Physics

A two-fluid model is derived from the plasma kinetic equations using the moment model reduction method. The moment method we adopt was recently developed with a globally hyperbolic regularization where the moment model attained is locally well-posed in time. Based on the hyperbolic moment model with well-posedness, the Maxwellian iteration method is utilized to get the closure relations for the resulted two-fluid model. By taking the Shakhov collision operator in the Maxwellian iteration, the two-fluid model inherits the correct Prandtl number from the plasma kinetic equations. The new model is formally the same as the five-moment two-fluid model except for the closure relations, where the pressure tensor is anisotropic and the heat flux is presented. This provides the model the capacity to depict problems with anisotropic pressure tensor and large heat flux.

We discuss a family of Vlasov-Maxwell equilibrium distribution functions for current sheet equilibria that are intermediate cases between the Harris sheet and the force-free (or modified) Harris sheet. These equilibrium distribution functions have potential applications to space and astrophysical plasmas. The existence of these distribution function had been briefly discussed in by Harrison and Neukirch 2009, but here it is shown that their approach runs into problems in the limit where the guide field goes to zero. The nature of this problem will be discussed and an alternative approach will be suggested that avoids the problem. This is achieved by considering a slight variation of the magnetic field profile, which allows a smooth transition between the Harris and force-free Harris sheet cases.

In magnetic fusion experiments, a simple technique to evaluate the heat flux on first wall components is a key to controlled plasma surface interaction. The heat flux can be characterized by the peaking factor which is the ratio of the peak heat flux to the average heat flux. The peaking factor can be calculated exactly using simple derivations and standard software tools. This analysis is applied to an Iter class experiment for plasma wall contact during start up phases at 15 MW, in idealised, realistic and misaligned situations. Even though the peaking factors are usually above 10, the peak heat load on the wall remains moderate at a few MW/m 2 .

Many high power electronic devices operate in a regime where the current they draw is limited by the self-fields of the particles. This space-charge-limited current poses particular challenges for numerical modeling where common techniques like over-emission or Gauss Law are computationally inefficient or produce nonphysical effects. In this paper we show an algorithm using the value of the electric field in front of the surface instead of attempting to zero the field at the surface, making the algorithm particularly well suited to both electromagnetic and parallel implementations of the PIC algorithm. We show how the algorithm is self-consistent within the framework of finite difference (for both electrostatics and electromagnetics). We show several 1D and 2D benchmarks against both theory and previous computational results. Finally we show application in 3D to high power microwave generation in a 13 GHz magnetically insulated line oscillator.

Detachment, an important mechanism for reducing target heat deposition, is achieved through reductions in power, particle and momentum; which are induced through plasma-atom and plasma-molecule interactions. Experimental research in how those reactions precisely contribute to detachment is limited. In this work, we investigate a new spectroscopic technique to utilise Hydrogen Balmer line measurements to 1) disentangle the Balmer line emission from the various plasma-atom and plasma-molecule interactions; and 2) quantify their contributions to ionisation, recombination and radiative power losses. During detachment, the observed Hα emission often strongly increases, which could be an indicator for plasma-molecule interactions involving H + 2 and/or H − . Our analysis technique quantifies the Hα emission due to plasma-molecule interactions and uses this to 1) quantify the Balmer line emission contribution due to H + 2 and/or H − ; 2) subsequently estimate its resulting particle sinks/sources and radiative power losses. Its performance is verified using synthetic diagnostic techniques of both detached TCV and MAST-U SOLPS-ITER simulations. Experimental results of this technique on TCV data show a bifurcation occurs between the measured total Hα and the atomic estimate of Hα emission, indicative of the presence of additional Hα due to plasma-molecule interactions with H + 2 (and/or H − ). An example analysis shows that the hydrogenic line series, even Lyα as well as the medium-n Balmer lines can be significantly influenced by plasma-molecule interactions by tens of percent during which significant Molecular Activated Recombination (MAR) is expected.

The role of flux-limited thermal conduction on the fusion performance of the uniaxially-driven targets studied by Derentowicz et al.; Jour. Tech. Phys. 18, 465 (1977) and Jour. Tech. Phys. 25, 135 (1977), is explored as part of a wider effort to understand and quantify uncertainties in ICF systems sharing similarities with First Light Fusion's projectile-driven concept. We examine the role of uncertainties in plasma microphysics and different choices for the numerical implementation of the conduction operator on simple metrics encapsulating the target performance. The results indicate that choices which affect the description of ionic heat flow between the heated fusion fuel and the gold anvil used to contain it are the most important. The electronic contribution is found to be robustly described by local diffusion. The sensitivities found suggest a prevalent role for quasi-nonlocal ionic transport, especially in the treatment of conduction across material interfaces with strong gradients in temperature and conductivity. We note that none of the simulations produce neutron yields which substantiate those reported by Derentowicz et al. Jour. Tech. Phys. 25, 135 (1977), leaving open future studies aimed at more fully understanding this class of ICF systems.

