Impurity Radiation Loss Research Articles

1D model of tokamak scrape-off layer and divertor plasmas: Dimensionless parameters and the results of numerical simulations

Solutions of a 1D model of the scrape-off layer and a divertor plasma in a tokamak are analyzed on the basis of dimensionless parameters governing the physics of both plasma and neutral transport and atomic physics processes. Numerical simulations based on fluid plasma and Monte Carlo neutral modeling confirm earlier results obtained with simplified analytic considerations and numerical simulations performed with fluid neutral description and demonstrate bifurcation of plasma parameters. These simulations also confirm that the major physical reasons for the rollover of the plasma flux to the target, which is the key feature of the phenomenon of divertor plasma detachment, are the impurity radiation loss and plasma recombination. It is suggested that simulations of the edge plasma based on dimensionless parameters provide a powerful tool for checking the consistency of the physics incorporated in existing complex 2D codes used for edge plasma modeling.

Divertor plasma detachment: roles of plasma momentum, energy, and particle balances

Contrary to statements from some recent papers, the bifurcation of the scrape-off layer and divertor plasma parameters, as described within a 1D model, does exist. The analysis of both numerical simulations and experimental data shows that the real physical reasons that can cause a strong reduction of the plasma particle flux to the target are impurity radiation loss and volumetric plasma recombination. Fitting the parameter describing the ‘plasma momentum removal’, based on the results of 2D numerical simulations, then using this fit in subsequent analytic expressions for , and finally interpreting the evolution of as the result of elastic plasma-neutral interactions break the causality principle and lead to a wrong physical picture of divertor plasma detachment.

Effect of impurity radiation and helium particle confinement on tokamak–reactor plasma performance

In this paper two important matters for assessments the D–T plasma performance of future tokamak–reactors such as ITER and EU–DEMO projects are discussed. The first issue is the heat removal via the radiation of intrinsic and seeded impurities. The second is the helium particle confinement and ash removal. To study these issues, a simple numerical 0.5D-model is proposed. The impurity radiation in the tokamak high–temperature plasma is described by the coronal model. Coronal radiative cooling used rates are based on the tabulated data published in the recent works. Helium particle confinement and ash exhaust efficiency are determined by the variable parameter ρ* characterizing the ratio between the global helium particle confinement time and the energy confinement time τ E . Such approach allows simultaneously obtaining solutions of the particle and energy balance equations in steady–state conditions, taking account of the effect of impurity radiation loss and ash poisoning. Using known energy confinement scaling τ E and design parameters of the ITER and EU–DEMO projects the particle and energy balance equations have been solved. For fixed values of impurities concentration and ratios ρ*, iso–curves of constant fusion power were obtained. These curves are similar to the well–known Plasma OPeration CONtours (POPCON). The POPCON curves are often used to study the operating points of tokamak–reactors. It is shown that for the ITER parameters there is a noticeable margin in the ratio ρ*. For the design parameters of the EU–DEMO project, the margin for ratio ρ* is very small, which casts doubt on the feasibility of the project in its current form (at least with regard to the EU–DEMO steady–state project). Further research in this area is required. The methods and results obtained in this paper can be used for further conceptual analysis of tokamak–reactors operating space.

Numerical analyses of impurity behaviors for CFETR advanced scenarios by core-edge integrated simulations

