Alfvén eigenmodes in reversed shear plasmas in JT-60U negative-ion-based neutral beam injection discharges

Takechi, M.; Fukuyama, A.; Ishikawa, Masatoshi; Cheng, C. Z.; Shinohara, K.; Ozeki, T.; Kusama, Y.; Takeji, S.; Fujita, T.; Oikawa, T.; Suzuki, Takahiro; Oyama, N.; Morioka, A.; Gorelenkov, N. N.; Kramer, G. J.; Nazikian, R.

doi:10.1063/1.1938973

Cited by 48 publications

(63 citation statements)

References 24 publications

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“…Figure 4 illustrates how an n = 1 instability sweeps up through the range 40-90 kHz while another sweeps down through 130-90 kHz over the period from 6-6.5 s, when both merge into a single mode [136]; the mode amplitude is enhanced when the frequency chirping saturates. The kinetic full-wave code TASK/WM has been used to interpret the measurements as reversed shear induced AEs (RSAE, an alternative name for Alfvén cascades) located immediately below the m = 2 and above the m = 3 Alfvén continua merging into a (3,2) TAE where q MIN ∼ = 2.5 [137].…”

Section: Identification Of Alfvénic Modes In Different Operating Scenmentioning

confidence: 99%

Chapter 5: Physics of energetic ions

et al. 2007

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This chapter reviews the progress accomplished since the redaction of the first ITER Physics Basis Nucl. Fusion 39 2137 in the field of energetic ion physics and its possible impact on burning plasma regimes. New schemes to create energetic ions simulating the fusion-produced alphas are introduced, accessing experimental conditions of direct relevance for burning plasmas, in terms of the Alfvénic Mach number and of the normalised pressure gradient of the energetic ions, though orbit characteristics and size cannot always match those of ITER. Based on the experimental and theoretical knowledge of the effects of the toroidal magnetic field ripple on direct fast ion losses, ferritic inserts in ITER are expected to provide a significant reduction of ripple alpha losses in reversed shear configurations. The nonlinear fast ion interaction with kink and tearing modes is qualitatively understood, but quantitative predictions are missing, particularly for the stabilisation of sawteeth by fast particles that can trigger neoclassical tearing modes. A large database on the linear stability properties of the modes interacting with energetic ions, such as the Alfvén eigenmode has been constructed. Comparisons between theoretical predictions and experimental measurements of mode structures and drive/damping rates approach a satisfactory degree of consistency, though systematic measurements and theory comparisons of damping and drive of intermediate and high mode numbers, the most relevant for ITER, still need to be performed. The nonlinear behaviour of Alfvén eigenmodes close to marginal stability is well characterized theoretically and experimentally, which gives the opportunity to extract some information on the particle phase space distribution from the measured instability spectral features. Much less data exists for strongly unstable scenarios, characterised by nonlinear dynamical processes leading to energetic ion redistribution and losses, and identified in nonlinear numerical simulations of Alfvén eigenmodes and energetic particle modes. Comparisons with theoretical and numerical analyses are needed to assess the potential implications of these regimes on burning plasma scenarios, including in the presence of a large number of modes simultaneously driven unstable by the fast ions.

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Section: Identification Of Alfvénic Modes In Different Operating Scenmentioning

confidence: 99%

Chapter 5: Physics of energetic ions

et al. 2007

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“…It was demonstrated that MHD theory can explain frequency chirping behavior of so-called Alfvén cascades observed in reversed shear plasmas. [18][19][20] Recently, the ␣ particle driven Alfvén eigenmode observed in the Tokamak fusion test reactor was reexamined and explained in terms of the cylindricallike MHD eigenmode. 21 In Ref.…”

