M. Liniers

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 × 1019 m−3 under Li-coated walls.

A flexible luminescent probe to monitor fast ion losses at the edge of the TJ-II stellarator

A mobile luminescent probe has been developed to detect fast ion losses and suprathermal ions escaping from the plasma of the TJ-II stellarator device. The priorities for its design have been flexibility for probe positioning, ease of maintenance, and detector sensitivity. It employs a coherent fiber bundle to relay, to the outside of the vacuum chamber, ionoluminescence images produced by the ions that impinge, after entering the detector head through a pinhole aperture, onto a screen of luminescent material. Ionoluminescence light detection is accomplished by a charge-coupled device camera and by a photomultiplier, both of which are optically coupled to the in-vacuum fiber bundle head by means of a standard optical setup. A detailed description of the detector, and the first results obtained when operated close to the plasma edge, are reported.

Overview of TJ-II flexible heliac results

The TJ-II is a four period, low magnetic shear stellarator, with high degree of configuration flexibility (rotational transform from 0.9 to 2.5) which has been operating in Madrid since 1998 (R = 1.5 m, a 0.22 m, B0 = 1T , PECRH600 kW, PNBI3 MW under installation). This paper reviews the main technical aspects of the TJ-II heliac as well as the principal physics results obtained in the most recent TJ-II experimental campaign carried out in 2000.

Thermographic calorimetry of the neutral beam injectors heating beams at TJ-II

A new beam diagnostic based on infrared thermography has been developed for the neutral beam injectors of the stellarator TJ-II. A highly anisotropic movable target intercepts the beam at its entrance into the stellarator. The thermal print of the beam is captured with a high resolution infrared camera. The infrared images of the target can be translated, with the appropriate analysis, into power density patterns of the beam. The system is calibrated in situ with two thermocouples adiabatically mounted in the target. The two-dimensional beam power density distribution can be accurately characterized allowing beam optimization with respect to the different parameters involved in the beam formation and transport.

Conceptual designs of fast-ion loss detectors for the TJ-II stellarator

We present conceptual designs for two compact, flexible fast-ion loss detectors for quantifying the direct loss term, which is predicted by theoretical calculations to be the most relevant one, from TJ-II plasmas heated by Neutral Beam Injectors. In both conceptual designs, we attempt to encompass the need for a very sensitive system with no imaging capabilities and that for a less sensitive system with some imaging capacity. Finally, we model fast-ion losses from this device in order to identify the optimal locations for such detectors, while also taking experimental constrains into account, and we estimate the ion currents in these detectors as well as the resultant output signal levels.

Development and tests of B4C-covered heat shields for TJ-II

Abstract The Neutral Beam Injection System for the Spanish Stellarator will start in 2001. This heating system consists of two beam lines and will inject about 2 MW into the plasma. The ‘groove region’ of the vacuum vessel (Proc. 18th Symp. On Fusion Technol. (1994) 771) will be exposed to thermal loads as high as 5 MW/m2 (Fus. Technol. Vol. 35, (1999)). A heat resistant material must cover the present stainless steel groove protection plates. Plasma-sprayed boron carbide (B4C) seems to fulfil all the requirements, in addition needs little room and therefore does not reduce the size of the plasma. The development of the B4C-covered plates was done in two steps. In the first one, stainless steel plates with simplified geometry were covered with B4C and tested at the JET Neutral Beam Test Bed. To analyse damages after the tests, surface analysis by means of SEM, ESCA and Auger techniques have been done at Ciemat. The second step comprises the plasma spraying of stainless steel plates with the real geometry. In order to achieve a sufficiently good adhesion of the B4C-layer an intermediate layer of NiCr was sprayed. Standard tests of these plates at the manufacturer have successfully been terminated. The heat load cycling remains to be done in the near future.

TJ-II neutral beam injectors control and data acquisition system

Abstract The Control and Data Acquisition System for the Neutral Beam Injectors of TJ-II are described in this paper. The DAQ and Control architecture is based on a distributed system including VME-OS9 real-time computers and personal computers (PCs) with Linux and Windows NT. One OS9-VME local computer located near each injector performs the timing, local signal processing, signal interlock, and operation of the injector mechanical and electrical components. A main OS9-VME interfaces and controls some peripheral systems, such as the high voltage power supplies, water cooling, safety and protection systems. Control is performed from the OS9-VME machines, in which the software elements are basically C/C++ programs performing real time processing and a web server enabling access to the HTML pages acting as user graphic interface.

High voltage power supplies for the neutral beam injectors of the stellarator TJ-II

Abstract Neutral beam injection will be available for the second experimental phase of TJ-II. Two injectors, set in co-counter configuration, will inject into the plasma two 40 keV H0 beams, each of up to 1 MW. The two high voltage power supplies to feed the acceleration grids of the injectors, described in this paper, are of the transformer–rectifier type, taking their primary energy from a pulsed flywheel generator, and are coupled to the acceleration grids through a switching device. This environment effectively sets the main operation limits and protection requirements of the power supplies.

TECHNICAL ASPECTS AND STATUS OF THE SPANISH STELLARATOR TJ-II

Abstract TJ-II is a medium-size stellarator of the heliac-type being under construction in Madrid, Spain. The project was granted EURATOM preferential support phase II in November 89 and is expected to start operation in 95. This device allows the exploration of a wide range of magnetic configurations. The rotational transform can be varied from 0.9 to 2.5. Shear variation is possible up to 10%. Theory predicts maximum beta values as high as 6%. The average major plasma radius is 1.5 m. The minor dimensions of the bean-shaped plasma are approximately 0.4 m by 0.2 m. Toroidal field: 1T. Pulse length: 0.2 s to 1 s. Pulse break: 5 min. Auxiliary heating: 400 kW ECRH at 53.2 GHz in stage I; 2 MW NBI will be added in stage II. In stage III 4 MW NBI heating is planned to be applied. The paper gives an overview on the technical features of the experiment, describes the main aspects of the recent engineering work and presents the status of the device.

Transport Properties in the TJ‐II Flexible Heliac

TJ‐II flexibility is exploited to perform an investigation on the transport properties of this device. Rotational transform can be varied in a wide range, which allows one to introduce low order rationals and to study their effect on transport. On the other hand, confinement properties can be studied at very different rotational transform values and for different values of magnetic shear: Experiments on influence of the magnetic shear on confinement are reported. Plasma potential profiles have been recently measured in some configurations up to the plasma core with the Heavy Ion Beam Probe (HIBP) diagnostic and the electric field values measured in low‐density plasmas are consistent with neoclassical calculations near the plasma core. 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.