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Radio-frequency (RF) inductively coupled negative hydrogen ion source (NHIS) has been adopted in China Fusion Engineering Test Reactor (CFETR) to generate negative hydrogen ions. By introduction the level-lumping method into a three-dimensional fluid model, the volume production and transportation of H- in the NHIS, which consists of a cylindrical driver region and a rectangular expansion chamber, are investigated self-consistently at a large input power (40 kW) and different pressures (0.3–2.0 Pa). The results indicate that with the increase of pressure, the H- density at the bottom of the expansion region first increases and then decreases. In addition, the effect of the magnetic filter is also examined. It is interesting to note that a significant increase in the H- density is observed when the magnetic filter is introduced. As the permanent magnets move towards the driver region, the H- density decreases monotonically, and the asymmetry is enhanced. This work could help to understand the H- distribution under various conditions, and thus to optimize the volume production of negative hydrogen ions in the NHIS.

We studied the work function of cesium deposition and co-deposition with the electronegative element on the plasma grid (PG) using the first-principles calculations. The impurity particles may exist in the background plasma and vacuum chamber wall, and the work function of the PG will be affected. The results indicate that the minimum work functions of pure cesium deposition on Mo (110), W (110), and Mo (112) are reached at a partial monolayer. They are 1.66 eV (σ = 0.56 θ), 1.69 eV (σ = 0.75 θ), and 1.75 eV (σ = 0.88 θ), respectively. An appropriate co-deposition model consisting of cesium with electronegative elements can further decrease the work function. The coverage of cesium and electronegative elements are both 0.34 θ in all the co-deposition models. The F-Cs co-deposition model where the Cs atom and F atom are aligned along the surface normal obtains the lowest work function. They are 1.31 eV for F-Cs on Mo (110), and 1.23 eV for F-Cs on W (110), respectively. The change in work function is linearly related to the change in dipole moment density with a slope of −167.03 VÅ. For pure cesium deposition, two factors control the change in dipole-moment density, one is the electron transfer between adsorbates and the substrate, and another one is the restructuring of surface atoms. There are two additional factors for the co-deposition model. One is the intrinsic dipole moment of the double layer, the other is the angle between the intrinsic dipole moment and the surface. The latter two factors play important roles in increasing the total dipole moment.

The structural, mechanical and thermodynamic properties of tungsten-based alloys, including W0.5Ti0.5,W0.67Zr0.33,W0.666Ti0.1667Zr0.1667,W0.67Hf0.33 and W0.666Ti0.1667Hf0.1667, have been investigated in this paper by first-principles calculations based on density functional theory (DFT). The calculated elastic constants and mechanical stability criteria of cubic crystals indicated that all of these cubic alloys are mechanical stable. The mechanical properties, including bulk modulus (B), shear modulus (G), Young’s modulus(E), ratio B/G, Poisson’s ratio, Cauchy pressure and Vickers hardness are derived from the elastic constants Cij. According to calculated elastic modulus and Vickers hardness, the W0.666Ti0.1667Hf0.1667 alloy has the greatest mechanical strength. The Vickers hardness of these cubic alloys rank as follows: W0.666Ti0.1667Hf0.1667 > W0.67Zr0.33 > W0.666Ti0.1667Zr0.1667 > W0.5Ti0.5 > W0.67Hf0.33. Moreover, calculated ratio B/G, Poisson’s ratio, Cauchy pressure indicated that the ductility of W0.666Ti0.1667Hf0.1667 alloy is the worst among these alloys. The ductility of these cubic alloys rank as follows: W0.67Hf0.33 > W0.5 Ti0.5 > W0.67Zr0.33 > W0.666Ti0.1667Zr0.1667 > W0.666Ti0.1667Hf0.1667. What is noteworthy is that both mechanical strength and ductility of W0.666Ti0.1667Hf0.1667 are greater than pure W. Finally, Debye temperature, melting point and thermal conductivity have been predicted through empirical formulas. All these results will provide scientific data for the study on new product development of electrode materials.

