Yoshiyuki Hashimoto

Electron beam behavior in an axially-extracted virtual cathode oscillator

High-power microwave generation in an axially-extracted virtual cathode oscillator has been studied experimentally. The microwave emission of 120 MW at frequencies in the range of 10-12 GHz is observed in the presence of annular electron beam operated at a 300 kV and 20 kA. The electron beam current in the diode region is well characterized by the electron space-charge-limited current in bipolar flow, which is well below the critical current defined for diode pinching. The observed microwave emission is accompanied by the strongly pinched electron beam. Essential feature of such a pinching effect for the microwave generation is confirmed by the absence of emission when the pinch of electron beam is suppressed by the application of the axial magnetic field. >

Amorphous Layer Formation on Nickel Alloy Surface by Intense Pulsed Ion Beam Irradiation

The formation of an amorphous layer due to irradiation of intense pulsed ion beams (PIBs) has been studied experimentally. A mixed carbon and fluorine PIB with energy of 180 keV, ion beam current density of 180 A/cm2 and pulse duration of 30 ns is irradiated on a Ni65Cr15P16B4 alloy, resulting in the formation of an amorphous layer on the substrate surface to a depth of 0.66 µ m which is consistent with the thermal diffusion length. Repeated irradiations of PIB lead to the formation of an amorphous phase at depths exceeding 0.66 µ m from the surface. PIB irradiation leads to rapid heating of the substrate surface to its boiling point (2732° C), and the cooling rate from the melting point to the glass transition point is estimated to be 2.2×109° C/s which is larger than the critical cooling rate for amorphous layer formation of nickel alloys.

Generation and Focusing of Intense Ion Beams with an Inverse Pinch Ion Diode

Generation and focusing of ion beams using an inverse pinch ion diode with a flat anode has been studied. The ion beams generated with the inverse pinch ion diode were found to be focused at 120 mm from the anode by the electrostatic field in the diode. The energy and maximum current density of the ion beams were 180 keV and 420 A/cm2, respectively. The focusing angle of the ion beams was 4.3°. The beam brightness was estimated to be 1.3 GW/cm2rad2. The focusing distance of the ion beams was found to be controllable by changing the diameters of the anode and cathode.

Effect of Adsorbed Matter on Intense Pulsed Ion Beam Generation

Generation of intense pulsed ion beams with high purity has been studied experimentally. Impurity ions in the ion beams were found to be caused by adsorbed matter on the anode and residual gas molecules in the diode chamber. The first few shots without breaking vacuum in the diode chamber were found to be effective to remove the adsorbed matter on the anode, and lead to higher purity of the ion beams. No effect on the ion beam current was observed upon changing the residual gas pressure in the range from 10-2 to 10-3 Pa or upon leaving the anode in atmosphere for ~10 h. Residual gas molecules in the diode chamber had little effect on the purity of the ion beams. After the first 5 shots, the current of the ion beams was found to be 3.0 kA. The most dominant species in the ion beams was F2+.

Stability of an Intense Pulsed Ion Beam during Successive Operation.

Stability of an intense pulsed ion beam during successive operation was studied experimentally with an inverse pinch ion diode. When an anode of metals or plastics is used for an ion source, the diode impedance and ion beam current were changed with every shot of operation. In the case of a copper anode, the ion current at 10 shots was reduced to one-half or one-third that at the first shot. Using a soda-lime glass ( Na2OCaO5SiO2), on the surface of which vacuum oil or Aquadag was coated as the ion source, the diode impedance and the ion current remained constant for 20–30 shots. With the vacuum oil ion source, the most dominant species of the ion beam was C3+, which has energy in the range of 150–200 keV. The current density of the ion beam at 120 mm behind the anode was about 100 A/cm2.

High-Frequency, High-Power Microwave Generation by a Virtual Cathode Oscillator

High-frequency, high-power microwave radiations by a virtual cathode oscillator have been observed using the “point pinch diode” which can produce a high electron beam density. The peak power of microwave pulses is typically 350 MW in the frequency region of 20-22 GHz. The radiation frequency increases almost linearly from 20 to 35 GHz with decreasing anode-cathode gap spacing. The mechanism for microwave emission is ascribed to the oscillation of the virtual cathode.