Recent Activities on JIPP T-IIU Tokamak

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The JIPP T-IIU, upgraded from JIPP T-II in FY 1982,is a mediumsize tokamak with major radius 91-93cm, plasma radius 23-25cm of circular cross section, and the maximum toroidal magnetic field 3T. The device has mainly been used for the studies of non-inductive current start-up by means of lower hybrid wave (LHW) and ion cyclotron range of frequency (ICRF) heating including ion Bernstein wave (IBW) heating. A typical set of device and plasma parameters are listed in Table 1,and a plan view of the device with heating and diagnostics is shown in Fig.1. By an injection of LHW into a cold and low density plasma produced by electron cyclotron resonance discharge, a plasma current is initiated and raised to a quasi-stationary value of about 20 kA with a certain characteristic time. The current rise time is found to be inversely proportional to the bulk electron density and to agree with the collision time of high energy electrons (T_<et> ∿ 30keV) with the bulk plasma. In the current rising phase the bulk electrons are rapidly heated to over 100eV by a return current of the bulk electrons. This electron heating has been evaluated from a time history of oxygen lines of various ionization states, and proved consistent with the results from Thomson scattering. The influx of impurities and the growth of m = 2 mode are serious problems to be solved when high power ICRF heating is carried out on a tokamak plasma. A strong gas-puffing and an additional rise of plasma current have been applied during an ICRF heating (D_2 + H_2 minority) to overcome these difficulties. A significant reduction has been observed in the influx of heavy impurities (Fe) as well as light impurities (C, O). It is regarded that the radial profiles of electron temperature and density and plasma current density are controlled favorably by this combination of the gas puffing and the current rise. Ion Bernstein wave heating, a new heating scheme, has been applied using a loop antenna with an rf-current component parallel to the toroidal confining magnetic field (Nagoya Type III Coil). A toroidal field of 1.8T is chosen so that the rf frequency of 40MHz be the third harmonic of ^4He cyclotron resonance at the center of the hydrogen plasma column. An efficient heating has been observed in perpendicular temperature (ΔT_<i⊥> ∿ 600eV) from an energy analysis of charge-exchanged fast neutrals, while rather moderate increase in parallel temperature which is regarded closer to the bulk ion temperature. Here a forbidden line 974.8 A of FeXVIII has been utilized in obtaining ion temperature since the intensity of this line is sensitive to the proton temperature. The result agrees with T_<i⊥> from the fast neutral energy analysis. Besides these spectroscopic works related to JIPP T-IIU experimental program, Plasma Spectroscopy Group and Plasma Diagnostics Group in the Institute have been making their own works such as a calibration of VUV system, X-ray spectroscopy on vacuum spark plasmas, Li beam probe spectroscopy, and laser-induced fluorescence spectroscopy.
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Recent Activities on JIPP T-IIU Tokamak

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