Observation of the spontaneous and stimulated emission from the ISAS compact FEL in the millimeter wave region

Observation of the spontaneous and stimulated emission from the ISAS compact FEL in the millimeter wave region

Nuclear Instruments and Methods in Physics Research A 483 (2002) 214–219 Observation of the spontaneous and stimulated emission from the ISAS compact...

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Nuclear Instruments and Methods in Physics Research A 483 (2002) 214–219

Observation of the spontaneous and stimulated emission from the ISAS compact FEL in the millimeter wave region S. Fujiia,*, H. Takedaa, T. Fujitab, T. Mizunoc, T. Ohshimac, M. Kawaid, H. Saitoc, S. Kurokib, K. Koshijia a

Science University of Tokyo, 2641, Yamazaki, Noda, Chiba 278-8510, Japan b Soka University, 1-236, Tangi, Hachioji, Tokyo, 192-8577, Japan c Institute of Space and Astronautical Science, 3-1-1, Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan d Kawasaki Heavy Industries Ltd., 118 Futatsuzuka, Noda, Chiba 278-8585, Japan

Abstract A compact FEL for submillimeter and far-infrared regions is studied in the Institute of Space and Astronautical Science. The FEL can be made compact by using an electrostatic accelerator and a micro wiggler. The FEL has a tabletop size with a total length of 2.5 m. At the preliminary phase, we aim to oscillate the FEL in the short millimeter wave. A photo cathode is used as the source of an electron beam. Characteristics of the electron beam were improved by means of designing the electron gun and the photo cathode. This electron beam was injected into an FEL resonator consisting of a waveguide and distributed Bragg reflectors and a spontaneous emission was observed. When we increased the current of the electron beam that passed through the resonator, a stimulated emission was observed with a frequency of 96 GHz and a peak power of 3.4 mW. r 2002 Elsevier Science B.V. All rights reserved. PACS: 41.60.Cr; 07.57.Hm Keywords: Free electron laser; Micro wiggler; Electrostatic accelerator

1. Introduction In a recent development, the FEL system is of very large scale and expensive. For instance, X-ray FELs using SASE method need the linac accelerator of several hundred meters length or several kilometers and long wiggler magnets of several hundred meters length. On the other hand, FELs that have laboratory size or tabletop size are very desirable for the application. *Corresponding author. Tel.: +81-42-759-8365. E-mail address: [email protected] (S. Fujii).

We study a compact FEL for submillimeter and far-infrared regions using an electrostatic accelerator and a micro wiggler at the Institute of Space and Astronautical Science, Japan. The size of the FEL including the accelerator and the wiggler magnets is very compact and the total length is 2.5 m. As an example of the compact FEL, the FEL using a classical microtron is developed at KAERI [1]. They operated the FEL at wavelengths of 97– 150 mm in 2000. This FEL system is compact and the size is just 4  4 m2. As examples of FEL with a similar accelerator as ours, the FELs with the

0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 2 ) 0 0 3 1 4 - 5

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the results of the experiment and future plans for improvement.

electrostatic accelerator of Van de Graff type have been developed at University of California, Santa Barbara and Tel-Aviv University [2,3]. Our electrostatic accelerator of DISKTRON is very compact compared with the electrostatic accelerator of Van de Graff type. In the electrostatic accelerator of DISKTRON, disks are used for the transportation of charges. Our goal is the oscillation of FEL in the submillmeter wave and far-infrared region. FELs that oscillate in this region are very useful since there are no wave sources with high power and high brightness. However, at the preliminary phase, we aim to oscillate the FEL in the millimeter wave of 96 GHz since measuring instruments are easy to obtain in this region. In Section 2, the system of the experimental apparatus and improvement of electron gun will be described. We measured characteristics of the spontaneous emission. The result of this measurement will be reported in Section 3. The generation and measurement of the stimulated emission will be shown in Section 4. In Section 5, we summarize

