High Fluence Plasma Research Articles

Multi-staged ERO2.0 simulation of material erosion and deposition in recessed mirror assemblies in JET and ITER

The deposition/erosion on optical diagnostic components—mirrors—is a critical issue in reactor class devices with long-pulsed high fluence plasma operation. The paper presents results of the three-dimensional Monte–Carlo code ERO2.0 for two diagnostic aperture and first mirror geometries to be deployed in ITER, along with a separate simulation study that aims to replicate results from an experimental first-mirror study carried out on JET. Promisingly, very little plasma and impurity deposition on mirrors for the anticipated plasma durations is found in the ERO2.0 modelling taking into account the current ITER Research Plan and a material mix with beryllium first wall and a tungsten divertor. The post-mortem analysis of mirrors exposed during the experiment and the initial benchmarking efforts on the JET mirror experiment are also broadly consistent, increasing the confidence in predictions for ITER.

Multi-staged ERO2.0 simulation of material erosion and deposition in recessed ITER mirror assemblies

In reactor class devices, with long pulse, high fluence plasma operation and with diagnostic systems that cannot easily be maintained, a critical issue is the deposition/erosion of wall materials and its impact on key optical diagnostic components — mirrors. A workflow for recessed volume simulation was developed for the three-dimensional Monte-Carlo code ERO2.0 (Romazanov, 2020), consisting of multiple simulation stages starting from first wall erosion to more focussed volumes around the key components. This complex workflow has been found to be mandatory to have meaningful statistics of interaction processes on the mirror because the physical fluxes of particles into recessed volumes are extremely low compared to first wall fluxes, leading to unsatisfying statistics in the case of standard Monte-Carlo simulations. The workflow is introduced with the example of a diagnostic mirror assembly in equatorial port 12 behind panels 14 and 15 in the diagnostic first wall in ITER, assuming a beryllium first wall of the main vessel, a stainless steel mirror unit and two molybdenum mirrors embedded in a port plug for neutron shielding. The simulation case corresponds to an H-mode burning plasma case from SOLPS modelling, for which the incoming flux distributions of deuterium neutrals into the recessed volume were simulated by the neutrals transport code EIRENE.

Influence of impurity gas seeding into deuterium plasma on the surface modification, sputtering erosion and deuterium retention in W and W-La2O3 alloy

Surface modification, sputtering erosion and deuterium (D) retention were investigated of tungsten (W) and W-1 wt.% La2O3 (WL10) that were exposed to pure D plasma and D plasma with different impurities (He, N2, Ar). After the plasma exposure, the surface of WL10 is covered by blisters with various shapes, and their size and density increase with D fluence. When the fluence was higher than 8.6 × 1024 D m−2, the blister rupturing occurred causing a partially- and/or fully-opened lid features. The phase interface between La2O3 grains and W matrix was served as the nucleation sites for blistering and large-scale crack propagation near the surface. Small blisters were preferentially appeared on the grains with surface orientation of nearly (111) and that originated from the intra-granular crack at depths much closer to the sample surface. Micro-sized pits were formed on the WL10 surface after exposure to high fluence plasma, which was due to a higher sputtering rate of La2O3 grains during plasma exposure. For both materials, seeding He in D plasma can effectively inhibit the blister formation, and most serious blistering occurs with N seeding in D plasma. When exposed to Ar + D plasma, sparse small blisters were visible and most of blisters were ruptured for W, whereas there are some big blisters with long strip shape for WL10. The difference in blistering behavior between W and WL10 was attributed to the differences in microstructure, composition as well as ductility due to the presence of La2O3 phases. The sputtering yields increased slightly with He seeding in D plasma. N2 and Ar seeding in D plasma leaded to a significant increase in sputtering yields. As compared to the W, the WL10 exhibits a better sputtering erosion resistance in terms of the sputtering yield. The D retention in WL10 increases with D fluence and that was equal to or lower than that in W when exposed to the same conditions.

