Torus Experiment For Technology Oriented Research Research Articles

Supersonic helium beam diagnostic for fluctuation measurements of electron temperature and density at the Tokamak TEXTOR

A supersonic helium beam diagnostic, based on the line-ratio technique for high resolution electron density and temperature measurements in the plasma edge (r/a > 0.9) was designed, built, and optimised at TEXTOR (Torus Experiment for Technology Oriented Research). The supersonic injection system, based on the Campargue skimmer-nozzle concept, was developed and optimised in order to provide both a high neutral helium beam density of n(0) = 1.5 × 10(18) m(-3) and a low beam divergence of ±1° simultaneously, achieving a poloidal resolution of Δ(poloidal) = 9 mm. The setup utilises a newly developed dead volume free piezo valve for operation in a high magnetic field environment of up to 2 T with a maximum repetition rate of 80 Hz. Gas injections are realised for a duration of 120 ms at a repetition rate of 2 Hz (duty cycle 1/3). In combination with a high sensitivity detection system, consisting of three 32 multi-channel photomultipliers (PMTs), measurements of edge electron temperature and density with a radial resolution of Δ(radial) = 2 mm and a maximum temporal resolution of Δt ≃ 2 μs (470 kHz) are possible for the first time. The diagnostic setup at TEXTOR is presented. The newly developed injection system and its theoretical bases are discussed. The applicability of the stationary collisional-radiative model as basis of the line-ratio technique is shown. Finally, an example of a fluctuation analysis demonstrating the unique high temporal and spatial resolution capabilities of this new diagnostic is presented.

Avalanche photodiode based detector for beam emission spectroscopy

An avalanche photodiode based (APD) detector for the visible wavelength range was developed for low light level, high frequency beam emission spectroscopy (BES) experiments in fusion plasmas. This solid state detector has higher quantum efficiency than photomultiplier tubes, and unlike normal photodiodes, it has internal gain. This paper describes the developed detector as well as the noise model of the electronic circuit. By understanding the noise sources and the amplification process, the optimal amplifier and APD reverse voltage setting can be determined, where the signal-to-noise ratio is the highest for a given photon flux. The calculations are compared to the absolute calibration results of the implemented circuit. It was found that for a certain photon flux range, relevant for BES measurements (≈10(8)-10(10) photons/s), the new detector is superior to both photomultipliers and photodiodes, although it does not require cryogenic cooling of any component. The position of this photon flux window sensitively depends on the parameters of the actual experimental implementation (desired bandwidth, detector size, etc.) Several detector units based on these developments have been built and installed in various tokamaks. Some illustrative results are presented from the 8-channel trial BES system installed at Mega-Ampere Spherical Tokamak (MAST) and the 16-channel BES system installed at the Torus Experiment for Technology Oriented Research (TEXTOR).

Transportation of radiation through opaque magnetoactive plasmas by the means of parametrically induced transparency

The concept of “parametrically induced transparency” (PIT) is introduced: a variant of electromagnetically induced transparency (EIT) in inhomogeneous media, in which the EIT regime takes place in only a limited spatial region in which the signal wave is scattered into a transporting wave by a high power drive wave. It is shown how PIT can be employed for the extraction of radiation from the core of overdense toroidal plasmas. Numerical examples are given for the TEXTOR (Torus Experiment for Technology Oriented Research) tokamak [U. Samm, Fusion Sci. Technol. 47, 73 (2005), Special Issue on TEXTOR], which use the existing gyrotron as the high power drive source.

Mapping of drift surfaces in toroidal systems with chaotic magnetic fields

Drift orbits of test particles are studied using a recently proposed Hamiltonian theory of guiding-center motion in toroidal systems. A symplectic mapping procedure in symmetric form is developed which allows a fast and accurate characterization of the Poincaré plots in poloidal cross sections. It is shown that the stochastic magnetic field acts differently on the onset of chaotic motion for co- and counterpassing particles, respectively. Resonant drift surfaces are shifted inward for the co-passing particles, and are shifted outward for the counterpassing particles, when compared with resonant magnetic surfaces. The overall result is an inward (outward) shift of chaotic zones of co-passing (counterpassing) particles with respect to the magnetic ergodic zone. The influence of a stationary radial electric field is discussed. It shifts the orbits farther inward for the co-passing particles and outward for the counterpassing particles, respectively. The shifts increase with the energies of the particles. A rotation of the magnetic field perturbations and its effect on drift motion is also investigated. Estimates for the local diffusion rates are presented. For applications, parameters of the dynamic ergodic divertor of the Torus Experiment for Technology-Oriented Research are used [Fusion Eng. Design 37, 337 (1997)].