Abstract The impurity behaviors for the advanced scenarios of CFETR with full tungsten divertor have been analyzed. Core plasma profiles for steady state and hybrid scenarios are simulated by consistently iterative transport calculations within the OMFIT framework. The core-SOL integrated COREDIV code is then used to evaluate the effects of different seeding impurities (Ne, Ar, Kr) with the OMFIT modelled core plasma parameters as input. The impurity behaviors for the two modelled scenarios are similar. Seeding by all the considered impurities can effectively reduce the power to the divertor below the acceptable level. For Ne and Ar seeding, it is possible to achieve H-mode plasma operation with power from the core plasma to the SOL higher than L-H transition power threshold. However, higher Ne seeding rate would strongly reduce the fusion power due to the dilution effect. Kr seeding seems to be infeasible because the power crossing the separatrix would be lower than the L-H transition threshold, when the seeding rate is high enough to meet the divertor requirements. Higher separatrix electron density or SOL diffusion can slightly reduce the core radiation. Although normal COREDIV simulations are performed with a fixed H98 factor, simulations with fixed transport indicate that higher impurity seeding rate leads to a decrease of confinement and fusion gain, due to the increase of impurity radiation loss in the core plasma.

Influence of Cross-Field Transport in a Divertor on Seeded Impurity Radiation and Divertor Plasma Detachment

The dissipative divertor for the power and particle exhaust in a tokamak relies heavily on strong impurity radiation inside the divertor volume. In the present paper, the influence of the cross-field transport inside the divertor volume on the radiation loss by nitrogen and neon impurity seeding is studied in the SOLPS4.3 2D transport code. The study is carried out in a simplified tokamak geometry with long divertor legs, aimed at amplifying the effect of impurity radiation. It is shown that despite the simple analytical models predicting the enhancement of the low-Z impurity radiation loss with increasing cross-field heat conduction inside the divertor volume, the detailed 2D simulations demonstrate such a trend only if the electron density at the separatrix is used as the principal parameter quantifying the density of the edge plasma. However, if the total amount of the hydrogen isotope particles, which is a more suitable parameter for the edge plasma characterization, is considered, then the cross-field transport shows practically no impact on the impurity radiation in the divertor. It is found also that for a fixed ratio of the impurity to the main plasma content in the edge, two stable solutions can exist in a rather wide interval of the densities of the edge plasma. These solutions are attributed to the different distributions of the seeded impurity between the outer and inner divertors. Jumps between these two stable branches may result in bifurcation-like transitions between the attached and detached divertor regimes.

Enhanced energy confinement after series of pellets in Wendelstein 7-X

A series of ice pellets was injected into the advanced stellarator Wendelstein 7-X (W7-X). Although the pellets were small and slow, deep and efficient particle fueling could be observed experimentally. The most striking feature appearing after the injection of the pellets, however, was a transient increase in the energy confinement time. This transient phase resembled in several aspects modes of enhanced confinement after gas-puff or pellet injection, as observed in other fusion experiments. All experimental attempts, to prolong this phase, failed. In this paper, discharges are described that show the enhanced energy confinement, and some conditions are summarized which seem to be essential in order to generate it. The focus here is on deep particle fueling by pellets, and shaping of the density profiles during and after the series of pellets. During this time, neutral gas particle re-fueling at the plasma edge is reduced, while density profile peaking and low impurity radiation losses are present.

Exploring SF- in-out asymmetry and detachment bifurcation in HL-2M with E × B by SOLPS

Abstract In this paper, the SF- (with the second X-point located near the outer target surface) in-out asymmetry and detachment bifurcation in HL-2M were explored by the edge plasma code SOLPS5.1 including electric but neglecting magnetic drifts. The modeling results reveal that the radial electric drift component (Epol × B) can play a key role on inducing the SF- in-out asymmetry due to the effective carbon (C) screen capacity and high heat dissipation. The Epol × B drift can enhance the poloidal electric drift component (Er × B), and their synergistic effects can induce a marked in-out asymmetry. Besides, the results showed that the detachment bifurcation in SF- also could occur under given conditions. The presence of four factors (including E × B, C impurity, normal-Bt, and relatively small radial anomalous transport coefficients) forms the necessary conditions for producing the detachment bifurcation. The root reason for the detachment bifurcation formation is due to the strong interaction of Te, E×B, and C impurity radiation loss in the outer divertor region with normal-Bt direction. The detachment bifurcation could be removed or mitigated by weakening their interaction. Finally, the time-dependent detachment bifurcation process is simulated in this work to better understand its physical picture.

