TJ-II Stellarator Research Articles

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 cyclotronwaves, 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.

Magnetic well and instability thresholds in the TJ-II stellarator

Experimental evidence for the controlled onset of fluctuations by varying the magnetic well in the plasma edge region of the TJ-II stellarator (major radius R=1.5 m, minor radius a<0.22 m, magnetic field B=1 T) has been obtained. The level of fluctuation, the degree of intermittency and the radial correlation of relevant quantities increase as the magnetic well is reduced. TJ-II possesses a high degree of flexibility, making it an ideal device for investigating the characteristics of transport close to instability thresholds.

Wall conditioning of TJ-II stellarator by glow discharge

The wall conditioning of the vacuum vessel has been a critical issue during the first experimental campaigns of TJ-II stellarator. In this paper, a new procedure for plasma density control in metallic wall scenarios, based on conventional glow discharge cleaning (GDC) techniques, is presented. After a first bakeout of the chamber, intensive He GDC at room temperature was applied with the aim of keeping the values of residual water as low as possible. A subsequent Ar GDC for 30 min before the TJ-II operation was required in order to remove an important fraction of He implanted on the wall, which hindered the density control in plasma discharges at injected powers above 300 kW. The Ar GDC removes a total amount of 10 21 He atoms from the TJ-II surface allowing a good control of plasma discharges. Detrapping cross sections for He atoms by Ar bombardment of 1.6×10 −15 cm 2, for high coverages and 4×10 −16 cm 2 at lower ones were estimated.

On the radial scale of fluctuations in the TJ-II stellarator

The TJ-II stellarator was designed to have a high degree of magnetic configuration flexibility. The rotational transform can be varied between 0.9 and 2.5, and the magnetic well may be changed from -1% to 6%. This flexibility is a powerful tool for physics studies in fusion plasmas. The magnetic-well scan experiments in TJ-II support the view that plasma turbulence displays universality. These results emphasize the importance of the statistical description of transport processes in fusion plasmas as an alternative approach to the study of transport based on the computation of effective transport coefficients. Comparative studies in TJ-II stellarator and other devices show that fluctuations and E × B sheared flows organize themselves to be close to marginal stability. This property should be considered as a critical test for L-H transition models. The magnetic-configuration scan experiments in TJ-II show the complex interplay between transport and radial electric fields in the proximity of rational surfaces.

Experimental evidence of fluctuation-induced inward transport linked to rational surfaces in the TJ-II stellarator

Radial profiles of electrostatic fluctuations and fluctuations-inducedtransport have been measured in the proximity of rational surfaces inthe plasma boundary region of the TJ-II stellarator. Structures inplasma profiles linked to the location of low-order rational surfaces(n = 8/m = 5, n = 4/m = 2) have been observed. Local turbulent E×Bdriven fluxes are significantly modified in the proximity of rationalsurfaces. In the case of low-order rationals (n = 4/m = 2) turbulenttransport reverses from radially outwards to radially inwards. Thesechanges are due to a modification in the phase relation between densityand electric field fluctuations.

Main results after regular operation of the HVSMPS for gyrotrons

A new high voltage power supply for gyrotrons using solid-state elements was developed by JEMA. In 1992, this power supply was put into operation on gyrotrons at CIEMAT. In this paper, the parameters of the power supply and its behaviour in the regular operation of the TJ-II stellarator are presented.

Calibration procedure and data processing for a TV Thomson scattering system

Calibration procedures for Thomson scattering systems based on television-like cameras, so-called TVTS systems, are described. The TVTS systems of the Rijnhuizen Tokamak Project (RTP), the TJ-II stellarator, and the Torus Experiment for Technology Oriented Research (TEXTOR) tokamak combine a 10–15 J ruby laser as a source with an intensified charge coupled device camera as a detector. A tungsten strip lamp in combination with an integrating sphere is used to calibrate all ∼105 pixels of the camera relatively to each other. Rayleigh scattering on hydrogen or nitrogen is used to perform an absolute calibration of the complete detection system. Great emphasis is placed on possible systematic errors on the determination of the electron temperature Te and density ne due to the calibration, such as tungsten lamp temperature, detoriation of the detection window, long term stability, laser beam alignment, and detector linearity. The long term stability of the system was tested by comparing different sets of calibration factors. Over a period of 1/2 yr the values of the calibration factors varied by less than 5%. Using the same Thomson scattering data but different sets of calibration factors the Te values varied even less than 1%. A two camera technique was used to search for possible unknown systematic errors in the determination of Te profiles. It appears that these systematic errors are about half of the observational error on Te. Density fluctuations can be determined with an accuracy equal to the statistical error of ∼3%, while the systematic error on ne appeared to be ∼10%, which can be corrected for using interferometer data. As a result, these TVTS systems can measure Te and ne profiles with ∼100 (RTP) or 120 (TEXTOR) spatial elements along the full plasma diameter with observational errors on Te of ∼6% in the range of 25 eV–6 keV, at ne=2×1019 m−3 and 10 J laser energy.

