Zhanliang Wang, Huanyu Wang, Ziyuan He, Zhigang Lu, Huarong Gong, Shaomeng Wang, Yubin Gong. S band radial beam coaxial grating backward wave oscillator[J]. High Power Laser and Particle Beams, 2023, 35(11): 113001
- High Power Laser and Particle Beams
- Vol. 35, Issue 11, 113001 (2023)
Abstract
High power microwave devices have been paid a lot of attention because of their potential applications, such as: advance radars, electromagnetic warfare systems, plasma chemistry reactors, material processing instruments, medical/biomedical research instruments[1-8]. At present, significant progresses have been made in this field. Many laboratories, institutes and universities around the world have obtained GW class HPM radiation[5-13]. However, the low efficiency, the short lifetime and the enormous volume of the HPM devices limit their applications. To extend the lifetime and reduce the volume, HPM devices should operate at high efficiency without focusing systems and explosive emission cathodes. Current HPM devices, such as conventional RBWO[6-10, 14-15] and RKO[5, 12-13, 16-17], Vircator[18- 19], MILO[20], can’t satisfy the requirements. In this paper, a radial beam coaxial grating BWO which satisfies the above requirement is proposed. The structure is described in detail in section 1, the PIC simulation is presented in Section 2, and section 3 gives a brief conclusion.
1 Structure of the radial beam coaxial BWO
The cross section of the coaxial grating BWO is shown in Fig.1. The BWO looks like a disk with a cover, and there are some coaxial gratings etched on the cover, which slow down the EM wave[21-22]. A radial electron beam is injected into the center of the BWO from the outside edge as shown in Fig.1. After the beam wave interaction in the “disk” device, the HPM radiation is produced and extracted from the coaxial output port on the cover, which is located at the first grating.
Figure 1.Cross section of the BWO
The dispersion relation of the coaxial grating SWS can’t be written easily as conventional “O” shape BWOs because of the radial structure[21]. Nonetheless, the eigen modes of the BWO can be analyzed by CST software, as shown in Fig.2. In Fig.2, the electric field distributions of the symmetric modes，whose corresponding frequencies are 1.05 GHz, 2.11 GHz, 2.92 GHz, 3.3 GHz, 3.8 GHz respectively, are shown. The dispersion relation is described in Fig.3 as that of the conventional “O” shape BWOs.
Figure 2.Electric field of the modes
Figure 3.Dispersion relation of the BWO
2 PIC simulation of the BWO
The PIC simulation of the BWO is carried out by CST Particle studio[11,19,23]. The parameters are provided in Table 1. The output power is 1.2 GW and the operating frequency is 3.8 GHz, as shown in Fig.4. The result shows that the BWO is operating in the π mode. Almost all the particles lost energy at the center of the BWO as shown in Fig.5.
| voltage of
| current of
| thickness
| magnetic
| thickness
| outer radius
| inner radius
| radius of
| radius of
| radius of
| radius of
| radius of
| depth of
|
| 460 | 6 | 5 | 0 | 30 | 180 | 60 | 166 | 140 | 115 | 100 | 75 | 12.5 |
Table 1. Parameters for the PIC simulation
Figure 4.Output power and corresponding frequency
Figure 5.Particle view of the BWO
To improve the stability, extend the BWO lifetime and reduce the volume, two optimizations are made. First, because of the low current density of the radial beam, the BWO does not require the focusing system in the simulation, which can reduce the volume and the power loss in practice.
Second, to improve the stability and the lifetime, a thermionic radial beam cathode instead of an explosive emission cathode is used in the BWO, because the lower beam current density can be achieved by the thermionic cathode[24-25]. A radial beam gun is designed, whose cross section is shown in Fig.6. The radial beam gun is transformed from the convenient pencil beam guns with the same cross section profile. The cathode is a ring located between the upper and the lower focus electrodes. The particles are emitted from the inner face of the ring cathode. The parameters of the gun are provided in Table 2. The beam current density of the BWO is 112 A/cm2.
Figure 6.Structure and potential of the radial beam gun
| voltage of the cathode, Uc/kV | voltage of the focus, Uf/kV | voltage of the anode, Ua/V | thickness of the cathode, Tc/mm | radius of the cathode, Rc/mm | thickness of the focus, Tf/mm | inner radius of the focus, Rf/mm | radius of the anode, Ra/mm |
| −460 | −460 | 0 | 17 | 209 | 23 | 197 | 180 |
Table 2. Parameters of the radial beam gun
The tracking simulation shows that the radial beam gun is with 17∶5 beam compression. Accordingly, the beam current density at the cathode surface (about 33 A/cm2) can be achieved by the conventional thermionic cathodes with long lifetime and high stability, which have been used for many years in the modern vacuum electronic devices (VEDs) .
To radiate the output power, a small sized Vlasov antenna is designed, as shown in Fig.7[1,8,23]. The antenna transforms the TEM mode to the TM01 mode and radiates the output power. The parameters of the antenna are shown in Table 3. The gain of the antenna is shown in Fig.8.
Figure 7.Cut view of the Vlasov antenna
| length of the transition, Zt/mm | radius of the terminal, Rt/mm | length of the antenna, La/mm | corner cut of the antenna Cc/(°) |
| −150 | 110 | 450 | 45 |
Table 3. Parameters of the Vlasov antenna
Figure 8.Gain of the Vlasov antenna
The BWO is designed as shown in Fig.9. It is expected to manufacture the BWO and carry out the test in the near future.
Figure 9.BWO structure for test
3 Summary and conclusion
A high power radial beam coaxial grating BWO is proposed in this paper. Driven by a radial electron beam (460 kV, 6 kA), the BWO can produce 1.2 GW microwave at frequency 3.8 GHz, corresponding to the efficiency of 43.5%. The simulation does not require the focusing system, which can reduce the volume and the power loss in future. A thermionic radial beam gun is designed to improve the stability and the lifetime. The Vlasov antenna with the maximum gain 17.5 dB is used to radiate the output power.
References
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[10] Hahn K, Schamiloglu E. Longpulse relativistic backward wave oscillat operation utilizing a disk cathode[C]28th IEEE International Conference on Plasma Science 13th IEEE International Pulsed Power Conference. 2001: 16181621.
[11] Hegeler F, Schamiloglu E, Kovin S D, et al. Recent advances in the study of a long pulse relativistic backward wave oscillat[C]Proceedings of the 12th IEEE International Pulsed Power Conference. 1999: 825828.
[17] Gan Yanqing, Huang Hua, Lei Lurong, et al. Experimental investigation on an S-band relativistic klystron oscillator[J]. High Power Laser and Particle Beams, 20, 815-818(2008).
[19] Ma Qiaosheng. A novel efficient vircator[J]. High Power Laser Part Beams, 27, 053005(2015).
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