Surface Heat Load Research Articles

Thermal hydraulic and material analysis of upgraded flat-type Graphite divertor mock-up for Pakistan Spherical Tokamak (PST)

The design of a divertor is critically dependent on managing power deposition, erosion effects, and plasma configuration. An upgraded double-null divertor configuration has been developed for the Pakistan Spherical Tokamak (PST), featuring graphite targets that are actively cooled and designed to withstand a peak heat flux of 0.3 MW/m² at a pressure of 0.1 MPa. This paper presents a comprehensive thermal-hydraulic and material analysis for the upgraded flat-type divertor mock-up system, covering aspects such as material surface heat load, peak temperature rise (∆T °C) on the mock-up, and surface temperature increase in the cooling channel. The analysis includes total deformation (mm) and equivalent strain for the inner vertical target (IVT), outer vertical target (OVT), and dome structure, along with material comparison. The recommended SST K-ω turbulence model is utilized in the pressure-based transient analysis, with an inlet velocity of 1.5 m s-1 and an inlet temperature of 16.8 °C. A comparative study of the material and thermal-hydraulic analyses was performed using CFD and RELAP5. The findings reveal that graphite is more suitable than tungsten for the PST's upgraded divertor system, demonstrating its effectiveness as the preferred surface material to address heat enhancement challenges in the PST.

Toroidal distribution of heat load on castellated plasma facing components for lower divertor target in EAST

In tokamak devices, the performance of plasma facing components (PFCs) under high heat loads is a key challenge for achieving long-pulse and high-performance plasma operations. The lower divertor of EAST adopts an ITER-like cassette modules (CMs) structure. However, regular distribution of damage along the toroidal direction have been observed on the divertor target plates, which not only affects the lifespan of PFCs but also limits the long-pulse and high-performance plasma operations. Therefore, investigating the toroidal distribution of the heat load on the divertor target in EAST is crucial for understanding the mechanism of damage on the target plate surface. The toroidal characteristics of heat loads on CMs of the lower divertor in EAST was analyzed using a three-dimensional magnetic field line tracing program (PFCFlux). For the inner vertical target (IVT) and outer vertical target (OVT) within a single CM, the heat load varies monotonically along the toroidal direction. In contrast, the heat load distribution on the outer horizontal target (OHT) is uniform, except at the chamfers, which is directly correlated with the target’s geometric structure and the magnetic field configuration. The surface heat load distribution exhibits toroidal non-uniformity, with the heat deposition pattern being generally consistent and displaying periodic variations. The toroidally uneven heat load distribution is attributed to the change in the magnetic field’s tilt angle. Additionally, the peak heat load is observed at the chamfer positions between cassettes, and an analysis is conducted on the relationship between peak heat load and the size of the chamfer. Smaller tilt angles result in lower surface heat load. Furthermore, a comparative analysis of peak heat loads at the chamfer under different plasma current conditions reveals that higher plasma currents generally result in increased peak heat loads. This investigation into the toroidal heat load distribution on the divertor surface contributes to understanding of the damage mechanism of W PFC as the target plate and provides valuable data for divertor design of future fusion devices.

Development and manufacturing of beryllium-armoring ITER enhanced heat flux FW toward series production in China

ITER’s enhanced heat flux (EHF) first wall (FW) provides thermal shielding to the other components behind it and limits its plasma boundary, consequently bearing high surface heat loads up to 4.7 MW m−2. China developed the EHF FW technologies in steady stages by making small mock-ups, semi-prototypes and a full-scale prototype (FSP), working toward series production at the Southwestern Institute of Physics. All the key technologies for bimetallic diffusion bonding, welding and assembly have been fully qualified. The Be armor tile size effects on thermal-fatigue performance have been evaluated using a pulsed high heat flux test with an EMS-400 electron beam facility. It was found that both the Be/CuCrZr bonding and its thermal-fatigue life are highly dependent on the tile size, and 12 × 12 mm2 Be tiles are acceptable for the required performance, either as individual tiles or in castellation by cutting larger ones. Small-scale mock-ups with such Be tiles survived 16 000 thermal cycles at 4.7 MW m−2, while EHF FW fingers with 16 × 16 mm2 Be tiles demonstrated unstable performance. In contrast, only BE tiles larger than 24 × 12 mm2 showed reliable diffusion bonding with the CuCrZr heat sink by hot isostatic pressing and consequently should be castellated into smaller ones. The manufacturing of the FSP finger goes through complex thermal cycles, using CuCrZr alloys in age-strengthening plus cold-rolling states, enabling it to maintain its tensile strength to the level of 285 MPa and its mean grain size less than 100 µm. The pipe welding for finger pairing and its assembly with the central beam are demonstrated, including enabling welding to be carried out twice for repair in a narrow space. The FSP showed good finger alignment, dimensional control and reliable vacuum tightness without any blockage of cooling channels.

