B. Brañas

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.

Atomic beam diagnostics for characterization of edge plasma in TJ-II stellarator

He and Li beams have been installed and set into operation at the TJ-II stellarator with the aim of measuring electron temperature and density profiles in the edge and scrape-off layer regions of the plasma. The effusive thermal Li beam penetrates up to a normalized effective radius of ρ=0.6–0.7 in typical electron cyclotron heated plasmas. Density profiles deduced from the Li beam are wider than those obtained from extrapolation of the profile measured by Thomson scattering. The He beam is a pulsed supersonic one. Simultaneous detection of three emission lines (667.2, 706.5, and 728.1 nm) is made with a set of three photomultipliers. First He beam measurements of ne and Te in TJ-II are presented and the agreement among results of different plasma edge diagnostics is discussed.

Edge-localized-mode-like events in the TJ-II stellarator

ELM-like activity has been recently observed in TJ-II, in plasmas with stored energy above 1xa0kJ. The plasma is observed to develop bursts of magnetic activity (seen in Mirnov coil signals) which are followed by a large and distinct spike in the Hα signal. An increase in electrostatic and magnetic fluctuations at the plasma edge and a cold pulse towards the plasma centre are also characteristics of these events. In addition, the electron temperature profile locally flattens at the plasma radius where the temperature is in the range 100-200xa0eV. This flattening can be explained in terms of enhanced electron heat conductivity. Between ELM-like events the electromagnetic turbulence at the edge decreases and the Te profiles recover their former shapes. This activity is probably triggered by a resonant m = 2, n = 3 mode.

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.

Multipulse supersonic helium beam diagnostic in the TJ-II stellarator

The well known He beam diagnostic, able to provide profiles of the electronic plasma edge parameters, has undergone a significant improvement by the application of supersonic expansion techniques in the last years. The use of pulsed fast piezoelectric valves was initiated in the TJ-IU and implemented in the TJ-II stellarators. In its former configuration the repetition rate of the beam was ultimately limited in the expansion chamber by the pressure buildup. In the actual upgraded version, however, a repetition rate up to 200u2002Hz can be achieved, and the recording of the electron temperature and density profiles with millisecond resolution is possible. In the present work, the technical development of the diagnostic required for optimized performance is described. In addition, the validation of the collisional–radiative model needed for the reconstruction of the edge profiles and the analysis of the complementary information given by the beam attenuation is also 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.

Confinement studies in the TJ-II stellarator

ECR (electron cyclotron resonance) heated plasmas have been studied in the low magnetic shear TJ-II stellarator (R = 1.5 m, a<0.22 m, B = 1 T, f = 53.2 GHz, PECRH = 300 kW, power density = 1-25 W cm-3). Recent experiments have explored the flexibility of the TJ-II across a wide range of plasma volumes with different rotational transforms and rational surface densities. In this paper, the main results of this campaign are presented and, in particular, the influence of iota and rational surfaces on plasma profiles is discussed.

Transport studies in the TJ-I tokamak from steady and perturbative methods

Transport analysis in the steady state and the natural perturbation of the sawteeth observed in electron temperature and density measurements in ohmic discharges of the TJ-I tokamak are used for studying the dependence of particle and electron energy fluxes on density and temperature gradients. The results of coupled analysis for propagation of density and temperature perturbations give a transport coefficient matrix with negligible off-diagonal terms. The effective electron heat conductivity chi is one order of magnitude greater than the effective particle diffusion coefficient D. The heat conductivity obtained from heat pulse measurements is close to that obtained from power balance analysis. The diffusion coefficient from density pulses is also close to that obtained from steady state analysis assuming a neoclassical inward pinch. A strong relationship of the deduced heat conductivity with q profile was found, which can be interpreted as due to tearing modes. A unified expression for particle and energy fluxes, which explains the results of pulse propagation and steady state profiles, is also given

Testing of the Collisonal‐Radiative Model by Laser Induced Perturbation of a Supersonic He beam in TJ‐II plasmas

A laser‐based technique for the direct testing of the predictions of the Collisional‐Radiative Model, of application to the analysis of the supersonic He beam diagnostic, is presented. The time evolution of the populations of some relevant excited levels upon the laser pumping of one of them is simulated by solving the corresponding system of coupled differential equations. Good sensitivity of the changes induced by the laser interaction to the particular values of the relevant collisional rate constants is observed for both, the singlet and triplet systems of the He atom. The available experimental set‐up and some previous results are also shown, and the required improvements for its application to the TJ‐II plasma conditions are addressed.

Reflectometry Density Profile Measurements in Turbulent Plasmas

One of the limiting factors in reflectometers when trying to measure the density profile in fusion plasmas is the existence of a fraction of experiments where the measurement becomes impossible: very noisy signals leading to nonsense density profiles. Those so called “bad shots” are usually linked with strongly turbulent plasmas and the remaining problem is to be able to assess “a priori” if the data coming from a given experiment will correspond to a normal profile or if they will be meaningless.