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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.

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. >

A simple method to focus ion beams using the "Inverse Pinch Ion Diode" with a flat anode has been studied. In this diode, an electric field in the diode is used as an electrostatic lens to focus ion beans. Ion beams were focused at 120 mm from an anode and it's power density of 0.1 GW/cm/sup 2/ obtained. And the focusing point of ion beams can be changed by varying the diameters of an anode and a cathode.

High-power microwave generation in an axially-extracted virtual cathode oscillator (VIRCATOR) is studied numerically using MAGIC which is a two-dimensional, finite-difference, time-domain particle-in-cell code. The simulation was motivated by the recent experiment of Yatsuzuka et al at the Himeji Institute of Technology in Japan. The Himeji experiment observed that, while pinching of the electron beam was found to be essential for microwave generation, the electron beam current remained below the critical pinching current when microwaves were detected. On the other hand, an earlier experiment by Benford et al showed that microwave emission occurred only when the diode current exceeded the critical current. In the simulation being developed for the Himeji VIRCATOR geometry, the preliminary phase space diagrams show the formation of a virtual cathode. A detailed simulation study is in progress on the effect of bipolar flow on the critical current for beam pinching and microwave emission. Bipolar flow consists of ion flow from the anode into the diode gap in addition to the electron flow and is known to increase the electron space charge limited current.

A new method for repetitive operation of a virtual cathode oscillator is developed. In this scheme, a virtual cathode oscillator in an external axial magnetic field consists of a carbon plate anode (1 mm in thickness) with a hole in the center and a knife-edged annular cathode with a diameter of D/sub c/ = 30 mm. The concept is similar to a reditron except no center part of an anode. We have observed characteristics of a repetitive-typed virtual cathode oscillator with a function of d in gap length, D/sub 0/ in a hole-diameter of an anode, and B in an axial magnetic field strength experimentally. Repetitive microwave radiations more than 450 times have been observed at D/sub 0/ = 32 mm, d = 3.8 mm and B = 0.74 T. The frequency is almost constant to about 250 shots, that is, the microwave frequency of each shot was contained in 11.8 to 12.8 GHz, however, some frequencies has been sometimes observed together in one shot after about 300 shots. The radiation of microwave has not observed rapidly after 480 operations.

Summary form only given. Development of high-brightness photocathodes for a compact infrared free electron laser operated on LEENA (Laser Emitted ElectroN Accelerator) with a plane LaB/sub 6/ cathode and third harmonics of a mode-locked Nd:YLF laser (micropulse width: 10 psec, jitter

Summary form only given. Effect of slow wave structure length on microwave emission from a backward-wave oscillator (BWO) with an intense relativistic electron beam was studied experimentally. The slow wave structure of a circular waveguide with square-wave wall of 57 periods maximum is constructed from individual rings for each period, then its length can be easily changed in any period. The inner and outer radii of the waveguide structure are 1.4 cm and 2.0 cm, respectively. The length of each ring, or one period, is fixed in 0.9 cm, while the length of inner radius section can be exchanged in different length. An annular electron beam with the energy of 300 kV, current of 2 kA, duration of 50 ns, and outer radius of 0.8 cm propagates in the axial magnetic field of 1.5 kG.

Summary form only given. A high-brightness needle photo cathode, being developed for a compact infrared free electron laser for industrial application is studied. A free electron laser operation on LEENA (Laser Emitted ElectroN Accelerator), 6 MeV small Linac at LASTI/HIT was demonstrated with plane LaB/sub 6/ cathode driven by third harmonics of mode-locked Nd: YLF laser. Experimental results and analytic calculations show that the electron beam quality is the major limit to generating sufficient high power FEL output using a compact linac such as LEENA. We are planning to use a high-field needle (tungsten or molybdenum) photo-cathode as a candidate for a new electron gun to improve beam energy spread, beam current and beam emittance. A preliminary experiment on the field-emission tungsten-needle-cathode was performed. Current and voltage properties were in good agreement with Fowler-Nordheim field-emission theory. This result show that generation of high peak current beam of about 10 A is possible by applying 10 to 20 kV on to a needle with diameter less than 1 micron. Matching with a linac may be achieved by the photo-emission mode.

Summary form only given. Metal surface characterization by irradiation of an intense pulsed ion beam (IPIB) has been studied experimentally and numerically. The IPIB irradiation to metals leads to rapid heating of the surface which is immediately followed by rapid cooling and resolidification, resulting in metal surface modification. The authors focus on the reduction of machining roughness on a titanium surface. The experiments were performed with the pulse power generator "HARIMA-II" (400kV, 3/spl Omega/, 50 ns) at the Himeji Institute of Technology. The IPIB was produced with an inverse pinch ion diode that consists of a ring anode of 50 mm in diameters and annular cathode projected from a circular plate of 16 mm in diameter. A 2-mm thick acrylic plate was attached to the anode surface as an ion source. The anode-cathode gap length was 3 mm. A diode chamber was exhausted by an oil diffusion pump to

Summary form only given. High-power microwaves with the peak power of 20 MW, frequency of 12 GHz and pulse duration of 14 ns, which are produced by a virtual cathode oscillation, are sent to a discharge electrode in a stainless steel vacuum vessel. A DC voltage less than a gap-breakdown voltage is applied to the discharge electrode, consisting of a pair of needle and plane electrodes. The gap separation is 1.7 cm. Time and spatial evolutions of breakdown luminosity are observed by a high-speed image converter camera. Microwave induced breakdowns occur at the pressure range less than 760 Torr for negative polarity and less than 600 Torr for positive polarity. An injection of microwaves at first leads to local luminescence at several spots between the gap. Each luminescence grows mainly toward the positive electrode. When these luminescences link together, the air breakdown occurs when the discharge current between the gaps grows suddenly.

