Coherent Vavilov-Cherenkov radiation generated by a 6.1MeV

bunched electron beam traveling in the vicinity of a solid dielectric target (PTFE and Paraffin) has been investigated experimentally. In addition, we have also demonstrated the simple scheme of the Cerenkov interferometer for non-invasive longitudinal electron bunch length diagnostics

Naumenko, G. A.

Popov, Yu. A.

Potylitsyn, A. P.

Shevelev, M. V.

Sukhikh, L. G.

English

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DOI 10.1393/ncc/i2011-10966-xColloquia: Channeling 2010IL NUOVO CIMENTO Vol. 34 C, N. 4 Luglio-Agosto 2011The coherent Vavilov-Cherenkov radiation for a bunch lengthdiagnosticM. V. Shevelev(∗), G. A. Naumenko, A. P. Potylitsyn, Yu. A. Popovand L. G. SukhikhInstitute of Physics and Technology, Tomsk Polytechnic University - Lenin Ave. 30634050 Tomsk, Russia(ricevuto il 22 Dicembre 2010; pubblicato online il 19 Settembre 2011)Summary. — Coherent Vavilov-Cherenkov radiation generated by a 6.1MeVbunched electron beam traveling in the vicinity of a solid dielectric target (PTFEand Paraffin) has been investigated experimentally. In addition, we have also demon-strated the simple scheme of the Cerenkov interferometer for non-invasive longitu-dinal electron bunch length diagnostics.PACS 41.60.Bq – Cherenkov radiation.1. – IntroductionThe efficiency of some modern accelerator complexes such as colliders or free-electronlasers, is determined not only by the transverse sizes of a bunch but also by the longitu-dinal size of a bunch. The optimization and detailed control of the longitudinal electrondistribution in the bunch is an important feature in order to maximize the luminosity inthe future complexes. Crucial requirement of beam diagnostic tools is the possibility tomeasure parameters of a single electron bunch without noticeable beam deformation. Wepropose to use coherent Cherenkov radiation (ChR) for noninvasive beam diagnostics.ChR that is emitted while a charged particle moves in a medium with velocity exceed-ing the speed of light in this medium, is developed theoretically and experimentally andis a widely used tool for detecting particles [1, 2]. The radiation cone in the transparentmedium id defined by the following condition:(1) cos θ =1n · β ,(∗) E-mail: s062014@yandex.ruc© Societa` Italiana di Fisica 297298 M. V. SHEVELEV, G. A. NAUMENKO, A. P. POTYLITSYN, YU. A. POPOV and L. G. SUKHIKHwhere θ is the radiation angle, β is the electron velocity in the speed of light units, n isthe refraction index of the medium.ChR may also appear while a charged particle travels in the vacuum nearby themedium due to the fact that this radiation is caused by the charged particle’s electro-magnetic field that has the transverse dimensions of about γλ (γ is the particle Lorentzfactor, λ is the radiation wavelength). This fact becomes clear if we remember that ChRis one of the kinds of the so-called polarization radiation [3]. Our geometry of radia-tion differs from the conventional one as diffraction radiation differs from the transitionone. The theoretical possibility of simultaneous appearance of Cerenkov radiation anddiffraction one was shown by Karlovets and Potylitsyn [4].When the bunch travels near the target, the longitudinal form-factor drasticallychanges the intensity of polarization radiation. The simple estimation proves the factthat the coherency of any kind of polarization radiation strongly depends on the lon-gitudinal bunch [5]. The total spectral-angular density of polarization radiation (bothcoherent and incoherent) from the bunch of Ne electrons may be written as (we neglecttransverse bunch size effects):(2)∂2Wcoh∂ω∂Ω=∂2Wincoh∂ω∂ΩNe(1 + (Ne − 1)|fz(σz)|2),∂2Wincoh∂ω∂Ω is the spectral-angular density of radiation from one electron, Ne is the numberof electrons in the bunch, |fz(σz)|2 is the longitudinal form factor of the electron bunch.For the Gaussian bunch with rms σz the form-factor looks like the following:(3) |fz(σz)|2 = exp[−ω2σ2zβ2c2].Most of existing bunch length measurement methods using a coherent radiation arebased on the measurement of the radiation spectral density distribution with the sub-sequent bunch length calculation using spectral density features. Usually to obtain aspectral density distribution a radiation beam is to be splattered into two beams andthe autocorrelation function (interferogram) is measured using some types of interferom-eter [6, 7].On the other hand, if one has a target material with frequency dispersion [8], adifferent radiation wavelength is generated under different angles. In this case one maychange the complicated spectra measurements for convenient angular measurements inorder to find coherent threshold and determine the bunch length [9].In the first part of the paper we show our experimental results devoted to charac-teristics of coherent ChR from two prisms with different refractive indexes in millimeterwavelength region. In