Beam diagnostic based on coherent Cherenkov radiation using dielectric tube
K. Kan #,A), J. YangA), A. OgataA), T. KondohA), T. ToigawaA), K. NorizawaA), H. KobayashiA), Y. YoshidaA),
M. HangyoB), R. KurodaC), H. ToyokawaC)
A)
Institute of Scientific and Industrial Research, Osaka University, Osaka, Japan
B)
Institute of Laser Engineering, Osaka University, Suita, OsakaC)
National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan

Abstract
Beam diagnostic for electron bunch length using spectrum analysis of multimode terahertz (THz) -wave on the orderof 0.1 THz was investigated. The multimode THz-wave was generated by coherent Cherenkov radiation (CCR) using
hollow dielectric tubes and femtosecond/picosecond electron bunches. The spectra of multimode THz-wave depended
on electron bunch length, which was measured by a streak camera, resulting in a possibility of a new beam diagnostic
based on CCR and a bunch form factor.

誘電体管コヒーレントチェレンコフ放射によるビーム診断の研究
1.

Introduction

Ultrashort, e. g. femtosecond [ 1 ] or picosecond,
electron bunches are used in accelerator physics
applications such as free electron lasers (FELs), laserCompton X-ray, and pulse radiolysis [2]. On the otherhand, electron bunches with short bunch lengths are also
useful for electro-magnetic (EM) radiation production interahertz (THz) range because the inverse of 1 ps bunch
length corresponding to the frequency of 1 THz. A shorter
electron bunch can emit EM radiation of higher frequency
according to the bunch form factor, which is described by
the Fourier coefficients of the bunch distribution [3].
Furthermore, the radiation intensity at lower frequency
than the inverse of the bunch length is proportional to theelectron number, e. g. ≈109 at 1 nC. As the result, anintense THz-wave can be generated by such electron
bunches. In the studies on EM radiation emitted fromelectrons, Cherenkov radiation (CR) has been studied
since the 1930s [4] and radiation yield dependence on
wavelength and angular distribution was reported. In the1990s, the bunch form factor [3] and angular distributionof coherent transient radiation (CTR) from electron
bunches were measured experimentally. Recently, notonly monochromatic THz-wave [5] but also multimodeone [ 6 , 7 ] of <0.4 THz were generated by coherent
Cherenkov radiation (CCR). Another THz-wave
generation method, which utilized a periodic electron
bunch distribution, generated narrow-band THz-wave up
to ≈0.86 THz for a bunch length diagnostic [8]. Alike
THz-wave generated by a laser, that generated by anaccelerator would be useful to a probe light, whichmonitors conductivity due to quasi-free electrons, orimaging for medical and security use.
In this paper, generation of multimode THz-wave usingmultimode CCR on the order of 0.1 THz was investigated.
The multimode CCR was generated by a hollow dielectric

tube covered by a metal and femtosecond/picosecond
electron bunches from a photocathode RF gun linac.
Finally, beam diagnostic for electron bunch length using
spectrum analysis of multimode THz-wave was proposed.

2.

The photocathode RF gun linac and SPR measurement
system are shown in Fig. 1. The details of the linac are
discussed in references [9,10,11]. Figure 1(a) shows thelinac system. The electron bunch was generated by a 1.6cell S-band (2856 MHz) RF gun with a copper cathodeand a Nd:YLF picosecond laser. The pulse width of theUV pulse from the laser was measured to be 5 ps in
FWHM as a Gaussian distribution. The UV light wasprojected onto the cathode surface at an incident angle ofapproximately 2° along the electron beam direction. Thebeam energy at the gun exit was 4.2 MeV. The
picosecond electron bunch produced by the RF gun wasaccelerated up to ~32 MeV by a 2 m long S-bandtravelling-wave linac with an optimal energy-phasecorrelation for the bunch compression, in which the headelectrons of the bunch have more energy than the bunch
tail. Finally, the energy-phase-correlated electron bunch iscompressed into femtosecond by rotating the phase space
distribution in the magnetic bunch compressor, which isconstructed with two 45°-bending magnets (B1 and B2),
four quadrupole magnets (Q3, Q4, Q5, and Q6), and two
sextupole magnets (S1 and S2). The femtosecond electron
bunch was obtained by adjusting the energy modulationin the linac, e. g., compressed bunch length of ~0.2 ps inrms measured by a streak camera at a bunch charge of~40 pC and a linac phase of 97.5°.
In the experiment, not only the picosecond electron
bunch at the linac exit but also the femtosecond one at thecompressor exit was used for multimode THz-wave
generated by CCR as shown in Fig 1(b). When suchelectron bunches move on the axis of a hollow dielectric___________________________________________#

