Contributions to Plasma Physics最新文献

The formation of longitudinal, nonlinear structures in an interacting ion beam, spin-1/2 quantum, electron-ion plasma is reported by employing a separate spin evolution quantum hydrodynamic model (SSE-QHD). The reductive perturbation method is followed to develop the KdV equations. In the presence of an ion beam and keeping the finite electron inertial effects, ion acoustic (IA) soliton and spin electron acoustic (SEA) soliton are found. The critical values of beam speed and spin polarization, required for the existence of solitary structures, are highlighted. It is prescribed that the present model supports rarefactive and compressive SEA soliton and compressive IA soliton. It is shown that the ion beam is essential for the existence of a compressive SEA solitary structure. We have also calculated and compared the energy densities of  both solitons. Furthermore, the dependence of the nonlinear structures on the beam parameters and spin polarization are also examined. Our findings would be useful to understand the new spin-dependent nonlinear structures as well as the features of nonlinear IA waves in ion beam–plasma interactions.

Algorithmic differentiation (AD) is a promising tool to accurately and efficiently compute sensitivities of several code outputs w.r.t. the code input parameters for the complex Monte Carlo code EIRENE. We illustrate the procedure by assessing the sensitivity of the total atom content w.r.t. the atomic and molecular cross-sections and rate coefficients. AD does not suffer from decorrelation between primal and perturbed particle trajectories, which easily happens for finite difference (FD) sensitivity calculations. Consequently, the resulting statistical errors on AD sensitivities are up to five orders of magnitude smaller than the statistical errors on FD sensitivities for a JET low-recycling background plasma. Although this statistical error reduction is promising for the application of the AD sensitivities in a future optimization or uncertainty quantification framework, we show that the AD sensitivities can blow up for a minority of long-lived particles, especially occurring in detachment. Finally, we conclude that the Monte Carlo simulation and estimator type possibly need to be adapted to improve the accuracy of the sensitivities.

Predictions of the scrape-off layer (SOL) plasma conditions from the 2D multi-fluid code EDGE2D-EIRENE are compared to the OEDGE 1D fluid code for JET low-confinement mode (L-mode) plasmas. In the low-recycling divertor conditions (divertor target plasma collisionality $��{\nu }_e�\approx �30�$), OEDGE and EDGE2D-EIRENE agree on the electron temperature, the ion temperature, and the electron density within 10% in the low-field side (LFS) divertor X-point region (divertor SOL) and within 25% in the LFS midplane region. In high-recycling conditions ($��{\nu }_e�\approx �55�,�000�$), the predictions of both codes agree on the electron temperature within 20%, while differences in the ion temperature and electron density increase to as high as 50% in the divertor SOL.

The discharge properties of post-cathode plasma generated by grid electrode discharge were investigated. Post-cathode plasma is the plasma, generated in an open discharge, at the other side of the cathode that is different from the electrode gap. Based on the unique electron transport, discharge chamber parameters were adjusted to compare their impact on the electron density of the post-cathode plasma. The peak of the electron flow in the grid cathode hole and the peak of the post-cathode plasma thickness appear in the same diameter of the grid electrode holes, which shows the crucial influence of the electron transport within the grid cathode on the post-cathode plasma. The study also investigated the combined effects of cathode thickness and gas pressure on post-cathode plasma thickness. The findings suggest that the primary factor influencing post-cathode plasma thickness is the electron transport within the grid cathode.

The self-organization of a magnetized multi-ion plasma, composed of inertialess electrons and inertial H⁺, He⁺, and O⁺ ions, leads to the formation of quadruple Beltrami (QB) field structures. The QB self-organized state is a linear combination of four single Beltrami fields, and it is a non-force-free state that shows strong magnetofluid coupling. Moreover, the QB state is characterized by four relaxed state structures of different length scales. The investigation reveals that the generalized helicities of plasma species and the densities of ion species have a significant impact on the characteristics of the self-organized vortices in the QB state. The study also highlights the potential consequences of QB field structures on earth's inner magnetosphere, including diamagnetic and paramagnetic trends as well as heating effects resulting from disparate length scales.

A grid refinement study is performed for mean-field plasma boundary simulations with fluid neutrals in an EU-DEMO geometry. In general, grid convergence of the simulations is achieved, with differences between the finest and second-finest grid of <5% for the majority of the quantities of interest. The estimated discretization errors for the original 96 × 36 grid are mostly in the 4%–25% range. However, it is also shown that exceptions are possible due to the non-linear nature of plasma boundary codes. Hence, it is advised to regularly perform grid refinement studies on subsets of cases.

