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AbstractThe linear plasma device PSI-2 serves as a pilot experiment for the development of components, operational regimes and control systems for the linear plasma device JULE-PSI, which will be located in the nuclear environment allowing studies of radioactive and toxic samples. PSI-2 is also used for fusion reactor relevant plasma-material interaction studies. This contribution describes the PSI-2 layout and parameters and summarizes the recent scientific and technical progress in the project, including the installation of a target station for the sample manipulation and analyses.

In this work we investigate the applicability of several optical emission spectroscopy methods to measure the electron density and temperature in deuterium plasma in the linear plasma generator PSI-2. The spectroscopy measurements are realized by an imaging spectrometer which delivers radial profiles of the emission lines. With the application of an inverse Abel transformation, spatially resolved measurements are obtained.The spectroscopy methods divide into two groups: The measurement of neby Balmer line ratios and by the rotational temperature of molecules is only suitable for ionizing plasmas; the measurement of neby the Stark broadening of Paschen lines and of Teby Paschen line ratios is only applicable for recombining plasmas.For the evaluation of these methods, different plasma conditions are produced in PSI-2. The plasma generator is capable of producing deuterium plasmas with electron densities of up to 1013cm-3and electron temperatures of up to 20 eV. Additional measurements with a Langmuir double probe are conducted for comparison with the spectroscopy measurements.A collisional-radiative model in the Yacora code is used to compare measured Balmer line emissions with the calculation and to investigate which reaction channels influence the recombination in PSI-2.

This paper discusses an ongoing effort to evaluate the use of Glow Discharge Optical Emission Spectroscopy (GDOES) for after-exposure, target surface characterization in a plasma-materials interactions (PMI) facility, without need to remove the sample from the facility’s vacuum chamber and/or to expose the sample’s surface to air. The effort includes testing of a compact, inductively-coupled plasma (ICP) source, which would eventually become part of an integrated, movable GDOES sub-chamber, including plasma source, gas-feed and pumping capability, configured to function inside the main vacuum chamber. The present, conceptual design for implementation on PSI-2 shows promise in meeting these challenges and is presented here.

This contribution gives an overview of different simulation methods for transient events and damages induced in tungsten. The investigations were focussed on the resulting crack networks and special attention was paid to crack distance, width and depth. The results indicate that the different techniques show, in general, similar damage behaviours and the same damage thresholds.

AbstractA series of neutral beam injectors for plasma heating and diagnostics in modern magnetic fusion devices has been developed in the Budker Institute of Nuclear Physics. In ion sources of these injectors arc discharge or RF plasma boxes are used. Ion optical systems are optimized to produce ion beams with a low enough angular divergence. In order to provide beam focusing, the grids are formed as spherical segments. Such ballistically focused beams are further neutralized in a gas target and subsequently are used to heat or diagnose plasma. Obtained diagnostic neutral beams with precise focusing are widely used to measure plasma parameters by beam emission spectroscopy methods in tokamaks, stellarators, reversed field pinches and open traps. High power focused beams with small divergence are also necessary for heating of localized regions of plasma and in the devices with narrow access ports through which only small size, high power density beams can be transported. Transition to steady state operation regime of the injectors is discussed.

AbstractPlasma can be studied and characterised by the analysis of its radiation. Signals obtained by passive spectroscopy contain much information about temperature, density and flux of the main species and impurities. The interpretation of measured line intensities requires the knowledge of atomic physics describing the specific radiation from the plasma. Tomographic methods are applied but they need symmetries for the calculation of local parameters. Additionally in magnetic confined plasmas the interpretation might be more difficult due to the Zeeman splitting.

