Local magnetic shear control in a tokamak via fast wave minority ion current drive: theory and experiments in JET

Bhatnagar, V.P.; Start, D.; Jacquinot, J.; Chaland, F.; Cherubini, Anna Maria; Porcelli, Francesco

doi:10.1088/0029-5515/34/12/i04

Cited by 66 publications

(113 citation statements)

References 18 publications

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“…It is shown that when a counterpropagating wave is deposited sufficiently accurately on the high field side, the fast ion effect is so strong that the internal kink mode is driven ideally unstable, and this in turn is consistent with the observed sawteeth [7,9] that are much shorter in period than those obtained in Ohmic plasmas. This should be contrasted with the classical [10] sawtooth control mechanism relating to the change in the magnetic shear, due to ICCD, which leads to a moderate effect on the threshold of an instability to resistive MHD (e.g., [19]), but with no realistic recourse to ideal instability. Furthermore, unlike the classical sawtooth control mechanism, the fast ion mechanism is independent of the electron drag, which is expected [20] to limit the current drive efficiency of the proposed ICRF system for ITER [21].…”

Section: Prl 102 065005 (2009) P H Y S I C a L R E V I E W L E T T Ementioning

confidence: 99%

“…Hence, hj 0 ðrÞi and hj 1 ðrÞi can be identified with, respectively, the passing and trapped ion currents, before the bulk plasma drag [10] has been subtracted. The asymmetry function cðr; y 2 Þ can be resolved if the passing ion current contribution is known.…”

Section: Prl 102 065005 (2009) P H Y S I C a L R E V I E W L E T T Ementioning

confidence: 99%

“…Such is the sensitivity to the location of deposition, and the magnitude of the effect, that this fast ion mechanism, i.e., non-MHD mechanism, dominates over the previously assumed classical mechanism (e.g., Refs. [9][10][11]) relating to the change in the magnetic shear due to the fast ions [12], and the resulting effect on MHD stability.…”

mentioning

confidence: 99%

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Sawtooth-Control Mechanism using Toroidally Propagating Ion-Cyclotron-Resonance Waves in Tokamaks

et al. 2009

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The sawtooth control mechanism in plasmas employing off-axis toroidally propagating ion cyclotron resonance waves in tokamaks is reinvestigated. The radial drift excursion of energetic passing ions distributed asymmetrically in the velocity parallel to the magnetic field determines stability when the rational q ¼ 1 surface resides within a narrow region centered about the shifted fundamental cyclotron resonance. Magnetohydrodynamic (MHD) stability of plasmas in the presence of energetic ions is a crucial issue for present and future large tokamak experiments. Such ions include 3.5 MeV fusion alpha particles, and energetic minority ions produced by auxiliary heating methods such as from ion cyclotron resonance frequency (ICRF) waves. Ions trapped outside the region of highest magnetic field strength have been shown [1] to stabilize a key core instability known as the sawtooth, located within the q ¼ 1 rational surface, thereby lengthening the period between sequential soft-xray relaxations [2]. Without an effective means of shortening the period of sawteeth, coupling can occur with instabilities located at rational surfaces closer to the tokamak edge. Indeed, interaction of long sawteeth and performance degrading neoclassical tearing modes has been observed [3] in the Joint European Torus (JET), while improved plasma confinement is often found to coincide with small regular sawteeth.Under certain conditions untrapped, or passing, energetic ions can also strongly influence sawteeth. Sawtooth control from energetic ions injected with near tangential unbalanced neutral beams has already been demonstrated analytically, for deeply passing ions [4], numerically and experimentally [5]. The mechanism responsible was found to be due to the contribution on the n ¼ 1 internal kink mode of passing particles intersecting the q ¼ 1 rational surface. An important fast ion effect is obtained when the distribution function of passing ions is asymmetrically distributed in the velocity parallel to the magnetic field. By extending the analysis of Ref.[4] to solve for the internal kink mode across all velocity space, including barely passing trajectories, it is shown in the present contribution that the sawtooth control mechanism responsible for localized off-axis toroidally propagating ICRF waves is essentially the same as for unbalanced neutral beam injection (NBI) scenarios. Moreover, the propagating ICRF waves are more effective than unbalanced NBI because the orbit widths of the energetic ions are larger, and the parallel asymmetry of the distribution function is more strongly radially sheared. Simulations using SELFO [6] for the ICRF wave field and distribution function are applied to a key demonstration JET discharge [7] with localized off-axis ion cyclotron current drive (ICCD) counter to the Ohmic current. Analytical and full numerical calculations [8] of the internal kink mode with the simulated JET ICRF distribution function [9] demonstrate ideal instability when the deposition of the resonating ions is very close to the ...

