OTR FROM NON-RELATIVISTIC ELECTRONS
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Abstract 2 OTR EMISSION FROM NON RELATIVISTIC ELECTRONS The CLIC Test Facility 3 (CTF3) injector will provide pulsed beams of high average current; 5A over 1.56μs at 140keV. For transverse beam sizes of the order of 1mm, as foreseen, this implies serious damage to the commonly used scintillating screens. Optical Transition Radiation from thermally resistant radiators represents a possible alternative. In this context, the backward OTR radiation emitted from an aluminium screen by a 80keV, 60nC, 4ns electron pulse has been investigated. The experimental results are in good agreement with the theoretical expectations, indicating a feeble light intensity distributed over a large solid angle. Our conclusions for the design of the CTF3 injector profile monitor are also given. We consider the transition between the vacuum and a material with a relative permittivity e. The screen is tilted with respect to the beam trajectory ( z ) by an angle ψ, as shown in figure 1. The OTR emission results from the contribution of the direct ( n ), the reflected ( ' n ) and the refracted ( ' n' r ) radiations emitted by the particle. Using the formalism developed by Wartski in [6], the backward OTR spectral and angular distribution emitted with polarizations parallel and perpendicular to the observation plane can be expressed by:Keywords:
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We propose and use a technique to measure the transverse emittance of a laser-wakefield accelerated beam of relativistic electrons. The technique is based on the simultaneous measurements of the electron beam divergence given by $v_{\perp}/v_{\parallel}$, the measured longitudinal spectrum $\gamma_\parallel$ and the transverse electron bunch size in the bubble $r_{\perp}$. The latter is obtained via the measurement of the source size of the x-rays emitted by the accelerating electron bunch in the bubble. We measure a \textit{normalised} RMS beam transverse emittance $<0.5$ $\pi$ mm$\:$mrad as an upper limit for a spatially gaussian, spectrally quasi-monoenergetic electron beam with 230 MeV energy in agreement with numerical modeling and analytic theory in the bubble regime.
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The commissioning of the 7-GeV Advanced Photon Source (APS) storage ring began in early 1995. Characterization of the stored particle beam properties involved time-resolved transverse and longitudinal profile measurements using optical synchrotron radiation (OSR) monitors. Early results include the observation of the beam on a single turn, measurements of the transverse beam sizes after damping using a 100 μs integration time (σx∼150±25 μm, σy∼65±25 μm, depending on vertical coupling), and measurement of the bunch length (στ ∼25 to 55 ps, depending on the charge per bunch). The results are consistent with specifications and predictions based on the 8.2 nm-rad natural emittance, the calculated lattice parameters, and vertical coupling less than 10%. The novel, single-element focusing mirror for the photon transport line and the dual-sweep streak camera techniques, which allow turn-by-turn measurements, will also be presented. The latter measurements are believed to be the first of their kind on a storage ring in the USA.
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In the research center Rossendorf, the radiation source ELBE, based on a super conducting LINAC, is under construction. In the year 2001 the first accelerating module was commissioned. The electron beam parameters like emittance, bunch length, energy spread were measured. Here we present results of the measurements as well as the methods used to make the measurements. In the ELBE injector, where electron beam energy is 250 keV, the emittance was measured with the aid of a multislit device. Emittance of the accelerated beam was measured by means of quadrupole scan method and is 8 mm×mrad at 77 pC bunch charge. Electron bunch length was measured using the coherent transition radiation technique. At the maximum design bunch charge of 77 pC the RMS bunch length was measured to be 2 ps. A set of online diagnostic systems is also under development. One these include a system of stripline beam position monitors is also described here. A BPM resolution of about 10 μm was achieved using logarithmic amplifier as the core element of the BPM electronics. A system of beam loss monitors based on the RF Heliax cable working as an ionization chamber is intended to be another online diagnostic system.
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The initial demonstrations over the last several years of the use of optical diffraction radiation (ODR) as non-intercepting electron-beam-parameter monitors are reviewed. Developments in both far-field imaging and near-field imaging are addressed for ODR generated by a metal plane with a slit aperture, a single metal plane, and two-plane interferences. Polarization effects and sensitivities to beam size, divergence, and position will be discussed as well as a proposed path towards monitoring 10-micron beam sizes at 25 GeV.
