J. H. Won

Hydrothermal route to ZnO nanocoral reefs and nanofibers

ZnO nanocoral reefs and nanofibers are synthesized on the glass substrate dip coated with ZnO seed with nanoparticles with an average size of 5 nm under a hydrothermal reaction. The ratios of length to diameter for the former and the latter are determined to be 100 and 1000, respectively. In addition, we found that a threshold power density for UV lasing action could be remarkably reduced from 40 kW/cm2 for the nanocoral reefs to 8 kW/cm2 for the nanofibers by increasing the cavity length of ZnO nanowires.

Soft-solution route to ZnO nanowall array with low threshold power density

ZnO nanowall array (ZNWA) has been directionally grown on the buffer layer of ZnO nanoparticles dip-coated on Si-wafer under a soft solution process. Nanowalls on substrate are in most suitable shape and orientation not only as an optical trap but also as an optical waveguide due to their unique growth habit, V[011¯0]⪢V[0001]≈V[0001¯]. Consequently, the stimulated emission at 384 nm through nanowalls is generated by the threshold power density of only 25 kW/cm2. Such UV lasing properties are superior to those of previously reported ZnO nanorod arrays. Moreover, there is no green (defect) emission due to the mild procedure to synthesize ZNWA.

Electron prebunching in microwave magnetron by electric priming using anode shape modification

Electric priming is proposed for electron prebunching in a microwave magnetron and is demonstrated using a particle-in-cell code. When the N∕2-fold perturbation of the radial electric field using an anode shape modification is imposed in the interaction space of the N-cavity magnetron along the azimuthal direction, the electrons initially develop into the desired π-mode resulting in rapid startup. The startup is hastened from 3.2 to 1.2 ns when the radial variation of the protrusion and recession is 6.5% of the anode radius with the angular width of 6°.

Reduction of noise in strapped magnetron by electric priming using anode shape modification

Noise reduction in a 2.45GHz strapped magnetron oscillator is experimentally demonstrated by electric priming using anode shape modification. The sideband noise is reduced by approximately 15dB at the nominal operating current and by 28dB at the start-oscillation current; this is due to electron prebunching into the π mode, resulting from the modulation of the drift velocity of the electrons by an azimuthally periodic electric field. In this experiment, a 4.3kV–330mA half-wave rectified input power is employed.

Dynamic transition between Zn-HDS and ZnO; growth and dissolving mechanism of dumbbell-like ZnO bipod crystal

The dumbbell (DB) ZnO bipod crystal is directly grown from acetate anion intercalated Zn based hydroxyl double salts (Zn-HDS). Our results suggest that the (0001)-to-(0001) plane orientation of the DB ZnO crystal comes from the acetate anions intercalated into the Zn-HDS. The DB ZnO crystal is dynamically changed to tube, nanoplate, and the new Zn-HDS with interlayer spacing of 20.0A by etching the (000-1) surface.

Experimental Investigation on High Order Mode Reflex Klystron using Cold Cathode

Using the property of photonic band gap, we are able to design photonic crystal resonator in which a high order TMmno mode can be stably supported and coupled to an external system without distortion of the high order resonance mode. The concept of this photonic crystal resonator with higher order mode is employed in X-band reflex klystron (PCRK) to demonstrate on experiment that the output power of PCRK is expected to be increased by m times n compared with that of conventional devices at the same operating frequency. Multiple electron beams with the beam voltage and current of 1kV, 100mA are used in PCRK using Spindt-type field emission cathode

Reduction of Noise in Strapped Magnetron by Electric Priming using Anode Shape Modification

Noise reduction in a 2.45 GHz strapped magnetron oscillator is experimentally demonstrated by electric priming using anode shape modification. The sideband noise are reduced by about 15 dB at the nominal operating current and 28 dB at the start-oscillation current due to electron prebunching into the pi-mode resulted from the modulation of the drift velocity of the electron using the azimuthally periodic electric field. In this experiment, a 4.3kV and 330mA half-wave rectified input power is employed

Nonstationary behavior in 10-vane strapped magnetron oscillator

In this paper, the study on the noise is investigated from the nonstationary behavior. The transition from the stationary state to the nonstationary state of the 10-vane strapped magnetron oscillator is observed as the normalized cathode current is increased using the three-dimensional particle-in-cell code, MAGIC3D.

Experimental Study on Electron Guns with High Current Density Beam for THz devices

The experimental study on electron guns with high current density beam for THz devices is carried out. A thermionic electron gun is designed and the fabrication is underway. The current density requirement at the circuit is >50 A/cm2 for 0.1 THz LIGA fabricated devices [1]. A rectangular sheet electron beam for 0.5THz needs to meet the beam current density requirement of >150 A/cm2 [2].

Experimental Study on 0.5THz Superradiant Smith-Purcell Radiation

A novel way of enhancing the Smith-Purcell radiation intensity by the use of counter-streaming electron beams and 1D slit array is introduced. The mechanism of the intensity enhancement is unveiled through numerical simulation. The experimental setup for 0.5 THz is being developed and the details of each part will be presented.