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Section: Prl 102 065005 (2009) P H Y S I C a L R E V I E W L E T T Ementioning

confidence: 99%

Section: Prl 102 065005 (2009) P H Y S I C a L R E V I E W L E T T Ementioning

confidence: 99%

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Sawtooth-Control Mechanism using Toroidally Propagating Ion-Cyclotron-Resonance Waves in Tokamaks

et al. 2009

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“…b͒ Invited speaker. c͒ nificantly reduced by choosing 3 He minority ICRH, since the current dragged 16,13 by the background plasma tends to cancel the 3 He current in the relevant region of the tokamak ͑provided that the effective charge of the plasma is close to that of 3 He, where Z ef f Ͻ 2 typically͒. It is shown that sawtooth control employing low concentration minority 3 He is nevertheless effective, thus strengthening the likelihood of the dominant effect of the fast ion mechanism, 9 and moreover, demonstrating the viability of sawtooth control using ICRH in ITER, 1 which is primarily and routinely expected to employ 3 He minority.…”

Section: Introductionmentioning

confidence: 99%

A new sawtooth control mechanism relying on toroidally propagating ion cyclotron resonance frequency waves: Theory and Joint European Torus tokamak experimental evidence

Graves¹,

Chapman²,

Coda³

et al. 2010

Physics of Plasmas

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The sawtooth control mechanism in plasmas employing toroidally propagating ion cyclotron resonance waves is extended. The asymmetrically distributed energetic passing ions are shown to modify the ideal internal kink mode when the position of the minority ion cyclotron resonance resides within a narrow region close to the q = 1 surface. An analytical treatment of the internal kink mode in the presence of model distribution function with parallel velocity asymmetry is developed. The fast ion mechanism explains the strong sensitivity of sawteeth to resonance position, and moreover is consistent with dedicated Joint European Torus ͓F. Romanelli, Nucl. Fusion 49, 104006 ͑2009͔͒ experiments which controlled sawteeth despite negligible current drive.

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“…5). The sawtooth period has been varied in ICRF-only discharges by more than one order of magnitude due to changes in the magnetic shear near q = 1, using hydrogen minority (ω ≈ ω cH ) ICCD [18][19][20][21]. Experiments have also been carried out with directed ICRF waves tuned to the second harmonic hydrogen resonance (ω ≈ 2ω cH ) to affect the sawtooth period and amplitude in discharges with ICRF only and in discharges with combined ICRF and NBI [3,[22][23][24].…”

Section: Modifications Via Ion Cyclotron Current Drivementioning

confidence: 99%

Modification of Sawtooth Oscillations with ICRF Waves in the JET Tokamak

Mantsinen

Alper

Angioni

et al. 2007

AIP Conference Proceedings

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Methods of modifying sawtooth oscillations using waves in the ion cyclotron range of frequencies (ICRF) in the JET tokamak are presented. Examples of sawtooth stabilization by ICRF-accelerated high-energy ions are shown, including experiments with ICRF-acceleration of 4 He-beam ions to simulate the effects of fusion born alpha particles. With high power ICRF heating in low-density plasmas, fast ion stabilization of sawteeth is lost and a new type of small-period and small-amplitude sawteeth appears. ICRF-induced radial pinch with toroidally asymmetric waves is found to be useful in affecting the radial profile of the ICRF-driven fast ion populations and thereby their influence on sawteeth. Ion cyclotron current drive (ICCD) applied close to the sawtooth inversion radius is effective in modifying the sawtooth period. The latest achievements include the successful application of ICCD to shorten the fast-ioninduced long-period sawteeth and thereby avoid triggering of neoclassical tearing modes (NTMs).

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Local magnetic shear control in a tokamak via fast wave minority ion current drive: theory and experiments in JET

Cited by 66 publications

References 18 publications

Sawtooth-Control Mechanism using Toroidally Propagating Ion-Cyclotron-Resonance Waves in Tokamaks

Sawtooth-Control Mechanism using Toroidally Propagating Ion-Cyclotron-Resonance Waves in Tokamaks

A new sawtooth control mechanism relying on toroidally propagating ion cyclotron resonance frequency waves: Theory and Joint European Torus tokamak experimental evidence

Modification of Sawtooth Oscillations with ICRF Waves in the JET Tokamak

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