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

The edge parameters of electron cyclotron resonance heated plasmas inthe TJ-II stellarator are reported. Data from atomic beam diagnostics andelectrical probes have been used for edge and scrape-off layercharacterization. Scans in heating power and plasma density for H and Heplasmas have been performed, for a given magnetic configuration. A linearincrease of the diffusion coefficient at the last-closed magnetic surfacewith the ratio of injected power to plasma density and a similar valueof that parameter for the two atomic species investigated were obtained.Global particle confinement times between 3 and 15 ms have been deduced, andtransition to an enhanced confinement mode in H plasmas has been observedunder some conditions. The role of high-energy particle losses, due totrapping into the relatively high magnetic ripple, in the global energybalance of TJ-II plasmas is addressed.

K spectra of Thomson scattering temperature profiles at the TJ-II stellarator

High resolution Thomson scattering temperature profiles at the TJ-II stellarator are subjected to spectral analysis. The obtained k spectrum is very robust in shape. While its amplitude has been shown previously to scale inversely with the collisionality, here it is found that its shape is independent of plasma configuration (magnetic well, iota), global plasma parameters (temperature, density) and radial position (centre, edge). Theshape is also found to be very similar to that of k spectra obtained with Langmuir probes in the edge of TJ-II and of k spectra obtained using other techniques in other devices. The spectral shape is characterized by a `knee' at k = 500-1000 m-1 and a steep decay at large k. Self-similarity analysis reveals the existence of a high degree of self-organization dominated by structures of around 2.5 cm in size.

Monte Carlo estimation of neoclassical transport for the TJ-II stellarator

The neoclassical transport properties of TJ-II stellarator [C. Alejaldre et al., Fusion Technol. 13, 521 (1988)] are studied with the monoenergetic Monte Carlo technique. A compromise between the number of modes and particles and the required computing time to obtain reliable estimates, from the computational point of view, of the monoenergetic diffusion coefficients is shown to be of one thousand particles and one hundred harmonics, because of the rich magnetic-field structure of TJ-II. Although, these requirements are probably too demanding in making the transport estimations. The data base containing the normalized monoenergetic diffusion coefficient for several radial positions, radial electric fields and collisionalities have been fitted using a neural network. This fit reduces the number of points necessary in the data base and allows a smooth interpolation and extrapolation to perform the convolutions of the monoenergetic coefficients with the Maxwellian. For two different typical TJ-II discharges the ambipolar radial electric field, and the neoclassical particle and heat fluxes are presented both showing rather large positive radial electric fields at the plasma core and small negative fields at the edge. The neoclassical particle and energy confinement time are in surprisingly good agreement with the experimental energy balance analysis and the international stellarator scaling. Although no satisfactory explanation is available yet the large neoclassical diffusion caused by the complex ripple structure of TJ-II magnetic field may be an important ingredient.

Wall conditioning and density control in the TJ-II stellarator

Helium glow discharge cleaning (GDC) at room temperature, applied during the overnight periods between experimental days, has been used as standard method of wall conditioning in the TJ-II stellarator during the initial operation with ECRH plasmas in an all stainless steel scenario. However, the density control at medium and high power injection (more than 300 kW) becomes difficult due to the contribution of He implanted on the wall during GDC and desorbed by plasma discharges. He desorption rates of ∼4×10 10 cm −3 ms −1 have been estimated from residual gas analyser (RGA) postpulse measurements. In order to overcome this effect, an Ar GDC of a duration of 30 min prior to the TJ-II operation, and after the overnight He GDC have been applied in the last campaign. The Ar GDC removes of the order of 10 21 He atoms (about one monolayer) from the walls allowing a good control of the plasma density by means of external gas puffing. Values of Z eff∼2.0–2.5 in H 2 plasmas and mass spectra measurements of thermal desorption show that the contribution of Ar implanted to the plasma fuelling is not important.

