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The effects of 3D geometry are explored in TJ-II from two relevant points of view: neoclassical transport and modification of stability and dispersion relation of waves. Particle fuelling and impurity transport are studied considering the 3D transport properties, paying attention to both neoclassical transport and other possible mechanisms. The effects of the 3D magnetic topology on stability, confinement and Alfvén Eigenmodes properties are also explored, showing the possibility of controlling Alfvén modes by modifying the configuration; the onset of modes similar to geodesic acoustic modes are driven by fast electrons or fast ions; and the weak effect of magnetic well on confinement. Finally, we show innovative power exhaust scenarios using liquid metals.

The boundary of the tokamak core plasma, or scrape-off layer, is normally characterized in terms of average parameters such as density, temperature and e-folding lengths suggesting diffusive losses. However, as is shown in this paper, localized filamentary structures play an important role in determining the radial efflux in both L mode and during edge localized modes (ELMs) on MAST. Understanding the size, poloidal and toroidal localization and the outward radial extent of these filaments is crucial in order to calculate their effect on power loading both on the first wall and the divertor target plates in future devices. The spatial and temporal evolution of filaments observed on MAST in L-mode and ELMs have been compared and contrasted in order to confront the predictions of various models that have been proposed to predict filament propagation and in particular ELM energy losses.

Electron cyclotron heating (ECH) is one of the most common methods used for heating stellarator plasmas, providing both ionization for startup and net-current free heating during the discharge. This paper presents the design of the ECH heating system for the Stellarator of Costa Rica (SCR-1), a small modular stellarator under construction at the Instituto Tecnologico de Costa Rica (ITCR). The SCR-1 is a small (Major radius 0.238 m, mean plasma radius 0.042 m) low budget device, using a 2 period magnetic field with flat rotational transform profile (i=0.3), a resonant strength of 8.78×10−2 T and expected to achieve a 15 eV temperature with an electron line density of 3.74×1015 m−3.

A study of the evolution of the filaments observed during Type I ELMs on ASDEX Upgrade and MAST is presented. The filaments start off rotating toroidally/poloidally with velocities close to that of the pedestal. This velocity then decreases as the filaments propagate radially. On both devices the ion saturation current e-folding lengths of the filaments show a weak, if any, dependence on the size of the ELM (δWELM/Wped). On MAST the measured radial velocities of the filaments also show at most a weak dependence on δWELM/Wped.

The response of transport in low density (line average ) plasmas to electron cyclotron resonance heating (ECH) power, 100 kW QECH 400 kW, is documented for the TJ-II Heliac-type stellarator. Radially resolved electron heat balance shows no significant differences between boron or lithium coating of the vacuum chamber walls. The main trends in electron heat transport are found to be similar to other stellarator/heliotron devices and are compatible with neoclassical calculations in the bulk of the plasma. According to our calculations the heat fluxes are anomalous near the edge, ρ 0.8 where ρ is the normalized minor radius, but the uncertainties there are large. Particle transport in the density gradient region, ρ ≈ 0.8, has little sensitivity to the variation of heating power and is compatible with neoclassical predictions. Neoclassical transport of particles and electron heat is found to be dominant in the gradient regions of typical ECH plasmas of the TJ-II Heliac.

Several improvements to the MAST plant and diagnostics have facilitated new studies advancing the physics basis for ITER and DEMO, as well as for future spherical tokamaks (STs). Using the increased heating capabilities P NBI ⩽ 3.8 MW H-mode at I p = 1.2 MA was accessed showing that the energy confinement on MAST scales more weakly with I p and more strongly with B t than in the ITER IPB98(y, 2) scaling. Measurements of the fuel retention of shallow pellets extrapolate to an ITER particle throughput of 70% of its original designed total throughput capacity. The anomalous momentum diffusion, χϕ, is linked to the ion diffusion, χi, with a Prandtl number close to P ϕ ≈ χϕ/χi ≈ 1, although χi approaches neoclassical values. New high spatial resolution measurements of the edge radial electric field, E r , show that the position of steepest gradients in electron pressure and E r (i.e. shearing rate) are coincident, but their magnitudes are not linked. The T e pedestal width on MAST scales with rather than ρpol. The edge localized mode (ELM) frequency for type-IV ELMs, new in MAST, was almost doubled using n = 2 resonant magnetic perturbations from a set of four external coils (n = 1, 2). A new internal 12 coil set (n ⩽ 3) has been commissioned. The filaments in the inter-ELM and L-mode phase are different from ELM filaments, and the characteristics in L-mode agree well with turbulence calculations. A variety of fast particle driven instabilities were studied from 10 kHz saturated fishbone like activity up to 3.8 MHz compressional Alfvén eigenmodes. Fast particle instabilities also affect the off-axis NBI current drive, leading to fast ion diffusion of the order of 0.5 m2 s−1 and a reduction in the driven current fraction from 40% to 30%. EBW current drive start-up is demonstrated for the first time in a ST generating plasma currents up to 55 kA. Many of these studies contributed to the physics basis of a planned upgrade to MAST.

Results on edge turbulence in periods separating edge localised modes (ELMs), i.e. inter-ELM periods, in Mega-Amp Spherical Tokamak (MAST) are presented. It is shown through combined measurements of fast camera images and reciprocating Langmuir probes that filamentary structures contribute to transport during these periods. Analysis of Dα light emission reveals that inter-ELM filaments are the lowest amplitude fluctuations in the MAST scrape-off layer (SOL) relative to L-mode and ELM filaments. Physical properties such as size, density and mode numbers have also been characterized, along with measurements of the spatio-temporal evolution: inter-ELM filaments are found to rotate in the vicinity of the last closed flux surface and propagate radially outwards. Motion of these filaments is found to depend strongly on plasma density such that with increasing density, there is an enhancement of the radial transport manifested by an increased number of filaments which leave the edge and travel faster into the SOL. Camera images show that intermittent fluctuations in ion saturation current signals correspond to inter-ELM filaments passing the probe. Measured radial e-folding lengths indicate larger decay lengths at higher densities. Similar trends are also obtained in simulations of a filament propagating radially and losing particles on ion parallel loss timescales. Finally, a discussion is presented on how the radial velocity and Isat measurements reported in this paper are used to test the velocity scalings predicted by different theories.