Introduction to the measurement of noise with application to particle accelerator beam stabilization.

Abstract

One of the most important figures of merit for a synchrotron radiation source, once specified beam intensity and energy have been achieved, is charged particle beam stability. While a significant effort has been expended at the Advanced Photon Source (APS) to reduce or eliminate undesirable sources of beam motion, it will be necessary to employ active feedback to stabilize the user photon beams to the very stringent levels required. This becomes especially important when one considers that transverse beam stability is generally quoted as a fraction of beam dimensions. Since source brightness tends to be inversely proportional to these transverse dimensions, it should be evident that x-ray beamline users in general will support any and all efforts to reduce the transverse charged particle beam dimensions. The obvious corollary to this is that coincident with emittance reduction efforts must come improvements in our ability to both measure and correct the particle beam trajectory. Presently, there are at least two active proposals at the APS for reducing both horizontal and vertical emittance. A simple change in lattice functions gives a factor of two reduction in horizontal and vertical beam emittance, while a machine studies program focusing on the correction of horizontal-vertical coupling will allow a reduction of vertical emittance by a factor of 100 or more. The Advanced Photon Source presently operates with the design natural emittance of 8.2 nm-rad, and without coupling correction, the vertical emittance is approximately 3% of this value. The APS design value for coupling is 10%. Given the design lattice functions, the effect of horizontal-vertical coupling on insertion device source beam size is shown in Table 1. Presently, the most stringent requirement on rf beam position monitors (RFBPMs) is derivable from the vertical orbit stability specification, namely, that the vertical charged particle beam trajectory must be stable to within 5% of the rms vertical beam size. At the time the APS beam position monitor system was designed, it was not known that such a specification was even achievable, and even with completely noise-free electronics, other sources of apparent beam motion come into play (e.g., thermo-mechanical effects impacting both the accelerator and the beamlines). Fortunately, due to a careful and extensive design specification process, the presently installed RFBPM system has been demonstrated to have an AC sensitivity that can resolve vertical orbit motions much smaller than 5% of the ''design'' vertical beam size, i.e., assuming 10% coupling [1]. The reason for qualifying the above statement as applying only to AC beam motions should be clear to those familiar with the theory of low frequency electronic noise. Long-term drift of electronic circuit characteristics is a very difficult problem. Ultimately, what the x-ray user is concerned about is the uniformity of x-ray flux striking the sample under study; this is the ultimate aim of our orbit correction and feedback efforts.