Table of contents

During plasma excitation by high power Alfven waves at TCA, signals at harmonics of the generator frequency are observed in the plasma scrape-off layer. Experimental investigations of the sheath effect and the excitation efficiency and dispersion properties of the harmonics are reported. The results indicate that the harmonics arise, either directly or indirectly, through the driven Alfven waves and not through the sheath effect at the exciting antenna. The RF ion saturation current is observed to have a nonnegligible peak amplitude in comparison with the time averaged ion saturation current and may provide evidence of a nonlinear evolution of the driven Alfven waves

In D-T and D-³He plasmas, ICRF heating at the second harmonic of deuterium results in a modification to the distribution function of the heated ions. The authors describe the results of such effects on the dynamic behaviour of plasmas. Using a 0-D plasma transport model, the effect of ICRF heating on an operating point is analysed for ITER and for a D-³He tokamak. To describe the dynamic behaviour, a model for the characteristic time of tail relaxation, τ_ξ, is developed and a feedback model based on auxiliary power is presented. The stabilization of temperature perturbations in a D-³He plasma is simulated for various values of τ_ξ

From the first principles of plasma physics, the local thermal diffusivity can be expressed in terms of the Bohm diffusivity χ_B and a function, F, of dimensionless parameters. Tests of transport models, theoretical, empirical or heuristic, expressed in terms of such parameters often produce ambiguous answers, which arise from collinearities in the experimental data. In experiments on dimensionally similar discharges, the scaling of confinement with one single dimensionless parameter can be examined in a more unambiguous fashion. Experiments on JET have been carried out in which only the normalized Larmor radius rho is varied. This is achieved by varying the plasma current, the toroidal field, the density, the ion cyclotron frequency and the power level in a predetermined pattern, such that other dimensionless parameters, for example the normalized collisionality ν_*, β and the safety factor q_ψ, are kept constant. The experiments on JET demonstrate that L mode confinement, both globally and locally, scales with ρ_* according to the long wavelength or Bohm scaling. The implications for theory and future experiments are outlined

Measurements of third harmonic electron cyclotron emission have been made on the Advanced Toroidal Facility (ATF). Electron temperatures have been extracted from third harmonic, optically thin, electron cyclotron emission (ECE) measurements. The radial and temporal evolutions of the electron temperature have been characterized for electron cyclotron resonance heated discharges and for the early stages of neutral beam heated plasmas. A slab based emission model incorporating the equation of radiation transfer and ray tracing has been used to calculate the expected ECE. The results have been compared with experimental measurements. Measurements of the polarization of the third harmonic ECE during 1 T discharges have been compared with theoretical calculations of wave polarization rotation due to magnetic shear. Significant polarization of the emission was observed, and correction factors for a given wave frequency and peak density were obtained

The effects of fast electron confinement and plasma temperature on the lower hybrid current ramp-up efficiency are investigated. The production of reverse runaways in a toroidal geometry is calculated with a Monte Carlo particle following code, which includes collisions and both neoclassical and relativistic effects. When anomalous fast electron diffusion in the configuration space is taken into account, the ramp-up efficiency is found to be enhanced for increasing loss rate of runaway electrons. The enhancement is in accordance with a simple analytical model that expresses the efficiency as a function of the average runaway electron confinement time and the runaway fraction of current carrying electrons. A larger runaway production rate is found for the electrons orbiting on outer magnetic surfaces, because their trapping region in velocity space is wider. The runaway rate is also significantly enhanced, as the thermal velocity is increased up to and above one sixth of the runaway velocity v_R. The range of the phase velocity spectrum where the waves efficiently ramp up the current in large tokamaks is limited by these effects to velocities below 1.3v_R

The ion temperature gradient (ITG) driven instability is investigated for a plasma with trapped ions and collisions. For the toroidal branch, the perpendicular temperature gradient is needed for the stability in contrast to the slab branch. The threshold value of the temperature gradient is higher with trapped ions and is further increased by electron and ion collisions. A theoretical model is used to explain the experimental results for a hybrid trapped ion ITG mode in the Columbia linear machine