AbstractPlasma can be studied and characterised by the analysis of its radiation. Signals obtained by passive spectroscopy contain much information about temperature, density and flux of the main species and impurities. The interpretation of measured line intensities requires the knowledge of atomic physics describing the specific radiation from the plasma. Tomographic methods are applied but they need symmetries for the calculation of local parameters. Additionally in magnetic confined plasmas the interpretation might be more difficult due to the Zeeman splitting.Asymmetries and steep gradients of plasma parameters as it appears in the plasma boundary of a tokamak or stellarator require the direct local measurement of these quantities. There are two methods to probe the plasma locally, by a laser or an atomic beam. In both cases, elastic collisions lead to scattering of light (Thomson scattering), respectively atoms (Rutherford scattering) and inelastic collisions cause the emission of light that is analysed (laser induced fluorescence, atomic beam diagnostics).In this article we will concentrate on the interaction of beam atoms with plasma, yielding to optical emission, which is observed with spectroscopic methods. After interaction with the bulk plasma the beam atoms or deuterons and impurity ions can be investigated. The first method is called beam emission spectroscopy (BES), the second charge exchange recombination spectroscopy (CXRS).Both techniques need two ports, one for the injection and a second for observation, which should be nearly perpendicular in order to get the best spatial resolution. The location of the measurement is determined by the intersection of the beam with the (perpendicular) line of sight of the detection systemThis paper is structured in four chapters. After this introduction the basic properties of atomic beam injection used for BES and CXRS are described in chapter II. The collisional-radiative model necessary for the interpretation of the measured line intensities is presented in the third part. Examples of atomic beam sources applied in tokamaks and evaluated signals are given in the last chapter.

AbstractPlasma can be studied and characterised by the analysis of its radiation. Signals obtained by passive spectroscopy contain much information about temperature, density and flux of the main species and impurities. The interpretation of measured line intensities requires the knowledge of atomic physics describing the specific radiation from the plasma. Tomographic methods are applied but they need symmetries for the calculation of local parameters. Additionally in magnetic confined plasmas the interpretation might be more difficult due to the Zeeman splitting.

With the dynamic ergodic divertor in TEXTOR fundamental effects of the coupling of external magnetic field perturbations to the confined plasma have been studied. The non-linear coupling between external (m/n = 12/4) and internal modes (m/n = 3/1) has been investigated. The critical perturbation field (m/n = 3/1) for the excitation of an m/n = 2/1 tearing mode depends not only on the magnitude but also on the direction of the toroidal angular momentum input by neutral beam injection (NBI). Below the excitation threshold of this mode a toroidal spin-up of the plasma has been observed, which only depends on the strength of the perturbation field. It is independent of both the rotation direction of the external perturbation field and the toroidal angular momentum supplied by the NBI.

Determination of both the radial and poloidal components of electron density fluctuation parameters in a tokamak edge plasma was achieved by operation of two narrow Li-beams (FWHM ?10?mm at a distance of 100?mm from the source), which were separated by 50?mm. Beams of thermal lithium atoms were generated using a newly developed oven, which provides a long operation time and a high reliability. The lithium atoms, with a mean velocity of ?1.7?km?s?1, were injected radially into the plasma edge, where they were excited and ionized by the plasma. The line radiation (resonance transition 2s-2p, ? = 670.8?nm) of the excited atoms was observed, which enables determination of the electron density profiles. The measurements are performed with high temporal (?t2?µs) and spatial (?r1?mm in the radial and ?p10?mm in the poloidal direction) resolution, thus also allowing determination of the fluctuations. This new diagnostic permits studies of both radial and poloidal fluctuation parameters like the correlation length, wavenumber, frequency spectrum, propagation velocity and lifetime. The method can be applied continuously over long discharges without disturbing the plasma.

AbstractThe exploration of plasma-wall–interaction physics is one of the major tasks of the tokamak TEXTOR. A characterization of the high-temperature plasma edge is essential to interpret the interaction processes of the different charged and uncharged particles in the boundary layer. In the design of the TEXTOR, much effort was made to optimize diagnostic access to the plasma edge for the best possible characterization. The major part of the plasma edge diagnostics presented here is based on passive and active spectroscopy, in addition to different types of electrical probes. Thereby, pioneering work has been achieved in both fields.In passive emission spectroscopy, the work concentrated on the determination of particle fluxes of different types of atomic (W, Si, C, …) and molecular (D2, CD, C2, …) species from the corresponding photon fluxes at different locations and on the visualization of the local impurity sources by means of two-dimensional imaging. The active spectroscopy with atomic beams was focused on the determination of plasma edge parameters (ne, Te, Ti, …) with good spatial and temporal resolution. Therefore, different techniques like thermal Li and He beams, suprathermal Li beams—realized by laser blow-off techniques—and hydrogen neutral beam injectors have been employed. Furthermore, laser-induced fluorescence measurements in the ultraviolet and in the vacuum ultraviolet ranges, which were for the first time performed in a fusion plasma, are presented. The continuous improvement of the different plasma edge diagnostics over more than a decade of TEXTOR plasma operation with different types of first-wall materials is discussed.

