Spatial Distribution of Fast Ion Loss Detectors in the ASDEX Upgrade Tokamak

Abstract

To ensure heating efficiency and device integrity in future fusion reactors, suprathermal (fast) ions need to be confined for a sufficient time so that their energy is transferred to the bulk plasma via Coulomb collisions. In order to understand the mechanisms responsible for the loss of fast-ion confinement, fast-ion loss detectors (FILD) provide velocity-space measurements at the reactor wall. These measurements are nevertheless spatially localized and do not provide enough information to characterize the spatial distribution of the losses required to fully understand (and therefore mitigate) the loss mechanism induced by MHD instabilities and externally applied 3D magnetic perturbations. In order to characterize the spatial distribution of the fast-ion losses, in the frame of this thesis, the FILD suite of the ASDEX Upgrade (AUG) tokamak is upgraded with two new fast-ion loss detectors, being both located below the tokamak midplane. One of these diagnostics, labelled FILD5, is installed right above the lower divertor, between two saddle coils. The other new detector, known as FILD4, is an in-situ probe consisting of an in-vessel coil that, when externally energized, creates a magnetic dipole that tries to align with the existing tokamak toroidal magnetic field, generating the torque required to move the probe head back an forth. The equations governing the coil dynamics and motion of the probe head are derived and linearized to optimize the coil parameters for fast-retraction. A multibody model is applied to estimate the induced stresses on the more sensitive components and the effect of friction on the diagnostic trajectory. The mechanical stresses induced by thermal expansion on the probe head under convective heat loads is analyzed to optimize the measurement cycle. A programmable power supply is employed to energize the magnetic coil, enabling real-time retraction when overheating is detected on the probe head. These new detectors enabled simultaneous measurements of multiple FILDs in quiescent plasmas and in the presence of MHD instabilities such as toroidicity induced Alfvén eigenmodes (TAEs) and magnetic islands. Moreover, the insertion of the magnetically-driven FILD4 is rapidly scanned during plasma discharges, enabling unprecedented radially-resolved velocity-space measurements of escaping ions generated by ICRH and NBI. The light-ion beam probe technique is employed to these scans to radially resolve the internal structure of MHD instabilities. The impact of the poloidal spectrum of the applied 3D magnetic perturbations on fast-ion losses and TAEs amplitude is investigated experimentally. The hybrid kinetic-MHD MEGA code is employed to model these experiments. The plasma response to the externally applied 3D perturbations is modelled using the MEGA code for the first time. The observed TAEs are reproduced in both axisymetric and 3D equilibriums, finding a phase-space overlap of the TAE fast-ion drive and the edge resonant transport layer induced by the 3D fields.