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Infrared Physics & Technology 51 (2008) 390–393

Generation of 0.1 THz coherent synchrotron radiation with compactS-band linac at AIST

R. Kuroda *, N. Sei, M. Yasumoto, H. Toyokawa, H. Ogawa, M. Koike, K. Yamada

Research Institute of Instrumentation Frontier, National Institute of Advanced Industrial Science and Technology (AIST), Central 2, 1-1-1

Umezono, Tsukuba, Ibaraki 305-8568, Japan

Available online 1 February 2008

Abstract

The 0.1 THz coherent synchrotron radiation (CSR) was successfully generated in the 90� bending magnet of the compact S-band linacwith the achromatic arc section using the ultra-short electron bunch which has the energy of 40 MeV, the bunch charge of about 1nc andthe bunch length less than 1 ps (rms). The electron bunch compression of 1 nC electron bunch was achieved less than 1 ps (rms) by con-trolling the Q-magnets in the achromatic arc section as the bunch length was measured by the rms bunch length monitor.� 2007 Elsevier B.V. All rights reserved.

Keywords: THz radiation; Coherent synchrotron radiation (CSR); Bunch compression; Electron linac

1. Introduction

Lack of the intense terahertz (THz) radiation source isthe major obstacle in progressing on biomedical and mate-rial studies [1,2]. SASE-FEL light source which is one ofthe high power THz radiation source has been designedbased on an S-band compact electron linac at AIST. TheFEL will be operated in the wavelength range of 100–300 lm, which corresponds to 1–3 THz using 40 MeV elec-tron beam and a small undulator which has number of per-iod of 20 and undulator period of 100 mm [3]. In thisdesign, the output power of the THz SASE-FEL is morethan about 2 W with the narrow spectral width of 2–3%,output stability of 2–3% and pulse width of 5–10 ps (1r)using multi-bunch electron beam which is about 1 nC/bunch � 100 bunches at repetition rate of 50 Hz.

As a preliminary experiment, we have performed exper-iments to generate 0.1 THz coherent synchrotron radia-tion (THz CSR) using a 40 MeV compact S-band linacwith a electron injector, an achromatic arc section and a

1350-4495/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.infrared.2007.12.010

* Corresponding author. Tel.: +81 29 861 5104; fax: +81 29 861 5683.E-mail address: ryu-kuroda@aist.go.jp (R. Kuroda).

90� bending magnet. The electron injector consists of alaser photo-cathode rf gun which has the BNL type S-band1.6 cell cavity and a solenoid magnet for emittance com-pensation. The electron linac has two 1.5-m-long accelerat-ing structures which is designed for 5 nC/bunch as a 1/2 pmode standing wave structure with an alternative periodicstructure [4]. The electron beam can be accelerated up toabout 42 MeV using the rf source of a 20 MW klystron.The achromatic arc section consists of two 45� bendingmagnets and four Q-magnets for the bunch compression.The 90� bending magnet was located after the Q-tripletdownstream from the achromatic arc section and CSRexperiment was performed at 20� direction in the 90� bend-ing magnet which had the curvature radius of 300 mm.

2. Theoretical electron bunch compression

The bunch compression study with the achromatic arcsection has been carried out using PARMELA andTRACE-3D so that it is found that an ultra short and highcharge electron bunch with less than 1 ps and with morethan 1 nC can be easily generated by optimizing injectorparameters, linac phases and Q-magnets parameters ofthe achromatic arc section [5]. The transformation in the

Table 1Parameters of electron beam

Maximum energy 30–42 MeV

Charge per bunch 1 nCEnergy spread for compression �5%Bunch length (after compression) 500 fs (rms)Rep rate 10–50 Hz

R. Kuroda et al. / Infrared Physics & Technology 51 (2008) 390–393 391

achromatic arc section can be easily expressed usingR-matrix by

zf ¼ z0 þ R56

DEE

� �; ð1Þ

where zf and z0 are longitudinal positions of electrons at theexit of the arc and at the entrance of the arc, respectively.R56 is the momentum compaction coefficient of the first-or-der effects. In case of a negative value of R56, it is foundfrom Eq. (1) that the high energy electron and low energyelectron are moved to backward and forward in the longi-tudinal direction, respectively. As a result of the simulationusing TRACE-3D, the bunch length was compressed toabout 500 fs (rms) with R56 of about 21 mm. The typicalelectron beam parameters using our system were describedin Table 1.

3. Theoretical CSR generation

Synchrotron radiation less than critical frequency xc iscoherently emitted from a ultra short electron bunch (rz).Its frequency is expressed by

xc ¼ pc=rz: ð2Þ

The total photons (Itot) with both of incoherent andcoherent radiation are derived from equations

I tot ¼ I incð1þ ðN � 1Þf ðxÞÞ; ð3Þ

and

f ðxÞ ¼ e�ðxrzÞ2

2 : ð4Þ

Fig. 1. Enhancement factor of CSR as a function of frequency bychanging electron rms bunch length (0.5 ps, 1 ps, 5 ps, 10 ps).

Here, Iinc is the photons of incoherent radiation, N is thenumber of electrons in the bunch and f(x) is the fouriertransform of the longitudinal electron density for Gaussianbunches with bunch length rz [6].