2. Apparatus of FEL The schematic diagram of the experimental apparatus is shown in Fig. 1. This compact FEL system consists of an electron gun with a photo cathode, an electrostatic accelerator (DISKTRON), an accelerating tube of short length, a beam line with control system of the electron beam, a micro wiggler and a waveguide resonator with distributed Bragg reflectors (DBRs) [4]. The waveguide resonator is applied as a resonator since the gap of the wiggler magnets is 3 mm and the total length of the wiggler magnets is 248 mm. The straight beam line is kept as short as possible to retain the high quality of the beam. We use DBRs as reflecting mirrors of resonator since the electron beam can pass through the DBR. In the design of our FEL system, the peak beam current, the pulse width of the beam, the beam diameter, the growth rate of the electromagnetic wave and the one pass

Electrostatic accelerator (DISKTRON) Electron gun

215

Accelerating tube Solenoid lens

Current monitor

DBRs

Micro wiggler

Steering magnet Waveguide resonator Photo tube Excimer laser (KrF)

Delay path

2.5m Fig. 1. Schematic diagram of the experimental apparatus.

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gain are 0.5 A, 42 ns, 2 mm, 6.0 m1 and 1.5, respectively [5]. The normalized emittance of the electron beam at the end of the accelerating tube is decreased from 4.4 to 1.5p mm mrad by redesigning the shape of the electrodes in the electron gun. In the previous design, all the surfaces of electrodes were flat. Some curved surfaces are used in this design. We used the code of E-GUN2W that calculates the trajectory of the electron beam. We can increase the output power of the electromagnetic wave by expanding the pulse width of the electron beam since the interacting time between the electron and the electromagnetic wave is increased. As shown in Fig. 1, the excimer laser that is used for irradiating the photo cathode was divided into two beams. One of the beams is given an additional optical path to irradiate the cathode with delay. The present electron beam has a pulse width of 42 ns, which is two times as long as the pulse width of the excimer laser. The main parameters of the system are listed in Table 1.

3. Spontaneous emission The millimeter wave radiated when we injected the electron beam into the waveguide with the wiggler field. We detected the radiation of the millimeter wave by using a W-band diode. The typical waveform of the detected signal is shown in Fig. 2. Figs. 2(a)–(c) show the waveform of the excimer laser for the photo cathode, the current of the electron beam entering the waveguide and the detected signal of the millimeter wave, respectively. The peak power of the millimeter wave is 1.7 mW. Fig. 3 shows the peak power of the millimeter wave against the energy of the electron beam. The bars in Fig. 3 indicate the typical range of shot-toshot reproducibility. In this measurement, the magnetic field in the beam line was constant in amplitude. When the energy of the electron beam is increased or decreased from the optimized energy of 680 keV, the current of the electron beam that can pass through the waveguide is decreased. The peak power of the millimeter wave without DBRs is decreased in the same manner. In

Table 1 Main parameters of FEL system Micro wiggler Type Period Number of periods Total length Gap K-parameter Magnets

Planer Halbach 8 mm 29 248 mm 3 mm 0.48 Nd–Fe–B

Resonator Type Width Height Length Resonator Q Reflectors Reflection index Center frequency FWHM of reflection

Rectangular waveguide 2.25 mm 3.5 mm 290 mm 2230 Distributed Bragg reflectors 0.91 (designed) 96.2 GHz 8.5 GHz

Electron beam Energy Peak current Pulse width Repetition rate Beam diameter Current density Energy spread Normalized emittance

B1 MeV 1.5 A 42 ns 1 Hz 2.5 mm 31 A/cm2 0.18% 1.5 pmm mrad (designed)

Fig. 2. Typical waveforms: (a) magnitude of the excimer laser; (b) current of the electron beam entering the waveguide; (c) detected signal of the millimeter wave.

