Ritoku Ando

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.

Effect of In Situ Carbon Coating on ICRF-Heated Tokamak Plasmas Relating to Radiation Loss by Iron-Impurities in JIPP T-IIU

The effect of the C-coating has been demonstrated for tokamak plasmas with high power heating. In situ carbon coating has been made with a glow discharge in a methane/hydrogen mixture gas. Without C-coating, iron contamination has been so severe in megawatts of ICRF heating that the total radiation loss exceeds the rf-heating power, and the electron temperature, after once heating up, then decreases continuously during the latter half of 55 ms rf pulse. With C-coating, the radiation loss is reduced to be one fifth of the rf power and the electron temperature Te becomes stationary at the end of the rf pulse. As a result of this change in Te, the total stored energy increases more than the one without C-coating.

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.

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.

Spectroscopic Measurements of Intense Relativistic Electron Beam Driven Turbulence

High frequency strong electric fields originating from interaction of an intense short-pulse relativistic electron beam with an unmagnetized plasma were measured using two optical diagnostic techniques; the Stark shift measurement and the plasma satellite method. Strong electric fields with Gaussian distribution existed in the plasma. The dimensionless electrostatic energy density W ∼1.1. So the plasma was in a strong Langmuir turbulent state. The strong field regions were found to occupy a few percent of the beam volume. The final scale of caviton was determined to be about 22 λ D . The turbulent state lasted about thirty times as long as the IREB duration. The photon intensity decreased exponentially, while the distribution of electric fields remained the same. These aspects are remain to be explored further.

10 channel grating polychromator for the measurements of electron cyclotron emission

A 10-channel grating polychromator has been developed and applied for the measurements of the second harmonic electron cyclotron emission from the JIPP T-IIU tokamak plasma. The frequency resolution measured at 150 GHz is about 3 GHz which corresponds to the spatial resolution of 1.8 cm near the center of the plasma column. The grating efficiency is about 80% for S-polarization around 160 GHz. Its excellent time resolution, Δt≤2 µs, is favorable for studying MHD activities in hot plasmas. It has been found from the measurements in JIPP T-IIU that the grating polychromator is a powerful diagnostic tool for the measurements of electron temperature of the plasma.

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.