One of the driving forces behind the development of cold plasma sources at atmospheric pressure is their application in the biomedical field. In this respect, radio-frequency (RF) plasma jets are of particular importance due to their possible safe operation on humans and the generation of the high amount of reactive species. For this reason, we designed an RF plasma jet in co-axial geometry with the possibility of aerosol introduction, where its characteristics were evaluated by electrical diagnostics, optical emission and laser scattering spectroscopy. The RF plasma jet operation and stability of diffuse mode were analysed based on energy balance. It was observed that α -mode diffuse discharge characterised by an effluent length up to 5 mm was sustained at a power density below 30 W cm ⁻³ . The gas and rotational temperature were determined by means of spectroscopy methods and compared with the results of direct laser scattering. It was established that the gas temperature obtained from N ₂ emission of transition C ³ П _u → B ³ П _g (0, 2) is highly overestimated whereas the gas temperature estimated from OH transition A ² Σ+ → X ² П _i (0, 0) gave a reasonable agreement with both Rayleigh and Raman spectroscopy. Based on the Rayleigh scattering method, uniform gas temperature distribution in the discharge effluent was found at a power below 15 W with the average temperature below 340 ± 15 K. The low gas temperature of argon plasma jets would allow use of this source in temperature-sensitive material applications including skin treatments.

A V-band coaxial relativistic transit time oscillator is presented. Different from the traditional transit time oscillator, in order to improve the power handling capacity, we replace the traditional standing wave extractor with a traveling wave output structure. The operation characteristic of modulation cavity is also different from traditional transit time oscillators. The electron load conductance of modulation cavity is designed to be positive to reduce the maximum surface electric field. The device is investigated by particle-in-cell (PIC) simulation and preliminary experiment. In PIC simulation, an output microwave of 689 MW at 60.16 GHz is obtained with a diode voltage of 400 kV and beam current of 5.0 kA. The saturation time is 22 ns and efficiency is 34.5%. In experiment, the radiation pattern of the system is measured, and output power is calculated. A 135 MW V-band microwave at 60.20 GHz is obtained with a diode voltage of 405 kV and beam current of 5.1 kA.

A Ku-band relativistic transit-time oscillator with a three-cavity extractor and a distance-tunable reflector was proposed in our previous work [Ling et al., Phys. Plasmas 24(1), 013103 (2017)]. Recently, a new experiment was carried out on this device with an intense relativistic electron beam generator that produces electron beams with a duration of 55 ns and voltage of 0.4–1 MV. In preliminary experiments, mode competition was found with three main frequencies of which the reasons are analyzed theoretically and experimentally. The results show that plasmas generated from the collector's surface are most likely to cause this problem. To solve this problem, the collector structure is extended axially. With an input voltage of 600 kV and a current of 11.7 kA, a microwave output power 1.9 GW with pulse width 26 ns and frequency 12.42 GHz is obtained under a 0.7 T guiding magnetic field. These experimental results effectively confirm the potential of high power capacity of this proposed Ku band transit-time oscillator.

In recent years, high-power microwave (HPM) generator with a low working magnetic field has caught more attention as this device has the merit of low energy consumption and the potential of using a permanent magnet package in the HPM system. However, the conversion efficiency of this device is generally low because the electron beam will fluctuate transversely and the modulation depth will decline when the guiding magnetic field is low. To obtain concurrently high-power, high-efficiency, and low operating magnetic field, a transit-time oscillator (TTO) with two bunchers is designed and fabricated. Two bunchers are adopted in the TTO to modulate the electron beam twice. By introducing the second buncher, the modulation depth of the electron beam increases by 23%, and the conversion efficiency increases from 25% to 40.5% in simulations. A microwave with a power of 3.65 GW and a frequency of 4.31 GHz is obtained with a 600-kV, 15-kA electron beam under the guidance of a 0.5-T magnetic field in simulations. Furthermore, the corresponding experiments are carried out to verify the two-buncher TTO, and a microwave with a power of 3.3 GW and a frequency of 4.31 GHz is obtained. The experimental efficiency is 32% with an external magnetic field of 0.48 T.

A novel dual-band nested transit time oscillator (TTO) is proposed in this paper, which consists of an outer X-band coaxial TTO and an inner Ka-band coaxial TTO. It is capable of generating dual-band microwaves simultaneously with three distinctive features. First, the emission processes of inner and outer annular electron beams are independent. Second, the inner and outer high frequency structures are spatially isolated. Third, the inner and outer annular electron beams share the guiding magnetic field. Particle-in-cell simulation is performed where a voltage of 450 kV and a guiding magnetic field of 0.6 T are used. Simulation results demonstrate that microwave output with a frequency of 8.4 GHz and a power of 1.72 GW is obtained for the X-band, corresponding to a conversion efficiency of 42% and a microwave saturation time of 30 ns. Meanwhile, microwave output with a frequency of 31.3 GHz and a power of 1.18 GW is achieved for the Ka-band, corresponding to a conversion efficiency of 40% and a microwave saturation time of 15 ns.