Charge Exchange Neutrals Research Articles

Microscopic damage of metals exposed to the helium discharges in TRIAM-1M tokamak and its impact on hydrogen recycling process

High-energy charge-exchange particles bombarding the plasma-facing wall may cause not only surface sputtering but also damage the materials inside due to their rather high energy. In the case of burning plasma, we should take into account the effects of helium because it is well known that helium atoms have much stronger effects on material damage than hydrogen atoms. In this paper, therefore, microscopic damage of metals exposed to long pulse discharges of helium plasma in TRIAM-1M was studied and the impact on the hydrogen recycling process was discussed by comparing it with helium ion irradiation experiments. Considerably larger amounts of dislocation loops and very dense fine bubbles were formed by the irradiation of rather low-energy charge-exchange neutrals of helium. According to the irradiation experiments with low-energy helium ions, formation of dense helium bubbles drastically enhances hydrogen trapping and makes the desorption difficult. These results indicate that the hydrogen recycling phenomenon during the burning plasma discharge must be quite different from that of the hydrogen plasma discharge experiments.

Neutral particle fueling at the midplane of DIII-D

Data from an array of eight tangential D α viewchords, located at the outer midplane of DIII-D, are inverted to obtain atomic neutral density profiles across the separatrix and are compared to results of B2.5 and DEGAS simulations. In H-mode and (low density) L-mode plasma conditions, charge-exchange neutrals reflected and desorbed from the outer midplane region of the vacuum vessel wall can account for the midplane neutral density that is deduced from the D α data, making it unnecessary to invoke main chamber ion recycling. The midplane measurements and DEGAS calculations are used to validate a slab-model of neutrals in the edge. In this model, cold neutrals from the wall fuel the edge and are attenuated by charge-exchange and ionization, and the results are in agreement with measured neutral profiles.

Sputtering and codeposition of silicon carbide with deuterium

Due to its excellent thermal properties, silicon carbide is being considered as a possible plasma-facing material for fusion devices. If used as a plasma-facing material, the energetic hydrogen isotope ions and charge-exchanged neutrals escaping from the plasma will sputter the silicon carbide. To assess the tritium inventory problems that will be generated by the use of this material, it is necessary that we know the codeposition properties of the redeposited silicon carbide. To determine the codeposition properties, the deuterium plasma experiment at Sandia National Laboratories in Livermore, California has been used to directly compare the deuterium sputtering and codeposition of silicon carbide with that of graphite. A Penning discharge at a flux of 6×10 19 D/m 2 and an energy of ≈300 eV was used to sputter silicon and carbon from a pair of 0.05 m diameter silicon carbide disks. The removal rate of deuterium gas from the fixed volume of the system isolated from all other sources and sinks was used to measure the codeposition probability (probability that a hydrogen isotope atom will be removed through codeposition per ion striking the sample surface). A small catcher plate used to capture a fraction of the codeposited film was analyzed using Auger spectroscopy. This analysis showed the film to begin with a high carbon to silicon ratio due to preferential sputtering of the carbon. As the film became thicker, the ratio of the depositing material changed over to the (1:1) value that must eventually be attained.

Theoretical interpretation of the toroidal rotation velocityobserved in Alcator C-Mod Ohmic H-mode discharges

