Noboru Yugami

Frequency upshifting of microwave radiation via resonant excitation of plasma oscillations in a thin layer of a time-varying plasma

Interaction of a p-polarized microwave radiation with a thin (compared to the wavelength) plasma layer, whose density slowly (compared to the wave period) grows in time due to ionization by an external source, e.g., laser pulse, is considered. Unlike previous papers on the frequency upshifting of electromagnetic radiation in a time-varying plasma, which treated mainly the cases of unbounded plasma or thick plasma slab, a novel mechanism of frequency upshifting is presented. It is connected with the excitation of free oscillations in the plasma layer at the moment when the plasma density passes the critical value and, thus, plasma resonance in the layer occurs. The frequency of the excited plasma oscillations increases in time following adiabatically the growing plasma frequency, whereas the oscillations themselves gradually radiate. Due to this mechanism, the upshifted electromagnetic radiation with the frequency equal to the maximum value of the plasma frequency in the layer can be effectively generated....

Laser wakefield in low density plasma

The laser wakefiled (LWF) and the relativistic and charge-displacement self-channeling in the comparatively low density plasma were studied. The dynamics of the electron plasma wave (EPW) in LWF was experimentally observed at the electron density of ∼10 16 cm -3 . The two dimensional instantaneous image of EPW was also obtained. The asymmetric 2D image was explained by the modified liner theory. These experimental results suggest that the intensity of ∼10 18 W/cm 2 was achieved in the low density static gas target. This means the possibility of not only the coherent control of EPW, but also the long relativistic and charge-displacement self-channeling with the low density static gas. For the self-channeling, the simple model tells us that the long channel without defocusing could be possible with a tens of TW, ps glass laser system.

Enlarged Frequency Upshift from DC to AC Radiation Converter using Boundary Effect of Plasma-filled Waveguide

Further frequency upshift from a DC to AC radiation converter is analyzed and experimentally observed by taking into account the boundary effect of a plasma-filled waveguide. The radiation frequency depends on both plasma density and cut-off frequency of the waveguide. The observed frequencies of the emitted radiation are in reasonable agreement with the expected theoretical values.

Generation of Sub-picosecond GeV Electron Bunches by Laser Acceleration in Vacuum

The interaction of free electrons with intense laser beams in vacuum was studied using 3D test particle simulation instead of solving analytically the relativistic Newton-Lorentz equation of motions. We found a group of solutions of the equation which show very interesting and unusual characteristics. When the laser intensity is strong enough a0 ≥100, where a0 ≡ eE0 / meωc is a measure of the laser intensity, an electron can be captured into the high-intensity region and accelerated to GeV energy with acceleration gradient of tens of GeV/cm. The accelerated GeV electron bunch is a macro-pulse composed of a lot of micro-pulses. The paraxial approximation equations of the Gaussian laser beam used in the simulation are of high accuracy and the contribution of the high-order correction is almost negligible when the laser beam width w0 ≥ 60.

New Electrodeless Light Source Intensified by Electron Cyclotron Resonance Heating

Brighter and electrodeless light sources are developed by introducing the electron cyclotron resonance (ECR) effect into the discharge area. In the present proof-of-principle demonstration, a microwave source with a frequency of 2.45 GHz, and a maximum power of 1 kW is used. A mercury lamp placed inside the microwave cavity is immersed in a uniform magnetic field, B. At B = 875 G, which satisfies the ECR condition, the light intensity is increased by 28% at an incident microwave power of 800 W. Short-wavelength components are effectively improved relative to long ones.

Ultra short high power microwave generation associated with short pulse laser

It is demonstrated that a short microwave pulse (/spl sim/6 ns) is generated from cw or long pulse microwave source (9 GHz) by the laser produced overdense plasma. The cw microwave propagating in the waveguide is sliced by laser produced plasma at two different points along the waveguide. The generated pulse duration is determined by the group velocity of the microwave in the waveguide and the length between two points. Furthermore, twice the power is generated by adding the two pulses by this mechanism. The results are in good agreement with theoretical prediction.

Further Frequency Upshift in DC to AC Radiation Converter by Perpendicular DC Magnetic Field

Theory of electromagnetic wave radiation by the interaction of a relativistic ionization front with a periodic electrostatic field using a perpendicular DC magnetic field has been developed. The frequency shifts, transmission and reflection coefficients of the radiation are shown to depend on both the plasma density and the magnetic field strength. Compared with an unmagnetized case, further frequency upshift of the emitted radiation is found, although the conversion rate decreases. This mechanism can be used as a new technique for tunable electromagnetic wave radiation sources.

Vp×B electron linear accelerator with TE-wave assisted by plasma for beam stabilization

Abstract The V p × B acceleration scheme with the use of a transverse electromagnetic wave is demonstrated experimentally, in which a pre-ionized plasma is supplemented for obtaining a stable electron beam. The slow wave structure employed here is a dielectric loaded waveguide, and an electron beam in the accelerator induces surface charges on the dielectric. The electron beam on account of acceleration also produces a dilute plasma to neutralize the surface charges. An initial energy gain of 2.5xa0keV for the electron beam is observed from an incident energy of 60xa0keV without any external vertical magnetic field. When an external vertical magnetic field of 1.5xa0G is applied under the same conditions for performing the V p × B scheme, an additional 1.5xa0keV energy gain is observed.