Thermal ions dilution and ITG suppression in ASDEX Upgrade ion ITBs

Tardini, G.; Hobirk, J.; Igochine, V.; Maggi, C. F.; Martin, P.; McCune, D.; Peeters, A. G.; Sips, A. C. C.; Stäbler, A.; Stöber, J.

doi:10.1088/0029-5515/47/4/006

Cited by 71 publications

(98 citation statements)

References 16 publications

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“…For n α /n e > 1.5%, the amplitude of ITG turbulence, evaluated (in line with the GENE's results) for all components with positive toroidal mode numbers n>20 as |δB r (ITG)| =(Σ n>20 |δB r (n)| decreases significantly when the αsh a v en o ty e tt h e rmalized (time-point T1). This result is consistent with theoretical predictions [32] and experimental observations [33]. For the data taken at the time-point T2, with fully thermalized αs, ITG turbulence has around four times larger amplitudes, decreasing only for n α /n e > 2.8%.…”

Section: -P2supporting

confidence: 91%

Evidence for a new path to the self-sustainment of thermonuclear fusion in magnetically confined plasmas

Testa¹,

Albergante²

2012

EPL

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In this work we provide the first explanation for observations made in 1997 on the Joint European Torus of unexpected ion heating with fusion-born alpha particles occurring over time scales much shorter than those theoretically foreseen. We demonstrate that non-thermal alpha particles above a critical concentration stabilize ion-drift-wave turbulence, therefore significantly reducing one of the main energy loss channels for thermal ions. As such ion heating occurs over times scales much shorter than those classically predicted, this mechanism opens new prospects on additional paths for the self-sustainment of thermonuclear fusion reactions in magnetically confined plasmas.

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Section: -P2supporting

confidence: 91%

Evidence for a new path to the self-sustainment of thermonuclear fusion in magnetically confined plasmas

Testa¹,

Albergante²

2012

EPL

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“…We also note that Tardini and his colleagues have similarly linked the formation of an ion transport barrier on the ASDEX-Upgrade tokamak to the suppression of ITG turbulence due to thermal ion dilution by the slowing-down population of fast NBI ions [53]. However, in our GENE simulations we have also included a fastion energy dependence on the ITG stabilization mechanism which is derived from the turbulence measurement, thus differentiating our work from that of [53]. For the data taken at the time-point T2 (maximum value of T e0 , figure 11(b)), the turbulence in the ion-drift direction (positive toroidal mode numbers) has larger amplitudes (by approximately a factor ∼2) and a larger spread (spectra centred around n ∼ 60 with a variance ∼35) than at the time-point T1 (the same T e0 ).…”

Section: Measurements Of the Ion And Electron Drift-wave Turbulence Smentioning

confidence: 78%

“…However, in [28] the role of tracer particles is discussed, whereas with our GENE simulations we have shown that fusion-born αs cannot always be considered as a tracer species. We also note that Tardini and his colleagues have similarly linked the formation of an ion transport barrier on the ASDEX-Upgrade tokamak to the suppression of ITG turbulence due to thermal ion dilution by the slowing-down population of fast NBI ions [53]. However, in our GENE simulations we have also included a fastion energy dependence on the ITG stabilization mechanism which is derived from the turbulence measurement, thus differentiating our work from that of [53].…”

Section: Measurements Of the Ion And Electron Drift-wave Turbulence Smentioning

confidence: 99%

A phenomenological explanation for the anomalous ion heating observed in the JET alpha-heating experiment of 1997

