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The tungsten spectrum in the extreme ultraviolet (EUV) wavelength range has been investigated in tokamak HL-2A to find line emissions from low-ionized tungsten ions which can be used for tungsten transport study in the plasma edge. For this purpose, tungsten is injected in the HL-2A plasma using the laser blow-off (LBO) technique with a newly developed double-layer tungsten target. The total number of injected tungsten particles can be accurately controlled by adjusting the distance from the focal lens to the tungsten target. A quasi-continuum spectrum called an unresolved transition array (UTA), consisting of many line emissions from several ionization stages of tungsten ions, is observed at around 50 Å as a typical indication of the tungsten spectrum. In the present HL-2A experiment the quasi-continuum of tungsten is mainly composed of line emissions from W14+ to W23+ ions. Analyzing the observed tungsten spectrum, two isolated line emissions of W VII (5p–5d: 216.219 Å) and W VII (5p–5d: 261.387 Å) from W6+ ions are successfully identified for the first time in tokamaks. The intensity ratio and the wavelengths of the two lines determined in the present experiment show good agreement with the intensity ratio calculated with a collisional-radiative model developed for the W6+ ion and a previous result by Sugar and Kaufman (1975 Phys. Rev. A 12 994), respectively. On the other hand, the W VII line emission occasionally reappears after the LBO injection, suggesting a re-entry of the tungsten deposited onto divertor plates and a movement of the strike point during the discharge to a certain area of the divertor plate where tungsten is deposited. Based on the W VII emission observed during the re-entry event of tungsten ions, an influx of the W6+ ion, , to the edge plasma is evaluated using the inverse photon efficiency calculated with the ADAS code and using an average-configuration option. As a result, it is found that is typically within the range (0.1–2.0)  ×  1013 cm−2 s−1. A brief discussion is also made on the influx evaluated in the present study. [ABSTRACT FROM AUTHOR]

In HL-2A, an inverse sawtooth oscillation is observed with a long-lasting m/n  =  1/1 mode during ECRH phase with power deposition inside sawtooth inversion radius (inner-deposited ECRH), while a normal sawtooth instead appears when the ECRH power is deposited outside sawtooth inversion radius (outer-deposited ECRH). Aluminum is then injected as a trace impurity with laser blow-off (LBO) method into the inner- and outer-deposited ECRH phases of HL-2A discharges to investigate the effect of ECRH on impurity transport. Temporal behavior of soft x-ray (SXR) array signals is analyzed with a 1D impurity transport code, and radial structures of impurity transport coefficients are obtained. The result shows that the radial transport of Al ions is strongly enhanced during the inner-deposited ECRH phase. In particular, an outward convection velocity is developed with positive values of 0  ⩽  V(ρ)  ⩽  3.8 m s−1 in ρ  ⩽  0.5, while the convection velocity is inward in ρ  ⩾  0.6. In the outer-deposited ECRH discharge, on the other hand, the convection velocity takes a big negative value in ρ  ⩽  0.4 and close to zero at ρ ~ 0.6. In ohmic discharges, an inward V(ρ) always appears in the whole plasma radii and gradually increases toward the plasma edge (−3.2 m s−1 at ρ  =  1). The simulation result also indicates that centrally-peaked Al ion density profiles presented in the outer-deposited ECRH discharge can be flattened by the inner-deposited ECRH. Modification of impurity transport is discussed in the presence of long-lasting m/n  =  1/1 MHD mode. [ABSTRACT FROM AUTHOR]