Development of Thomson Scattering Measurement System for Long Duration Discharges on the QUEST Spherical Tokamak

Abstract

The Thomson scattering control system has been modified to measure the time evolution of the electron density and temperature profiles during long duration discharges in the QUEST spherical tokamak. The system consists of a signal generator and a control circuit. The former accepts a QUEST main trigger and provides multiple triggers, each of which starts a short-term (e.g. 15 s) measurement. The latter provides triggers to synchronize the oscilloscope and laser oscillation during a short-term measurement. The system was used in 1000 s long duration discharges in QUEST, and the temporal evolutions of density and temperature profiles were obtained successfully. It was found the profiles are stationary after about 300 s. c

Comparison of electron temperature and density measured by helium line intensity ratio and Thomson scattering methods in ECH spherical tokamak plasma

“…Furthermore, the measurement in this study was performed at every 1 s with accumulating ten spectra measured at every 100 ms interval to improve the signal-to-noise ratio of the spectra. The temporal stability of the measurement was previously confirmed in a long-duration discharge plasma, having T e and n e similar to the present plasma [20]. The hydrogen and helium atom densities evaluated using a quadrupole mass spectrometer at the vacuum chamber [22] were 7 × 10 16 m −3 and 5 × 10 14 m −3 , respectively, whereas the helium atom density evaluated by spectroscopy in this study was 2-5 × 10 14 m −3 .…”

“…The discharges became steady after ∼20 s, providing good reproducibility of plasma parameters and helium atom spectra. T e and n e profiles in the midplane measured by the Thomson scattering method [19,20] at 62.5 s are indicated by symbols in figure 3. T e reached up to 60 eV on the inboard side near the ECH resonance layer, and n e had a relatively flat profile of 2-5 × 10 17 m −3 .…”