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2 OTR FOR RELATIVISTIC PARTICLES Optical Transition Radiation (OTR) is widely used in beam diagnostics. The most common application is the acquisition of the transverse and longitudinal beam profiles. Other beam parameters, like divergence and energy, can also be deduced from the angular distribution of the OTR emission (“Doughnut”). In order to investigate the possibilities and the limits offered by this technique we have performed a test on the 48MeV, 1nC electron beam of the CLIC Test Facility 2 (CTF2.). Beam divergences between 2 and 6mrad were measured with an accuracy of a few percent. A good agreement was also found between the energy measurements obtained with a classical spectrometer and the OTR based technique. We conclude by describing some possible applications of OTR based diagnostics for CLIC. Let us consider the interface between vacuum and a material with a relative permittivity e. Let us also assume that this interface is tilted with respect to the beam trajectory by an angle ψ, as shown in figure 1. Using the formalism developed in [4], the backward OTR spectral and angular distribution emitted with polarizations parallel and perpendicular to the observation plane can be expressed by: ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) 2 2 2 4 2 2 2 2 2
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We have developed several analytic and experimental techniques to measure the divergence and emittance of charged particle beams, which employ optical transition radiation (OTR) produced from thin intercepting foils. OTR's directionality, promptness, linearity, polarization, and the sensitivity of its angular distribution to energy and divergence, can be all exploited to diagnose the spatial distribution, energy, and emittance of a charged particle beam. We describe the techniques we have developed to separately determine the x and y emittances of a beam at an x or y waist using OTR from a single foil or a two foil OTR interferometer. These methods have proven to be especially valuable for diagnosing low emittance electron beams produced by FEL accelerators, which range in energy from 17 to 110 Mev. However, we have shown that there is no inherent theoretical limit to the utility of these methods for much higher energy lepton or hadron beams. The advantages of OTR methods over those commonly used to diagnose beam properties are described.
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Beam divergence
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We have developed single and double foil techniques to measure current density, energy, and divergence of intense relativistic charged particle beams from the transition radiation produced at a foil-vacuum interface. Single foil optical transition radiation (OTR) measurements have been made using a high intensity beam of lo-25 MeV electrons from the EG&G/EM linac, in which the entire OTR distribution is captured with an imaging system.' Here we describe the results of similar experiments utilizing a twofoil interferometer, which has potential for making high precision energy and emittance measurements of very cold beams.
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Particle beam
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Charged particle beam
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Divergence (linguistics)
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Diffraction radiation (DR) can be described as the scattering or diffraction of pseudo-photons associated with the transverse electromagnetic field of a relativistic charged particle, when the particle passes through an aperture or near an object. DR is closely related to transition radiation (TR) which is produced when a charged particle crosses a boundary between media with different dielectric constants. DR is also associated with “beam loading” which occurs when beams pass through discontinuties in their transport lines. DR is very similar in its properties to TR, so that much of what has been developed for TR beam diagnostics can readily be adapted to DR diagnostics. We will present concepts for using DR as a compact, nondestructive diagnostic that can measure the beam divergence, beam profile, and thus emittance in real time. These concepts, together with the previously demonstrated use of DR for longitudinal bunch length measurement, make an attractive suite of noninterceptive, time-resolved diagnostics which parallel those based on TR.
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Optical transition radiation (OTR) plays an important role in beam diagnostics for high energy particle accelerators. Its linear intensity with beam current is a great advantage as compared to fluorescent screens, which are subject to saturation. Moreover, the measurement of the angular distribution of the emitted radiation enables the determination of many beam parameters in a single observation point. However, few works deals with the application of OTR to monitor low energy beams. In this work we describe the design of an OTR based beam monitor used to measure the transverse beam charge distribution of the 1.9-MeV electron beam of the linac injector of the IFUSP microtron using a standard vision machine camera. The average beam current in pulsed operation mode is of the order of tens of nano-Amps. Low energy and low beam current make OTR observation difficult. To improve sensitivity, the beam incidence angle on the target was chosen to maximize the photon flux in the camera field-of-view. Measurements that assess OTR observation (linearity with beam current, polarization, and spectrum shape) are presented, as well as a typical 1.9-MeV electron beam charge distribution obtained from OTR. Some aspects of emittance measurement using this device are also discussed.
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Beam emittance
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A number of particle physics experiments are being proposed as part of the Department of Energy HEP Intensity Frontier. Many of these experiments will utilize megawatt level proton beams onto targets to form secondary beams of muons, kaons and neutrinos. These experiments require transverse size measurements of the incident proton beam onto target for each beam spill. Because of the high power levels, most beam intercepting profiling techniques will not work at full beam intensity. The possibility of utilizing optical transition radiation (OTR) for high intensity proton beam profiling is discussed. In addition, previous measurements of OTR beam profiles from the NuMI beamline are presented.
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