Density measurements in the edge, divertor and X-point regions of Alcator C-Mod from Balmer series emission

Abstract

The electron density in the edge, divertor and X-point regions of Alcator C-Mod [Proceedings of the IEEE 13th Symposium on Fusion Engineering (Institute of Electrical and Electronic Engineers, New York, 1990), Vol. 1, p. 13] has been measured using the Stark broadening of high-n (n=8 through 11) transitions of the Balmer series of deuterium. These measurements have been made during typical single null, diverted operation and during gas-puffing through capillaries located within the first wall and divertor. Electron densities up to 1.6×1021 m−3 have been measured in the X-point region and slightly lower densities (5−7×1020 m−3) have been measured in the divertor and edge regions. These results are factors of 2 to 5 larger than the density measurements from the CO2 laser interferometer system in the main chamber (3×1020 m−3) and comparable with probe measurements in the divertor. The ratios of the widths of adjacent lines within the Balmer series have been compared with ratios calculated for the Stark broadening of these lines. These transitions can be used to measure electron densities from 3×1019 to 2×1021 m−3 with a modest resolution spectrograph [(Δλinst=1 Å full width at half maximum)].

Electron Density

“…(1.44) and using it for the conditions of the low-density discharge at Alcator C-Mod [12], where deuterium Balmer lines up to n max = 16 were observed and the magnetic field was B = 8 Tesla, we obtain T = 6 eV.…”

“…In many types of plasmas -such as, tokamak plasmas (see, e.g., [12]), laser-produced plasmas (see, e.g., [13]), capacitor-produced plasmas [14], astrophysical plasmas [15,16] -there are strong magnetic fields. There are two major effects of a strong magnetic field on the SB of low-n hydrogenic spectral series.…”

“…The simplicity of this method is the reason why, despite the existence of more sophisticated (but more demanding experimentally) spectroscopic methods for measuring N e , this method is still used in both laboratory and astrophysical plasmas. For example, Welch et al [12] used it (with the constant in the right side of Eq. (1.39) suggested by Griem [31]) for determining the electron density in the low-density discharge at Alcator C-Mod.…”

“…The three most frequent applications are edge plasmas of magnetic fusion devices [12,21], where N e ∼ (3 × 10 13 -10 15 ) cm −3 , plasmas of radiofrequency discharges [22], where N e ∼ 10 13 cm −3 , as well as the plasma of the solar chromosphere, where in the quiet Sun N e ∼ 10 11 cm −3 [23] (in solar flares typically N e ∼ 10 13 cm −3 , but the SB can be controlled by the electrostatic plasma turbulence -as presented in Chapter 6.1). While the absolute intensity of the lines decreases along the spectral series, the SB (and thus the sensitivity to plasma microfields) increases.…”

Electron density measurements in the National Spherical Torus Experiment detached divertor region using Stark broadening of deuterium infrared Paschen emission lines

“…We use the astrophysical notation for the series lines: for example, H10 is used for the Balmer 10 − 2 transition, and P10 for the Paschen 10 − 3 transition. The corresponding ultraviolet (UV) H6-H14 and near-infrared (NIR) P8-P14 lines have been observed in many divertor MCF experiments [3][4][5][6]. While the Stark-broadened Balmer lines, as well as the Balmer and Paschen continua emissions have been extensively used for diagnosing tokamak detached divertor plasma conditions, no application of the NIR Paschen lines has been reported.…”