It is shown that the toroidal ion temperature gradient (ITG) mode has an upper η i (higher η i ) stability regime for experimentally relevant parameter values in addition to the lower regime with the stability threshold (η ilow ) at η i around one (η i = L n /L T i where L n and L T i are the characteristic lengths for the density and ion temperature gradients). The ITG mode is studied with a focus on the upper η i stability regime and the β dependence (β = plasma pressure/magnetic pressure). The results of a gyrokinetic and a two-fluid model as well as a semilocal approximation are compared. It is shown that the upper stability threshold (η iup ) is very sensitive to and considerably reduced by finite-β effects. It is also sensitive to finite Larmor radius (FLR) effects and to ε n = 2L n /L B (L B is the characteristic length for the toroidal magnetic field gradient). Predictions and comparisons are made with data from a joint European torus (JET) optimized shear discharge and a JET hot ion H-mode (high-performance mode) discharge.

Experimental results concerning the plasma response to the biasing of the tokamak ISTTOK localized limiters, on a strong flat-top plasma current reference discharge, are reported. Modifications of central beta as well as of energy confinement time are determined through timeresolved measurements of the line-averaged plasma density, electron density profile, electron temperature and ohmic power. Gross particle confinement variations are confirmed by the associated changes of the ratio between the line-averaged electron density and the H α radiation level. Plasma stability modifications are analysed by measurements of the plasma column transverse displacement, plasma poloidal rotation frequency and sliding fast Fourier transform spectra of both the magnetic and the electron density fluctuations. The evolution of the amplitude as well as the frequency of the most important tearing modes is determined. Negative bias leads to better particle and energy confinement, and improved stability. Positive bias reduces both confinement and stability, causing a significant transitory vertical displacement of the plasma column as well as of its current axis.

The electromagnetic waves in a partially-filled plasma waveguide, placed in an external d.c. magnetic field, are numerically and experimentally investigated. Compared with the homogeneous filled waveguide. two new effects are observed. The inhomogeneous structure improves the cyclotron mode resolution. When the plasma filling of the waveguide is decreased, the coupling of both the EH and HE high-frequency modes with the cyclotron modes is possible. It is shown experimentally that increasing both the magnetic field and the discharge current (i.e. the plasma density) improves the cyclotron mode resolution.

The ion temperature gradient driven ballooning mode is investigated using two-fluid, gyrofluid, and gyrokinetic descriptions. The linear eigenmode equation is solved numerically in a model equilibrium with shifted circular magnetic surfaces. The localization of the eigenmodes, which persist in the magnetohydrodynamic (MHD) second stability region, and the mode structure, are displayed. The role of finite-Larmor radius (FLR) and magnetic drift resonance effects on the growth rate are elucidated. Negative magnetic shear is found to have a stabilizing effect on the mode.

The influence of the trapped electrons fraction on the growth-rate of the ion temperature gradient (ITG) modes is studied numerically for JET relevant conditions. As a possible mechanism for increasing the ITG mode growth rate with an increase in the fraction of trapped electrons, a shift in the real frequency is assumed that leads to less Landau damping. Another possible mechanism is the destabilizing contribution of dissipation effects.