Fumiko Otsuka

High-Density Helicon Plasma Sources: Basics and Application to Electrodeless Electric Propulsion

The development of unique, high-density helicon plasma sources is described. Characterization of the largest and the smallest source sizes is made along with a discussion of particle production efficiency using Ar gas. Next, we describe an application of helicon sources to plasma propulsion using a new advanced concept without any eroding electrodes, as a review of our Helicon Electrodeless Advanced Thruster (HEAT) project.

CROSS FIELD DIFFUSION OF COSMIC RAYS IN A TWO-DIMENSIONAL MAGNETIC FIELD TURBULENCE

Cross field diffusion of energetic particles (cosmic rays) in a two-dimensional static magnetic field turbulence is studied performing test particle simulations. Qualitatively different diffusion processes are observed depending on the ratio of Larmor radius (ρ) to the correlation length (λ) of the magnetic field fluctuations. The diffusion is found to be composed of several regimes with distinct statistical properties, which can be characterized using Levy statistics.

Anomalous diffusion of cosmic rays in magnetic field turbulence - Linkage between diffusion statistics and turbulence statistics -

Anomalous diffusion of energetic charged particles (cosmic rays) is studied using a simple two-dimensional cross field diffusion model. Both super-diffusion and sub-diffusion can take place in the model. When typical Larmor radius of the particles is much less than the field correlation length, the particles essentially gradient-B drift along equi-contour lines of the magnetic field strength, and thus the diffusion in this parameter regime can essentially be understood by analyzing statistics of the magnetic field islands composed of these equi-contour lines. We numerically evaluate the statistics of the field islands such as the probability distribution function of island radius and fractal dimension of the island contour lines, as functions of the power-law index of the magnetic field turbulence. We find numerically and analytically the scaling laws of time-scale dependent diffusion coefficients using the parameters obtained by analysis of the field islands statistics.

Penetration of a radio frequency electromagnetic field into a magnetized plasma: one-dimensional PIC simulation studies

Penetration of a radio frequency (rf) electromagnetic field into a magnetized plasma is discussed by performing one-dimensional particle-in-cell (PIC) simulations. We consider two models: an electrostatic (ES) model using an external rf voltage and an electromagnetic (EM) model using an external rf current. The background magnetic field is perpendicular to the one-dimensional system. In the ES model, the external rf voltage across the plasma produces the rf electric field into the plasma. When ωrf=Ωe/2, where ωrf and Ωe are the externally applied field and electron gyrofrequencies, respectively, the ion-rich sheath is formed due to the electron wall loss caused by the electron polarization drift. When ωrf=ωLH/2, where ωLH is the lower hybrid frequency, the electron-rich sheath is formed due to the ion wall loss caused by its larger gyroradius than the electron. In the EM model, an induced electromagnetic field via the external rf current forms a standing wave in the plasma region bounded by the external antennas. When the diameter of the plasma is equal to the wave length, the spatial/temporal profiles of the rf standing waves are well explained by a cold collisionless plasma linear theory except for the existence of the sheath. When the rf magnetic field is increased to 10 % of the background magnetic field, the waveforms become irregular and complex because of the boundary effects.

Development of electrodeless electric propulsion systems using high-density helicon plasmas: The HEAT project

In order to develop completely electrodeless next generation plasma thrusters for deep space missions, we have initiated the HEAT (Helicon Electrodeless Advanced Thruster) project. In our scheme, source plasmas are generated by means of the highly efficient helicon-wave discharge; they are then electromagnetically accelerated using external antennas to yield a thrust. The entire process can be achieved without using any eroding electrodes, leading to plasma thrusters of a limitless lifetime.

Cross-Field Diffusion of Cosmic Rays in Two-Dimensional Magnetic Field Turbulence Models

We discuss cross-field diffusion of energetic particles using compressional and noncompressional two-dimensional turbulence models by performing test particle simulations. For both models, the diffusion coefficient, defined in the classical way, exhibits a timescale dependence, suggesting that the underlining physical process should be described by Levy statistics. The diffusion coefficient for long timescales is classified in terms of the Kubo number, K = bL ⊥/ρ, where b is the standard deviation of magnetic field fluctuation in units of the background field, ρ is the particle Larmor raduis, and L ⊥ is the field turbulence scale length. While the well-known JKG theorem predicts that a particle cannot move more than about one gyroradius normal to the magnetic field in a system with two or less spatial dimensions, the cross-field diffusion does take place in our models since they are exceptions to the theorem: we argue that particle motion on the flux surface is not prohibited in general, and in particular, it is not bounded when the background magnetic field is exactly parallel to the ignorable coordinate.

Effect of field-aligned beam on upstream wave excitation and particle scattering in the earth's foreshock: One-dimensional PIC simulation

The Earths foreshock extends to a large domain of upstream quasi-parallel bow shock, and is characterized by a presence of field-aligned beams (FABs), diffuse ions, ultra-low frequency (ULF) waves, high frequency whistler waves, shocklets, and so on. Kinetic self-consistent numerical simulation is one of the key tools to analyze detailed physics of the foreshock which has not been clearly understood. Because of the necessity of the large simulation domain, a full particle-in-cell (PIC) simulation of quasi-parallel shock has seldom been performed. In this paper we show preliminary results of a long-term and large-scale one-dimensional full PIC simulation of the quasi-parallel collisionless shock with the Alfven Mach number 6.6 and shock angle

Cross Field Diffusion of Cosmic Rays: Dependence on 2‐D Field Turbulence Models

20

Research and Development of Electrodeless Plasma Thrusters Using High-Density Helicon Sources: The Heat Project

degrees. The FAB component is observed far upstream with the beam velocity of 10.5 times the Alfven velocity and the beam density of 0.5 % of the background plasma. This FAB generates right-handed Alfven waves in the plasma rest frame via resonant mode instability, and the excited waves are amplified as approaching the shock during the plasma convection. The number densities of energetic particles for both electrons and ions also increase as approaching the shock.

Numerical Modeling of Electrodeless Electric Thruster by Ion Cyclotron Resonance/Ponderomotive Acceleration

Cross field diffusion is important in efficient diffusive shock acceleration of cosmic rays for quasi‐perpendicular geometry. Cross field diffusion of cosmic rays have been discussed theoretically by quasi‐linear approximation for ‘slab’ geometry of magnetic turbulence (for example, reference [1]). In the solar wind, the presence of quasi‐two dimensional, nearly incompressional turbulence was reported by Matthaeus et. al .We discuss cross field diffusion of energetic particles numerically using compressional (C) and non‐compressional (NC) two dimensional turbulence models. For both cases of C and NC models, the diffusion is found to be composed of several regimes with different statistics. The diffusion coefficient defined in the usual way in each regime exhibits dependence to the time scale, suggesting that the underlining physical process should be described by Levy statistics. When ρ/Lb << 1, where ρ is particle Larmor radius and Lb is field turbulence correlation length, particles tend to follow equi‐...