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The Chinese first quasi-axisymmetric stellarator (CFQS), which will be the first quasi-axisymmetric (QA) stellarator in the world, is now under construction. The primary task of the CFQS project is to realize a QA configuration and to examine its physical properties. Based on this task, two important issues were investigated in this work in order to estimate the robustness of the CFQS design from a physical perspective. One was the toroidal field (TF) ripple due to the discreteness of modular coils (MCs) which could potentially degrade the charged particle confinement in the CFQS configuration. The other was a possible MC misalignment in the assembly that would affect the magnetic field and charged particle confinement in the CFQS. Moreover, since the stellarator symmetry might be broken by the MC misalignment, such a case was also investigated in this work. By performing a magnetic field line tracing and charged particle orbit tracing calculation, it was found that the TF ripple does not affect the confinement property significantly and the magnetohydrodynamics equilibrium was robust against possible MC misalignments. These results are helpful in defining the reasonable tolerance of assembly accuracy.

A nonlinear simulation of the energetic particle driven instabilities in the Chinese First Quasi-axisymmetric Stellarator (CFQS) has been conducted for the first time. MEGA, a hybrid simulation code for energetic particles interacting with a magneto-hydrodynamic fluid, was used in the present work. Both the m/n = 3/1 energetic-particle-mode (EPM) like mode and the m/n = 5/2 toroidal Alfvén eigenmode (TAE) were found, where m is the poloidal mode number and n is the toroidal mode number. Four important results were obtained as follows. First, the instability in the CFQS in three-dimensional form was shown for the first time. Second, strong toroidal mode coupling was found for the spatial profiles, and it is consistent with the theoretical prediction. Third, the resonant condition caused by the absence of axial symmetry in CFQS was demonstrated for the first time. The general resonant condition is f mode = Nf ϕ − Lf θ , where f mode, f ϕ , and f θ are mode frequency, particle toroidal transit frequency, and particle poloidal transit frequency, respectively; N and L are arbitrary integers, represent toroidal and poloidal resonance numbers. For EPM-like mode, the dominant and subdominant resonant conditions are f mode = 3f ϕ − 7f θ and f mode = f ϕ − f θ , respectively. For TAE, the dominant and subdominant resonant conditions are f mode = 4f ϕ − 9f θ and f mode = 2f ϕ − 3f θ , respectively. On the one hand, the toroidal resonance numbers are different from the toroidal mode numbers by 2. This indicates that the two-fold rotational symmetry affects the resonance condition. On the other hand, the subdominant resonances satisfy N = n, which is expected for the axisymmetric plasmas and most of the toroidal plasmas including stellarators. Fourth, the nonlinear frequency chirpings in CFQS were demonstrated for the first time. Hole and clump structures were formed in the pitch angle and energy phase space, and the particles comprising the hole and clump were kept resonant with the modes during the mode frequencies chirping.

In the presence of energetic particles (EPs), the Long-Lived Mode (LLM) frequency multiplication with n = 1,2,3 or higher is often observed on HL-2A, where n is the toroidal mode number. Hybrid kinetic-MHD model simulations of the energetic particle (EP) driven kink/fishbone modes on a static HL-2A-like tokamak using NIMROD code find that, when the background plasma pressure is relatively high, and the EP pressure and the beam energy are relatively low, the mode frequency increases almost linearly with EP pressure, and the frequency is proportional to n ("frequency multiplication"), even in absence of any equilibrium plasma rotation. In addition, the frequency multiplication persists as the safety factor at magnetic axis q0 varies. In absence of EPs, the growth rate of the 1/1 mode is the largest; however, as the EP pressure increases, the growth rate of 2/2 modes or 3/3 modes becomes dominant, suggesting that the higher-n modes are more vulnerable to EPs. These results may shed light on the understanding about the toroidal mode number dependence of kink/fishbone modes in the advanced scenarios of tokamaks with weak or reversed central magnetic shear., Comment: 28 pages, 13 figures

The effects of positive and negative bias voltages on the turbulence characteristics in the edge plasmas of J-TEXT are studied using Langmuir probe arrays. The experimental results illustrate that the turbulent particle transport fluxes are suppressed in both positive and negative bias cases, while the suppression mechanisms are quite different. For positive bias (+300 V), long-range correlation (LRC) structures with frequencies of about 10–30 kHz are generated, which are beneficial for turbulent transport suppression. In the −300 V bias case, no LRCs appear, while a strong mean flow shear exists which can reduce the amplitudes of potential and density fluctuations and thus decrease the turbulent transport.

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.