Conceptual design of poloidal horseshoe limiter layout for JA DEMO

In the development of the JA DEMO fusion reactor, the proposed poloidal horseshoe limiter aims to mitigate the surface heat load localized on the breeding blanket. The limiter protrudes from the first wall toward the plasma to shade the blanket from the charged particles, which account for the majority of the surface heat load. To concretize the limiter design, this study presents a limiter layout that implements a concept that allows for sufficient cooling and remote maintenance. The limiter shape was determined by evaluating the surface heat load using 3-D magnetic field line tracing. A fully poloidal limiter with a width of 300 mm, which corresponds to 2.4 % of the FW area, was selected. The limiter heating, consisting of surface heat load and nuclear heating, was evaluated to discuss the required cooling, and the concept of a limiter cassette and modules for cooling was proposed. For the limiter replacement method, the outboard limiter is designed to be extracted from the upper port with the mounting segment. The inboard limiter is then grasped and transported to reach the upper port after the outboard limiter extraction. Both the inboard and outboard limiter are expected to be replaced without affecting other in-vessel components in this study.

The Tools and Parameters to Consider in the Design of Power Transformer Cooling Systems

Transformers are the most important elements of electric power systems. Many conditions must be met for power transformers to work properly. One of them is a low operating temperature. This condition will be met if the transformer cooling system is properly designed. One of the components of a cooling system is insulating liquid. The heat transfer coefficient α of liquid determines its ability to cool the transformer. The higher its value, the more effectively the liquid transfers heat to the environment. This article describes the influence of the position of the heat source, which is usually in the windings of the transformer, on the coefficient α value of the insulating liquid. The vertical and horizontal positions of the heat source were analyzed. The coefficient α was analyzed at different points of the heat source. The tests were carried out for mineral oil and various esters. Heat transfer coefficient measurements were carried out for various surface heat loads of the heat source. It has been proven that, in the case of a horizontal heat source, the coefficient α has a value several dozen percent higher than in the case of a vertical source. It has been proven that the coefficient α has different values in different places of the heat source. Regardless of the location, the highest value of the coefficient α occurred in the lower part of the heat source.

Urban Surface Thermal Runoff Generation Mechanism and Scenario Simulation

AbstractWhen precipitation happens in summer, the thermal runoff from impervious surfaces flow into urban lakes, causing the rise of the surface water temperature. Such a process eventually leads to the short‐term effects of cyanobacteria blooms in eutrophic lakes and the long‐term effects of changes in the structure and quantity of lake species. It is of great scientific significance to understand the heat exchange process between surface air, underlying surface, and surface runoff and reveal the thermal runoff formation mechanism. In this study, the heat transfer models between land, surface‐air, and surface‐water were implemented, aiming to quantify the heating process of surface runoff caused by surface heat load under real conditions. The results show that the two established models can accurately simulate the heat exchange process. Based on our model simulations and field measurements, we analyzed the impact of weather conditions, hydrological conditions, and underlying surface types on runoff temperature. The study finds that the initial surface temperature, wind speed, rainfall intensity, thermal conductivity, and the underlying surface's water permeability can explain 95.4% and 91.9% of the observed differences in surface runoff heating and temperature increase rate, respectively. The initial surface temperature is the most critical factor in the heat exchange process. The reduction of urban ventilation will aggravate the heat effect of surface runoff. Compared with the underlying permeable surface, the impervious surface is the main driving force for the formation of surface thermal runoff pollution.