Experimental arrangementkoichi81@sanken.osaka-u.ac.jp- 623 -Figure 1: (a) Photocathode RF gun linac with a magnetic bunch compressor of an achromatic arc. (b) Diagram ofmultimode CCR and measurement system. OAP denotes an off-axis parabolic mirror; M, a plane mirror; BS, abeam splitter.
tube covered by a metal, partially periodic electric field,i.e., multimode THz-wave, is induced as shown in Fig.
1(b). This slow-wave structure of the hollow dielectric
tube supports modes with phase velocity equal to thebeam velocity, which contain fundamental and higher
modes. The inner and outer radii of the tube, made offused silica, were 5 mm and 7 mm, respectively, resulting
in the tube wall thickness of 2 mm. The hollow tube wascovered by a copper conductive tape for a metal boundary
condition, which reflects and stores EM radiation in thetube. In order to measure the intensity and frequency of
CCR, a Michelson interferometer was set downstream of
the tube in the air. The distance between the tube exit andan off-axis parabolic mirror (OAP1) was fixed to theeffective focal length of the mirror from a viewpoint ofcollimating. The THz-wave was separated by a beam
splitter (BS) and one of the THz light was reflected by amoving mirror (M2). The two THz light joined together at
a 4.2K silicon bolometer (Infrared Laboratories Inc.). Theintensity and frequency of CCR were analyzed by the fast
Fourier transform (FFT) of an interferogram, which is adependence of the bolometer output on the moving mirror(M2) position.

3.

Results and discussion

3.1

Analytical frequency of TM mode

3.2

Dependence on tube length

In the multimode CCR experiment, the dependence ofthe intensity on the tube length was investigated as shownin Fig 2. In this experiment, the picosecond electron
bunch at the linac exit was used. An interferogram with
128 data points and 1ps time step was measured for theFFT calculation. Figure 2(a) shows the interferograms forthree different tube lengths. The periodic oscillation froma 150 mm long tube decayed more slowly than that from a50 mm long tube because the tube length decided theenergy of CCR stored in the tube. Figure 2(b) shows thefrequency spectra for three different tube lengths.
Analytical frequency of TM0n mode was also shown ascrosses. All the spectra indicated sharp peaks atfrequencies of 0.09 and 0.14 THz, which corresponded toTM03 and TM04 modes, respectively. The absence of thelower modes, e. g. TM01 or TM02, would be caused by thefrequency characteristics of the bolometer and the beam
splitter, the mirror diameters, and the loss in the fusedsilica. The intensity increased nonlinearly at a tube lengthof >100 mm. Saturation was observed at a tube length of~150 mm. The saturation would be caused by the balance
between the EM radiation production due to beam energy
loss and the dielectric loss in the fused silica.
20.2(b)(a)Intensity [a. u.]

Output [V]

0.1

0.05

0-40

1.5

10.5

0-30-20-100102030

40

0

0.1

0.2

0.3

0.4

Frequency [THz]Time [ps]1.4(c)

Figure 2: (a) Interferogramsfor 150, 100 and 50 mm tube
1.2
1lengths with offsets andfactors adjusted for comparison.
0.8(b) Frequency spectra
for 150, 100 and 50 mm tube
0.6lengths with offsets 0.4and factors adjusted. The theoreticalfrequencies for TM0n0.2 modes (cross) according to Eq. (1)0050
100150200were shown.

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Intensity [a. u.]