Detector system with high time resolution for the continuous measurement of spectra in the vacuum ultraviolet wavelength range

A new detector system with high time resolution (1 ms) has been developed and applied for the continuous measurement of spectra in the vacuum ultraviolet (VUV) and extreme ultraviolet (EUV) wavelength region at the fusion plasma experiment Torus Experiment for Technology-Oriented Research (TEXTOR). The system consists of an open multichannel-plate (MCP) detector with subsequent first generation (Gen I) light amplifier and a camera head which is based on a linear photodiode array with 1024 elements (pixels). The camera head provides the output signals of the individual pixels sequentially as an analog voltage with a full spectra rate of 1000 per second, which are measured using a PC-based data acquisition system. Three vacuum spectrometers operating in the VUV/EUV region (10–130 nm) have been equipped with the new system and a successful campaign of measurements from about 4000 discharges at TEXTOR has been performed. Spectra are recorded with a usable linear dynamic range of 10 bit and a wavelength resolution corresponding to a width of 3–4 pixels.

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.

Fractal structure of the magnetic field in the laminar zone of the Dynamic Ergodic Divertor of the Torus Experiment for Technology-Oriented Research (TEXTOR-94)

Magnetic field properties at the plasma edge in the Dynamic Ergodic Divertor (DED) for the Torus Experiment for Technology Oriented Research (TEXTOR-94) [Fusion Eng. Des. 37, 337 (1997)] have been studied. This is done by using a generalized symplectic mapping method of integration of field line equations. It is shown that by the radial shift of the resonant magnetic surfaces implemented by change of the plasma current (or the toroidal field) one can strongly vary the plasma edge regimes from the ergodic zone dominated one to the laminar zone dominated regimes in which the field lines with short wall to wall connection lengths are predominant. The embedded in the laminar zone narrow ergodic zones have fractal structures which are investigated in detail. The fractal structures of the magnetic footprints on the divertor plate are also studied.

Novel instrumentation for spectrally resolved soft x-ray plasma tomography: Development and pilot results on TEXTOR

A novel instrumentation for wavelength- and time-resolved plasma emission tomography in the range 0.1–4 keV has been demonstrated on the Torus Experiment for Technology Oriented Research (TEXTOR). The technique is intended for reconstruction of distributions of local emission coefficients (LEC) for selected spectral lines of impurity ions. Further determination (with additional data on electron density and temperature) of spatial distributions of impurity ions at particular ionization stages will become feasible. Spectrally selective plasma images at several viewpoints around plasma are obtained with miniature pinhole cameras supplemented with multilayer mirrors as dispersion elements. The x-ray image is converted to a visible image and transported by a fiber bundle to a gain enhanced recording camera with an electron bombarded charge coupled device tube. A part of the system has been installed on temporary TEXTOR ports. First demonstration results have been obtained on plasma imaging and on subsequent LEC reconstruction using a modified iterative sinogram restoration tomography algorithm. The complete diagnostics will be operational after the TEXTOR shutdown in 2001.

Three-dimensional fluctuation imaging diagnostic for TEXTOR (abstract)

After more than 4 decades of fusion research, plasma transport in tokamaks remains an outstanding issue. The standard hypothesis, that the observed anomalous transport is caused by small scale turbulence, is consistent with experimental observations, but it is neither based on a self-consistent theory of plasma turbulence nor on an exhaustive set of turbulence measurements. One of the major obstacles to the development of a satisfactory anomalous transport theory is the inability of standard fluctuation diagnostics to provide the full spectrum of turbulent fluctuations. The result is a nearly complete reliance on numerical simulations. Unfortunately, in spite of the enormous capabilities of today’s computers, this is extremely unsatisfactory since any theory or simulation of plasma turbulence can only be driven by direct experimental observations. The first step in achieving such data was the highly successful UC Davis Electron Cyclotron Emission (ECE) Imaging system, which was first developed for use on the TEXT-U tokamak and later modified for use on the Rijnhuizen Tokamak Project tokamak.1 Here, correlation techniques are applied to spatially resolved second harmonic ECE signals to provide detailed information about the microturbulence associated with Te fluctuations. The present work extends this work by developing a similar technique for spatially resolved ne fluctuations via microwave imaging reflectometry. The result is a diagnostic capable of the simultaneous measurement of both Te and ne fluctuations (both turbulent and coherent) and profiles on toroidal devices such as tokamaks and stellarators. A 16 channel ECE imaging system has recently been installed on the Torus Experiment for Technology Oriented Research (TEXTOR) tokamak. A prototype millimeter wave imaging reflectometry system will be installed on TEXTOR in July 2000, which will operate in V band (50–75 GHz) and reflect from the X-mode cutoff. Data collected with this system will be utilized in the design of the full three- dimensional (3D) imaging system, which is scheduled for installation on TEXTOR in April 2001. System details and laboratory characterization results of the prototype reflectometric imaging system will be presented, along with a preliminary design for the full 3D system.

