Search

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.

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 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.

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 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.

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.

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.