Impurities In Tokamaks Research Articles

Effect of toroidal rotation on impurity transport in tokamak improved confinement

The centrifugal force effects from toroidal rotation in improved confinement plasmas are analyzed on high-Z impurities in tokamaks. Tungsten (W) transport simulations are performed using the impurity transport code developed in the integrated code TASK. The geometric factors PA and PB are introduced into the neoclassical transport coefficients to include the effects of the toroidal rotation, which come from poloidal asymmetry in the high-Z impurity profiles. Inward neoclassical particle pinch driven by the main ion density gradient is enhanced by the poloidal asymmetry to be the dominant mechanism for W accumulation in the plasma central region. Simulations with experimental plasma profiles show good agreement with the experimental results and first-principle simulation results in the H-mode. In the hybrid mode and advanced mode, the impurity accumulation is enhanced in the internal transport barrier (ITB) regions. The condition to suppress impurity accumulation is investigated by calculating dependencies on the toroidal rotation velocity and ITB position. The neoclassical transport is sufficiently small with the prospected ITER condition of the Mach number of main ions Mi ∼ 0.1. The impurity transport inside the ITB is strongly influenced by competition between the density peaking effect and the temperature screening effect, and the present simulations show suppression of the impurity accumulation with the outer ITB position to improve the plasma performance, due to the relatively larger temperature gradient of the main ion.

Neoclassical transport of impurities in tokamaks with non-axisymmetric perturbations

The effect of resonant magnetic perturbations (RMPs) on neoclassical transport of impurities is calculated in DIII-D and NSTX. The neoclassical fluxes are evaluated using the NEO code with nonlinear one-fluid nonaxisymmetric equilibrium calculated using M3D-C1. Neoclassical fluxes of impurities show significant changes with RMPs if the impurity resides in low-collisionality regime, but are weakly affected by RMPs in the Pfirsch–Schlüter (P–S) regime. Charge number (Z) of the impurity affects the collisionality of impurity species, which then determines the effect of 3D fields on neoclassical impurity transport. This suggests that RMPs can be possibly used for exhaust of low-Z impurities in these facilities, but have little effect for impurities with charge number greater than 10 or so. Additionally, it is shown that it is the change in the convective fluxes which is a main contributor in modifying the neoclassical impurity fluxes in the presence of RMPs.

Multiple Analytical Approach to Isotopic Transport Analysis in Magnetic Fusion Devices

High-Z impurities released from plasma-material interactions have been shown to limit the performance of fusion plasmas, and understanding these impurity transport mechanisms throughout the plasma scrape-off layer is a major challenge. Presented herein is a study of tungsten (W) erosion and transport by uniquely measuring absolute quantities of isotopic W in order to determine the source of natural and enriched 182W isotopes that have traveled throughout the tokamak discharges on the DIII-D National Fusion Facility at General Atomics. Two primary analysis methods have been implemented to characterize this W on graphite collector probes that were inserted into DIII-D’s outboard midplane. Results from experiments using Rutherford backscattering spectrometry (RBS) have measured W particle areal densities down the centerline of the probes as high as 6E14 atoms/cm2 with a detection limit of 1E12 atoms/cm2. Laser ablation inductively coupled plasma mass spectrometry (LAMS) has confirmed the elemental trends found with RBS and has provided additional insight into collector probe surface profiles. Two-dimensional elemental and isotopic maps from LAMS are used to reveal new collector probe features and further refine the source of collected W. Variations in isotopic profiles and total W content are coupled to (a) the face of the probe being analyzed, (b) the dimensions of the probe, and (c) the plasma pulse parameters that were used during probe exposure. These results provide one-of-a-kind empirical evidence that is now being utilized for validation of tokamak impurity transport through theoretical models and in codes such as 3D-LIM and OEDGE.

3D two-temperature magnetohydrodynamic modeling of fast thermal quenches due to injected impurities in tokamaks

An integrated model for the ionization, radiation, and advection of impurities in the extended-magnetohydrodynamic code M3D-C1 is described. This implementation makes use of the KPRAD model, which calculates bremsstrahlung radiation and impurity ionization, recombination, and radiation rates using a model in which the density of each charge state is advanced separately. The integrated model presented here allows the independent evolution of electron and ion temperatures, which is necessary to accurately model cases where the electron temperature drops more quickly than the electron–ion thermal equilibration time. This model is used to simulate the disruption of a model NSTX discharge caused by the introduction of argon impurities, using physically realistic resistivity. Despite well-mixed impurities, contraction of the current channel is found to lead to magnetohydrodynamic instabilities that result in stochastization of the magnetic field, a fast thermal quench, and localized parallel electric fields that can exceed the axisymmetric values by a factor of five for brief periods.

