REVIEW OF SCIENTIFIC INSTRUMENTS 79. 064701(2008) A series of tufted carbon fiber cathodes designed for different high power microwave sources ie Liu, Limin Li, Jun Zhang, Xiaoping Zhang, Jianchun Wen, and Yonggui Liu College of Photoelectric Science and Engineering, National Universiry of Defense Technology (Received 17 October 2007; accepted 3 May 2008; published online 4 June 2008) We report the fabrication technique of tufted carbon fiber cathodes for different microwave sources Three carbon fiber cathodes were constructed, including a planar cathode, an annular cathode, and a cylindrical cathode for radial emission. Experimental investigations on these cathodes were erformed in a reflex triode virtual cathode oscillator(vircator), a backward wave oscillator(Bwo) and a magnetically insulated transmission line oscillator(MILO), respectively. The pulse duration of microwave emission from the reflex triode vircator was lengthened by using the planar carbon fiber cathode. In the bwo with the annular carbon fiber cathode. the uniform electron beam with a kA/cm current density was observed. In addition, carbon fiber has great promise as field emitter for MILOs. These results show that the carbon fiber cathodes can be utilized for electron emission in high power diodes with different structures. o 2008 American Institute of Physics [DO:10.1063/12936870 L INTRODUCTION that carbon fiber cathodes can be widely used in various microwave sources. The purpose of this study is to confirm Explosive emission cathodes have been used extensively whether the carbon fiber cathodes are suitable for different in high power microwave(HPM) tube research and have sources. In this paper, we constructed carbon fiber cathodes thus been the subject of many experimental efforts. by using commercial carbon yarns. First, we proposed a These cathode materials ferroelectric to carbon fiber. Particularly, carbon fiber In addition, the test experiments on carbon fiber cathodes cathodes have shown great promise for explosive emission, were conducted in a reflex triode vircator, a Bwo, and in that they have light weight and require no heating for MILO, respectively. operation. Coating cesium iodide salt can enhance the per- formance of carbon fiber cathodes by decreasing the emis- .FABRICATION TECHNIQUE sion threshold, increasing the emission uniformity, and in The carbon fiber cathodes were fabricated by the pro creasing the cathode lifetime. The emission property of the cesses as follows. First, the design of the cathode structure cathodes is considered to be a major issue for high-quality depends on the diode geometrical structure. High-quality car- lectron beam generation. bon fiber yarns(T300-3K, 5 um) and high-density graphite High-current, relativistic electron beams are required for were selected as initial materials for constructing carbon pulsed-power technology and HPM generation. Generally, fiber cathodes. The following processes were, in turn,the the cathode structure depends on the types of microwave mold design, the cathode substrate machining, and the re- sources. Even with extremely careful diode design, no cath- moval of fiber epoxy, respectively. The mold machining,the ode materials are suitable for all high power microwave substrate drilling, and the formation of fiber tufts were the tubes. Then, the application of different cathodes(i.e key processes. A lot of holes were drilled so uniformly that different cathode materials and structures)for electron emis- carbon fiber tufts were uniformly distributed on the substrate sion in different hPm sources needs to be solved. Hence it Prior to this process, the carbon fiber yarns were woven into is necessary for developing HPM source technology to grope a piece of cloth in a single direction through a winding ma- for explosive emission cathodes suitable for diverse micro- chine, then tailored and arranged in a designed mold to ob- wave tubes The HPm radiation is based on the interaction of elec- tain the fiber preforms. The fiber epoxy inherent on the fiber surface was eliminated at 300C under the protection of beam is required for all microwave sources. The emission 10 atm.99.95% nitrogen gas. The fabricated fiber tufts were property of explosive emission cathodes directly affects the mounted into the holes drilled on the graphite substrate and electron beam quality. Therefore, the cathode technology the back of the substrate. The following processes were is a bottleneck of developing HPM sources. Although carbon removal of the mold and the cleaning of the whole cathode. fiber cathodes have excellent performances, it is uncertain To remove the contaminations on the fiber surface, including the epoxy for fastening fiber tufts, water, oil films, and ab- eleCtronicmailnewages1979@yahoo.com.cn sorbed gases, the fabricated cathodes were baked for several 00346748/2008/796/064701/4s23.0 79,064701-1 o 2008 American Institute of Physics
