Internal transport barrier triggering by rational magnetic flux surfaces in tokamaks

Joffrin, E.; Challis, C.; Conway, G. D.; Garbet, X.; Gude, A.; Günter, S.; Hawkes, N.; Hender, T. C.; Howell, D. F.; Huysmans, G.; Lazzaro, E.; Maget, P.; Marachek, M.; Peeters, A. G.; Pinches, S. D.; Sharapov, S. E.; Contributors, Jet‐Efda

doi:10.1088/0029-5515/43/10/018

Cited by 144 publications

(173 citation statements)

References 42 publications

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“…The presence of several low order rational q surfaces in this region may echo results found on several other experiments connecting ITB formation to the presence of specific low order rational q surfaces in the plasma, although it lies well inside of the q=2 and q=3/2 surfaces which have been important in other devices. 24 The rise of the central density and impurity accumulation in the ITB phase of the discharge are clamped by the addition of small amounts of central ICRF power, keeping the central Zeff level typically below 2. To date, however, exceeding a certain threshold power destroys the ITB profile.…”

Section: Discussionmentioning

confidence: 99%

Control of internal transport barriers on Alcator C-Mod

et al. 2004

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Recent studies of internal transport and double transport barrier regimes in Alcator C-Mod [I. H. Hutchinson, et al., Phys. Plasmas 1, 1511] have explored the limits for forming, maintaining, and controlling these plasmas. C-Mod provides a unique platform for studying such discharges: the ions and electrons are tightly coupled by collisions and the plasma has no internal particle or momentum sources. The double-barrier mode comprised of an edge barrier with an internal transport barrier (ITB) can be induced at will using off-axis ion cyclotron range of frequency (ICRF) injection on either the low or high field side of the plasma with either of the available ICRF frequencies (70 or 80 MHz). When enhanced D α high confinement mode (EDA H-mode) is accessed in Ohmic plasmas, the double barrier ITB forms spontaneously if the Hmode is sustained for ~2 energy confinement times. The ITBs formed in both Ohmic and ICRF heated plasmas are quite similar regardless of the trigger method. They are characterized by strong central peaking of the electron density, and reduction of the core particle and energy transport. Control of impurity influx and heating of the core plasma in the presence of the ITB have been achieved with the addition of central ICRF power in both Ohmic H-mode and ICRF induced ITBs. The radial location of the particle transport is dependent on the toroidal magnetic field but not on the location of the ICRF resonance. A narrow region of decreased electron thermal transport, as determined by sawtooth heat pulse analysis, is found in these plasmas as well. Transport analysis indicates that reduction of the particle diffusivity in the barrier region allows the neoclassical pinch to drive the density and impurity accumulation in the plasma center. Examination of the gyrokinetic stability at the trigger time for the ITB suggests that the density and temperature profiles are inherently stable to ion temperature gradient (ITG) and trapped electron (TEM) modes in the core inside of the ITB location.

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Section: Discussionmentioning

confidence: 99%

Control of internal transport barriers on Alcator C-Mod

et al. 2004

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“…Within the uncertainties of the measurement one finds that the minimum q value, q min , in the plasma is just below 2. So-called grand-cascades of Alfvén modes [18,8] have been seen in discharge 69670 at t=3.8s and another one at t~5s, while for 69668 these modes are only seen at t=3.7s. The integer q values that appear in the plasma and trigger these modes may be identified by q=3 and q=2 for the two times in 69670.…”

Section: Ex/8-3mentioning

confidence: 99%

“…This feature, identical to those observed in ref. [8,22], cannot be predicted by transport models since usually no role is given to rational surfaces. On the other hand rarefaction of rational flux surfaces, i.e.…”

Section: Ex/8-3mentioning

confidence: 99%

“…Nevertheless, studies identified two factors that play a crucial role: magnetic and rotational shear [5,6]. It was found for example that ITBs form more easily in plasmas that have a particular current density profile and safety factor, q, profile, such that there exists a region of low or even negative magnetic shear, r·q'/q [7], furthermore, the initial formation or triggering of ITBs, is often closely linked to detailed characteristics of the q profile [8]. The role of rotational shear, usually given as the shearing rate: ω ExB =RB θ /B φ · d(E r /RB θ )/dr, has been pointed out in experiments in JT-60U and TFTR which applied balance neutral beam injection (NBI) in order to create low torque plasmas with ITBs [9,10].…”

Section: Introductionmentioning

confidence: 99%

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Internal transport barrier dynamics with plasma rotation in JET

et al. 2009

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Abstract. At JET the dynamics of Internal Transport Barriers (ITBs) has been explored by trying to decouple the effects of heating on one hand and torque on the other with the ultimate objective of identifying the minimum torque required for the formation of transport barriers. The experiments shed light on the physics behind the initial trigger for ITBs, which often shows to be linked to the shape of the q profile and magnetic shear, while the further development was influenced by the strength of the rotational shear. In discharges with a small amount of rotational shear ITBs were triggered, which suggest that the rotational shear is not the dominant factor in the triggering process. However, the subsequent growth of the barrier was limited if the rotational shear was too low at the time of triggering. This growth phase may be highly non-linear, with several possible positive feedback loops, such as the increases of the toroidal and poloidal component of the rotational shear caused by the ITB itself.

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“…ITB's are not anything like LCS's, but indicate an experimental tokamak regime with locally, strongly reduced radial transport. 9,10 It is suggested that an ITB may be identified with broken KAM surfaces with noble values of the magnetic winding number q (Ref. 11) or that this reduced transport regime could be explained by the beneficial effect of the qprofile on the chaoticity due to a broad spectrum of magnetic perturbations.…”

Section: Phys Plasmas 18 102308 (2011)mentioning

confidence: 99%

Barriers in the transition to global chaos in collisionless magnetic reconnection. II. Field line spectroscopy

et al. 2011

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The transitional phase from local to global chaos in the magnetic field of a reconnecting current layer is investigated. The identification of the ridges in the field of the finite time Lyapunov exponent as barriers to the field line motion is carried out adopting the technique of field line spectroscopy to analyze the radial position of a field line while it winds its way through partial stochastic layers and to compare the frequencies of the field line motion with the corresponding frequencies of the distinguished hyperbolic field lines that are the nonlinear generalizations of linear X-lines.

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Internal transport barrier triggering by rational magnetic flux surfaces in tokamaks

Cited by 144 publications

References 42 publications

Control of internal transport barriers on Alcator C-Mod

Control of internal transport barriers on Alcator C-Mod

Internal transport barrier dynamics with plasma rotation in JET

Barriers in the transition to global chaos in collisionless magnetic reconnection. II. Field line spectroscopy

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