Global migration of impurities in tokamaks

The migration of impurities in tokamaks has been studied with the help of tracer-injection (13C and 15N) experiments in JET and ASDEX Upgrade since 2001. We have identified a common pattern for the migrating particles: scrape-off layer flows drive impurities from the low-field side towards the high-field side of the vessel. Migration is also sensitive to the density and magnetic configuration of the plasma, and strong local variations in the resulting deposition patterns require 3D treatment of the migration process. Moreover, re-erosion of the deposited particles has to be taken into account to properly describe the migration process during steady-state operation of the tokamak.

Internal Transport Barrier Analysis Including Impurities in Tokamak and Helical Reactor Plasmas

The operation with Internal Transport Barrier (ITB) is expected as a high performance operation. ITB is utilized to improve core plasma confinement in the reversed magnetic shear. It is considered that the changes of core plasma profile by the ITB cause changes of impurity transport. In a large fusion reactor, high-Z materials will be used as plasma facing components because high loads of heat and particles concentrate there. However, high-Z impurities from these components cause large radiation loss and dilute the fuel even if the amount of impurities is small. Therefore, in this study, firstly, the ITB formation which includes the effects of the magnetic shear and perturbed profiles by the pellet injection was simulated using the Toroidal Transport Analysis Linkage code TOTAL. Secondly, we analyzed transport of the tungsten impurities using an impurity model in TOTAL code, and compared the impurity profile in the case with ITB to the one without ITB in the tokamak reactor. The impurities decreased in the ITB formation region when ITB was formed, and the outward flux of total impurity density was observed there. It can be expected that outward flux of impurities is generated by the temperature and the density gradients.

Drift-kinetic simulation of neoclassical transport with impurities in tokamaks

Plasmas in modern tokamak experiments contain a significant fraction of impurity ions in addition to the main deuterium background ions. A new multiple ion-species δf particle simulation capability has been developed to self-consistently study the nonlocal effects of impurities on neoclassical transport in toroidal plasmas. A new algorithm for an unlike-particle collision operator, including test-particle and conserving field-particle parts, is described. Effects of the carbon impurity on the main deuterium species heat flux as well as an ambipolar radial electric field in a National Spherical Torus Experiment (NSTX) [M. Ono, S. M. Kaye, Y.-K. M. Peng et al., Nucl. Fusion 40, 557 (2000)] configuration were studied. A difference between carbon poloidal rotation found from simulation and from conventional theoretical estimates has been investigated and was identified to be a nonlocal finite orbit effect. In the case of large-aspect ratio tokamak configurations with steep toroidal flow profiles, we propose a theoretical model to describe this nonlocal effect. The dominant mechanisms captured by the model are associated with ion parallel velocity modification due to steep toroidal flow and radial electric field profiles. We present simulation results for carbon poloidal velocity in NSTX. Comparisons with neoclassical theory are discussed.

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Fluorescence spectroscopy of low-Z-materials: Application of Raman-converted VUV-radiation to beryllium and oxygen

Fluorescence experiments with beryllium and atomic oxygen are described. VUV-radiation generated by high-order anti-Stokes Raman scattering in molecular hydrogen is used to excite resonance transitions at 235 nm and 166 nm (Be) and at 130 nm (O). The results show that the available VUV power and the spectral properties allow applications of this VUV-source to fusion relevant material studies on particle-wall interactions in laboratory experiments as well as applications to concentration measurements of impurities in tokamaks.

Continuous-wave laser fluorescence spectroscopy of impurities in tokamaks: C E Young et al,J Appl Phys, 53 (7), 1982, 4726–4733

Continuous-wave laser fluorescence spectroscopy of impurities in tokamaks: C E Young et al,J Appl Phys, 53 (7), 1982, 4726–4733

On the Development of Accident Sequences Involving Tokamak Impurity Control Systems

A set of initiating events are identified and the associated accident sequences are developed for impurity control systems in tokamak fusion reactors. The master logic diagram concept used in risk analysis is developed for a tokamak. The notion of the fusion island is used as the central power producing element in fusion power plants. Differences in the severity of initiating events and/or accident sequences that are associated with the two leading candidate impurity control concepts, the pump limiter and the poloidal divertor, are identified. They reflect both safety-related and/or financial consequences.

