Design and Upgrade of a Compact Imaging System for the APS Linac Bunch Compressor

We present the design, performance, and recent upgrade of a high-resolution, highcharge-sensitivity imaging camera and beam position monitor (BPM) system for the APS linac beam profile measurement. Visible light is generated from the incoming electron beam using standard YAG or optical transition radiation (OTR) converter screens. Two CCD cameras share the light through a beam splitter, each with its own imaging optics. Normally, one camera is configured with high magnification and the other with large field of view. In a different lens configuration, one of the cameras focuses at the far field, allowing the measurement of beam divergence using an OTR screen, while the other camera simultaneous measures the beam size. A four-position actuator was installed recently to provide the option of two screens, a wakefield shield, and an in situ calibration target. A compact S-band beam position monitor electrode was designed to mount directly on the flag. The BPM rf circuit was fabricated from a machinable ceramic (MACOR) cylinder substrate, and the copper electrodes were deposited on the substrate. The new design and precision fabrication process make it viable to explore more complex microstrip components printed on the substrate and higher frequency applications. The proximity of the BPM and the camera (< 5 cm) will provide a precise calibration platform to study shot-toshot jitter, long-term stability of both systems, and the dependence of BPM signal on beam properties (size, charge distribution) due to nonlinearity. INTRODUCTION A chicane bunch compressor was designed and implemented at the Advanced Photon Source (APS) in 2000 [1] to increase the peak current of the bunch and improve the performance of the low-energy undulator test line (LEUTL) free-electron laser (FEL). It is expected to operate at ~200 MeV with normalized emittance in the range of 1 π to 4 π mm·mrad. Coherent synchrotron radiation (CSR) effects are expected to be significant at these emittance levels. Their study calls for accurate emittance measurement at the level of several percent or better. TABLE 1. Electron Beam Parameters at the Three Screen Section ELECTRON ENERGY (MEV) 200 (γ = 400) Single bunch charge (nC) 0.2 -1.0 x Normalized emittance (π mm⋅mrad) 4 .0 1.0 Emittance ε (π mm⋅mrad) 0.010 0.0025 Beta function at beam waist β (m) 1.00 Rms beam size, εβ (μm) 100 50 Rms beam size, / ε β (μrad) 100 50 A compact, modular imaging system [2] was designed and implemented to support studies of the low-emittance beams, with the high resolution, reliability, reproducibility, and accuracy needed. In this paper, we report several implemented and planned upgrades to the system. A new screen mount was designed and installed. It includes a wakefield shield, a YAG screen mount, an optical transition radiation (OTR) screen mount, and a calibration target. A new set of optics was designed to focus in the far field, or image the angular divergence of the electron beam when the OTR screen is employed. A compact electronic beam position monitor (BPM) was designed to mount directly on the flag. The BPM design uses a new fabrication process, which is expected to make it more viable to explore complex electrode configurations and for higher frequency applications. FOUR-POSITION SCREEN MOUNT Due to high-energy spread used for chirping the beam, the rms beam size in the horizontal direction can reach several millimeters in the midsection of the chicane. This is comparable to the aperture of the vacuum vessel of 25 mm. A smooth transition of the beam pipe is desired to reduce wakefield effects to the electron bunch. Figure 1 shows the design of the wakefield shield. Figure 2 shows an ACAD rendering of the screen mount assembly. The wakefield shield / transition is mounted at position no. 1 on a four-position vacuum feedthrough, powered by a pair of pneumatic actuators connected back-toback. The stroke of the short actuator is 38.4 mm and that of the long actuator is 76.4 mm. Four screen positions (38.4 mm spacing) can be reached by extending or contracting these two pneumatic actuators. FIGURE 1. Four-position screen mount assembly: (1) Long-stroke actuator, (2) short-stroke actuator, (3) mounting block and linear bearing housing, (4) vacuum bellow, (5) wakefield shield / transition, (6) OTR screen mount, (7) YAG screen mount, (8) mirror, (9) calibration target, and (10) viewport for target illumination. 2 1 3 9 8 7 6 5 4 1 0 FIGURE 2. ACAD 3-D rendering of the four-position screen mount assembly. At position no. 2, a 45-degree mirror is mounted as an OTR screen. It is used for high-intensity (charge density) electron beams. At position no. 3, a YAG scintillator screen (20 mm × 15 mm × 0.1 mm) is mounted with a mirror. It is used for lowintensity electron beams. At position no. 4, an aluminum-coated prism is used as a first surface mirror, in conjunction with a calibration target. The target is designed to have a dark metallic coating with an etched dot matrix at 0.25 mm spacing. The illumination light is fed from outside of the vacuum through a viewport. Images of these bright dots in the dark background are convenient for the video digitizer to pick out and perform profile analysis. The center coordinates of the dots are used to calibrate pixel sizes, while the rms image size of these 10-μm-diameter dots are used to obtain optical resolution of the camera. Since the optical distance from the calibration target is the same as from the YAG screen, the target is also used as a focusing aid. OPTICS FOR BEAM DIVERGENCE MEASUREMENT By design, the camera module has two cameras sharing the light with a beam splitter. Normally, each camera has its own imaging optics, one is used for highresolution imaging but covers only a part of the screen, while another is used for lowresolution imaging and covers the full view of the screens. By reconfiguring the optics, however, one camera could be equipped with optics for the far field, i.e., with object space at infinity. Measurement Using OTR Angular Distribution To measure relatively large beam divergence (σx' ~ 0.1/γ), we can use the features in the angular distribution of the optical transition radiation. A linear polarizer installed in the turntable can be used to select the direction of polarization to be along the x or y-axis. When the x-polarization is selected, the OTR angular distribution is, ( ) ( ) 2 2 2 2 2 2 , x x x y x y A I θ θ θ π γ θ θ − = + + , (1) where (θx, θy) is the angle from the specular direction, and A is a constant. For an electron beam with Gaussian divergence distribution, with rms divergence of (σx', σy'), ( ) 2 2