Yasushi Nishida

Excitation of ion‐acoustic rarefactive solitons in a two‐electron‐temperature plasma

Rarefactive ion‐acoustic solitons have been observed in a two‐electron‐temperature plasma. Some of the characteristics can be interpreted by the solution of the Kortweg–de Vries (K–dV) equation. The Mach number of the solitons is a function of the temperature ratio of hot and cold components.

Vacuum electron acceleration by an intense laser

Using 3D test particle simulations, the characteristics and essential conditions under which an electron, in a vacuum laser beam, can undergo a capture and acceleration scenario (CAS). When a{sub 0} {approx}> 100 the electron can be captured and violently accelerated to energies {approx}> 1 GeV, with an acceleration gradient {approx}> 10 GeV/cm, where a{sub 0} = eE{sub 0}/m{sub e}{omega}c is the normalized laser field amplitude. The physical mechanism behind the CAS is that diffraction of the focused laser beam leads to a slowing down of the effective wave phase velocity along the captured electron trajectory, such that the electron can be trapped in the acceleration phase of the wave for a longer time and thus gain significant energy from the field.

Reflection of a planar ion‐acoustic soliton from a finite plane boundary

Experimental observations on the partial reflection of a planar ion‐acoustic soliton from a plane rigid boundary with sharp density gradients are presented. The reflected wave attenuates rapidly, forming a spherical wave front apart from a small reflector. The reflection coefficient of about 25% at the boundary has no apparent dependence on the incident wave amplitude and the sheath thickness in front of the reflector in the case of the disk reflector, while it has a dependence in the case of the mesh reflector, having even larger values of up to about 50%.

Fast plasma shutdown scenarios in the JT-60U tokamak using intense mixed gas puffing

Fast plasma shutdown without runaway electron generation by gas puffing is investigated in the JT-60U tokamak. Argon-only injection enables a fast shutdown; however, it induces runaway electron generation. Hydrogen-only injection generates much less runaway electrons; however, the shutdown time is considerably longer. Mixed injection of hydrogen and argon achieves a fast plasma shutdown without runaway generation. Argon atoms contribute to radiate the energy of plasma leading to a fast plasma shutdown, whilst hydrogen atoms contribute to increase the electron density for avoiding runaway electron generation.

Study of plasma termination using high-Z noble gas puffing in the JT-60U tokamak

Argon, krypton and xenon were puffed with and without simultaneous hydrogen gas puffing into Ohmically heated plasmas of the JT-60U tokamak with low plasma currents in order to study the capability of disruption mitigation. It was found that krypton gas puffing can provide a plasma termination with smaller amounts of runaway electrons in comparison to argon and xenon gas puffing.

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.

Accurate description of Gaussian laser beams and electron dynamics

In this paper, the higher order corrections to the description of a Gaussian laser field are derived and expressed as power functions of the parameter s ¼ 1=kw0, where k is the laser wave number and w0 the beam width at the focus center. Using the test particle simulation programs, the electron dynamics obtained using the paraxial approximation, the fifthorder correction, and the seventh-order correction are compared. Special attention is given to electron acceleration in vacuum by intense laser beams. The results reveal that, when kw0 J 50, the paraxial approximation field is good enough to reproduce all the electron dynamic characteristics. In the range of 40 K kw0 < 50, the fifth-order corrected field should be used. For very tightly focused laser beams kw0 K 30, one has to utilize seventh-order or higher order corrections to describe more accurately the field of a Gaussian beam. 2002 Published by Elsevier Science B.V.

Plasma wake-field accelerator experiments at KEK☆

Abstract We report on a plasma wake-field accelerator experiment using a high-intensity 250 MeV electron beam of the linac at KEK (National Laboratory for High Energy Physics, Japan). The experiment provedthe plasma wake-fields excited by a train of several bunches accelerate a trailing bunch. We observed an approximately 4 MeV shift of beam energy in a 1 m long plasma with a density of 4 × 1011 cm−3. We describe the present status and the future plan of experimets based on a new scheme of a plasma wake-field accelerator driven by a train of multiple bunches.

Dynamic of ion density perturbations observed in a microwave-plasma interaction

The dynamical behavior of ion density perturbations propagated at low-frequency wave nature is experimentally observed in microwave-plasma interaction. An unmagnetized, inhomogeneous laboratory plasma irradiated by an obliquely incident microwave with maximum power P=10kW and pulse width approximately ion plasma period (τpi≈2π∕ωpi) is studied. The p-polarized electric-field component of the interacted microwave of frequency ω0 leads to a nonlinear phenomenon driven by the ponderomotive force by the process of resonance absorption at the critical layer where ω0=ωp is satisfied. The nonlinear ion density perturbations are created from the resonant layer and propagated to an underdense plasma as an electrostatic wave nature.

A Proof-of-principle Experiment of Laser Wakefield Acceleration

A principle of the laser wakefield particle acceleration has been tested by the Nd : glass laser system with the peak power of 30 TW and the pulse duration of 1 ps. The particle acceleration up to 18 MeV/c has been demonstrated by injecting 1.0 MeV/c electrons emitted from a solid target by an intense laser impact. The corresponding field gradient achieves 1.7 GeV/m.