High-speed VUV spectroscopy for edge impurity line emission measurements in HL-2A tokamak

A 20 cm focal length normal incidence vacuum ultraviolet （VUV_20 cm） monochromator with a fast time response has been developed for measuring edge impurity line emission in the wavelength range of 300-2000 A on an HL-2A tokamak. An aberration corrected concave holographic grating with 1200 grooves/mm is adopted in the monochromator, which provides a wavelength dispersion of 40 A mm-1. The aperture is f/4.5. A channel electron multiplier is used as a detector. The time resolution of the system is 17 μs. Wavelength calibration of the system has been done by using a hollow cathode light source in the laboratory with helium and argon gases. The obtained signals of helium and argon spectra are very strong since the inner surface of the monochromator vacuum chamber is blackened and the stray light level is then significantly reduced. The optical property of the system has been examined by scanning the width of the entrance and exit slits. The system is then installed at the mid-port of the HL-2A tokamak and typical line emissions from the HL-2A plasma are measured. Time behaviors of edge impurity line emissions are observed with the fast time response system in different plasma confinement regimes, especially in the H-mode discharges. The result shows that the VUV_20 cm system works very well to measure the edge impurity line emissions in the edge localized modes phase of H-mode discharges.

Improvement of Plasma Performance Using Carbon Pellet Injection in Large Helical Device

A cylindrical carbon pellet with a size of 1.2L x 1.2φ mm to 1.8L x 1.8φ mm and a velocity of 100 m/s to 300 m/s was injected into large helical device （LHD） for an efficient fueling based on its deeper deposition instead of hydrogen gas puffing and ice pellet injection. Electron density increment of Ane = 10^14 cm^-3 is successfully obtained by single carbon pellet injection without plasma collapse. Typical density and temperature of the ablation plasma of the carbon pellet, e.g., 6.5× 10^16 cm^-3 and 2.5 eV for CII, are examined respectively by spectroscopic method. A confinement improvement up to 50% compared to ISS-95 stellarator scaling is clearly observed in a relatively low-density regime of ne = 2 × 10^13 cm^-3 to 4×10^13 cm^-3, and high ion temperature Ti（0） of about 6 keV is also observed with an internal transport barrier at ne = 1.2 × 10^13 cm^-3. In particular, the improvement in the ion temperature largely exceeds that observed in hydrogen gas-puffed discharges, which typically ranges below 3 keV.

Observation of Magnetic Dipole Forbidden Transitions in LHD and Its Application to Burning Plasma Diagnostics

Magnetic dipole forbidden （M1） transition was studied in large helical device （LHD） and F-, Si- and Ti-like M1 transitions are successfully observed for highly ionized Ar, Kr, Mo and Xe ions. The wavelengths measured in visible range for the heavy elements, which are carefully determined with extremely small uncertainties of 0.02 - 0.05 A as a standard wavelength of usual electric dipole （El） plasma emissions, are compared with theoretical predictions. The result shows a good agreement with recent Hatree-Fock calculation including semi-empirical adjustment. The M1 intensity for the F-like ions is examined by analyzing the intensity ratio of M1 to El. Density dependence of the ratio is experimentally verified by comparing with collisional- radiative model calculation on level population. The M1/E1 line ratio for the F-like ions is applied to the α （He^2＋） particle diagnostics in ITER, in which a steady-state operation of burning plasmas based on D-T fusion reaction is expected with α particle heating. Unfortunately, the present estimation suggests a negative result for the α particle measurement because the ratio is largely enhanced by the collisional excitation with bulk ions due to high ion temperature of ITER of 10 keV as assumed and the resultant effect of the collisional excitation with α particles becomes less. Meanwhile, the M1 transition, in particular, Ti-like WLIII （W^52＋） transition （3627 A） emitted in visible range, is very useful for diagnostics of the impurity behavior and the core plasma parameters in ITER.