Optimization of the active MHD spectroscopy system on JET for the excitation of individual intermediate and high- n Alfvén eigenmodes (original) (raw)
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Experimental studies of plasma-antenna coupling with the JET Alfvén Eigenmode Active Diagnostic
Nuclear Fusion, 2020
This paper presents a dedicated study of plasma-antenna (PA) coupling with the Alfvén Eigenmode Active Diagnostic (AEAD) in JET. Stable AEs and their resonant frequencies f, damping rates γ < 0, and toroidal mode numbers n are measured for various PA separations and limiter versus X-point magnetic configurations. Two stable AEs are observed to be resonantly excited at distinct low and high frequencies in limiter plasmas. The values of f and n do not vary with PA separation. However, |γ| increases with PA separation for the low-f, but not high-f, mode, yet this may be due to slightly different edge conditions. The high-f AE is detected throughout the transition from limiter to X-point configuration, though its damping rate increases; the low-f mode, on the other hand, becomes unidentifiable. The linear, resistive MHD code CASTOR is used to simulate the frequency scan of an AEAD-like external antenna. For the limiter pulses, the high-f mode is determined to be an n = 0 GAE, while t...
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.
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.
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.
The JET upgraded toroidal Alfvén Eigenmode Diagnostic System
Fusion Engineering and Design, 2019
The JET Toroidal Alfvén Eigenmode (AE) diagnostic system is undergoing a major upgrade which will provide for a state of the art excitation and real-time detection system. Experimental measurements and studies of AE at JET have been done successfully first with the saddle coil system and then with purpose built in-vessel antennas with a real time mode tracking algorithm. Complete new excitation and digital control systems have been developed for this upgrade and are currently being installed to provide JET with a unique diagnostic to study AE in the planned DT experimental campaign with relevance to ITER. New exciters consisting of 4kW class D power switching amplifiers, one for each antenna, have been developed in collaboration with the industry to cover the frequency range of operation 10-1000 kHz with RF pulse duration of 15s and repeatability ≥ 15min. Due to the varying transmission line impedance throughout the frequency band, a design solution with high resilience to reflected power was implemented with VSWR>>10:1. A completely new digital amplifier control system has been implemented which is based on FPGA modules for amplifier frequency and phase control with frequency resolution <1Hz and phase <0.3 degrees at 100 kHz. Gain control along with timing, gating, and trip management is done using RT LabVIEW. New capabilities, such as independent antenna current and phase control, will allow improved excitation control and better definition of the antenna spectrum combined with enhanced system reliability.
Nuclear Fusion
The interaction of Alfvén eigenmodes (AEs) and energetic particles is one of many important factors determining the success of future tokamaks. In JET, eight in-vessel antennas were installed to actively probe stable AEs with frequencies ranging 25–250 kHz and toroidal mode numbers |n| < 20. During the 2019–2020 deuterium campaign, almost 7500 resonances and their frequencies f 0, net damping rates γ < 0, and toroidal mode numbers were measured in almost 800 plasma discharges. From a statistical analysis of this database, continuum and radiative damping are inferred to increase with edge safety factor, edge magnetic shear, and when including non-ideal effects. Both stable AE observations and their associated damping rates are found to decrease with |n|. Active antenna excitation is also found to be ineffective in H-mode as opposed to L-mode; this is likely due to the increased edge density gradient’s effect on accessibility and ELM-related noise’s impact on mode identification...