S. Fujii et al. / Nuclear Instruments and Methods in Physics Research A 483 (2002) 214–219

frequency of the millimeter wave is measured to be 98 GHz from the peak interval in the resonance length. The finesse of the Fabry–Perot resonator and the bandwidth of the millimeter wave causes the spread of the resonance length. The FWHM bandwidth of the millimeter wave Dl=l0 is approximately expressed as qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð2=nÞ ðDln Þ2  ðl0 =2F Þ2 Dl ¼ ð1Þ l0 l0

Peak power of output (mW)

1.8

1.2

0.6

640

680

720

760

Energy of electron beam (keV)

Fig. 3. Peak power of millimeter wave versus the energy of electron beam. The bars indicate the typical range of shot-toshot reproducibility.

the case of using the waveguide with DBRs in Fig. 3, the peak power of the millimeter wave is decreased at a beam energy of around 680 keV. This decrease is due to the fact that the millimeter wave is reflected by the DBR at the end of the waveguide since the frequency of the millimeter wave radiating at this beam energy is the reflecting frequency of the DBR. We measured the frequency of the millimeter wave by means of a Fabry–Perot resonator. The Fabry–Perot resonator consists of a silicon plate and an aluminum plate. The silicon plate is used as the input side of the millimeter wave. The thickness of the silicon plate is 240 mm. The aluminum plate is used as the output side of the millimeter wave. The aluminum plate has a tiny hole that is the same as the W-band rectangular waveguide (2.54  1.27 mm2) and connected with the waveguide. The minimum resolution of the Fabry–Perot resonator is 1.5 GHz and the finesse is 11, being calibrated by means of an IMPATT diode as a monochromatic source. When the energy of the electron beam is 680 keV, the center

where l0 ; F ; and Dln are the wavelength of the millimeter wave, the finesse of the Fabry–Perot resonator and the FWHM spread of the nth resonance length, respectively. In Eq. (1), both the spectrum of the millimeter wave and the distribution of the resonance length due to the resonator finesse are assumed to be Gaussian distributions. The spectral spread of the millimeter wave is calculated to be 8% by using Eq. (1). Furthermore, we measured the frequency of the millimeter wave radiating at the beam energies of 630 and 730 keV. The result of this measurement is shown in Fig. 4. The theoretical curve is shown in Fig. 4 as a solid line [6]. The difference between the experimental results and the theoretical curve is

130 Theory Experiment

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Frequency (GHz)

with DBRs without DBRs

0 600

217

110

100

90

80 600

650

700

750

800

Energy of electron beam (keV)

Fig. 4. Center frequency of the millimeter wave versus the energy of the electron beam: result of the measurement value (dot); theoretical curve (solid line).

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about 40 keV. We think that this difference is caused by the error of the accelerating voltage as well as the fabrication error of the wavguide width. The accelerating voltage is measured by using a capacitor in the accelerator.

4. Stimulated emission

Fig. 5. Typical waveforms. Guide magnetic field was supplied in this measurement: (a) magnitude of the excimer laser; (b) current of the electron beam entering the waveguide; (c) detected signal of the millimeter wave.

with DBRs without DBRs

3.5

3

Peak power of output (mW)

To generate the stimulated emission, we increased the beam current that can pass through the waveguide. In the case of the measurement as described in the previous section, the peak current of the electron beam after passing through the waveguide resonator is only 0.28 A. The rate of transmission is 19%. In this case, the peak growth rate of the electromagnetic wave is theoretically calculated to be 5.1 m1 and the one pass gain of the FEL is o1 [5]. To increase the beam current that can pass through the waveguide resonator, we fabricated a guide magnet that supplies the guide field in the wiggling region. By using this guide field, the peak current of the electron beam after passing through the waveguide resonator was increased to 0.36 A. In this case, the growth rate of the electromagnetic wave is 5.3 m1 and the one pass gain of the FEL is 1.1. We generated and detected the millimeter wave in this condition. When the energy of the electron beam is 690 keV, the peak power of the millimeter wave is 3.4 mW. Typical waveforms measured at the beam energy of 690 keV are shown in Fig. 5. Figs. 5(a)–(c) show the waveform of the eximer laser, the current of the electron beam and the detected signal of the millimeter wave, respectively. Fig. 6 shows the peak power of the millimeter wave against the energy of the electron beam. The bars in Fig. 6 indicate the typical range of shot-toshot reproducibility. The magnetic field in the beam line was constant in amplitude. As shown in Fig. 6, a rapid increase in the peak power of the millimeter wave was measured by using the waveguide with DBRs and the electron beam with the energy of 670–720 keV. The frequency of the millimeter wave was measured by using the Fabry– Perot resonator. When the energy of the electron beam was 690 keV, the center frequency was measured to be 96 GHz. The interferogram data

