Localization of MHD and fast particle modes using reflectometry in ASDEX Upgrade

Graca, S. da; Conway, G. D.; Lauber, P.; Maraschek, M.; Borba, D.; Günter, S.; Cupido, L.; Sassenberg, K.; Serra, F.; Manso, M. E.

doi:10.1088/0741-3335/49/11/007

Cited by 27 publications

(33 citation statements)

References 61 publications

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“…A frequency hopping millimeter-wave reflectometer [26] installed in ASDEX Upgrade is now capable of measuring density fluctuations profiles with a fast (∼100 ms) time scale. Recent results have been obtained for the determination of the radial eigenfunction of MHD and fast particle modes [27]. As a future work, the radial structure of the MHD modes present in type II-ELM discharges could be determined in the pedestal region.…”

Section: Phenomenologymentioning

confidence: 94%

Identifying the MHD signature and power deposition characteristics associated with type-II ELMs in ASDEX Upgrade

Thun

Maraschek

Graca

et al. 2008

Plasma Phys. Control. Fusion

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Abstract. The properties of ASDEX Upgrade's type-II ELM regime, which combines good confinement with benign temporal variation of the power exhaust, make it an attractive candidate for ITER baseline scenario operation. As yet it is not understood what the physics origin of this regime is. In previous type-II ELM studies [Stober J et al 2001 Nucl. Fusion 41 1123, Stober J et al 2005 Nucl. Fusion 45 1213] a whole range of fluctuations have been reported. In this article the properties of these fluctuations are inspected in more detail, and their relevance for the regime investigated. For high frequency (∼ 150-220 kHz) magnetic fluctuation activity, from its absence in certain type-II ELM scenarios and behaviour during regime transitions it is concluded that it is not an essential ingredient of type-II ELM regimes on ASDEX Upgrade. A stronger contender to explain the type-II ELM regime appears to be given by small repetitive electromagnetic bursts, and low frequency (∼ 5-25 kHz) magnetic precursor activity associated with these bursts. It is demonstrated that these bursts give rise to small but frequent heat loads of the right order to influence ELM cycles. These are best detected in the vicinity of the inner strike point in the lower divertor through infra-red thermography and Langmuir probes. Their properties further suggest that the bursts are neither small type-I ELMs nor type-III ELMs. However, it remains to be shown that the losses associated with these bursts are high enough to replace the type-I ELM losses. Other enhanced density fluctuation activity at low frequency (10-30 kHz) is detected by reflectometry near the plasma boundary. The information available about this mode is more limited, but so far a causal implication of this mode in the type-II ELM regime cannot be excluded. Overall it can be concluded that, of the three potential dissipation mechanisms, the low frequency electromagnetic burst behaviour seems to be the most promising candidate to account for type II ELMs and their energy losses.Identifying the MHD signature associated with type-II ELMs 2

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Section: Phenomenologymentioning

confidence: 94%

Identifying the MHD signature and power deposition characteristics associated with type-II ELMs in ASDEX Upgrade

Thun

Maraschek

Graca

et al. 2008

Plasma Phys. Control. Fusion

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“…In a frequency hopping mode -under conditions of stationary TAE activity -reflectometry measurements probe the density fluctuations at different radial locations, using correlation techniques to reconstruct the envelope of the TAE eigenfunction [36,37]. The experimental eigenfunction shown in Fig.…”

Section: Damping Of Tae Modesmentioning

confidence: 99%

“…The experimental eigenfunction shown in Fig. 15 gives the cross phase between magnetic and reflectometry signals for an n = 4 TAE perturbation excited by ICRH heating in a low density discharge, where the peaked density profile allows for reflectometry measurements up to the plasma core (for details see [38]). …”

Section: Damping Of Tae Modesmentioning

confidence: 99%

Interaction of energetic particles with large and small scale instabilities

Günter

Conway

Graca³

et al. 2007

Nucl. Fusion

124

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Beyond a certain heating power, measured and predicted distributions of NBI driven currents deviate from each other even in the absence of MHD instabilities. The most reasonable explanation is a redistribution of fast NBI ions on a time scale smaller than the current redistribution time. The hypothesis of a redistribution of fast ions by background turbulence is discussed. Direct numerical simulation of fast test particles in a given field of electrostatic turbulence indicates that for reasonable parameters fast and thermal particle diffusion can indeed be similar. -High quality plasma edge density profiles on ASDEX Upgrade and the recent extension of the reflectometry system allow for a direct comparison of observed TAE eigenfunctions with theoretical ones as obtained with the linear, gyrokinetic, global stability code LIGKA. These comparisons support the hypothesis of TAE-frequency crossing the continuum at the plasma edge in ASDEX Upgrade H-mode discharges. -A new fast ion loss detector with 1 MHz time resolution allows frequency and phase resolved correlation between the observed losses and low frequency magnetic perturbations such as TAE modes and rotating magnetic islands.Whereas losses caused by TAE modes are known to be due to resonances in velocity space, by modelling of the particle drift orbits we were able to explain losses caused by magnetic islands as due to island formation and stochasticity in the drift orbits.

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“…Recently, a new operation technique has been developed, that is called frequency hopping. [6][7][8] It is a sort of frequency scanning operation. The source frequency is swept step by step in the whole frequency range.…”

Section: Introductionmentioning

confidence: 99%

Multifrequency channel microwave reflectometer with frequency hopping operation for density fluctuation measurements in Large Helical Device

Tokuzawa

Ejiri

Kawahata

2010

Review of Scientific Instruments

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In order to measure the internal structure of density fluctuations using a microwave reflectometer, the broadband frequency tunable system, which has the ability of fast and stable hopping operation, has been improved in the Large Helical Device. Simultaneous multipoint measurement is the key issue of this development. For accurate phase measurement, the system utilizes a single sideband modulation technique. Currently, a dual channel heterodyne frequency hopping reflectometer system has been constructed and applied to the Alfvén eigenmode measurements.

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Localization of MHD and fast particle modes using reflectometry in ASDEX Upgrade

Cited by 27 publications

References 61 publications

Identifying the MHD signature and power deposition characteristics associated with type-II ELMs in ASDEX Upgrade

Identifying the MHD signature and power deposition characteristics associated with type-II ELMs in ASDEX Upgrade

Interaction of energetic particles with large and small scale instabilities

Multifrequency channel microwave reflectometer with frequency hopping operation for density fluctuation measurements in Large Helical Device

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