A comparison of drift wave stability in stellarator and tokamak geometry

The linear stability of the ion-temperature-gradient modes (ITG) in the electrostatic limit is examined in the short wavelength region by using a two fluid reactive model in fully three-dimensional Wendelstein 7-X (W7-X) stellarator [G. Grieger et al., Plasma Physics and Controlled Nuclear Fusion Research, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 3, p. 525] geometry. The spectrum of stable and unstable modes and their real frequencies and eigenfunctions are calculated. The effects of density gradients, temperature gradients, temperature ratios, wavevector, ballooning angle, curvature and local magnetic shear on the ITG mode are also investigated. The frequency and growth rate of the most unstable ITG mode is calculated and visualized for a specific magnetic flux surface. For the equilibrium under investigation both localized and extended eigenmodes are found. The effect of small and large temperature ratios, small and large density gradients as well as large local magnetic shear are all found to be stabilizing. The highest growth rates are found at the outer part of the surface where the local magnetic shear is small and normal curvature is unfavorable.

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“…7͑a͒ of Ref. 27. We find helical ripples in the W7-X case, which decreases the connection length between bad and good curvature region.…”

Section: Numerical Resultssupporting

confidence: 49%

Unstable ion-temperature-gradient modes in the Wendelstein 7-X stellarator configuration

2002

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“…The drift waves have been found to exist universally in magnetically confined plasmas which exhibit anomalously short confinement times caused by pressure gradient driven instabilities. These have been observed by experimentalists [7,29,30] and calculated by theoreticians [5,6,31,32] for different plasma environments. The plasma instability frequencies of the order of kHz are in agreement with the theoretical work by Liu et al [33] but their excitation source is the lower hybrid waves of finite wave number.…”

Section: Theoretical Justificationmentioning

confidence: 52%

Experimental investigation of plasma instabilities by Fourier analysis in an electron cyclotron resonance ion source

Kumar

Sharma

et al. 2018

Phys. Rev. Accel. Beams

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The plasma instabilities play an important role in an electron cyclotron resonance (ECR) ion source for the production of intense heavy ion beams in high charge states for particle accelerators. The geometrical and operational constraints of ECR sources hinder the trapping of ions for a sufficient time to get fully ionized with maximum efficiency. This problem is looked at in detail by studying the plasma instabilities in ECR ion sources. The ECR environment is full of complex rearrangements of various electric and magnetic fields to define a sustainable trap for the ions. The maximum frequency of plasma instability has been observed to be of 122.5 kHz under a set of sustainable plasma parameters. However, this limit may be pushed further if the plasma is overdriven in terms of source parameters. The instabilities cover a full regime of few tens of Hz to few hundreds of kHz under various operating conditions of radio frequency (rf), negative bias voltage, rf power and injection gas pressure. The rigorous details of frequencies and amplitudes of plasma instabilities are being reported by studying the Fourier spectrum of extracted and analyzed beam intensity. The plasma instabilities are attributed as drift waves in an inhomogeneous ECR plasma generated by the application of radio-frequency fields.

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“…In the present study, a simple s-α geometry is assumed for the simulation domains. The effects of different tokamak geometries on drift wave turbulence have been studied in both fluid 42,43 and gyrokinetic descriptions. 44,45…”

Section: Simulationsmentioning

confidence: 99%

Impurity transport in temperature gradient driven turbulence

Skyman¹,

Nordman²,

Strand³

2012

Physics of Plasmas

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In the present paper the transport of impurities driven by trapped electron (TE) mode turbulence is studied. Non-linear (NL) gyrokinetic simulations using the code GENE are compared with results from quasilinear (QL) gyrokinetic simulations and a computationally efficient fluid model. The main focus is on model comparisons for electron temperature gra- dient driven turbulence regarding the sign of the convective impurity velocity (pinch) and the impurity density gradient R/LnZ (peaking factor) for zero impurity flux. In particular, the scaling of the impurity peaking factors with impurity charge Z and with driving temper- ature gradient is investigated and compared with the results for Ion Temperature Gradient (ITG) driven turbulence. In addition, the impurity peaking is compared to the main ion peaking obtained by a self-consistent fluid calculation of the density gradients corresponding to zero particle fluxes. For the scaling of the peaking factor with impurity charge Z, a weak dependence is obtained from NL GENE and fluid simulations. The QL GENE results show a stronger dependence for low Z impurities and overestimates the peaking factor by up to a factor of two in this region. As in the case of ITG dominated turbulence, the peaking factors saturate as Z increases, at a level much below neoclassical predictions. However, the scaling with Z is weak or reversed as compared to the ITG case. The scaling of impurity peaking with the background temperature gradients is found to be weak in the NL GENE and fluid simulations. The QL results are also here found to significantly overestimate the peaking factor for low Z values. For the parameters considered, the background density gradient for zero particle flux is found to be slightly larger than the corresponding impurity zero flux gradient.Comment: 23 pages, 13 figures. Submitted to AIP: Physics of Plasma

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A comparison of drift wave stability in stellarator and tokamak geometry

Cited by 16 publications

References 21 publications

Unstable ion-temperature-gradient modes in the Wendelstein 7-X stellarator configuration

Unstable ion-temperature-gradient modes in the Wendelstein 7-X stellarator configuration

Experimental investigation of plasma instabilities by Fourier analysis in an electron cyclotron resonance ion source

Impurity transport in temperature gradient driven turbulence

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