AEROTHERMODYNAMICS OF COMBINED SPIKE AND COUNTERFLOW JET TECHNIQUE FOR REACTING HYPERSONIC FLOWS

Computational investigation is carried out to estimate the drag force and surface heating load for hypersonic reacting flows. An in-house viscous nonequilibrium finite volume-based reacting gas solver has been utilized. This solver is capable of investigating 11 chemical elementary reactions and temperature-dependent specific properties to reveal the effect of lower as well as higher freestream stagnation enthalpy conditions. Initially, the calorically perfect-gas and real-gas model-based simulations are carried out to understand the real-gas effects in the presence of a metallic spike. Computed surface pressure and heat flux are compared for the freestream stagnation enthalpy of 2 MJ/kg. The real-gas model predicts a 5% higher drag and 57.21 kW/m<sup>2</sup> higher peak heat flux compared to the perfect-gas model. However, lower enthalpy conditions predict almost the same drag force for any spike length. Further, a counterflowing jet is installed at the root of the spike, and flow field alterations are studied for this proposed integrated configuration. The root jet further pushes the conical shock in the upstream direction and provides an extra-large recirculation zone. Here, the possibility of drag and surface heat flux reduction is very much evident due to the decrease in surface pressure and presence of low-temperature jet gas in the vicinity of the object. Various freestream stagnation enthalpies, as well as the jet pressures, are considered to investigate the performance alterations by the combination technique. It is observed that the drag and surface heat load reduction efficiency of the combined configuration decreases with an increase in the freestream stagnation enthalpy. Moreover, it increases when increasing the root jet pressure for given enthalpy conditions. Hence, instead of attaching a long spike at the stagnation region of a blunt-shaped object, the use of a short spike and low-pressure root jet is recommended for a better reduction in drag and surface heat load.

STABILITY OF THIN QUASI-CRYSTALLINE Ti-Zr-Ni FILMS AND RELATED CRYSTALLINE PHASES UNDER LOW-ENERGY TRANSIENT PLASMA IRRADIATION

The properties of Ti41Zr38.3Ni20.7 thin films under radiation-thermal action of hydrogen plasma with a surface heat load of 0.2 MJ/m2 was studied at the QSPA Kh-50 quasi-stationary plasma accelerator (NSC KIPT).The phase composition, structural state, and surface morphology were studied using X-ray diffraction and scanning electron microscopy. It was found that the quasicrystalline phase and related crystalline phases, the Laves phase, the α-solid solution, and the 2/1 phase of the Ti-Zr-Ni approximant crystal were stable under irradiation with up to 20 hydrogen plasma pulses. The phase composition did not change. It is shown that the changes in the coatings mainly manifest themselves as changes in the substructure of the observed phases. With an increase in the plasma exposure dose, the structure of the quasicrystalline icosahedral phase improves, and the size of the coherence regions increases. In the films consisting of crystalline phases, a partial phase transformation is observed with a redistribution of components between the 2/1 phase of the approximant crystal and the α-solid solution phase. It was found that thin films of the TiZr-Ni system containing a quasicrystalline icosahedral phase, irradiated with radiation-thermal plasma pulses, are less prone to cracking than coatings with crystalline phases of the same system.

Ultrashort pulse laser ablation of steel in ambient air

This paper explores large area laser ablation of steel in air at ambient pressure and compares the influence of different pulse duration regimes from femtosecond to nanosecond. It is demonstrated that irradiation with 70 ns long pulses induces sufficient surface heat load to damage the steel by producing a network of microcracks on the entire surface which propagate deep into the bulk. In stark contrast, ultrashort pulse laser ablation maintains the integrity of the steel bulk structure. The first appearance of cracks on surfaces processed at the optimal fluence occurs when the pulse duration is increased to 15 ps. The laser fluence dependence of the ablation rate in the ultrashort pulse regime with 275 fs pulses is investigated and the ablation threshold is determined at 0.25 ± 0.05 J/cm2. The optimal ablation regime in terms of volume removal is established to be an ablation efficiency of 0.20 ± 0.02 mm3/min/W at a fluence of ∼ 1.2 ± 0.3 J/cm2 . Finally, the surface roughness for fluences from the ablation threshold up to 2.5 J/cm2 is analysed and it is demonstrated that the best smoothness of the laser processed surface occurs in the same range of fluences where ablation is most efficient.

