山下 裕介

Multiparametric study of a thrust force exerted on a model electric (E) sail spacecraft by the solar wind is conducted with a particle-in-cell method. Several effects relevant to the thrust performance are examined with the focus on the accuracy of the obtained results, reflected in the improved initial conditions and physically realistic boundary conditions set at the external boundaries and the near field of a tether. These effects include thrust sensitivity to the solar wind density, temperature, and relative velocity between the solar wind and the spacecraft as well as the influence of the tether potential on the resulting thrust force. The dependence of thrust [Formula: see text] on the distance between the spacecraft and the sun [Formula: see text] is found to have an exponential form approximated by [Formula: see text]. Numerical estimates of the electric current to the E-sail tether mesh give values that are on average about 50% higher than those predicted by the orbital motion limited theory. In addition to single tethers, tether arrays are also studied and shown to have lower thrust per tether that that of a single tether.

Abstract A multi-fluid moment model, a drift-diffusion (DD) model, and a particle-in-cell/Monte Carlo collision (PIC-MCC) model are compared to investigate direct-current breakdown accounting only for singly charged ions and electrons. The key difference between the moment and DD models is that the electron inertial terms are taken into account in the moment model while the DD models neglect the inertia terms. The breakdown voltage results obtained from the multi-fluid moment and PIC-MCC models are in good agreement with each other over a wide range of pd values, where p is the gas pressure and d is the distance between the cathode and anode. The steady-state electron momentum balance reveals the importance of the electron inertial term at low values of pd, showing the invalidity of the DD approximation under such conditions. The results also show that the main electron energy loss mechanism transitions from volumetric (collisional) losses at high pd regime, which corresponds to low reduced electric field, to convective heat to the anode at low pd regime, where the reduced electric field is high.

Ionic liquid electrospray thrusters represent an alternative propulsion method for spacecraft to conventional plasma propulsion because they do not require plasma generation, which significantly increases the thrust efficiency. The porous emitter thruster has the advantages of simple propellant feeding and multi-site emissions, which miniaturize the thruster size and increase thrust. However, the multi-scale nature, that is, nano- to micrometer-sized menisci on the millimeter-size porous needle tip, makes modeling multi-site emissions difficult, and direct observation is also challenging. This paper proposes a simple model for multi-site emissions, which assumes that the ionic conductivity or ion transport in the porous media determines the ion-emission current. The conductivity was evaluated by comparing the experimental and numerical data based on the model. The results suggest that the ionic conductivity of the porous emitter is suppressed by the ion–pore wall friction stress. Additionally, the model indicates that the emission area expansion on the porous emitter creates the unique curve shape of the current vs voltage characteristics for multi-site emissions.

Abstract In electron cyclotron resonance (ECR) thrusters, the plasma mode transition is a critical phenomenon because it determines the maximum thrust performance. In ECR ion thrusters, ionization generally occurs in the magnetic confinement region, where electrons are continuously heated by ECR and confined by magnetic mirrors. However, as the flow rate increases, ionization is also observed outside the magnetic confinement region, and this induces the plasma mode transition. In our previous work, two-photon absorption laser-induced fluorescence (TALIF) analysis revealed that the stepwise ionization from the metastable state plays an important role in the ionization process. However, the distribution of the stepwise ionization has not yet been revealed because of the long lifetime of the metastable state. In this study, this distribution was investigated using one experimental and two numerical approaches. First, TALIF was applied to two types of gas injection with clear differences in thrust performance and ground-state neutral density distribution. In the first simulation, the metastable state particle simulation was used to estimate the excitation rate distribution. In the second study, simulations of the electric field of microwaves were used to estimate the contribution of the stepwise ionization to the plasma density. The experimental and numerical results revealed that the stepwise ionization spreads outside the magnetic confinement region because of the diffusion of metastable particles, and this spread induces the plasma mode transition, explaining the difference between the two types of gas injection.

<p>The authors investigate the discharge chamber of the microwave ion thruster μ10 by using kinetic particle simulation. First, to investigate the plasma phenomena qualitatively, we conduct a particle-in-cell (PIC) simulation model. The simulation results indicate that the distribution of ion density is ring-shaped. To verify the simulation result with the experimental result, the simulation result is compared with the optical emission distribution. In low propellant flow rates, the distribution of ion density agrees with the optical emission distribution. However, in high propellant flow rates, the optical emission distribution is different from simulation results in the waveguide due to the excited neutral particles. In the thruster, the performance strongly depends on the location of injecting the propellant. Hence, to develop the plasma simulation for quantitative comparison with the experiment, the distribution of the neutral density is evaluated by using direct Monte Carlo simulation (DSMC). The results show the neutral density in the waveguide increases corresponding to the ratio of waveguide injection, which indicates that the density is one of the most important parameters for quantitative evaluation with the experiment.</p>