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Experimental verification of a low-impedance transit-time oscillator without foils

Published online by Cambridge University Press:  25 September 2012

Yibing Cao ,
Juntao He ,
Jiande Zhang  and
Junpu Ling
Yibing Cao
Affiliation:
College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha, People's Republic of China
Juntao He*
Affiliation:
College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha, People's Republic of China
Jiande Zhang
Affiliation:
College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha, People's Republic of China
Junpu Ling
Affiliation:
College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha, People's Republic of China
*
Address correspondence and reprint requests to: Juntao He, College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha 410073, People's Republic of China. E-mail: hjt0731@163.com
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Abstract

The low-impedance transit-time oscillator without foils is a new high-power microwave generator. In a previous report, a radiation power of 2.7 GW at 1.64 GHz has been achieved and the corresponding power conversion efficiency is 18.75%. Recently, the further experiments are continued in our laboratory. By increasing the cathode-anode gap properly, the operating voltage of device is enhanced to about 628 kV, and the corresponding radiation power reaches 3.6 GW. The device efficiency approaches 23%. In the newest experiments, because of the higher power level, the radiation power has been obviously influenced by RF breakdown in the vicinity of the dielectric window. By using a plastic bag filled with sulfur-hexafluoride (SF6), such an influence can be minimized.

Keywords

High power microwaveLow-impedanceRF breakdownTransit-time
Type
Research Article
Information
Laser and Particle Beams , Volume 30 , Issue 4 , December 2012 , pp. 613 - 619
Copyright
Copyright © Cambridge University Press 2012

1. INTRODUCTION

Although many new directions are becoming more and more attractive in the high-power microwave (HPM) domain (Fan et al., Reference Fan, Zhong, Li, Shu, Zhang, Zhang, Zhang, Yang and Luo2007; Ge et al., Reference Ge, Zhong, Qian, Zhang, Gao, Jin, Fan and Yang2010; He et al., Reference He, Cao, Zhang, Wang and Ling2011; Korovin et al., Reference Korovin, Kurkan, Loginov, Pegel, Polevin, Volkov and Zherlitsyn2003; Li et al., Reference Li, Shu, Yuan, Zhu, Liu, Wang and Zhang2010; Zhang et al., Reference Zhang, Yuan and Liu2010), advancing the peak power is always one of the most important considerations. Effective methods are to increase the input electric power or enhance the device efficiency. In pursuit of high power conversion efficiency, many efforts have been done and significant progress has been achieved (Barker et al., Reference Barker and Schamiloglu2001; Shkvarunets et al., Reference Shkvarunets, Carmel, Nusinovich, Abu-elfadl, Rodgers, Antonsen and Granatstein2002; Teng et al., Reference Teng, Liu, Shao and Tang2009; Xiao et al., Reference Xiao, Chen, Zhang and Sun2009, Reference Xiao, Zhang, Zhang, Li, Zhang, Song, Hu, Sun, Huo, Chen, Zhang and Liu2010, Reference Xiao, Chen, Sun, Zhang and Zhang2011). The highest power so far was obtained by the multiwave Cerenkov generator and relativistic klystron amplifier in the 1990s (Bugaev et al., Reference Bugaev, Cherepenin, Kanavets, Klimov, Kopenkin, Koshelev, Popov and Slepkov1990; Serlin et al., Reference Levine and Harteneck1994). The corresponding efficiencies are respectively 50% and 40%. However, it seems to be very difficult for the HPM devices to further enhance the power conversion efficiency with the exception of the sources based on super radiance (Anton et al., Reference Anton, Sergei, Vladislav, Igor, Gennady, Sergei, Valery, Michael and Naum2003; Bandurkin et al., Reference Bandurkin and Savilov2011).