the second part of the paper we discuss the possibility of themeasurement of the bunch length.2. – ExperimentWe have carried out our experimental research on the extracted beam of TomskPolytechnic University microtron with parameters listed in table I.The experimental scheme is shown in fig. 1. The electron beam extracted into airthrough the 40μm Be foil was used. The train of bunches travels near a target. Thedetecting system consisted of the parabolic telescope, which represented a paraboloidalTHE COHERENT VAVILOV-CHERENKOV RADIATION FOR A BUNCH LENGTH DIAGNOSTIC 299Table I. – Electron beam parameters.Electron energy 6.1MeV(γ = 12) Bunch spacing 380 psTrain duration τ ≈ 4μs Bunch population Ne = 6 · 108Bunches in a train nb ≈ 1.6 · 104 Bunch length σz ≈ 2mmmirror (diameter 170mm, focal distance 145mm) in the focus of which the detector wasset up. This tool provides the possibility to measure the angular radiation characteristicsin the far-field zone [10]. The radiation from each train has been detected using DP-21Mdetector. The last is based on a wide-band antenna, high-frequency low barrier Shottkydiode and preamplifier. The average sensitivity of the detector in the radiation wave-length region 11–17mm is approximately equal to 0.3V/mW [11]. The wavelength regionwas limited by coherent threshold in the smaller wavelengths and by the beyond-cutoffwaveguide (diameter 15mm) that passes wavelengths lower than 25mm and is used todecrease accelerator RF background. Incoherent radiation cannot be measured by thedetector. The measured region yield was averaged over 20 trains. The statistic errorwas less than 10% during the experiments. The Faraday cup signal allowed measuring azero point of impact-parameter using the electron beam intensity suppression when thewire scanner crossed the electron beam. The impact-parameter was equal to h = 25mm.ChR in diffraction geometry should be polarized only in the horizontal plane, i.e. theplane of electron velocity vector and radiation wave-vector. During the experiment agrid polarizer was used.In our experiment we used two various solid dielectric targets with different refractiveindexes. Teflon (Polytetrafluoroethylene -PTFE) target with the length equal to 247mmand the height equal to 74mm was used during the experiment. The target was a prismbased on a right-angled isosceles triangle in order to decrease ChR refraction losses.Paraffin target with length equal to 253mm and height equal to 80mm was also used.The target was a prism with 40 deg. The refractive indexes of Teflon and paraffin forthe millimeter wave region were measured to be equal to 1.41 and 1.49 for λ > 10mm,respectively.At first, we have measured the ChR peak positions while scanning over θ angle for bothtargets. Figure 2 shows the result of θ scan. The measured distribution of the radiationFig. 1. – Experimental scheme.300 M. V. SHEVELEV, G. A. NAUMENKO, A. P. POTYLITSYN, YU. A. POPOV and L. G. SUKHIKHFig. 2. – The angular dependence of ChR: the circle—horizontal polarization component ofradiation from the Paraffin target, the squares—horizontal polarization component of radiationfrom the PTFE target, the inclined squares—vertical polarization component of radiation fromthe PTFE target, the solid lines—theoretical calculations.from the Teflon target is shown in fig. 2. by the squares and from the paraffin target bythe circles. The corresponding curves in this figure show the theoretical calculation ofChR [4]. It should be noted the angular width of ChR cone θ is defined by a dispersionlaw and target sizes but not by the Lorentz factor. From fig. 2 one may clearly see thatthe vertical polarization component is suppressed as expected (inclined squares).As a second step, we have checked the coherency of radiation. In order to find whetherthe radiation was coherent or not the beam current dependence has been measured.This dependence is presented in fig. 3. Measured radiation is shown by dots and thesolid line shows the fit by the function y = a + bx + cx2. One may see from fig. 3Fig. 3. – The current dependence of CCR: the dots—experimental data, the solid line—quadraticfit function.THE COHERENT VAVILOV-CHERENKOV RADIATION FOR A BUNCH LENGTH DIAGNOSTIC 301Fig. 4. – The dependence on the impact parameter: the dots—experimental data, the solidline—fit function.that the fit function agrees very well with measured data and, therefore, radiation iscoherent.The measured coherent Cherenkov radiation (CChR) intensity as a function of impact-parameter is presented in fig. 4. The solid line in fig. 4 is the fit for CChR intensity witha function y = 1085.5 · exp[−0.02 · h].Figure 5 shows by the histogram bars the spectrum of CChR measured at the angle ofradiation maximum. For the spectral measurements the low-pass filters, described in [12]were used. The filters were set up at (before) the detector horn. The solid line presentsthe radiation form-factor from eq. (3) for σz = 2mm.3. – Longitudinal beam diagnosticsAs a rule, in order to measure the spectrum of coherent radiation and reconstruct thebunch length one should use an interferometer. We propose to use a compound CChRFig. 