K. Kadota

Space- and time-resolved study of impurities by visible spectroscopy in the high-density regime of JIPP T-II tokamak plasma

Time-resolved spatial profiles of Zeff in JIPP T-II tokamak plasmas have been determined from the measurement of bremsstrahlung in the visible spectral region. The effective ion charge Zeff increases at the plasma centre and decreases at the plasma periphery in the high-electron-density regime. The emissions from low ionization states of light impurities increase notably in the region of lower electron temperatures in this regime. As a result, impurities accumulate in the plasma centre in the high-density regime. The results obtained with respect to the plasma periphery support the interpretation of the behaviour of the impurities, which is based on VUV spectroscopic studies.

Current density profile control by programming of gas puffing and plasma current waveform in the JIPP T-II tokamak

In the resistive-shell tokamak, JIPP T-II , a control of the current density profile has been attempted by programming both gas puffing and plasma current waveform. A stable high-density plasma has been obtained with the following parameters: the maximum line-average electron density is e = 8.5 × 1013cm−3, the minimum q(a)-value is 2.2, and the relative amplitude of the m/n = 2/l mode is suppressed to an extent less than 10−3. A derivation of the current density profile by solving the magnetic-diffusion equation on the basis of the experimental data shows that the current density profile favourable to the stability of low-m kink and tearing modes is realized by combining the effects of cooling through an increase in density and of heating by a current rise in the outer plasma region. The results of kink and tearing modes analysis agree well with the experimental observations. The criterion that the current density profile is successfully controlled by this method is derived as a function of the ratio of plasma current to electron density in the current-rise phase, i.e. 20 × 10−13 Ip/e 30 × 10−13 kA·cm3. The major disruption due to the density increase is completely suppressed by the method proposed in this paper. The major disruption due to a reduction of q(a) to less than 2.2 has, however, not yet been suppressed. In future, the current density profile should be maintained more precisely at its optimum shape by using a feedback-control technique and a control of the plasma boundary with titanium gettering, etc.

Observations of low frequency density fluctuations in a tokamak edge plasma

Density fluctuations in the outer edge of the plasma, behind the limiter, have been investigated in the TEXTOR tokamak by a new technique using a probing beam of thermal neutral lithium. With this method, radial profiles of the electron density and its fluctuations are obtained and thus also the relationship between turbulence and plasma parameters, especially the density gradient scale lengths. The turbulences are found to consist of coherent fluctuations in the frequency range of sawtooth oscillations (<100 Hz) and fluctuations having a broadband spectrum of frequencies up to ~1 MHz. The frequency spectra vary, depending on radial position and the density gradient scale length. It is also shown that the fluctuation level is well correlated with the density gradient scale length.

Limiter H-mode and other improved confinement regimes with ICRF and NBI heating in JIPP T-IIU

The H-mode, an improved confinement regime, was attained recently in JIPP T-IIU high power heating experiments, in a limiter configuration without any shaping of the plasma cross-section. This H-mode is unusual because it was obtained with heating in the ion cyclotron range of frequencies (ICRF). It was also attained with combined ICRF and neutral beam injection (NBI) heating. The threshold power level obtained with ICRF alone is similar to or less than that obtained in the combined heating case. The dependence of the power threshold of the H-mode on various plasma parameters has been studied. It increases with the plasma current and is insensitive to the plasma density, and there is an optimum value of the toroidal field intensity. The power deposition profile for ICRF heating has been analysed with a ray tracing code and used to explain the observed dependence on the toroidal magnetic field. The paper also discusses a class of discharges with improved confinement observed in the same series of experiments. These discharges had a power level close to the H-mode threshold power and exhibited a marked improvement of confinement. They were, however, different from H-mode discharges in the time evolution of the profiles.

High power ICRF heating experiments on the JIPP T-IIU tokamak

In the JIPP T-IIU tokamak, a high power ICRF heating experiment has been conducted, up to an extremely high power density (~2 MW·m−3), with a total RF power of PRF = 2 MW. Great attention has initially been paid to the problem of impurities, and it has been found that (a) the adoption of low Z materials for the limiter, (b) in situ carbon coating (i.e. carbonization) and (c) adequate gas puffing synchronized to the RF pulse are very effective in suppressing radiation loss. With these methods, a remarkable reduction in metal impurities (especially in iron impurity) has been achieved; the total radiation loss has been reduced to less than 30-40% of the input power. In these reduced radiation loss plasmas, the characteristics of ICRF heated plasmas have been studied intensively. With an increase in the ICRF heating power, a deterioration of the energy confinement time has been observed, indicating quantitative agreement with the Kaye-Goldston L-mode scaling. It is shown that the so-called profile consistency, which is the leading feature in neutral beam heated plasmas, also holds in ICRF heated plasma. It has been observed that the electron temperature profile only responds to the safety factor q(a) and does not change when the deposition profile is controlled by tailoring the k1 spectrum.