Multimodal Interaction in the ALS Longitudinal Feedback Kicker RF Cavity

RF cavities are essential components in particle accelerators not only for beam acceleration, but also for control purposes (bunch lengthening/shortening, deflecting and crabbing, transverse and longitudinal kickers) and for beam diagnostics (BPM). Normally, only a single resonating mode is actively used, although other modes can be excited by the circulating beam. Cavities used as longitudinal kickers for bunch-by-bunch feedback systems are designed with an axial mode, which, appropriately excited, provides a kick to the circulating bunches for maintaining beam stability. To provide the necessary bandwidth this mode has to be strongly damped resulting in quality factors of just a few units. In the longitudinal feedback kicker cavity just installed on the ALS we have detected a second axial mode which, although a few hundreds of MHz below the 1.4 GHz design mode, is also strongly damped and has a shunt impedance high enough to be appreciably excited by the feedback amplifier coupling to the first mode. In this paper we show bench measurements on the cavity and with beam during its commissioning and discuss the interaction of the two modes resulting in a modulation of shunt impedance and phase response. INTRODUCTION The Advanced Light Source main storage ring relies on a longitudinal feedback system to suppress multibunch instabilities and be able to operate at its design current of 500 mA [1]. At the end of 2015 the original transmission line longitudinal kicker was replaced by a much shorter device, based on a heavy loaded RF cavity, in order to make space for a new insertion device. Table 1 lists the ALS relevant parameters. The maximum voltage that the kicker needs to supply can be estimated from Vmax = 2 1 τε T0 E0 e Δεmax ≈ 400 V (1) where τε is the longitudinal damping time, T0 the ring's revolution period, E0 its energy and Δεmax the maximum energy spread. The kicker cavity consists of a stainless steel pillbox cavity loaded by four ridged waveguides on each side (Fig.1). Table 1: Relevant ALS Parameters RF Frequency 499.65 MHz Harmonic Number 328 Beam Energy 1.9 GeV Beam Current 500 mA Long. Damping Time 6 ms Max. Energy Spread < 1⋅10 Req. Kicker Voltage ~ 400 V The four upstream feedthroughs are connected to the feedback amplifier and the others to external loads. Figure 1: Transverse and longitudinal sections of the ALS longitudinal feedback kicker cavity. The resulting quality factor decreases to a value around 5, which allows satisfying the minimum bandwidth requirement given by Δf−3dB ≥ (p ± 0.25) fRF (2) where p is any non-zero integer and fRF the main RF frequency. The cavity is not too dissimilar from the one in use in the BESSY II storage ring [2], with the exception of a larger, circular beampipe aperture, to match the ALS vacuum chamber shape in the installation straight, and the addition of "nosecones" to partly compensate the consequent reduction in shunt impedance. It should be noted that a larger pipe aperture is beneficial from the point of view of trapped high-order modes. The design parameters of the ALS cavity are summarized in Table 2. A first series of computer simulations using SuperLANS [3] and HFSS [4] were carried out yielding results in line with those reported for BESSY II and DAΦNE [5]. * Work supported by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 † sdesantis@lbl.gov Proceedings of IPAC2016, Busan, Korea WEPOW052 06 Beam Instrumentation, Controls, Feedback and Operational Aspects T05 Beam Feedback Systems ISBN 978-3-95450-147-2 2965 C op yr ig ht © 20 16 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s Table 2: Kicker Cavity Specifications Pillbox Length 72 mm Overall Length 283 mm Pillbox Radius 83 mm Beampipe Radius 35 mm No. of Waveguides 8 Center Frequency 1383 MHz Bandwidth (-3 dB) 276 MHz Shunt Impedance 657 Ω