It is shown that neoclassical theory explains quite well the origin of the co-current toroidal rotation velocity measured in the core of stationary Alcator C-Mod edge localized mode (ELM)-free Ohmic high confinement (H)-mode discharges. Both edge and core toroidal rotation velocity profiles are determined to a good approximation by the edge ion temperature and density pedestals, where the gradients are large and the plasma is in the high collisionality regime. Under these conditions, the predicted radial electric field profile is similar to those measured in the DIII-D tokamak whereas the usual expression for the poloidal velocity is modified by finite Larmor radius (FLR) effects. Over the entire plasma cross section, the expression of the toroidal velocity can approximately be cast as the product of a dimensionless non-local functional of the pedestal normalized profiles Ti(r)/Ti(rinf) and Ni(r)/Ni(rinf) with powers of the plasma density, temperature, safety factor and magnetic field at the pedestal inflexion point rinf provided the FLR related corrections are independent of the latter parameters. The collapse of the core toroidal rotation velocity when either an internal transport barrier forms (that leads to impurity accumulation), or the plasma experiences a transition from the H- to the low confinement (L)-mode, or ELMs appear, and the spin up at the L-H transition are also explained. In the edge region, power balance is consistent with the prediction from sub-neoclassical ion energy transport theory at high collisionality. The role of charge exchange neutrals is discussed and the critical density above which they are expected to noticeably slow down the rotation is estimated. The toroidal velocity gradient predicted by theory at the edge of the ELM-free Ohmic H-mode discharge mainly under study (qs = 3.4) is near the onset value for the Kelvin-Helmholtz (K-H) parallel velocity shear (PVS) instability; this result is very interesting since a transition from ELM-free to enhanced Dα (EDA) H-modes occurs at q≅3.5-4; the PVS K-H instability appears to have the characteristics of the `quasi-coherent' mode that is present in all EDA plasmas, but not in ELM-free H-modes.

Nonmonotonic Spatial Distributions of Charge-Exchange Neutral Fluxes and Optical Radiation from Plasma in the DAMAVAND Tokamak

Results are presented from investigations of the nonmonotonic spatial distributions of charge-exchange neutral fluxes and optical radiation from plasma in the DAMAVAND tokamak. It is shown that, during ohmic heating of the plasma, the regions with enhanced confinement of both the background plasma particles and heavy impurity ions arise near rational magnetic surfaces with q = 1 and 2. These regions are characterized by enhanced emission of accelerated charge-exchange neutrals and optical radiation from impurity ions.

Ion kinetics in a magnetized plasma source

There are regimes of plasma source operation in which the ion motion is controlled by the ambipolar electric field and ion–atom collisions. This paper presents a derivation of the corresponding ion distribution function and plasma density profiles in the source for a given electron temperature and a given gas ionization rate. Conditions are discussed under which the ion flux is predominantly radial or predominantly axial. It is found that the presence of plasma can substantially change the neutral gas pressure due to production of fast charge–exchange neutrals.

Impact of charge exchange neutrals on plasma facing components of Tore Supra

The inner vessel of a tokamak is submitted to high particle fluxes bombardment, resulting in a contamination of the core plasma and in a limited lifetime for the plasma facing components. In the Tore Supra tokamak, a major upgrade in terms of power exhaust and pulse length, called the CIEL project, is taking place (power coupled to the plasma up to 25 MW, pulse duration up to 1000 s). In this paper, the effect of charge exchange neutrals on the stainless steel protection panels covering the inner vessel is investigated in the CIEL configuration. Based upon simulations of full power CIEL discharges by a neutral transport code, it is found that erosion by charge exchange neutrals might lead to the contamination of the plasma by heavy impurities. Therefore, various methods allowing to shield the protection panels from the charge exchange neutrals, such as carbonisation, boronisation, B 4C coating, are discussed.

Anisotropic radiation damage by charge exchange neutrals under tokamak discharges in TRIAM-1M

Thin foil specimens were exposed to tokamak discharges in TRIAM-1M. As a result of the microstructural observation, accumulated radiation damage (dislocation loops) due to charge exchange (CX) neutrals were observed only in the specimens pointing to the lower half of plasma and almost no damage in the specimens pointing to the upper side. These results indicate that the lower half of the plasma is the major source for energetic CX neutrals. This anisotropy can be explained by the effect of the gradient B drift of high-energy ions trapped in a toroidal ripple. The mean flux was estimated to 4.2×10 17 particles m −2 s −1 sr −1 from the lower side by using the areal density (A.D.) of dislocation loops. The depth distribution of the dislocation loops shows a significant contribution of the high-energy CX neutrals. The energetic CX neutrals will give a great impact on material degradations.