A quasi-axisymmetric stellarator, the CFQS, has been designed as a joint project of the National Institute for Fusion Science and Southwest Jiaotong University to prove intrinsic advantages of quasi-axisymmetry. Principal parameters of the CFQS are as follows: the major radius is 1 m, the magnetic field strength is 1 T, the aspect ratio is 4, and the toroidal periodic number is 2. The magnetic field configuration is designed based on that of the CHS-qa. Enhanced confinement properties within the context of neoclassical theory are achieved by its quasi-axisymmetric configuration. In the entire radial range, the magnetic well is retained to keep favourable stability features in the magnetohydrodynamic equilibrium. A magnetic field coil system was designed for the CFQS, which consists of 16 modular coils, 12 toroidal field coils, and 4 poloidal field coils. The supporting structure is designed to withstand strong electromagnetic force under 1 T operation, maintaining enough space for heating and diagnostic systems. The mock-up modular coil with the most complicated shape was constructed by Hefei Keye Electro Physical Equipment Manufacturing Co., Ltd. to check manufacturability and the achieved accuracy. A heat-run test was performed to check the temperature rise of conductors, and the capability of 1 T operation was confirmed. After various tests for the mock-up coil, construction of actual modular coils and the vacuum vessel has begun.

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.

The toroidal flow damping has been investigated in the edge plasma of J-TEXT tokamaks with an electrode biasing (EB). The estimated viscosity of the toroidal flow after the bias turning off is about 67 times of the classical viscosity and four times of the neo-classical viscosity, which infers that the flow damping is anomalous in the transition phase. In the equilibrium phase, the torque densities driven by the EB, the momentum transport, and the neo-classical toroidal viscosity (NTV) under different bias conditions are calculated, which show that the NTV torques are comparable with the other terms. According to the angular momentum balance equation, the remnant damping torque densities and the effective damping coefficients have been obtained. Considering that the remnant damping is mainly contributed by the neutral particle damping, this investigation provides a method to estimate the neutral particle density ( nH), which gives that the nH in the edge plasma of J-TEXT is in the range of 1–4 (×1017 m−3).

In the presence of energetic particles (EPs) from auxiliary heating and burning plasmas, fishbone instability and Alfven modes can be excited and their transition can take place in certain overlapping regimes. Using the hybrid kinetic-magnetohydrodynamic model in the NIMROD code, we have identified such a transition between the fishbone instability and the β-induced Alfven Eigenmode (BAE) for the NBI heated plasmas on HL-2A. When the safety factor at magnetic axis is well below one, typical kink-fishbone transition occurs as the EP fraction increases. When q0 is raised to approaching one, the fishbone mode is replaced with BAE for sufficient amount of EPs. When q0 is slightly above one, the toroidicity-induced Alfven eigenmode (TAE) dominates at lower EP pressure, whereas BAE dominates at higher EP pressure.

The ion (T i) and electron (T e) temperatures at the last closed flux surface (LCFS) during density scanning discharges and in the scrape-off layer (SOL) with a constant plasma density have been measured in the J-TEXT tokamak using a retarding field analyzer and four-tip Langmuir probe, respectively. Both the LCFS T i and T e are found to reduce with increasing central line-averaged density ( n ˉ e ). The temperature ratio (τ = T i/T e) is about 2 for a low n ¯ e (1.5 → 2.5 × 1019 m−3), decays in a moderate n ˉ e (2.5 → 3.5 × 1019 m−3) and tends to unity at a high n ˉ e (3.5 → 4.2 × 1019 m−3). This suggests that the ions and electrons at the LCFS (even in the SOL) are thermally decoupled at low or moderate n ˉ e and tend to full thermal coupling in the high-density plasmas. In addition, both T i and T e decrease with the radius and T e falls more steeply than T i due to faster parallel energy loss of electrons in the SOL. In contrast, τ rises with the radius, which indicates that the ions and electrons are much more thermally decoupled in the deeper SOL.

The effect of the toroidal flow and flow shear on the m/n = 2/1 tearing mode (TM) has been investigated in J-TEXT tokamak, where m and n are the poloidal and toroidal mode numbers. It is found that the toroidal flow increment is linearly modulated by the bias current/return current. The frequency of the 2/1 TM is also found to be proportional to the toroidal flow during the bias phase, which is consistent with the theoretical expectation. The electron diamagnetic drift frequency is almost constant before and during the bias phase. Moreover, the statistical analysis shows that the saturated island width of the TM is more correlated to the toroidal flow amplitude than the flow shear, which reveals that the effect of the TM depends on the magnitude of the toroidal flow rather than the flow shear. This result may illustrate the process of the 2/1 TM controlled by the electrode biasing, and suggests that changing the magnitude of toroidal flow is a possible method for stabilizing the TM.

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.