Edge and divertor plasma: detachment, stability, and plasma-wall interactions

The paper presents an overview of the results of studies on a wide range of the edge plasma related issues. The rollover of the plasma flux to the target during progressing detachment process is shown to be caused by the increase of the impurity radiation loss and volumetric plasma recombination, whereas the ion-neutral friction, although important for establishing the necessary edge plasma conditions, does not contribute per se to the rollover of the plasma flux to the target. The processes limiting the power loss by impurity radiation are discussed and a simple estimate of this limit is obtained. Different mechanisms of meso-scale thermal instabilities driven by impurity radiation and resulting in self-sustained oscillations in the edge plasma are identified. An impact of sheared magnetic field on the dynamics of the blobs and ELM filaments playing an important role in the edge and SOL plasma transport is discussed. Trapping of He, which is an intrinsic impurity for the fusion plasmas, in the plasma-facing tungsten material is considered. A newly developed model, accounting for the generation of additional He traps caused by He bubble growth, fits all the available experimental data on the layer of nano-bubbles observed in W under irradiation by low energy He plasma.

Energy balance in plasma detachment

Divertor plasma detachment is analyzed from the viewpoint of energy and particle balance in the edge plasma. It is shown that volumetric recombination and impurity radiation losses are responsible for the transition to the detached plasma regime, whereas “momentum removal” plays although important, but auxiliary role providing conditions necessary for the first two to become efficient. A criterion of the local (on an isolated flux tube) detachment onset is studied for both pure and impurity-seeded plasmas.

Stability of divertor detachment

The 2D simulations of edge plasma transport show that unlike some earlier publications, the impurity radiation loss per se does not cause the bifurcation-like transition to detached divertor regime. However, for the case where anomalous plasma transport is increasing with advancement to detachment, like it was recently observed experimentally, the transition to detachment exhibit the bifurcation-like character. Some other plausible reasons for similar bifurcation-like evolution to detachment are discussed. It is demonstrated that the current convective instability can be triggered in detached inner divertor plasma for the condition when outer divertor is still attached. This can explain the fluctuations of radiation loss observed recently experimentally for similar conditions. The self-sustained oscillations observed recently in numerical simulations and related to the interplay of the thermal force effects in impurity transport and impurity radiation loss are further investigated. It is shown that for some conditions these oscillations are ubiquitous, since no stable solutions possible.

Divertor plasma detachment

Regime with the plasma detached from the divertor targets (detached divertor regime) is a natural continuation of the high recycling conditions to higher density and stronger impurity radiation loss. Both the theoretical considerations and experimental data show clearly that the increase of the impurity radiation loss and volumetric plasma recombination causes the rollover of the plasma flux to the target when the density increases, which is the manifestation of detachment. Plasma-neutral friction (neutral viscosity effects), although important for the sustainment of high density/pressure plasma upstream and providing the conditions for efficient recombination and power loss, is not directly involved in the reduction of the plasma flux to the targets. The stability of detachment is also discussed.

Modelling of plasma behaviour in the vicinity of intensive impurity sources

Plasma behaviour in the vicinity of strong sources of impurities is considered by taking into account quasi-neutrality, Coulomb collisions of background particles with impurity ions, radiation losses and sinks of charged particles to bounding material surfaces. If several ion species are present in the plasma the generalized Bohm criterion requires that a certain function of the main ion velocity changes its sign when moving along the magnetic field from the distant plasma through the impurity source to the surface. This allows us to formulate the ‘regularity’ conditions for the motion equation of the main ions and a numerical approach to find the only physical solution without singularities is elaborated. Calculations of plasma parameters in the shadow of a limiter, through which impurities are injected, have been done for the conditions of impurity seeding experiments in the tokamak TEXTOR.