A series of tufted carbon fiber cathodes designed for different high power microwave sources Lie Liu, Limin Li,a Jun Zhang, Xiaoping Zhang, Jianchun Wen, and Yonggui Liu College of Photoelectric Science and Engineering, National University of Defense Technology, Changsha 410073, People’s Republic of China Received 17 October 2007; accepted 3 May 2008; published online 4 June 2008 We report the fabrication technique of tufted carbon fiber cathodes for different microwave sources. Three carbon fiber cathodes were constructed, including a planar cathode, an annular cathode, and a cylindrical cathode for radial emission. Experimental investigations on these cathodes were performed in a reflex triode virtual cathode oscillator vircator, a backward wave oscillator BWO, and a magnetically insulated transmission line oscillator MILO, respectively. The pulse duration of microwave emission from the reflex triode vircator was lengthened by using the planar carbon fiber cathode. In the BWO with the annular carbon fiber cathode, the uniform electron beam with a kA/cm2 current density was observed. In addition, carbon fiber has great promise as field emitter for MILOs. These results show that the carbon fiber cathodes can be utilized for electron emission in high power diodes with different structures. © 2008 American Institute of Physics. DOI: 10.1063/1.2936870 I. INTRODUCTION Explosive emission cathodes have been used extensively in high power microwave HPM tube research and have thus been the subject of many experimental efforts.1–3 These cathode materials range from polymer velvet1 to ferroelectric4 to carbon fiber.5–7 Particularly, carbon fiber cathodes have shown great promise for explosive emission, in that they have light weight and require no heating for operation.8 Coating cesium iodide salt can enhance the performance of carbon fiber cathodes by decreasing the emission threshold, increasing the emission uniformity, and increasing the cathode lifetime.9 The emission property of the cathodes is considered to be a major issue for high-quality electron beam generation. High-current, relativistic electron beams are required for pulsed-power technology and HPM generation.10 Generally, the cathode structure depends on the types of microwave sources. Even with extremely careful diode design, no cathode materials are suitable for all high power microwave tubes.11 Then, the application of different cathodes i.e., the different cathode materials and structures for electron emission in different HPM sources needs to be solved. Hence, it is necessary for developing HPM source technology to grope for explosive emission cathodes suitable for diverse microwave tubes. The HPM radiation is based on the interaction of electron beam and microwave field.12 Thus, the uniform electron beam is required for all microwave sources. The emission property of explosive emission cathodes directly affects the electron beam quality.13,14 Therefore, the cathode technology is a bottleneck of developing HPM sources. Although carbon fiber cathodes have excellent performances, it is uncertain that carbon fiber cathodes can be widely used in various microwave sources. The purpose of this study is to confirm whether the carbon fiber cathodes are suitable for different sources. In this paper, we constructed carbon fiber cathodes by using commercial carbon yarns. First, we proposed a technique for the construction of the carbon fiber cathodes. In addition, the test experiments on carbon fiber cathodes were conducted in a reflex triode vircator, a BWO, and a MILO, respectively. II. FABRICATION TECHNIQUE The carbon fiber cathodes were fabricated by the processes as follows. First, the design of the cathode structure depends on the diode geometrical structure. High-quality carbon fiber yarns T300-3K, 5 m and high-density graphite were selected as initial materials for constructing carbon fiber cathodes. The following processes were, in turn, the mold design, the cathode substrate machining, and the removal of fiber epoxy, respectively. The mold machining, the substrate drilling, and the formation of fiber tufts were the