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Proposal for Using Optical Transition Radiation for Electron Beam Alignment and Emittance Measurement for the Free Electron Laser Experiments at ATF *

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Proposal for Using Optical Transition Radiation for Electron Beam Alignment and Emittance Measurement for the Free Electron Laser Experiments at ATF * BNL-60999 Proposal for using optical transition radiation for electron beam alignment and emittance measurement for the free electron laser experiments at ATF * Xu Z. Qiu, Xijie Wang and Ilan Ben-Zvi Building 725C National Synchrotron Light Source Brookhaven National Laboratory- Upton, N.Y., 11973 Abstract Optical trnasiton radiation (OTR) produced from thin intercepting foils have been employed to image the spatial profile of the electron beam in several free electron laser experiments. It was found that the images from OTR were significantly sharper than the images produced from phosphor screens. Furthermore, OTR's sensitivity of its angular distribution and polarization to energy and divergence of the electron beam was exploited to diagnose energy and emittance of the electron beam. OTR has been proven to be vital in electron beam alignment in FEL experiments. In this report, we will give a summary of the basic theory of transition radiation and techniques using transition radiation for electron beam imaging and emittance measurement. We will then explored the possibility for employing these techniques in the HGHG FEL and the visible FEL experiments in ATF. *Work performed under the auspices of the U.S. Department of Energy /,,;.? >-i v^ **r* sr^. DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED ..... ,....$ DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. 1 Introduction The earliest observation of optical transition radiation (OTR) were those made in connection with studies of x-ray tubes at the turn of the cen• tury. The cause of the light, emitted by electrons impinging on the anodes of the tubes, was not at first understood. In 1919 Lilienfeld investigated the spectra of the emission and showed that they were continuous. For a time OTR was referred to as Lilienfeld radiation. The theory of transition was de• veloped by Ginzburg and Frank in 1945. They predicted that radiation would be produced by a charged particle traveling at constant velocity across the boundary between two media with different dielectric properties. The radia• tion was considered to be a special case of Cherenkov radiation. Goldsmith and Jelley observed TR from 1.5 MeV protons in 1958, but did not consider their results to be a definitive demonstration of the existance of transition ra• diation. Gradually during the 1960's, the experimental proof of the existance of OTR became established. Aitken et. al. observed and imaged the angu• lar distribution of the OTR produced by 26-29 MeV electron beams produced from the windows of a gas filled Cherenkov detector when the gas pressure was reduced to below the Cherenkov threshold condition. They also observed the polarization of the radiation, and raised the possibility of using the angular distribution to measure beam energy. In the 1970's, Wartski intensively studied the properties of OTR and its application to charge particle beam diagnostics. He demonstrated that the angular distribution of OTR from a single foil could be used to measure the energy of of linac beams (35-72MeV), and that OTR images of the beam could be used to make high resolution beam profile measurements. He further showed how the interference pattern from a two foils OTR inteferometer could be used as a sensitive (1%) beam energy diagnostic. Wartski experimentally investigated the effect of mutiple scattering on single foil and two foil interfer• ometer radiation patterns. In 1983, Fiorito and Rule of Naval Surface Warfare Center started to apply the techniques developed by Wartski to diagnosis high current electron beam. They studied the effect of beam divergence on the angular distribution pattern and polarization of OTR from a single foil and two foil interferometer. They applied the OTR emittance measurement techniques for electron beams from several linacs including both the Boeing and LANL HIBAF FEL accel• erators. The result obtained from this technique is consistant with those from variable quadrupole method and mutiple screen method. 