AbstractLimiter lock systems on the top and the bottom of the TEXTOR vessel are essential elements for experimental investigations of plasma-wall interaction in a tokamak. The lock systems are designed as user facilities that allow the insertion of wall elements (limiter) and tools for diagnostic (electrical probes, gas injection) without breaking the TEXTOR vacuum. The specially designed holder on top of the central carrier and a powerful vacuum pump system permit the exchange of components within ~1 h. Up to ten electrical signals, four thermocouples, and a gas supply can be connected at the holder interface. Between discharges, the inserted component can be positioned radially and turned with respect to the toroidal magnetic field. Additionally, the central carrier is electrically isolated to apply bias voltages and currents up to 1 kV and 1 kA, respectively.An important feature of the lock system is the good access for optical spectroscopic observation of the inserted components in the vicinity of the edge plasma. The whole spectrum from ultraviolet to infrared is covered by spectrometers and filters combined with cameras. Toroidally and poloidally resolved measurements are obtained from the view on top of the probes while the tangential poloidal view delivers radially and toroidally resolved information.A programmable logic controller (Simatic S5) that is operated inside the TEXTOR bunker and from remote locations outside the concrete wall drives all possible features of the lock system.

AbstractRecently, the Dynamic Ergodic Divertor (DED) with 18 helically wound coils at the high field side has been installed on TEXTOR. The DED allows static and dynamic operation up to 10 kHz. The specific features of ergodization and the open laminar zone are discussed. The dynamic feature leads to induced electrical currents and to a force transfer from the external coils to the plasma. The structures due to the DED near field are described, which result in a stripelike pattern seen both in the light of recycling particles (Hα, impurities) and in the heat deposition pattern. The ergodization leads either to an enhanced plasma rotation—probably due to edge electric fields—or to a reduction of the central rotation if a tearing mode is excited; the result depends on the sense of DED rotation.

Recently, the dynamic ergodic divertor (DED) of TEXTOR has been studied in an m/n= 3/1 set-up which is characterized by a relatively deep penetration of the perturbation field. The perturbation field creates (a) a helical divertor, (b) an ergodic pattern and/or (c) excitation of tearing modes, depending on whether the DED current is static, rotating in the co-current direction or in the counter-current direction. Characteristic divertor properties such as the high recycling regime or enhanced shielding have been studied. A strong effect of the ergodization is spin up of the plasma rotation, possibly due to the electric field at the plasma edge. Tearing modes are excited in a rather reproducible way and their excitation threshold value, their motion and their reduction due to the ECRH/ECCD have been studied. The different scenarios are characterized by strong modifications of the toroidal velocity profile and by a reduced or enhanced radial transport.

AbstractPlasma can be studied and characterised by the analysis of its radiation1,2. Signals obtained by passive spectroscopy contain much information about temperature, density and flux of the main species and impurities3. The interpretation of measured line intensities requires the knowledge of atomic physics describing the specific radiation from the plasma. Tomographic methods are applied but they need symmetries for the calculation of local parameters. Additionally in magnetic confined plasmas the interpretation might be more difficult due to the Zeeman splitting.

The parallel and radial transport properties of the plasma edge of TEXTOR are studied using radial electron density and temperature profiles as well as ion temperatures and poloidal velocities in the scrape-off layer (SOL). These quantities are measured by thermal helium beams at the low (LFS) and high field side (HFS) by emission and beam driven charge exchange recombination spectroscopy. We investigate the influence of the safety factor and of a magnetic field reversal on these edge parameters. Especially the field reversal leads to clear effects: a decrease of the density at the LFS, a significant change in the poloidal density distribution, which is identified by comparing LFS and HFS densities, and an increase in the density e-folding length at both poloidal positions. The poloidal ion rotation changes sign in reversed field configuration and thus gives a hint on the role of poloidal drifts in the SOL. For further analysis, we present a simple fluid model including poloidal drift velocities and local ionization sources. With this model we can show that the poloidal E×B drift clearly influences the poloidal density distribution. However, although the model results show the same tendencies as the experimental findings, the impact of the field reversal on the density asymmetry is not that pronounced as in the experiment. The dependence of the density e-folding length on the poloidal and radial drifts as well as on the source distribution is discussed within the frame of analytical estimates.