Plasma boundary and SOL studies of ECH-plasmas in TJ-II stellarator with diagnosed mobile poloidal limiters

TJ-II is a medium size (major radius R=1.5 m, average minor plasma radius a<0.2 m , on axis magnetic field B=1 T) helical axis stellarator operating in its first phase with up to 600 kW of ECH power. Two mobile poloidal limiters can control the last closed magnetic surface (LCMS) and diagnose the plasma boundary with a set of Langmuir probes and with CCD-cameras equipped with interference filters. In the described experiments, plasmas with different minor radii interact either with the toroidal limiter or with the mobile poloidal limiters. The electron density and temperature profiles are characterised in the plasma centre, boundary and scape-off layer (SOL) for the different configurations. The global energy for the different configurations is compared and the ratio of the energy confinement time to that obtained by the LHD-scaling-law, seem to be improved when inserting the poloidal limiters. Finally, it is discussed whether the large connection lengths obtained in certain poloidal limiter SOL regions effectively reduce the plasma size, making the definition of the plasma minor radius ambiguous.

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 <1.7×10 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.

Laser interferometric experiments for the TJ-II stellarator electron-density measurements

Laser two-color heterodyne interferometry is a proven method to measure electron density in fusion plasmas. Though only used in tokamaks with high electron densities, the idea of also using two-color laser interferometers for stellarators and small machines to replace far-infrared laser interferometers is being proposed. This will lead to low-cost, reliable, and easy to operate diagnostics for electron-density measurements. In this article, we present the interferometric experiments we have used for calibration of a two-color laser heterodyne system for electron-density measurements in the TJ-II stellarator. These experiments have been based on the use of a novel interferometric scheme: the heterodyne/homodyne interferometer. Finally, we describe the interferometer we have installed in the TJ-II stellarator and present the first results on mechanical-vibration subtraction and electron-density measurements with a two-color laser interferometer in a stellarator (TJ-II, Madrid, Spain).

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.

Installation of an advanced heavy ion beam diagnostic on the TJ-II stellarator

An advanced 200 keV heavy ion beam diagnostic has been developed for the TJ-II stellarator based on the simultaneous utilization of a 30° Proca–Green electrostatic energy analyzer and a multiple cell array detector. This innovative design allows instantaneous measurements of plasma potential and electron density profiles together with their respective fluctuations. In this article we present a description of the main parts of a heavy ion beam diagnostic (injection system, detectors, and control and data acquisition system) and the results obtained during the first operation on TJ-II. The problems of plasma loading of the detectors and hard x-ray generation associated with the probing beam are reported.

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.

Electron cyclotron emission measurements on TJ-II stellarator plasmas

Electron cyclotron emission (ECE) measurements are used to investigate the changes in the electron temperature profile during on axis and off axis EC-heating in TJ-II stellarator. The modifications of the electron temperature profiles are discussed and compared with the predictions of ray tracing calculations. Recently, the two gyrotrons (PECRH ≤600 kW) were simultaneously applied for the plasma heating in TJ-II. ELM-like events are clearly observed in these plasmas. A first description of these phenomena is also presented.

Filamentary current detection in stellarator plasmas

In this work we present the experimental setup, combining edge, and x-ray diagnostics, used to localize the currents generated by fast electrons confined near the plasma periphery in TJ-II plasmas. The method is similar to that used for the experimental determination of the magnetic field mapping in stellarators, just intercepting the trajectories of test electrons. The fact that stellarators can keep magnetic structures “frozen” in vacuum during the field plateau enables the detection of local currents near the plasma periphery in the TJ-II stellarator.

Profile structures of TJ-II stellarator plasmas

Fine structures are found in the TJ-II stellarator electron temperature and density profiles, when they are measured using a high spatial resolution Thomson scattering system. These structures consist of peaks and valleys superimposed to a smooth average. Some irregularities remain in an ensemble average of discharges with similar macroscopic parameters such as line density, central temperature, and plasma current. They are found in all the magnetic configurations explored in plasmas heated by electron cyclotron waves. Their characteristics are shown and their possible origin is discussed.