- Email: [email protected]
Direct observation of beam bunching in BWO experiments
Nuclear Instruments and Methods in Physics Research A 475 (2001) 509–513
Direct observation of beam bunching in BWO experiments I. Morimotoa,*, X.D. Zhengb, S. Maebarac, J. Kishirod, K. Takayamad, K. Horiokaa, H. Ishizukae, S. Kawasakic, M. Shihoa,c a
Tokyo Institute of Technology, 4259 Nagatsuda, Midori-ku, Yokohama 226-8502, Japan b Beijing Normal University, 19 out-of-Xinjiekou Street, Beijing 100875, China c Japan Atomic Energy Research Institute, Fusion Research Establishment, Naka-machi, Ibaraki 311-0193, Japan d High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan e Fukuoka Institute of Technology, Wajiro, Higashi-ku, Fukuoka 811-0295, Japan
Abstract Backward Wave Oscillation (BWO) experiments using a Large current Accelerator-1 (Lax-1) Induction Linac as a seed power source for an mm-wave FEL are under way. The Lax-1 is typically operated with a 1 MeV electron beam, a few kA of beam current, and a pulse length of 100 ns. In the BWO experiments, annular and solid beams are injected into a corrugated wave guide with guiding axial magnetic field of 1 T. In the BWO with annular beam an output power of 210 MW at 9.8 GHz was obtained. With a solid beam the output power was 130 MW, and an electron beam bunching with the frequency of 9.6–10.2 GHz was observed by a streak camera. r 2001 Published by Elsevier Science B.V. PACS: 84.40.Fe Keywords: BWO; Beam bunching; X-band FEL; Seed power source; Pre-buncher
1. Introduction At JAERI, we started research on a GW-class X-band FEL using the induction linac (JLA; B4 MeV, B3 kA, B160 ns) [1]. In this FEL system, a Backward Wave Oscillator (BWO) is expected to work as a seed power source for the FEL and a pre-buncher (Fig. 1) [2,3]. For this purpose, using the smaller induction linac Lax-1 a preliminary study is carried out. The output power of 210 MW was obtained at 9.8 GHz [4]. The RF power is enough as a seed power of FEL, but beam bunching characteristics in the BWO are not *Corresponding author. Tel.: +81-29-282-5102. E-mail address: [email protected] (I. Morimoto).
clearly observed in previous work. In this paper the first observation of beam bunching with the BWO is presented.
2. BWO characteristics In this BWO, the corrugated wave guide as a slow wave structure is designed as in Fig. 2 [4]. The corrugated function is RðzÞ ¼ R0 þ h cos ð2pz=15:0 mmÞ; R0 ¼ 13:70 mm
h ¼ 2:8 mm;
pitch number is 10. Using this set up in the BWO, from the dispersion relation the output RF is only TM01 mode at 9.8 GHz (Figs. 3 and 4). Using the
0168-9002/01/$ - see front matter r 2001 Published by Elsevier Science B.V. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 6 0 2 - 3
510
I. Morimoto et al. / Nuclear Instruments and Methods in Physics Research A 475 (2001) 509–513 Beam JLA
Bunching beam BWO
Bunching beam Wiggler
RF
Application Amplified RF
Fig. 2. The corrugated wave guide as a slow wave structure for 9.8 GHz BWO (The mean radius R0 is 13.7 mm, the depth h is 2.8 mm, and the pitch number is 10).
R 0 =15.0 mm, z 0 =15.0 mm
FREQUENCY f (GHz)
11
-1
33.0mm
GROWTH RATE ω i (ns )
21.8mm
the electron beam
FREQUENCY f (GHz)
Fig. 1. Schematic view of JLA-X-band FEL.
10
5
0 0.5 0.4 0.3 0.2 0.1 0
0
1
2
3
4
WAVENUMBER k z (cm-1)
Fig. 4. Wave number vs. frequency and growth rate. The output frequency is 9.8 GHz in this BWO.
10 9 8 h=2.5 mm h=3.0 mm
7
h=3.5 mm 6 0
1
2
3
4
WAVENUMBER k z (cm-1 )
Fig. 3. The corrugated wave guide as a slow wave structure for 9.8 GHz BWO (The mean radius R0 is 13.7 mm, the depth h is 2.8 mm, and the pitch number is 10).
MAGIC code, an output power of 270 MW is expected (Fig. 5). The characteristics of the BWO are shown in Table 1.
