Isabel Garcia-Cortes

In this paper, we study electron to ion root confinement transitions triggered by Electron Cyclotron Resonant Heating switch-off when the line-averaged electron density is sufficiently high and while Neutral Beam Injection heating is on. We use a Heavy Ion Beam Probe system to document the transition in considerable detail. The transition occurs in two phases. An initial, fast phase is characterized by a rapid, global decay of the plasma potential and the concomitant establishment of an edge radial electric field shear layer. This phase is induced by the pump-out effect, as documented using simultaneous measurements of the evolution of the plasma potential at different radial locations. This initial, fast phase is followed by a slow phase during which the profiles adjust to the new edge and heating conditions via transport.

Efficient plasma core fuelling is critical for magnetic confinement fusion (MCF). The envisaged technology is cryogenic pellet injection where small cylinders of fuel ice are injected at high speed into the confined plasma. Indeed, injectors are operated currently on many tokamak and stellarator devices. As a pellet penetrates into plasma its surface is ablated by particle impacts, the pellet’s ablation rate and, hence penetration depth, being determined primarily by plasma electron temperature and, to a lesser extent, by electron density. During ablation, Balmer light emitted from the neutral cloud that surrounds a pellet provides a means to quantify ablation and to determine its penetration depth. Reconstructed emission profiles often exhibit overlying structures such as striations. In the stellarator TJ-II, ablation light has been recorded for injections into a broad range of its magnetic configurations. After reviewing signals in its pellet database for selected magnetic configurations heated by balanced/unbalanced neutral beam injection, large reproducible transient structures, that cannot be explained by plasma parameter variations alone, are found occasionally in signals. It is argued here that their occurrence can be associated with the radial locations of low-order rational surfaces in the core region. Moreover, it is shown that structures appear in, or disappear from, ablation light signals as the rotational transform profile is modified by the net plasma current. Examples of ablation emission profiles with and without structures are presented for selected magnetic configurations and Neutral Beam Injector heating set-ups and the radial locations of significant structures are compared with predicted locations for low-order rational surfaces. Finally, implications of these observations for pellet injection are discussed.

We provide an overview of activities carried out at the TJ-II stellarator aimed at understanding transport from first principles and power balance analysis. These tasks include gyrokinetic simulations with the codes stella and EUTERPE, neoclassical simulations with the code SFINCS, particle deposition calculations with the code HPI2, and heat source estimates with ASCOT5. All these numerical simulation efforts converge, together with transport analyses, to address the transport mechanisms in plasmas with improved confinement through pellet injection—a scenario studied during the past few TJ-II campaigns bearing resemblance to the pellet-fueled high-performance plasma scenarios of observed in W7-X.

The Large Helical Device (LHD), the largest superconducting helical system in the world, is equipped with advanced heating and diagnostic tools, facilitating plasma control and physics research. Data assimilation was employed for electron temperature control using a real-time Thomson scattering system and real time prediction code. A virtual LHD environment enabled visualization of escaping high-energy tritium ions and demonstrated that these ions impact the rear side of the divertor plate. Pioneering results crucial to plasma control have also been achieved. Real-time wall conditioning using Lithium granule dropping improved bulk ion energy and particle transport while simultaneously enhancing the heavy impurity transport. Progress has also been made in the investigation of turbulence-driven transport. At the confinement bifurcation, ion-scale turbulence decreased, while electron-scale turbulence increased. A change in the anisotropy of turbulent eddies was also observed at the confinement bifurcation. Coexistence of local and non-local turbulence was identified in electron-scale turbulence. Non-local turbulence exhibited the rapid spatial propagation of perturbations throughout the plasma, while local turbulence followed the temperature gradient. A transition between drift-wave turbulence and magnetohydrodynamics (MHD) turbulence was observed with the turbulence minimized at the transition condition. Machine learning analysis was employed to evaluate the temperate and density conditions of this turbulence transition. Then, real-time control of fueling and heating was applied to maintain the turbulence transition condition, improving the energy confinement enhancement factor by 20%. In addition, evidence was obtained for collisionless ion heating by energetic-ion-driven geodesic acoustic modes and MHD bursts. These achievements represent unique contributions to the development of fusion reactors.

In the stellarator TJ-II, an enhanced-confinement (PiEC) phase is induced by pellet injections (PIs) into the NBI heated phase of its plasmas. As a consequence, its operational regime is extended significantly, with improvements approaching a factor of 1.5 when compared with fuelling by beam, gas-puff and/or wall recycling. The main characteristics of this PiEC phase are: steepened density gradients, increased ion temperatures, record stored energy and particle confinement times and reduced turbulence. In this work, studies are extended to a broadened range of magnetic configurations and heating powers. For instance, an updated dataset confirms the enhancement of energy confinement times following PIs when these are compared with empirical stellarator scaling law predictions (Yamada et al 2005 Nucl. Fusion 45 1684) and demonstrates, that such improvements have a magnetic configuration dependency. Thus, in the TJ-II, pellet fuelling defines the current upper operational boundary for NBI-heated plasmas. In addition, a systematic analysis of plasma potential, radial electric field (Er) and its shear is performed for PiEC phases. It reveals that plasma potential becomes more negative across the whole plasma radius after a single injection while additional injections induce steeper density gradients and broaden profiles. In parallel, it shows that the observed increases in radial electric field in the gradient region, from ~-3 kV/m (reference) to ~-8 kV/m (2 pellets) at normalized radius = 0.6, is consistent with neoclassical simulations. Finally, the work presented here indicates also that consecutive injections maintain the radial electric field shear at maximum level (at ρ = 0.75) while steepening density profiles and improving confinement. Thus results indicate that PIs not only acts as plasma fuelling in TJ-II, but also contribute to strengthening shear-driven confinement in the gradient region.

