Central flattening of the fast-ion profile in reversed-shear DIII-D discharges

Abstract

Neutral beam injection into a plasma with negative central shear produces a rich spectrum of toroidicity-induced and reversed-shear Alfvén eigenmodes in the DIII-D tokamak. The application of fast-ion D α (FIDA) spectroscopy shows that the central fast-ion profile is flattened in the inner half of the discharge. Neutron and equilibrium measurements corroborate the FIDA data. The temporal evolution of the current profile is also strongly modified. Studies in similar discharges show that flattening of the profile correlates with the mode amplitude and that both types of Alfvén modes correlate with fast-ion transport. Calculations by the ORBIT code do not explain the observed fast-ion transport for the measured mode amplitudes, however. Possible explanations for the discrepancy are considered.

Phenomenology of energetic-ion loss from the DIII-D tokamak

“…The losses correlate more strongly with the low-frequency radial magnetic field ( figure 17(b)) than with the high-frequency AE activity ( figure 17(d)). Another example of losses during AE activity appears in figure 15 of [15]. As in the case shown here, large losses occur during a phase of the discharge with strong Alfvén activity but the temporal correlation with the core mode amplitude is weak.…”

Particle Distribution Modification by Low Amplitude Modes

“…In plasmas without appreciable MHD activity, the FIDA profiles agree well [10] with the profiles predicted by the NUBEAM module [9] of the TRANSP code but, in the presence of the strong TAE and RSAE activity, the profile in the inner half of the plasma is much flatter than classically expected [7,8].…”

“…Likewise, the same scaling factor gives quantitative agreement with the electron density fluctuations measured by BES [4], confirming that the mode amplitudes are accurately determined. The resultant beam-ion transport is measured by five independent techniques [7,8], including spatially resolved fastion D-alpha (FIDA) spectroscopy. The data imply strong central flattening of the fast-ion profile during the early phase of the discharge when many Alfvén modes are unstable [7,8].…”

“…Since the Alfvén activity and fast-ion transport are correlated [7,8], it is natural to assume that they are causally related. To test the assumption that the Alfvén modes cause the additional fast-ion transport, in previous works [7,8], we inserted the magnetic part of the NOVA calculated eigenfunctions that were experimentally validated by ECE measurements into the guiding center code ORBIT [11] and calculated the expected fast-ion transport.…”

Combination of fast-ion diagnostics in velocity-space tomographies

“…Measurements from any other fast-ion diagnostic could be included in our joint tomography prescription, if quantitative weight functions describing the measurements such as those for CTS [34] or FIDA [20,56] can be formulated. Our joint tomography method would then also be applicable to other tokamaks with many-view FIDA systems and additional fastion diagnostics, for example DIII-D [57,58], NSTX [59] and MAST. Here we make a start by combining CTS and FIDA.…”