Identification of MHD Instabilities in Experiments

Gude, A.

doi:10.1007/978-3-662-44222-7_3

Cited by 8 publications

(9 citation statements)

References 36 publications

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“…To examine the changes in the radial structure, a radial mapping of the oscillation amplitudes was constructed. This means that each LOS is labelled with the normalized poloidal flux ( pol ρ ) of the magnetic flux surface to which the LOS is tangential [36]. To distinguish between the LOSs that pass above and below the magnetic axis, the radial coordinate of the latter is assigned with pol ρ − .…”

Section: Beta-induced Alfvén Eigenmodesmentioning

confidence: 99%

“…The radial location of the mode was also estimated from the frequency modulated continuous wave (FMCW) reflectrometry measurements. Four channels (K, Ka, Q and V bands) were used with fixed frequencies (24,33,36 and 53 GHz correspondingly) from each the low and high field sides. The corresponding density values are 0.71, 1.35, 1.61 and 3.48 10 19 ⋅ m −3 .…”

Section: Energetic Particle Driven Gamsmentioning

confidence: 99%

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Experimental investigation of the radial structure of energetic particle driven modes

et al. 2016

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Abstract. Alfvén eigenmodes (AEs) and energetic particle modes (EPMs) are often excited by energetic particles (EPs) in tokamak plasmas. One of the main open questions concerning EP driven instabilities is the non-linear evolution of the mode structure. The aim of the present paper is to investigate the properties of betainduced AEs (BAEs) and EP driven geodesic acoustic modes (EGAMs) observed in the ramp-up phase of off-axis NBI heated ASDEX Upgrade (AUG) discharges. This paper focuses on the changes in the mode structure of BAEs/EGAMs during the nonlinear chirping phase. Our investigation has shown that in case of the observed downchirping BAEs the changes in the radial structure are smaller than the uncertainty of our measurement. This behaviour is most probably the consequence of that BAEs are normal modes, thus their radial structure strongly depends on the background plasma parameters rather than on the EP distribution. In the case of rapidly upward chirping EGAMs the analysis consistently shows shrinkage of the mode structure. The proposed explanation is that the resonance in the velocity space moves towards more passing particles which have narrower orbit widths.

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Section: Beta-induced Alfvén Eigenmodesmentioning

confidence: 99%

Section: Energetic Particle Driven Gamsmentioning

confidence: 99%

Experimental investigation of the radial structure of energetic particle driven modes

et al. 2016

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“…The metal wall and the high heating power provided by 8 neutral beam injection (NBI) sources delivering up to 20 MW, 6 MW of ion cyclotron heating, and 4 MW of electron cyclotron heating make this tokamak well suited for power exhaust studies relevant for future fusion reactors. The SXR diagnostic in AUG is regularly used for the analysis of the impurity transport 15,16 , magnetohydrodynamic (MHD) instabilities 17 and fast particles in the plasma 18 . All of these measurements can benefit from the high-quality tomographic methods described in this article.…”

Section: Introductionmentioning

confidence: 99%

Optimized tomography methods for plasma emissivity reconstruction at the ASDEX Upgrade tokamak

Odstrčil

Pütterich

Odstrčil

et al. 2016

Review of Scientific Instruments

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The soft X-ray (SXR) emission provides valuable insight into processes happening inside of high-temperature plasmas. A standard method for deriving the local emissivity profiles of the plasma from the line-of-sight integrals measured by pinhole cameras is the tomographic inversion. Such an inversion is challenging due to its ill-conditioned nature and because the reconstructed profiles depend not only on the quality of the measurements, but also on the inversion algorithm used. This paper provides a detailed description of several tomography algorithms, which solve the inversion problem of Tikhonov regularization with linear computational complexity in the number of basis functions. The feasibility of combining these methods with the Minimum Fisher Information Regularization is demonstrated, and various statistical methods for the optimal choice of the regularization parameter are investigated with emphasis on their reliability and robustness. Finally, the accuracy and the capability of the methods are demonstrated by reconstructions of experimental SXR profiles, featuring poloidal asymmetric impurity distributions as measured at the ASDEX Upgrade tokamak.

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“…These values are close to the inversion and mixing radii of the fastion redistribution but significantly different from the current redistribution radii. The inversion radius of the T e profile is often used as an indication of the q = 1 surface, but it has been demonstrated that they are not necessarily at the same position [55,56]. Generally, for a measured quantity (here T e ) significantly less peaked than the current profile, the inversion occurs at larger radii than ρ q=1 [56].…”

Section: Pressure Profiles and Fast-ion Redistributionmentioning

confidence: 99%

“…The inversion radius of the T e profile is often used as an indication of the q = 1 surface, but it has been demonstrated that they are not necessarily at the same position [55,56]. Generally, for a measured quantity (here T e ) significantly less peaked than the current profile, the inversion occurs at larger radii than ρ q=1 [56].…”

Section: Pressure Profiles and Fast-ion Redistributionmentioning

confidence: 99%

Sawtooth inducedq-profile evolution at ASDEX Upgrade

Fischer¹,

Bock²,

Burckhart³

et al. 2019

Nucl. Fusion

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The sawtooth induced q-profile evolution was modelled at ASDEX Upgrade and validated with imaging motional Stark effect (IMSE) measurements. The equilibrium current distribution was estimated by the coupling of a Grad-Shafranov solver and the current diffusion equation. At ASDEX Upgrade an extensive diagnostic suite allows for a reliable modelling of the current density profile and its temporal evolution as long as neo-classical current diffusion can be assumed. The sawtooth-induced central q change, ∆q 0 , was estimated from equilibrium reconstruction between the sawtooth events without using IMSE measurements. The sawtooth-induced current relaxation is modelled with two different reconnection models, a Kadomtsev full reconnection and a newly developed fully stochastic flat-current model. Both models result in about the same sawtooth-induced ∆q 0 ≈ 0.10-0.12 but with a small difference in the absolute q 0 values. The sawtooth-induced evolution of the forward modelled MSE angles agrees well with the IMSE measurements. This confirms the assumption of neoclassical current diffusion between the sawtooth events. A sensitivity study reveals two major ingredients for a reliable interpretation of MSE measurements. Firstly, the sawtooth-induced redistribution of fast ions has to be taken into account for a reliable estimation of the Shafranov shift. The fast-ion redistribution is modelled with a particle-conserving homogeneous fast-ion pressure profile in the plasma core and a subsequent recovery time of 50 ms. Secondly, a reliable estimation of the electric field is necessary for a credible interpretation of the MSE angles.

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Identification of MHD Instabilities in Experiments

Cited by 8 publications

References 36 publications

Experimental investigation of the radial structure of energetic particle driven modes

Experimental investigation of the radial structure of energetic particle driven modes

Optimized tomography methods for plasma emissivity reconstruction at the ASDEX Upgrade tokamak

Sawtooth inducedq-profile evolution at ASDEX Upgrade

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