The prototype for a negative hydrogen ion source for neutral beam injection of China Fusion Engineering Test Reactor is being developed at the Southwest Institute of Physics. To study the physics of negative ion beam transport and to optimize the design of the source, the stripping loss and the stray particles’ impacts on the one-RF-driver prototype are analyzed. Collision simulation, including both the beam-gas collisions and the particle-grid collisions, is carried out basing on the results of gas flow evaluation and particle tracing. The stripping loss, the distribution of stray particles and the heat loads are calculated, comparing two configurations of grounded grid (GG) (multiaperture or multislot). At the source filling pressure of 0.3 Pa and the vessel pressure 0.05 Pa, the extraction voltage being 8 kV and the acceleration voltage 200 kV and the extraction grid (EG) magnet peak of ±45 mT, the stripping loss of the 200 A m−2 H− beam can be reduced from 25% to 20% by changing GG from multiapertures to multislots. The H− proportion in the total current at 40 mm after GG, however, shows smaller change than the reduction of the stripping loss possibly because the multislot GG’s larger transparency increases the chance for the stray particles to pass through GG. The total heat load on EG in the two cases with different GG configurations are both around 66 kW, while the GG heat load is reduced from 45 kW for multiapertures to 17 kW for multislots. The study provides good comprehension of the transport process and useful guidance for practical operations.

A negative ion source for the Neutral Beam Injection (NBI) system of China Fusion Engineering Test Reactor (CFETR) is under development in the Southwestern Institute of Physics. The aim is to extract an H– beam of 20 A for a pulse duration of 3600 s at an energy of 200 keV. In this work, the beam optics of the source is investigated with a single beamlet model in the COMSOL environment. The effects of varying the main electromagnetic parameters are studied. The boundary between plasma and beam, namely the meniscus, is calculated with a self-consistent method. The optimal condition at which the beam has a minimum divergence angle is found. Beam divergence is within limits. The magnetic field produced by permanent magnets in the extraction grid is verified to deflect co-extracted electrons effectively, while inducing a slight offset of the ion beam. As a conclusion, the system is capable to operate as required.

In this study, we have modeled the sputtering process of energetic He+ ions colliding with W nano-fuzz materials, based on the physical processes, such as the collision and diffusion of energetic particles, sputtering and redeposition. Our modeling shows that the fuzzy nanomaterials with a large surface-to-volume ratio exhibit very high resistance to sputtering under fusion-relevant He+ irradiations, and their sputtering yields are mainly determined by the thickness of fuzzy nano-materials, the reflection coefficients and mean free paths of energetic particles, surface sputtering yields of a flat base material, and the geometry of nano-fuzz. Our measurements have confirmed that the surface sputtering yield of a W nano-fuzz layer with the columnar geometry of nano-fuzz in cross-section is about one magnitude of order lower than the one of smooth W substrates. This work provides a complete model for energetic particles colliding with the nano-fuzz layer and clarifies the fundamental sputtering process occurring in the nano-fuzz layer.

The thermal diffusion of nanostructured W fuzz layers formed over W divertor targets due to fusion-relevant He+ irradiations remain unclear so far. By analyzing the collision process of energetic He+ and D+ ions in the fuzz layers, we have calculated the temperature distribution and heat flux of fuzz layers under low-energy (10–200 eV) and high-flux (1022–1025/m2⋅s) He+/D+ irradiations. Both the heat flux of fuzz layers and the temperature difference ( Δ T ) between the top and bottom of fuzz layers are determined by the radius of W nanofibers, the thickness of fuzz layers, and the heat load over W targets. Our simulation predicts that under fusion-relevant He+/D+ irradiations at the energy of Δ T is lower than 35 K when the heat load over the divertor varies in the range of