Suppression of net erosion in the DIII-D divertor with detached plasmas

The ability to withstand disruptions makes carbon-based materials attractive for use as plasma-facing components in divertors. However, such materials suffer high erosion rates during attached plasma operation which, in high power long pulse machines, would give short component lifetimes and high tritium inventories. We present the results from recent experiments in DIII-D, in which the divertor materials evaluation system (DiMES) was used to examine erosion and deposition during short exposures to well-defined plasma conditions. These studies show that during operation with detached plasmas, produced by gas injection, net erosion is suppressed everywhere in the divertor. Net deposition of carbon with deuterium was observed at the inner and outer strike points and in the private flux region between strike points. For these low temperature plasmas ( T e<2 eV), physical sputtering is eliminated. These results show that with detached plasmas, the location of carbon net erosion and the carbon impurity source, probably lies outside the divertor. Physical or chemical sputtering by charge-exchange neutrals or ions in the main plasma chamber is a probable source of carbon under these plasma conditions.

Studies of tungsten erosion at the inner and outer main chamber wall of the ASDEX Upgrade tokamak

A critical issue for the choice of main chamber first wall materials in future fusion devices such as ITER is the erosion rate due to bombardment by charge-exchange (CX) neutrals. Due to the relatively small flux density of impacting particles, respective measurements are only possible using long term samples (LTS) exposed for a full experimental campaign. In ASDEX Upgrade, CX erosion has been studied extensively for tungsten on the inner heat shield by placing four W coated tiles at different poloidal positions in one toroidal sector. During the same campaign, several LTS were placed at different poloidal and toroidal positions of the outer wall. 13C and Cu coated graphite probes were also used in order to test and compare W low and medium Z alternatives. The erosion results from the probes are compared with the calculated erosion [W. Eckstein, C. Garcia-Rosales, J. Roth, W. Ottenberger IPP Report, IPP 9/82]; [H. Verbeek, J. Stober, D. Coster, W. Eckstein, R. Schneider Nucl. Fus. 38 (1998) 12] and a figure of merit (F. of M.) between several materials is proposed which also takes into account the plasma isotope effect in CX erosion.

Particle trapping in carbon walls during ICRH heating in Tore Supra

The role of high-energy charge exchange (CX) particles during radio-frequency (RF) heating is examined in an attempt to develop a predictive model for wall saturation and desaturation processes. Tore Supra discharges in which an apparent saturated wall, previously obtained through a succession of ohmic discharges, was transformed into a pumping wall with the application of RF heating are analyzed. Direct measurements show the increased mean energy of the CX spectrum upon application of RF heating. This leads to deeper CX neutrals penetration reaching the remaining a:C–D unsaturated layer and restoring the wall pumping efficiency. The conversion from a saturated to a pumping state with the application of RF heating, and the time scales for saturation during the ohmic and the heating phases, are reasonably well explained by several existing models. The deeper particle wall penetration observed during RF plasma experiments points to a potential mechanism which can increase tritium trapping in a fusion reactor.

Deposition and erosion in local shadow regions of TEXTOR-94

Carbon erosion and deposition were investigated on the surface of a flat target covered with an a-C:H film and exposed for 197 s in the SOL of TEXTOR-94. The target was declined by 20° with respect to the toroidal direction and partly protected by an aluminum (3 mm) plate which created an 8 mm wide local shadow. Thickness changes were measured by colorimetry after each plasma discharge. Carbon is eroded from surface areas near the plasma edge (LCFS +1 cm) and transported into the local shadow regions. Accumulation rates up to ≈ 7 nm/s were found. The erosion in the local shadow regions (about -0.1 nm/s) is due to charge exchange neutrals. The observations are confirmed by ion beam analyses and by preliminary calculations with the B2-EIRENE and ERO-TEXTOR code.