key processes. A lot of holes were drilled so uniformly that carbon fiber tufts were uniformly distributed on the substrate. Prior to this process, the carbon fiber yarns were woven into a piece of cloth in a single direction through a winding machine, then tailored and arranged in a designed mold to obtain the fiber preforms. The fiber epoxy inherent on the fiber surface was eliminated at 300 °C under the protection of 10 atm., 99.95% nitrogen gas. The fabricated fiber tufts were mounted into the holes drilled on the graphite substrate and these tufts were well fastened by using conductive epoxy on the back of the substrate. The following processes were the removal of the mold and the cleaning of the whole cathode. To remove the contaminations on the fiber surface, including the epoxy for fastening fiber tufts, water, oil films, and absorbed gases, the fabricated cathodes were baked for several a Electronic mail: newages1979@yahoo.com.cn. REVIEW OF SCIENTIFIC INSTRUMENTS 79, 064701 2008 0034-6748/2008/796/064701/4/$23.00 © 2008 American Institute of Physics 79, 064701-1
064701-2 Liu et al Rev. Sci Instrum. 79, 064701(2008) Anode grid Vacuum pump Rogowski coil Cathode Radiation window FIG. 2. Experimental configuration of a reflex triode vircator driven by a h 1. A series of tufted carbon fiber cathodes: (a)a planar tufted carbon 400 ns, 450 kV accelerating pulse cathode, (b)an annular tufted carbon cathode and (c) a tufted carbon fiber cathode for radial emission voltage and current for the reflex triode vircator. The ampli- ude and pulse duration of the accelerating voltage were hours at 300-400C under forevacuum. Finally, the carbon 450 kV and 400 ns, respectively. Moreover, the diode volt- fiber cathodes were completely constructed. The processes age had a flattop of about 200 ns. In addition, it can be seen described here can ide the carbon fiber cathodes with that, from the current waveform. the maximum emission cur- uniformly distributed. It should be noted that rent was 11 kA. The increase in the current amplit Ide during slightly technique changes in the cathode fabrication are the"plateau"of the accelerating pulse as well as the large made for different microwave tubes We tested three cathodes based on carbon fiber emitters. returned zero were observed, indicating the expansion of the The carbon fiber cathodes consisted of discrete bundles of cathode plasma toward the anode. We also investigated the carbon fiber which protruded through the cathode substrate. effects of the planar carbon fiber cathode on the microwave The carbon fiber cathode was constructed by mounting car- emission. as shown in Fig. 3. The pulse duration for the bon fiber tufts in a graphite substrate. The carbon fiber tufts stainless steel cathode was approximately 150 ns. Under the on the planar carbon fiber cathode were 5 mm, with a same accelerating voltage pulse, the microwave pulse dura- 1.5 mm diameter and spaced by 9 mm, shown in Fig. 1(a). tion. for the carbon fiber cathode. was increased to about The fiber tuft was composed of thousands of carbon fibers 180 ns. The lengthening of the microwave pulse duration is with 2.5 um radius. Moreover, the annular carbon fiber cath- to a great extent attributed to the improvement of the elec- ode had an inner diameter of 68 mm and an outer diameter tron beam qualit of 71 mm, shown in Fig. 1(b), and the planar and annular The schematic for an overmoded Bwo is presented in cathodes were used for the reflex triode vircator and the Fig. 4. Here, the tested cathode is the annular carbon fiber cathode in Fig. 1(b). It mainly includes two slow-wave struc- The tufted carbon fiber cathode for the MILO operation was constructed as follows. A 5.2 cm diameter cylinder of high-density graphite was drilled with an array of 1.5 mm diameter holes. Carbon fiber tufts were threaded through the holes and secured to the graphite cylinder substrate. These fiber tufts were cut to the uniform length of 4 mm. The load 30M of 5.2 cm diameter was also fabricated by the same process as above. Finally, these two parts were connected into one assembly by a screw on the graphite substrate s shown in 300MW Fig. 1(c) 450k IlL RESULTS AND DISCUSSIONS The experiments on the planar cathode were performed in the reflex triode vircator in Fig. 2. The base pressure was maintained in the range of(7.5-10)x 105 torr. A high voltage pulse was applied to the anode-cathode(A-K) gap of 19 mm. The planar diode was operated at moderate 200 ns/div 200 kV/cm electric field, depending on the Marx bank grid of 200 mm in diameter and transparency of 95, eel FIG 3. Beam current(channel 1)and diode voltage (channel 2)for the charging voltage and the diode impedance. A stainless reflex triode vircator at the A-K gap of 1.9 cm. The upper two traces are the microwave waveforms for the carbon fiber cathode(channel 3)and the used as an anode. Figure 3 presents typical waveforms of stainless steel cathode (channel 4), respectively