Neutral beam injection (NBI) heating is a significant auxiliary heating method used in Tokamak fusion devices. The material of faraday shield (FS) and accelerator grids in the NBI inductively coupled plasma (ICP) source can be damaged during operation by the high-density hydrogen plasma irradiation, and thus affecting the stability of the NBI system. In this paper, a series of hydrogen plasma exposure experiments are performed on oxygen-free copper (OFC) specimens at 400 K–850 K with ion energy of 20–200 eV and irradiation fluence up to 1.0 × 1025 m−2. Meanwhile, the rate equation model is adopted for numerical simulation of the bubble growth and hydrogen retention. The influence of OFC surface temperature, hydrogen ion energy and fluence on OFC damage are experimentally and numerically investigated. Surface observations show that swell and exfoliation are formed on the OFC samples at 400 K and 600 K by scanning electron microscopy. The hydrogen ion energy varying from 20 to 200 eV at 400 K is observed to have little effect on OFC surface microstructure. The simulative results show that there exist different critical temperatures when the initial bubble radius changes. The bubble surface density rises and the bubble size decreases with increasing temperature (below the critical temperature). In addition, adjacent bubbles get closer to each other with the growth of hydrogen bubbles, and the strong tensile stress is produced inside the surrounding material of hydrogen bubbles. Some cracks caused by hydrogen bubbles appear on the surface of the OFC to relax the pressure-induced stress, ultimately leading to OFC FS/grids material damage. This investigation helps to understand hydrogen retention and failure mechanisms of OFC materials under extreme operation conditions in the NBI devices.

In this work, the O impurity particle adsorption on the Cs/Mo(110) surface, to determine the influence of O on Cs stability on Mo(110) surface and also the work function of Cs/O/Mo(110) surface, has been investigated by performing density functional theory (DFT). There are four independent adsorption sites on Cs/Mo(110) surface for O atom. No matter which adsorption site O was adsorbed, the surface adsorption energy of Cs increased. O atoms attract more charges due to the difference in electronegativity. Although this weakens the interaction between the Cs and Mo atoms, the O atom does connect them like a bridge, resulting in enhanced adsorption of Cs atom. In our model, the surface work function of Cs/Mo(110) is calculated as 1.835 eV. In some cases, the adsorption of O can dramatically reduce the surface work function to 1.633 eV. Conversely, O may also increase it, but this increase is not pronounced. This result suggests that the appropriate addition of O atoms may be beneficial to the development of high-efficiency negative ion sources.

By using scanning electron microscopy and atomic force microscopy, we observe the microscopic surface evolution of W crystals exposed to low-energy (50 eV) He+ irradiations at the temperature of 1050 K. We find that the He+ irradiation leads to the growth and emergence of nanoscale undulations over W (110), and the surface regions with less undulations are dominating the microscopic evolution of W crystals. Analysis shows that the thermodynamic instability of W crystals irradiated is attributed to the He nanobubble driven W surface growth, which has been confirmed by our experimental measurements and He reaction rate model in W. Our study will play a crucial role in understanding the growth process of undulating surface microstructures under fusion-relevant He+ irradiations.

Based on the physical processes, such as the collisions of energetic He ions in a fuzz layer, He+ implantations into W bulk, and surface erosion of W fuzz layer, we have modeled the growth of W fuzz layer over polycrystalline W targets under fusion-relevant He/D ion irradiations. From this modeling, we have found that the W fuzz growth results from He+ implantations into W bulk beneath the fuzz layer, and their subsequent erosion into W bulk. The fast-growing, slow-growing, and equilibrium regions are clearly observed during the growth of fuzz layer. In the equilibrium region, the maximum fuzz layer thickness is formed while the rate of W fuzz growth resulting from He+ implantations into W bulk is quite close to the erosion rate. Our study shows that the W fuzz layer with a large surface-to-volume ratio provides an entire protection against the He+ erosion into W bulk. In this study, we have predicted the maximum fuzz layer thickness at the fusion-relevant He+ dose of ~1030/m2, which is strongly dependent on the free distance of He+ ions in fuzz layer and the surface erosion yield of fuzz layer.