5. – The spectrum of CCR measured at radiation peak angle θ = 44.5◦. The histogrambar—experimental data, the solid line—form factor from eq. (3).302 M. V. SHEVELEV, G. A. NAUMENKO, A. P. POTYLITSYN, YU. A. POPOV and L. G. SUKHIKHFig. 6. – The scheme of Cherenkov interferometer.target as an interferometer. The scheme of a compound CChR target is shown in fig. 6.As can be seen in fig. 6, the target consists of two parts, and the parts volumes are equal.When the beam moves near the CChR target, the CChR is generated by each part of thetarget. If the detector is installed at the angle of radiation maximum, one may measureCChR yield moving the second part of the target. In this case one may obtain a detuningcurve (interferogram).In our experiment the detector measures radiation yield in some wavelength regionthat is limited by coherent threshold in the smaller wavelengths and by the beyond-cutoffwaveguide that passes wavelengths lower than 25mm. In this case, the expression for adetuning dependence may be described by the following formula:(4) P (L, σz) =∫ λmax0exp[−4π2σ2zλ2](1− cos[2πLλ(1β− sin[α])])WCR(λ)dλ,where WCR(λ) is the spectral-angular density of radiation from one target, L is themovement distance.Calculated detuning dependence for the experimental conditions (γ = 12, λmax =25.5mm, paraffin target (α = 40◦), h = 10mm, σz = 2mm) is presented by solid (blue)curve in fig. 7. The measured detuning dependence is shown by the (yellow) dots infig. 8.In order to obtain information from the detuning curve one may use differentways. Let us empirically introduce the following way to obtain the bunch length.τ = P (0)−P (d1)−P ′(d1) , ε =(P ′(d2))2P (d2)P ′′(d2)are some functions, where d1 is the distance valueat the fist derivative minimum for the measured dependence, d2 is the distance value atthe second derivative maximum for the measured dependence. Using these functions onemay obtain bunch rms as(5) σz(τ, ε) = 0.65 · τ · exp[ −ε0.785].THE COHERENT VAVILOV-CHERENKOV RADIATION FOR A BUNCH LENGTH DIAGNOSTIC 303Fig. 7. – (Colour on-line) The calculated dependence of P on L.Fig. 8. – (Colour on-line) The measured dependence on L.Fig. 9. – The σz as a function of τ and ε: the dots—calculated dots, the surface—approximatedfunction σz(τ, ε) = 0.65 · τ · expˆ −ε0.785˜.Figure 9 shows by the dots the calculation of τ and ε functions from the simulateddetuning curves (eq. (4)) for different bunch lengths. The surface in fig. 9 shows theproposed function to estimate the bunch length (eq. (5)).304 M. V. SHEVELEV, G. A. NAUMENKO, A. P. POTYLITSYN, YU. A. POPOV and L. G. SUKHIKHFor our experimental interferogram the bunch length is equal to σz = 2.45± 0.3mmbasing on eq. (5).4. – ConclusionWe would like to point that eq. (1), as expected, is useful for the coherent ChRgenerated by the relativistic electron bunch passing in the vicinity of the target. We havedemonstrated experimentally the square dependence of Cherenkov radiation intensity onthe bunch population, which confirms the coherent character of radiation.We also have demonstrated the possibility to measure the bunch length without abeam distortion. This scheme was tested in the millimeter wavelength region on the6.1MeV bunched electron beam of microtron. The measured beam length is in goodagreement with the experimental data.∗ ∗ ∗This work was partly supported by the Russian Ministry of Science and Educationunder contract No. I790.REFERENCES[1] Jelly J., Phys. Usp., 58 (1957) 231.[2] Bolotovsky B., Phys. Usp., 62 (1958) 201.[3] Amusia M., Rad. Phys. Chem., 75 (2006) 1232.[4] Karlovets D. V. and Potylitsyn A. P., submitted to Phys. Rev. ST Accel. Beams,arXiv: 0908.23336v2.[5] Shchelkunov S., Marshall T., Hirshfield J. et al., Phy. Rev. ST Accel. Beams, 8(2005) 062801.[6] Scholott V., Loos H., Gentz H. et al., Part. Accel., 52 (1994) 74.[7] Castellano M., Verzilov V. et al., Phys. Rev. E, 63 (2001) 056501.[8] Meijer A., Pijpers J., Nienhuys H. et al., J. Opt. A: Pure Appl. Opt, 8 (2008) 095303.[9] Potylitsyn A., Gogolev S., Karlovets D. et al., Processing of IPAC, (2010) 176.[10] Kalinin B., Naumenko G., Potylitsyn A. et al., JETF Lett., 84 (2006) 110.[11] Cha V., Kalinin B., Monastyrev E. et al., in Proc. Intern. Conf. on Charged andNeutral Particles Channeling Phenomena II, 6634 (2007) 663416.[12] Hanke K., Schlott V., Aulenbacher K. et al., Beam Diagnostics Using CoherentTransition Radiation at CLIC Test Facility, CERN CLIC Note 298 (1996).

The coherent Vavilov-Cherenkov radiation for a bunch length

diagnostic

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The coherent Vavilov-Cherenkov radiation for a bunch length diagnostic

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