ITER monoblock performance under lifetime loading conditions in Magnum-PSI

The ITER divertor will be exposed to extremely high plasma fluences over its lifetime, and it is known that plasma exposure can lead to a variety of particle-induced surface-morphology and microstructure changes in tungsten. However, no data exists at fluences comparable to those expected over extended ITER operations (1030−31 m−2) and so it is uncertain how these changes will evolve and affect the divertor performance over such long timescales. Six monoblocks were exposed to high flux plasma comparable to partially-detached plasma conditions in the ITER divertor in Magnum-PSI. Different exposures used different plasma species (H, He, D or D + He) and aimed to replicate conditions similar to those during different phases of the ITER staged approach. The highest fluence achieved was 1030 D m−2, comparable to around one year of ITER Fusion Power Operation. Post-mortem analysis by Nuclear Reaction Analysis revealed very low deuterium retention throughout the blocks, while surface analysis showed no cracking or damage, but did observe helium fuzz growth at low ion energies of 8–18 eV, below typically assumed ion energy requirements for such growth to occur. Metallographic sectioning revealed recrystallization up to 2.2 mm below the surface of monoblocks exposed at peak surface temperatures of up to 1580 °C for different durations up to ∼20 h. Finite Element Method analysis coupled to metallographic and Vickers Hardness identification of the boundary of the recrystallized region identified a faster recrystallization process compared to literature expectations, reinforcing that recrystallization dynamics is an important criterion for tungsten grade selection for the ITER divertor. Overall, no major damage or failure was identified, indicating that the design is capable of fulfilling its steady-state performance requirements under high flux, high fluence plasma loading conditions in the ITER divertor.

Impact of Kr and Ar seeding on D retention in ferritic-martensitic steels after high-fluence plasma exposure

Total deuterium (D) retention from the bulk material of reduced-activation ferritic-martensitic (RAFM) steel Eurofer’97 (EU’97) and commercial ferritic-martensitic grade P92 was traced experimentally by means of thermal desorption spectroscopy (TDS) and linked to the role of krypton (Kr) and argon (Ar) seeding during high-fluence plasma exposure. The influence of impurity seeding on the steel microstructure was determined using scanning electron microscopy (SEM) and extended with focused ion beam (FIB) cross-sectioning. D capture at depths in the µm range was measured by nuclear reaction analysis (NRA). Plasma exposure of the steel samples occurred at 470 K with ion energy of 30–40 eV in the linear plasma device PSI-2 with up to 10% simultaneous impurity admixture in plasma. D inventory achieved values in the 1020 D/m2 range after plasma exposure with high fluences of up to 1 × 1026 D+/m2. In pure and mixed plasmas, the majority of D was trapped in the steel bulk. The Kr and Ar seeding of D plasma resulted in the population of multiple already existing D trapping sites. A similarity of D desorption spectra suggests that D trapping in P92 follows the same mechanism as in EU’97. Kr and Ar seeding mostly contributed to the surface sputtering of the steel samples, yielding less D retention due to the material loss.

Deuterium retention in RAFM steels after high fluence plasma exposure

Deuterium retention and detrapping behavior in the ferritic-martensitic steels EUROFER’97 and P92 after exposure to plasma at high fluences ≥ 1026D/m2 was studied using thermal desorption spectroscopy (TDS), supported by nuclear reaction analysis. Low-temperature irradiation at 450K and fluences ≥ 1026D/m2 with low impact energy D+ / D++He+ ions of 40eV at PSI-2 resulted in a deuterium inventory of 7–18 ×1019D/m2 predominantly at depths ≥8.6µm. Helium admixture led to a reduction of total D retention in both steels, irrespective of surface erosion and composition. The deuterium spectra of both steels displayed one D2 desorption peak at ∼ 540–570K and HD maxima at 540–590, 700–730 and 900–930K. It is suggested that deuterium is mostly retained in the bulk of steel material on interfaces of carbide precipitates and on grain boundaries.

Retention in tungsten resulting from extremely high fluence plasma exposure

PISCES-B was used for a series of high-fluence plasma exposures to investigate the deuterium fuel retention properties of tungsten, when exposed to continuous plasma irradiation. The goal was to determine whether the fuel retention in the tungsten saturates at sufficiently high fluence, or continues to increase as a function of the plasma fluence. During pure deuterium plasma exposure, up to a maximum deuterium fluence of 2×1028m−2, retention results indicate that saturation is not reached and that retention scales as the square root of time, indicative of diffusion dominating the fuel uptake of the tungsten. However, measurements performed while sculpting the PISCES plasma to replicate a burning plasma, by adding a small amount (5%) of helium ions to the incident deuterium plasma, indicate the deuterium uptake in the target is severely inhibited.