hours at 300– 400 °C under forevacuum. Finally, the carbon fiber cathodes were completely constructed. The processes described here can provide the carbon fiber cathodes with fiber tufts uniformly distributed. It should be noted that slightly technique changes in the cathode fabrication are made for different microwave tubes. We tested three cathodes based on carbon fiber emitters. The carbon fiber cathodes consisted of discrete bundles of carbon fiber which protruded through the cathode substrate. The carbon fiber cathode was constructed by mounting carbon fiber tufts in a graphite substrate. The carbon fiber tufts on the planar carbon fiber cathode were 5 mm, with a 1.5 mm diameter and spaced by 9 mm, shown in Fig. 1a. The fiber tuft was composed of thousands of carbon fibers with 2.5 m radius. Moreover, the annular carbon fiber cathode had an inner diameter of 68 mm and an outer diameter of 71 mm, shown in Fig. 1b, and the planar and annular cathodes were used for the reflex triode vircator and the BWO, respectively. The tufted carbon fiber cathode for the MILO operation was constructed as follows. A 5.2 cm diameter cylinder of high-density graphite was drilled with an array of 1.5 mm diameter holes. Carbon fiber tufts were threaded through the holes and secured to the graphite cylinder substrate. These fiber tufts were cut to the uniform length of 4 mm. The load of 5.2 cm diameter was also fabricated by the same process as above. Finally, these two parts were connected into one assembly by a screw on the graphite substrate, as shown in Fig. 1c. III. RESULTS AND DISCUSSIONS The experiments on the planar cathode were performed in the reflex triode vircator in Fig. 2. The base pressure was maintained in the range of 7.5– 1010−5 torr. A highvoltage pulse was applied to the anode-cathode A–K gap of 19 mm. The planar diode was operated at moderate 200 kV/cm electric field, depending on the Marx bank charging voltage and the diode impedance. A stainless steel grid of 200 mm in diameter and transparency of 95% was used as an anode. Figure 3 presents typical waveforms of the voltage and current for the reflex triode vircator. The amplitude and pulse duration of the accelerating voltage were 450 kV and 400 ns, respectively. Moreover, the diode voltage had a flattop of about 200 ns. In addition, it can be seen that, from the current waveform, the maximum emission current was 11 kA. The increase in the current amplitude during the “plateau” of the accelerating pulse as well as the large amplitude of the current pulse as the voltage pulse gradually returned zero were observed, indicating the expansion of the cathode plasma toward the anode. We also investigated the effects of the planar carbon fiber cathode on the microwave emission, as shown in Fig. 3. The pulse duration for the stainless steel cathode was approximately 150 ns. Under the same accelerating voltage pulse, the microwave pulse duration, for the carbon fiber cathode, was increased to about 180 ns. The lengthening of the microwave pulse duration is to a great extent attributed to the improvement of the electron beam quality. The schematic for an overmoded BWO is presented in Fig. 4. Here, the tested cathode is the annular carbon fiber cathode in Fig. 1b. It mainly includes two slow-wave strucFIG. 1. A series of tufted carbon fiber cathodes: a a planar tufted carbon fiber cathode, b an annular tufted carbon cathode, and c a tufted carbon fiber cathode for radial emission. FIG. 2. Experimental configuration of a reflex triode vircator driven by a 400 ns, 450 kV accelerating pulse. FIG. 3. Beam current channel 1 and diode voltage channel 2 for the reflex triode vircator at the A-K gap of 1.9 cm. The upper two traces are the microwave waveforms for the carbon fiber cathode channel 3 and the stainless steel cathode channel 4, respectively. 064701-2 Liu et al. Rev. Sci. Instrum. 79, 064701 2008
