Shaomeng Wang

A Wideband Microfabricated Ka-Band Planar Helix Slow-Wave Structure

A Ka-band planar helix slow-wave structure (SWS) which is suitable for microfabrication is proposed and its design is described in this paper. A wideband design is achieved by using dispersion control techniques. The design shows an

Design of a Sheet-Beam Electron-Optical System for a Microfabricated

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better than −20 dB over a 42.8% cold-test bandwidth with discrete input and output ports. Cold-test results for coplanar waveguide ports are also presented. The hot-test parameters of the SWS with discrete ports are investigated using CST Particle Studio. For an elliptical cross-sectional electron beam with the beam voltage of 3.72 kV and the current of 50 mA, a 3-dB small-signal gain bandwidth of 48.5% and a maximum gain of 42 dB are achieved. Saturated power of 26.5 W is achieved by using pitch tapering. Hot-test performance of the proposed SWS including a sever for high gain applications is also presented. Thermal simulations using CST have been carried out showing that the proposed SWS can have very good thermal dissipation properties. RF electric field values at some critical locations in the SWS have also been examined from the point of view of dielectric breakdown.

-Band Traveling-Wave Tube Using a Cold Cathode

A design of a sheet-beam electron-optical system (EOS) for a microfabricated W-band traveling-wave tube (TWT) is presented. The proposed TWT is based on a rectangular field-emitter array (FEA) and a planar helix slow-wave structure with straight-edge connections (PH-SEC). The PH-SEC is designed to work with a 5-kV and 10-mA sheet electron beam. The particle-in-cell simulation results show that the TWT can give 3.7-W peak power at 110 GHz with a 3-dB bandwidth of 60%. The EOS consists of a novel beam-forming electrode, a novel anode, a magnetic circuit to provide solenoidal focusing magnetic field, and a depressed collector. Except for the magnetic circuit, all components are compatible with microfabrication. Moreover, the anode and the collector can be fabricated together with the PH-SEC using the same process steps. Using an FEA with the dimensions of 1000 × 500 μm2, a beam of cross section 160 × 40 μm2 is generated and under ideal conditions 100% transmission is achieved through a 25-mm-long drift tunnel of cross section 240 × 100 μm2. The effects of misalignments of the FEA, anode, and the magnetic circuit are investigated, and the limits on misalignments are determined to achieve satisfactory beam transmission.

Design of a sheet-beam electron gun for a Ka-band microfabricated traveling-wave tube

A novel design of a sheet beam electron gun, suitable for a Ka-band microfabricated traveling-wave tube (TWT), is described in this paper. Simulations take into account emission from a CNT field-emitter array (FEA) and the formation of the sheet beam. The rectangular FEA cathode of 2250×300 μm2 area produces a 10 mA sheet electron beam which is compressed into a 350×150 μm2 area. The configuration of the focusing electrodes and the anode is suitable for microfabrication. For an anode voltage of 3.7 KV, the presented simulation results show a uniform beam at the anode output.

Field emission properties of SiO2-wrapped CNT field emitter

Carbon nanotubes (CNTs) exhibit unstable field emission (FE) behavior with low reliability due to uneven heights of as-grown CNTs. It has been reported that a mechanically polished SiO2-wrapped CNT field emitter gives consistent FE performance due to its uniform CNT heights. However, there are still a lack of studies on the comparison between the FE properties of freestanding and SiO2-wrapped CNTs. In this study, we have performed a comparative study on the FE properties of freestanding and SiO2-wrapped CNT field emitters. From the FE measurements, freestanding CNT field emitter requires lower applied voltage of 5.5 V μm-1 to achieve FE current density of 22 mA cm-2; whereas SiO2-wrapped field emitter requires 8.5 V μm-1 to achieve the same current density. This can be attributed to the lower CNT tip electric field of CNTs embedded in SiO2, as obtained from the electric field simulation. Nevertheless, SiO2-wrapped CNTs show higher consistency in FE current than freestanding CNTs. Under repeated FE measurement, SiO2-wrapped CNT field emitter achieves consistent FE behavior from the 1st voltage sweep, whereas freestanding field emitter only achieved consistent FE performance after 3rd voltage sweep. At the same time, SiO2-wrapped CNTs exhibit better emission stability than freestanding CNTs over 4000 s continuous emission.

Spatial power combining of Ka-band microfabricated TWTs based on a planar helix slow-wave structure

Spatial power combining of two Ka-band microfabricated traveling wave tubes (TWTs) is reported. The TWTs are based on planar helix slow-wave structure with straight-edge connections (PH-SEC). Transition from a waveguide to the PH-SEC forms the basis for division of input power from a waveguide to the TWTs and for combining the output power into a waveguide. For a 3.7 KV and 10 mA beam, the PIC simulation results show that the output power can reach 13 W at 30GHz, with a calculated combining efficiency of 98%.

Backward wave oscillator using a planar helix slow-wave structure with straight-edge connections

A novel BWO based on a planar helix slow-wave structure with straight-edge connections is presented. Simulation results show that the operating frequency of the BWO varies from 15.5-20 GHz when the beam voltage is tuned from 4.5 KV to 9KV. A cylindrical annular electron beam is applied to improve the interaction between the beam and the em field.

Design of a Ka-band microfabricated PH-SEC slow-wave structure with coplanar waveguide couplers

An improved Ka-band microfabricated planar helix slow wave structure (SWS) with straight-edge connections (PH-SEC) together with novel coplanar waveguide (CPW) couplers is proposed. The proposed design is mechanically more stable and is simpler to fabricate. The Eigenmode characteristics of the proposed PH-SEC SWS are provided. The proposed CPW couplers can be fabricated together with one of the layers of the PH-SEC SWS, requiring no additional steps in the fabrication process. CST Microwave Studio has been used to study the transmission characteristics of the PH-SEC SWS with 50 periods and the CPW couplers at the input and the output. The simulation results show that for the entire structure, Sn can be less than −20 dB in the frequency range 23–35 GHz, corresponding to a cold-test bandwidth of 40% centered at 29 GHz.

Magnetic circuit for a sheet electron beam Ka-band microfabricated traveling wave tube

A magnetic circuit for a sheet beam Ka-band microfabricated traveling wave tube (TWT) is presented in this paper. The magnetic circuit is designed to provide a uniform solenoidal magnetic field to keep the sheet beam focused. The design of the magnetic circuit has been optimized using the CST EM STUDIO. Neodymium Magnets and Consumet® Electronic Iron have been selected as the materials of magnets and pole pieces in this design, respectively. Simulation results are presented in the form of 3-d vector magnetic field plots. The results show that the magnetic circuit can produce a uniform magnetic field of 0.21 T over a distance of 40 mm. To assemble the magnetic circuit, an aluminum fixture is designed and fabricated to hold the magnets and pole pieces together. Measured results show an excellent match with the simulation results.