3. Beam bunching observation As schematically shown in Fig. 6, an extracted electron beam (700 keV, 2B3 kA, 100 ns) using the
Fig. 5. Microwave power calculation in outlet of slow wave structure using MAGIC code.
induction linac Lax-1 is transported to the corrugated wave guide. The quartz window is set in the outlet of the corrugated wave guide. The Cherenkov light is radiated when the beam hits the quartz window. The radiated Cherenkov light is
I. Morimoto et al. / Nuclear Instruments and Methods in Physics Research A 475 (2001) 509–513
measured by a streak camera (time resolution is 4.0 ps, sweep speed is 30B670 ps/mm). Simultaneously, the RF output power is measured. The streak data are measured around the peak current of the beam (Fig. 7). We studied two types of beams in the beam bunching experiment. One was an annular beam, the other was a solid beam. Using the annular beam (the inner cathode diameter was 12.0 mm, the outer cathode diameter was 16.0 mm, the beam current was 2.7 kA, the accelerated voltage was 710 kV), a beam bunching was measured in the RF output power of 210 MW [4]. The typical waveform of the beam current and the RF output power is shown in Figs. 7 and 8, respectively. The frequency of 9.8 GHz was estimated by FET procedure described elsewhere [3]. The result of a bunching measurement is shown in Fig. 9a. The upper figure of Fig. 9 is the position of the beam in the corrugated wave guide. The position of the camera was adjusted so that the camera did not observe the diode part directly (Fig. 6), this means that only the Cherenkov light
511
was observed from the quartz window. The lower figure of Fig. 9 is the picture from a streak camera (sweep speed is 67 ps/mm). From that picture a bunching of an annular beam was not clearly observed. When the solid beam (the cathode diameter is 16 mm, the beam current is 1.9 kA, the accelerated voltage is 710 kV) was injected, the RF output power was 60% of 210 MW. The frequency in this case is the same. The picture from a streak camera is shown in Fig. 9b. In this case, the beam bunching was clearly obtained. Fig. 10 shows a time dependence of the light intensity. The beam bunching frequency was estimated from 9.6 to 10.2 GHz. This corresponded to the RF frequency of 9.8 GHz within the experimental error.
4. Discussion In the experiments, we always observe the bunching with the solid beam but with the annular
50ns/div Beam Current Table 1 BWO parameters Accelerated voltage Beam current Pitch length of slow wave structure Pitch number of slow wave structure Inner radius of wave guide Outer radius of wave guide Wave guide length Axial magnetic field Designed frequency Expected output power
1
900 kV 1.5 kA 15 mm 10 10.9 mm 16.5 mm 15 cm 1T 9.6 GHz 270 MW
2
RF Signal
Streak data Fig. 7. The upper wave is the beam current 2.7 kA. The down wave is the RF output power of 210 MW. Streak data is measured around the peak current.
Fig. 6. Schematic view of the BWO.
512
I. Morimoto et al. / Nuclear Instruments and Methods in Physics Research A 475 (2001) 509–513 80
Intensity
70
60
50
40
0 Fig. 8. FFT of oscilloscope data, The output frequency is monochromatic 9.8 GHz.
0.1
0.2
0.3 0.4 0.5 Time [ns]
0.6 0.7
Fig. 10. A time dependence of the intensity of the Cherenkov light.
Fig. 11. A longitudinal electric field distribution of 210 MW TM01 mode in a cylindrical wave guide (radius 10.9 mm).
Fig. 9. Typical example of optical measurement for 67 ps/mm at the outlet of the corrugated wave guide: (a) in the annular beam the beam bunching was not clear and (b) in the solid beam the beam bunching was clear.
the bunching is not clearly seen. The bunching is possibly caused by the coupling of the RF of TM01 mode with a beam. When we calculate electric field strength within the corrugated wave guide, it is
found that the strength of the electric field in the central part is four times higher than that of the peripheral part (Fig. 11). At present, we think that the bunching is closely related to the strength of the electric field. Considering the fact that we can always observe higher RF output power with annular beams, why the beam bunching is not clearly observed in the annular beams is open for future study.
I. Morimoto et al. / Nuclear Instruments and Methods in Physics Research A 475 (2001) 509–513
513
From the viewpoint of using BWO as a prebuncher for FELs, to what extent beam bunching is maintained as another important item to be determined.
From the engineering point of view we can reasonably expect that a BWO with the solid beam works as a seed RF power and pre-buncher for Xband FELs.
5. Conclusion
References
We always observed the beam bunching with the solid beam. Although the RF output power is not maximum, the BWO with the solid beam always produced a high RF output power of more than 100 MW level, which is enough as a seed power of FEL. Moreover, the BWO with solid beam always produced strong beam bunching.
[1] S. Maebara, et al., Nucl. Instr. and Meth., in this conference, WE-4-06. [2] S. Kawasaki, et al., Nucl. Instr. and Meth. A 341 (1994) 316. [3] X. Zheng, et al., Nucl. Instr. and Meth. A 407 (1998) 198. [4] X.D. Zheng, et al., 13th International Conference on HighPower Particle Beams, Nagaoka, Japan, June 25–30, 2000.