In order to obtain higher output microwave power, increasing the input electric power may be a more feasible method (Arman, Reference Arman1996; Cao et al., Reference Cao, Zhang and He2009; He et al., Reference He, Zhong and Liu2004; Yang et al., Reference Yang and Ding2005). With the fast development of pulsed power technology, an intense relativistic electron beam (IREB) over 20 GW electric powers can be easily provided by many pulsed power sources (Adam et al., Reference Adam, Anders, Hans and Mats2007; Ouyang et al., Reference Ouyang, Liu, Liu, Wang and Feng2008; Yatsui et al., Reference Yatsui, Shimiya, Masugata, Shigeta and Shibata2005; Zhang et al., Reference Zhang, Chang, Xiang, Song, Kang, Luo, Li and Gong2007; Zou et al., Reference Zou, Liu, Zeng, Han, Yuan, Wang and Zhang2006). Even if the efficiency of HPM devices is only 10%, a microwave power of 2.0 GW can still be obtained. However, most HPM devices so far operate at a high diode impedance (about 100 Ω) (Fan et al., Reference Fan, Liu, Chen, Tan and Zhou2004; Hahn et al., Reference Hahn, Fuks and Schamiloglu2002; Levine et al., Reference Serlin and Friedman1994; Zhang et al., Reference Zhang, Zhong and Luo2004, Reference Zhang, Jin, Yang, Zhong, Shu, Zhang, Qian, Yuan, Li, Fan, Zhou and Xu2011), which is disadvantageous to match with the low-impedance pulsed power sources providing high electric power (about 20 Ω) (Liu et al., Reference Liu, Li, Zhang, Li and Wang2006, Reference Liu, Yin, Ge, Zhan, Cheng, Feng, Shu, Zhang and Wang2007a, Reference Liu, Zhan, Zhang, Liu, Feng, Shu, Zhang and Wang2007b, Reference Liu, Cheng, Qian, Ge, Zhang and Wang2009; Zou et al., Reference Zou, Liu, Zeng, Han, Yuan, Wang and Zhang2006). This has greatly limited further improvement of the microwave power.

In our previous work, a low-impedance transit-time oscillator without foils (LITTO) has been put forward (Cao et al., Reference Cao, Zhang and He2009). The device impedance is about 20 Ω and can thus utilize ultrahigh-current electron beams to reach high peak power. With a 600 kV, 24 kA electron beam guided by an external magnetic field of 0.5 Tesla, a radiation power of 2.7 GW at 1.64 GHz has been achieved in the experiments (Cao et al., Reference Cao, He and Zhang2012). Recently, the further experiments are continued in our laboratory. This article briefly introduces the newest experimental results.

2. EXPERIMENTAL IMPROVEMENT

According to the particle-in-cell (PIC) simulation (Cao et al., Reference Cao, Zhang and He2009), the power conversion efficiency peaks at about 700 kV, which is treated as the optimal working voltage of the LITTO. Operating at the optimal voltage, the highly modulated electron beam can deliver more possible power to the radio frequency (RF) field because of perfect synchronism between the electron transit time and the RF period. However, in previous experiments, the operating voltage of the LITTO is basically lower than 600 kV. We failed in all our attempts to further increase the diode voltage.

From previous experimental results (Cao et al., Reference Cao, He and Zhang2012), it is found that the saturated diode voltage gradually decreases with the increasing time and the beam current is just the reverse. Figure 1 shows the variations of the voltage and current versus time. Maybe the varying diode impedance is responsible for these. With the expansion of the cathode plasma (Roy et al., Reference Roy, Menon, Mitra, Kumar, Sharma and Nagesh2009), the cathode-anode gap is gradually decreased and thus lowering the corresponding diode impedance (see Fig. 2). Supposing the electric power provided by the accelerator is stabilized at P, the diode voltage V and the beam current I can be respectively expressed as

(1)$$\eqalign{& V=\sqrt {PR} \cr & I=\sqrt {P/R} }\comma \;$$V=PRI=P/R,

where R is the diode impedance. Therefore, it is inevitable that the voltage and the current will, respectively, decrease and increase because of the lowering diode impedance (see Fig. 2). In the newest experiments, we tried to slightly enhance the impedance by enlarging the cathode-anode gap from previous 15 mm to 20 mm. Eventually, the average operating voltage is indeed increased to about 630 kV.

Fig. 1. (Color online) Variations of diode voltage and current versus time.

Fig. 2. (Color online) Variation of diode impedance versus time.

3. EXPERIMENTAL RESULTS

Figure 3 illustrates our overall experimental arrangement. The operating voltage provided by the IREB accelerator can be varied by changing the trigger time of the main switch. The measures of the voltage, current and microwave have been introduced in detail in our previous paper (Cao et al., Reference Cao, He and Zhang2012). The directional couplers and the microwave crystal detectors are also the same with those used there. In the newest experiments, the signals from the directional couplers are, respectively, attenuated 37.87 dB and 43.90 dB before passing through the crystal detectors.

Fig. 3. (Color online) Photograph of the experimental layout.