Effect of SOL Decay Length on Modeling of Divertor Detachment by Using Simple Core‐SOL‐Divertor Model

AbstractTaken impurity radiation and momentum loss in the divertor region into consideration, the applicability of Core‐SOL‐Divertor(CSD) model to the divertor detachment is investigated. Three solutions for the low recycling state, the high recycling state and the detached state can be obtained from the CSD model. The dependence of the ratio of the decay length for the density (Δn ) and temperature (ΔT ) in the SOL region is also examined, and it is found that Δn /ΔT ∼ T0.5s scaled by the SOL temperature (Ts), which is based on the experimental data, has a good potential to reproduce B2‐EIRENE results. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Latest Results from the Damavand Tokamak Experiments

Damavand is a small tokamak (a = 7 cm, R = 36 cm) with an elongated plasma cross section (k ≃ 2) and a poloidal divertor. Its passive coils within the vacuum chamber provide the plasma formation at the torus center and act as a passive stabilizer for the plasma current. The experimental measurements presented here are the latest results related to the Damavand discharge main behavior with graphite limiters (before the device modification) during the ohmic heating phase. In this respect, we have evaluated some of the characteristic parameters such as edge safety factor (qa ≃ 3.1), energy confinement (τE = 0.4 ms), electron temperature (Te = 161 eV), and so on, during normal operation of the plasma current. The assessment of disruption phase of the plasma current has been considered by estimating the characteristic parameters of thermal and current quenches to be about 6 eV and 6 MA/s, respectively. Here, also we have monitored the evolution of the line emission of impurity (C, O) ions in the central deuterium plasma. The relative density of carbon and oxygen impurity levels has been estimated to be ≃2.6 and ≃1.4%, respectively. It is concluded that the impurity radiation loss is much higher during disruption phase of plasma current.

The effect of charge exchange with neutral deuterium on carbon emissionin JET divertor plasmas

High-density, low-temperature divertor plasma operation in tokamaks results inlarge neutral deuterium concentrations in the divertor volume. Under theseconditions, low-energy charge transfer reactions between neutral deuterium andthe impurity ions can, in principle, enhance the impurity radiative losses andthus help to reduce the maximum heat load to the divertor target. Auantitative study of the effect of charge exchange on carbon emission ispresented, and applied to the JET divertor. Total and state-selectiveeffective charge exchange recombination rate coefficients were calculated inthe collisional radiative picture. These coefficients were coupled to divertorand impurity transport models in order to study the effect of charge exchangeon the measured carbon spectral emission in JET divertor discharges. Thesensitivity of the effect of charge exchange to the assumptions in theimpurity transport model was also investigated. A reassessment of fundamentalcharge exchange cross section data in support of this study was made.

Intrinsic molybdenum impurity density and radiative power losses with their scalings in ohmically and ICRF heated Alcator C-Mod and FTU tokamak plasmas

A new radiative cooling curve for molybdenum, determined from the Hebrew University, Jerusalem - Lawrence Livermore atomic code (HULLAC) has been used to estimate the radiative power losses from various ion cyclotron resonance frequency (ICRF) and ohmically heated Alcator C-Mod tokamak plasmas and ohmically heated Frascati tokamak upgrade (FTU) plasmas. This cooling curve has been found to accurately predict the radiative losses from molybdenum ions in plasma. Radiative losses from molybdenum computed by using the HULLAC cooling curve could typically account for greater than 60% of the total radiative losses as measured by bolometry in Alcator C-Mod tokamak plasmas. The molybdenum density was found to be as high as particles in ICRF heated plasmas. Plasmas after botonization of the plasma facing surfaces had the lowest molybdenum density and radiated power of all the plasmas examined; ICRF heated plasmas had the highest. The molybdenum density and radiative power losses were found to increase roughly linearly with input ICRF power during different heating schemes in plasmas with both H-mode and L-mode confinement. Although the molybdenum densities were found to decrease with increasing electron density, the radiative power losses did not change significantly for all types of plasmas studied in Alcator C-Mod and FTU: diverted plasmas heated with ICRF and both limited and diverted plasmas heated ohmically.

