J. Guasp

First plasmas in the TJ-II flexible Heliac

The first experimental campaign of the TJ-II stellarator has been conducted using electron cyclotron resonance heating (, ) with a pulse length of . The flexibility of the device has been used to study five different configurations varying plasma volume and rotational transform. In this paper, the main results of this campaign are presented and, in particular, the influence of plasma-wall interaction phenomena on TJ-II confinement is briefly discussed.

Multi-scale physics mechanisms and spontaneous edge transport bifurcations in fusion plasmas

The magnitude of radial transport in magnetic confinement devices for controlled nuclear fusion suffers spontaneous bifurcations when specific system parameter values are exceeded. Here we show, for the first time, that the correlation length of the plasma potential becomes of the order of the machine size during the edge bifurcation itself, quite unlike the density fluctuations. The mechanism governing the development of this bifurcation, leading to the establishment of an edge transport barrier, is still one of the main scientific conundrums facing the magnetic fusion community after more than twenty years of intense research. The results presented here show the dominant role of long-range correlations when approaching the Low to High confinement edge transition in fusion plasmas. This is in line with the expectation that multi-scale interactions are a crucial ingredient of complex dynamics in many non-equilibrium systems.

Ion kinetic transport in the presence of collisions and electric field in TJ-II ECRH plasmas

The ion collisional transport is estimated for the TJ-II device without the assumption of radially narrow particle trajectories, which is usually made in neoclassical transport studies. One million particles are followed in a realistic TJ-II magnetic configuration, taking into account collisions and the electric field. Global features of transport, not present in the customary neoclassical models, appear. The main results reached by considering the actual particle geometries are the monotonic increasing of heat and particle fluxes with minor radius, the non-diffusive character of transport, the appearance of asymmetries on the magnetic surfaces and the non-Maxwellian character of the distribution function.

Transitions to improved core electron heat confinement triggered by low order rational magnetic surfaces in the stellarator TJ-II

Transitions to improved core electron heat confinement are triggered by low order rational magnetic surfaces in TJ-II electron cyclotron heated (ECH) plasmas. Experiments are performed changing the magnetic shear around the rational surface n = 3/m = 2 to study its influence on the transition; ECH power modulation is used to look at transport properties. The improvement in the electron heat confinement shows no obvious dependence on the magnetic shear. Transitions triggered by the rational surface n = 4/m = 2 show, in addition, an increase in the ion temperature synchronized with the increase in the electron temperature. Ion temperature changes had not been previously observed either in TJ-II or in any other helical device. SXR measurements demonstrate that, under certain circumstances, the rational surface positioned inside the plasma core region precedes and provides a trigger for the transition.

Neoclassical global flux simulations in stellarators

Neoclassical transport theory describes local diffusive transport. For those magnetic configurations where this approach is valid it has the benefit of being independent of the plasma profiles. In contrast for configurations where many particle orbits have large deviations from their birth surface, non-local contributions to the particle flux have to be considered. In this work a newly developed Monte Carlo code has been used to compute global ion fluxes for the same plasma parameters for two devices with very different magnetic configurations, namely the Wendelstein 7-X helias, which is under construction at Greifswald (Germany), and the TJ-II heliac, in operation at Madrid (Spain). Whereas the results indicate that the local diffusive approach is valid for W7-X under the considered plasma conditions, a non-local treatment is mandatory for TJ-II. These global flux simulations show the appearance of fluxes in regions where there are no gradients; larger fluxes than those expected from local estimations in the gradient regions; and fluxes which are increasing towards the edge.

Review of confinement and transport studies in the TJ-II flexible heliac

TJ-II is a four period, low magnetic shear stellarator (R = 1.5 m, a < 0.22 m, B0 ≤ 1.2 T) which was designed to have a high degree of magnetic configuration flexibility. In the most recent experimental campaign, coupling of the full ECRH power (PECRH ≤ 600 kW) to the plasma has been possible using two ECRH transmission lines which have different power densities. Both helium and hydrogen fuelled plasmas have been investigated. The article reviews the latest physics results in particle control, configuration effects, and transport and fluctuation studies.

Confinement and stability on the TJ-II stellarator

This paper describes a series of experiments performed in TJ-II stellarator with the aim of studying the influence of magnetic configuration on stability and transport properties of TJ-II plasmas. Plasma potential profiles have been measured in several configurations up to the plasma core with the heavy ion beam probe diagnostic. Low-order rational surfaces have been positioned at different plasma radii observing the effect on transport features. Plasma edge turbulent transport has been studied in configurations that are marginally stable due to decreased magnetic well. Results show a dynamical coupling between gradients and turbulent transport. Experiments on the influence of magnetic shear on confinement are reported. Global confinement issues as well as enhanced confinement regimes found in TJ-II are discussed as well.

Flux-expansion divertor studies in TJ-II

The flux-expansion divertor concept is theoretically explored in TJ-II. This concept seems to be most suitable for this flexible stellarator in which the positions and the order of the resonances can be varied. As a first step, a family of configurations that exhibit flux expansion is identified. Then, a three-dimensional map of the collisional particle flux has been obtained for two different plasma regimes using the full-f Monte Carlo code ISDEP (Integrator of Stochastic Differential Equations in Plasmas), which computes the ion guiding-centre trajectories. The particle trajectories rather than the field lines must be considered since ion orbits can drift from the field lines in TJ-II and the plasma electric field and the collisionality must be considered in such calculations. We have checked that it is possible to strongly reduce the heat and particle fluxes by intercepting the trajectories at a given zone of the space. These calculations are done for thermal ions and for fast particles coming from the neutral beam injection heating. Future studies, possibly including the creation of an ergodic zone, will determine the best strategy for intercepting the trajectories.

Loss cone structure for ions in the TJ-II helical axis stellarator Part II: Radial electric field effects

Previous studies are extended of losses and trapping for collisionless ions of low and intermediate energy (0.1-1 keV roughly), done for the TJ-II helical axis stellarator, to the case where radial electric fields are present. The modifications produced by the electric field on the radial profile of losses, the angular distributions at the plasma exit and the impact patterns on the TJ-II vacuum vessel are analysed showing a progressive peripheral concentration and a slow rotation of the distributions of lost particles at the plasma exit and on the vacuum vessel in the direction of the poloidal rotation induced by the electric field. Changes in radial and angular trapping distributions are discussed, showing the relative insensitivity to the electric field of the angular positions where the maxima and minima of trapping occur. The effect of the electric field resonances is analysed and there is a discussion of the order of appearance of the different TJ-II magnetic configurations and the influence on trapping and losses. The modifications produced by the electric field and the resonances on the different loss cone diagrams are analysed and explained.

Kinetic simulations of fast ions in stellarators

The steady-state distribution function of neutral beam injection (NBI) fast ions is calculated numerically for the LHD and TJ-II stellarators using the code ISDEP (Integrator of Stochastic Differential Equations for Plasmas). ISDEP is an orbit code that solves the guiding centre motion of fast ions using Cartesian coordinates in position space, allowing arbitrary magnetic configurations and the re-entering of particles in the plasma. It takes into account collisions of fast ions with thermal ions and electrons using the Boozer and Kuo-Petravic collision operator. The steady-state distribution function is computed with a time integral following Greens function formalism for a time-independent source. The rotation profiles of the fast ions are also estimated, thus computing their contribution to the total plasma current. In addition, energy slowing down time and escape distribution are studied in detail for both devices.