An enhancement of core impurity transport is observed in high-density plasmas of the stellarator large helical device, heated by neutral beam injection, when continuous lithium (Li) granule dropping is performed. In the reported experiments, in which the tracer-encapsulated solid pellet is employed to inject trace amounts of titanium (Ti) and molybdenum (Mo) into the plasma core, confinement times for these impurities are seen to reduce significantly when Li dropping is applied, this reduction being more notable for Mo. To gain some initial insight into these observations, simulations are performed using the drift-kinetic transport code SFINCS for the Mo case. These simulations indicate that, while neoclassical transport prevails for the main plasma components (electrons, majority ions, and low-Z impurities), the classical contribution appears dominant for transporting Mo impurities. In summary, this work reports the first experimental observation of the degradation of mid-Z and high-Z impurity confinement induced by the continuous dropping of Li granules into a high-density stellarator plasma. In the case of the Mo impurity, simulations suggest that classical transport is the key mechanism underlying the enhanced impurity transport.

Massive injection of impurities, e.g., due to unforeseen mechanical failures, into the core region of fusion plasmas can lead to their termination. Resulting thermo-mechanical loads may affect both the integrity of plasma-facing components and safe plasma operation. In order to examine the mechanism of impurity-induced plasma termination in stellarators, experiments were conducted in LHD by intentionally injecting impurities. To this end, TESPEL, loaded with various defined amounts of tungsten, exceeding typical tracer amounts, was used. 3×1017 tungsten atoms are found to be sufficient to induce radiative collapses of plasmas with diamagnetic energy of ∼500 kJ. The termination is governed by the formation of inwards-propagating cold fronts. Additional ECRH was applied in response to the TESPEL injection with the goal of mitigating detrimental heat loads. It was found that additional electron heating prolonged plasma termination up to ∼100 ms. This investigation documents a high resilience of helical plasmas against strong perturbations and suggests approaches to mitigate the criticality of heat loads in fusion grade stellarator plasmas.

A pellet-induced enhanced confinement (PiEC) phase, with general characteristics similar to those reported for the stellarator W7-X, is observed after single pellet injection (>10¹⁹ H atoms) into the neutral beam injection heated phase of plasmas in the mid-sized heliac-type stellarator TJ-II. In addition to a step-like increase in density, plasma diamagnetic energy content rises significantly with respect to that of reference discharges, energy confinement time is similarly enhanced when compared to International Stellarator Scaling law predictions (Yamada et al 2005 Nucl. Fusion 45 1684) renormalized for TJ-II, and the triple product, n _e · T _i · τ _E, exhibits a clear bifurcation towards an improved confinement branch when compared to the branch product predicted by the same law. In this work, multiple pellets are injected in series into NBI-heated plasmas in the TJ-II and post-injection plasma performance is reported and discussed. For instance, a charge-exchange recombination spectroscopy diagnostic reveals significantly increased core ion temperatures after pellet injection compared to temperatures achieved in comparable reference plasmas, this pointing to increased ion energy content and improved ion energy confinement during a PiEC phase. It is also found that enhanced performance is independent of whether co- or counter-NBI heating beam is employed. Finally, record stored diamagnetic energy content and plasma beta values are achieved when the largest available pellets are employed. The results indicate that pellet injections extend the operational regime well beyond limits previously achieved in TJ-II without pellets.

Pellet injection is currently the primary candidate for achieving efficient plasma fuelling, one of the key issues for steady-state operation in large fusion devices. In this paper, pellet injection experiments are performed for several magnetic configurations of the TJ-II stellarator. The aim of this study is to increase the understanding of the role played by rational surfaces in plasmoid drift and deposition profiles in stellarators. The analysis of experimentally observed plasmoid drifts is supported by simulations of such cases made with the HPI2 code. Plasmoid drift is found to be significantly reduced, as in tokamaks, in the vicinity of rational surfaces. This is attributed to the fact that plasmoid external charge reconnection lengths are shorter near rational surfaces, resulting in a more effective damping of the plasmoid drift. Although the effect of plasmoid external currents on the drift is expected to be negligible in stellarators, compared with those caused by plasmoid internal currents, the effect observed in TJ-II is clearly measurable. In addition, simulations show that enhanced drift reductions near rational surfaces lead to significantly different deposition profiles for the magnetic configurations included in this study. This implies that it should be possible to select the magnetic configurations to obtain more efficient pellet fuelling.