Observation of Reversed Shear Alfvén Eigenmodes between Sawtooth Crashes in the Alcator C-Mod Tokamak

Abstract

Groups of frequency chirping modes observed between sawtooth crashes in the Alcator C-Mod tokamak are interpreted as reversed shear Alfvén eigenmodes near the q ¼ 1 surface. These modes indicate that a reversed shear q profile is generated during the relaxation phase of the sawtooth cycle. Two important parameters, q min and its radial position, are deduced from comparisons of measured density fluctuations with calculations from the ideal MHD code NOVA. These studies provide valuable constraints for further modeling of the sawtooth cycle. The sawtooth cycle has long been recognized as a robust feature of tokamak plasmas [1]. The cycle is composed of an abrupt crash in the central electron temperature and a relaxation, or reheat, phase (see Fig. 1, t ! 0:25 s). The sawtooth crash is a process whereby the magnetic field lines interior to the q ¼ 1 surface (where q ¼ rB =R 0 B is the magnetic safety factor, r is the minor radial coordinate, B is the toroidal magnetic field, R 0 is the tokamak major radius, and B is the poloidal magnetic field) reconnect to lines outside of the q ¼ 1 surface. In this process the magnetic field ''frozen-in'' laws are broken leading to rapid radial transport wherein the temperature, density, and current profiles are flattened [2][3][4]. Following the cessation of magnetic reconnection, the plasma relaxation phase is characterized by a nearly linear increase in the central electron temperature and an inward diffusion of parallel current formerly displaced by the reconnection. Understanding the evolution of the current profile and its relation to sawtooth phenomena is important, especially for future burning plasma experiments such as ITER, where an accurate model of the physics of the crash event and the subsequent relaxation process is crucial for developing predictive capability for sawtooth control. In this Letter we report observations of frequency chirping Alfvénic modes excited during the sawtooth cycle in Alcator C-Mod tokamak plasmas, herein identified as reversed shear Alfvén eigenmodes (RSAEs, also Alfvén cascades) located near the q ¼ 1 surface. RSAEs should be of great interest for future burning plasma experiments both for their use as a form of MHD spectroscopy and because Alfvénic modes may negatively impact the confinement of the energetic ions in the plasma core [5]. Importantly, the presence of RSAEs near the q ¼ 1 surface is interpreted as evidence of a hollow current profile during the sawtooth cycle.The phase contrast imaging (PCI) diagnostic [6], a type of internal reference beam interferometer, is an outstanding tool for the study of core localized Alfvénic activity. The output signal of the PCI system is a 1D image, decomposed into 32 elements of approximately 4.5 mm chord separation in the direction of the major radius, which is linearly proportional to the line integral of the electron density perturbations along the beam path. The reporting of q ¼ 1 RSAEs in Alcator C-Mod, and also in JET [7,8] (see also [9]), is due to the availability of diagnostics sensiti...

Convective beam ion losses due to Alfvén eigenmodes in DIII-D reversed-shear plasmas

“…Future work should perform simulations using a larger number of Monte Carlo particles with the aim of providing adequate statistics in the perturbation field of a single RSAE. Given that RSAEs can be excited due to the equilibrium perturbation of sawtooth crashes [42], it is important to consider that they may be capable of contributing to the energetic ion transport observed in sawtoothing plasmas [43,44]. Ongoing investigations of sawtooth-induced transport [45,46] will be able to incorporate energetic ion loss measurements and core transport measurements simultaneously.…”

Asymmetric radiative damping of low shear toroidal Alfvén eigenmodes

“…The important role of low shear TAEs as triggers of monster sawtooth crashes was underlined in experiments on TFTR, 15 DIII-D, 16 JET, 17,18 and C-MOD. 19 Moreover, in addition to the sawtoothing low shear scenarios typical for present-day tokamaks, novel so called hybrid scenarios of enhanced plasma performance were established as good candidates for burning ITER plasmas. 20 These scenarios also require extended regions of low shear, typically satisfying s 2 .…”

Low-frequency Alfvén gap modes in rotating tokamak plasmas

“…The radial group velocity of these standing waves vanishes because of interference with either a counterpropagating wave or with its own reflection off some 'potential well' in the plasma [12]. An experimentally well-established example of the first type is the toroidal Alfvén eigenmode (TAE) [13] while the latter category includes the global Alfvén eigenmode [14] and the reversed-shear Alfvén eigenmode (RSAE) [15][16][17]. Global modes called beta-induced Alfvén eigenmodes (BAEs) [8,18] can exist near the GAM frequency.…”