Table of contents

Studies of the relaxation and confinement of hot anisotropic ions are considered to be the key elements of the gas dynamic trap (GDT) experimental research programme. The method of confinement study described consists essentially in the comparison of measured ion parameters with those predicted by computer simulations. To realize this approach a set of diagnostics for the measurements of local and global parameters of the fast ions has been developed. In particular, this set includes diagnostics to measure the local energy and the angular distribution functions. For numerical studies of the fast ion dynamics a Monte Carlo code based on the theory of two body Coulomb collisions has been elaborated. Comparison of the experimental data with the results of the simulation clearly demonstrates that the fast ion characteristic relaxation times in the warm target plasma are close to those determined by binary Coulomb collisions. Significant anomalous energy losses or scattering of fast ions have not been observed as yet. The measurements provide a maximum density of the fast ions with mean energy of about 8 keV up to 10¹³ cm^-3, in good agreement with computer simulations. The increase of the neutral beam power and improved vacuum conditions of GDT made possible the access to plasma β of as high as 30%.

Growth rates of the axisymmetric instability of elongated plasmas in TCV are measured, in open loop configuration, for a wide variety of plasma shapes with elongations and triangularities in the ranges 1.3 < κ < 2.2 and -0.3 < δ < 0.6, respectively, and for stability margins covering the range 1.03 < f < 1.60. In addition, the stability of the vertical position control system is investigated experimentally, under closed loop conditions, by varying the feedback gains and by measuring the oscillation frequencies close to the stability limit. The results of these measurements are compared with theoretical calculations, based on a rigid plasma model (RPM) and on a deformable plasma model (DPM). While RPM underestimates the open loop growth rate for large values of δ and overestimates the size of the stable domain in gain space, DPM gives very good agreement with the experimental results.

Fast particle driven instabilities are investigated at low toroidal field B₀ ≈ 1 T in tritium, deuterium and hydrogen plasmas. Tritium, deuterium and hydrogen neutral beams with energies of 160, 140 and 110 keV were injected, respectively, into tritium, deuterium and hydrogen plasmas. The destabilization of Alfvén eigenmodes was observed during injection of tritium beams into a tritium plasma and during the injection of deuterium beams into a deuterium plasma but not during the injection of hydrogen beams into a hydrogen plasma. At high beta, β_N = β/(I_p/B_T a) > 2.5, high amplitude fishbones were observed and appeared to be correlated with the degradation of confinement.

In the l = 2/m = 19 Heliotron E heliotron/torsatron device, with a currentless plasma heated by NBI and ECH, the poloidal distributions were measured of diverted plasma flows near the vacuum chamber wall in eight sections of one helical field pitch. A vertical asymmetry of these distributions was observed, contrary to what had been expected from the helical symmetry of the `ideal' (unperturbed) vacuum divertor magnetic configuration. Under NBI conditions, the density normalized divertor plasma flow J_d/_e was a rising function of the density normalized absorbed power P_abs/_e. This is considered to be a consequence of the power degradation of confinement. With second harmonic ECH, where the main part of the launched power was absorbed in the central region, a similar power dependence as for the NBI was observed. With fundamental ECH, where a considerable fraction of the microwave power was absorbed at the periphery, J_d/_e for some areas of the divertor trace distinctly exceeded those obtained under NBI-only conditions with the same P_abs/_e. On the other hand, two different time responses (fast and slow) of J_d were observed for the perpendicular NBI and fundamental ECH cases in some particular positions. The slow response is considered to be caused by a diffusion-like outflow of the bulk plasma to the divertor. The fast outflow of particles to the divertor might be caused by a loss of locally trapped high energy electrons and ions.

The scaling of the transport rate of particles with normalized gyroradius has been measured for L mode discharges in the DIII-D tokamak. Both the helium diffusivity and the effective electron particle diffusivity were measured. In these L mode discharges, the particle diffusivities tend to scale like the ion thermal diffusivity, which scales between Bohm-like and Goldston-like. Non-zero inward pinch velocities are measured. Like the diffusivities, the pinch velocities increase with gyroradius.