2.5

2

1.5

1

0.5

0 600

640

680

720

760

Energy of electron beam (keV)

Fig. 6. Peak power of the millimeter wave versus the energy of the electron beam. Guide magnetic field was supplied in this measurement. The bars indicate the typical range of shot-toshot reproducibility.

of the resonance length for this stimulated emission are measured to be the same as those for the monochromatic IMPATT source. This result indicates that the reproducibility of the center

S. Fujii et al. / Nuclear Instruments and Methods in Physics Research A 483 (2002) 214–219

frequency and the bandwidth are much smaller than one due to the resonator finesse.

5. Summary In the configuration without the guide magnetic field, we generate the millimeter wave radiation from the wiggler field. The peak power, the center frequency, and the bandwidth are 1.7 mW, 98 GHz and 8%, respectively. Furthermore, no radiation is observed from the electron beam when there is no wiggler field. We consider this radiation as a spontaneous emission. The spread of the spectral is comparatively broad and even for the waveguide without DBRs, the radiation is generated. The spontaneous emission is the partial coherent radiation with the spectral spread of 1=2N; where N is the number of the wiggler period. This spread is 2% in our micro wiggler. The radiating power of spontaneous emission is calculated to be about 2 mW [7]. The center frequency of the spontaneous emission depends on the energy of the electron beam as shown in Fig. 4. The measured frequencies in Fig. 4 approximately agree with the theoretical curve. To generate the stimulated emission, we increased the beam current by using the guide magnet. The millimeter wave with a peak power of 3.4 mW was generated. The center frequency is 96 GHz by using the electron beam with an energy of 690 keV and a peak current of 0.36 A after passing through the resonator. The bandwidth is

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significantly narrower than one of the spontaneous emission. We consider this radiation as the stimulated emission because of the rapid increase in the peak power of the millimeter wave and the narrow bandwidth. To increase the power of the stimulated emission, we plan to increase the beam current by means of improvement of the guide magnetic field. Also, we plan to expand the pulse width of the electron beam. The electromagnetic wave can grow by expanding the pulse width. To expand the pulse width, we plan to replace the photo cathode by a thermionic cathode. This cathode can generate an electron beam with a pulse width of 1–5 ms.

References [1] Y.U. Jeong, et al., First lasing of the KAERI compact farinfrared free-electron laser driven by a magnetron-based microtron, Presented at the 22nd International FEL Conference, Durham, USA, August 2000, Nucl. Instr. and Meth. A 475 (2001) 47. [2] G. Ramian, et al., Nucl. Instr. and Meth. A 393 (1997) 220. [3] A. Abramovich, et al., Nucl. Instr. and Meth. A 407 (1998) 220. [4] S. Fujii, et al., Proceedings of the 22nd International FEL Conference, II-9, Elsevier, Amsterdam, 2000. [5] S. Fujii, et al., Experimental study of compact FEL with micro wiggler and electrostatic accelerator, Presented at the 22nd International FEL Conference, Durham, USA, August 2000, Nucl. Instr. and Meth. A 475 (2001) 281. [6] R.M. Phillips, IRE Trans. Electron Dev. ED-7 (1960) 231. [7] W.B. Colson, et al. (Eds.), Laser Handbook, Vol. 6, Elsevier, Amsterdam, 1990.