Speaker
Description
Resonant energy exchange between waves and particles is a fundamental concept in plasma physics. It was previously measured in satellite and basic laboratory experiments but has never been measured in the core of a magnetic fusion device. Now, however, thanks to DIII-D’s diagnostic suite, resonant energy exchange between energetic particles and Alfvén waves has been measured. This work establishes a validated experimental methodology for extracting energy transfer rates directly from diagnostic fluctuations without requiring absolute calibration.
The DIII-D imaging neutral particle analyzer (INPA) [1] systems were upgraded with a new set of 16 fast channels, achieving temporal resolution of 350 kHz while maintaining a good resolution in phase space (7 keV in energy and 10 cm in radius). These new capabilities allowed to capture the fluctuations of the confined population of energetic particles both in the Alfvenic frequency range (~100kHz for typical DIII-D parameters) and the tearing and fishbones mode (TM) frequency range (~15 kHz for typical DIII-D discharges). In both frequency ranges, fluctuations of a few percent over the baseline signals are observed.
The measured fluctuation of the confined EP, together with the electric fields extracted from the density and temperature fluctuations [2] allows for the characterization of the energy exchanged between the energetic particles and the waves [3]. The secular energy exchange is mostly carried by the interaction of EP drift velocities with the perpendicular (poloidal) electric field; parallel and radial components are negligible, directly confirming the expected behavior due to large drift of EP orbits. Comparison with ASCOT simulations using a mode structure predicted by the NOVA code agree with the observed energy exchange within the approximation of the calculation.
ASCOT scans show that a magnetic fluctuation amplitude of δB/B ≈ 2×10⁻⁶ (corresponding to ~O(1 eV) temperature fluctuation) produces non-overlapping phase-space islands, which is compatible with coherent INPA signals; a mere ~3× increase causes island overlap and likely diffusive transport, explaining why large-amplitude TAEs (observed in the ramp-up and other scenarios) are invisible to the INPA fast channel.