Vacuum UV spectroscopy and impurity behavior in tokamak and stellarator plasmas

An overview of applications of vacuum UV (VUV) spectroscopy to studies of fusion plasmas is presented. This spectral region is usually considered to extend from 10 to 2000 Å. Most impurity studies in core plasmas and in divertors rely on measuring the intensities of the bright resonance lines that lie in this wavelength range. In the core, these data are supplemented by emissions from charge exchange excitation to determine concentrations of fully stripped low- Z ions. Determinations of individual impurity radiative losses and densities are carried out with the aid of one-dimensional codes in which transport coefficients are adjusted until the modelled intensities are well matched to experimental observation. Analysis of divertor spectra presents special difficulties, because of the inherently two-or three-dimensional nature of the radiation patterns. In addition to impurity studies, VUV lines are used to determine electron and ion temperatures, and plasma rotation.

Electromagnetic instability in nonuniform resistive electron magnetohydrodynamics

A local dispersion relation for electromagnetic modes in a nonuniform collisional magnetized electron plasma with fixed ion background is derived, taking into account equilibrium magnetic field and pressure gradients, as well as impurity radiation losses. The dispersion relation is then analyzed both analytically as well as numerically. It is found that for a low-β plasma, the principal source for the generation of unstable modes is the impurity radiation loss; whereas for a high-β plasma, the various effects such as the electron streaming, the electron–ion collisions, finite electron thermal conductivity, and impurity radiation losses are shown to be responsible for unstable perturbations. The results should be useful in the interpretation of nonthermal electromagnetic fluctuations in nonuniform collision-dominated magnetoplasmas with impurities.

Effect of Perpendicular Transport on Edge Plasma Energy Loss due to Impurity Radiation

AbstractWe consider the effect of perpendicular energy transport in the low temperature divertor region on impurity radiation loss from the SOL plasma. We show that the perpendicular energy transport results in enlargement of the volume with relatively low temperature and a very high density of the plasma. High plasma density causes a strong energy loss due to impurity radiation peak in this low temperature region. For low Z impurity the energy transport due to plasma convection and neutrals also can strongly influence the volume of low temperature region.

Calculations of energy losses due to atomic processes in tokamaks with applications to the International Thermonuclear Experimental Reactor divertor

Reduction of the peak heat loads on the plasma facing components is essential for the success of the next generation of high fusion power tokamaks such as the International Thermonuclear Experimental Reactor (ITER) [Rebut et al., Plasma Physics and Controlled Nuclear Fusion Research (International Atomic Energy Agency, Vienna, in press)]. Many present concepts for accomplishing this involve the use of atomic processes to transfer the heat from the plasma to the main chamber and divertor chamber walls and much of the experimental and theoretical physics research in the fusion program is directed toward this issue. The results of these experiments and calculations depend upon a complex interplay of many processes. In order to identify the key features of these experiments and calculations and the relative role of the primary atomic processes, simple quasianalytic models and the latest atomic physics rate coefficients and cross sections have been used to assess the relative roles of central radiation losses through bremsstrahlung, impurity radiation losses from the plasma edge, charge exchange and hydrogen radiation losses from the scrape-off layer, and divertor plasma and impurity radiation losses from the divertor plasma. This analysis indicates that bremsstrahlung from the plasma center and impurity radiation from the plasma edge and divertor plasma can each play a significant role in reducing the power to the divertor plates, and identifies many of the factors which determine the relative role of each process. For instance, for radiation losses in the divertor to be large enough to radiate the power in the divertor for high power experiments, a neutral fraction of 10−3 to 10−2 and an impurity recycling rate of neτrecycle of ∼1016 s m−3 will be required in the divertor.