Ryo Sugihara

Non-Stochastic Heating of Magnetized Plasma by Electrostatic Wave

When an electrostatic wave is suddenly applied, initially trapped particles suffer a rapid, large acceleration as well as the stochastic heating, while initially untrapped particles suffer only the stochastic heating. When the bounce frequency ω B is larger than (ω c ω) 1/2 , the initially trapped particles mainly absorb the wave energy and form a high energy tail. The results are applied to the problem of energetic ion creation in laser fusion plasma and of a high energy tail formation in a plasma heated by the lower hybrid wave.

Electron acceleration by electromagnetic waves in a weakly magnetized inhomogeneous plasma

Suprathermal electrons produced at the resonance absorption layer in a nonuniform plasma are accelerated more efficiently in the direction across both the density gradient and magnetic field in microwave–plasma interaction experiments. The experimental results are interpreted by a new Vp×B0 acceleration mechanism.

Unlimited acceleration of a charged particle by an electromagnetic wave with a purely transverse electric field

Abstract It is shown that a charged particle can be trapped in a magnetic neutral sheet produced by the combination of a static magnetic field B 0 and an electromagnetic wave propagating perpendicularly to B 0 with its magnetic field component parallel to B 0 . The neutral sheet as well as the trapped particle moves with the phase velocity of the wave and the trapped particle feels a constant force. Once the particle is trapped it never detraps and the acceleration continues unlimited. Possible ways of creating such a slow electromagnetic wave which can interact with the particles are briefly discussed.

Electron Acceleration by Longitudinal Electric Field of a Gaussian Laser Beam

It is analytically shown that the longitudinal electric field of a Gaussian laser beam in vacuum accelerates an electron to an ultra-relativistic energy. The most favorable electron is accelerated in a length of twice the Rayleigh range. It is found that the ultimate energy increment of the electron with a single laser beam is given by the product of transverse field intensity and the beam waist. Also stability of the electron orbit and the trapping condition are discussed. The gain can be of the order of 100 MeV per single stage and then a multi-stage acceleration enables TeV-order-acceleration in a length of a few kilometers with the present state of the art.

An Asymptotic Method for the Vlasov Equation. III. Transition from Amplitude Oscillation to Linear Landau Damping

The nonlinear behaviour of Landau damping is investigated numerically. The basic equations are able to describe the amplitude oscillation, linear Landau damping, and intermediate regions of wave behaviour. If q ≡γ L /ω B , where γ L is the linear Landau damping coefficient and ω B the bouncing frequency of electron trapped in the wave, we find that for q ≪1 a plateau is formed in the time dependence of the amplitude of the electric field D , while for \(q{\lesssim}0.5\) a plateau may also be formed. Another kind of plateau appears for q ≈0.77, but for q <0.77 the wave is damped (not necessarily Landaudamped). The time evolution of the number of “trapped” electrons and also of the distribution of resonant electrons is obtained. It is found that even when D reaches its constant asymptotically for q ≪1 the corresponding distribution of resonant electrons is not necessarily uniform in the trapped regions.

New acceleration mechanism of electrons by an electromagnetic wave in a weakly magnetized plasma

Abstract Acceleration of electrons by an electromagnetic wave has been observed in a weakly magnetized (ωce/ω0 ≲ 10−2) inhomogeneous plasma. This acceleration is interpreted as a Vp × B0 acceleration, which is a new concept for heating or accelerating electrons very efficiently to high energy.

Enhancement of damping of a large amplitude wave

By evaluating the exact population difference between resonant electrons with velocities smaller and larger than the phase velocity, a correction to Landau damping proportional to the square of the wave amplitude is obtained theoretically.

Interaction between particles and a wave in a system of magnetic-trapping acceleration by an electromagnetic wave

The wave-particle interaction is investigated for Vp×B acceleration or magnetic-trapping acceleration by an electromagnetic wave. We present a condition for efficient particle acceleration: the trapped particles are accelerated efficiently if the number density of the imposed particles and the time interval for the acceleration are appropriate. When the condition is violated, severe modification of the electric field of the wave takes place and the efficient particle acceleration cannot be accomplished.

Initial damping of large amplitude waves

Nonlinear damping rates of finite amplitude electrostatic waves are obtained by using an invariant perturbation method. The result is valid in an initial phase defined by 0<t≲tb≡r/wb (wb is the bounce frequency). As e‖f‖/T (f is the wave potential, and T is the temperature) increases, the nonlinear initial damping becomes significant and it dominates the linear damping rate gl when, for example, e‖f‖/T≳0.3 for k = 0. 2kD (kD the Debye shielding constant) and e‖f‖/T≳0.7 for k = 0.3kD. The theory does not assume a constant slope of the velocity distribution function f0 at the phase velocity vp since higher order derivatives of f0 at vp, ∂nf0/∂vpn (n = 3,5) play an essential role in enhanced damping. First, a dispersion relation is obtained, is solved for the Langmuir wave, and the theory is applied to an ion‐acoustic wave. A simulation study is carried out on the latter wave. The result confirms the validity of the theory.

Gaussian Laser Beam in a Magnetized Plasma

Properties of a laser beam propagating perpendicular to the external magnetic field in a plasma are investigated theoretically. The beam is represented by a Helmholtz equation in almost all plasma parameter space. For the upper hybrid wave region the wave equation turns to a hyperbolic partial differential equation. It is shown that both equations can be exactly solved by using a method of separation of variables and that solutions having Gaussian profiles lateral to the propagation direction are obtained. An appreciable difference between two solutions appears in their phase factors. Owing to the asymmetry of the configuration around the axis of wave propagation the cross sections of beams take shapes of ellipse and each beam has two different focus points.