Collisionless microinstabilities in stellarators. Part 4. The ion-driven trapped-electron mode

Plunk, G. G.; Connor, J. W.; Helander, P.

doi:10.1017/s0022377817000605

Cited by 24 publications

(38 citation statements)

References 15 publications

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“…The first one of these cases holds in a maximum-J stellarator and implies that ordinary density-gradient-driven trapped-electron modes are linearly stable. Proll et al (2012) and Helander, Proll & Plunk (2013) concluded that any collisionless instability must then draw energy from the ions rather than the electrons, and Plunk, Connor & Helander (2017) showed that there is indeed a remnant ion-driven trapped-electron mode if η i > 0. From our considerations of the available energy, this is not surprising, since it is apparently impossible to achieve a non-trivial ground state for both species simultaneously in the ordering (1.5).…”

Section: P Helandermentioning

confidence: 99%

Available energy of magnetically confined plasmas

Helander

2020

J. Plasma Phys.

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The concept of the available energy of a collisionless plasma is discussed in the context of magnetic confinement. The available energy quantifies how much of the plasma energy can be converted into fluctuations (including nonlinear ones) and is thus a measure of plasma stability, which can be used to derive linear and nonlinear stability criteria without solving an eigenvalue problem. In a magnetically confined plasma, the available energy is determined by the density and temperature profiles as well as the magnetic geometry. It also depends on what constraints limit the possible forms of plasma motion, such as the conservation of adiabatic invariants and the requirement that the transport be ambipolar. A general method based on Lagrange multipliers is devised to incorporate such constraints in the calculation of the available energy, and several particular cases are discussed for which it can be calculated explicitly. In particular, it is shown that it is impossible to confine a plasma in a Maxwellian ground state relative to perturbations with frequencies exceeding the ion bounce frequency.

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Section: P Helandermentioning

confidence: 99%

Available energy of magnetically confined plasmas

Helander

2020

J. Plasma Phys.

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“…2016; Helander, Proll & Plunk 2013; Plunk et al. 2014; Plunk, Connor & Helander 2017; Plunk et al. 2019).…”

Section: Introductionmentioning

confidence: 99%

“…Turbulence in magnetised plasmas is usually generated by temperature and density gradient driven electrostatic microinstabilities, the most important ones being the ITG, electron temperature gradient (ETG) and trapped electron mode (TEM) (Doyle et al 2007). Their respective role for turbulence and transport has been explored analytically and by means of gyrokinetic simulations for stellarators and W7-X in particular (Proll et al 2012;Proll, Xanthopoulos & Helander 2013;Proll et al 2016;Helander, Proll & Plunk 2013;Plunk et al 2014;Plunk, Connor & Helander 2017;Plunk et al 2019). Besides gyrokinetic simulations with codes like GENE (Jenko et al 2000), key tools on W7-X to investigate turbulence in the core of the plasma experimentally are diagnostics like phase contrast imaging (PCI) (Edlund et al 2018;Huang et al 2021) and Doppler reflectometry (DR) (Carralero et al 2019;.…”

Section: Introductionmentioning

confidence: 99%

Phase contrast imaging measurements and numerical simulations of turbulent density fluctuations in gas-fuelled ECRH discharges in Wendelstein 7-X

et al. 2021

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The fundamental nature of turbulent density fluctuations in standard Wendelstein 7-X (W7-X) stellarator discharges is investigated experimentally via phase contrast imaging (PCI) in combination with gyrokinetic simulations with the code GENE. We find that density fluctuations are ion-temperature-gradient-driven and radially localised in the outer half of the plasma. It is shown that the line-integrated PCI measurements cover the right range of wavenumbers and a favourable toroidal and poloidal location to capture some of the strongest density fluctuations in W7-X. Due to the radial localisation of fluctuations, measured wavenumber–frequency spectra exhibit a dominant phase velocity, which can be related to the $\boldsymbol {E\times B}$ rotation velocity at the radial position of a well in the neoclassical radial electric field. The match is robust against variations of heating power and line-integrated density, which is partly due to the localisation of fluctuations and partly due to effects of the radial gradient in the $\boldsymbol {E\times B}$ velocity profile on the wavenumber–frequency spectrum. The latter effect is studied with a newly built synthetic PCI diagnostic and global gyrokinetic simulations with GENE-3D.

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“…The neoclassical transport induced by the bounce-averaged radial drift of trapped particles is strongly reduced. The magnetic field strength and the local curvature κ are shifted with respect to each other, and the turbulent structure can be distinguished between the ion-temperature gradient (ITG) and trapped-electron mode (TEM) by examining the scale length of the turbulence [18].…”

Section: Introductionmentioning

confidence: 99%

Experimental characterization of a quasi-coherent turbulent structure in the edge plasma in Wendelstein 7-X

et al. 2019

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A quasi-coherent mode (QCM) near the plasma edge is characterized experimentally by measuring the density fluctuation via a poloidal correlation reflectometer (PCR) in Wendelstein 7-X. This QCM frequency descends from 25 kHz near the separatrix to 10 kHz as the cutoff position penetrates toward the core. A similar mode structure is also observed in the spectra of ECE edge channels and Mirnov coil signals, which shows a clear correlation with respect to the PCR measurement. The mode evaluation yields a range for the poloidal mode number of 8 ≤ m ≤ 25, and for the normalized mode scale k ⊥ ρ s ≤ 0.1, which suggest a drift type instability. The QCM structure appears in the standard and narrow-mirror configurations (both with the rotational transform ι = 5/5 in the boundary), but is absent in the high-ι configuration (boundary ι = 5/4). The impact of plasma parameters on the QCM is discussed. It is observed that the QCM frequency and amplitude can be affected by the edge magnetic topology and plasma parameters. The f mode shifts upward during the negative and non-stellarator-symmetric control coil current scans, which may be attributed to the effect of the E × B shear, E r , and n e in the plasma edge region.

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Collisionless microinstabilities in stellarators. Part 4. The ion-driven trapped-electron mode

Cited by 24 publications

References 15 publications

Available energy of magnetically confined plasmas

Available energy of magnetically confined plasmas

Phase contrast imaging measurements and numerical simulations of turbulent density fluctuations in gas-fuelled ECRH discharges in Wendelstein 7-X

Experimental characterization of a quasi-coherent turbulent structure in the edge plasma in Wendelstein 7-X

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