The frequency of multimode CCR depends on thehollow dielectric tube conditions. Assuming azimuthally
symmetric Transverse Magnetic (TM) mode along thetube axis is induced, frequency of TM0n mode can be
expressed as [5,7,12]s I1 ka  0,k I 0 ka  1(1)where k and s denote the radial wave numbers in thevacuum and dielectric regions; a, the inner radius of thetube; Ψ0 and Ψ1 [12], functions composed of the inner andouter radii and Bessel functions of the first and secondkinds. If Eq. (1) is satisfied, phase velocity is equal to thebeam velocity. The theoretical frequencies of TM0n modes
were calculated for the tube with the relative permittivityof 3.8 and the inner and outer radii of 5 mm and 7mm.

0.15

Tube length [mm]3.2

Acknowledgement

Dependence on bunch length

The spectra of multimode THz-wave depends onelectron bunch length from a view point of a bunch form
factor [3], which indicates that shorter electron bunch canemit THz-wave of higher frequency. The tube length wasfixed to 150 mm because of intensity as mentioned in Sec.
3.2. Furthermore condition of bunch compression wasvaried by the linac phase, which changed energy-phasecorrelation of electron bunch. Bunch charge was fixed to
45 pC to avoid saturation of bolometer output. Whenbunch charge was decreased by the linac phase, thebolometer output was increased, e.g., intensity factor of~8 at bunch lengths of 1.3 ps and 0.21 ps. In order toinvestigate dependence of spectra of THz-wave on bunch
length, electron bunch length was also measured by afemtosecond streak camera. Streak camera detected
Cherenkov light in air by imaging using 2 lenses of focal
length of 300 mm and a band pass filter of 480 ±5 nm.
Figure 4 shows the dependence of spectra on bunch
length. Short bunch length induced higher order of CCR,e.g., TM09 (0.36 THz) and TM017 (0.72 THz) modes at abunch length of 0.21 ps. On the other hand, intensity ratioof lower modes, e.g., TM03 and TM04, would be changed
by a spectrum of incoherent radiation [3] at a bunch
length of 0.21 ps. According to spectra of CCR and bunch
length measurement using a streak camera, a new bunch
length measurement using spectrum analysis of CCR
would indicate a possibility of a new beam diagnostic
based on a bunch form factor.

We thank the staff of the Radiation Laboratory at theInstitute of Scientific and Industrial Research (ISIR),
Osaka University for the linac operation. This work was
supported by KAKENHIs (23109507, 21226022) andfunded research from AIST (generation of high-powerTHz-wave).

References[1] G. Berden et al., Phys. Rev. Lett. 99, 164801 (2007).
[2] J. Yang et al., Nucl. Instr. and Meth. A 629, 6 (2011).
[3] T. Takahashi et al., Phys. Rev. E 50, 4041 (1994).
[4] I. E. Tamm et al., Dokl. Akad. Nauk SSSR 14, 107 (1937)[5] A. M. Cook et al., Phys. Rev. Lett. 103, 095003 (2009).
[6] K. Kan et al., Radiat. Phys. Chem. 80, 1323 (2011).
[7] K. Kan et al., Appl. Phys. Lett. 99, 231503 (2011).
[8] P. Piot et al., Appl. Phys. Lett. 98, 261501 (2011).
[9] J. Yang et al., Nucl. Instrum. Methods A 556, 52 (2006).
[10] K. Kan et al., Nucl. Instrum. Methods A 597, 126 (2008).
[11] K. Kan et al., Nucl. Instrum. Methods A 622, 35 (2010).
[12] B.M. Bolotovskii, Sov. Phys. Usp. 4, 781 (1962). A

Figure 3: Frequency spectra for 0.21, 0.55, and 1.3 psbunch lengths. Factors and offsets were adjusted forcomparison. The theoretical frequencies for TM0n modes(cross) according to Eq. (1) were shown.

4.

Conclusions

Beam diagnostic for electron bunch length using
spectrum analysis of multimode THz-wave based on CCR
was investigated. Picosecond electron bunches induced
TM03 (0.09 THz) and TM04 (0.14 THz) modes. The
intensity of CCR was maximized at a tube length of 150
mm. When bunch length was compressed to ~0.2 ps,
TM09 (0.36 THz) and TM017 (0.72 THz) modes were also
observed. A new bunch length measurement using
spectrum analysis of CCR would indicate a possibility ofa new beam diagnostic based on a bunch form factor.

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