Phase contrast imaging measurements of reversed shear Alfvén eigenmodes during sawteeth in Alcator C-Mod

Edlund, E.; Porkoláb, M.; Kramer, G. J.; Lin, L.; Lin, Y.; Wukitch, S.J.

doi:10.1063/1.3086869

Cited by 12 publications

(22 citation statements)

References 31 publications

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“…In an ICRF heated plasma with giant sawtooth crashes on JT-60U, q 0 was evaluated just before and after the crash with the help of MHD spectroscopy using EP driven TAEs and EAEs which were thought to be of a core-localized nature [479]. Here, q 0 was successfully predicted from the time sequence of multiple TAEs and EAEs with different toroidal numbers n. On C-Mod, RSAEs excited near the q = 1 rational surface in ICRF heated plasmas were observed by a phase contrast imaging system with a CO 2 laser beam [480]. These RSAEs also exhibited a characteristic frequency sweeping across the sawtooth crash.…”

Section: Mhd Spectroscopymentioning

confidence: 99%

Energetic particle physics in fusion research in preparation for burning plasma experiments

2014

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The area of energetic particle (EP) physics in fusion research has been actively and extensively researched in recent decades. The progress achieved in advancing and understanding EP physics has been substantial since the last comprehensive review on this topic by Heidbrink and Sadler (1994 Nucl. Fusion 34 535). That review coincided with the start of deuterium-tritium (DT) experiments on the Tokamak Fusion Test Reactor (TFTR) and full scale fusion alphas physics studies. Fusion research in recent years has been influenced by EP physics in many ways including the limitations imposed by the 'sea' of Alfvén eigenmodes (AEs), in particular by the toroidicity-induced AE (TAE) modes and reversed shear AEs (RSAEs). In the present paper we attempt a broad review of the progress that has been made in EP physics in tokamaks and spherical tori since the first DT experiments on TFTR and JET (Joint European Torus), including stellarator/helical devices. Introductory discussions on the basic ingredients of EP physics, i.e., particle orbits in STs, fundamental diagnostic techniques of EPs and instabilities, wave particle resonances and others, are given to help understanding of the advanced topics of EP physics. At the end we cover important and interesting physics issues related to the burning plasma experiments such as ITER (International Thermonuclear Experimental Reactor).

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Section: Mhd Spectroscopymentioning

confidence: 99%

Energetic particle physics in fusion research in preparation for burning plasma experiments

2014

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“…As a final note we would like to mention that the presence of q > 1 following the sawtooth crash may allow for down-chirping RSAEs to exist. This possibility has been explored with NOVA with the result that relatively undamped and unstable solutions have been discovered [24]. The existence of the down-chirping solutions seems to require a local maximum in q near the core, equivalent to a peak in the Alfvén continuum, around which the RSAEs form.…”

Section: Prl 102 165003 (2009) P H Y S I C a L R E V I E W L E T T Ementioning

confidence: 99%

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

Porkoláb

Kramer

et al. 2009

Phys. Rev. Lett.

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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...

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“…NOVA is an ideal MHD eigenvalue solver and TAEFL is non-perturbative gyrofluid code that can be run as initial value or eigenvalue solver. Both of these codes have successfully modeled AEs in many DIII-D plasmas [2,37,45,46], as well as other devices including JT60 [62], TFTR [63], JET [28], NSTX [64], CMOD [29] and ASDEX-Upgrade [50].…”

Section: Nova and Taefl Simulationsmentioning

confidence: 99%

Electron cyclotron heating can drastically alter reversed shear Alfvén eigenmode activity in DIII-D through finite pressure effects

et al. 2016

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A recent DIII-D experiment investigating the impact of electron cyclotron heating (ECH) on neutral beam driven reversed shear Alfvén eigenmode (RSAE) activity is presented. The experiment includes variations of ECH injection location and timing, current ramp rate, beam injection geometry (on/off-axis), and neutral beam power. Essentially all variations carried out in this experiment were observed to change the impact of ECH on AE activity significantly. In some cases, RSAEs were observed to be enhanced with ECH near the off-axis minimum in magnetic safety factor (q min ), in contrast to the original DIII-D experiments where the modes were absent when ECH was deposited near q min . It is found that during intervals when the geodesic acoustic mode (GAM) frequency at q min is elevated and the calculated RSAE minimum frequency, including contributions from thermal plasma gradients, is very near or above the nominal TAE frequency ( f TAE ), RSAE activity is not observed or RSAEs with a much reduced frequency sweep range are found. This condition is primarily brought about by ECH modification of the local electron temperature (T e ) which can raise both the local T e at q min as well as its gradient. A q-evolution model that incorporates this reduction in RSAE frequency sweep range is in agreement with the observed spectra and appears to capture the relative balance of TAE or RSAE-like modes throughout the current ramp phase of over 38 DIII-D discharges. Detailed ideal MHD calculations using the NOVA code show both modification of plasma pressure and pressure gradient at q min play an important role in modifying the RSAE activity. Analysis of the ECH injection near the q min case where no frequency sweeping RSAEs are observed shows the typical RSAE is no longer an eigenmode of the system. What remains is an eigenmode with poloidal harmonic content reminiscent of the standard RSAE, but absent of the typical frequency sweeping behavior. The remaining eigenmode is also often strongly coupled to gap TAEs. Analysis with the non-perturbative gyro fluid code TAEFL confirms this change in RSAE activity and also shows a large drop in the resultant mode growth rates.Nuclear Fusion

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Phase contrast imaging measurements of reversed shear Alfvén eigenmodes during sawteeth in Alcator C-Mod

Cited by 12 publications

References 31 publications

Energetic particle physics in fusion research in preparation for burning plasma experiments

Energetic particle physics in fusion research in preparation for burning plasma experiments

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

Electron cyclotron heating can drastically alter reversed shear Alfvén eigenmode activity in DIII-D through finite pressure effects

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