Rotation profile flattening and toroidal flow shear reversal due to the coupling of magnetic islands in tokamaks
[2];
[1];
[2];
- Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
- Univ. of California, Davis, CA (United States)
- DIFFER, Rhinjuizen (Netherlands). Dutch Inst. for Fundamental Fusion Energy Research
- Univ. of Texas, Austin, TX (United States)
- General Atomics, San Diego, CA (United States)
The electromagnetic coupling of helical modes, even those having different toroidal mode number, modifies the distribution of toroidal angular momentum in tokamak discharges. This can have deleterious effects on other transport channels as well as on magnetohydrodynamic (MHD) stability and disruptivity. Analogous to entrainment in an external field, modes that are nearly stationary with respect to one another exert a mutual force that encourages alignment in their mutually least stable orientation. Furthermore, nonlinear torque develops amongst rotating modes that slip rapidly past each other toroidally. These torques modify the distribution of toroidal angular momentum such that differential rotation of the fluid at the two rational surfaces decays, ultimately bifurcating to a phase-locked state near half the initial slip frequency. At low levels of externally injected momentum, the coupling of core-localized modes initiates a chain of events whereby flattening of the core rotation profile inside successive rational surfaces leads to the onset of a large m/n=2/1 tearing mode and locked-mode disruption. However, with increased torque from neutral beam injection (NBI), neoclassical tearing modes (NTMs) in the core may phase-lock to each other without locking to external fields or structures that are stationary in the laboratory frame. This allows for momentum transport analysis to be performed, revealing a significant torque density that peaks near the 2/1 rational surface. As the coupled rational surfaces are brought closer together by reducing q95, additional momentum transport in excess of that required to attain a phase-locked state is sometimes observed. Rather than maintaining zero differential rotation (as is predicted to be dynamically stable by single-fluid, resistive MHD theory), these discharges develop hollow toroidal plasma fluid rotation profiles with reversed plasma flow shear in the region between the m/n = 3/2 and 2/1 islands. Analysis of these data highlights the impact of mode coupling on torque balance and the challenges associated with predicting the rotation dynamics of a fusion reactor—a key issue for ITER.
- Research Organization:
- Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States); General Atomics, San Diego, CA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Fusion Energy Sciences (FES)
- Grant/Contract Number:
- AC02-09CH11466; FC02-04ER54698; FG02-99ER54531; SC0012551
- OSTI ID:
- 1295403
- Alternate ID(s):
- OSTI ID: 1247892; OSTI ID: 1372490
- Journal Information:
- Physics of Plasmas, Vol. 23, Issue 5; Conference: 57th Annual Meeting of the APS-Division-of-Plasma-Physics (DPP) , Savannah, GA (United States), 16-20 Nov 2015; ISSN 1070-664X
- Publisher:
- American Institute of Physics (AIP)Copyright Statement
- Country of Publication:
- United States
- Language:
- English
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