Development of Alfvén Spectroscopy in Advanced Scenarios on JET
Measurements of low-amplitude Alfvén Eigenmodes (AEs) excited by fast ions represent an attractive form of MHD spectroscopy of fusion plasmas, which can provide data on plasma equilibrium and plasma parameters. Techniques for detecting Alfvén Cascades (ACs) and Toroidal Alfvén Eigenmodes (TAEs) with the use of interferometry and X-mode reflectometry demonstrate on JET an attractive approach to such measurements, complementary and sometimes superior to the magnetic measurements with Mirnov coils. With the use of O-mode interferometry, allowing a very high time resolution for detecting ACs, the correlation between the Internal Transport Barrier (ITB) triggering events and low rational values of qmin (t) has been assessed in JET plasmas with non-monotonic q(r) -profiles. It was found that in the majority of cases where the ITB were triggered near integer qmin (t) in plasmas with non-monotonic q(r) -profiles, the ITB triggering event preceded a grand ACs. Since grand ACs mark the time of qmin (t) = integer, this observation indicates strongly that the ITB triggering event is associated with the depletion of rational magnetic surfaces just before qmin (t) = integer, rather than with the presence of an integer qmin (t) value itself. The correlation between ITB triggering events and grand ACs has been found to exist in JET plasmas with high densities, up to 5¥1019 m-3, showing that the timing of ITB triggering from AC diagnosis may facilitate scenario development in machines with high plasma densities such as C-MOD and ITER. ACs driven by sub-Alfvenic NBI were detected on JET using O-mode interferometry in JET experiments with Tritium NBI. Although considerable NBI-driven AC activity was present, a good agreement was found both in the radial profile and in the time evolution of DT neutrons between the neutron measurements and TRANSP code, indicating the ACs have at most a small effect on fast particle confinement in this case. Transitions from TAEs to ACs have been observed on JET when an ITB forms. It was found that the observed TAE-to-AC transition is caused by a reversal of the magnetic shear, mainly associated with the bootstrap current. ACs were also observed in high-temperature JET plasmas with monster sawteeth, and the use of Alfvén spectroscopy shows that the sawtooth crashes happen at qmin @ 0.85, while qmin > 1 is deduced for the post-crash phase. The theory of ACs now includes thermal plasma effects and fast ion response, which enable determination of kinetic plasma parameters from AC measurements.