Decoupled recovery of energy and momentum with correction ofn  =  2 error fields

Abstract

Experiments applying known n = 2 'proxy' error fields (EFs) find that the rotation braking introduced by the proxy EF cannot be completely alleviated through optimal n = 2 correction with poorly matched poloidal spectra. This imperfect performance recovery demonstrates the importance of correcting multiple components of the n = 2 field spectrum and is in contrast to previous results with n = 1 EFs despite a similar execution. Measured optimal n = 2 proxy EF correction currents are consistent with those required to null dominant mode coupling to the resonant surfaces and minimize the neoclassical toroidal viscosity (NTV) torque, calculated using ideal MHD plasma response computation. Unlike rotation braking, density pumpout can be fully corrected despite poorly matched spectra, indicating density pumpout is driven only by a single component proportional to the resonant coupling. Through precise n = 2 spectral control density pumpout and rotation braking can thus be decoupled. Rotation braking with n = 2 fields is also found to be proportional to the level of co-current toroidal rotation, consistent with NTV theory. Plasmas with modest counter-current rotation are insensitive to the n = 2 field with neither rotation braking nor density pumpout observed.

Empirical scaling of the n = 2 error field penetration threshold in tokamaks

“…An intrinsic error field equivalent to approximately 1.03kA in 0 • phasing (or "even parity") I-coils has been taken into account in all the DIII-D data. This approximation of the error field is consistent with previous measurements of the n = 2 DIII-D EF and confirmed using 2-point scans at fixed phase in these devoted experiments [59] . Uncertainties in the DIII-D n = 2 EF however, may still be on the order of the slight offset between the scans shown here.…”

“…H-mode threshold data, for example, would enable a wider parametric scaling and determine if the pressure scaling linking the high and low confinement regimes reverses as in the n = 1 case. In addition, none of the machines in question have constrained the intrinsic n = 2 EF as thoroughly as the intrinsic n = 1 EF [59]. Uncertainties in the intrinsic n = 2 could skew the scalings between machines or even within a single machine using multiple coil configurations (internal and external coils, changes in the phasing between multiple coil sets, or even just different absolute phases of the ramped coil currents).…”

Evidence of Toroidally Localized Turbulence with Applied 3D Fields in the DIII-D Tokamak

“…While complete ELM suppression only occurs in certain ranges of edge safety factor q [3], density pumpout is a more consistent feature of DIII-D H-mode plasmas with applied 3D fields [4]. Recent experiments have linked both ELM suppression and density pumpout to resonant excitation of modes on magnetic surfaces with rational q values [5][6][7]. Understanding the physics of density pumpout has the potential to significantly improve the performance of fusion reactors if this confinement loss can be minimized while using 3D fields as an actuator for ELM and rotation control.…”

“…It is speculated that n =3 error fields play a role in the observations by the second system, which would be expected to measure nearly zero difference in fluctuations between phases for the modeled n=3 perturbations that are dominant elsewhere. Uncorrected n=2 error fields are estimated to contribute the equivalent of ≤ 1 kA of I-coil current [6], while uncorrected n=1 error fields are smaller due to the standard application of n=1 error field correction.…”

DIII-D research to address key challenges for ITER and fusion energy

“…Thus ideal MHD plays a major role in 3D field response, and further experiments show that this is primarily governed by a single 'least stable' ideal mode for n = 1 fields. This is confirmed by experiments measuring neoclassical toroidal viscosity (NTV) braking of plasma rotation [59][60][61]. figure 20(a) shows measured toroidal angular momentum (from CER rotation Thomson density profiles) in plasmas where C-or I-coils are applied separately, and then when combined together in such a way as to cancel out ('null') coupling to the q = 2 surface through ideal MHD.…”