Section: Introductionmentioning

confidence: 99%

Nonlocal energetic particle mode in a JT-60U plasma

Todo

Shinohara²,

Takechi³

et al. 2004

Physics of Plasmas

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Energetic-ion driven instability in a Japan Atomic Energy Research Institute Tokamak-60 Upgrade (JT-60U) [S. Ishida et al., Phys. Plasmas 11, 2532(2004] plasma was investigated using a simulation code for magnetohydrodynamics and energetic particles. The spatial profile of the unstable mode peaks near the plasma center where the safety factor profile is flat. The unstable mode is not a toroidal Alfvén eigenmode (TAE) because the spatial profile deviates from the expected location of TAE and the spatial profile consists of a single primary harmonic m / n =2/1 where m and n are poloidal and toroidal mode numbers. The real frequency of the unstable mode is close to the experimental starting frequency of the fast frequency sweeping mode. Simulation results demonstrate that energetic-ion orbit width and energetic-ion pressure significantly broaden radial profile of the unstable mode. For the smallest value among the investigated energetic-ion orbit width, the unstable mode is localized within 20% of the minor radius. This gives an upper limit of the spatial profile width of the unstable mode which the magnetohydrodynamic effects alone can induce. For the experimental condition of the JT-60U plasma, energetic ions broaden the radial width of the unstable mode spatial profile by a factor of 3. The unstable mode is primarily induced by the energetic particles.

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“…RSAEs and characteristic frequency sweeping were observed in many tokamaks during ion cyclotron resonance frequency (ICRF) heating and NBI in the current ramp-up phase [23][24][25][26][27][28][29][30][31]. The RSAE frequency is swept unidirectionally, that is, upward from a certain minimum when q min decreases in time from a rational value.…”

Section: Aes In Reversed Magnetic Shear Plasmasmentioning

confidence: 99%

“…In RS tokamak plasmas, characteristic energetic-iondriven modes called RSAEs or Alfvén cascade modes are observed [23][24][25][26][27][28][29][30][31]. Moreover, a GAM having an n = 0 mode structure is sometimes excited by energetic ions [32,33].…”

Section: Aes In Reversed Magnetic Shear Plasmasmentioning

confidence: 99%

Energetic-Ion-Driven Global Instabilities Observed in the Large Helical Device and Their Effects on Energetic Ion Confinement

Toi

2013

Plasma and Fusion Research

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This paper reviews various global instabilities destabilized by tangential neutral beam injection (NBI) in the Large Helical Device (LHD) plasmas. These global modes are toroidal Alfvén eigenmodes (TAEs), which are also observed in tokamak plasmas, and helicity-induced Alfvén eigenmodes (HAEs) which are observed only in three-dimensional plasmas such as LHD plasmas. Moreover, reversed magnetic shear Alfvén eigenmodes (RSAEs) are observed in a reversed magnetic shear (RS) plasma in the LHD, where the sign of the magnetic shear changes from positive in the plasma central region to negative in the plasma peripheral region. The RSAEs exhibit a characteristic frequency sweeping due to temporal evolution of the rotational transform profile. In the RS plasma, the energetic-ion-driven geodesic acoustic mode (GAM) is also excited. The GAM interacts nonlinearly with the RSAEs and generates a multitude of frequency sweeping modes through a three-wavecoupling process. The TAEs and GAM exhibit various types of nonlinear evolution, that is, pitchfork splitting and rapid frequency chirp-up and/or chirp-down. The linear and nonlinear characteristics of these energetic-iondriven global instabilities in the LHD are compared with those observed in tokamak plasmas. TAE bursts having rapid frequency chirp-down induce redistribution and/or loss of energetic ions. Future important issues are briefly described.

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Alfvén eigenmodes in reversed shear plasmas in JT-60U negative-ion-based neutral beam injection discharges

Cited by 48 publications

References 24 publications

Chapter 5: Physics of energetic ions

Chapter 5: Physics of energetic ions

Nonlocal energetic particle mode in a JT-60U plasma

Energetic-Ion-Driven Global Instabilities Observed in the Large Helical Device and Their Effects on Energetic Ion Confinement

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