Summary form only given. Surface modification of metals with an intense pulsed ion beam (IPIB) was studied experimentally. When the temperature rise of metal surfaces by IPIB irradiation exceeds their boiling point, it is found that machining roughness on surfaces is removed. The experiments were performed with the pulsed power generator "HARIMA-II" (400 kV, 3 /spl Omega/, 50 ns) at Himeji Institute of Technology. The IPIB was produced with the inverse pinch ion diode (IPD) which consists of a ring anode of 50 mm in diameters and an annular cathode projected from a circular plate of 16 mm in diameter. A Teflon plate in thickness of 2 mm was attached to the anode surface as an ion source. The anode-cathode gap length was 3 mm. A diode chamber is exhausted by oil diffusion pump to

Summary form only given. Recently, intense pulsed ion beams (PIBs) have been interested as a tool for surface modification of metals, because irradiation of PIBs to metals leads to rapid heating of the near surface which is immediately followed by rapid cooling and resolidification. In this paper formation of an amorphous layer on a Ni/sub 65/Cr/sub 15/P/sub 16/B/sub 4/ alloy by a PIB irradiation is successfully demonstrated. A PIB composed of C/sup 2+/ and F/sup 2+/ was produced by an inverse pinch ion diode (IPD) with the pulsed power machine "HARIMA-II". Ion beams produced by the IPD were focused by the electrostatic field in the diode region. A shot of the mixed carbon and fluorine PIB was irradiated on a Ni/sub 65/Cr/sub 15/P/sub 16/B/sub 4/ alloy to make amorphous structure. The amorphous nature of the PIB-processed surface was examined by the X-ray diffractometry (Cu K/sub /spl alpha//, 50 kV, 200 mA). The diffraction pattern of the non-processed substrate reveals the crystalline phase which is characterized by the narrow spectrum. On the other hand, the PIB-processed Ni/sub 65/Cr/sub 15/P/sub 16/B/sub 4/ alloy surface exhibits the typical diffraction pattern of the amorphous structure, when the maximum X-ray diffraction depth is within 0.66 /spl mu/m.

Summary form only givem, as follows. Electron beam pinching in relativistic diodes has been widely observed. This pinching of electrons is understood to be caused by the flow of ions from the anode together with the flow of electrons from the cathode. The anode flow is created by the heating of the anode by the electron beam. Such a counter flow of electrons and ions is known as a bipolar flow. A vircator experiment at the Himeji Institute of Technology suggested that microwave emission in the vircator was due to a strongly pinched electron beam caused by bipolar flow. A MAGIC particle-in-cell simulation is being developed to study the effects of electron beam pinching on microwave emission in a vircator. Cathode emission from an annular cathode is modelled in the simulation by placing a plasma on the surface of the cathode and an electric field is applied to accelerate the electrons extracted from the plasma. To model the anode emission, the anode is divided into segments. The ion current is then taken to be a fraction of the electron current through each segment. Preliminary results suggest that the pinched electron beam would form a larger virtual cathode potential inside the waveguide behind the diode which should enhance microwave production. The effect of an axially applied magnetic field will also be studied to determine if the magnetic field would suppress electron pinching and microwave emission as was observed in the Himeji experiment.

Summary form only given. High-power microwave radiation from an axially extracted virtual cathode oscillator has been studied experimentally. The experiments were performed using the pulse power generator HARIMA-II (400 kV, 3 /spl Omega/, and 50 ns). The microwave emission occurs when the beam current exceeds the space-charge-limiting current. The peak microwave power is typically 100 MW in the frequency region of 10-12 GHz using 300 kV, 80 kA annular beam of radius 1 cm. The radiation frequency increases with decreasing A-K gap spacing or cathode radius. The microwave emission is suppressed by an axially applied magnetic field. The microwave radiation was dependent on residual gas pressure. The largest radiation power appears at the gas pressure of p /spl ap/ (1.5-4) /spl times/ 10/sup -5/ Torr. No emission occurs in the region of p /spl les/ 1 /spl times/ 10/sup -5/ and p /spl ges/ /spl times/ 10/sup -5/ Torr. These results suggests that a low-density plasma generated by the electron beam leads to efficiency propagation of the beam, resulting in effective microwave emission.

Summary form only given. Generation of intense pulsed ion beams and improvement of beam quality have been studied for the purpose of ion implantation in material developments. It is found that a deterioration of ion beams quality is caused by adsorbed matter on an anode surface. As the adsorbed matter was removed by bombarding the anode with electrons, the improvement of beam quality was studied by successive shots. The experiments were performed using the pulse power generator HARIMA-II (400 kV, 3 /spl Omega/, 50 ns) at the Himeji Institute of Technology. The ion beams were produced by successive shots at intervals of 7 minutes without breaking vacuum. It was found that cleaning of the anode surface by a few initial successive shots was useful for removal of the adsorbed matter on the anode and for generation of high-purity ion beams.