MONOCHROMATICITY OF GRATING TRANSITION RADIATIONA. AryshevA,B), S.ArakiA), K.ArtyumovC,G), M.FukudaA,B), Y. MorikawaA), R. MoritaD),
A.KotlyarovC), G. NaumenkoC), A. PotylitsynC), D.Yu. SergeevaE), M. ShevelevC),D.ShkitovC), L. SukhihC), Y-I. TadenumaD), T. TanbaD), N. TerunumaA,B), A.A. TishchenkoE,F),
J. UrakawaC), M. WashioD)
A)KEK: High Energy Accelerator Research Organization, Tsukuba, Ibaraki, JapanB)SOKENDAI: The Graduate University for Advanced Studies, Tsukuba, Ibaraki, Japan
C)Tomsk Polytechnic University, Tomsk, RussiaD)Research Institute for Science and Engineering, Waseda University, Tokyo, Japan
E)National Research Nuclear University (MEPhI), Moscow, RussiaF)National Research Centre โKurchatov Instituteโ, Moscow, Russia
G)Institute of High Current Electronics SB RAS, Tomsk, Russia
AbstractA strong interest for developing of intense monochromatic
THz radiation sources is explained by its unique features,such as non-ionizing interaction with matter, weak absorp-tion in dielectrics, etc. The KEK: LUCX facility can produceTHz/subTHz radiation via coherent transition/diffraction ra-diation (CTR/CDR) mechanisms as the rms electron bunchlength is of the order of 0.15 mm. Spectral characteristics ofCTR when the electron beam interacts with a grating insteadof a flat metal foil usual for conventional CTR were studiedand CTR continuous spectral distribution transformation intodiscrete spectral lines (so-called Grating Transition Radia-tion, GTR [1]) was confirmed. Moreover, GTR spectral linesplitting for orientation angles much larger than the inverseLorentz factor was observed. In this report, spectra measure-ment results and its comparison with Smith-Purcell radiationis presented and further developments are discussed.
INTRODUCTIONElectromagnetic radiation in the terahertz (THz) range
attract attention due to its potential application in a differ-ent applied fields: biology, medicine, cargo inspection, etc.Many of existing THz sources based on the compact lin-ear accelerators [1, 2] employ the coherent transition radia-tion mechanism to generate broadband emission spectrum.However, large number of applied investigations requiremonochromatic sources and additional devices for radia-tion monochromatization are currently considered. In ourprevious experiment [3] we have showed that the short elec-tron bunch passing through a grating instead a conventionalfoil generates radiation, spectrum of which consists of thenarrow-band spectral lines (so-called grating transition radi-ation, GTR).
The dispersion relation which sets the connection betweenwavelength of the GTR spectral lines, observation angle ๐and grating inclination (with respect to an electron beam)angle ๐ has the following form:
๐๐ = ๐๐ (cos ๐
๐ฝ โ cos (๐ โ ๐)) , ๐ฃ๐ = ๐๐๐
(1)
Here ๐ is the diffraction order, ๐ is the grating period,๐ฝ = ๐/๐. Evidently, for the grating orientation ๐ = 0the relation Eq. (1) reduces to the well-known Smith-Purcellformula. We have investigated a monochromaticity of theSmith-Purcell radiation (SPR) before [4] and in this report,we have compared it with the GTR monochromaticity.
RESULT OF SIMULATIONS
The generalized surface current method [5] to simulateGTR characteristics for conditions of the LUCX facility wasused. The GTR spectral-angular distribution is calculatedfrom the field strength obtained by integration over a gratingsurface:
๐2๐๐๐๐ฮฉ = ๐๐2 โฃ๐ธ๐ท
๐ (๐๐ท, ๐)โฃ2 (2)
๐ธ๐ท๐ (๐๐ท, ๐) = 1
2๐ โซ โซ [[๐ (๐๐) , ๐ธ๐๐ (๐๐, ๐)] ,
โ๐บ (๐๐, ๐๐ท, ๐)] ๐๐๐(3)
For calculations of the integral โซ โซ ๐๐๐ in Eq. (3) the realgrating surface (see Fig. 1) for which we found a sum ofintegrals over surface of each period was taken. As it wasshown in the paper [5] the Green function can be presentedas following:
โ๐บ (๐๐, ๐๐ท, ๐) = ๐๐ท โ ๐๐
โฃ๐๐ท โ ๐๐โฃ2๐๐๐(๐๐ทโ๐๐) ( 1
โฃ๐๐ท โ ๐๐โฃ โ ๐๐) ,
and the normal to the grating surface in Eq. (3) was cal-culated for each period as ๐ (๐๐) = ๐ด (๐) {0, 0, 1}, where๐ด (๐) is the rotation matrix for the angle ๐ (see Fig. 1).Grating parameters are presented in Table 1.
