A simple monitor has been built to measure the profile of the high power beam (800 kw) delivered by the CEBAF accelerator at Jefferson Lab. The monitor uses the optical part of the forward transition radiation emitted from a thin carbon foil. The small beam size to be measured, about 100 pn, is challenging not only for the power density involved but also for the resolution the instrument must achieve. An important part of the beam instrumentation community believes the radiation being emitted into a cone of characteristic angle l/y is originated from a region of transverse dimension roughly hy; thus the apparent size of the source of transition radiation would become very large for highly relativistic particles. Our monitor measures 100 pm beam sizes that are much smaller than the 3.2 mm hy limit; it confms the statement of Rule and Fiorito that optical transition radiation can be used to image small beams at high energy. The present paper describes the instrument and its performance. We tested the foil in, up to 180 pA of CW beam without causing noticeable beam loss, even at 800 MeV, the lowest CEBAF energy. Existing wire scanners measure profiles of only pulsed or low-current-CW beams to avoid melting their wire and also to prevent the beam loss monitors from triggering the machine protection system.We present an operational prototype of a profile monitor that uses the optical part of the forward transition radiation emitted from a thin carbon foil when the beam passes through it. Previous studies [l,
indicated the heat the beam deposits on tbe foilshould not damage it and also that the beam scattering through 0.25 pm of carbon should not deteriorate the beam characteristics down to 800 MeV. Here, we report on the successful operation of the instrument at 180pA without damaging the foil and without any detectable beam loss in the range of CEBAF energy delivery. An important issue for measuring small beam profiles is the resolution of the instrument. A relativistic particle beam yields a forward Optical Transition Radiation (Om) with a peak intensity on a cone of characteristic angle lly. The thinking has been that there is an uncertainty = the OTR photon emission. Rule and FGrito have challenged this concept [3]. We prove them to be right, after measuring a 100 pm rms size of a 3.2 GeV beam in the optical domain (hy = 3.2 mm).In the near future, we are going to monitor the energy spread in the experimental beam lines with the same kind of profile monitor installed in a dispersive region. We mention a few improvements that will make these instruments better than the prototype from the operational point of view.#snilSUTlON OF THIS DOCUMENT IS UNUM .
DESIGN CONSIDERATIONS
I Forward versus Backward OTRTransition radiation is generated on both sides of a conductive or dielectric foil when relativistic charged particles go through it, as shown in Figure 1.
