductive current drive. However, in scenarios with q min > 2 that target the typical range of q 95 ¼ 5-7 used in next-step steady-state reactor models, Alfv en eigenmodes cause greater fast-ion transport than classical models predict. This enhanced transport reduces the absorbed neutral beam heating power and current drive and limits the achievable b N . In contrast, similar plasmas except with q min just above 1 have approximately classical fast-ion transport. Experiments that take q min > 3 plasmas to higher b P with q 95 ¼ 11-12 for testing long pulse operation exhibit regimes of better than expected thermal confinement. Compared to the standard high-q min scenario, the high b P cases have shorter slowing-down time and lower rb fast , and this reduces the drive for Alfv enic modes, yielding nearly classical fast-ion transport, high values of normalized confinement, b N , and noninductive current fraction. These results suggest DIII-D might obtain better performance in lower-q 95 , high-q min plasmas using broader neutral beam heating profiles and increased direct electron heating power to lower the drive for Alfv en eigenmodes. V C 2015 AIP Publishing LLC. [http://dx.

Phase-space dependent critical gradient behavior of fast-ion transport due to Alfvén eigenmodes

“…The same modes from #162753 were then scaled to match similar modes in shots at lower beam powers. Scaling factor values are normalized (×10 4 ) and are proportional to mode amplitude. Table A1.…”

“…In DIII-D, significant fast-ion transport due to AEs has been observed in high q min steady-state reactor scenarios where the measured neutron rate approaches 60% of the classical rate predicted by the TRANSP NUBEAM code [3] assuming no anomalous transport, limiting the achievable β N . Some scenarios with q min closer to one still have AE activity, yet fast ions behave classically [4,5]. Therefore, understanding the regimes where AEs lead to losses and reduced fusion performance is important for developing a practical long-pulse fusion power plant.…”

Compatibility of internal transport barrier with steady-state operation in the high bootstrap fraction regime on DIII-D

“…The pedestal density is particularly effective at slowing down the fast ions because of the relatively low temperature at large minor radius (T~0.5 i keV at ρ~0.95). Once the density has reached a sufficiently high flattop, the inferred fast ion losses are reduced to near classical levels, despite the very high values of q min which can be associated with stronger AE drive through coupling to higher order resonances [28]. In addition to the slowing down effect of the density discussed here, recent analysis with the Nova code [28] indicates that the Alfvén continuum in the core of these plasmas during the high density phase is relatively closed, which may also contribute to the reduced AE activity and resulting fast ion transport.…”

“…Once the density has reached a sufficiently high flattop, the inferred fast ion losses are reduced to near classical levels, despite the very high values of q min which can be associated with stronger AE drive through coupling to higher order resonances [28]. In addition to the slowing down effect of the density discussed here, recent analysis with the Nova code [28] indicates that the Alfvén continuum in the core of these plasmas during the high density phase is relatively closed, which may also contribute to the reduced AE activity and resulting fast ion transport. These results are similar to the observations in [29,30] that AE modes are stable in negative shear discharges with large density gradient at the ITB, although the regime under consideration here is significantly different, with a q-profile that is flatter in the core and overall significantly higher [see figure 3(c)].…”

Control of power, torque, and instability drive using in-shot variable neutral beam energy in tokamaks

“…Recent DIII-D upgrade plans focus on increasing the beam energy in order to input more beam power and current drive into steady state scenario plasmas [11]. Some of these scenarios exhibit reduced confinement linked to the existence of beam ion driven instabilities [12,13], however, and there is a sizable collection of such instabilities that cause enhanced transport of injected beam ions [14] and reduce the effective heating and current drive from the beams. In a fundamental shift in thinking, the DIII-D neutral beams have been modified to vary their injection energy during plasma shots with the ultimate intention of tailoring the velocity space distribution of beam ions to produce continuously varying power and torque curves, and to temporarily reduce the drive for undesirable modes while remaining capable of reaching peak power input later in the plasma shot.…”