Table of contents

Potential and density/temperature fluctuations at the L-H transition are measured by the heavy ion beam probe on JFT-2M. It has been observed that the timescale of the potential change is as fast as 10-100 μs when the input power (P_in) is larger than the L-H threshold power (P_th). When P_in ∼ P_th, the confinement is improved gradually step by step with sawtooth crashes. After a few sawtooth crashes, the potential drops rapidly to the level of the ELM free H mode. From the gradual change of the potential, assuming that dE_r/dr is a key to forming and sustaining the transport barrier, the criterion of dE_r/dr is less than (1.2±0.4) × 10³ kV/m². At an ELM just before the H-L transition, the potential inside the separatrix also shows a rapid positive jump. The timescales of the positive jump and of the recovery of the potential to its negative value are about 40 and 150 μs, respectively. Before the H-L transition, the time between ELMs and/or dithering transition becomes shorter and the plasma finally goes back to the L mode.

A comparative experiment on low aspect ratio (A ≈ 1.5) torus plasmas, including compact reversed field pinches (RFPs), spheromaks and spherical tokamaks (STs), has been performed in the TS-3 and TS-4 compact toroid (CT) devices. In particular, a compact RFP with q₀ as large as 0.3 was found to have a low n dynamo mode, n = 3, while a spheromak with q₀ ≈ 0.5 had an n = 2 mode. Under OH current sustainment, the magnetic fluctuation and loop voltage of the CT were observed to increase inversely with its q value. A new method of edge current drive was developed using axial CT merging, which together with the OH current drive created a balanced current drive. This current drive was found to reduce the dynamo fluctuations of compact RFPs and spheromaks by a factor of 3. Multiple CT merging is a promising method of edge current drive because of its intermittent current drive and its detailed control of current profiles using varied size of colliding CTs.

The seeding mechanism by which neoclassical tearing modes (NTMs) are triggered is crucial to understanding the onset conditions for these modes. However, it is difficult to decouple measurements of the MHD activity involved in the mode onset from the seed perturbation itself, making direct verification of the onset criteria difficult. It is important to examine this, in order to validate the underlying physics being used to predict the requirements for larger devices such as ITER-FEAT. Following earlier work on COMPASS-D identifying NTM behaviour, and development of mode avoidance and removal techniques, new studies have focused on validating models of onset criteria. Using `resonant magnetic perturbation' windings, clear, identifiable and measurable rotating islands are produced in the plasma. By varying the timing and size of the heating power ramp-up, applied during the (natural) decay of such an island, the island evolution can be observed as a function of the key parameters - the values of normalized plasma pressure and island width. The resulting variation confirms the form predicted by theory, provided threshold mechanisms are incorporated, and favours the ion polarization model in particular.

Large scale transport events are studied using two different 3-D simulation codes related to resistive ballooning and ion temperature gradient turbulence. The turbulence is driven by a constant incoming flux. In the case of resistive ballooning simulations, the underlying structures are found to be radially elongated on the low field side and distorted by magnetic shear in the parallel direction (streamers). The non-linear character of these structures is emphasized. Bursty transport is investigated in the presence of zonal flows and internal transport barriers generated either by a strong shear flow or with a magnetic shear reversal. In deriving a low dimensional model that captures the main features of bursty transport dynamics, it is found that E × B shear flow is necessary to trigger the bursts.

The transmission/escape probability (TEP) method for neutral particle transport has recently been introduced and implemented for the calculation of 2-D neutral atom transport in the edge plasma and divertor regions of tokamaks. The results of an evaluation of the accuracy of the approximations made in the calculation of the basic TEP transport parameters are summarized. Comparisons of the TEP and Monte Carlo calculations for model problems using tokamak experimental geometries and for the analysis of measured neutral densities in DIII-D are presented. The TEP calculations are found to agree rather well with Monte Carlo results, for the most part, but the need for a few extensions of the basic TEP transport methodology and for inclusion of molecular effects and a better wall reflection model in the existing code is suggested by the study.

The ICRF heating experiments conducted in 1999 in the third experimental campaign on LHD are reported, with an emphasis on the optimization of the heating regime. Specifically, an exhaustive study of seven different heating regimes was carried out by changing the radiofrequency relative to the magnetic field intensity, and the dependence of the heating efficiency on H minority concentration was investigated. It was found in the experiment that both ion and electron heating are attainable with the same experimental set-up by properly choosing the frequency relative to the magnetic field intensity. In the cases of both electron heating and ion heating, the power absorption efficiency depends on the minority ion concentration. An optimum minority concentration exists in the ion heating case while, in the electron heating case, the efficiency increases with concentration monotonically. A simple model calculation is introduced to provide a heuristic understanding of these experimental results. Among the heating regimes examined in this experiment, one of the ion heating regimes was finally chosen as the optimized heating regime and various high performance discharges were realized with it.

An advanced asymptotic matching method of ideal and resistive MHD stability analysis in tokamaks is reported. A solution method for the two dimensional Newcomb equation, a dispersion relation for an unstable ideal MHD mode in tokamaks and a new scheme for solving resistive MHD inner layer equations as an initial value problem are reported.

In order to preserve the conditions for an environmentally safe machine, at present the selection of materials for the structural components of fusion reactors is made not only on the basis of adequate mechanical properties, behaviour under irradiation, and compatibility with other materials and cooling media, but also on their radiological properties, i.e. radioactivity, decay heat and radiotoxicity. These conditions strongly limit the number of suitable materials to a few families of alloys, generically known as low activation materials. The criteria for making decisions about such materials, the alloys resulting from the application of these ideas and the main issues and problems with their use in a fusion environment are discussed.

