Beam-Size Measurement with Optical Diffraction Radiation at KEK Accelerator Test Facility Pavel Karataev, 1, * Sakae Araki, 1 Ryosuke Hamatsu, 2 Hitoshi Hayano, 1 Toshiya Muto, 1 Gennady Naumenko, 3 Alexander Potylitsyn, 3 Nobuhiro Terunuma, 1 and Junji Urakawa 1 1 KEK: High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan 2 Tokyo Metropolitan University, 1-1 Minamiohsawa, Hachioji, Tokyo 192-0397, Japan 3 Tomsk Polytechnic University, 634050, pr. Lenina 2a, Tomsk, Russia (Received 17 March 2004; published 8 December 2004) An experiment on the investigation of optical diffraction radiation (ODR) from a slit target as a possible tool for noninvasive electron beam-size diagnostics has been performed at the KEK accelerator test facility. The experimental setup has been installed at the diagnostics section of the extraction line. We have performed the first incoherent ODR observation from a slit target. The measured angular distributions are in reasonable agreement with the theoretical expectation. The beam-size effect onto the ODR angular pattern has been observed. Moreover, the sensitivity to the beam size as small as 14 m has been achieved. DOI: 10.1103/PhysRevLett.93.244802 PACS numbers: 41.75.Ht, 41.60.–m Nowadays a few techniques for measuring an electron beam size as small as 10 m with resolution of 1 m are being developed at the accelerator installations with dif- ferent beam energies. That is the requirement for the accelerator installations of the next generations such as linear collider [1], where a small beam size at the inter- action point is required to achieve a reasonable luminos- ity. A beam with 10 m vertical size will be accelerated in the main linac. Therefore, it is extremely necessary to control the beam size at every stage of the accelerating process from injection to extraction. The modern accel- erators preclude the use of any invasive techniques like wire scanners or transition radiation monitors due to a significant emittance growth. However, a simple nonin- vasive method for beam-size measurement with a single shot is still absent. Optical diffraction radiation (ODR) appearing when a charged particle moves in the vicinity of a medium [2] is a very promising effect for beam-size monitoring. If the particle passes through a slit between two semiplanes (see, for instance, Fig. 1), it induces currents changing in time on both half planes. Those currents give rise in radiation. Two cones propagate in the direction of spec- ular reflection producing an interference pattern, which is very sensitive to the transversal beam parameters. Since the particles do not directly interact with the target it allows keeping the beam parameters almost the same as the initial ones. The first observation and investigation of the ODR effect from a single edge target has been per- formed by us and represented in [3]. The experimental results were in reasonable agreement with the theory of ideally conducting infinitely thin target. In this Letter we represent ODR measurements from a slit and the beam- size effect. The first theoretical consideration on the beam-size diagnostics using ODR technique appeared a few years ago [4,5]. In those papers it was shown that for the geometry shown in Fig. 1 only the vertical polarization component is sensitive to the vertical beam size. The angular distribution of it convoluted with a Gaussian could be represented in the form d 2 W slit y d!d   2 4 2 expz  1  t 2 x p  1  t 2 x  t 2 y   exp  2z 2  2 y a 2 1  t 2 x    cosh  2zb a  1  t 2 x q   coszt y  2  (1) Here   1=137 is the fine structure constant,  is the particle Lorentz factor, a is the effective slit size (the real one is a= sin 0 , where  0 is the target tilt angle), t x;y   x;y are the observation angles measured from the di- rection of specular reflection (see Fig. 1) in units of the  1 ;z  !=! c  2a= is the emitted photon energy in units of the ODR characteristic energy ! c ,  y is the vertical rms beam size, b is the beam offset with respect to the slit center (see Fig. 1), and   arctant y =  1  t 2 x p . Figure 2(a) illustrates the two-dimensional angular distribution of the vertical polarization component. The intensity between two peaks strongly depends on the e θ 0 θ y ODR pattern a b θ y 0 FIG. 1. Geometry of the ODR production. PRL 93, 244802 (2004) PHYSICAL REVIEW LETTERS week ending 10 DECEMBER 2004 0031-9007= 04=93(24)=244802(4)$22.50 244802-1  2004 The American Physical Society