Table of contents

The scaling of turbulent density fluctuations with drift-wave characteristics has been examined in the PDX tokamak using scattering of 2 mm microwaves. The authors have measured how the frequency spectra, the k_⊥ spectra and the magnitude of the fluctuations vary with toroidal field, neutral beam heating power, plasma current, position and confinement regime in PDX. Since turbulence is a possible cause of anomalous heat loss in PDX and other tokamaks, the scaling of the turbulence is compared with energy confinement scalings. In many cases, an increased fluctuation level is correlated with decreased energy confinement. In one case, the fluctuation level does not change as the confinement properties of the plasma change. The scaling of the experimental fluctuation levels is compared with various drift-wave parameters. It is found that the fluctuation level scales as the mixing-length limit, ñ/n ∼ l/k_⊥L_n, in some sets of data, but not in other sets.

The flux of energetic (E ≥ 100 eV) atoms emanating from the outer target plate in the divertor chamber of ASDEX is measured by classical corpuscular diagnostic techniques. The atom flux is caused by ions from the main plasma which are neutralized and backscattered at the target plate. The spatial profile of backscattering atoms is analysed in terms of parallel ballistic flight of ions onto the target plate and perpendicular diffusion. The diffusion coefficient D in the scrape-off layer is found to be anomalous and to agree with D-values as deduced from impurity transport analysis for the main plasma, both for Ohmically heated hydrogen and deuterium discharges and beam-heated L- and H-plasmas. The transient phases of density decay experiments, the plasma build-up phase, and MHD effects are also investigated.

Relativistic electron beam current drive experiments in the Macrotor tokamak achieved current drive levels (increases in the toroidal current) which were consistent with the zero-dimensional theory. Injection of a 2–4 kJ relativistic electron beam into Macrotor produced an additional toroidal current of 4.5–7.5 kA, which persisted for more than 800 μs. The beam-induced increase in toroidal current was observed to be independent, to first order, of the initial tokamak plasma current, I_p, and the number density, n_p, over the entire range of plasma parameters available in Macrotor (3 kA > I_p > 60 kA; 5 × 10¹² > n_p > 2 × 10¹³). The 1 μs rise-time of the current implied an enhanced plasma collision frequency of 10² – 10³ over classical. Comparison of co- and counter-injection data demonstrated that the initial increase in current was entirely due to beam-driven currents while later in time the increased current was sustained by the OH system.

A review of measurements of microscopic fluctuations and theories of turbulence and anomalous transport for tokamaks is given, and some comparisons between theory and experiment are presented. The results of the measurements indicate that all tokamaks have rather similar, broadband, incoherent microscopic fluctuations. Such fluctuations have been measured in the density, potential, electric field, and magnetic field. In the edge regions of three tokamaks, the particle transport caused by the turbulent electric field fluctuations has been measured directly. Although tokamak microturbulence has been studied extensively, neither its source nor its role in anomalous energy transport is yet understood. The incoherent, turbulent nature of the fluctuations has made it difficult to understand them theoretically. Recently, however, significant theoretical progress has been made in several areas including non-linear models of drift wave turbulence and transport, models of anomalous electron thermal conduction by stochastic magnetic field fluctuations, and non-linear models of localized resistive-MHD instabilities.

The addition of an ℓ = 1 helical winding to the heliac central conductor adds a significant degree of flexibility to the configuration by making it possible to control the rotational transform and shear. Such control is essential for an experiment because the presence of low-order resonances in the rotational transform profile can cause breakup of the equilibrium magnetic surfaces. The use of the additional winding also permits a reduction of the total central conductor current and can deepen the magnetic well.

In the absence of an emf, the parallel current, j_||, in a steady-state tokamak will consist of a neoclassical portion plus a wave-driven contribution. Using the drift kinetic equation, the quasi-linear (wave driven) current is computed for high-phase-speed waves in a torus, and this is combined with the neoclassical term to obtain the general expression for the flux surface average ⟨j_||B⟩. For a given pressure profile this technique fully determines the MHD equilibrium, permitting the study of a new class of toroidal equilibria.

The first Symposium on Fusion Technology (SOFT) took place at Harwell in 1960. Since that time, the Symposia were organized every other year in different countries of the European Community. Key issues of the earlier conferences were the exchange of experience concerning the design, construction and operation of small devices intended to investigate the basic problems related to plasma heating and confinement. During the last ten years the interest of participants has focused more and more on large-scale machines and on technological aspects of fusion reactors.