Imagescope to Photodiode Beam-Profile Imaging System

Transverse beam-distribution measurements of high-current cw accelerators must be obtained from noninterceptive sensors. For the 100-mA H2 or D beam of the Fusion Materials Irradiation Test (FMIT) accelerator, these transverse properties may be obtained by detecting the visible radiation resulting from beam interactions with residual gas. A system of mirrors, intensified TV cameras, digitizers, and tomographic reconstruction codes has been reported previously. This report describes a new technique for sensing and digitizing the light projected transversely from the beam of the FMIT accelerator at Los Alamos National Laboratory. Figure 1 is a sketch of the main components of the Imagescope to photodiode beam-imaging system. An input lens focuses the beam-profile image onto a bundle of optical fibers that are spatially coherent from end to end. The output end gathers the fibers from the four input legs into a small double-row format. Thus, four different beam-profile images are transferred from this fiber-optic bundle (Imagescope) through a microchannel-plate (MCP) image intensifier with a format arranged so that two linear photodiode arrays (Reticons) receive the intensified images. The photodiode arrays have 512 elements each; thus, each beam profile has a 256-element resolution. An electronic package accepts the electric signal from the photodiodes, digitizes the contribution of each sensitive element, and transmits the digitized profile information over a fiber-optic data link to a memory unit accessible by the CAMAC data-acquisition and processing system. Details of this new beam-profile imaging and digitizing system are presented in the following sections.