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 Chinese First Quasi-axisymmetric Stellarator (CFQS) will be the first operational quasi-axially symmetric stellarator in the world. The physical and engineering complexities led to the cancellation of two famous quasi-axisymmetric stellarators, CHS-qa and NCSX. Therefore, the major mission of the CFQS is to experimentally achieve the canonical quasi-axisymmetric configuration. The CFQS has been designed to possess a number of advanced features in fixed and free-boundary equilibria. It is a compact stellarator with an aspect ratio R/a ∼4.0. The neoclassical diffusion coefficient is similar to that of tokamaks in the collisionless regime. The MHD equilibrium of the CFQS configuration is stable up to volume-averaged normalized pressure β ∼1.1%. A region of the second ballooning stability exists in this facility with a large region of plasma, becoming second stable for β ∼2.7% in free-boundary equilibria. The gap between the first and second stability boundaries is very narrow, which is greatly beneficial for the CFQS operation in the second stable regime with high β plasma. A modular coil system with 16 coils is designed which robustly reproduces the standard quasi-axisymmetric magnetic field.

To date, almost all coil-design codes, e.g. NESCOIL, COILOPT, FOCUS codes, etc, have been primarily attributed to the optimization of filament coils for stellarators. However, evolving to a practical/finite-sized coil from a filament coil, the finite-size effect of coils significantly constrains the fabrication tolerances of a coil system. This paper presents a novel approach that emphasizes the optimization of practical modular coils to reduce sensitivity to fabrication tolerances and to achieve the expected magnetic configurations precisely. A new evaluation parameter, surface twist, is defined in this paper and applied to the optimization sequence in addition to the practical coil line torsion and curvature. The approach has been applied to the framework of the filament coil scheme in the Chinese first quasi-axisymmetric stellarator. This practical coil system without surface twists has been accomplished. Compared to the original finite-sized coil design, the new result is a more considerable simplification of coil shapes, such that in a certain direction view each finite-sized coil becomes a planar-like one. Moreover, this method can also be implemented for the estimation of stochastic deviations of practical coils during the fabrication and assembly of the coil system.

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 retarding field analyzer (RFA) is considered as one of the most effective tools to measure the ion temperature (T i) in the plasma boundary. However, there are some inaccuracies in the measured T i by RFA, due to impurities in the plasma. In this paper, the influences of impurities on the measured T i by RFA have been analyzed in hydrogen isotope plasmas with both low-Z and high-Z impurities. The analytical results show that the T i is underestimated in H+ (also D+ and T+) plasma when the fuel ion charge is taken as the charge number and the analyzed T i in H+ are all larger than that in D+ and T+ plasmas, exhibiting the isotope effect. With the concentration of low-Z impurity (η l) increasing, the analyzed T i decreases and it is about 20% underestimated with η l~12%. Comparison of different charge numbers and masses of low-Z impurities, it is found that larger charge number and smaller mass of low-Z impurities lead to the analyzed T i being more underestimated. The high-Z impurities make almost no impact on the T i, due to their small current contribution. Moreover, the T i is measured by RFA and edge rotational diagnostic (ERD) system through experiments. The results show good agreement with the analytical evaluations.

The effects of misalignment of modular coils on various physical properties in the Chinese first quasi-axisymmetric Stellarator (CFQS) are discussed in this study. To estimate the effects quantitatively, simple assumptions are made regarding the structure of coil displacement. We consider the following three cases: displacement in radial direction (Case A), displacement in the vertical direction (Case B), and displacement by tilting (Case C). In all cases, we assume that the displacement structure has a stellarator symmetry. These assumptions are employed to calculate the change in the magnetic surfaces, rotational transform profile, magnetic well depth profile, and the effective helical ripple. Calculation results show that if the magnitude of displacement is less than 10 mm, the effects on these physical properties are small, and the good neoclassical transport property is retained.

Numerical calculations of resistive internal kink mode with effects of energetic particles (EPs) on HL-2A have been performed using the hybrid kinetic-MHD model inplemented in the NIMROD code. The m/n = 1/1 resistive internal kink mode is unstable in MHD limit. However, with kinetic effects of energetic ions, a fishbone mode is excited with mode frequency around 10 kHz. We calculate the impact of resistivity on the growth rate and frequency of ion fishbone mode, and the results are in good agreement with the analytic solutions, which are obtained by solving the fishbone dispertion relation including resistivity effect. The effects of β frac and cut off velocity of EP on fishbone mode are calculated in detail, where β frac is the ratio of EP pressure to background plasma pressure. This work presents a clear explanation of the stabilizing effect of ECRH on ion fishbone, which is first observed on HL-2A.

The effect of spatial coherence on laser beam self-focusing in the atmosphere to assist delivering powerful laser beams from orbit to the ground is studied. It is found that a fully coherent beam is more strongly compressed on the ground than a partially (spatial) coherent beam (PCB), even so, for a PCB the compressed spot size on the ground may be reduced below the diffraction limit due to self-focusing effect, and a PCB has higher threshold critical power than a fully coherent beam. Furthermore, an effective design rule for maximal compression without beam splitting of the transported PCB from orbit to the ground is presented.