Multiphysics Optimization for First Wall Design Enhancement in Water-Cooled Breeding Blankets

The commercial feasibility of the first fusion power plant generation adopting D-T plasma is strongly dependent upon the self-sustainability in terms of tritium fuelling. Within such a kind of reactor, the component selected to house the tritium breeding reactions is the breeding blanket, which is further assigned to heat power removal and radiation shielding functions. As a consequence of both its role and position, the breeding blanket is heavily exposed to both surface and volumetric heat loads and, hence, its design requires a typical multiphysics approach, from the neutronics to the thermo-mechanics. During last years, a great deal of effort has been put in the optimization of the breeding blanket design, with the aim of maximizing the tritium breeding and heat removal performances without undermining its structural integrity. In this paper, a derivative-free optimization method named “Complex method” is applied for the design optimization of the European DEMO Water-Cooled Lithium Lead breeding blanket concept. To this purpose, a potential performances-based objective function, focusing on the maximization of the tritium breeding, is defined and a multiphysics numerical model of the blanket is developed in order to solve the coupled thermo-mechanical problem, while the optimization algorithm leads the design towards a minimum optimum point compliant with the prescribed requirements. Once the optimized design is obtained, its nuclear and thermo-structural performances are assessed by means of specific neutron transport and multiphysics simulations, respectively. Finally, the structural integrity is verified by means of the application of the RCC-MRx design criteria. • A tool for First Wall optimization in water-cooled breeding blankets is developed. • The optimization is performed using the gradient-free method named “Complex method”. • An objective function aiming at tritium breeding maximization is defined. • A FE model is used to check the compliance with the thermo-structural requirements. • The optimized design is finally assessed by proper, more detailed numerical analyses.

Deuterium and helium retention and corresponding modifications of W under heat loads relevant to ITER transient plasma events: Part I. The power load below the tungsten melting temperature

In order to simulate ITER transient events with surface heat load parameters relevant to edge-localized-mode (ELM) impacts, tungsten samples were exposed to pulsed heat loads using pure deuterium (D) and with 10% helium (He) seeding plasmas in quasi-stationary high-current plasma gun QSPA-T. The pulse duration was 1 ms that is relevant to ELMs and number of pulses was varied from one to thirty. The power load was 0.7 MJ/m2 that is below the tungsten melting temperature (Tm). Two tungsten samples were used, namely, polycrystalline tungsten without (W) and with pre-existing He-induced W ‘fuzz’ (Wf). Similar to the steady state plasma, the presence of He in tungsten leads to a reduction of the D retention during transient events at temperature below Tm. We explained it as interruption of the D diffusion towards to the bulk of tungsten by (i) the strain field induced by He bubbles and (ii) the formation of interconnected He bubbles at high temperature which leads to an open porosity for accelerated D desorption, thus, decreasing the D influx into the tungsten bulk. But as the He retention in Wf decreases below 1019 He/m2, the effect of He on the D retention after the plasma gun irradiation disappears: the D retention in W and Wf is the same after 30 pulses of the exposure to pure D plasma. In both cases of pure D and He seeded D plasma gun exposures, the D retention is higher compared to the steady state plasma exposure at sample temperature above 600 K.

Vapour shielding of liquid-metal CPS-based targets under ELM-like and disruption transient loading

