ICRF physics aspects of wall conditioning plasma characterization in TEXTOR

This paper focuses on encouraging results obtained on the characterization of RF produced plasmas during pulsed-mode wall conditioning discharges in ion cyclotron resonance frequency (ICRF) regime in the limiter tokamak TEXTOR. Recent Ion Cyclotron Wall Conditioning (ICWC) experiment carried out in TEXTOR tokamak, lead to the identification of various dependences of the antenna-plasma coupling efficiency on the plasma parameters for possible ICWC-discharge cleaning in ITER at half field. Our ICWC experiments emphasize on (i) study of antenna coupling during the mode conversion scenario, (ii) reproducible generation of ICRF plasmas for wall conditioning, by coupling RF power from one or two ICRF antennas and (iii) effect of application of an additional (along with toroidal magnetic field) stationary vertical (B_V ? B_T) or oscillating poloidal magnetic field (B_p ? B_T) on antenna coupling and relevant plasma parameters.     

Investigation of liner samples exposed to an in-situ carbidization in TEXTOR

A plasmachemical in-situ technique for the deposition of carbon on limiters and first wall of the tokamak TEXTOR has been developed. RG-discharge in a throughflow of hydrogen with admixed 1.5% methane has been used to implant methane ions into the limiters and wall. Different-samples (Inconel 600) in liner position have been exposed and afterwards analyzed with SIMS and AES.It has been shown that during the plasmachemical carbon deposition at 240°C predominantly chromium carbide was formed in the surface near layer. Nickel carbide was traced in a negligibly small amount. Depth profiles indicate an exponential dependence of the chromium carbide concentration up to a depth of about 10 nm. This is in agreement with the estimated diffusion profile of carbon; literature values of the carbon diffusivity in Ni, Ni-Cr and Ni-Co alloys yield a mean characteristic diffusion length of about 8 nm. The total amount of deposited carbon as determined from AES measurements is $\text{6 × 10}{}^{15}\text{atoms cm}{}^{\mathrm{?3}}$, in reasonable agreement with $\text{1.2 × 10}{}^{16}\text{cm}{}^{\mathrm{?2}}$ as determined from the partial pressure balance during the carbidization process.     

The TEXTOR program at the KFA,Jülich

The problem of impurity generation in fusion devices has, as a consequence of rapid development in the confinement sector, reached a high priority level. In collaboration with the KFA institutes for surface and vacuum physics, nuclear chemistry, solid state physics, reactor development, reactor materials, and others, the Institute for Plasma Physics of the KFA has initiated a program for a systematic study of the interaction of hot confined plasmas with the surrounding first wall. Because of the nonlinear character of the problem, not only simulation measurements but also the availability of a tokamaktype flexible confinement device, TEXTOR, with adequate access to all domains of relevance is held indispensible. Essential parameters of TEXTOR are major and minor plasma radii 175 and 50 cm, toroidal field 2 T initially, 2.6 T later, flux swing 4.1 Vs, wall temperature up to 600°C, and easy exchange ability of first wall.     

Investigations of single crystal and polycrystalline metal mirrors under erosion conditions in TEXTOR

Metal mirrors are planned for optical diagnostic systems of ITER. However, erosion, deposition and particle implantation can change the performance of mirrors. Mirrors made from the single crystal (SC) materials are among the main candidates for use in ITER diagnostic systems operating under erosion-dominated conditions. Laboratory tests have confirmed good optical performance of SC mirrors under erosion, but the dedicated direct comparative test in tokamak environment was missing.Such a direct test was performed in TEXTOR. Single crystal molybdenum, tungsten and polycrystalline (PC) molybdenum mirrors were exposed under the same conditions in the SOL plasma of TEXTOR. Surface and optical properties of mirrors were characterized before and after exposure. Before exposure glow discharge cleaning in hydrogen restored the reflectivity of mirrors oxidized during storage on air.No significant changes in total reflectivity were observed for all mirrors after exposure. Drastic increase of diffuse reflectivity was measured for PC Mo mirror, no change for a SC one. Thus, specular reflectivity of single crystal is higher than of polycrystalline one. The most affected wavelength range is 250-1000 nm, no significant change of reflectivity was noticed in the range 1000-2000 nm. Negligible effect of the exposure on polarization characteristics was observed.     