Laser-driven atomic-probe-beam diagnostics

A new laser-driven atomic-probe-beam diagnostic (LAD) is proposed for local, time-resolved measurements of electric field and ion dynamics in the accelerating gap of intense ion beam diodes. LAD adds new features to previous Stark-shift diagnostics which have been progressively developed in several laboratories, from passive observation of Stark effect on ion species or fast (charge-exchanged) neutrals present naturally in diodes, to active Stark atomic spectroscopy (ASAS) in which selected probe atoms were injected into the gap and excited to suitable states by resonant laser radiation. The LAD scheme is a further enhancement of ASAS in which the probe atoms are also used as a local (laser-ionized) ion source at an instant of time. Analysis of the ion energy and angular distribution after leaving the gap enables measurement, at the chosen ionization location in the gap, of both electrostatic potential and the development of ion divergence. Calculations show that all of these quantities can be measured with sub-mm and ns resolution. Using lithium or sodium probe atoms, fields from 0.1 to 10 MV/cm can be measured.

Modeling of particle and energy transport in the edge plasma of Alcator C-Mod

In the present study recycling and transport in the edge plasma of Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] is modeled and analyzed with the multi-fluid code UEDGE [T. D. Rognlien et al., J. Nucl. Mater. 196–198, 347 (1992)]. Matching the experimental plasma density profiles in the scrape-off layer (SOL) requires a spatially dependent effective anomalous diffusion coefficient D⊥ growing rapidly towards the wall. The midplane pressure of neutral gas, Pmid, is a key parameter that reflects the magnitude of anomalous transport of plasma from the core. Recycling of plasma on the main chamber wall appears to be quite significant, especially in the case of high Pmid∼0.3 mTorr when the main wall provides ∼70% of recycling neutrals in the main chamber. In the upper SOL (well above the x point) draining of particles by the poloidal flow is weak and thus the particle balance is predominantly radial. For the radial heat transport it is found that energy flux carried by radial plasma convection and by charge-exchange (CX) neutrals is quite significant in SOL. In the high Pmid case, heat conduction by CX neutrals along with radial heat convection by plasma carries most of the power flux (∼75%) across the last closed flux surface. Even in the low Pmid case, heat conduction by CX neutrals dominates the radial heat flux far out in the SOL.

Carbon influx in He and D plasmas in DIII-D

Differences in the carbon behavior between He and D plasmas during VH-mode, L-mode and L-mode with excess gas puffing are reported and inferences on the importance of the various carbon sources during these modes of operation are discussed. During a VH-mode phase, VUV and visible charge exchange spectroscopy indicates that for both He and D operation the carbon behavior is very similar. In the edge plasma, carbon build up is quite rapid, and the carbon influx represents a large fraction of the total plasma density increase until the termination of the VH phase. During cold divertor operation induced by puffing the primary fueling gas, D and He discharges show a difference in the carbon behavior. The core carbon density is seen to be approximately constant during a D discharge as it transitions from an attached to a cold divertor. However in a He discharge, the core carbon density disappears soon after the cold divertor transition. Arguments are made that the primary carbon source in the ELM free H-mode period is physical sputtering by ion impact at the divertor strike point. In L-mode, both attached and cold divertor, the primary source is from the divertor region and two possibilities for this source are chemical sputtering or charge exchange neutral sputtering. Existing data supports charge exchange neutrals as dominant.

Review of recent works in development and evaluation of high-Z plasma facing materials

This paper reviews the recent activities and results in development and evaluation of new high-Z materials focusing on tungsten and its alloys. VPS-W coating on graphite and CFC showed superior high heat load properties by inserting sophisticated multilayer diffusion barriers (W/Re) between tungsten and substrate. For successful application it is necessary to know the temperatures for recrystallization and interdiffusion of rhenium and tungsten, which may degrade bonding properties. Sputtering by plasma impurities and damage by energetic charge-exchange neutrals must be considered. Retention of hydrogen isotopes strongly depends on material grade and production process. Dense bubbles and dislocations formed by helium bombardment may act as effective traps for hydrogen. It is anticipated that long range diffusion of helium induces embrittlement. Neutron irradiation is one of the major concerns for tungsten materials in fusion reactor application. The tungsten materials developed so far perform poorly under neutron irradiation and their brittle nature could limit their application as a fusion material. Further efforts are needed to improve ductility while keeping other desirable properties such as high thermal conductivity.