The geometrical effect of a noncircular plasma cross-section on the loss of alpha particles induced by a toroidal field ripple is investigated theoretically and numerically by using an orbit following a Monte Carlo code. The ripple-enhanced banana drift and the critical field ripple for ergodic orbits are very sensitive to the elongation. The effect of the triangularity on these parameters is not as important as that of the elongation. The ripple-enhanced banana drift loss shows a drastic decrease with plasma ellipticity. As the banana drift loss is reduced with ellipticity, the ripple trapping begins to dominate the total loss and shows a sharp increase with ellipticity. The resulting beneficial effect of elongation on reducing ripple-enhanced losses is weakened. Two key parameters that affect the fraction of ripple-trapped loss, the nonuniformity of the elongation and the gap between the plasma surface and the first wall, have been found. The geometrical effect is very important, not only for the basic loss mechanisms but also for the global ripple losses of alpha particles in connection with the distribution of field ripple

The PHAEDRUS-T tokamak was operated with radiofrequency power near the ion cyclotron frequency at 90 degrees phasing between two current straps with and without a stainless steel Faraday screen covering the antenna. In both cases, the sides of the antenna were protected by insulating limiters. The plasma parameters in the scrape-off layer were measured and were shown to be essentially the same when radiofrequency power was applied from the Faraday screen covered antenna as compared with the antenna without a Faraday screen. The intensity of Fe(XVI) light dropped an order of magnitude alter the screen was removed

Electron heat pulse propagation following sawtooth collapse has been simultaneously observed along two opposite radial directions. For a given flux surface, the arrival of the heat pulse at the high field side (HFS) is delayed by up to 1 ms as compared with the arrival at the low field side (LFS). The observed delay agrees with an estimate of the time required for parallel electron heat transport along the magnetic field lines connecting the corresponding LFS and HFS isobars. This suggests that the primary radial heat transport occurs on the LFS, whereas the HFS is fed subsequently from the LFS by temperature equilibration along the connecting magnetic field lines

The density limits were determined in Ohmic and L mode discharges over a wide range of parameters: B_T = 0.8-2.1 T, I_p = 0.5-1.9 MA, an elongation of 1.09 to 1.92, Z_eff < 2, P_beam ≲ 8 MW and n_e,max 1.2 × 10²⁰ m^-3. The density limits in ELMing H mode discharges were also determined and found to be comparable with those of the Ohmic and L mode discharges. In Ohmic plasmas, the density limit increased linearly with I_p and decreased with minor radius (~1/a^2.4); the density limit showed little, if any, dependence on either B_T or plasma elongation. These results are in good qualitative agreement with Greenwald density limit scaling, (n_e,max(10²⁰ m^-3) ~ I_p (MA)/a² (m)). In discussing the density limits of detached plasmas, the 'effective' minor radius must be considered. The density limit data for L and H mode discharges were also consistent with Greenwald scaling. The density limits in the L and H modes were insensitive to the heating power. In the Ohmic and L mode cases, rotating MHD modes, typically with m/n=2/1, existed in high density discharges with no perceptible adverse impact on particle confinement until they locked, precipitating global disruptive behaviour. To date, the data show that the ELMing H mode density limit is not disruptive in itself

Carbon test limiters have been exposed in TEXTOR to high heat loads up to about 30 MW/m². The evolutions of the surface temperature distribution and of the carbon release have been observed by means of local diagnostics. A sudden acceleration of the rise of the surface temperature has been found at a critical temperature of approximately= 2400 degrees C. The increase of the rate of the temperature rise is consistent with an enhancement of the power loading by a factor of 2.5-3. Following the temperature jump (hot spot), a quasi-equilibrium temperature establishes at approximately= 2700 degrees C. The development of the hot spot is explained by an increase of the local power loading due to the breakdown of the sheath potential by thermal emission of electrons from the carbon surface. Simultaneously with the appearance of the hot spot, the carbon release from the surface increases sharply. This increase can be explained by normal thermal sublimation. Sublimation cooling contributes to the establishment of the quasi-equilibrium temperature at about 2700 degrees C

Through the use of an auxiliary function of the usual poloidal magnetic stream function, ψ, anisotropic axisymmetric equilibria subjected to the condition σ_- = (p_|| - p_⊥)/|B|² = σ_-(ψ) can be constructed starting from solutions of the Grad-Schluter-Shafranov equation for isotropic plasmas