Proceedings of the 16th Annual Meeting of Particle Accelerator Society of JapanJuly 31 - August 3, 2019, Kyoto, Japan
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The field of the initial electron is given by the followingexpression:
๐ธ๐๐ (๐๐, ๐) = 2๐๐๐ ๐
๐ฝ ๐ง๐
๐ฝ2๐๐พ๐
โง{{{{{{{โจ{{{{{{{โฉ
๐ฅโ๐ฅ2
๐+๐ฆ2๐
ร
๐พ1 ( ๐๐ฝ๐พโ๐ฅ2
๐ + ๐ฆ2๐)
๐ฆ
โ๐ฅ2๐+๐ฆ2
๐
ร
๐พ1 ( ๐๐ฝ๐พโ๐ฅ2
๐ + ๐ฆ2๐)
โ ๐๐พ๐พ0 ( ๐
๐ฝ๐พโ๐ฅ2๐ + ๐ฆ2
๐)
โซ}}}}}}}โฌ}}}}}}}โญ(4)
Figure 1: Geometry of experiment. Here ๐ โ grating tiltangle; ๐ - radiation wavelength; ๐พ โ Lorenz-factor.
Table 1: Grating Parameters
Material Al
Period numbers 15
Period (AC) 4 mm
Full length 59.46 mm
Width 30 mm
Strip dimensions 3.46 (BC)ร30 mm2
Strip tilting angle (๐) 30โ
Target height (BD) 1.73 mm
AB and DC 2 mm and 3 mm
Further we calculate the horizontal polarization compo-nent (HP) of the field ๐ธ๐ท
๐ , because the vertical componentis equal to zero since the electron beam passes through agrating center.
Figure 2a-c shows simulation results obtained for the grat-ing under consideration, electron beam energy 8 MeV andthe observation angle ๐ = 90โ. As one can see, for a smallinclination angle ๐ = 5โ there is a tendency for spectral line
splitting only for a low diffraction orders. For inclinationangles ๐ โณ ๐พโ1 such a splitting effect becomes sharper.
Figure 2: GTR spectra for different grating inclination an-gles.
It should be noted that the dispersion relation (1) gives avalue ๐๐ corresponding to a minimum between split peaks.Despite such a splitting, widths of the spectral lines remainsmall enough for a large inclination angles. As an example,characteristics of spectral lines for the angle ๐ = 15โ arepresented in Table 2.
EXPERIMENTAL RESULTSExperimental layout is shown in Fig. 3. Its detailed de-
scription can be found in [4]. KEK LUCX beam parametersare presented in the inset of the figure. We investigatedGTR spectral characteristics using Michelson interferom-eter [6] and Schottky barrier diodes (SBD) with spectralsensitivity ranges 60 โ 90 GHz and 140 โ 220 GHz. Pre-
Proceedings of the 16th Annual Meeting of Particle Accelerator Society of JapanJuly 31 - August 3, 2019, Kyoto, Japan
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Table 2: GTR Spectra Lines Parameters
Diffraction order, k 1 2 3 4
๐๐, GHz 98 197 295 394
ฮ๐๐ ๐๐, GHz 10.6 12.1 15.1 18.1
ฮ๐ (๐น๐๐ป๐), GHz 19 23 28 37
ฮ๐/๐๐ 0.19 0.12 0.10 0.09
RF Gun
LinacSoleniodLaser pulse
Screen
M1
M2
BS PM
Detector
Beam directionSapphirewindow
Grating
q
90
Electron beam energy 8 MeV
Bunch length (r.m.s.) < 0.15 mm
Bunch size (r.m.s.) ~100um
Bunch population ~25pC
Bunch per train 1
Observation angle 090
Figure 3: Experimental set-up.
liminary adjustment of the grating with respect to electronbeam was performed by measuring bremsstrahlung yieldalong electron beam direction and after that, โzerothโ targetorientation was determined by backward transition radiationscan (see, Fig. 4). The reconstruction procedure of the GTR
Figure 4: Dependence of the CTR yield on the target rotationangle.
spectra was conducted in analogy with Smith-Purcell radia-tion reconstruction described in [4]. Typical GTR spectraare presented in Fig. 5 and Fig. 6. One can see that there isa clear evidence of the spectral line splitting.
DISCUSSIONWe have observed GTR lines with no splitting for a small
inclination angles in agreement with simulations performedfor the same experimental geometry and beam parameters.For ๐ โผ ๐พโ1 angles, experimental spectral lines demon-strate just a โweakโ dip between peaks in a contrast withthe simulation results, where one can see almost completeseparations between peaks (see, Fig. 2c). We suppose thatthis fact can be explained by a finite aperture of the interfer-
Figure 5: GTR spectral lines measured for grating inclinationangles a: ๐ = 5โ and b: ๐ = 7.2โ.
Figure 6: GTR spectral lines measured for grating inclinationangles a: ๐ = 13.5โ and b: ๐ = 17.5โ.
ometer system and imperfections of the Si splitter used inthe interferometer and off-axis parabolic mirror as well aslimited SBD aperture. Despite such a splitting effect, thespectral lines widths remains small enough and GTR can stillbe considered as potentially monochromatic source allow-ing for a fine spectral lines frequency tuning by the gratingrotation for the fixed direction of the emitted radiation.
ACKNOWLEDGMENTS
The work was supported by the JSPS and RFBR underthe Japan-Russia Research Cooperative Program (18-52-50002 YaF_a), the Competitiveness enhancement programof Tomsk Polytechnic University and the Competitivenessprogram of National Research Nuclear University โMEPhIโ.
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Proceedings of the 16th Annual Meeting of Particle Accelerator Society of JapanJuly 31 - August 3, 2019, Kyoto, Japan
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