Boronization in future devices – protecting layer against tritium and energetic neutrals

A thin boron film is attractive as a deuterium/tritium free wall, and as a protecting layer against impact of energetic charge-exchange neutrals in future fusion devices with long pulse operation. New experimental evidence is given for desorption of hydrogen isotopes from these films at relatively low temperature. Most hydrogen atoms in a boron-coated layer are re-emitted to the plasma side below 400°C without penetration into the substrate of stainless steel. The maintainability of a thin boron layer during a long pulse operation may be a problem. Boron atoms are hardly removed by pumping because their hydrides are easily disintegrated and redeposited. Gross migration of boron atoms inside the vessel is a concern. A condition required for avoiding the migration is discussed.

Characteristics of carbon sheet pump in application experiments to a high-temperature plasma device

Carbon sheet pump (CSP), which has been developed for reduction of hydrogen recycling in the large helical device (LHD) of National Institute for Fusion Science, is applied to an actual plasma device for the first time to evaluate the pumping characteristics. A small scale of pump module is manufactured and installed in the GAMMA-10 tandem mirror device and the module is exposed to charge-exchange neutrals emitted from ICRF-heated hot-ion plasmas. Temporal evolution of the pressure is measured with a fast ionization gauge installed in the test chamber during the plasma discharge and a significant difference between CSP-on and CSP-off is observed under good reproducibility. The dependence on the ICRF power is investigated and the pressure difference between on and off is found to increase with the ICRF power. These phenomena indicating the pumping ability of CSP are observed to continue during more than 300 plasma shots, which shows the effectiveness of CSP in controlling hydrogen recycling. The result of thermal desorption experiments of CSP after being exposed to charge-exchange neutrals is also described.

Wall erosion and material transport to the Mark I carbon divertor of JET

Erosion and deposition at the vessel walls of the main chamber of JET were measured with long term samples during the whole operation period of the Mark I carbon divertor from April 1994 until March 1995. Assuming toroidal symmetry, about 70 g Ni + Cr + Fe was found to be eroded from the inner torus wall and about 55 g Be from the outer torus wall due to sputtering by energetic charge-exchange neutrals. Deposition has been measured on a poloidal section of the Mark I C divertor and on tiles from the inner and outer wall limiters. Eroded material is redeposited in the divertor and in the main chamber where it is found predominantly at the sides of the poloidal limiters and at the outer vessel wall. At these deposition-dominated areas in total about 25 g Ni + Cr + Fe, 28 g Be and 370 g C are found. The carbon originates mostly from the carbon limiters; additionally some may be originating from the divertor plates.

A unified theory of transport barriers and of subneoclassical transport

“Subneoclassical” heat fluxes are shown to be rigorous consequences of the revisited neoclassical theory published earlier [Phys. Plasmas 1, 619 (1994)]. Including finite Larmor radius and inertia effects, this theory also provides a nondegenerate ambipolarity constraint, which, together with the parallel momentum equation, defines unambiguously the radial electric field Er and the parallel velocity U∥,i. It is shown that the stationary solution of those equations features, under conditions that are discussed, highly sheared Er profiles as observed in edge transport barriers. The operation regime is determined by a competition between nonlinear spin up of the rotation (which is interpreted) and momentum loss via charge exchange neutrals. The position of the transport barrier—near the last closed magnetic surface (LCMS)—is explained. The local threshold condition is analyzed, including the role of recycling neutrals and of the isotope mass. The width of the shear layer, as well as the predicted jumps and negative values of Er in front of the LCMS, agrees with experimental data. The time-dependent equations have solutions propagating from the edge to the core; the time scale associated with the toroidal rotation scales as and is usually comparable to the neoclassical heat transport time scale. Although the theory is so far limited to the high collisionality regime, a clear physical interpretation of the results allows extrapolation to low collisionality plasmas.