In order to identify the operating mode in the new state, the microwave frequency is first measured in the experiments. Figure 4 gives the relationship between the diode voltage and the operating frequency. From the figure, it can be seen that the microwave frequency varies between 1.63 GHz and 1.64 GHz with the changed voltage. After voltage is beyond 530 kV, the microwave frequency is stabilized at 1.639 GHz, which agrees well with the simulation result. With an input electron beam of 553 kV voltage and 21 kA current guided by an external magnetic field of 0.5 Tesla, the experimental radiation pattern is shown in Figure 5, which is basically consistent with the numerical result, indicating that the radiation mode is TM01. Typical experimental waveforms of the diode voltage, diode current, and two output microwave signals measured in the far-field are shown in Figure 6. The integrated power was 2.45 GW, which is slightly lower than the simulation result (about 2.6 GW). The corresponding pulse width was about 32 ns, close to that obtained by the PIC simulation, indicating that there was no obvious pulse shortening.

Fig. 4. (Color online) Microwave frequency versus diode voltage.

Fig. 5. (Color online) Radiation pattern of the system.

Fig. 6. (Color online) Waveforms of the diode voltage (C1), diode current (C2), microwave detector signals (C3, C4) (diode voltage 553 kV, diode current 21 kA, microwave power 2.45 GW, 20 ns/div).

According to the PIC simulation (Cao et al., Reference Cao, Zhang and He2009), the microwave power should gradually increase with the increasing diode voltage and the power conversion efficiency peaks at about 700 kV. However, when the experimental voltage is increased up to 628 kV (the corresponding current is 25 kA), the radiated microwave power has not increased obviously. The corresponding waveforms of the diode voltage, diode current, and two output microwave signals measured in the far-field are shown in Figure 7. The evaluated microwave power was 2.40 GW, with an efficiency of only 15.5%. Several shots showed similar results. With the similar voltage and current, a 3.7 GW microwave has been obtained in the simulation. In order to draw a comparison, the microwave powers of the experiment and simulation are shown in Figure 8. Obviously, the experimental power is much lower than the simulation result. One probable explanation is that the RF breakdown in the vicinity of the dielectric window has limited the radiation of HPM.

Fig. 7. (Color online) Waveforms of the diode voltage (C1), diode current (C2), microwave detector signals (C3, C4) (diode voltage 628 kV, diode current 25 kA, microwave power 2.40 GW, 20 ns/div).

Fig. 8. (Color online) Microwave output powers from the simulation and experiment.

In previous experiments, the RF breakdown has been observed. Fortunately, the RF breakdown there didn't obviously influence the microwave radiation and the breakdown phenomenon was only used to judge the operating modes (He et al., Reference He, Cao, Zhang and Ling2012). Nevertheless, with the improved radiation power, the rapidly increasing plasma caused by the RF breakdown could reflect a fraction of HPM radiation. As shown in Figure 9, the light caused by the RF breakdown is obviously much more intense than those in previous experiments.

Fig. 9. (Color online) Photographs of window RF breakdown in the atmosphere.

The electric field intensity on the interface between dielectric and air is first analyzed by numerical calculation. With a 3.5 GW input at 1.64 GHz, Figure 10 gives the electric field variation along the radial position on the interface, corresponding to the TM01 mode radiation. As shown in the figure, the maximum axial electric field is about 120 kV/cm and the maximum radial electric field approaches 60 kV/cm. Both of them are greatly beyond the air breakdown threshold (20–30 kV/cm) (Booske, Reference Booske2008). Therefore, it is very probable that fierce RF breakdown would happen in the vicinity of window. Figure 11 displays the electric field distributions on the interface. From this figure, it can be concluded that the RF breakdown caused by the axial and radial field components would happen at different position. In combination with Figure 10, it can be concluded that the present breakdown is mainly caused by the axial field components.

Fig. 10. (Color online) Electric field components versus radius r.

Fig. 11. (Color online) Distributions of axial electric field (a) and radial electric field (b) on the dielectric-air interface.

After using a clear plastic bag filled with SF6, the RF breakdown is effectively suppressed and the corresponding result is shown in Figure 12. Although a trifling surface flashover caused by the radial field still exists (corresponding to Fig. 11b), it hardly affects the radiated microwave and the integrated power is increased from 2.5 GW to 3.6 GW, which agrees well with the simulation results (about 3.7 GW). Figure 13 gives the typical experimental waveforms of the diode voltage, diode current, and two output microwave signals measured in the experiments. Several shots showed similar results. After taking off the clear plastic bag filled with SF6, the microwave power measured in the far-field is again decreased to about 2.5 GW and the corresponding RF breakdown as shown in Figure 9 is still observed.

Fig. 12. (Color online) Photographs of window RF breakdown in the atmosphere with a clear plastic bag filled with SF6.

Fig. 13. (Color online) Waveforms of the diode voltage (C1), diode current (C2), microwave detector signals (C3, C4) (diode voltage 628 kV, diode current 25 kA, microwave power 3.6 GW, 20 ns/div).

