Use of Fast Ion Loss Detectors to Probe Internal Perturbations

Kenneth Ryan Gage
UC Irvine
Tuesday, May 23, 2023
11:00 am
FRH 4135 + Remote



Fast Ion Loss Detectors (FILDs) are edge diagnostics used to analyze the
distribution of energetic particles (EPs) lost to the vessel walls in a
plasma device. More complex probes are capable of distinguishing
populations of losses by their velocity, pitch angle, and frequency. FILDs
can be utilized as a Light Ion Beam Probe (LIBP) if the observed losses are
EPs that are ejected promptly after passing through a magnetic perturbation
(MP). This technique allows for inference of the orbit displacement caused
by a single pass through the perturbation.

On DIII-D and AUG, rigidly rotating n=1 magnetic fields were externally
applied to the plasma. The upper and lower sets of coils used to generate
the MPs were held with a phase difference between the two sets to analyze
how the MP spectra affected losses. On DIII-D, the normalized pressure was
also varied to modify the strength of the plasma response to applied MPs.
Simulations of AUG discharges show a significant dependence of orbit
displacement on initial pitch angle, and confirm that density perturbations
can lead to signals in the FILD that pollute LIBP signal from the MP.
DIII-D simulations show that relative fluctuation of lost ions is not
significantly diminished when the plasma response decreases for certain MP
spectra. Simulations also show a concentration of prompt losses to the
outer midplane, a poloidal location usually more susceptible to potential
damage due to the use of diagnostics here.

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William Heidbrink