Experiment Group LHD

H. Funaba

M. Kobayashi

National Institute for Fusion Science (NIFS-Repository)

The compression ratio of helium at the divertor regionis important for the helium exhaust in a fusion reactor. Twoof ten torus inboard side divertors have the bafﬂe structureon the Large Helical Device (LHD) [1]. In order to evaluatethe helium compression ratio at the divertor regions with andwithout the bafﬂe structure, two penning gauge spectroscopysystems are newly installed to the torus inner ports of LHD.The penning gauge spectroscopy can measure the partial pres-sures of neutral gases at each local position from the spectro-scopic observation of the penning discharge inside the gauge.This diagnostic was developed at TEXTOR in order to distin-guish the partial pressures of deuterium and helium [2]. Thepenning gauge spectroscopy is ﬁrst installed on LHD at 2004.Since it is located at the upper port, it mainly measures theperipheral pressure of the vacuum vessel.Two penning gauges are installed at the torus inner portsof LHD. One is located at the bafﬂe structured divertor and theother is located at the divertor region without the bafﬂe struc-ture. The same type (Leybold PR36) of the gauge heads areused and the high voltage from the same power supply is usedfor them. Although the penning gauge originally has perma-nent magnets of about 0.13 T in order to make the magneticﬁeld for the penning discharge, the permanent magnets areremoved because the magnetic ﬁeld strength at the torus in-side is up to about 2 T when the magnetic ﬁeld strength at themagnetic axis, Bax, is 2.75 T. It is possible to start penningdischarge with the magnetic ﬁeld of LHD. The dependenceof signal intensity on the magnetic ﬁeld strength is small atthe region of Bax ≥ 2T. The light in the penning gauges aretransferred to the visible spectrometer by optical ﬁbers. Thesignals from two different positions are simultaneously de-tected by a CCD camera. As the wavelength range is about580 ∼ 730 nm, Hα and 4 lines of He I are observed in thisrange. For the helium partial pressure measurement, the He Iline of 667.8 nm is used. The time resolution is usually 200msfor high density plasmas and 500ms for low density plasmas.Figure 1 shows an example of temporal evolutions ofsome parameters and signals in a plasma where helium gaswas injected: (a) averaged electron density, (b) hydrogen gas-puff, (c) helium gas-puff, (d) H-pressure without the bafﬂestructure, (e) He-pressure without the bafﬂe structure, (f) H-pressure with the bafﬂe structure, and (g) He-pressure withthe bafﬂe structure. The helium gas was injected by the gas-puff at t = 3.8 s during the hydrogen gas puff (Fig. 1 (b, c)).One penning gauge spectroscopy system is located at the baf-ﬂe structured divertor and the other is located at the divertorregion without the bafﬂe structure. The hydrogen and heliumpartial pressures (Fig.1 (d) ∼ (g)) are derived from the Hαand He I (668nm) line intensity in the penning gauges, respec-tively, with the calibration where hydrogen or helium gas wasﬁlled in the vacuum vessel. Both H- and He-pressures at thebafﬂe structured divertor are higher than those at the positionwithout the bafﬂe structure by about one order.02×10194×10196×10198×1019<ne>[m−3 ]<ne>(a)gas puffgas puff (H)(b)gas puffgas puff (He)(c)0.000.020.040.06P [Pa](d) Neutral Pressure (w/o baffle)H20.000.020.040.06P [Pa](e) Neutral Pressure (w/o baffle)He0.000.20P [Pa](f) Neutral Pressure (with baffle)H22.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.50.000.20time [s]P [Pa](g) Neutral Pressure (with baffle)HeFig. 1. Temporal evolutions of some parameters and signalsin a plasma where helium gas was injected.(a) averaged electron density, (b) hydrogen gas-puff, (c) he-lium gas-puff, (d) H-pressure without the bafﬂe structure, (e)He-pressure without the bafﬂe structure, (f) H-pressure withthe bafﬂe structure, and (g) He-pressure with the bafﬂe struc-ture.[1] S. Masuzaki, M. Kobayashi et al., Plasma and Fusion Res.6, (2011) 1202007.[2] T. Denner, K.H. Finken, et al., Rev. Sci. Instrum. 67,(1996) 3515.53§32. Helium Pressure Measurement by the Penning Gauge Spectroscopy in the Divertor RegionFunaba, H., Kobayashi, M.,  LHD Experiment Group

Helium Pressure Measurement by the Penning Gauge Spectroscopy in the Divertor Region

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