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.
High Power Laser and Particle Beams, Volume. 35, Issue 11, 113001(2023)
S band radial beam coaxial grating backward wave oscillator
High power microwave devices are investigated extensively, because of their potential applications, such as advanced radars, electromagnetic warfare systems. However, low efficiency, enormous volume, huge weight and short lifetime limit their applications. In this paper, a coaxial grating slow wave structure backward wave oscillator (BWO) driven by radial beam is proposed. The focusing system is eliminated in the particle in cell simulation, which can reduce the volume and the power loss in practice. The lifetime of the BWO can also be improved with the thermionic radial beam cathode instead of the explosive emission cathode. After optimization, the BWO driven by 460 kV, 6 kA radial beam can produce 1.2 GW at frequency 3.8 GHz, with the efficiency of 43.5%.
Keywords
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.
Table 1. Parameters for the PIC simulation
Table 1. Parameters for the PIC simulation
voltage of the sheet beam, U/kV current of the sheet beam, I/kA thickness of the beam, Sbeam/mm magnetic field, B/T thickness of the BWO, H/mm outer radius of the BWO, Rout/mm inner radius of the BWO, Rin/mm radius of the first grating, R1/mm radius of the second grating, R2/mm radius of the third grating, R3/mm radius of the fourth grating, R4/mm radius of the fifth grating, R5/mm depth of the grating, D/mm 460 6 5 0 30 180 60 166 140 115 100 75 12.5
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
Table 2. Parameters of the radial beam gun
Table 2. Parameters 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
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
Table 3. Parameters of the Vlasov antenna
Table 3. Parameters 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
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.
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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
Paper Information
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Received: Jun. 28, 2023
Accepted: Oct. 23, 2023
Published Online: Dec. 26, 2023
The Author Email: Wang Zhanliang (46969449@qq.com)
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