This paper presents experimental studies of plasma-surface interactions during powerful plasma impacts of a quasi-stationary plasma accelerator (QSPA) on the Sn capillary porous systems (CPSs) in conditions simulating disruption and edge localized modes (ELM) like loads. Experiments were carried out using two QSPA devices. ELM-like plasma exposures were performed with QSPA-M test-bed facility. A large-scale QSPA Kh-50 device was used to simulate plasma disruptions and giant ELMs. Variation of the plasma stream energy density has been performed to study the onset of vapour shield. It is shown that during plasma exposures of a Sn-CPS target with the QSPA plasma load <1 MJ m−2, single dust particles traces have been registered. A further increase in the heat load leads to the splashing of the eroded material. For ELM-like impacts, a rather weak melt motion was observed on the target surface. A post-mortem analysis has shown that the CPS structure was not destroyed in the course of many repetitive ELM-like pulses. Surface morphology has changed from a smooth surface to corrugation structures with the formation of some cavities in mesh cells due to the influence of the surface tension and capillary effects. Spectral lines of Sn I and Sn II have been identified by optical emission spectroscopy in the near-surface plasma. A plasma shield, that consists mostly of Sn neutrals appears at Q ∼ 0.1 MJ m−2. An increase in the surface heat load resulted in the intensive emission of Sn II lines, which started to be observed at Q ∼ 0.3 MJ m−2. The plasma electron density near the surface increases significantly at Q > 0.5 MJ m−2, which corresponds to the strong vapour shielding of the exposed surface. A comparison between the obtained results on the vapour shielding of Sn CPS and available numerical simulation using the TOKES code has been performed.

Implementation of Soft Computing Technique for Recovery of Impulsive Heat Loads

The knowledge of surface heat flux over aerodynamic surfaces is highly desirable for high-speed applications. Impulse test facilities like shock tubes and shock tunnels are invariantly employed for this where the aerodynamic test models experience step/ramp heat loads. Contrary to conventional methods, the usage of an advanced soft computing technique through an adaptive neuro-fuzzy inference system (ANFIS), for recovery of such surface heat loads, is theme of this paper. A coaxial thermal sensor is fabricated in house from chromel and constantan alloy. This E-type thermal probe is subjected to known heat flux (2–3.5 W) of laser light in an exclusive experimental setup, and the temperature responses are recorded. The simulations are also performed to get the temperature history for these heat loads. The experimental and computational results, either separately or together, are used to train the ANFIS network. The time-averaged values of heat flux obtained from ANFIS-based recovery shows excellent agreement in trend and magnitude (uncertainty band of ) with the applied heat load. The present studies demonstrate the possible use of a soft computing technique for heat flux recovery in short-duration experiments within a desired accuracy level by using training data obtained experimentally or computationally or both.

Perspective of analogy between heat loads and impurity production in L-mode discharges with ICRH in WEST

• RF sheaths due to Ion Cyclotron Resonance Heating (ICRH) on tokamaks. • RF sheaths induce DC potential rectification and increase incident ions energy. • We can be in an ion-dominated regime when magnetically connected to ICRH antennas. • Then heat fluxes and sputtering are carried by ions and an analogy can be relevant. • In ion-dominated regime, infrared and spectroscopic data can become correlated. This study compares experimental observations on plasma surface interactions and heat loads induced by Ion Cyclotron Resonance Heating (ICRH) in WEST medium size tokamak. When powering ICRH, core contamination by metallic impurities systematically results in an increase of the power radiated, often hardening the achievement of long discharges in H-mode at high power. The radiofrequency (RF) sheaths are excited mainly at proximity of ICRF antennas, resulting in an increase of metallic impurity sources. In the presence of RF sheaths, we show that most of the power reaching the wall can become carried by ions. Therefore, both heat loads and material sputtering are the result of ion fluxes on the various plasma facing components, such that some analogy between both phenomena can be made. By combining infrared with spectroscopic measurements, we show experimental evidences from WEST of RF-induced heat fluxes and their correlation with local production of impurity.

