H. Song

Theory and experiment of a 94 GHz gyrotron traveling-wave amplifier

Experimental results are presented on the first W-band gyrotron Traveling-Wave Tube (gyro-TWT) developed to exploit the 94 GHz atmospheric window for long-range, high-resolution radar applications. The gyro-TWT is designed to operate in the higher order TE01 mode and is driven by a 100 kV, 5 A electron beam with a pitch angle of v⊥/vz=1 and velocity spread of Δvz/vz=5%. Large-signal simulations predict 140 kW output power at 92 GHz with 28% efficiency, 50 dB saturated gain, and 5% bandwidth. The stability of the amplifier against spurious oscillations has been checked with linear codes. To suppress the potential gyro-BWO interactions involving the TE02, TE11, and TE21 modes, the interaction circuit with a cutoff frequency of 91 GHz has been loaded with loss so that the single-path, cold-circuit attenuation is 90 dB at 93 GHz. A coaxial input coupler with 3% bandwidth is employed with a predicted and measured coupling of 1 dB and 2 dB, respectively. The operating voltage is limited to below 75 kV because o...

Design of a W-band TE/sub 01/ mode gyrotron traveling-wave amplifier with high power and broad-band capabilities

A high-power gyrotron traveling-wave amplifier operating in the low-loss TE/sub 01/ mode has been constructed at the University of California, Davis that will be driven by a 100-kV, 5-A electron beam with a pitch angle (/spl upsi//sub /spl perp////spl upsi//sub z/) of unity and velocity spread of 5%. The amplifier is predicted by large-signal simulations to generate 140 kW at 92 GHz with 28% efficiency, 50-dB saturated gain and 5% bandwidth. The stability of the amplifier from oscillation has been investigated with linear codes. The threshold current for the absolute instability of the TE/sub 01/ operating mode for the chosen operating parameters is predicted to be 10 A. To suppress the potential gyro-backward-wave oscillator interactions, the interaction circuit with a cutoff frequency of 91 GHz has been loaded with distributed loss so that the single-pass attenuation is 90 dB at 93 GHz. The coaxial input coupler has a predicted and measured coupling of 1 and 2 dB, respectively.

A microfabricated klystron amplifier for THz waves

An emerging need for a powerful source of sub-millimeter radiation, also known as terahertz (THz) waves has attracted a lot of attention and research interests. Applications include bio-chemical detection, stand-off explosives detection, and improved communication bandwidth. However, present sources of sub-THz and THz radiation are physically large, such as free electron laser systems, making them less suitable for many applications in the field. Micro-electro-mechanical systems (MEMS) would offer a compact source of sub-millimeter radiation, but traditional micro-fabrication techniques do not allow the precision and accuracy required to fabricate a device such as a klystron with the required tolerances [1]. But recently, new advances in the MEMS fabrication technique of deep reactive-ion etching (DRIE) have made feasible the possibility of a compact, micro-fabricated sub-millimeter radiation source.

A LARGE ORBIT ELECTRON GUN DESIGN FOR A TERAHERTZ HARMONIC GYROTRON

Design of an axis-encircling large-orbit electron gun for a terahertz harmonic gyrotron is presented. Based on a canonical momentum conservation theory, analytical calculations are carried out to determine critical electron gun parameters. This procedure provides a fast optimization process to determine electron gun parameters. The calculated results are compared with electron gun code.

Disease diagnostics of biological tissues using free-space technique in terahertz frequency range

The sensitivity of terahertz absorption to water, biological tissue structure content, and the degree of hydration of diseased tissues are exploited to distinguish between normal and malignant human skin tissues. This can be achieved using free-space technique in terahertz frequency range (110-170 GHz), with the reflection and transmission coefficients, which are converted into the dielectric constant.

New Method of Integrating Periodic Permanent Magnet (Ppm) Assembly in Traveling Wave Tubes (TWTs)

In traveling wave tube (TWT) ampliflers, an axial focusing magnetic fleld is required to keep electrons traveling in a narrow, pencil-like beam over the considerable length of the circuit. Conventionally, this focusing has been accomplished by using a periodic permanent magnet system housing axially polarized ring magnets. Making the structure to this point has been a complicated process consisting of brazing multiple metals together and honing the piece to the desired speciflcations. We present a new method of fabricating this housing structure monolithically using iron, developing magnetically oversaturated housing regions, and saving processing time and efiort.

Absolute instabilities in a high-order-mode gyrotron traveling-wave amplifier

We present a theoretical study of the diversity of the absolute instability behavior and the physical processes involved in a 94-GHz, TE/sub 01/-mode gyro-TWT with distributed wall losses. Methods of absolute-instability control and their significance to the current gyro-TWT experiment at UC Davis are discussed.

Ka-Band Klopfenstein Tapered Impedance Transformer for Radar Applications

The design, fabrication, and characterization of an amulti- section impedance transformer using Klopfenstein tapering method is presented. The transformer is employed in a Ka-band traveling-wave tube (TWT)for radar applications. The Klopfenstein tapering provides the shortest length between the two difierent impedance levels with continuous tapering sections.

Design of a microfabricated TunneLadder slow-wave structure for a 50 GHz millimeter traveling-wave tube

Design and analysis of a slow-wave structure potentially applicable to high power traveling-wave tube (TWT) amplifiers at millimeter-wave frequencies are presented. The circuit characteristics including dispersion relation is predicted. Hot test particle-in-cell (PIC) simulation results will be described employing VORPAL that is based on the conformal finite difference time domain (CFDTD) method.

Stable microwave coaxial cavity plasma system at atmospheric pressure

We present a systematic study of the development of a novel atmospheric microwave plasma system for material processing in the pressure range up to 760 torr and the microwave input power up to 6 kW. Atmospheric microwave plasma was reliably produced and sustained by using a cylindrical resonator with the TM(011) cavity mode. The applicator and the microwave cavity, which is a cylindrical resonator, are carefully designed and optimized with the time dependent finite element Maxwell equation solver. The azimuthal apertures are placed at the maximum magnetic field positions between the cavity and the applicator to maximize the coupling efficiency into the microwave plasma at a resonant frequency of 2.45 GHz. The system consists of a magnetron power supply, a circulator, a directional coupler, a three-stub tuner, a dummy load, a coaxial cavity, and a central cavity. Design and construction of the resonant structures and diagnostics of atmospheric plasma using optical experiments are discussed in various ranges of pressure and microwave input power for different types of gases.