In high density Ohmically heated discharges in the tokamak TEXTOR a transition from the saturated Ohmic confinement (SOC) to the improved Ohmic confinement (IOC) was observed triggered by a sudden reduction of the external gas flow. The SOC–IOC transition was investigated regarding the influence of the toroidal ITG instability driven by the ion temperature gradient (ITG). The ion temperature profiles were measured with high radial resolution by means of charge-exchange recombination spectroscopy (CXRS) with a high-energetic diagnostic hydrogen beam recently installed at TEXTOR. On the basis of the measured ion temperature distributions the η_i parameter (ratio of the density and ion temperature decay lengths) and the growth rate of the toroidal ITG instability were calculated. After the SOC–IOC transition η_i drops and lies in a noticeably smaller radial region over the threshold for the toroidal ITG. In consequence of it, the IOC regime is characterized by a clear reduction of the ITG growth rate γ_ITG which was calculated including finite Larmor radius effects. The steepening of the plasma density profile after the decrease of the external gas flow is the main reason for the reduction of the ITG growth rate and the subsequent confinement transition to the IOC regime.

Hydrogen isotopes are released from the walls, divertor plates and limiters of fusion devices in different forms, as atoms or molecules. The density and velocity distributions of these particles upon release can heavily influence the boundary plasma, especially through their penetration depth, and, indirectly, the plasma and its confinement properties as a whole. Recent experiments on different tokamaks have brought to light the deep interdependence between atomic and molecular species in this respect. Investigating this complexity calls for sophisticated spectroscopic diagnostics and new data on molecules like D₂ and HD. Hydrogen and deuterium atoms with extremely low energies, definitely below 1 eV, are observed and give indications on the release processes, i.e. on those where molecules are involved. It turns out that corrections to the estimated hydrogen flux may be required - a procedure is proposed in the present work to obtain values for an effective \textit{S/XB} coefficient for atomic hydrogen (&neq;15), which denotes the ratio of the collisional ionization (S) to the excitation (X) rate coefficients, divided by the branching ratio B. Moreover, attention is drawn to a possible heating mechanism of these very cold atoms by the proton/deuteron background (from 0.2 up to 10 eV). The derived information on the detailed release mechanisms should contribute towards improving the various numerical codes in which neutrals play a role. Eventually, the strong influence of the surface temperature on the ratio of atoms to molecules has to be considered in the choice of materials for plasma-facing components.

An overview is presented of recent experiments with radiating mantles on limiter and divertor machines, realizing simultaneously high confinement and high density at high-radiation levels. A variety of operational regimes has been observed and the characteristics of each are documented. High-performance plasmas (i.e. edge localized mode (ELM)-free H-mode confinement quality and normalized beta values simultaneously) with radiating mantles have been demonstrated under quasistationary conditions during the maximum flattop time of the machine (equal to tens of confinement times) on DIII-D and TEXTOR-94. Maximum values for up to 4 and for the advanced tokamak confinement-stability product up to 13, have been obtained in very high confinement mode (VH-mode) like discharges with radiating mantles in DIII-D. There is a striking similarity between improved ohmic confinement discharges (with or without Ne seeding) and radiating mantle discharges, indicating a possible common origin for the confinement improvement observed. Possible scenarios for the application of radiating mantles on larger machines such as JET and JT-60U are indicated.

Sputtering investigations of an Al/Li alloy containing 9.1 at-% of lithium have been performed for 6 keV helium ion bombardment. Absolute particle densities and velocity distributions of the sputtered neutral lithium atoms were measured with laserinduced fluorescence. The amount of sputtered lithium was found to be constant for target temperatures ranging from room temperature up to 500° C. The mean transport velocity and the sputtering yield of the Li component have been calculated from the measurements. Thermal evaporation of neutral Li atoms could be measured independently of the presence of the helium beam for target temperatures above 300° C. The experimental results indicate that the surface is covered by lithium with at least several atomic layers even under highcurrent ion irradiation.

