V. Tribaldos

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.

Doppler reflectometer system in the stellarator TJ-II

A Doppler reflectometer system has recently been installed in the stellarator TJ-II. The system is optimized for the Q-band (33-50 GHz) and the high-curvature plasmas produced in TJ-II. The launch angle of the microwave beam can be controlled by a steerable mirror to obtain angles between +/-20 degrees enabling the measurement of perpendicular wave numbers in the range of 3-15 cm(-1). The available angular range allows for comparisons between positive and negative values and additionally for calibration of the system. Localization and k(perpendicular)-estimation is done via the three-dimensional ray/beam-tracing code TRUBA. First measured spectra and radial profiles of the perpendicular velocity of plasma density fluctuations are presented.

Status and prospects for mm-wave reflectometry in ITER

Reflectometry with wavelengths in the centimetre to millimetre-wave range will be used in ITER to measure the density profile in the main plasma and divertor regions and to measure the plasma position and shape in order to provide a reference for the magnetic diagnostics in long pulses. In addition, it is expected to provide key information for the measurement of density fluctuations. A set of reflectometers to meet the relevant ITER measurement requirements has been included in its present outline as part of the ITER design since 2001 and is being adapted to the present ITER baseline and to accommodate progress with reflectometry techniques and measurement capabilities. It comprises low and high field side (HFS and LFS, respectively) ordinary (O-) mode systems for the measurement of the density profile in the gradient regions, a LFS extraordinary (X-) mode system for the detailed study of the edge profile, an HFS X-mode system operating in the left hand cutoff to measure the core profile, a dedicated O-mode system for plasma-wall gap measurement and a multi-band, multiple line of sight O-mode system to measure divertor density profiles. This paper describes the evolution of the design, in particular some recent improvements in the engineering implementation and improvements aimed at enhancing the measurement capability. It concludes with a brief assessment of the likely measurement performance against the ITER measurement requirements for the parameters of interest and the overall confidence that the technique will be implanted on ITER.

The role of a fast ion component on the heating of the plasma bulk

Ion temperatures have been measured in the TJ-II stellarator using passive emission spectroscopy in parallel and perpendicular directions. A high-energy component was observed from the tails of the Hα line emission in both directions, suggesting a non-negligible population of suprathermal ions. The role of this fast ion energy component, which has been observed in both stellarator and tokamak plasmas when heated by electron cyclotron resonance heating in the additional heating of the plasma bulk is investigated by means of a simple model. The origin of such a fast component is discussed in terms of different mechanisms.

Electron cyclotron emission radiometer upgrade on the Joint European Torus (JET) tokamak

The capabilities of the Joint European Torus (JET) electron cyclotron emission (ECE) diagnostics have recently been extended with an upgrading of the heterodyne radiometer. The number of channels has been doubled to 96 channels, with a frequency separation corresponding to 2.6 m for the X-mode due to harmonic overlap) at almost all magnetic field values used at JET (1.7 T<BT<4 T), while maintaining the high sensitivity and spectral resolution of the previous system. In this paper an overview of the upgraded radiometer is presented along with some results showing its performance.

Enhanced heat confinement in the flexible heliac TJ-II

Recent experimental results show that the core electron temperature in the TJ-II stellarator almost doubles previously obtained values for the same heating power. These plasmas, heated with electron cyclotron waves, are characterized by their low density, and by having highly peaked electron temperature profiles and flat, or even hollow, density profiles. The conditions for obtaining these high electron temperature discharges regarding their density, injected power and dependence on plasma species are described. Neoclassical and experimental transport analyses are performed for these discharges, showing a reduction in the electron heat conductivity at the plasma core. The relations of this observed confinement enhancement to the CHS internal transport barrier and the W7-AS neoclassical electron root feature are discussed.

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.

Edge characteristics and global confinement of electron cyclotron resonance heated plasmas in the TJ-II Stellarator

The edge parameters of electron cyclotron resonance heated plasmas in the TJ-II stellarator are reported. Data from atomic beam diagnostics and electrical probes have been used for edge and scrape-off layer characterization. Scans in heating power and plasma density for H and He plasmas have been performed, for a given magnetic configuration. A linear increase of the diffusion coefficient at the last-closed magnetic surface with the ratio of injected power to plasma density and a similar value of that parameter for the two atomic species investigated were obtained. Global particle confinement times between 3 and 15 ms have been deduced, and transition to an enhanced confinement mode in H plasmas has been observed under some conditions. The role of high-energy particle losses, due to trapping into the relatively high magnetic ripple, in the global energy balance of TJ-II plasmas is addressed.

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.

Novel passive spectroscopy system for absolutely referenced plasma rotation measurements in clean plasmas

A novel method for measuring absolutely calibrated rotation velocities in fusion plasmas by means of passive spectroscopy, using a single plasma view, is presented. The method consists of simultaneously recording the emission lines from the plasma and from a calibration lamp by means of a double fiber-fiber guide. A software simultaneously analyzes the plasma and calibration lines, which are obtained with a high-resolution spectrometer equipped with an intensified linear detector array, and provides absolute rotation measurements. Experimental results of carbon impurity ion rotation measured in the TJ-II stellarator are reported, demonstrating an accuracy of better than 0.3km∕s.