Scintillator based detector for fast-ion losses induced by magnetohydrodynamic instabilities in the ASDEX upgrade tokamak

Abstract

A scintillator based detector for fast-ion losses has been designed and installed on the ASDEX upgrade (AUG) tokamak [A. Herrmann and O. Gruber, Fusion Sci. Technol. 44, 569 (2003)]. The detector resolves in time the energy and pitch angle of fast-ion losses induced by magnetohydrodynamics (MHD) fluctuations. The use of a novel scintillator material with a very short decay time and high quantum efficiency allows to identify the MHD fluctuations responsible for the ion losses through Fourier analysis. A Faraday cup (secondary scintillator plate) has been embedded behind the scintillator plate for an absolute calibration of the detector. The detector is mounted on a manipulator to vary its radial position with respect to the plasma. A thermocouple on the inner side of the graphite protection enables the safety search for the most adequate radial position. To align the scintillator light pattern with the light detectors a system composed by a lens and a vacuum-compatible halogen lamp has been allocated within the detector head. In this paper, the design of the scintillator probe, as well as the new technique used to analyze the data through spectrograms will be described. A last section is devoted to discuss the diagnosis prospects of this method for ITER [M. Shimada et al., Nucl. Fusion 47, S1 (2007)].

Scintillator-based diagnostic for fast ion loss measurements on DIII-D

“…Measuring the twodimensional light pattern from the scintillator yields information on both the perpendicular gyroradius and the pitch angle of the escaping fast ions, an approach first used to detect escaping fusion products 5 and more recently used to study losses of energetic ions from rf heating and neutral beam heating. 6 Figure 2 shows a schematic drawing of the FILD diagnostic. The detector head has a graphite heat shield ͑diameter ϳ8.9 cm͒ to protect the detector from the plasma and beam ion heat loads.…”

“…The TG-Green scintillator, first used on the AUG fast ion loss detector, 6 has a 490 ns decay time, high light generation ͑ionoluminescense͒ efficiency, and high saturation levels. For fast time response measurements, a beam splitter cube couples ϳ50% of the scintillation light to a second 10-110 mm zoom lens designed to image the entire scintillator onto the input of a single 1500 m diameter fiber.…”

Gamma-ray spectroscopy measurements of confined fast ions on ASDEX Upgrade

“…Frequencies due to Toroidal Alfvén Eigenmodes (TAE) driven by energetic protons appear in the range 100-250 kHz, from 1.11 s. The toroidal mode numbers, obtained by measurements of the phase shifts among coils displaced at different toroidal locations, are n=3-6 between 1 and 2 s; TAEs with lower toroidal numbers were also excited after the NBI blip at 2 s. A complex loss pattern in phase space was correspondingly detected with FILD [17] (figure 6b). Prompt losses with pitch angles in the range 70-75° and Larmor radii between 80-100 mm were observed at t=1.06 s, i.e.…”

Multi-view fast-ion D-alpha spectroscopy diagnostic at ASDEX Upgrade

“…Their distribution function must consequently be investigated by diagnostics that can access different parts of the corresponding six dimensional phase space. Several approaches have been developed to monitor the fast ions, such as the measurement of neutrons 3 , gamma rays 4 , Doppler shifted micro waves 5 and fast-ion losses 6 . In addition, a widely applied technique to obtain information on the fast ions is based on their charge exchange reactions with neutrals.…”