T. Estrada

Plasma performance and confinement in the TJ-II stellarator with lithium-coated walls

In the last campaign, the TJ-II heliac has been operated under lithium-coated walls, representing the first stellarator ever working under these boundary conditions. Enhanced density control and discharge reproducibility, leading to the drastic enlargement of the operational window, have been obtained. A strong decrease in recycling together with changes in the shot by shot fuelling characteristics and in the wall particle inventory have been recorded. These changes, associated with the new wall scenario, had led to a long-lasting good density control. The new conditions were also mirrored in the plasma profiles under NBI heating scenarios with increased peaking of the electron density profiles. Fuelling rates corresponding just to the nominal beam current were obtained for the first time, and transitions from bell to dome-type plasma profiles, with different collapsing limits, were observed and tentatively ascribed to changes in the local edge power balance. ELM-type activity was observed in concomitance to reduced fluctuation levels and confinement improvement. Record values of plasma energy content were measured at central densities up to 8 × 1019u2009m−3 under Li-coated walls.

Integrated data analysis at TJ-II: the density profile.

An integrated data analysis system based on Bayesian inference has been developed for the TJ-II stellarator. It reconstructs the electron density profile at a single time point, using data from interferometry, reflectometry, Thomson scattering, and the Helium beam, while providing a detailed error analysis. In this work, we present a novel analysis of the ambiguity inherent in profile reconstruction from reflectometry and show how the integrated data analysis approach elegantly resolves it. Several examples of the application of the technique are provided, in both low-density discharges with and without electrode biasing, and in high-density discharges with an (L-H) confinement transition.

Doppler reflectometry studies using a two-dimensional full-wave code

A two-dimensional full-wave numerical code in the extraordinary mode of propagation has been developed to simulate reflectometry. The code uses the finite-difference time-domain technique to solve wave propagation in magnetized and turbulent plasmas. The code has been used to study the viability of the Doppler reflectometry technique in determining the perpendicular rotation velocity of density fluctuations. The influence of parameters like plasma curvature, probing beam characteristics, turbulence wave-number spectral width and antenna tilt angle is studied. Accurate Doppler measurements can be obtained using Gaussian beams with low divergence and optimum beam waist in slab plasmas and also in plasmas with high curvature. Gaussian beam antennas with optimum spot size provide accurate Doppler frequency values for a wide range of wave-number spectral widths of the turbulence. Doppler measurements at large tilt angles—high probed wave-numbers—are able to give accurate results on the Doppler frequency keeping the enhancement factor of the probing electric field close to the cutoff layer and therefore its contribution to the spatial localization of the measurement; however, the efficiency of the Bragg backscattering process—defined as the ratio between the backscattered and incident power—is low at large tilt angles.

Multichannel electron cyclotron emission radiometry in TJ-II stellarator

Electron cyclotron emission measurements are routinely performed in TJ-II stellarator by means of a multichannel heterodyne radiometer. The radiometer is absolutely calibrated and measures the temperature profile with high temporal resolution. The description of the radiometer, the calibration procedure, and some data taken during the occurrence of fast phenomena in the plasma (edge localized mode-like events and central temperature crashes) are presented in the article.

Impact of wall conditioning and gas fuelling on the enhanced confinement modes in TJ-II

Abstract The TJ-II helical stellarator has been operated under full metallic first wall conditions until mid 2001. This scenario has proven fully compatible with the ECR production and heating of plasmas. Under those conditions, several modes with enhanced confinement have been found. In the last experimental campaigns, however, boronisation of the first wall of the TJ-II has been carried out. This has lead to strong changes in edge parameters and density control. Namely, a rise of the electron temperature at the limiter of a factor of ∼3, and a strong (

Two-dimensional full-wave code for reflectometry simulations in TJ-II

A two-dimensional full-wave code in the extraordinary mode has been developed to simulate reflectometry in TJ-II. The code allows us to study the measurement capabilities of the future correlation reflectometer that is being installed in TJ-II. The code uses the finite-difference-time-domain technique to solve Maxwell’s equations in the presence of density fluctuations. Boundary conditions are implemented by a perfectly matched layer to simulate free propagation. To assure the stability of the code, the current equations are solved by a fourth-order Runge–Kutta method. Density fluctuation parameters such as fluctuation level, wave numbers, and correlation lengths are extrapolated from those measured at the plasma edge using Langmuir probes. In addition, realistic plasma shape, density profile, magnetic configuration, and experimental setup of TJ-II are included to determine the plasma regimes in which accurate information may be obtained.

Status of the diagnostics development for the first operation phase of the stellarator Wendelstein 7-X

An overview of the diagnostics which are essential for the first operational phase of Wendelstein 7-X and the set of diagnostics expected to be ready for operation at this time are presented. The ongoing investigations of how to cope with high levels of stray Electron Cyclotron Resonance Heating (ECRH) radiation in the ultraviolet (UV)/visible/infrared (IR) optical diagnostics are described.

Density control and plasma edge characterisation of ECRH heated plasmas in the TJ-II stellarator

In the 1999 experimental campaign, the Spanish stellarator, TJ-II (R = 1.5 m, a < 0.22 m, B 0 < 1 T) has been operated under a broad range of parameters, including changes in the magnetic configuration, working gas (H 2 vs He), microwave heating power (100-600 kW, two independent lines at 53.2 GHz, second harmonic X-mode) and plasma-wall interaction conditions (wall conditioning, poloidal vs toroidal limiter). Although a close coupling between the plasmas and the TJ-II vacuum vessel is naturally present in most conditions, a good control of central plasma values has been achieved for both atomic species even under conditions close to the highest power density (n e0 < 1.7 x 10 13 cm 3 , T e0 < 1.3 keV). For this purpose, a careful control of wall conditions has been required. In addition, the low electron density and temperature of TJ-II edge plasmas have led to a significant reduction of the expected plasma-wall interaction. In this work, the issues of plasma density control and edge characteristics for the different plasma species and heating power are addressed. Results of new edge diagnostics, as a thermal lithium beam and a supersonic helium beam, among others are presented.

Influence of magnetic well on electromagnetic turbulence in the TJ-II stellarator

A magnetic well scan has been performed in the TJ-II stellarator to investigate the confinement properties with different values of the well, or even of the hill, and to explore the properties of electromagnetic turbulence. Stable plasmas have been obtained in theoretically Mercier-unstable configurations, and the electrostatic turbulence levels in the edge are increased. Three families of modes appear during the experiments: (1) a family of modes of Alfvenic nature with high frequencies; (2) a second set of modes of middle frequencies (tens of kHz) and (3) an oscillation at f ≈ 10–20 kHz happens in several cases. In spite of the fact that the vacuum rotational transform is very similar in all of the cases, the Alfvenic mode family changes drastically when decreasing the magnetic well, showing a non-monotonic behaviour of the amplitude, and a decrease of the typical frequencies. This behaviour cannot be explained only by current or density variations, so the effect of the modification of the configuration is playing a key role. Regarding the intermediate frequencies, a coherent mode appears with decreasing frequency as the magnetic well decreases. This mode is a candidate for a GAM, which can survive in these TJ-II plasmas, despite of the strong damping these modes should suffer in this device.