A comprehensive investigation has been performed of the static and dynamic behaviour of detached recombining plasmas in the linear divertor plasma simulator NAGDIS-II. For stationary plasma detachment, the transition from electron-ion recombination (EIR) to molecular activated recombination (MAR) has been observed by injecting hydrogen gas into high density helium plasmas. The particle loss rate due to MAR is found to be comparable to that of EIR. Experiments have also been performed by the injection of a plasma heat pulse produced by RF heating into the detached helium plasma to demonstrate the dynamic behaviour of volumetric plasma recombination. Negative spikes in the Balmer series line emission were observed and found to be similar to the so called negative ELM observed in tokamak divertors. Observed Balmer spectra were analysed in detail using the collisional-radiative model. A rapid increase of the ion flux to the target plate was observed associated with the re-ionization of the highly excited atoms generated by EIR.

The article reports on recent developments in the theory of secondary instability in drift-ion temperature gradient turbulence. Specifically, the article explores secondary instability as a mechanism for zonal flow generation, transport barrier dynamics and avalanche formation. These in turn are related to the space-time statistics of the drift wave induced flux, the scaling of transport with collisionality and β, and the spatio-temporal evolution of transport barriers.

Coaxial helicity injection (CHI) on the National Spherical Torus Experiment (NSTX) has produced 240 kA of toroidal current without the use of the central solenoid. Values of the current multiplication ratio (CHI produced toroidal current/injector current) up to 10 were obtained, in agreement with predictions. The discharges, which lasted for up to 200 ms, limited only by the programmed waveform, are more than an order of magnitude longer in duration than any CHI discharges previously produced in a spheromak or a spherical torus.

The formation and release of particle agglomerates, i.e. debris and dusty objects, from plasma facing components and the impact of such materials on plasma operation in controlled fusion devices has been studied in the Extrap T2 reversed field pinch and the TEXTOR tokamak. Several plasma diagnostic techniques, camera observations and surface analysis methods were applied for in situ and ex situ investigation. The results are discussed in terms of processes that are decisive for dust transfer: localized power deposition connected with wall locked modes causing emission of carbon granules, brittle destruction of graphite and detachment of thick flaking co-deposited layers. The consequences for large next step devices are also addressed.

At and around minima or maxima of the safety factor q, overlapping, and therefore toroidicity induced coupling of drift eigenmodes centred on neighbouring rational surfaces r_l,m defined by q(r_l,m) = m/l and r_l,m±1 defined by q(r_l,m±1) = (m±1)/l, is generally negligible; l and m (m±1) are the toroidal and (main) poloidal mode numbers, respectively. Under these conditions, a careful analysis shows that the growth rate of the ion temperature gradient (ITG) mode is proportional to the absolute value of the magnetic shear parameter; its radial width and frequency (in the E × B rotating frame) are larger in the axisymmetric torus than in the plasma slab or cylinder, especially at small values of k_θ a_i (k_θ is the poloidal mode number and a_i the characteristic ion Larmor radius). These results provide a straightforward theoretical interpretation of the origin of internal ion transport barriers (ITBs) in discharges with negative central shear if the ITG instability is indeed the main channel for ion energy transport. In agreement with recent experiments, the mechanism does not degrade for T_e∼T_i, an important result for extrapolation to reactors. Once initiated, an ITB will locally increase the curvature of the ion temperature profile and, consequently, the gradients of the radial electric field and E × B rotation profiles (until neoclassical (or sub-neoclassical) transport comes into play). The damping associated with velocity shear yields an upper bound on the unstable mode number range, whereas Landau damping is expected to introduce a lower bound. In cases where toroidal effects are subdominant, the ratio of the damping rate associated with velocity shear to the growth rate associated with magnetic shear leads naturally to the Hamaguchi-Horton parameter (divided by the factor η_i-2/3 which characterizes the departure from the instability threshold (η_i is the ratio of the density and temperature length scales)) when introducing the most unfavourable mode number. Toroidal effects are always dominant for sufficiently weak magnetic shear or sufficiently large values of |L_N|/R (where L_N is the density gradient length), and therefore before steepening of the profiles has occurred near the zero magnetic shear layer; the counterpart of the Hamaguchi-Horton characteristic parameter appropriate to that case is also obtained, considering again the most unfavourable mode number.

Limited available pellet velocities have so far restricted the refuelling performance of efficient launch schemes from the tokamak magnetic high field side (HFS). Although pellet injection during H mode has resulted in more peaked density profiles and enhanced performance with respect to gas puff refuelling, prompt particle and energy losses caused by pellet induced ELM bursts have still limited the extension of the operational area. Now, the preliminary version of a new optimized pellet injection set-up at ASDEX Upgrade allows for significantly higher injection speeds when launching pellets from the magnetic HFS. Intact pellets with velocities up to v_P = 560 m/s were successfully injected instead of the v_P = 240 m/s available with the previous set-up. The velocity increase results in a deeper pellet penetration and seems to follow the v_P^1/3 scaling derived from a multimachine study using conventional pellet launch from the torus outside. The inward shift of the pellet particle deposition profile with respect to the penetration depths turned out to be approximately the same for both launch velocities. The respectively achieved deeper particle deposition inside the plasma column reduced the particle and energy loss rates during the immediate post-pellet phase. Thus, further enhancement of tokamak operation in the high density regime seems feasible by means of high speed pellets launched from the torus inner side.