Behaviour of central plasma relaxation oscillations during localized electron cyclotron heating on the TCV tokamak

Pietrzyk, Z.A.; Pochelon, A.; Goodman, T. P.; Henderson, M.; Hogge, J.‐P.; Reimerdes, H.; Tran, M.Q.; Behn, R.; Furno, I.; Moret, J.-M.; Nieswand, C.; Rommers, J.; Sauter, O.; Toledo, W. van; Weisen, H.; Porcelli, Francesco; Razumova, K.A.

doi:10.1088/0029-5515/39/5/303

Cited by 52 publications

(58 citation statements)

References 41 publications

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“…Structures with poloidal mode numbers up to m = 3 are resolved by the SVD, and coherent rotating structures such as MHD modes can be identified. It has been shown [4,13,14] that there is a strong correlation between the sawtooth shape and the heating location relative to the sawtooth inversion surface, in the sense that small changes in the location of the ECH deposition with respect to the inversion surface lead to large changes in sawtooth period, amplitude and MHD activity. The SVD method also provides a reliable tool in the determination of the sawtooth inversion surface of the local soft X ray emissivity.…”

Section: Experimental Set-upmentioning

confidence: 99%

“…In Ref. [4], a classification of the different types of observed sawteeth was proposed, from the usual triangular sawteeth, obtained with on-axis power deposition, to non-standard behaviour when the location of the resonance position is moved off-axis. In particular, when the ECH power is deposited close to the q = 1 surface, the line integrated soft X ray traces can exhibit 'humpback'-like features.…”

Section: Introductionmentioning

confidence: 99%

“…In previous TCV papers on this subject [4,7,8], an understanding of the sawtooth dynamics was hampered by the limited temporal resolution of the available diagnostics which did not allow us to resolve the sawtooth crash phase. This has now been overcome by an improvement of the time resolution of the TCV soft X ray tomographic system [9], which allows monitoring of the sawtooth crash ∼10 times faster than the crash time, which is typically ∼100 µs.…”

Section: Introductionmentioning

confidence: 99%

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Understanding sawtooth activity during intense electron cyclotron heating experiments on TCV

et al. 2001

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Abstract. Different types of central relaxation oscillations are observed in the presence of ECH depending on the location of the deposited power. In the TCV tokamak, normal sawteeth, i.e. triangular sawteeth similar to ohmic sawteeth, and saturated sawteeth are observed with central ECH power deposition, while giant sawteeth and 'humpback' oscillations occur when heating close to the sawtooth inversion surface of the local soft X ray emissivity. New measurements with high temporal resolution show that the crash phase of these sawtooth types is accompanied by a reconnection process associated with an m/n = 1 resistive internal kink mode. After the sawtooth crash, full magnetic reconnection is observed in normal and in saturated sawteeth, while for giant and humpback sawteeth the reconnection process is incomplete and poloidally asymmetric temperature profiles persist after the crash. The detailed dynamics of the magnetic island associated with the resistive internal kink mode are described by a displacement function which is inferred from the experimental data. In normal sawteeth, the kink mode is destabilized just before the crash, while in all other sawtooth types a magnetic island exists for a significant fraction of the sawtooth period. The different types of sawteeth have been simulated using a numerical code based on a theoretical model which describes the evolution of the electron temperature in the presence of localized heat sources and of a magnetic m/n = 1/1 island.

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Section: Experimental Set-upmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Understanding sawtooth activity during intense electron cyclotron heating experiments on TCV

et al. 2001

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“…These modes are known to be responsible for the well known internal plasma sawtooth relaxation oscillations [5]. In TCV experiments with intense ECRH, sawtooth oscillations acquire a non-standard character, as reported in Refs [6,7]. In particular, when the ECRH power is deposited close to the q = 1 surface, the line integrated soft X ray temporal traces exhibit peculiar shapes, which suggested the nickname 'humpback' when they were first observed in T-10 experiments [8].…”

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confidence: 79%

“…1(b) presents a tomographic reconstruction of the soft X ray emissivity isocontours. With off-axis ECRH, different types of nonstandard oscillations are observed [6]. For instance, so-called 'saturated sawteeth' are often observed when the wave energy is deposited within the magnetic surface where the magnetic winding index (safety factor), q, equals unity.…”

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Model for humpback relaxation oscillations

et al. 2000

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Abstract. An interpretation of non-standard, central MHD events in the TCV tokamak during localized ECRH is presented. It is shown that the non-standard behaviour is a consequence of specific features in the electron temperature profile produced when ECRH power is deposited close to the q = 1 surface and by the advection and mixing of electron thermal energy resulting from a resistive MHD instability.Experimental research on controlled thermonuclear fusion requires the production of hot plasmas with temperatures in the multi-keV range. One effective scheme providing intense localized heating is ECRH [1]. Resonant electromagnetic waves are launched into a magnetically confined plasma at a frequency matching a low integer multiple of the local EC frequency, ω ce = eB/mc. In recent years, record values of ECRH power density have been attained. This has revealed new and peculiar plasma behaviour. One example is the observation of multipeaked electron temperature profiles and sharp temperature gradients in the RTP [2] and TEXT Upgrade [3] experiments, explained in Ref.[4] as being the consequence of interaction between ECRH and resistive MHD modes with toroidal n = 1 and dominant poloidal m = 1 mode numbers. These modes are known to be responsible for the well known internal plasma sawtooth relaxation oscillations [5]. In TCV experiments with intense ECRH, sawtooth oscillations acquire a non-standard character, as reported in Refs [6,7]. In particular, when the ECRH power is deposited close to the q = 1 surface, the line integrated soft X ray temporal traces exhibit peculiar shapes, which suggested the nickname 'humpback' when they were first observed in T-10 experiments [8]. The purpose of this Letter is to propose a theoretical interpretation of the humpback phenomenon. We argue that specific features in the electron temperature profile are produced by localized ECRH. Then, the humpback behaviour is consistent with the advection and mixing of electron thermal energy in the plasma core associated with the excitation of resistive m/n = 1 internal modes.TCV is a tokamak with major radius R = 0.88 m, minor radius a = 0.25 m, vacuum vessel elongation κ = 3 and central magnetic field B = 1.43 T in vacuum. Up to three 82.7 GHz, 500 kW gyrotrons, each with a pulse length of 2 s, for heating at the cyclotron second harmonic resonance via the extraordinary mode, were used in this study. The vertical microwave beam diameter near the plasma centre is of the order of 5 cm, while the horizontal width of the deposition region is O (1 cm). ECRH power densities in excess of 1 × 10 2 MW/m 3 can be obtained. New insight into the temporal evolution of the different types of MHD modes observed in TCV has become possible because of the recently upgraded, 200 channel soft X ray tomographic system, allowing for a sampling time of 13 µs. The spatial reconstruction of the central emissivity profile is based on a pixel method with characteristic spatial resolution of about 3 cm, to be compared with a typical sawtooth inversion radius of 5-10...

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Identification of MHD Instabilities in Experiments

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2014

Active Control of Magneto-Hydrodynamic Instabilities in Hot Plasmas

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Behaviour of central plasma relaxation oscillations during localized electron cyclotron heating on the TCV tokamak

Cited by 52 publications

References 41 publications

Understanding sawtooth activity during intense electron cyclotron heating experiments on TCV

Understanding sawtooth activity during intense electron cyclotron heating experiments on TCV

Model for humpback relaxation oscillations

Identification of MHD Instabilities in Experiments

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