1 2 Transition radiation from single particle at normal incidence If a single particle of charge e crosses an interface (from the medium to the vacuum) at normal incidence, the intensity of the transition radiation emitted into the vacuum (forward emission), in a frequency range du> and a solid angle dfl, is given by the following formula. [3]: d2W e2j32 sin29cos20 (e-l)[l-02~/3(e-sin29)i} 2 2 (1) dwdn TT2C (l-(3 cos 9f [ecos9 + (c - sin29)i][l - j3{t - sin29)*) where e is the complex dielectric constant of the medium, 6 the angle of emis• sion with respect to the direction of the charge velocity v, and ft the particle velocity expressed in unit of the speed of light. For extreme relativistic particles, and in the case where |e| > 1 as for a metal, the third term in (1) tends to unity and (1) reduces to d2W e2/32 sin26 2 2 2 2 (2) dudti 7TC (1 - /3 COS 9) The maximum intensity for (2) occurs at the angle, 1-4, sin0m = (3) \ r -l 9r 7 (4) where 7 is the Lorentz factor. The OTR radiation is polarized in the plane of observation. The inten• sity of transition radiation depends strongly on the particle energy. It can be shown that the total intensity is proportional to 7 [5]. When the particle goes from vacuum to the medium, the intensity of the transition radiation emitted into the vacuum (backward emission) is easily obtained by changing j3 to — 0 in Eq. (1). For 7 2> 1, it takes the form, 2 2/32 2 2 1 d W e /3 sin 6 €2 2 2 2 2 (5) dudn T C (1 - (3 cos 6) C2 + 1 which is just Eq. (1) times the Fresnel reflection formula for light normally incident on a medium. 2 3 The formation zone The formation zone of OTR radiation is defined as the distance mea• sured along the particle tracjectory for which the phase difference between the radiation field and the Coulomb field of the particle is just equal to 1 radian, /3c -1 Lm — f3^cos9 (6) In a vacuum e = 1 and T Pc 1 ?X 1 (7\ _ u> 1 - (3cos9 ~ 2TT 1 - /3cos0 for 6 < 1, 2 2 1 Lm « -(7- + ft )' (8) for 50 MeV electron, Lm is about 0.16 cm at A = 500nra. 4 Transition radiation for particle at oblique incidence 4.1 Single particle theory The angular distribution for the transition radiation from a particle crossing the boundary from vacuum into medium at oblique incidence can easily be derived using image charge method [8]. As shown in figure 1, an electron is approaching the interface from left. The electron induces an positive image charge in the medium (the magnitude of its charge is not necessarily equal to that of electron). The image charge is moving in the direction of specular reflection. Both the electron and its image charge contribute to the radiation. The fourier transform of the magnetic field of the radiation is give by [8], (3xh0 0 x h where r is the Fresnel reflection coefficient for the medium. n0 is along the direction of observation, n is the reflection direction corresponding to ii0. The two vectors define the plane of observation.The first term corresponds to the 3 radiation from the electron, and the second term is that from the image charge. If the plane of incidence does not coincide with the plane of observa• tion, the radiation is circularly polarized. Let C\\(u>) be the magnetic field component perpendicular to the plane of observation and C±(u>) the compo• nent parallel to the plane of observation. Assuming the two planes nearly coincide and the particle is relativistic and approximately 45° incident, we have [8]: ^ / x „ sinB SucosB .. where f3\\ (/?j.) is the component of {3 parellel (perpendicular) to the plane of observation, B is the angle between the reflection of /3 and no projected into the plane of observation (the angle between the reflection of f3\\ and re), r\\ (r±) is the Fresnel coefficient for the particular polarization. Using small angle approximation for sinusoidal function, it can be shown that the intensity per unit frequency, per steradian, is given by [8], \r\\\2B2 ReruB ' *-o —— —— 2 2 2 2 (12) dwdSl c •K _(7- + 0 )2 7- + 02 2 2 2 2 d W± e (3 IT-JJ Rerx rs^ (13) duidSl c 7r2 + The intensity distribution for the parallel component around the reflection of /?ll exhibits a similar characteristic as the case with normal incident, which has a maximun intensity at 0m = -. Nevertheless, the pattern of OTR is no longer symmetric because r\\ (|T"|||2) is not symmetric as B changes sign. In figure 2, we plot the intensity distribution for particles with different energy and 45° incident observed in the plane of incidence at A = 500nm.
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