ANL, Argonne, USA
The APS storage ring beam position monitor system presently is impacted by transverse electric (TE) longitudinal resonances trapped in the beam vacuum chamber. These TE-like modes are excited in the large-aperture sections and become trapped between the bellow end flanges. Ideally the chamber would be in cut-off for frequencies well above the BPM operating frequency of 352 MHz, but due to the geometry of the chamber the cut-off frequency is nominally 330 MHz. The modes are vertically-oriented and are superimposed on the beam position signals resulting in erroneous step changes in beam position measurements and systematic intensity dependence in the vertical plane. This problem places a fundamental limitation on vertical beam position monitor performance. In this paper we will discuss the mode suppression design, simulation, installation, and commissioning results of the first three sectors installed in the APS storage ring.
ANL, Argonne, USA
It has been shown that cavity BPMs are sensitive not only to beam centroid position but also longitudinal beam tilt* . Button-style BPMs also should in principle be sensitive to beam tilt that may impact their perfor- mance when used to measure the beam centroid. For the APS up- grade project, beam stability at a level better than 0.5 micron (0.1 - 200 Hz) is required. Simplified models of the button geometry are used used in the calculation and simulation. For the experiment, tilt oscillations were induced by kicking the beam vertically and letting it decohere** . Tilt oscillations were simultaneously observed using a streak camera and a specially-instrumented set of button- type BPM pickup electrodes. Experimental results are compared with calculation and simulations to quantify the impact of beam tilt on BPM centroid resolution performance.
* Marc Ross et al., RF Cavity BPMs as Beam Angle and Beam Correlation Monitors, Proc. of the PAC03, pp. 2548-2550, 2003.
** W. Guo et al. Phys. Rev. ST Accel. Beams 10, 020701 (2007).