An optimized divertor design is crucial to maximize the lifetime of plasma-facing components and reduce costs of future fusion power plants. Numerical shape optimization could be a powerful tool to obtain improved designs in an automated way. However, it is not trivial to apply due to the need of a field-aligned and boundary-fitted grid for simulating the plasma and quantifying the heat load. This paper shows how a grid deformation tool can automate the gridding process while safeguarding the grid quality. Additionally, sensitivities of shape parameters are computed using finite differences and compared to those obtained by remeshing using standard tools such as CARRE2. The plasma and neutral behavior is simulated using the unstructured SOLPS-ITER code with the latest advanced fluid neutrals model for an ASDEX Upgrade test case. The comparison shows that, contrary to the remeshing strategy, the grid deformation approach yields smoother sensitivities. Furthermore, it is shown that the deformed grids have better mesh quality in terms of poloidal cell size ratio compared to the grid generated with CARRE2, which improves the accuracy of the simulation. This supports the use of the grid deformation tool for automated shape design in future work.

The influence of the non-ideality (NI) of the classical plasmas on the doubly excited singlet S states of the helium atom (He) embedded in the plasma has been investigated theoretically. A pseudopotential containing the Debye length and the non-ideality parameter (NIP) as its characteristics is used to represent the screened interaction potentials in the plasma. Using a large wavefunction within the framework of the stabilization method, it has been possible to recognize six doubly excited singlet S states (five lying below the He⁺(2S) excitation threshold and one lying below the He⁺(3S) excitation threshold) for the plasma-free case. The energies and the autoionization life-times of those states are computed by fitting the density of states to the Lorentzian form. Convergence of the computed results is corroborated by increasing the number of terms in the employed wavefunction. For the plasma-free case, these results are in excellent agreement with the established results in the literature. A comprehensive analysis has been made on changes induced on those doubly excited states by varying NI over a wide range. It has been observed that the energies of the states gradually approach the corresponding threshold energies with the increasing NI of the plasma, whereas the change in the life-times (alternatively the widths of the states) of the states shows distinctive features depending on the angular momentum of an individual electron.

A new microwave electrothermal thruster (MET) with small dimensions (length of 25 mm and diameter of 6 mm), utilizing surface wave (SW) plasma at low average power consumption P_av ˜ 2 W, is made. It uses microwaves at a frequency of 2.45 GHz for plasma generation within a dielectric chamber enclosed in a metal shield. The SW is excited at the plasma-dielectric interface by a coaxial launcher and a standing wave is created due to the wave reflection from the shield. The high-density plasma produced in this resonant chamber heats up the argon gas, which expands through the nozzle in the axial direction. The temperature of the gas decreases from 1900 K at atmospheric pressure to 1200 K at 10 Torr ambient pressure. This small-sized MET shows stable low-pressure operation with a thrust of 4 mN and an efficiency of 8% and is suitable for nanosatellite orbit control.

The processes in far scrape-off layer (SOL) and the plasma interaction with the first-wall (FW) elements may notably affect the tokamak discharge, since they define fuel and impurity recycling, material erosion and redeposition, wall surface heating, etc. For a long time, the far SOL description in most plasma edge transport codes was insufficient or absent at all, and so particle and heat fluxes onto the FW (except divertor plates) were out of consideration. Recently some codes, for example, SOLEDGE and SOLPS-ITER are upgraded allowing for the extension of the computational grid up to real walls and for corresponding account of the vacuum vessel shape and all in-vessel elements. The new release of SOLPS-ITER (the version 3.2.0) required a development of a new code data structure and new approach to numerical approximation of fluid equations compatible with unstructured non-orthogonal computational grid. Intensive testing of the new code in different conditions is still required. In the present contribution, such a testing is performed for the EAST disconnected double null (DDN) L-mode discharge. For the first time, the SOLPS-ITER 3.2.0 modeling results with drifts and currents turned on are presented, and a comparison to former SOLPS-ITER version (3.0.8) is performed. The far SOL transport and its effects on the discharge performance are studied by comparing the computational results obtained on several meshes which differ by their width in equatorial midplane. A single null (SN) one (with mesh width limited by distance to the secondary separatrix) was examined versus two DDN meshes (one with actual and one with artificially extended targets to make mesh wider) and a true unstructured (the widest) mesh. The notable difference in results obtained on different meshes appears in those places where plasma density does not vanishes at the computational domain boundaries. For the cases on true unstructured (the widest) mesh, the particle and heat fluxes onto central column, limiters, far SOL part of targets, dome umbrella and other EAST far SOL in-vessel structures are calculated for the first time by SOLPS-ITER, allowing assessment of the plasma interaction with those surfaces.