4. DISCUSSIONS

High input electric power and output microwave power is treated as a main merit of the LITTO, which has been testified by the simulation and experimental results. Dielectric window breakdown is demonstrated to be a major issue in the transmission of HPM radiation (Krile et al., Reference Krile, Neuber, Krompholz and Gibson2006; Kuo et al., Reference Kuo, Zhang and Kossey1990, Reference Kuo and Zhang1991; Liu et al., Reference Liu, Liu, Huang, Zhou, Song and Ning2000; Nam et al., Reference Nam, Lim and Verboncoeur2009; Woo et al., Reference Woo and Degroot1984). Plasma caused by RF breakdown will bring some negative impacts on the HPM, such as reflection, absorption attenuation, and so on. Consequently, the measured microwave power in the experiments is obviously decreased. In order to depress RF breakdown, a clear plastic bag filled with SF6 has been used and the microwave power is significantly increased.

However, further investigation is limited by the available voltage level and its pulse width. In fact, it is not obvious to enhance the diode voltage by enlarging the cathode-anode gap. The present voltage is still less than 700 kV. Besides, due to the limited pulse width, it may be difficult for the plasma to build up to the extent that it cuts-off the radiated microwave. Along with an increasing voltage pulse width, the influence from space-time-dependent plasma would probably get more and more significant.

The LITTO is also expected long-pulsed and repetitive operation. Although no obvious pulse shortening is observed in the present experiment, the ability of long-pulsed and repetitive operation is still indistinct because of the limited voltage pulse width. A fatal shortcoming is the design of the electron collector, which has been greatly neglected in previous investigation (Cao et al., Reference Cao, Zhang and He2009). Experiment indicated that the material of the electron collector has great influence on the generation of HPM (He et al., Reference He, Cao, Zhang and Ling2012). When the collector is stainless steel, intense electrons reflection and plasma formation on the collector would result in high-order mode excitation and pulse shortening. The mode competition has been effectively depressed by using a graphite collector instead of the stainless steel collector. However, as the voltage pulse width increases, the dust from the IREB impact on the graphite collector also probably contaminates the device and results in pulse shortening.

5. CONCLUSIONS

The LITTO is a new low-impedance HPM source. A simple structure makes such a device attractive. Simultaneously, due to a short length (about 50 cm), it is very advantageous to the transportation of IREB (Konoplev et al., Reference Konoplev, Cross, MacInnes, He, Whyte, Phelps, Robertson, Ronald and Young2006). The experiments have demonstrated that such a device can allow high input and output powers. With an IREB of 628 kV voltage and 25 kA current, guiding by a magnetic field of 0.5 Tesla, a 3.6 GW radiated microwave at frequency 1.639 GHz has been obtained in the experiments. The device efficiency approaches 23%. The experimental results agree well with those of simulation. No obvious pulse shortening phenomenon has been observed in the present experiment. The intensive investigation will be continued on a long-pulsed IREB generator.

ACKNOWLEDGMENT

This work was supported by the National Natural Science Foundation of China under Grant No. 61171021.

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Figure 0

Fig. 1. (Color online) Variations of diode voltage and current versus time.

Figure 1

Fig. 2. (Color online) Variation of diode impedance versus time.

Figure 2

Fig. 3. (Color online) Photograph of the experimental layout.

Figure 3

Fig. 4. (Color online) Microwave frequency versus diode voltage.

Figure 4

Fig. 5. (Color online) Radiation pattern of the system.

Figure 5

Fig. 6. (Color online) Waveforms of the diode voltage (C1), diode current (C2), microwave detector signals (C3, C4) (diode voltage 553 kV, diode current 21 kA, microwave power 2.45 GW, 20 ns/div).

Figure 6

Fig. 7. (Color online) Waveforms of the diode voltage (C1), diode current (C2), microwave detector signals (C3, C4) (diode voltage 628 kV, diode current 25 kA, microwave power 2.40 GW, 20 ns/div).

Figure 7

Fig. 8. (Color online) Microwave output powers from the simulation and experiment.

Figure 8

Fig. 9. (Color online) Photographs of window RF breakdown in the atmosphere.

Figure 9

Fig. 10. (Color online) Electric field components versus radius r.

Figure 10

Fig. 11. (Color online) Distributions of axial electric field (a) and radial electric field (b) on the dielectric-air interface.

Figure 11

Fig. 12. (Color online) Photographs of window RF breakdown in the atmosphere with a clear plastic bag filled with SF6.

Figure 12

Fig. 13. (Color online) Waveforms of the diode voltage (C1), diode current (C2), microwave detector signals (C3, C4) (diode voltage 628 kV, diode current 25 kA, microwave power 3.6 GW, 20 ns/div).