A stationary high level of edge radiation ( gamma =P_rad/P_tot up to ~90% with peak radiation up to ~1 MW/m³) has been obtained in TEXTOR by using silicon and/or neon as radiating impurities. The confinement and neutron reactivity are not degraded but can even be improved at high plasma densities. Stationary reactor relevant heating and radiated power flows with a figure of merit f_H/q_a=0.6 have been achieved. The interpretation of these results shows a reduction of the bulk transport in the presence of edge radiation cooling. The properties of the radiatively cooled discharges are interpreted or modelled mainly by the self-consistent radiative transport code RITM, and also by the codes TRANS and PRETOR. From these modelling studies an enhancement of the bulk confinement is found in terms of the reduction of the convective losses and the decrease of the edge electron temperature, which results in a peaking of the current profile. The code RITM also predicts self-consistently the detailed properties of the radiating layer for different injected impurities as a function of their incoming flux, and shows that the optimal conditions to obtain confinement improvement as well as minimum fuel dilution by the radiating impurity are obtained at high density

We have examined the behavior of high Z limiters exposed to TEXTOR edge plasma and found that under certain conditions high Z materials are compatible with plasmas. In high density Ohmic plasmas the accumulation of a high Z impurity in the plasma center with significant radiation is observed, whereas an auxiliary heating like NBI and ICRH enhances the impurity exhaust with saw tooth activity. For a practical use of high Z plasma facing materials, extremely high heat load from the plasma becomes a serious concern. In the present work we have conducted the high heat load tests of tungsten (W) using two different heat sources, one is the W limiter exposed to TEXTOR plasma and the other is various W samples heat loaded with an intense E-beam using the JEBIS facility in Japan Atomic Energy Research Institute (JAERI). From the test results we have to conclude that W, if applied in the form of the bulk material, should be used above the ductile brittle transition temperature (DBTT) but below about 1500°C to avoid the recrystallization. Maximum heat load tolerable without surface melting is about 20 MW/m for several seconds. The monocrystalline used at high temperatures shows very good performance, though the production of the monocrystalline with a desired shape is not easy. Considering its brittle nature, hard machining and heavy mass, bulk W cannot be a structure material but be used as a thin tile or deposited film on some structure materials. Unfortunately, however, the thermal expansion coefficient of W is so small that brazing of W to a heat sink material like Cu which has a much larger thermal expansion coefficient would easily result in cracking due to the large thermal stress. Thus the development of tungsten plasma facing component (PFC) needs much effort in future.

To study local particle transport, silane was injected through a hole in a test limiter surface during discharges in TEXTOR-94. The gas injection lead to the deposition of a thin layer around the injection hole which was close to the area of the highest heat load. The layer was eroded in discharges without silane injection. The layer was not only formed by the injected silicon but also by a large amount of carbon which must originate from other plasma facing components. The preferential direction (symmetry line) of the deposition pattern is tilted by ∼ 25° relative to the magnetic field lines Calculations with the ERO-TEXTOR code recover the general shape of the Si-deposition pattern. To get accordance between the calculations and the experimental results a strong electric field parallel to B and a sticking coefficient much smaller than 1 has to be considered to reproduce the low silicon deposition efficiency, whereas the tilting of the symmetry line can be explained by the existence of a radial electric field.

A W test limiter has been inserted into the plasma edge of TEXTOR beyond the magnetic surface of the main ALT graphite limiter, and the release of neutral W as well as carbon and oxygen are examined spectroscopically. The penetration depth of neutral W has decreased significantly with increasing plasma density and it is as small as 1.2 mm when the local plasma density at the magnetic surface of the limiter increases up to 1.5 × 10 m. The flux of neutral W decreases as the local density increases, while the C and D fluxes increase. Radial intensity distributions of CII line spectrum emission have indicated larger penetrations of C ions emitted from a W limiter than those from a C limiter. The penetration length of OII emission from a W limiter is similar to that from a C limiter and much smaller than that of CII.

