Direct Measurements of the Damping of Alfvén Eigenmodes for an Assessment of their Stability Limits in Tokamak Plasmas

Direct damping rate measurements of AEs are obtained using the active MHD spectroscopy system installed on the JET tokamak. The system was recently equipped with new antennas, designed to study especially the modes of intermediate toroidal mode number n, |n| = 3 – 15, as the AEs of this range are most prone to destabilization by the fast particles in JET and in future burning plasma experiments such as ITER. The broad n-spectrum that is driven by the new antennas and the more localized structure of intermediate-n AEs has important implications for the ability to measure damping rates of intermediate n. To obtain an extended database of high accuracy individual-n measurements, experimental work on technical and engineering aspects was indispensable both on the excitation side and on the detection side. On the excitation side, the electrical model of the AE exciter has been constructed during this thesis. The model is used to determine the operational capabilities of the exciter with the new antennas, to optimize the antenna currents and to design the relevant impedance matching circuits. On the detection side, the excitation of multiple-n, degenerate AEs at close frequencies prompted for a sophisticated method to correctly estimate the n-spectrum of the plasma response. To this end, a sparse spectrum representation method was adapted to deal with the complex and real-time data produced by the active MHD spectroscopy system. The n-decomposition of the plasma response requires an accurate relative calibration of the magnetic pick-up coils. An in situ method was developed and applied for the calibration of the coils using the direct coupling to the new AE antennas. A large collection of damping rate measurements of, mainly, toroidal AEs (TAEs) was obtained during the 2008/2009 JET experimental campaigns following the technical optimization of the antenna system. Selected measurements of |n| = 3, 4 and |n| = 7 TAEs are compared to the plasma models of the numerical codes LEMan and CASTOR. The robustness of the results of the simulations is tested against uncertainties in density and safety factor profiles. The antenna-driven modes are characterized and the dominant damping mechanisms are identified in a number of cases, and confirm in general for a wide n- range the stabilizing role played by the edge magnetic shear. These comparisons underline the importance of the kinetic effects in order to achieve a realistic estimation of the TAE damping. A database of approximately 3000 TAE damping rate measurements in ohmic plasmas is studied. The dependence of the damping rate measurements of |n| = 2 – 7 TAEs on the edge magnetic shear, the edge elongation and the q profile is investigated. The analysis provides experimental evidence that the damping properties of TAEs with |n| = 2 – 7 change as the toroidal mode number n increases, showing that medium-n modes tend to be less damped than low-n modes. In JET plasmas, the turning point for the damping properties to change from low-n to medium-n behavior is found to be at |n| ≈ 3 – 4.

Related material