We present a solver of 3D two-fluid plasma model for the simulation of short-pulse laser interactions with plasma. This solver resolves the equations of the two-fluid plasma model with ideal gas closure. We also include the Bhatnagar-Gross-Krook collision model. Our solver is based on PseudoSpectral Time-Domain (PSTD) method to solve Maxwell's curl equations. We use a Strang splitting to integrate Euler equations with source term: while Euler equations are solved with a composite scheme mixing Lax-Friedrichs and Lax-Wendroff schemes, the source term is integrated with a fourth-order Runge-Kutta scheme. This two-fluid plasma model solver is simple to implement because it only relies on finite difference schemes and Fast Fourier Transforms. It does not require spatially staggered grids. The solver was tested against several well-known problems of plasma physics. Numerical simulations gave results in excellent agreement with analytical solutions or with previous results from the literature.

A number of physical processes in laser-plasma interaction can be described with the two-fluid plasma model. We report on a solver for the three-dimensional two-fluid plasma model equations. This solver is particularly suited for simulating the interaction between short laser pulses with plasmas. The fluid solver relies on two-step Lax-Wendroff split with a fourth-order Runge-Kutta scheme, and we use the PseudoSpectral Analytical Time-Domain (PSATD) method to solve Maxwell's curl equations. Overall, this method is only based on finite difference schemes and fast Fourier transforms and does not require any grid staggering. The PseudoSpectral Analytical Time-Domain method removes the numerical dispersion for transverse electromagnetic wave propagation in the absence of current that is conventionally observed for other Maxwell solvers. The full algorithm is validated by conservation of energy and momentum when an electromagnetic pulse is launched onto a plasma ramp and by quantitative agreement with wave conversion of p-polarized electromagnetic wave onto a plasma ramp.

We propose a filter method to generate electron beams of high polarization from bubble and blow-out wakefield accelerators. The mechanism is based on the idea to identify all electron-beam subsets with low-polarization and to filter them out by an X-shaped slit placed right behind the plasma accelerator. To find these subsets we investigate the dependence between the initial azimuthal angle and the spin of single electrons during the trapping process. This dependence shows that transverse electron spins preserve their orientation during injection if they are initially aligned parallel or anti-parallel to the local magnetic field. We derive a precise correlation of the local beam polarization as a function of the coordinate and the electron phase angle. Three-dimensional particle-in-cell simulations, incorporating classical spin dynamics, show that the beam polarization can be increased from 35% to about 80% after spin filtering. The injected flux is strongly restricted to preserve the beam polarization, e.g. <1kA in Ref.[27]. This limitation is removed by employing the proposed filter mechanism. The robust of the method is discussed that contains drive beam fluctuations, jitters, the thickness of the filter and initial temperature. This idea marks an efficient and simple strategy to generate energetic polarized electron beams based on wakefield acceleration

In this work we provide experimental insights into the impact of plasma-molecule interactions on the target ion flux decrease during divertor detachment achieved through a core density ramp in the TCV tokamak. Our improved analysis of the hydrogen Balmer series shows that plasma-molecule processes are strongly contributing to the Balmer series intensities and substantially alter the divertor detachment particle balance. We find that Molecular Activated Recombination (MAR) ion sinks from H + 2 and/or H − are a factor ∼ 5 larger than Electron-Ion Recombination (EIR) and are a significant contributor to the observed reduction in the outer divertor ion target flux. Molecular Activated Ionisation (MAI) may also be significant during detachment. Plasma-molecule interactions enhance the Balmer line series emission strongly near the target as detachment proceeds. This indicates enhancements of the Lyman series, potentially affecting power balance in the divertor. As those enhancements vary spatially in the divertor and are different for different transitions, they are expected to result in a separation of the Lyβ and Lyα emission regions. This may have implications for the treatment and diagnosis of divertor opacity. The demonstrated enhancement of the Balmer series through plasma-molecule processes potentially poses a challenge to using the Balmer series for understanding and diagnosing detachment based only on atom-plasma processes.