Continuous-wave laser fluorescence spectroscopy of impurities in tokamaks

Laser-induced fluorescence spectroscopy has been applied as an in-situ diagnostic for impurity atoms in the edge region of the plasma in the Argonne Plasma Engineering Experiment (APEX) tokamak. Zirconium atoms introduced from a moveable probe were excited by a cw single-mode ring dye laser and monitored on lines of the a3F-z3F0 manifold. The fluorescence signal from a 0.03 cm3 volume was recorded at 1-ms intervals with a computer-controlled 4-channel 100-MHz scaler system. Acousto-optic modulation of the laser beam at 100 kHz allowed subtraction of plasma background light. Absolute calibration by Rayleigh scattering gave a detectability limit ∼1010 Zr atoms/cm3 in this apparatus. The detectability limit was determined by a detailed consideration of power and transit time broadening. The effects of several experimental parameters were examined and suggestions for increasing detection sensitivity are presented. Doppler-shift experiments indicated a thermal-velocity distribution for the detected Zr atoms. Intrinsic-velocity resolution of the experiments, calculated from effective excitation linewidths, was ∼25 m/s.

Study of the motion of vacuum arcs in high magnetic field

An investigation into the motion of low current vacuum arcs in a high transverse magnetic field is described with regard to arcing impurities in tokamaks. Cathode spot velocity measurements using streak camera studies are presented. The effects of magnetic field, arc current, material and surface finish are reported in some detail. The arc tracks are subsequently analysed using optical and scanning electron microscopes. Arc erosions observed are similar to those found in DITE at Culham Laboratory. Surface cleanliness of the electrodes seems to play an important role in establishing the type of arc. Prolonged glow discharge cleaning of the electrodes in hydrogen is found to drastically reduce the arcing tendency.

A method for the determination of confinement time for highly ionized impurities by numerical simulation in Tokamaks

A numerical simulation allows a quantitative interpretation of spatially and time resolved X-ray diagnostics relative to highly ionized impurities in Tokamaks. These impurities are injected during the plateau of the discharge in such a small quantity that the electron density ne as well as the electron temperature Te can be considered to be constant. It is shown that, for argon at Te>or approximately=2 keV, a precision better than 10% can theoretically be obtained for the determination of the local diffusion time constant.

Impurities in Tokamaks: Sources, Effects and Control (Report on the IAEA Technical Committee Meeting, Alushta, 1979)

The IAEA Technical Committee on Impurities in Tokamaks met at Alushta (Ukrainian SSR) from 8 to 12 October 1979. It discussed the present status of knowledge on the behaviour of impurities in tokamaks – their sources and dynamics, and the methods of controlling them. The meeting was convened at the initiative of the IAEA and organized by the Kharkov Physico-Technical Institute. At Alushta, 23 participants from seven countries (eight from USA, seven from the Federal Republic of Germany, three from France, two from Great Britain and one each from Hungary, the Netherlands and Japan) came together with 14 participants from the USSR. In more than 30 papers and during the extensive discussions which followed the presentations practically all the main aspects of the impurity problem were considered. As a result of the meeting, it became clear that in the last five years substantial advances had been made towards obtaining, in tokamaks, a sufficiently pure plasma with Zeff close to unity. This has been achieved by using new methods of purifying the discharge chamber walls, titanium gettering, graphite limiters and programmed filling of gas. Active methods of impurity control – with divertors and asymmetric injection of neutral hydrogen – were also found to be successful. The principal conclusions arrived at by the participants are reviewed below.

Surface cleanup and sputtered ion production: Observations with AES and SIMS

Surface cleanup of 304 stainless steel by 10 keV H 3 + is monitored with Auger electron spectroscopy and the concurrent variations in the relative yield of sputtered Cr + is measured. The most pronounced effect on Cr + production is attributed to surface oxygen: a 20% surface concentration of O enhances the Cr + signal by 2 orders of magnitude over that for a clean surface. The possible effect of surface carbon on the Cr + yield is also discussed. The possible contribution of oxygen enhanced sputtered ion production to tokamak impurity control is discussed with emphasis on the fate of the sputtered ion in a tokamak environment.

Suppression of Metallic Impurities by Electron Injection in a Tokamak

We found that the sheath potential plays a dominant role in the plasma-wall interaction in the Macrotor tokamak. An increase in the sheath potential, produced by removing electrons from the plasma, results in the enhancement of the metal influx. When the sheath potential is reduced by electron injection, the metal influx is suppressed. This mechanism may be used as a tool in controlling metallic impurities in tokamaks.

Dust particles as a possible source of impurities in Tokamaks.

A model is proposed where the dust particles are the source of impurity atoms that are present in the early phase of a Tokamak discharge. The model seems to fit the optical observation of the Doublet HA discharge. Also it explains the relative cleaniness of Alcator compared with the other tokamaks. The direct observations are needed to confirm the hypothesis.

Poloidal divertor experiment

Construction has begun on the Poloidal Divertor Experiment at Princeton University's Plasma Physics Laboratory. The $18‐million device, expected to begin operation by December 1977, will be used to test the effectiveness of poloidal field divertors and magnetic limiters to control impurities in tokamaks.