Thermal management of a quasi-optical mirror in a millimetre-wave gyrotron

• A millimetre-wave mirror of gyrotron was thermally analysed considering three different surface heat loads on mirror surface. • The heat load on reflecting surface of the mirror were considered: i) over the full reflecting surface, ii) over the circular cross section similar to that of RF beam, and iii) over the circular cross section in Gaussian distribution. • Thermal images were captured under these three considerations. Values and locations of highest temperature attained on the mirror were observed. Maximum temperature reached in three considerations of heat load were 348.72, 330.53 and 391.64 °C, respectively. • Further, three cooling channel routings: i) cooling cavity, ii) rectangular serpentine, and iii) circular serpentine were implemented behind the mirror. The rectangular serpentine cooling channel was the most effective for the considered millimetre-wave mirror. • The maximum temperature reached under consideration of Gaussian heat load was below 400 °C (allowable maximum temperature). Therefore, the present cooling proposal would be suitable for the heat load of 5 kW on reflecting surface of the mirror. A quasi-optical mirror of a millimetre-wave gyrotron was thermally analysed considering three different surface heat loads on mirror surface. The heat load on reflecting surface of the mirror were considered: i) over the full reflecting surface, ii) over the circular cross section similar to that of RF beam, and iii) over the circular cross section in Gaussian distribution. Thermal images were captured under these three considerations. Values and locations of highest temperature attained on the mirror were observed. Maximum temperature reached in three considerations of heat load were 348.72, 330.53 and 391.64 °C, respectively. Further, three cooling channel routings: i) cooling cavity, ii) rectangular serpentine, and iii) circular serpentine were implemented behind the mirror. The rectangular serpentine cooling channel was the most effective for the considered millimetre-wave mirror. The maximum localised temperature reached under consideration of Gaussian heat load was below 400 °C (allowable maximum temperature). Therefore, the present cooling proposal would be suitable for the heat load of 5 kW on reflecting surface of the mirror.

Fast-ion physics in SPARC

Potential loss of energetic ions including alphas and radio-frequency tail ions due to classical orbit effects and magnetohydrodynamic instabilities (MHD) are central physics issues in the design and experimental physics programme of the SPARC tokamak. The expected loss of fusion alpha power due to ripple-induced transport is computed for the SPARC tokamak design by the ASCOT and SPIRAL orbit-simulation codes, to assess the expected surface heating of plasma-facing components. We find good agreement between the ASCOT and SPIRAL simulation results not only in integrated quantities (fraction of alpha power loss) but also in the spatial, temporal and pitch-angle dependence of the losses. If the toroidal field (TF) coils are well-aligned, the SPARC edge ripple is small (0.15–0.30 %), the computed ripple-induced alpha power loss is small ( ${\sim } 0.25\,\%$ ) and the corresponding peak surface power density is acceptable ( $244\ \textrm{kW}\ \textrm {m}^{-2}$ ). However, the ripple and ripple-induced losses increase strongly if the TF coils are assumed to suffer increasing magnitudes of misalignment. Surface heat loads may become problematic if the TF coil misalignment approaches the centimetre level. Ripple-induced losses of the energetic ion tail driven by ion cyclotron range of frequency (ICRF) heating are not expected to generate significant wall or limiter heating in the nominal SPARC plasma scenario. Because the expected classical fast-ion losses are small, SPARC will be able to observe and study fast-ion redistribution due to MHD including sawteeth and Alfvén eigenmodes (AEs). SPARC's parameter space for AE physics even at moderate $Q$ is shown to reasonably overlap that of the demonstration power plant ARC (Sorbom et al., Fusion Engng Des., vol. 100, 2015, p. 378), and thus measurements of AE mode amplitude, spectrum and associated fast-ion transport in SPARC would provide relevant guidance about AE behaviour expected in ARC.

ITER upper visible/infrared wide angle viewing system: I&C design and prototyping status

The ITER Upper Visible/Infrared Wide Angle Viewing System is a key diagnostic system for divertor surface temperature monitoring and heat load calculation during plasma operation. This system will be installed after the first plasma phase in five upper ports with front-end optics under vacuum, relay optics in the interspace, back-end optics and sensors in the port-cells, and data acquisition and processing units in the Diagnostic building. The system is currently in the Preliminary Design phase which includes some prototyping activities. This paper describes the current design status of the plant system Instrumentation and Control (I&C) comprising camera pre-selection, hardware, and software architecture for camera data acquisition and image processing. The proposed design has been elaborated by following the ITER CODAC methodology for Plant System I&C design. On-going prototyping activities addressing identified R&D challenges such as camera high throughput data acquisition and real-time image processing are also presented.