In Fig. 1, the enhancement factor as a function of fre-quency was calculated by changing the electron bunchlength from 0.5 ps to 10 ps with 1 nC and 30 MeV againstthe incoherent synchrotron radiation yield of about0.1 THz which is normalized to 1. In this experiment, ourrf detector is a W-band rf detector (WiseWave FAS-10SF-01) which has a sensitive range of 0.075–0.11 THzso that the requirement of the electron bunch length is lessthan about 5 ps (rms).

4. Experimental 0.1 THz CSR generation

The coherent synchrotron radiation (CSR) was gener-ated using an ultra short and high charge electron bunchat the 90� bending magnet located after Q-triplet down-stream from the achromatic arc section. The setup ofexperimental components for the CSR generation is shownin Fig. 2. Fig. 3 shows the setup of the detection of 0.1 THzCSR. The CSR was extracted from a crystal quarts windowlocated at 20� direction in the 90� bending magnet andcollected by the parabolic antenna and passed through aW-band wave guide (WR-10), an E-bend and an attenuatorand guided to the W-band rf detector which signal of500 mV corresponded to 1 mW for about 0.1 THz radia-tion. To generate the THz CSR, the bunch length was com-pressed less than 1 ps using Q-magnets (Q1, Q2, Q3, Q4) inthe achromatic arc section as it was measured with the rmsbunch length monitor using two-frequency analysis tech-nique [7]. Fig. 4 shows one of the CSR generation results.

Fig. 2. Setup of 0.1 THz CSR generation using the achromatic arc section,bunch length monitor and 90� bending magnet.

Fig. 3. Setup of 0.1 THz CSR detection using quarts window, parabolicantenna, W-band wave guide, W-band attenuator and W-band RFdetector.

Fig. 6. CSR signal and Bunch length measured by W-band detector andthe bunch length monitor as a function of Q4 current.

392 R. Kuroda et al. / Infrared Physics & Technology 51 (2008) 390–393

It is clearly found that 0.1 THz radiation was emittedcoherently against electron bunch charge.

Fig. 5 shows the simulation result of TRACE-3D that thebunch length as a function of Q4 magnetic field. It means thatthe bunch length can be easily controlled from less than 1 psto more than 10 ps by changing the Q4 field. Minimumbunch length was about 500 fs (rms) with R56 = �21 mm.Initial bunch length of about 8.8 ps was equal to the bunchlength at the exit of the achromatic arc section withR56 = 0. The decrease of Q4 current means the increase of

Fig. 4. Power of 0.1 THz CSR generation as a function of electron bunchcharge.

Fig. 5. Bunch length as a function of Q4 field calculated with TRACE-3D.

R56 to more than 0 and the positive energy chirp of the elec-tron bunch. The increase of Q4 current means the decrease ofR56 to less than 0 and the negative energy chirp of the electronbunch for the bunch compression.

Fig. 6 shows 0.1 THz CSR power and rms bunch lengthas a function of Q4 current with 1 nC, 40 MeV electronbunch. The CSR signal and Bunch length were measuredby W-band detector and the bunch length monitor, respec-tively. As a result, the 0.1 THz CSR was observed with themaximum peak power of about 0.23 mW. However, themaximum power point is slightly deferent from the mini-mum bunch length point. Its deference is due to the loca-tion deference between the bunch length monitor and theCSR emission point. Another reason is considered thatthe CSR emission point is located at 20� of 90� bendingmagnet so that the electron bunch compressed by the ach-romatic arc section is slightly expanded in the 90� bendingmagnet. The minimum bunch length at the 20� emissionpoint is around R56 = 0 of the achromatic arc section dueto the misalignment of the Q-triplet downstream of theachromatic arc section. To solve this problem, the horizon-tal beam size which is coupled with the bunch length at the20� emission point should be adjusted by the Q-magnetlocated at the middle of Q-triplet and the electron bunchshould be focused right before the 20� emission points. Incase of R56 > 0, even if the bunch length was less than8 ps, the remarkable reduction of the CSR occurred dueto the remarkable increase of the horizontal beam size.

5. Summary

The generation of the 0.1 THz coherent synchrotronradiation (CSR) in the 90� bending magnet has been suc-cessfully performed using the electron bunch which hasthe energy of 40 MeV, the bunch charge of about 1ncand the bunch length less than 1 ps (rms). The electronbunch compression of 1 nC electron bunch was achievedless than 1 ps (rms) by controlling the Q-magnets in the

R. Kuroda et al. / Infrared Physics & Technology 51 (2008) 390–393 393

achromatic arc section as the bunch length was measuredby the rms bunch length monitor. However, the deferenceof the optimum Q4 current between the minimum bunchlength and maximum CSR power was found. To solve thisproblem, we will observe the CSR in the direction of 0� ofthe 90� bending magnet.

References

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[3] N. Sei et al., in: Proceedings of the 3rd Annual Meeting ParticleAccelerator Society of Japan (2006) 705.

[4] F. Sakai et al., Jpn. J. Appl. Phys. 41 (2002) 1589.[5] R. Kuroda et al., in: Proceedings of the 3rd Annual Meeting Particle

Accelerator Society of Japan (2006) 553.[6] E.B. Blum et al., Nuc. Inst. Meth. A 307 (1991) 568.[7] R. Kuroda et al., Jpn. J. Appl. Phys. 43 (2004) 7747.