Error field induced modes have been a significant concern for next step devices. Experiments on JET, using the lower saddle coils to simulate such fields, have determined the critical scaling dependences of error field sensitivity in terms of global plasma parameters, under the conditions most relevant to next step operation. These experiments indicate relatively weak toroidal field scaling for the error field strength required to induce a locked mode, while confirming the linear density scaling observed on other devices. Correction of intrinsic errors improves low density access and assists end-of-pulse disruption avoidance. Additional heating studies have shown a strong effect of q = 2 rotation on the threshold, but a much weaker dependence on β and the H mode transition. Error field modes have also led to neoclassical tearing modes at appropriate collisionality and β. Other parameter dependences have been examined: plasma inductance has a strong effect, but plasma configuration (limiter or divertor) is less important. Application of a dimensional scaling constraint enables determination of machine size scaling, which is found to be weak. This leads to the relatively tolerable error field sensitivity expected for devices such as ITER FDR. However, there remain some important questions to be answered about the effects of rotation on error field sensitivity, and for ITER FDR some means of correction remains prudent.

The β limit in radiative improved (RI mode) discharges where energy confinement is enhanced due to an increased radiation level from the edge by impurity seeding of neon has been investigated. Under these discharge conditions spontaneous confinement transitions from higher to lower energy confinement can be observed when the β limit is approached. The empirically determined limit of the normalized toroidal beta, β_N, is 2.2, β_p reaches values up to 1.5 in the circular limiter tokamak TEXTOR-94. The sawtooth activity is often stabilized and replaced by central mode oscillations around q = 1 when high values (⩾ 1) of the confinement enhancement factor f_H93 with respect to the ELM-free ITER-93H mode scaling are reached. Measurements of the plasma current distribution show that already before the confinement transition the central current density decreases, i.e. the current profile broadens and magnetic shear is decreased. The deterioration of energy confinement has been found to correlate with the onset of MHD activity either at the q = 1.5 and/or the q = 2 surface. The enhanced transport after the β drop is caused by the MHD mode activity in the plasma; the favourable stabilization of the ITG mode under RI mode conditions is not altered. The possible role of pressure driven contributions to the destabilization of the observed MHD modes will be discussed.

The possibility of controlling the radial electric field of toroidal plasmas by injecting high energy electrons along the reversible loss cone orbit of helical magnetic traps is investigated. It is well known that the radial electric field plays an important role in the confinement improvement scenario, especially in the low collisional regime in the physics picture of neoclassical theory. For this purpose, it is made clear that the most suitable particles are transit particles, which show a transition from helically trapped orbits to blocked ones. It is also found that a parallel AC electric field launched from outside assists this transition and makes it possible for particles to penetrate deeply into the plasma. In addition it is confirmed that the viscosity of the plasma coupled with the helical field configuration provides a bifurcation of plasma states and its stable solution results in an improvement of confinement.

The calculated radiative cooling rate coefficient for krypton as a trace impurity in low to moderate density plasmas is calculated. Collisional-radiative line emission, dielectronic recombination, radiative recombination and bremsstrahlung are considered as the principal radiative loss channels. Collisional-radiative models and the calculated charge state distribution for krypton have been benchmarked against measured ion brightness profiles in the FTU plasma. The calculated radiative loss rate is compared with two measurements of the radiative cooling coefficient for krypton in the FTU plasma. The measurements differ in how the krypton density profile is experimentally constrained. The krypton density profile is found (to be flat) from spectroscopic observations and (to be increasing radially outwards) from measurements of visible bremsstrahlung emission. The calculations show excellent agreement with the former set of measurements. Polynomial fits to the total radiative cooling coefficient are provided for ease of use in plasma modelling codes. Tables of ion emissivities are provided for use in modelling radiative losses from non-equilibrium plasmas.

Analytic approximations to the p-¹¹B fusion rate coefficient suitable for use at both low (1 keV ⩽ T ⩽ 70 keV) and high (50 keV ⩽ T ⩽ 500 keV) temperatures are developed. Comparisons between these analytic approximations and calculations of the p-¹¹B fusion rate coefficient based on numerical integration indicate that the overall error in these approximations is dominated by errors in the experimental measurement of the underlying p-¹¹B fusion cross-sections.

Report on the IAEA Technical Committee Meeting held at Kloster Seeon, Germany, 21-23 June 1999

Report on the 4th International Workshop held at Jekyll Island, Georgia, United States of America, 18-21 October 1999