Keiichi Kamada

Gyrotron Backward Wave Oscillator Experiments with a Relativistic Electron Beam Using an X-Band Rectangular Waveguide

A mildly relativistic electron beam (500keV, 200A, 10ns) injected into an X-band rectangular waveguide immersed in a uniform axial magnetic field (4-10kG) produced magnetically tunable microwave radiation in the 9-13 GHz frequency range with an estimated output power of 1MW. The frequency range and tunability of the radiated microwave agreed with a theoretical model for a gyrotron backward wave oscillator taking into account the low energy component of the beam electron.

Dependence of Output Power on Cavity Length in a Gyrotron Backward Wave Oscillator

A relativistic electron beam (500 keV, 200 A, 10 ns) generated magnetically tunable microwave radiation in a frequency range of 9-13 GHz when it is injected into an X-band rectangular waveguide immersed in a uniform axial magnetic field (4-10 kG). The mechanism of the microwave radiation was identified as the gyrotron backward wave interaction. The output power of the radiated microwave increased exponentially with the increase of the cavity length.

Collective Acceleration of Barium Ions in a Preformed Anode Plasma

Barium ions in a preformed anode plasma were collectively accelerated up to a peak energy of 270 MeV (2.0 MeV/amu) using an electron beam pulse with peak energy of about 0.4 MeV, peak diode current 16 kA and pulse-width of 10 ns (FWHM).

Cross-Field Propagation of Intense Neutralized Proton Beam near a Metal Plate

An intense neutralized proton beam (70 keV, 18 A/cm2, 200–300 ns) is injected into a magnetic field of 2 kG across the magnetic lines of force where a thin metal plate is placed with its surface parallel to the beam axis and perpendicular to the magnetic lines of force. It is found that the polarization electric field in the beam is shorted out by the plate only when the beam is in contact with the plate, and then the beam is deflected according to gyro-motions of the beam protons; otherwise, the polarization electric field is retained and the beam penetrates into the magnetic field without deflection by polarization drift.

Microwave Power Emission from a Beam-Plasma System

The production of a broadband microwave pulse from the interaction of an intense relativistic electron beam (IREB) with a plasma was studied experimentally. The beam-to-plasma density ratio ( n b / n p ) is an important parameter for the beam-plasma interaction. We found that there was an optimum value for n b / n p at which the power density was maximized. The optimum n b / n p was found to be ∼0.01, when varied in a range from 0.1 to 0.001. A brief consideration for the radiation intensity will be presented in this paper.

Correlation between High-Power Broadband Microwave Radiation and Strong Langmuir Turbulence in an Intense Relativistic Electron Beam-Plasma System

A direct experimental evidence was given for that high-power broadband microwaves radiated from the plasma at the injection of an intense relativistic electron beam strongly correlates to the electric fields in cavitons induced in the plasma. This radiation did not increase linearly with the field energy density in cavitons, which differed from the trend in accordance with the collective Compton boosting model.

Propagation of Intense Pulsed Ion Beams across a Magnetic Field

The propagation of space-charge neutralized pulsed ion beams across the magnetic field was studied experimentally. The beam was generated by a reflex triode, and the ion energy peaked at 130 keV. A transverse magnetic field B ⊥ of up to 3 kG was applied to the drift tube over a distance of 20 cm. The maximum ion current density was approximately 10 A/cm 2 at the entrance of this region. The beam was not impeded by the magnetic field for \(\text{B}_{\bot}{\lesssim}2\) kG in agreement with a theoretical prediction.

Four-stage autoacceleration for a subnanosecond, intense relativistic electron beam

Experiments on four-stage autoacceleration were carried out to generate a subnanosecond, intense relativistic electron beam (IREB). An annular electron beam with energy of 500 keV, current of 5 kA, and pulse length of 12 ns was injected into a series of four coaxial cavities with decreasing lengths. The energy and pulse length of the most accelerated part of the beam electrons were 1.1 MeV and 0.8 ns, respectively. The transmission line theory that was used to explain single-stage autoacceleration process was found to be applicable to the multistage autoacceleration process.

Measurements of broad-band millimeter-wave radiation from an IREB-plasma interaction system

High-power broad-band millimeter-wave radiation is emitted from a plasma in a strong Langmuir turbulence state driven by an intense relativistic electron beam. We measured directivity and spectrum of this radiation with a filter-bank spectrometer, a heterodyne spectrometer, and a filter-waveguide-combination spectrometer covering 18-140 GHz. The directivity measurement indicated that the radiation was relativistically beamed. The observed spectra were nearly flat up to about 40 GHz and declined steeply above 40 GHz. Discussion is given on the experimental results in connection with the collective Compton boosting model proposed by Benford and Weatherall (1992).

Time Evolution of Beam Energy Distribution and Perpendicular Velocity Scattering due to Strong Langmuir Turbulence

Our recent works, which were based on the spectroscopic measurement of strong high frequency electric fields in a plasma, showed that the plasma became a strong Langmuir turbulence state when an intense relativistic electron beam was injected into it. To further confirm this the energy spread and the perpendicular velocity scattering of beam electrons after passing the plasma were measured as well as the strong high frequency electric fields and the electron temperature. The theory of transit-time interactions which deals with the beam scattering in strong Langmuir turbulence was applied to interpret the experimental data. The result again shows that the plasma becomes a strong Langmuir turbulence state. The broadband microwave radiation was also observed simultaneously with the measurement of the perpendicular scattering of the beam electrons. The wider the energy spread and the perpendicular scattering, the stronger the microwave radiation.