ORCID Profile
0000-0002-2230-457X
Current Organisation
University of California, San Diego
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Publisher: IOP Publishing
Date: 16-07-2013
Publisher: AIP Publishing
Date: 05-2017
DOI: 10.1063/1.4982688
Abstract: In several tokamaks, non-axisymmetric magnetic field studies show that applied magnetic fields with a toroidal harmonic n = 2 can lead to disruptive n = 1 locked modes. In Ohmic plasmas, n = 2 magnetic reconnection thresholds in otherwise stable discharges are readily accessed at edge safety factors q ∼ 3, low density, and low rotation. Similar to previous studies with n = 1 fields, the thresholds are correlated with the “overlap” field computed with the IPEC code. The overlap field quantifies the plasma-mediated coupling of the external field to the resonant field. Remarkably, the “critical overlap fields” at which magnetic islands form are similar for applied n = 1 and 2 fields. The critical overlap field increases with plasma density and edge safety factor but is independent of the toroidal field. Poloidal harmonics m & nq dominate the drive for resonant fields while m & nq harmonics have a negligible impact. This contrasts with previous results in H-mode discharges at high plasma pressure in which the toroidal angular momentum is sensitive to low poloidal harmonics. Together, these results highlight unique requirements for n & 1 field control including the need for multiple rows of coils to control selected plasma parameters for specific functions (e.g., rotation control or ELM suppression).
Publisher: AIP Publishing
Date: 18-09-0440
DOI: 10.1063/1.5000276
Abstract: Experiments have been executed in the DIII-D tokamak to extend suppression of Edge Localized Modes (ELMs) with Resonant Magnetic Perturbations (RMPs) to ITER-relevant levels of beam torque. The results support the hypothesis for RMP ELM suppression based on transition from an ideal screened response to a tearing response at a resonant surface that prevents expansion of the pedestal to an unstable width [Snyder et al., Nucl. Fusion 51, 103016 (2011) and Wade et al., Nucl. Fusion 55, 023002 (2015)]. In ITER baseline plasmas with I/aB = 1.4 and pedestal ν* ∼ 0.15, ELMs are readily suppressed with co-Ip neutral beam injection. However, reducing the beam torque from 5 Nm to ≤ 3.5 Nm results in loss of ELM suppression and a shift in the zero-crossing of the electron perpendicular rotation ω⊥e ∼ 0 deeper into the plasma. The change in radius of ω⊥e ∼ 0 is due primarily to changes to the electron diamagnetic rotation frequency ωe*. Linear plasma response modeling with the resistive MHD code m3d-c1 indicates that the tearing response location tracks the inward shift in ω⊥e ∼ 0. At pedestal ν* ∼ 1, ELM suppression is also lost when the beam torque is reduced, but the ω⊥e change is dominated by collapse of the toroidal rotation vT. The hypothesis predicts that it should be possible to obtain ELM suppression at reduced beam torque by also reducing the height and width of the ωe* profile. This prediction has been confirmed experimentally with RMP ELM suppression at 0 Nm of beam torque and plasma normalized pressure βN ∼ 0.7. This opens the possibility of accessing ELM suppression in low torque ITER baseline plasmas by establishing suppression at low beta and then increasing beta while relying on the strong RMP-island coupling to maintain suppression.
Publisher: Elsevier BV
Date: 03-2021
Publisher: IOP Publishing
Date: 27-07-2018
Publisher: IOP Publishing
Date: 20-06-2013
Publisher: IOP Publishing
Date: 06-12-2022
Abstract: We assess the toroidal magnetic field B t asymmetry in DIII-D due to a misalignment of the toroidal field coils with respect to the poloidal magnetic field coils and vacuum vessel. The peak-to-peak variation of the ertor strike point (SP) radius is measured to be 1 cm, with an n = 1 toroidal pattern. We use the centre of a narrow carbon deposition band on tungsten-coated ertor tiles just inside the outer strike point (OSP) as a proxy for the ertor SP location. The band occurred in a series of reverse B t discharges with the OSP positioned on the ertor inserts due to strong E × B drift transport of C from the inner to the outer SP through the private flux region. The variation in band radius (and hence the magnetic SP) is a (4.89 ± 0.31) mm shift toward (310 ± 4)° toroidal direction. These measurements agree well with previous measurements of the 3D magnetic field distribution (Luxon 2003 Nucl. Fusion 43 1813), simulations performed by the mafot field line integration code, and recent Langmuir probe measurements in the small-angle-slot (SAS) ertor (Watkins et al 2019 Nucl. Mater. Energy 18 46). Comparison of these measurements in the SAS ertor also indicates that there is the possibility of a tilt (in conjunction with the shift) of the B t coil field of (0.04 ± 0.07)° towards the toroidal angle of (215 ± 25)°. Previous measurements suggested a field misalignment of (4.6 ± 0.3) mm in the 270° toroidal direction, and a tilt of (0.06 ± 0.02)° toward the 114° toroidal direction, which is similar to the results reported here. These studies will be important for better understanding the radial variation of the toroidal strike line in DIII-D, for designing the new generation of SAS ertor, and for developing an understanding of the impact of error fields on tokamaks with tightly baffled slot ertors.
Publisher: American Physical Society (APS)
Date: 12-03-2015
Publisher: AIP Publishing
Date: 05-2015
DOI: 10.1063/1.4918359
Location: United States of America
Location: Russian Federation
No related grants have been discovered for Dmitri Orlov.