Measurements on the formation of hydrocarbons on plasma exposed surfaces performed by mass- and optical emission spectroscopy in TEXTOR is reported. The temperature dependence of hydrocarbon formation and the contribution of the hydrocarbon source to the CII ion densities near the limiter has been observed using a graphite limiter which is externally heatable up to 1400 K. It has been found that hydrocarbon formation occurs in a broad temperature region decreasing only for target temperatures above 1300 K and that hydrocarbons contribute to about 40% to the CII light. Strong methane release has been observed on copper and stainless steel limiters positioned at the LCFS while it is negligible on molybdenum and tungsten limiters under similar plasma edge conditions. Local transport and redeposition of molecules have been studied by gas injection of methane and silane through holes in the limiter surfaces and by local Monte Carlo calculations. Local deposition efficiencies between 4 and 7% have been measured for injected methane and silane. Monte Carlo calculations show, in general, a larger redeposition probability depending only little on local plasma parameters but significantly on the assumptions of the sticking and release properties of redeposited ions and radicals on the surface. For higher surface temperatures possible carbon release by radiation enhanced sublimation (RES) has been investigated. No increase of carbon release could be observed demonstrating that carbon release from RES is negligible under these conditions. Possible reasons for this behavior are discussed.

A well-diagnosed test limiter has been used in TEXTOR-94 to study -- by means of spectroscopic techniques -- the fluxes and the spatial distribution of hydrocarbons, which are formed by plasma interaction on carbon, silicon and titanium doped (SiC30, RGTi) graphites, B 4 C as well as from carbon coated metal limiters. On a special graphite limiter, which could externally be heated up to temperatures of 1400 K, the methane formation remained approximately constant up to 1050 K with a yield of about 3.5% and dropped well below 1% at 1350 K. Simultaneously the carbon ion fluxes into the plasma decreased by about 50%. By inserting the limiters deeper into the plasma the D-flux increased between 10/ms and 3 × 10/ms and the maximum methane production yield decreased to about 1%. On bulk doped graphites as well as Cu- and SS-limiters a similar methane formation behavior as pure graphite has been observed. However, a weak hydrocarbon formation rate is found for Mo- and W-limiters. No discrepancy between radial CH- and C 2 -intensity profiles from injected and sputtered CH 4 and C 2 H 4 gas could be detected. The measured radial intensity profiles for CD were compared with a local erosion and deposition model (ERO-TEXTOR) using measured plasma parameters and atomic data taken from the Langer model. The calculations showed a reasonable agreement with the experimental results both in radial and toroidal direction.

When neon is injected into a tokamak plasma with the aim of reducing the peak heat flow into the SOL by enhanced edge radiation, both P rad and Z eff increase. The ratio of the incremental P rad to the incremental Z eff , ΔP rad ΔZ eff , can change on TEXTOR from 0.1 MW to about 2 MW depending on n e , on T e (a) and on the transport regime. The measured ΔP rad ΔZ eff is higher for I-mode auxiliary heated (higher diffusion) than for OH discharges (lower diffusion), consistent with model calculations. For I-mode discharges ΔP rad ΔZ eff is a decreasing function of T e (a) and it scales approximately as n e . The I-mode regime on TEXTOR, which displays very good energy confinement scaling, can represent, for suitable conditions, a satisfactory compromise between a high τ E and not excessive particle confinement.

Characteristic quantities for impurity release from limiters and the penetration of atoms into the plasma boundary have been determined by optical methods on the tokamak TEXTOR-94. The velocity distribution of neon atoms released from a tungsten limiter has been measured. A drastic increase of fast particles could be observed in comparison to the velocity distribution of neon released from a carbon limiter under comparable conditions. As a result the penetration depth of neon increased by a factor of 1.7. The influence of local electron density and temperature near the limiter has been demonstrated for various species as He, C, Ne, Si and Mo comparing different central electron densities. Both penetration depth and neutral fueling efficiency decrease with increasing density. For high Z materials such as Mo prompt redeposition of singly ionized ions is found to be of major importance. The measured radial profile of the ion source distribution of molybdenum under neutral beam heated conditions has been used to calculate the Mo-density in the plasma center for different line averaged central electron densities n e , taking into account both the ionization processes inside and outside the LCFS. The decrease of the calculated Mo-density with n e is in agreement with the experimentally observed variation of the central Mo-density.