MHD mode structure and propagation in the ASDEX tokamak

Klueber, O.; Zohm, H.; Bruhns, H.; Gernhardt, J.; Kallenbach, A.; Zehrfeld, H. P.

doi:10.1088/0029-5515/31/5/008

Cited by 61 publications

(47 citation statements)

References 16 publications

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“…This assumes that the tearing mode field perturbation is frozen within the plasma; hence, the frequency is related to the plasma flow velocity. This interpretation has been experimentally confirmed elsewhere with a comparison of flow velocities determined from Doppler spectroscopy and tearing modes [5,25]. However, Doppler spectroscopy measurements are unavailable on Phaedrus-T.…”

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confidence: 71%

Experimental Observation of rf Driven Plasma Flow in the Phaedrus-T Tokamak

et al. 1996

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We have observed frequency shifts of the 2͞1 tearing mode in a tokamak that are consistent with injecting momentum with low frequency rf waves. The 2͞1 frequency increased for current drive antenna phasing, decreased for anticurrent drive phasing, and was linear with rf power and driven current. The change in the toroidal velocities derived from the frequency shifts and the calculated rf momentum is consistent. The frequency change also increased with magnetic field, independent of antenna phasing. This is the first demonstration of modifying the 2͞1 frequency through rf momentum input. [S0031-9007(96)00568-6] PACS numbers: 52.55. Fa, Both theory and experiment suggest that plasma rotation can significantly modify the stability and transport characteristics of tokamak plasmas. Simultaneous improvement of the plasma confinement and stability has been identified in reactor studies as the most significant means for improving the economic attractiveness of the tokamak concept [1,2].Plasma rotation's impact on plasma stability has been examined recently. In tokamak plasmas surrounded by a resistive shell, toroidal plasma rotation has been found to increase tearing mode stability [3]. Theoretical analysis suggests that external kink modes are also stabilized by toroidal rotation [4]. Experimental evidence of this stabilization has been reported by Strait et al. where the stabilization was achieved with rotation velocities a fraction of the Alfvén speed [5]. External control of the momentum input can be utilized to prevent locked modes which cause degradation of confinement and often are precursors to plasma disruptions [6,7]. In addition, sheared plasma rotation has been predicted to cause linear coupling of ballooning modes. This increases damping of these modes which may allow tokamak plasmas to reach the second stability regime [8,9].Models developed to describe the transport in experimentally observed enhanced confinement modes have identified the importance of plasma rotation and sheared rotation in the formation of transport barriers [10]. These models are consistent with the observation of shear flows in H-mode plasmas [11] and negative central shear mode plasmas in the DIII-D tokamak [12]. Experiments also have shown degradation of VH mode with decreasing local toroidal plasma flow [13]. In addition, the power threshold for transport barrier formation is predicted to decrease with increased shear [14].External control of the momentum input has been demonstrated with neutral beam injection. While the net momentum transfer is efficient, the deposition profile is broad. In addition, calculations suggest that plasma rotation driven by radio-frequency (rf) waves can be more efficient than neutral beams if the parallel index of refraction N k $ 30 [15]. Several experiments have reported changes in the plasma toroidal rotation velocity with the application of ion cyclotron heating [16][17][18]. The rotation is generated by the spatial diffusion or loss of fast, resonant ions which leads to a modification of the radial...

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confidence: 71%

Experimental Observation of rf Driven Plasma Flow in the Phaedrus-T Tokamak

et al. 1996

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“…where θ and φ are the geometric poloidal and toroidal angles respectively (toroidal effects are not considered in this ansatz [35]). In tokamaks, the toroidal geometry breaks the continuum Alfvén spectra and permits discrete toroidal Alfvén Eigenmodes (TAEs) with frequencies inside the continuum gaps [36].…”

Section: Overview Of Alfvén Eigenmodesmentioning

confidence: 99%

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

Graca¹,

Conway²,

Lauber³

et al. 2007

Plasma Phys. Control. Fusion

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Abstract. The radial structure of Toroidal Alfvén Eigenmodes (TAEs) is of great importance for comparison with theoretical predictions. A dual-channel fast frequency hopping millimeter-wave reflectometer installed on the ASDEX Upgrade tokamak is capable of measuring density fluctuations from the plasma edge to core, allowing the radial eigenfunction of n = 4 TAE and edge MHD modes to be obtained using phase perturbation and coherence data analysis techniques. The two techniques reveal similar results, and in particular the radial structure of the n = 4 TAE is found to be in good agreement with numerical predictions from linear gyrokinetic simulations. First results of the radial localization of Alfvén cascades are also presented.

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“…In figure 1, plasma current (in the unit of 10 kA) and loop voltage are plotted alongwith one of the Mirnov coils raw data. The points (1), (2) and (4) We have carried out SVD analysis of Mirnov coil data from 12 pickup coils to understand the progress of tokamak discharge. In addition, data from pickup coils are also analysed at point (3) in the loop voltage trace where no conspicuous change in loop voltage is seen.…”

Section: Analysis Of Mirnov Coil Datamentioning

confidence: 99%

“…However, the signal can be complicated when several modes are significant and they are interacting. In such a case, two numerical techniques have been developed to determine the mode numbers and the coherent mode structure [2,3]. In the first method, mode number and structure are determined by obtaining phase angles from a set of Mirnov coils at the frequency of interest.…”

Section: Introductionmentioning

confidence: 99%

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Mirnov coil data analysis for tokamak ADITYA

et al. 2000

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The spatial and temporal structures of magnetic signal in the tokamak ADITYA is analysed using recently developed singular value decomposition (SVD) technique. The analysis technique is first tested with simulated data and then applied to the ADITYA Mirnov coil data to determine the structure of current peturbation as the discharge progresses. It is observed that during the current rise phase, current perturbation undergoes transition fromÑ poloidal structure to Ñ and then to Ñ ¿ . At the time of current termination, Ñ ¾ perturbation is observed. It is observed that the mode frequency remains nearly constant ( 10 kHz) when poloidal mode structure changes from Ñ to Ñ ¾ . This may be either an indication of mode coupling or a consequences of changes in the plasma electron temperature and density scale length.

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MHD mode structure and propagation in the ASDEX tokamak

Cited by 61 publications

References 16 publications

Experimental Observation of rf Driven Plasma Flow in the Phaedrus-T Tokamak

Experimental Observation of rf Driven Plasma Flow in the Phaedrus-T Tokamak

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

Mirnov coil data analysis for tokamak ADITYA

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