Direct observation of beam bunching in BWO experiments I. Morimotoa,*, X.D. Zhengb, S. Maebarac, J. Kishirod, K. Takayamad, K. Horiokaa, H. Ishizukae, S. Kawasakic, M. Shihoa,c a
Tokyo Institute of Technology, 4259 Nagatsuda, Midori-ku, Yokohama 226-8502, Japan b Beijing Normal University, 19 out-of-Xinjiekou Street, Beijing 100875, China c Japan Atomic Energy Research Institute, Fusion Research Establishment, Naka-machi, Ibaraki 311-0193, Japan d High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan e Fukuoka Institute of Technology, Wajiro, Higashi-ku, Fukuoka 811-0295, Japan
Abstract Backward Wave Oscillation (BWO) experiments using a Large current Accelerator-1 (Lax-1) Induction Linac as a seed power source for an mm-wave FEL are under way. The Lax-1 is typically operated with a 1 MeV electron beam, a few kA of beam current, and a pulse length of 100 ns. In the BWO experiments, annular and solid beams are injected into a corrugated wave guide with guiding axial magnetic field of 1 T. In the BWO with annular beam an output power of 210 MW at 9.8 GHz was obtained. With a solid beam the output power was 130 MW, and an electron beam bunching with the frequency of 9.6–10.2 GHz was observed by a streak camera. r 2001 Published by Elsevier Science B.V. PACS: 84.40.Fe Keywords: BWO; Beam bunching; X-band FEL; Seed power source; Pre-buncher
1. Introduction At JAERI, we started research on a GW-class X-band FEL using the induction linac (JLA; B4 MeV, B3 kA, B160 ns) [1]. In this FEL system, a Backward Wave Oscillator (BWO) is expected to work as a seed power source for the FEL and a pre-buncher (Fig. 1) [2,3]. For this purpose, using the smaller induction linac Lax-1 a preliminary study is carried out. The output power of 210 MW was obtained at 9.8 GHz [4]. The RF power is enough as a seed power of FEL, but beam bunching characteristics in the BWO are not *Corresponding author. Tel.: +81-29-282-5102. E-mail address: [email protected] (I. Morimoto).
clearly observed in previous work. In this paper the first observation of beam bunching with the BWO is presented.
2. BWO characteristics In this BWO, the corrugated wave guide as a slow wave structure is designed as in Fig. 2 [4]. The corrugated function is RðzÞ ¼ R0 þ h cos ð2pz=15:0 mmÞ; R0 ¼ 13:70 mm
h ¼ 2:8 mm;
pitch number is 10. Using this set up in the BWO, from the dispersion relation the output RF is only TM01 mode at 9.8 GHz (Figs. 3 and 4). Using the
0168-9002/01/$ - see front matter r 2001 Published by Elsevier Science B.V. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 6 0 2 - 3
510
I. Morimoto et al. / Nuclear Instruments and Methods in Physics Research A 475 (2001) 509–513 Beam JLA
Bunching beam BWO
Bunching beam Wiggler
RF
Application Amplified RF
Fig. 2. The corrugated wave guide as a slow wave structure for 9.8 GHz BWO (The mean radius R0 is 13.7 mm, the depth h is 2.8 mm, and the pitch number is 10).
R 0 =15.0 mm, z 0 =15.0 mm
FREQUENCY f (GHz)
11
-1
33.0mm
GROWTH RATE ω i (ns )
21.8mm
the electron beam
FREQUENCY f (GHz)
Fig. 1. Schematic view of JLA-X-band FEL.
10
5
0 0.5 0.4 0.3 0.2 0.1 0
0
1
2
3
4
WAVENUMBER k z (cm-1)
Fig. 4. Wave number vs. frequency and growth rate. The output frequency is 9.8 GHz in this BWO.
10 9 8 h=2.5 mm h=3.0 mm
7
h=3.5 mm 6 0
1
2
3
4
WAVENUMBER k z (cm-1 )
Fig. 3. The corrugated wave guide as a slow wave structure for 9.8 GHz BWO (The mean radius R0 is 13.7 mm, the depth h is 2.8 mm, and the pitch number is 10).
MAGIC code, an output power of 270 MW is expected (Fig. 5). The characteristics of the BWO are shown in Table 1.