Evolution of arsenic in high fluence plasma immersion ion implanted silicon: Behavior of the as-implanted surface

High fluence (>1015ions/cm2) low-energy (<2keV) plasma immersion ion implantation (PIII) of AsH3+ on (100) silicon was investigated, with the focus on stability and retention of the dopant. At this dose, a thin (∼3nm) amorphous layer forms at the surface, which contains about 45% arsenic (As) in a silicon and oxygen matrix. The presence of silicon indicates that the layer is not only a result of deposition, but predominantly ion mixing. High fluence PIII introduces high concentration of arsenic, modifying the stopping power for incoming ions resulting in an increased deposition. When exposed to atmosphere, the arsenic rich layer spontaneously evolves forming arsenolite As2O3 micro-crystals at the surface. The micro-crystal formation was monitored over several months and exhibits typical crystal growth kinetics. At the same time, a continuous growth of native silicon oxide rich in arsenic was observed on the exposed surface, suggesting the presence of oxidation enhancing factors linked to the high arsenic concentration at the surface.

Impact of surface morphology on sputtering during high-fluence plasma exposure

The erosion behavior of beryllium targets subjected to deuterium, or argon, plasma is compared. Erosion by deuterium plasma results in the development of a cone-like surface morphology and the resultant measured erosion rate is almost an order of magnitude less than expected from sputtering calculations. The morphology that develops has been characterized as a function of incident ion energy and fluence to the target. The sputtering yield measured from targets exposed to argon plasma agrees with sputtering calculations and no surface morphology is observed after the exposure. A simulation of the sputtered atoms ejected into the plasma column is developed in order to understand the changing axial penetration distance measured spectroscopically as the morphology emerges. Redeposition of atoms sputtered at shallow angles on adjacent cone structures can result in a lower erosion rate and can account for an increase in the axial penetration distance of atoms into the plasma.

Plasma-induced morphology of beryllium targets exposed in PISCES-B

The surface morphology that results from exposure of beryllium targets in the PISCES-B linear plasma device to light-ion (D or He) plasma is investigated as a function of the incident ion energy distribution, the incident ion angle with respect to the surface and the temperature of the surface. Nearly identical cone-like surface structures result regardless of the ion energy distribution or the incident angle of the ions. Increasing the target temperature on the other hand decreases the resultant surface roughness. Differences in the shape of the cones that develop on the surface due to either deuterium or helium plasma exposure agree with models of ion surface interactions. Ion trenching at the base of the cones has been observed and may explain the resulting dense array of cones across the surface following high-fluence plasma exposure.

Surface modification at tungsten and tungsten coated graphite due to low energy and high fluence plasma and laser pulse irradiation

We have investigated the influence of low energy helium and hydrogen/deuterium plasma irradiation to tungsten or tungsten coated graphite samples. The experimental conditions of helium hole/bubble formation and hydrogen blister formation were summarized. The tungsten coated graphite exposed to a helium plasma shows different surface modification from those of bulk tungsten. We also demonstrated a transient heat load onto a damaged tungsten surface due to a helium plasma exposure by a laser-beam irradiation. Simultaneous helium plasma and laser pulse irradiations lead to a further roughing of the tungsten surface.

Erosion and redeposition behavior of selected NET-candidate materials under high-flux hydrogen, deuterium plasma bombardment in pisces

Plasma erosion and redeposition behavior of selected candidate materials for plasma-facing components in the NET-machine have been investigated using the PISCES-A facility. Materials studied include SiC-impregnated graphite, 2D graphite weaves with and without CVD-SiC coatings, and isotropic graphite. These specimens were exposed to continuous hydrogen or deuterium plasmas under the following conditions: electron temperature range from 5 to 35 eV; plasma density range from 5 × 10 11 to 1 × 10 12 cm −3; flux range from 5 × 10 17 to 2 × 10 18 ions cm −2 s −1; fluence of the order from 10 21 to 10 22 ions/cm 2; bombarding energies of 50 and 100 eV; target temperature range from 300 to 1000°C. The erosion yield of SiC-impregnated graphite due to deuterium plasma bombardment is found to be a factor of 2 to 3 less than that of isotropic graphite materials. A further factor of 2–3 reduction in the erosion yield is observed in when redeposition associated with reionization of sputtered particle becomes significant. From post-bombardment surface analysis with AES, the surface composition in terms of the Si/C of SiC-impregnated graphite ratio is found to increase from 0.15 to 0.7 after hydrogen plasma bombardment to a fluence around 4 × 10 21 ions/cm 2 at 350 ° C. However, the final surface composition appears to remain unchanged up to 4 × 10 22 ions/cm 2, the highest fluence in the present study. Significant surface morphological modifications of SiC-impregnated graphite are observed after the high-fluence plasma exposure. Several structural problems such as coating-substrate adhesion have been pointed out for SiC-coated 2D graphite weave.