Extended electron tails in electrostatic microinstabilities and the nonadiabatic response of passing electrons

Hardman, Michael; Parra, Félix I.; Chong, Ching Lok; Adkins, T.; Tzanis, M S Anastopoulos; Barnes, Michael; Dickinson, David; Parisi, Jason; Wilson, H. R.

doi:10.1088/1361-6587/ac4e9e

Cited by 11 publications

(43 citation statements)

References 62 publications

(205 reference statements)

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“…These ETG modes, which are unstable at k y ρ s 1 and very extended along the magnetic field line, are similar to the ones characterised in Ref. [34]. We note that decreasing the collisionality decreases the electron detrapping frequency and therefore increases the drive at lower k y and ω values, as shown in appendix A of Ref.…”

Section: Appendix C Binormal Ion Scale Etg Instabilitysupporting

confidence: 85%

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Numerical investigation of microtearing modes in the core of experimentally relevant spherical tokamak scenarios

Giacomin¹,

Dickinson²,

Kennedy³

et al. 2023

Preprint

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Linear and nonlinear gyrokinetic simulations are performed in experimentally relevant scenarios built from a MAST case where a microtearing mode instability dominates at ion Larmor radius scale (these microtearing modes are only weakly unstable and nonlinear simulations indicate they do not cause an experimentally significant level of transport across the chosen surface). This collisional microtearing mode instability appears only when a velocity dependent electron collision frequency is considered. Electrostatic potential fluctuations are found to provide a strong destabilising mechanism. The sensitivity to the electron collision frequency is investigated in both linear and nonlinear simulations. While the effect of electron collision frequency is moderate in linear simulations, a strong dependence on this parameter is found in nonlinear simulations. The effect of magnetic islands generated by microtearing modes and their interaction is analysed, showing that the radial extension of the stochastic region caused by islands overlapping plays an important role in determining the saturation level of the microtearing mode driven heat flux. Nonlinear simulations on the chosen surface find that the radial extend of the stochastic region and the mocrotearing mode heat flux both increase with decreasing electron collision frequency. The magnetic shear is found to play an important role in the formation of a stochastic layer.1/2 e ] (n e is the electron density, λ is the Coulomb logarithm, T e is the electron temperature and m e is the electron mass): a collisionless

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Section: Appendix C Binormal Ion Scale Etg Instabilitysupporting

confidence: 85%

“…The instability appearing at large a/L Te is an ETG mode characterised by k y ρ s ∼ 1 and k x ρ s > 1, which is similar to the long wavelength ETG instability described in Ref. [34] (see Appendix C for further details on this instability).…”

Section: Linear Characterisation Of the Mtm Instabilitysupporting

confidence: 67%

Numerical investigation of microtearing modes in the core of experimentally relevant spherical tokamak scenarios

Giacomin¹,

Dickinson²,

Kennedy³

et al. 2023

Preprint

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show abstract

“…An analogue of such a system in full tokamak geometry is the electromagnetic extension of Hardman et al. (2022).…”

Section: Discussionmentioning

confidence: 99%

Electromagnetic instabilities and plasma turbulence driven by electron-temperature gradient

et al. 2022

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Electromagnetic (EM) instabilities and turbulence driven by the electron-temperature gradient (ETG) are considered in a local slab model of a tokamak-like plasma. Derived in a low-beta asymptotic limit of gyrokinetics, the model describes perturbations at scales both larger and smaller than the electron inertial length $d_e$ , but below the ion Larmor scale $\rho _i$ , capturing both electrostatic and EM regimes of turbulence. The well-known electrostatic instabilities – slab and curvature-mediated ETG – are recovered, and a new instability is found in the EM regime, called the thermo-Alfvénic instability (TAI). It exists in both a slab version (sTAI, destabilising kinetic Alfvén waves) and a curvature-mediated version (cTAI), which is a cousin of the (electron-scale) kinetic ballooning mode. The cTAI turns out to be dominant at the largest scales covered by the model (greater than $d_e$ but smaller than $\rho _i$ ), its physical mechanism hinging on the fast equalisation of the total temperature along perturbed magnetic field lines (in contrast to kinetic ballooning mode, which is pressure balanced). A turbulent cascade theory is then constructed, with two energy-injection scales: $d_e$ , where the drivers are slab ETG and sTAI, and a larger (parallel system size dependent) scale, where the driver is cTAI. The latter dominates the turbulent transport if the temperature gradient is greater than a certain critical value, which scales inversely with the electron beta. The resulting heat flux scales more steeply with the temperature gradient than that due to electrostatic ETG turbulence, giving rise to stiffer transport. This can be viewed as a physical argument in favour of near-marginal steady-state in electron-transport-controlled plasmas (e.g. the pedestal). While the model is simplistic, the new physics that is revealed by it should be of interest to those attempting to model the effect of EM turbulence in tokamak-relevant configurations with high beta and large ETGs.

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“…i.e. at perpendicular scales much smaller that those at which electromagnetic effects become important (the 'flux-freezing scale'; see Adkins et al 2022), but much larger than those on which one encounters the effects of electron thermal diffusion due to the finite Larmor motion of the electrons (Hardman et al 2022;Adkins 2023) -both of these bring in a special perpendicular scale that would break the drift-kinetic scale invariance 1 . In other words, (3.8) and (3.9) describe physics on scales…”

Section: Scale Invariancementioning

confidence: 99%

“…are the coefficients calculated in, e.g. Hardman et al (2022) (and references therein). Truncating (B28) at p = 3, and demanding that the functional (B29) be stationary with respect to variations in the coefficients a p , we find that…”

Section: B32 First Order: Parallel Flowsmentioning

confidence: 99%

Scale invariance and critical balance in electrostatic drift-kinetic turbulence

2023

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The equations of electrostatic drift kinetics are observed to possess a symmetry associated with their intrinsic scale invariance. Under the assumptions of spatial periodicity, stationarity, and locality, this symmetry implies a particular scaling of the turbulent heat flux with the system's parallel size, from which its scaling with the equilibrium temperature gradient can be deduced under some additional assumptions. This macroscopic transport prediction is then confirmed numerically for a reduced model of electron-temperature-gradient-driven turbulence in slab geometry. The system realises this scaling through a turbulent cascade from large to small perpendicular spatial scales. The route of this cascade through wavenumber space (i.e. the relationship between parallel and perpendicular scales in the inertial range) is shown to be determined by a balance between nonlinear-decorrelation and parallel-dissipation timescales. This type of ‘critically balanced’ cascade, which maintains a constant energy flux despite the presence of parallel dissipation throughout the inertial range (as well as order-unity dissipative losses at the outer scale) is expected to be a generic feature of plasma turbulence. The outer scale of the turbulence, on which the turbulent heat flux depends, is determined by the breaking of drift-kinetic scale invariance due to the existence of large-scale parallel inhomogeneity (the parallel system size).

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Extended electron tails in electrostatic microinstabilities and the nonadiabatic response of passing electrons

Cited by 11 publications

References 62 publications

Numerical investigation of microtearing modes in the core of experimentally relevant spherical tokamak scenarios

Numerical investigation of microtearing modes in the core of experimentally relevant spherical tokamak scenarios

Electromagnetic instabilities and plasma turbulence driven by electron-temperature gradient

Scale invariance and critical balance in electrostatic drift-kinetic turbulence

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