China’s technological achievements on ITER enhanced heat flux first wall in the pre-PA qualification

The Chinese Domestic Agency (DA) is procuring ITER Enhanced Heat Flux (EHF) First Wall (FW) panels which shall withstand a surface heat load up to 4.7 MW/m2 during ITER operation. Prior to the implementation of the ITER Procurement Arrangement (PA), several key technologies in manufacturing the EHF FW panel have been qualified. In the pre-PA task, ITER grade CN-G01 high-purity beryllium has been developed; its chemical composition, mechanical properties and grain size satisfy the ITER requirements. The strict quality control in the manufacturing of bimetallic plate was aimed at increasing the reliability the reliability of hypervapotron (HVT) heat sink for the EHF FW fingers, while a design improvement of the HVT cooling channel improved the thermal fatigue performance. Investigation showed that magnetron sputtering Ti/Cu coating on the beryllium tiles helps in reducing possible defects and results in better quality of Be-Cu hot isostatic press (HIP) bonding interfaces. As a result of such measures, China completed two EHF FW semi-prototypes with one successfully passing the high heat flux test (HHFT) at 4.7 MW/m2 for 7500 cycles and 5.9 MW/m2 for 1500 cycles. Neither damage to the semi-prototype nor off-normal surface temperature increase was observed, which brought the semi-prototype qualification campaign to a successfully conclusion.

CHARACTERIZATION OF MACROSCOPIC EROSION OF CASTELLATED AND FLAT TUNGSTEN SURFACES UNDER ITER-LIKE TRANSIENT PLASMA LOADS

The damages of tungsten targets with different geometries under repetitive transient hydrogen plasma loads have been studied with a quasi-stationary plasma accelerator QSPA Kh-50. The results of the experiments on target with geometry close to ITER divertor reference design have been compared to results of previous experiments on flat target. The plasma stream parameters were relevant to ITER ELMs (surface heat load above the melting (0.6 MJ/m2) and below the evaporation (1.1 MJ/m2) thresholds of tungsten and pulse duration of 0.25 ms). Surface erosion and dynamics of erosion products have been investigated in the course of repetitive plasma pulses. The crack networks and progressive corrugation occurred on the surface of all the targets exposed to a large number of plasma pulses. Melt motion leads to grow of protuberances on edges of castellated target units. Unlike the flat targets, the separation of liquid/solid particles from the edges of the units is the most significant source of the castellated targets erosion.

Deuterium and helium retention in W with and without He-induced W ‘fuzz’ exposed to pulsed high-temperature deuterium plasma

In the present work, helium (He) was incorporated into tungsten (W) samples by inductively coupled plasma (ICP) source above the threshold of He-induced W ‘fuzz’ formation on W surface. Then, W samples with and without nano-structured W ‘fuzz’ were exposed to pulsed heat loads using deuterium (D) plasma in quasi-stationary high-current plasma gun QSPA-T. The pulse duration was 1 ms and number of pulses was varied from one to thirty to simulate ITER transient events with surface heat load parameters relevant to edge-localized-mode (ELM) impacts. The irradiation was performed below and above the W melting threshold. The D and He retention in each irradiated sample was measured by a method of thermal desorption spectroscopy. We examined the impact of (i) ELMs-like events and (ii) formation of He-induced nano-structured ‘fuzz’ on the D retention in W. We found that the D retention was the highest for samples irradiated by plasma gun above the melting threshold after thirty pulses. Moreover, the D retention after 10 pulses of deuterium plasma gun exposure was higher than that after stationary low-energy plasma exposure at sample temperature of either 600 or 700 K indicating the dominate influence of ELM's-like events on the D retention compared to normal operation regime. The D retention in W samples with the presence of He-induced W ‘fuzz’ was slightly smaller than without that after one pulse of plasma gun exposure with heat load below the W melting temperature. The W ‘fuzz’ was not disappear in this loading conditions, only the length and thickness of nano-structured W fibres were reduced by factors of ∼4 and ∼2, respectively. The He concentration in W with W ‘fuzz’ was decreased by a factor of about 3 after one pulse of plasma gun exposure. The results obtained give possibility to assess the particle retention in divertor areas subjected to high thermal loads at different operation regimes.