Imaging x-ray crystal spectrometers for the National Spherical Torus Experiment

A new type of high-resolution x-ray imaging crystal spectrometers is described for implementation on the National Spherical Torus Experiment (NSTX) to provide spatially and temporally resolved data on the ion temperature, toroidal and poloidal plasma rotation, electron temperature, impurity ion-charge state distributions, and impurity transport. These data are derived from observations of the satellite spectra of heliumlike argon, Ar XVII, which is the dominant charge state for electron temperatures in the range from 0.4 to 3.0 keV and which is accessible to NSTX. Experiments at the Torus Experiment for Technology Oriented Research (TEXTOR) demonstrate that a throughput of 2×105 photons/s (corresponding to the count-rate limit of the present detectors) can easily be obtained with small, nonperturbing argon gas puffs of less than 1×10−3 Torr l/s, so that it is possible to record spectra with a small statistical error and a good time resolution (typically 50 and 1 ms in some cases). Employing a novel design, which is based on the imaging properties of spherically bent crystals, the spectrometers will provide spectrally and spatially resolved images of the plasma for all experimental conditions, which include ohmically heated discharges as well as plasmas with rf and neutral-beam heating. The conceptual design, experimental results on the focusing properties, and relevant spectral data from TEXTOR are presented.

Studies on a helical near field divertor

The proposed Dynamic Ergodic Divertor (DED) on the Torus Experiment for Technology Oriented Research (TEXTOR) has inspired the analysis of the near field of the perturbation current. For simplicity, the analysis is performed in cylindrical geometry. In the near field of a resonant perturbation current the field lines are diverted such that a helical divertor is formed. The configuration can be transformed topologically into the poloidal divertor. The required perturbation amplitude is about an order of magnitude smaller than the one required for the poloidal divertor. At this rather low perturbation current, the particle diversion efficiency is lower than that of a poloidal divertor, but amounts to more than 50%, which is quite considerable

Particle balance in the Tokamak Fusion Test Reactor

Plasma particle pumping effects with carbon limiters have been seen in Tokamak Fusion Test Reactor (TFTR), Joint European Torus (JET), and Torus Experiment for Technology Oriented Research (TEXTOR). The pumping effects in TFTR were induced by conditioning the large-area axisymmetric bumper limiter with low-density, helium initiated discharges. As a result of the conditioning ∼150 Torr l of gas are removed from the carbon limiter tiles. The conditioning has reduced the particle recycling coefficient from near unity to values as low as 0.5. The low recycling conditions are not permanent. A pumping capacity of ∼100 Torr l has been determined from wall loading experiments. The total particle input has been compared to the total outgassing after a discharge for Ohmic plasmas in both helium and deuterium, and for deuterium neutral beam heated discharges. Ohmic deuterium plasmas have less outgassing than input. Ohmic helium plasmas have equal or greater outgassing compared to gas input. Neutral beam heated discharges have a slight deficit in outgassing, but it is much less than that for Ohmic shots. The low recycling conditions last much longer with neutral beams than in Ohmic discharges. The limiter pumping effect and the reduction in recycling is a prerequisite for the enhanced confinement neutral beam heated discharges observed on TFTR.

The High-Power (3-MW) Long-Pulse (3-s) Radio-Frequency System for Ion Cyclotron Resonance Heating Experiments on TEXTOR

A multimegajoule ion cyclotron resonance heating (ICRH) experiment was installed on the Torus Experiment for Technology-Oriented Research (TEXTOR) tokamak. The system consists of two independent power lines each designed to generate and launch 1.5 MW of radio-frequency (rf) power into the machine during a 3-s period in the 25- to 29-MHz frequency range. Each power line consists of the following items: a 1.5-MW transmitter, a transmission line system, and an interface linking the transmission line to the antenna of the shielded strip-line type placed along the tokamak's hot liner. Details of the line and antenna diagnostics and data acquisition system together with the subsequent impedance characteristic calculations are given. The rf radiation shielding for the ICRH experiment is explained. The control of the rf setup as a TEXTOR sub-system and the generator pulse control and operation modes are outlined. The antenna loading and power limitation in the presence of plasma and the conditioning procedure are discussed. Finally, the new rf system compatible with the toroidal pump limiter Advanced Limiter Test-II is presented.