The lattice structure, mechanical properties, electronic structure and thermodynamic properties of W1−xVx alloys (x = 0, 0.0625, 0.125, 0.25, 0.3125, 0.5 and 1) were calculated and simulated by first-principles method based on DFT. The mechanical properties including Poisson’s ratio, elastic moduli and B/G ratio were analysed via V concentration. The simulated results showed the ductility of alloys getting improved while the strength decreased with the addition of vanadium. And we calculated the anisotropy of Young’s modules and ideal tensile strength. The projected density of states is calculated to analyse the electronic structure. Furthermore, thermodynamic properties like Debye temperature and low temperature heat capacity were also calculated in this work. All the results can provide reference for selecting plasma facing materials in fusion reactor design.

Under fusion-relevant He+ irradiations, the W surface temperature is one of the most important parameters for controlling the fuzz growth over the W divertor targets, which is associated with the surface bursting of He nano-bubbles. Using He reaction rate model in W, we investigate the effect of temperature on the growth and surface bursting of He nano-bubbles under low-energy (100 eV) and large-flux (∼1022/m2⋅s) He+ irradiations. Increasing the irradiation temperature from 750 to 2500 K leads to a significant change in both the radius of He nano-bubbles and He retention. At an elevated temperature, He solute atoms prefer to rapidly diffuse into He nano-bubbles, thus affecting their concentration, growth and surface bursting. The decrease in He retention is attributed to an increase in the hop rate of solute He atoms in the W top layer, resulting in the significant He release from the W surface. The radius and density of He nano-bubbles calculated by our model are consistent with our experimental observation.

In order to support the design and manufacture the cesium (Cs) oven for the prototype negative ion source in the Chinese Fusion Engineering Test Reactor-Negative Ion Based Neutral Beam Injection (CFETR-NNBI) system, an advanced cesium oven is being set up at IFS. The design scheme is proposed based on the characteristics of the prototype negative ion source in the CFETR-NNBI system and combined with the international frontier research related to this aspect. Based on the reference to the Padova cesium oven, this article has designed a dedicated nozzle for the cesium oven and a transport channel more conducive to steam transport. A corrosion-resistant Cs reservoir and delivery tube is utilized. Precise control of cesium flux is achieved via remotely controlled switching valves and heating systems. The surface ionization detector is used to detect the cesium flux of the nozzle. And the thermal stability analysis, thermal transient analysis, thermal stress analysis and thermal deformation analysis were carried out on the design. The feasibility of the design is demonstrated through finite element analysis, which provides a theoretical basis for the next step of manufacturing. The cesium flux of oven is simulated and calculated, which provides reference for the subsequent device testing.

The structural, elastic constants, mechanical properties and thermodynamic properties of WTi, WV, W2Zr, WVTi and WVZr alloys were investigated in this paper by using first-principles calculation based on density functional theory(DFT). Bulk modulus, shear modulus, radio B/G, Poisson's ratio and Young’s were derived from elastic data Cij. And the Debye temperature, melting point and minimum thermal conductivity can be obtained by empirical formulas. The Vickers hardness and Cauchy pressure of WTi, WV, W2Zr, WVTi and WVZr alloys were also calculated in this work. This work provides a theoretical basis for employing and further developing WTi, WV, W2Zr, WVTi and WVZr alloys.

The intermittent He/D ion irradiations of polycrystalline W have been performed at the ion energy of 50 eV by changing the time of the single irradiations and the irradiation temperature. All irradiated W specimens have been observed by scanning electron microscopy, and the effect of intermittent He/D ion irradiations on the W fuzz growth has been analyzed. The W fuzz growth over W targets does not show the clear dependence on the intermittent He/D ion irradiations, where the He/D ion fluence of the single irradiations typically varies from 5.0 × 1024 to 2.5 × 1025/m2. However, a slight change in the W surface temperature during the single He ion irradiations significantly affects the W fuzz growth rate. Analysis indicates that W fuzz growth is significantly affected by the total He ion fluence varying from 5.0 × 1024 to 5.0 × 1025/m2 and the irradiation temperature varying from 1100 to 1450 K. This current study will play a crucial role in understanding the W fuzz growth under the periodic He/D ion irradiations of W divertor in fusion reactors, such as ELMs.