064701 Carbon fiber cathodes design Rev. Sci Instrum. 79, 064701(2008) Magnetic field coils Drift tube Output waveguide Collimator c。 FIG. 4. Schematic drawing of the BwO. The beam quality of the annular carbon fiber cathode was tested in this bed FIG. 6. Target images bombarded by electron beams from two cathodes: (a) a stainless steel cathode and(b) the annular carbon fiber cathode ture(SwS) sections separated by a drift tube, a resonant cavity, and a tapered waveguide. The electron beam was on the bombarded targets corresponds to the uniformity of propagated into the interaction region under a uniform mag- plasma formed on the cathode surface. It can be seen that the netic field provided by magnetic field coils. The voltage and electron emission of the carbon fiber cathode is more uni- current waveforms as well as the output microwave signal form than that of the stainless steel cathode. With respect to for the Bwo are presented in Fig. 5. The amplitude and he nature behind experimental results, the low threshold pulse duration of the accelerating voltage pulse were ap- electric field for electron emission may be responsible for the proximately 600 kv and 60 ns, respectively. In addition, the uniformity of electron emission maximum current was 8 kA and the -400 MW, 30 ns mi- The experiments on the carbon fiber cathode for radial crowave emission was obtained. At the A-K gap of 1.86 cm, emission were performed in the MILO in Fig. 7. It consists the maximum electric field attained the level of of a two-cavity choke section, a five-cavity SwS, and a cou- 300-20kvIcm. Since the inner and outer diameters of this pling cavity. The voltage, current, and microwave waveforms cathode were 68 and 71 mm, respectively, the maximum for the miLO are presented in Fig. 8. The amplitude and emission current density exceeding kA/cm- can be esti- pulse duration of the voltage were 500 kV and 100 ns, re- mated. The current density obtained here can meet the re- spectively. The maximum emission current was 42 kA and ents for HPm sou the rise time was 70 ns. The -800 Mw, 20 ns microwave We also made a quality test of electron beam generated emission from two main radiation lobes was observed. In from the annular carbon fiber cathode. In Fig. 6(a), the ab- addition, the above results are only primary experiments for lated marks were unevenly distributed on the nylon target for he MILO. However, carbon fiber has shown a potential can- the stainless steel cathode: some places were eroded very didate for the milo cathode materials badly, others were ablated relatively weakly, and even some The ideal cathode for hPm device would be one that places seemed not bombarded by the electrons For the an- exhibits the following properties: high-current densities nular carbon fiber cathode, an ablated circle with an inner (kA/cm?), rapid current turn on, little or no plasma produc- diameter of 7 cm and an outer diameter of 7.4 cm was ob- tion, and survivability under repetitively pulsed operation served.The marks were uniformly distributed with an iden- However, these ideal cathode properties appeared unattain tical ablated degree along the whole circle. This indicates able until recently. Indeed, not all of HPM tubes require the that the electron emission occurred along the whole cathode explosive emission cathodes to entirely exhibit the above urface. We can hold that the uniformity of the ablated marks properties, i.e., one of them may be enough to meet the re quirements of a given HPM tube. As for the performances of the carbon fiber cathodes fabricated here. the increase in the pulse duration of output microwave, the uniform electron Choke cavity Slow wave structure 600k∨ 8.KA Tufted carbon fiber cathode FIG. 5. Typical waveforms of the diode voltage(channel 1), beam current (channel 2), and microwave signal(channel 3) for the Bwo FIG. 7. Schematic of experimental setup for the MILO
ture SWS sections separated by a drift tube, a resonant cavity, and a tapered waveguide. The electron beam was propagated into the interaction region under a uniform magnetic field provided by magnetic field coils. The voltage and current waveforms as well as the output microwave signal for the BWO are presented in Fig. 5. The amplitude and pulse duration of the accelerating voltage pulse were approximately 600 kV and 60 ns, respectively. In addition, the maximum current was 8 kA and the 400 MW, 30 ns microwave emission was obtained. At the A–K gap of 1.86 cm, the maximum electric field attained the level of 30020 kV/cm. Since the inner and outer diameters of this cathode were 68 and 71 mm, respectively, the maximum emission current density exceeding kA/cm2 can be estimated. The current density obtained here