A novel measurement of marginal Alfvén eigenmode stability during high power auxiliary heating in JET (original) (raw)
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First Measurement of the Damping Rate of High-n Toroidal Alfvén Eigenmodes in JET Tokamak Plasmas
After many years of successful operation, the JET saddle coil system was dismantled during the 2004-2005 shutdown. A new antenna system was installed to replace it and excite MHD modes in the Alfvén frequency range . Due to their geometry, the saddle coils could drive only low toroidal mode numbers, |n|=0-2. Conversely, the Alfvén Eigenmodes (AEs) that can be driven unstable in JET (and ITER: [2]) by fusion generated alphas or other fast particles have toroidal mode numbers in the range n~5-20. This, and because most of the previous JET measurements were obtained in plasmas with low edge magnetic shear, makes it difficult to extrapolate the low-n results to ITER. These reasons prompted the design of a new system of compact antennas for excitation and measurements of high-n modes.
Identifying Alfvén eigenmodes in the early phase of advanced tokamak plasmas
Plasma Physics and Controlled Fusion, 2006
We examine the fast-ion driven Alfvén eigenmodes (AEs) in the frequency range 0-500 kHz during the current ramp-up phase of advanced JET plasma before the main heating starts (the preheating phase). Modelling of AEs using equilibrium reconstruction and the ideal MHD spectral code MISHKA shows that three types of mode commonly found during the preheating phase correspond to: global toroidal Alfvén eigenmodes (TAEs) found in regions of moderate to high magnetic shear; core-localized TAEs found in regions of low magnetic shear; and Alfvén cascades found in regions of zero shear. MHD spectroscopy diagnosis of the time evolution of the plasma current within the central region of the plasma and of the minimum safety factor value based upon the frequency evolution of these unstable AEs is discussed.
Experimental Study of the Stability of Alfvén Eigenmodes on JET
Over the last few years, experiments have been performed on JET to study the dependence of the AE stability limits on the main plasma parameters in different operating scenarios. The measurements are compared with theoretical models with the aim of improving the prediction capabilities for burning plasma experiments, such as ITER. An increase in the edge magnetic shear provides a significant stabilising contribution for AEs in plasmas characterised by a monotonic q-profile. Conversely, with non-monotonic q-profiles and Internal Transport Barriers, multiple weakly damped modes exist in the Alfvén frequency range even in the presence of a high edge magnetic shear, with possible negative implications for the AE stability in ITER. The dependence of the frequency and damping rate of n=1 TAEs on the bulk plasma β was also analysed using NBI heating in limiter plasmas. The mode frequency decreases for increasing β, in agreement with fluid and gyrokinetic predictions. Conversely, contrary t...
Nuclear Fusion, 2011
This paper reports the results of recent experiments performed on the JET tokamak on Alfvén Eigenmodes (AEs) with toroidal mode number (n) in the range n=3-15. The stability properties and the use of these medium-n AEs for diagnostic purposes is investigated experimentally using a new set of compact in-vessel antennas, providing a direct and real-time measurement of the frequency, damping rate and amplitude for each individual toroidal mode number. First, we report on the development of a new algorithm for mode detection and discrimination using the Sparse Signal Representation theory. The speed and accuracy of this algorithm has made it possible to use it in our plant control software, allowing real-time tracking of individual modes during the evolution of the plasma background on a 1ms time scale. Second, we report the first quantitative analysis of the measurements of the damping rate for stable n=3 and n=7 Toroidal AEs as function of the plasma elongation. The damping rate for these modes increases for increasing elongation, as previously found in JET for n=0-2 AEs. A theoretical analysis of these JET data has been performed with the LEMan, CASTOR and TAEFL codes. The LEMan and TAEFL results are in good agreement with the measurements for all magnetic configurations where there is only a minor up/down asymmetry in the plasma poloidal cross-section. The CASTOR results indicate that continuum damping is not the only mechanism affecting the stability of these medium-n AEs. The diagnostic potential of these modes has being confirmed during the recent gas change-over experiment, where independent measurements of the effective plasma isotope ratio A EFF have been provided in addition to the more routinely employed spectroscopic and gas-balance ones. These data shows a slight difference in the measurement of A EFF when using n<5 and n>7 modes, suggesting a radial dependence in the effective plasma isotope ratio. * Appendix of F.Romanelli, 23 rd IAEA Fusion Energy Conference, paper OV1/3, this conference.
Plasma Physics and Controlled Fusion, 2010
The stability properties of Alfvén Eigenmodes (AEs) are investigated directly using external antenna excitation and detection of stable modes in a variety of plasma configurations in different devices. Dedicated methods to measure the AE damping rate separately from the fast ion drive have been pioneered at JET, using low toroidal mode number internal saddle coil antennas. Other experiments have since installed localised in-vessel antennas to drive and detect MHD modes in the Alfvén frequency range, first on C-Mod, then on MAST.
Monitoring Alfvén Cascades with Interferometry on the JET Tokamak
Physical Review Letters, 2004
A microwave interferometry technique is applied for the first time for detecting a discrete spectrum of Alfvén cascade (AC) eigenmodes excited with fast ions in reversed magnetic shear plasmas of the Joint European Torus. The interferometry measurements of plasma density perturbations associated with ACs show an unprecedented frequency and time resolution superior to that obtained with external magnetic coils. The measurements of ACs are used for monitoring the evolution of the safety factor and density of rational magnetic surfaces in the region of maximum plasma current.
Linear kinetic effects of core plasma on low frequency Alfven and acoustic eigenmodes in tokamaks
Cornell University - arXiv, 2022
The resonant and non-resonant effects of core plasma on the excitation of low frequency modes with ω < ω BAE , such as Beta-induced Alfvén Acoustic Eigenmodes (BAAEs) and Kinetic Ballooning Modes (KBMs) are examined in the framework of the generalized fishbone-like dispersion relation. The formalism of the fishbonelike equation contains all the necessary ingredients to describe the features of these low frequency fluctuations, and explain experimental findings. Core plasma properties (diamagnetic frequency and precession resonance with trapped ions) strongly affect the excitation of the modes, and in the case of BAAEs more effectively than the energetic particles. The diamagnetic frequency of the core plasma also contributes to the coupling of the BAAEs with the KBMs, thus affecting the excitation and polarization of both modes. Energetic particles can still provide a non-resonant drive to some of the low frequency modes.