Selected journal articles

Achieving impurity and helium ash control is a crucial issue in the path towards fusion-grade magnetic confinement devices, and this is particularly the case of helical reactors, whose low- collisionality ion-root operation scenarios usually display a negative radial electric field which is expected to cause inwards impurity pinch. In this work we discuss, based on experimental measurements and standard predictions of neoclassical theory, how plasmas of very low ion collisionality, similar to those observed in the impurity hole of the large helical device can be an exception to this general rule, and how a negative radial electric field can coexist with an outward impurity flux.

  • T Klinger, A Alonso, S Bozhenkov, R Burhenn, A Dinklage, G Fuchert, J Geiger, O Grulke, A Langenberg, M Hirsch, G Kocsis, J Knauer, A Krämer-Flecken, H Laqua, S Lazerson, M Landreman, H Maaßberg, S Marsen, M Otte, N Pablant, E Pasch, K Rahbarnia, T Stange, T Szepesi, H Thomsen, P Traverso, J L Velasco, T Wauters, G Weir, T Windisch, and The Wendelstein 7-X Team. Performance and properties of the first plasmas of wendelstein 7-x. Plasma Physics and Controlled Fusion, 59(1):014018, 2017. PDF

The optimized, superconducting stellarator Wendelstein 7-X went into operation and delivered first measurement data after 15 years of construction and one year commissioning. Plasma operation was started with 5 MW electron cyclotron resonance heating power and five inboard limiters. Core plasma values of Te > 8 keV, Ti > 2 keV at line-integrated n ≈ 3 × 10^19 m^−3 densities were achieved, exceeding the original expectations by about a factor of two. Indications for a core-electron-root were found.

The largest and most sophisticated stellarator in the world, Wendelstein 7-X (W7-X), has just started operation. Here we show the first physics results, obtained before plasma operation: that the carefully tailored topology of nested magnetic surfaces needed for good confinement is realized, and that the measured deviations are smaller than one part in 100,000. This is a significant step forward in stellarator research, since it shows that the complicated and delicate magnetic topology can be created and verified with the required accuracy

  • J L Velasco, K McCarthy, N Panadero, S Satake, D López-Bruna, J A Alonso, I Calvo, T Estrada, J M Fontdecaba, J Hernández, R García, F Medina, M A Ochando, I Pastor, S Perfilov, E Sánchez, A Soleto, B Ph Van Milligen, A Zhezhera, and the TJ-II Team. Particle transport after pellet injection in the TJ-II stellarator. Plasma Physics and Controlled Fusion, 58(8):084004, 2016. arxiv / PDF

Core plasma fuelling and density control is a critical issue for developing steady-state scenarios in fusion reactors. In this work we study radial particle transport in stellarator plasmas using cryogenic pellet injection. By means of perturbative experiments, we estimate the experimental particle ux and compare it with neoclassical simulations. Experimental evidence is obtained of the fact that core depletion in helical devices can be slowed-down even by pellets that do not reach the core region. This phenomenon is well captured by neoclassical predictions with DKES and FORTEC-3D.

  • J A Alonso, J L Velasco, I Calvo, T Estrada, J M Fontdecaba, J García-Regaña, J Geiger, M Landreman, K McCarthy, F Medina, B Ph Van Milligen, M A Ochando, and F I Parra. Parallel impurity dynamics in the TJ-II stellarator. Plasma Physics and Controlled Fusion, 58(7):074009, 2016. arxiv / PDF

We review in a tutorial fashion some of the causes of impurity density variations along field lines and radial impurity transport in the moment approach framework. We apply the fluid model including main ion-impurity friction and inertia to observations of asymmetric emissivity patterns in neutral beam heated plasmas of the TJ-II stellarator. The model is able to explain qualitatively several features of the radiation asymmetry, both in stationary and transient conditions, based on the calculated in-surface variations of the impurity density.

Direct observations of electrostatic potential variations along the flux surfaces of the TJ-II stellarator are presented. Its effect on and/or combination with impurity density variations can give rise to substantial changes of radial impurity fluxes. Measurements taken with two distant Langmuir probe arrays show differences in the edge floating potentials profiles of several tens of volts in electron-root wave-heated plasmas. Neoclassical Monte Carlo simulations estimate the correct order of magnitude for the overall variation in potential and predict the trend observed with the radial electric field.

The drift-kinetic equation is solved for low density plasmas close to the transition from electron to ion root. We show that the range of frequencies of plasma potential fluctuations in which zonal flows are experimentally observed is neoclassically undamped in a neighbourhood of the transition. This makes the electron root regime of stellarators, close to the transition to ion root, a propitious regime for the study of zonal-flow evolution.

Rotation is favorable for confinement, but a stellarator can rotate at high speeds if and only if it is sufficiently close to quasisymmetry. This article investigates how close it needs to be; it contains the first step towards a formulation to calculate the rotation profile of a stellarator close to quasisymmetry.

A comparative study of energy transport for medium- to high-density discharges in the stellarator-heliotrons TJ-II, W7-AS and LHD is carried out. The chosen discharges exhibit significant ion energy transport, and ion-root conditions, i.e. a small negative radial electric field, were found. Within a core region, the predicted neoclassical energy fluxes comply with experimental findings .

The drift kinetic equation is solved for low density TJ-II plasmas employing slowly varying, time-dependent profiles. This allows us to simulate density ramp-up experiments and describe from first principles the formation and physics of the radial electric field shear layer. The main features of the transition are perfectly captured by the calculation, and good quantitative agreement is also found. The results presented here, that should be valid for other non-quasisymmetric stellarators, provide a fundamental explanation for a wealth of experimental observations connected to the shear layer emergence in TJ-II.