Optimised plasma stabilisation at TEXTOR with an advanced, real-time digital control scheme

The tokamak TEXTOR at the Research Centre in Jülich is in operation since more than 25 years. The various control systems at the start, in 1982, were based on analogue techniques, a standard at the time, and were later partly replaced by specially developed digital systems. These systems proved their robustness over the years. As a replacement for the old system, off-the-shelf products were used to ensure continuity, reliability and to reduce the development cost. To provide advanced control scenarios, the new system allows the implementation of more sophisticated algorithms for magnetic and kinetic control. The LabVIEW Real-Time (RT) modules and real-time hardware from National Instruments satisfy these requirements to a large extent. The new system has already been successfully commissioned at TEXTOR and is used to calculate in real-time the plasma density profile (10 ms), the Shafranov shift (10 ms), the plasma vertical and horizontal position (20 $\mu$s) and to control the plasma shape (1 ms). TEXTOR has circular plasmas and has an iron core. Its central part is operated in saturation. During the saturation phase, stray fields change the plasma shape from nearly circular to slightly triangular. By using a shape-control coil set, we can control and adjust the plasma form. The new real-time system is presented as well as the implemented control applications.     

Velocity distribution of helium and neon atoms released from graphite and tungsten limiters in TEXTOR

The release mechanisms of noble gases from plasma-facing components were observed spectroscopically in the TEXTOR plasma boundary by determining the velocity distribution from Doppler broadening. For the first time, three different mechanisms for helium and neon release from graphite and tungsten limiters could be distinguished quantitatively in a tokamak: thermal desorption, ion-induced desorption and particle reflection. Under the assumption that the thermal desorption follows a Maxwellian velocity distribution, the ion-induced desorption can be expressed by a Thompson velocity distribution. Calculating the particle reflection by the Monte-Carlo code TRIM, these processes could be separated in the measured velocity distribution.     

Magnetic Islands Observed by a Fast-Framing Tangentially Viewing Soft X-Ray Camera on LHD and TEXTOR

The formation of magnetic islands within plasmas confined magnetically within the tori has significant influence upon their confinement and stability. To obtain an experimental insight into the formation and dynamics of such island structures we employed a fast framing camera viewing the plasma tangentially in the toroidal direction. The toroidal viewing direction gives the advantage in that the islands are viewed almost tangentially and this greatly facilitates the reconstruction of the local data from the line integrated ones. We discuss an effective method to do inversion. To study the fluctuations seen in the video images we perform a singular value decomposition, and then we use a truncated least square method to infer their pictures in space.     

Prediction of impurity spectral emission in plasmas

This paper summarises the development of a set of general purpose computational procedures for the prediction of spectral emission from plasmas, with emphasis on fusion plasmas. The first stage [1] was concerned with the calculation of populations of low levels of impurity ions in a statistical balance approximation in thermal plasmas of arbitrary electron and proton temperatures and densities. This was merged with associated calculations of ionisation stage abundances in equilibrium, time dependent and spatially inhomogeneous conditions to yield spectrum line emissivities of direct relevance for comparative and diagnostic studies of observed spectra. The integrated computer program package draws upon sets of basic atomic data. In the present work, the compilation of this basic data is addressed.A set of computer programs has been developed and used to convert systematically atomic rate data, drawn from the literature, to standard forms and parameter ranges. Regularities in this data along isoelectronic sequences are exploited to infer rates for an arbitrary ion from a set of representative data (termed the ‘general Z’ database). From this, the input for the spectral prediction codes above is generated.Presently data in the H, He, Li and Be isoelectronic sequences is prepared. The operation of the procedures is illustrated.     

An impurity beam trapping instability in Tokamaks

We consider the sensitivity of neutron energy production to the trapping by impurities of injected neutral beams. The beam-trapping process is affected by inherent low-Z contamination of the tritium plasma, by the species composition of the neutral beam, and by the entrance angle of the beam. The sensitivities of the process to these variables, and to variation with wall material, are discussed. We find that use of a low-Z, low-sputtering material could retard a possible beam-trapping instability.     