The Chinese First Quasi-axisymmetric Stellarator (CFQS) is a joint project of international collaboration. It is designed and fabricated by Southwest Jiaotong University (SWJTU) in China and National Institute for Fusion Science (NIFS) in Japan. In this work, a plasma boundary and modular-coil system for the CFQS have been designed and optimized via scan of the magnetic configurations with various aspect ratios. The present target parameters of the CFQS are as follows: toroidal periodic number, aspect ratio, magnetic field strength and major radius are 2, 4.0, 1.0 T, and 1.0 m, respectively. The VMEC and NESCOIL codes are employed to obtain the optimum coil design via minimizing the normal component of the magnetic field on the target plasma boundary surface generated by the modular coils. In order to estimate the accuracy of the filament coils, the shape of the plasma boundary, rotational transform, depth of magnetic well and main Fourier components of magnetic field strength produced by the modular-coil system are compared with the target plasma boundary, rotational transform, depth of magnetic well and main Fourier components, correspondingly. This comparison between properties of the coil induced configuration and the target configuration shows a good agreement.

The effects of resonant magnetic perturbation on the triggering and the evolution of the double-tearing mode are investigated by using nonlinear magnetohydrodynamics simulations in a slab geometry. It is found that the double-tearing mode can be destabilized by boundary magnetic perturbation. Moreover, the mode has three typical development stages before it reaches saturation: the linear stable stage, the linear-growth stage, and the exponential-growth stage. The onset and growth of the double-tearing mode significantly depend on the boundary magnetic perturbations, particularly in the early development stage of the mode. The influences of the magnetic perturbation amplitude on the mode for different separations of the two rational surfaces are also discussed.

The effects of asymmetric magnetic perturbation on the triggering and evolution of double tearing modes (DTMs) are investigated using nonlinear magnetohydrodynamics simulations in a slab geometry. We find that for reversed magnetic shear plasmas the resistive reconnection process induced by the initial perturbation at one rational surface can drive a new island at the other rational surface with the same mode number. The four typical states of the mode for the time evolution are found, and include: (i) a linear growth stage; (ii) a linear/nonlinear stable stage; (iii) an interactively driving stage; and (iv) a symmetric DTM stage. These differ from previous simulation results. Moreover, nonlinear DTM growth is found to strongly depend on the asymmetric magnetic perturbation, particularly in the early nonlinear phase. The initial perturbation strength scale of island width suggests that the left island enters into a Sweet–Parker growth process when the right island is sufficiently large to effectively drive the other. These results predict that although externally applied magnetic perturbations can suppress the neoclassical tearing mode they can also trigger new instabilities such as asymmetric DTMs.

From a significant view, considering the thermal non-equilibrium factor, we investigate Kappa (κ) ion temperature anisotropies induced by a low-frequency parallel propagating Alfven wave by combining quasi-linear theory and test particle simulation. Analytic expressions for the ion temperature ratios and are derived for the solar wind, where and denote the perpendicular temperature of species i, parallel temperature of species i, and perpendicular temperature of species j, respectively. The results of our model are broadly consistent, compared to observations of solar-wind measurements. Solar wind helium that meets the condition for strong core heating is nearly seven times hotter than hydrogen, on average. Furthermore, we note that we are able to predict the temperature anisotropies of ions based on their thermal non-equilibrium factors.

The cusp-mirror magnetic structure in a compact fusion reactor (CFR) is investigated to understand the properties of the particle confinement for the first time. Compared with a cascade magnetic mirror device, its advanced performance is shown by means of test particle simulations. Some interesting results are obtained as follows: the adiabatic region and non-adiabatic region are found in the CFR's magnetic configuration. In the non-adiabatic region, due to the magnetic field-free space existing, the ions are scattered stochastically and are not directly guided into the loss cone, unlike the particles around the fixed magnetic lines in the adiabatic region, which decrease the ion loss fraction. The CFR's configuration, combining advantages of cusp-magnetic configuration and mirror-magnetic configuration, leads to confine particles longer than cascade magnetic mirror's. This phenomenon may be relevant to the construction of advanced magnetic-confinement devices.

The effect of spherical aberration on the laser beam self-focusing in the atmosphere to assist delivering powerful laser beams from orbit to the ground is studied. It is found that for the small orbital mirror size, a Gaussian beam with negative spherical aberration might be more strongly compressed without filamentation than that without spherical aberration, and its spot size on the ground can be reduced well below the diffraction limit. However, for the large orbital mirror size, spherical aberration will result in an increase of receiver size on the ground.