Although W fuzz is formed in the divertor region of the fusion reactor, no theory may clearly explain the W fuzz growth mechanism. In this study, we observe the growth process of W fuzz over W crystal under ITER-relevant He ion irradiations. We propose the tensile stress-driven cracking of nano-structured fuzz during the initial growth of W fuzz. We demonstrate that the existence of tensile stress is due to the swelling of He nano-bubbles in the fuzz. After this cracking, the W fuzz breaks away from the planar network and grows over the W surface, where the micro-stress in the W surface layer acts as the driving force.

Faraday shield (FS), a thin-wall metal structure with slits, is used in inductive coupled plasma (ICP) sources to protect the ceramic tube from plasma erosion and eliminate capacitive coupling. Electromagnetic energy is coupled into the driver through the slits, which makes FS an important component of ICP sources. Material and structure, which are critical for FS, should have effects on the distribution of electromagnetic field and temperature. In this contribution, discussions on various materials and structures’ impact on electromagnetic and thermal performance arise. FS made of copper, aluminum, molybdenum and stainless steel, as well as FS of different thickness, slit width, slit numbers and slit shapes, are investigated by numerical simulations referring to the parameters of an actual ICP source. Electromagnetic field, RF power loss, temperature and stress of different FS are compared. The results show that Mo has similar electromagnetic performance with Cu but better thermal performance than Cu for FS. As for structure, RF power transfer efficiency decreases slightly as FS thickens. Appropriate slit width and number are required to allow a sufficient RF coupling and acceptable plasma heat flux to ceramic tube. RF efficiency of z-shaped slotted FS and normally slotted FS is almost the same.

Hα (Balmer-alpha), Hβ (Balmer-beta) and Hγ (Balmer-gamma) spectral line intensities in atomic hydrogen plasma are investigated by using a high-power RF source. The intensities of the Hα, Hβ and Hγ spectral lines are detected by increasing the input power (0–6 kW) of ICPs (inductively coupled plasmas). With the increase of net input power, the intensity of Hα improves rapidly (0–2 kW), and then reaches its dynamic equilibrium; the intensities of Hβ can be divided into three processes: obvious increase (0–2 kW), rapid increase (2–4 kW), almost constant (4–6 kW); while the intensities of Hγ increase very slowly. The energy levels of the excited hydrogen atoms and the splitting energy levels produced by an obvious Stark effect play an important role in the results.

In this study, polycrystalline W, W-Re alloys, and La2O3 and Y2O3 dispersion-strengthened W have been irradiated by our large-power materials irradiation experimental system (LP-MIES) at the irradiation temperature of 1360–1460 K. Our measurements show that the W nano-fuzz layer which is

He-induced W nanofuzz growth over the W divertor target is one of the main limiting factors affecting the current design and development of fusion reactors. In this paper, based on He reaction rate model in W, we simulate the growth and evolution of He nanobubbles during W nanofuzz formation under fusion-relevant He+ irradiation conditions. Our modeling unveils the existence of He nanobubble-enriched W surface layer ( 10 nm). The formation of W nanofuzz at the surface is attributed to surface bursting of high-density He bubbles with their radius of ~4 nm, and an increase in the surface area of irradiated W. Our findings have been well confirmed by the experimental measurements.

A high-density RF ion source is an essential part of a neutral beam injector. In this study, the authors attempt to retrofit an original regular RF ion source reactor by inserting a thin dielectric tube through the symmetric axis of the discharge chamber. With the aid of this inner tube, the reactor is capable of generating a radial magnetic field instead of the original transverse magnetic field, which solves the E × B drift problem in the current RF ion source structure. To study the disturbance of the dielectric tube, a fluid model is introduced to study the plasma parameters with or without the internal dielectric tube, based on the inductively coupled plasma (ICP) reactor. The simulation results show that while introducing the internal dielectric tube into the ICP reactor, both the plasma density and plasma potential have minor influence during the discharge process, and there is good uniformity at the extraction region. The influence of the control parameters reveals that the plasma densities at the extraction region decrease first and subsequently slow down while enhancing the diffusion region.