can meet the requirements for HPM sources. We also made a quality test of electron beam generated from the annular carbon fiber cathode. In Fig. 6a, the ablated marks were unevenly distributed on the nylon target for the stainless steel cathode: some places were eroded very badly, others were ablated relatively weakly, and even some places seemed not bombarded by the electrons. For the annular carbon fiber cathode, an ablated circle with an inner diameter of 7 cm and an outer diameter of 7.4 cm was observed. The marks were uniformly distributed with an identical ablated degree along the whole circle. This indicates that the electron emission occurred along the whole cathode surface. We can hold that the uniformity of the ablated marks on the bombarded targets corresponds to the uniformity of plasma formed on the cathode surface. It can be seen that the electron emission of the carbon fiber cathode is more uniform than that of the stainless steel cathode. With respect to the nature behind experimental results, the low threshold electric field for electron emission may be responsible for the uniformity of electron emission. The experiments on the carbon fiber cathode for radial emission were performed in the MILO in Fig. 7. It consists of a two-cavity choke section, a five-cavity SWS, and a coupling cavity. The voltage, current, and microwave waveforms for the MILO are presented in Fig. 8. The amplitude and pulse duration of the voltage were 500 kV and 100 ns, respectively. The maximum emission current was 42 kA and the rise time was 70 ns. The 800 MW, 20 ns microwave emission from two main radiation lobes was observed. In addition, the above results are only primary experiments for the MILO. However, carbon fiber has shown a potential candidate for the MILO cathode materials. The ideal cathode for HPM device would be one that exhibits the following properties: high-current densities kA/cm2 , rapid current turn on, little or no plasma production, and survivability under repetitively pulsed operation. However, these ideal cathode properties appeared unattainable until recently. Indeed, not all of HPM tubes require the explosive emission cathodes to entirely exhibit the above properties, i.e., one of them may be enough to meet the requirements of a given HPM tube. As for the performances of the carbon fiber cathodes fabricated here, the increase in the pulse duration of output microwave, the uniform electron FIG. 4. Schematic drawing of the BWO. The beam quality of the annular carbon fiber cathode was tested in this bed. FIG. 5. Typical waveforms of the diode voltage channel 1, beam current channel 2, and microwave signal channel 3 for the BWO. FIG. 6. Target images bombarded by electron beams from two cathodes: a a stainless steel cathode and b the annular carbon fiber cathode. FIG. 7. Schematic of experimental setup for the MILO. 064701-3 Carbon fiber cathodes design Rev. Sci. Instrum. 79, 064701 2008
064701-4 Liu et al Rev.Sci. strun.79,064701(2008) that the carbon fiber cathode can provide long pulse electron beams. The lengthening of the output microwave pulse can display the slow plasma expansion velocity of carbon fiber cathodes. The uniform electron beam with a current density 42kA of more than kA/cm was obtained in the bwo with the annular carbon fiber cathode. In addition. the carbon fiber cathode was used for radial emission in the milo. These results show that the carbon fiber cathodes can be widely used for electron emission in high power diodes ACKNOWLEDGMENTS 800M This work was supported by the National Technology Research and Development Program of Ch 100 ns/div R. B. Miller, J. AppL. Phys. 84, 3880(1998) FIG.8. Channels I and 2 are, respectively, the voltage and current wave. R. J. Umstattd. C. A. Schlise, and F. Wang, IEEE Trans. Plasma Sci. 33 for the MILO. Channels 3 and 4 are the microwave signals from the 901(2005) E. Garate. R. D. Mcwilliams. D. E. Voss. A. L. Lovesee K. J. Hendricks nain radiation lobes T.A. Spencer, M. C. Clark, and A Fisher, Rev. Sci Instrum. 66. 252 beam with the current density of kA/cm 2 level. and the long Temkin, and C Pralong, Appl. Phys. Lett. 74, 335(199 Advani.R.J R. Drori, M. Einat. D. Shur, E. Jerby, G. Rosenman, R. lifetime were observed in the reflex triode vircator, the sY. E. Krasik, A. Dunaevsky, and J. Felsteiner, Phys. Plasmas 8,2466 BWO, and the MILO, respectively. Although the entire prop- (2001) erties of carbon fiber cathodes were not displayed in each L. Liu, L. Li, X. Zhang, J. Wen, H. Wan, and Y. Zhang, IEEE Trans HPM tube. the carbon fiber cathode can still be considered to Plasma Sci.35.361(2007) D. A. Shiffler, J. Luginsland, M. Ruebush, M. Lacour, K. Golby, K be a promising explosive emission cathode for high-current Cartwright, M. Haworth, and T. Spencer, IEEE Trans. Plasma Sci. 32. lectron beam generation SD. Shiffler, M. Ruebush, D. Zagar, M. LaCour, M. Sena, K. Golby, M. IV CONCLUSIONS R. Umstattd, J. Appl. Phys. 91. 