High density plasma production using m= +I and m=-1 helicon waves is studied. Characteristics of cylindrical helicon waves including effects of a vacuum gap between the plasma and the conducting wall and of a non-uniform separately excited by a helical antenna, and the dependences of plasma density and antenna loading resistance on RF power are shown to be different for these modes.

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 n̄e = 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/n̄e ⪅ 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.

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.

Helium I line intensity ratios obtained by a collisional radiative model, including new recommended excitation rate coefficients and the effects of hot electrons, enable us to measure electron density (ne) and temperature (Te) in high density plasma. Measured ne and Te using 492.2 nm (4 1D→2 1P)/471.3 nm (4 3S→2 3P), 504.8 nm (4 1S→2 1P)/471.3 nm, and 492.2 nm/504.8 nm line intensity ratios are in good agreement with the Langmuir probe results in the helium discharge plasma in the NAGDIS-I linear device (Nagoya University Divertor Simulator) for ne and Te regions of 1011–4×1012 cm−3 and 5–20 eV. Hot electrons in the plasma are important for the He I line emissions when Te is below the excitation energy ≊20 eV. A resonance scattering effect included in the calculation accounts for the experimental result of the enhanced 501.6 nm (3 1P→2 1S) line emission.

We measured temporal variations of the distributions of C2 and C3 radical densities in carbon plumes produced by laser ablation of graphite in ambient He gas. Laser-induced fluorescence imaging spectroscopy was used for the measurement. The temporal variations of total numbers of C2 and C3 contained in plumes were evaluated by integrating the density distributions. The experimental observations have shown that the gas-phase production of C2 is comparable to the direct production from the target, while C3 is mainly produced in gas phase by three-body reactions between C and C2. In addition, we have discussed a scenario for the temporal evolution of heavy clusters (Cn with n⩾4). The present results are useful for understanding initial formation processes of carbon clusters in laser-ablation plumes.