3. Beam bunching observation As schematically shown in Fig. 6, an extracted electron beam (700 keV, 2B3 kA, 100 ns) using the
Fig. 5. Microwave power calculation in outlet of slow wave structure using MAGIC code.
induction linac Lax-1 is transported to the corrugated wave guide. The quartz window is set in the outlet of the corrugated wave guide. The Cherenkov light is radiated when the beam hits the quartz window. The radiated Cherenkov light is
I. Morimoto et al. / Nuclear Instruments and Methods in Physics Research A 475 (2001) 509–513
measured by a streak camera (time resolution is 4.0 ps, sweep speed is 30B670 ps/mm). Simultaneously, the RF output power is measured. The streak data are measured around the peak current of the beam (Fig. 7). We studied two types of beams in the beam bunching experiment. One was an annular beam, the other was a solid beam. Using the annular beam (the inner cathode diameter was 12.0 mm, the outer cathode diameter was 16.0 mm, the beam current was 2.7 kA, the accelerated voltage was 710 kV), a beam bunching was measured in the RF output power of 210 MW [4]. The typical waveform of the beam current and the RF output power is shown in Figs. 7 and 8, respectively. The frequency of 9.8 GHz was estimated by FET procedure described elsewhere [3]. The result of a bunching measurement is shown in Fig. 9a. The upper figure of Fig. 9 is the position of the beam in the corrugated wave guide. The position of the camera was adjusted so that the camera did not observe the diode part directly (Fig. 6), this means that only the Cherenkov light
511
was observed from the quartz window. The lower figure of Fig. 9 is the picture from a streak camera (sweep speed is 67 ps/mm). From that picture a bunching of an annular beam was not clearly observed. When the solid beam (the cathode diameter is 16 mm, the beam current is 1.9 kA, the accelerated voltage is 710 kV) was injected, the RF output power was 60% of 210 MW. The frequency in this case is the same. The picture from a streak camera is shown in Fig. 9b. In this case, the beam bunching was clearly obtained. Fig. 10 shows a time dependence of the light intensity. The beam bunching frequency was estimated from 9.6 to 10.2 GHz. This corresponded to the RF frequency of 9.8 GHz within the experimental error.
4. Discussion In the experiments, we always observe the bunching with the solid beam but with the annular
50ns/div Beam Current Table 1 BWO parameters Accelerated voltage Beam current Pitch length of slow wave structure Pitch number of slow wave structure Inner radius of wave guide Outer radius of wave guide Wave guide length Axial magnetic field Designed frequency Expected output power
1
900 kV 1.5 kA 15 mm 10 10.9 mm 16.5 mm 15 cm 1T 9.6 GHz 270 MW
2
RF Signal
Streak data Fig. 7. The upper wave is the beam current 2.7 kA. The down wave is the RF output power of 210 MW. Streak data is measured around the peak current.
Fig. 6. Schematic view of the BWO.
512
I. Morimoto et al. / Nuclear Instruments and Methods in Physics Research A 475 (2001) 509–513 80
Intensity
70
60
50
40
0 Fig. 8. FFT of oscilloscope data, The output frequency is monochromatic 9.8 GHz.
0.1
0.2
0.3 0.4 0.5 Time [ns]
0.6 0.7
Fig. 10. A time dependence of the intensity of the Cherenkov light.
Fig. 11. A longitudinal electric field distribution of 210 MW TM01 mode in a cylindrical wave guide (radius 10.9 mm).
Fig. 9. Typical example of optical measurement for 67 ps/mm at the outlet of the corrugated wave guide: (a) in the annular beam the beam bunching was not clear and (b) in the solid beam the beam bunching was clear.
the bunching is not clearly seen. The bunching is possibly caused by the coupling of the RF of TM01 mode with a beam. When we calculate electric field strength within the corrugated wave guide, it is
found that the strength of the electric field in the central part is four times higher than that of the peripheral part (Fig. 11). At present, we think that the bunching is closely related to the strength of the electric field. Considering the fact that we can always observe higher RF output power with annular beams, why the beam bunching is not clearly observed in the annular beams is open for future study.
I. Morimoto et al. / Nuclear Instruments and Methods in Physics Research A 475 (2001) 509–513
513
From the viewpoint of using BWO as a prebuncher for FELs, to what extent beam bunching is maintained as another important item to be determined.
From the engineering point of view we can reasonably expect that a BWO with the solid beam works as a seed RF power and pre-buncher for Xband FELs.
5. Conclusion
References
We always observed the beam bunching with the solid beam. Although the RF output power is not maximum, the BWO with the solid beam always produced a high RF output power of more than 100 MW level, which is enough as a seed power of FEL. Moreover, the BWO with solid beam always produced strong beam bunching.
[1] S. Maebara, et al., Nucl. Instr. and Meth., in this conference, WE-4-06. [2] S. Kawasaki, et al., Nucl. Instr. and Meth. A 341 (1994) 316. [3] X. Zheng, et al., Nucl. Instr. and Meth. A 407 (1998) 198. [4] X.D. Zheng, et al., 13th International Conference on HighPower Particle Beams, Nagaoka, Japan, June 25–30, 2000.