5599(2002) 9D. A Shiffer, M. Lacour, K. Golby, M.Sena,M.Mitchell,M.Haworth We present the fabrication technique of carbon K. Hendricks, and T. Spencer, IEEE Trans. Plasma Sci. 29, 445(2001). cathodes for high power diodes. Three types of carbon A. E. Blaugrund, Appl. Phys. Lett. 83, 1264(2003) ID Price and J N. Benford, IEEE Trans. Plasma Sci. 26, 256(1998) cathodes(planar, annular, and cylindrical) were constructe J. Benford, H. Sze, W. Woo, and B. Harteneck, Phys. Rev. Lett. 56, 344 Experimental investigations on the three cathodes were per-(1986) formed in a reflex triode vircator. a bwo. and a milo suzuka, M. Nakayama, M. Tanigawa, S Nobuhara, D. Young, and hihara, IEEE Trans. Plasma Sci. 26. 1314(1998). respectively. The diode voltage with a flattop of 200 ns was Krasik, A Dunaevsky, A. Krokhmal, J. Felsteiner, A. V Gunin, I v observed by using the planar carbon fiber cathode, indicating el, and S. D. Korovin, J. Appl. Phys. 89, 2379(2001
beam with the current density of kA/cm2 level, and the long lifetime were observed in the reflex triode vircator, the BWO, and the MILO, respectively. Although the entire properties of carbon fiber cathodes were not displayed in each HPM tube, the carbon fiber cathode can still be considered to be a promising explosive emission cathode for high-current electron beam generation. IV. CONCLUSIONS We present the fabrication technique of carbon fiber cathodes for high power diodes. Three types of carbon fiber cathodes planar, annular, and cylindrical were constructed. Experimental investigations on the three cathodes were performed in a reflex triode vircator, a BWO, and a MILO, respectively. The diode voltage with a flattop of 200 ns was observed by using the planar carbon fiber cathode, indicating that the carbon fiber cathode can provide long pulse electron beams. The lengthening of the output microwave pulse can display the slow plasma expansion velocity of carbon fiber cathodes. The uniform electron beam with a current density of more than kA/cm2 was obtained in the BWO with the annular carbon fiber cathode. In addition, the carbon fiber cathode was used for radial emission in the MILO. These results show that the carbon fiber cathodes can be widely used for electron emission in high power diodes. ACKNOWLEDGMENTS This work was supported by the National High Technology Research and Development Program of China. 1 R. B. Miller, J. Appl. Phys. 84, 3880 1998. 2 R. J. Umstattd, C. A. Schlise, and F. Wang, IEEE Trans. Plasma Sci. 33, 901 2005. 3 E. Garate, R. D. Mcwilliams, D. E. Voss, A. L. Lovesee, K. J. Hendricks, T. A. Spencer, M. C. Clark, and A. Fisher, Rev. Sci. Instrum. 66, 2528 1995. 4 R. Drori, M. Einat, D. Shur, E. Jerby, G. Rosenman, R. Advani, R. J. Temkin, and C. Pralong, Appl. Phys. Lett. 74, 335 1999. 5 Y. E. Krasik, A. Dunaevsky, and J. Felsteiner, Phys. Plasmas 8, 2466 2001. 6 L. Liu, L. Li, X. Zhang, J. Wen, H. Wan, and Y. Zhang, IEEE Trans. Plasma Sci. 35, 361 2007. 7 D. A. Shiffler, J. Luginsland, M. Ruebush, M. Lacour, K. Golby, K. Cartwright, M. Haworth, and T. Spencer, IEEE Trans. Plasma Sci. 32, 1262 2004. 8 D. Shiffler, M. Ruebush, D. Zagar, M. LaCour, M. Sena, K. Golby, M. Haworth, and R. Umstattd, J. Appl. Phys. 91, 5599 2002. 9 D. A. Shiffler, M. Lacour, K. Golby, M. Sena, M. Mitchell, M. Haworth, K. Hendricks, and T. Spencer, IEEE Trans. Plasma Sci. 29, 445 2001. 10A. E. Blaugrund, Appl. Phys. Lett. 83, 1264 2003. 11D. Price and J. N. Benford, IEEE Trans. Plasma Sci. 26, 256 1998. 12 J. Benford, H. Sze, W. Woo, and B. Harteneck, Phys. Rev. Lett. 56, 344 1986. 13M. Yatsuzuka, M. Nakayama, M. Tanigawa, S. Nobuhara, D. Young, and O. Ishihara, IEEE Trans. Plasma Sci. 26, 1314 1998. 14Y. E. Krasik, A. Dunaevsky, A. Krokhmal, J. Felsteiner, A. V. Gunin, I. V. Pegel, and S. D. Korovin, J. Appl. Phys. 89, 2379 2001. FIG. 8. Channels 1 and 2 are, respectively, the voltage and current waveforms for the MILO. Channels 3 and 4 are the microwave signals from the two main radiation lobes. 064701-4 Liu et al. Rev. Sci. Instrum. 79, 064701 2008
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