Cryogenic pellet injection is a widely used technique for delivering fuel to the core of magnetically confined plasmas. Indeed, such systems are currently functioning on many tokamak, reversed field pinch and stellarator devices. A pipe-gun-type pellet injector is now operated on the TJ-II, a low-magnetic shear stellarator of the heliac type. Cryogenic hydrogen pellets, containing between 3 × 10 18 and 4 × 10 19 atoms, are injected at velocities between 800 and 1200 m s −1 from its low-field side into plasmas created and/or maintained in this device by electron cyclotron resonance and/or neutral beam injection heating. In this paper, the first systematic study of pellet ablation, particle deposition and fuelling efficiency is presented for TJ-II. From this, light-emission profiles from ablating pellets are found to be in reasonable agreement with simulated pellet ablation profiles (created using a neutral gas shielding-based code) for both heating scenarios. In addition, radial offsets between recorded light-emission profiles and particle deposition profiles provide evidence for rapid outward drifting of ablated material that leads to pellet particle loss from the plasma. Finally, fuelling efficiencies are documented for a range of target plasma densities (~4 × 10 18 -~2 × 10 19 m −3 ). These range from ~20%-~85% and are determined to be sensitive to pellet penetration depth. Additional observations, such as enhanced core ablation, are discussed and planned future work is outlined.

A cryogenic pellet injector (PI) and tracer encapsulated solid pellet (TESPEL) injector system has been operated in combination on the stellarator TJ-II. This unique arrangement has been created by piggy-backing a TESPEL injector onto the backend of a pipe-gun–type PI. The combined injector provides a powerful new tool for comparing ablation and penetration of polystyrene TESPEL pellets and solid hydrogen pellets, as well as for contrasting subsequent pellet particle deposition and plasma perturbation under analogous plasma conditions. For instance, a significantly larger increase in plasma line-averaged electron density, and electron content, is observed after a TESPEL pellet injection compared with an equivalent cryogenic pellet injection. Moreover, for these injections from the low-magnetic-field side of the plasma cross-section, TESPEL pellets deposit electrons deeper into the plasma core than cryogenic pellets. Finally, the physics behind these observations and possible implications for pellet injection studies are discussed.

A tracer-encapsulated solid pellet (TESPEL) injection system for the TJ-II stellarator was recently developed. In order to reduce the time and cost for the development, we combined a TESPEL injector provided by National Institute for Fusion Science with an existing TJ-II cryogenic pellet injection system. Consequently, the TESPEL injection into the TJ-II plasma was successfully achieved, which was confirmed by several pellet diagnostics including a normal-incidence spectrometer for monitoring a tracer impurity behavior.

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.

This paper presents the last results on confinement studies in the TJ-II stellarator. The research of the dependence of spatially resolved transport coefficients on plasma parameters for ECH plasmas with Boronised wall shows that the heat confinement increases linearly with density while particle confinement increases sharply a factor four above a certain density threshold associated with the positive electric field. Remarkably, lowest order magnetic resonances, even in a low shear environment, reduce locally the effective diffusivities. The inherently strong plasma wall interaction of TJ-II has been successfully reduced after Lithium coating by vacuum evaporation. Besides H-retention and low Z, Li was chosen because there exists a reactor-oriented interest in this element, thus giving especial interest to the investigation of its properties. The Li-coating has led to important changes in plasma performance. Particularly, the effective density limit in NBI plasmas has been extended reaching central values of 8 x 10 19 m-3 and Te≈250-300 eV, with peaked density, rather flat Te profiles and increased ion temperatures. Alfvén modes are destabilised and their influence on fast ion confinement is studied in NBI discharges. Due to the achieved density control, a second type of transitions has been added to the low density ones previously observed in boronised wall. The high density transitions, under NBI with Li-coated walls are characterised by the fall of Hα emission, the onset of steep density gradient, and the reduction of the turbulence, which are characteristics of transition to H mode. TJ-II is therefore a unique device where first and second order phase transitions can be investigated.