Research on 3-GW Repetitively Operating Relativistic Backward Wave Oscillator

Relativistic backward wave oscillator (RBWO) is a promising high-power microwave (HPM) generator. In past decades, many efforts have been devoted to improving its practicality, in which the repetitively operating RBWO has drawn much attention. However, two bottlenecks once limited its development. One is the generation of massive anode plasmas or even damage of the collector, and the other is the RF breakdown of electrodynamic structures. This article describes an RBWO which can operate repetitively with 3-GW microwave power by mitigating these two problems. First, a titanium collector, which shows better performance in resisting electron beam bombardment than the conventional stainless steel collector, is adopted. Meanwhile, water-cooling technology is applied to expedite heat dissipation of the collector. Second, since our investigations demonstrate that an anodic mechanism, not the conventional cathodic mechanism, is the main cause for RF breakdown of electrodynamic structures under strong guiding magnetic field, the titanium material is also used to manufacture the electrodynamic structures of the RBWO in order to increase its power capacity. The advantage of the titanium RBWO is verified in experiments. The application of these techniques, as well as some important surface treatment techniques such as polishing, ultrasonic washing, and vacuum baking, leads to the stable repetitive operation of the RBWO with 3-GW output power and 25-ns microwave duration under a repetition rate of 50 Hz. The successive operation time of the device is up to 20 s.

I. Introduction

In past decades, the high-power microwave (HPM) technology has achieved prominent progress, gradually approaching toward realistic applications in civil and military fields. A variety of HPM devices have been developed, among which the relativistic backward wave oscillator (RBWO) possesses the advantages of high microwave power, high conversion efficiency, and reliable repetitive operation mainly due to the good electron beam quality and the application of foil-less diode [1]–[4]. Since the first experiment of RBWO in 1970 [5], the output power and conversion efficiency of this device have experienced enormous increase. The output power has exceeded 10 GW in numerical simulations [6], [7], while the highest power in experiments has reached 8 GW with a frequency of 4.3 GHz, although severe pulse shortening occurred under this microwave power [8]. The conversion efficiency has broken through 70% in numerical simulations [9], coming up with the achievements of relativistic magnetron [10], but with much higher output power. The experimental conversion efficiency of RBWO has reached 47% [11], [12]. Another important performance of RBWO, namely the repetitive operation ability, has also attracted much attention, because it directly determines the application prospect of this device. Since the 1980s in which an RBWO with a microwave power of 450 MW and a repetition rate of 100 Hz was investigated [13], many efforts have been devoted to improve the repetitive operation stability under higher power [14]–[20]. The reported highest power for the repetitive RBWO is 2.5 GW [16], while the highest repetition rate is up to 3.5 kHz [15]. Most repetitive RBWOs operate with a repetition rate of 20–150 Hz. In many experiments, the guiding magnetic field of the RBWO is provided by cooled solenoid, which limits the successive operation time to be not over 1 s [14]–[18]. Even when a superconducting magnet is used, the successive operation time is still limited to be about 1 s by failure of the beam collector and RF breakdown of the electrodynamic structure, while the microwave power is over 1 GW [19].