A Highly Overmoded Structure for Hundred-Kilowatt-Class <italic>Ka</italic>-Band Extended Interaction Klystron

Here, we report on the development of highly overmoded structure for a millimeter-wave (MMW) extended interaction klystron (EIK). To enhance electron beam loading, a new method for designing an oversized beam tunnel in a large cavity by concentrating the axial field is demonstrated. The transmission and oscillation characteristics of the interaction circuit operating in the quasi-TM04 mode are tested by the developed mode conversion circuit. Results suggest that a five-cavity EIK based on this highly overmoded structure can achieve an output power of 289 kW at 32.92 GHz with a saturated gain of 51.6 dB by injecting a 3.3-mm-diameter electron beam with a current of 18 A. The output power exceeds 100 kW at a bandwidth of 100 MHz.</p

NH3-Sensing Mechanism Using Surface Acoustic Wave Sensor with AlO(OH) Film

In this study, AlO(OH) (boehmite) film was deposited onto a surface acoustic wave (SAW) resonator using a combined sol-gel and spin-coating technology, and prepared and used as a sensitive layer for a high-performance ammonia sensor. The prepared AlO(OH) film has a mesoporous structure and a good affinity to NH3 (ammonia gas) molecules, and thus can selectively adsorb and react with NH3. When exposed to ammonia gases, the SAW sensor shows an initial positive response of the frequency shift, and then a slight decrease of the frequency responses. The sensing mechanism of the NH3 sensor is based on the competition between mass-loading and elastic-loading effects. The sensor operated at room temperature shows a positive response of 1540 Hz to 10 ppm NH3, with excellent sensitivity, selectivity and stability

Study of an overmoded structure for megawatt Ka-band extended interaction klystron

For most applications in the millimeter wave band, corresponding to Ka and higher-frequency bands, relatively high atmospheric absorption necessitates the use of high-power sources. Here, a new approach for projecting an oversized beam tunnel in an overmoded structure by concentrating the axial field is demonstrated to meet the high-frequency and high-power demands of compact devices. Due to the enhanced intense beam loading capability of the interaction circuit, a six-cavity Ka-band extended interaction klystron with a four-coupling-hole disk-loaded structure is designed that can stably obtain high output power. An analysis of optimization tradeoffs from introducing high order modes for allowing the application of more powerful beams to improving high order mode field distribution for enhancing the electron-wave coupling and suppressing mode competition is reported. 3D particle-in-cell simulations show attainable output powers of 1.11 MW at 32.94 GHz with a saturated gain of 57 dB by injecting a 3.3 mm diameter electron beam with a current of 24 A.</p

Laser-Induced Damage Initiation and Growth of Optical Materials

The lifetime of optical components is determined by the combination of laser-induced damage initiation probability and damage propagation rate during subsequent laser shots. This paper reviews both theoretical and experimental investigations on laser-induced damage initiation and growth at the surface of optics. The damage mechanism is generally considered as thermal absorption and electron avalanche, which play dominant roles for the different laser pulse durations. The typical damage morphology in the surface of components observed in experiments is also closely related to the damage mechanism. The damage crater in thermal absorption process, which can be estimated by thermal diffusion model, is typical distortion, melting, and ablation debris often with an elevated rim caused by melted material flow and resolidification. However, damage initiated by electron avalanche is often accompanied by generation of plasma, crush, and fracture, which can be explained by thermal explosion model. Damage growth at rear surface of components is extremely severe which can be explained by several models, such as fireball growth, impact crater, brittle fracture, and electric field enhancement. All the physical effects are not independent but mutually coupling. Developing theoretical models of multiphysics coupling are an important trend for future theoretical research. Meanwhile, more attention should be paid to integrated analysis both in theory and experiment

Real-time monitoring of airborne molecular contamination on antireflection silica coatings using surface acoustic wave technology

Real time monitoring of contamination on antireflection (AR) silica coatings in high peak power laser systems (HPLs) is critically needed in order to avoid reductions of transmission and laser damage to optical surfaces. Herein we proposed to apply a surface acoustic wave (SAW) sensor to real-time monitor trace amounts of airborne molecular contaminants (AMCs) adsorbed on the AR silica coatings. The silica coating is found to be susceptible to AMCs because of its mesoporous structure, huge surface area and polar nature. The adsorbed AMCs caused the increased mass on the silica coating of the SAW sensor, which resulted in a significant increase of its frequency shift. The fabricated sensor showed a high sensitivity of ∼-490 mm2 ng−1Hz and an excellent linearity vs. the areal density of adsorbed AMCs since the frequency shift of the sensor is linearly related to the change of mass of the silica coating

Surface acoustic wave ammonia sensor based on SiO2-SnO2 composite film operated at room temperature

Sensitive thin film layers of SnO2, SiO2 and SiO2-SnO2 were deposited on a SAW resonator using sol-gel method and spin coating techniques. Their ammonia-sensing performance operated at room temperature was characterized and their sensing mechanisms were comprehensively studied. When exposed to ammonia, the sensors made of SnO2 and SiO2-SnO2 films exhibit positive frequency shifts, whereas the SiO2 film sensors exhibit a negative frequency shift. The positive frequency shift is related to the dehydration and condensation of hydroxyl groups, which make the films stiffer and lighter. The negative frequency shift is mainly caused by the increase of mass loading due to the adsorption of ammonia. The gas sensor based on SiO2-SnO2 film shows a positive frequency shift of 631 Hz when it is exposed to ammonia with a low concentration of 3 ppm, and it also shows good repeatability and stability, as well as a good selectivity to ammonia compared with gases of C6H14, C2H5OH, C3H6O, CO, H2, NO2, and CH4

Elastic loading enhanced NH3 sensing for surface acoustic wave sensor with highly porous nitrogen doped diamond like carbon film

We proposed a surface acoustic wave (SAW) NH3 gas sensor based on nitrogen doped diamond like carbon (N-DLC) film. The N-DLC film, prepared using a microwave electron cyclotron resonance plasma chemical vapor deposition (ECR-PECVD) method, is highly porous and physically and chemically stable, and have active polar groups on its surface, which can selectively absorb polar NH3 gas molecules. These features of the film lead to the high sensitivity, low noise and excellent stability of the sensor. The sensor can achieve capabilities of in-situ monitoring NH3 in a concentration range from 100 ppb to 100 ppm with fast response (∼5 s) and recovery (∼29 s) at room temperature. The NH3 sensing mechanism is attributed to the decreased porosity of the N-DLC film caused by adsorbed NH3 molecules on its polar groups, which leads an increase of the elastic modulus of the film