Sadao Nobuhara

Plasma effects on electron beam focusing and microwave emission in a virtual cathode oscillator

The effect of anode and cathode plasmas on the electron beam dynamics in a virtual cathode oscillator is investigated. A cathode plasma is formed immediately after the rise of the electron beam current and is followed by an anode plasma. The anode plasma formation occurs well before beam focusing and microwave emission. Each plasma expands in the diode region with approximately the speed of 2.0 cm//spl mu/s. The electron beam current in the diode region is well characterized by the electron space-charge-limited current in bipolar flow with expanding plasmas in the anode-cathode gap. Particle-in-cell computer simulation reveals that in the presence of anode plasma the annular electron beam is focused down to small radius while oscillating between a real and a virtual cathode. These simulation results agree qualitatively with X-ray measurements of the electron beam current density profile across the anode. Such a focused beam is found to be responsible for the formation of a strong virtual cathode and microwave emission.

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. >

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+.

Beat-Wave Excitation of an Electron Plasma Wave by Counterpropagating Microwaves

An electron plasma wave driven by optical mixing of two oppositely propagating microwaves is experimentally studied. The wave amplitude is resonantly enhanced when the difference in frequency between two incident electomagnetic waves equals the electron plasma frequency. The absolute amplitude and the growth time of the excited waves are found to be 0.8% and 150 nsec ( f p t =50, f p =340 MHz), respectively, at the incident microwave power of 40 kW. The wave amplitude obtained experimentally is in good agreement with that expected from the optical mixing theory with the damping term. The dominant damping term is ascribed to the Landau damping of high-energy electrons in the background plasma.

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.

Parametric Excitation of an Ion Cyclotron Wave and Plasma Heating by a Modulated Electron Beam

An electron beam modulated near the lower-hybrid frequency is injected into a target plasma immersed in a longitudinal magnetic field. The parametric excitation of the electrostatic ion cyclotron wave and lower-hybrid wave is observed. The threshold and the excited wave potentials are measured by the use of a capacitive probe with a small tip capacitance. Substantial heating of ions and electrons is observed with growth of the instability. Such a rapid heating of ions is ascribed to the electrostatic ion cyclotron wave.

Parametric Excitation of a Drift Wave and Associated Ion Heating by a Modulated Ion Beam

An ion beam modulated near the lower-hybrid frequency is injected into a low-β plasma immersed in a longitudinal magnetic field. Parametric excitation of a drift wave and a lower-hybrid wave is observed. From the measurement of the threshold electric field, an rf field parallel to the magnetic field is found to play an important role in the excitation of the instability. Substantial increase of the ion and electron temperatures has been observed with the growth of the instability. Such a rapid heating may be principally due to the drift wave instability driven parametrically.

Parametric excitation of the drift wave by a modulated ion beam

Abstract Parametric excitation of the drift wave is observed by an ion beam modulated near the lower-hybrid frequency. An rf field parallel to the magnetic field is found to play an important role in the excitation of the instability. Substantial increase of the ion and electron temperatures is observed.