EAST钨杂质输运特性分析

用基于蒙特卡罗方法的杂质输运程序DIVIMP模拟了EAST在加热功率Pin=8 MW的情况下边界层钨杂质输运过程并分析了钨杂质输运特性,其目的是为了合理设计第一壁和偏滤器靶板部件。DIVIMP模拟得到了低再循环、高再循环和脱靶运行条件下钨杂质在边界层的分布情况。模拟结果显示,钨在脱靶情况下所产生的杂质密度比其他运行模式所产生的杂质密度小。与碳杂质输运特性的模拟结果相比,钨杂质的密度有明显的下降,总体分布在1015m-3数量级上。     

Time resolved metal impurity fluxes in the dite tokamak

Measurements are presented of the time resolved flux of metallic impurities to a carbon probe placed at the limiter radius of the DITE tokamak. Average fluxes of titanium and molybdenum of the order 4 × 10¹² mm^?2 s^?1 are measured. It is concluded that peaks in the metal flux are correlated with arcing and with disruptions initiated by neutral beam injection. The decrease in flux by almost one order of magnitude compared with earlier studies may be related to a recent redesign of the molybdenum limiters which reduces the minor radius of the plasma. It is observed that the deposit density on the probe is not a linear function of exposure to the plasma and also that insertion of the probe causes a change to the local plasma environment.     

杂质对X射线在炸药中衰减规律影响的研究

对某炸药（纯炸药和含金属片的炸药）进行了X射线检测,并对穿透率进行拟合,通过对拟合结果分析得出了在管电压和管电流一定的条件下X射线在某炸药中随药柱的增加其衰减规律符合三次拟合。同时得到杂质的密度越大,衰减越明显,不同厚度的同一杂质,厚度大的衰减明显。     

Simulation studies of tungsten impurity behaviors during neon impurity seeding with tungsten bundled charge state model using SOLPS-ITER on EAST

An investigation into tungsten (W) impurity behaviors with the update of the EAST lower W divertor for H-mode has been carried out using SOLPS-ITER. This work aims to study the effect of external neon (Ne) impurity seeding on W impurity sputtering with the bundled charge state model. As the Ne seeding rate increases, plasma parameters, W concentration (), and eroded W flux () at both targets are compared and analyzed between the highly resolved bundled model 'jett' and the full W charge state model. The results indicate that 'jett' can produce divertor behaviors essentially in agreement with the full W charge state model. The bundled scheme with high resolution in low W charge states (20+) has no obvious effect on the Ne impurity distribution and thus little effect on W sputtering by Ne. Meanwhile, parametric scans of radial particle and thermal transport diffusivities (and) in the SOL are simulated using the 'jett' bundled model. The results indicate that the transport diffusivity variations have significant influences on the divertor parameters, especially for W impurity sputtering.     

Gettering of copper impurity in silicon by aluminum precipitates and cavities

Al precipitates as well as cavities (or open-volume defects) are known for their ability to getter impurities within Si. In order to compare their relative gettering strength we produced both Al precipitates and cavities at different depths within one Si wafer. This was done by H+ and Al+ implantation with different energies and subsequent annealing process, resulting in Al-Si alloy and cavities at depth of 300 nm and 800 nm, respectively. Cu was then implanted with an energy of 70 keV to a fluence of 1 X 1014 / cm". The Cu implanted samples were annealed at temperature from 700C to 1200C. It was found that Cu impurities were gettered primarily by the precipitated Al layer rather than by cavities at the temperature of 700~1000C, while gettering of Cu occured in both regions at the temperature of 1200C. The secondary ion mass spectrometry and transmission electron microscopy analyses were used to reveal the interaction between Cu impurities and defects at different trap sites.     

Homogeneity study of an Fe impurity in unalloyed Zn using PIXE

We have studied the distribution of an Fe impurity in unalloyed Zn material along line scans in spots of 1 mm diameter each using PIXE. In order to achieve the necessary precision (1%) in the determination of this low Fe content (about 100 mg/kg), a sufficient counting rate of Fe X-rays is needed without overloading the counting electronics with Zn X-rays at the same time. Therefore, the development of our PIXE analysis system had to take into account: (a) optimising the thickness of a Ni absorber foil in order to selectively reduce the counting rate of Zn without adding an unproportionally high Ni counting rate, (b) implementing a triggered ion beam deflection system capable of handling very high counting rates (up to 8 kcps), (c) developing the spectrum evaluation code to take account of the selective absorption by the Ni foil. The results of our PIXE measurements showed an inhomogeneity in Fe of up to 10%, considered too high to accept this zinc material as a new reference material.     

Investigation .on impurity transport using LBO technique

1. IntroductionIt is a common phenomenon in the tokamak devices that impurities have a concentrated trend towards the plasma central region. Impurity sourcesand its transport are very complicated, traditionalpassive methods can not be satisfied for the investigation of impurity transport. So transient perturbation methods have been developed and recognizedas the most appropriate for impurity transport measurement.The LBO technique is just the right transient perturbation method for this task…