Testa¹,

Albergante²

2012

Nucl. Fusion

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In the so-called 'alpha-heating' experiment performed on the JET tokamak during the deuterium-tritium campaign of 1997, the ion temperature was found to be far exceeding (both in absolute value and in its rise time) the level that could have been expected from direct collisional heating by the fusion-born alpha particles themselves and energy equipartition with the electrons. To date, no explanation has been put forward for this long standing puzzle, despite much work having been performed on this subject in the early 2000s.Two analysis methods that have recently become available have been employed to re-analyse these observations of an anomalous ion heating. First, an algorithm based on the sparse representation of signals has been used to analyse magnetic, reflectometry and electron-cyclotron emission measurements of the turbulence spectra in the drift-wave range of frequencies. This analysis has then been complemented with turbulence simulations performed with the GENE code.We find, both experimentally and in the simulations, that the presence of a minority, but sufficiently large, population of fusion-born alpha particles that have not yet fully thermalized stabilizes the turbulence in the ion-drift direction, but practically does not affect the turbulence in the electron-drift direction. We link such stabilization of the ion-drift-wave turbulence to the increase in the ion temperature above the level achieved in similar discharges that did not have (at all or enough) alpha particles. When the fusion-born alpha particles have fully thermalized, the turbulence spectrum in the ion-drift direction reappears at somewhat larger amplitudes, which we link to the ensuing reduction in the ion temperature. This phenomenological dynamics fully corresponds to the actual experimental observations. By taking into account an effect of the alpha particles that had not been previously considered, our new analysis finally presents a phenomenological explanation for the so-far-unexplained anomalous ion heating observed in the JET alpha-heating experiment of 1997.Through the formulation of an empirical criterion for ion-drift-wave turbulence stabilization by fusion-born alpha particles, we also show why similar observations were not made in the other deuterium-tritium experiments run so far in JET and TFTR. This allows assessing the operational domain for this stabilization mechanism for ion-drift-wave turbulence in future burning plasma experiments such as ITER, which may open a new path towards the sustainment of a high energy gain in such forthcoming devices.

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“…One such mechanism is the dilution of the main ion species by the fast ions. In ASDEX Upgrade strong evidence has pointed to a fast ion dilution mechanism for ITB formation at low density [49]. In addition, an increase in local α = β ′ q 2 R due to fast ion suprathermal pressure gradients also stabilises ITG modes through electromagnetic effects.…”

Section: Inclusion Of Fast Particlesmentioning

confidence: 99%

Ion temperature profile stiffness: non-linear gyrokinetic simulations and comparison with experiment

et al. 2014

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Abstract. Recent experimental observations at JET show evidence of reduced ion temperature profile stiffness, hypothesised to be due to concomitant low magnetic shear (ŝ) and significant toroidal rotational flow shear. Non-linear gyrokinetic simulations are performed, aiming to investigate the physical mechanism behind the observations. A comprehensive set of simulations are carried out, comparing the impact on the ion heat flux of various parameters that differ within the data-set. These parameters include q,ŝ, rotation, effect of rotation on the magnetohydrodynamic (MHD) equilibrium, R/L n , β e , Z ef f , and the fast particle content. The effect of toroidal flow shear itself is not predicted by the simulations to lead to a significant reduction in ion heat flux, due both to an insufficient magnitude of flow shear and significant parallel velocity gradient destabilisation. It is however found that non-linear electromagnetic effects due to both thermal and fast-particle pressure gradients, even at low β e , can significantly reduce the profile stiffness. A total of five discharges are examined, at both inner and outer radii. For all cases studied, the simulated and experimental ion heat flux values agree within reasonable variations of input parameters around the experimental uncertainties.PACS numbers: 52.30. Gz, 52.35.Ra, 52.55.Fa, 52.65.Tt Ion temp. profile stiffness: non-lin. gyrokinetic simu. and comparison with exp.

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Thermal ions dilution and ITG suppression in ASDEX Upgrade ion ITBs

Cited by 71 publications

References 16 publications

Evidence for a new path to the self-sustainment of thermonuclear fusion in magnetically confined plasmas

Evidence for a new path to the self-sustainment of thermonuclear fusion in magnetically confined plasmas

A phenomenological explanation for the anomalous ion heating observed in the JET alpha-heating experiment of 1997

Ion temperature profile stiffness: non-linear gyrokinetic simulations and comparison with experiment

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