In this paper, nitrogen dielectric barrier discharge (DBD) plasma was generated in a quartz tube with coaxial wire-cylinder electrodes at atmospheric pressure. By varying the nitrogen gas flow (FN) in the range of 0-1 m 3 /h, the plasma optical emission spectra (OES) were measured and studied. The vibration (T vib) and rotation temperature (Trot) of nitrogen were obtained, by fitting the rovibronic bands of N2(C 3 Πu − B 3 Πg, 0-1), and by the Boltzmann plot method for purposes of comparison. T vib increased up to 2481 K with increasing nitrogen flow till 0.2 m 3 /h, and then decreased with further increasing FN , while Trot decreased monotonously and approached to ∼350 K for FN ≥ 0.6 m 3 /h. The intensity of N2(C 3 Πu − B 3 Πg, 0-0, 1-0, 0-3) and N + 2 (B 2 Σ + u − X 2 Σ + g , 0-0) exhibited similar evolution with increasing FN to those of the T vib and Trot, respectively. The discharge photos revealed that the discharge filaments gradually decreased with increasing FN , and eventually disappeared, which implied that a discharge mode transition emerged with increasing FN. The possible mechanism for the discharge mode transition is studied in detail according to the vibration (T vib) and rotation temperature (Trot) of nitrogen.

The issue of asymmetric modes output of a Ka-band super overmoded coaxial Cerenkov oscillator is analyzed in this paper. Due to serious passband overlapping in a super overmoded coaxial slow wave structure (SWS), the asymmetric competition mode EH11 can hardly be suppressed thoroughly by the methods adopted in moderately overmoded devices, especially in the startup of oscillation. If the output structures reflect the asymmetric modes, the asymmetric mode competition in SWS will be aggravated and the normal operation state will be destroyed. In order to solve this problem, a taper waveguide is inserted at a specific position to achieve the destructive interference of the reflected TM11, and a special support structure is designed to avoid reflection of TE11. With these methods, asymmetric mode competition can be successfully eliminated, and the oscillator is capable of achieving a steady fundamental mode operation performance.

Compactness and miniaturization have become increasingly important in the development of high-power microwave devices. Based on this rising demand, a novel C-band coaxial transit-time oscillator (TTO) with a low external guiding magnetic field is proposed and analyzed. The proposed device has the following advantages: simple structure, short axial length, high power conversion efficiency, and low external guiding magnetic field, which are of great significance for developing the compact and miniaturized high-power microwave devices. The application of a shorter axial length is made possible by the use of a transit radiation mechanism. Also, loading the opening foil symmetrically to both ends of the buncher helps reduce the external magnetic field of the proposed device. Unlike traditional foils, the proposed opening foil has a circular-hole; therefore, the electron beam will not bombard the conductive foil to generate plasma. This makes it possible to realize long pulse and high repetition rate operation of the device in future experiments. Through numerical calculation and PIC particle simulation, the stability of the intense relativistic electron beam (IREB) and the saturation time of the device are improved by using the conductive foil. The voltage and current of the diode are 548 kV and 11.4 kA, respectively. Under a 0.4-T external guiding magnetic field, a C-band output microwave with a frequency of 4.27 GHz and power of 1.88 GW can be generated. The power conversion efficiency of the proposed device is about 30%.

The disk-beam relativistic klystron oscillator (DB-RKO) with the radial-line high frequency structure is one of the attractive high power microwave (HPM) sources due to its specific virtues of weak space-charge effect, high power handling capacity and strong electron collection ability. However, such a device generally exhibits a bulky volume in the radial direction, retarding its actual applications in some compact occasions. Besides, it usually saturates slowly because of the insufficient beam-wave coupling strength. For this purpose, a compact DB-RKO with a rapid saturation process is proposed and physically designed in this paper. By compressing the axial width of the electron beam drift tube suitably, the beamwave coupling strength is enhanced clearly and the saturation process is accelerated significantly. The whole beam-wave interaction structure is shortened with a radial length of about 5 cm. Besides, in order to increase the device compactness further, an improved magnetic-excited method is presented to enhance the magnetic field in the diode area by introducing a pair of soft-magnets. Furthermore, with the guiding of a low magnetic field of 0.4 T, the compact DB-RKO is experimentally demonstrated to be capable of generating Ku-band HPM radiations with a peak power of 810 MW, a power conversion efficiency of 28%, a center frequency of 14.19 GHz and a pulse duration of about 27 ns. The proposed DB-RKO is promising for the package of the permanent magnets which are preliminarily designed in this paper.INDEX TERMS High power microwave, relativistic klystron oscillator, disk electron beam, low guiding magnetic field.

Spatial coherent combination of multiple high power microwave (HPM) sources is considered as a promising scheme to improve the equivalent radiation power of the HPM system dramatically. Relativistic klystron amplifier (RKA) is one of the most suitable sources for the coherent power combination owing to its specific capabilities of stable microwave frequency and controllable phase. However, the RKAs operating at high frequency-band are severely limited by the problems of the intense space-charge effect and radio frequency breakdown. The radial-line HPM sources driven by the disk-shape electron beam may provide the potential to alleviate this issue due to its attractive features of the weak space-charge effect, the high power handling capacity, and the strong electron collection ability. In this paper, a disk-beam relativistic klystron amplifier (DB-RKA) is proposed and physically designed aiming to generate long-pulse HPM radiation at Ku-band. The physical idea, design principles, and simulation results are presented in detail. In a preliminary experiment, the disk-shape intense electron beam is well focused with an axial-width of 1.2 mm by an improved magnetic-excited method. Furthermore, the DB-RKA is demonstrated to be capable of generating Ku-band HPMs typically with peak power of 320 MW, pulse duration of 100 ns, and gain of 42 dB.