Transverse Impedance Measurement in Rhic and the Ags

The RHIC luminosity upgrade program aims for an increase of the polarized proton luminosity by a factor 2. To achieve this goal a significant increase in the beam intensity is foreseen. The beam coupling impedance represents a source of detrimental effects for beam quality and stability at high bunch intensities. In this paper, we evaluate the global transverse impedance in both the AGS and RHIC with measurements of tune shift as a function of bunch intensity. The results are compared to past measurements and the present impedance model. First attempts at transverse impedance localization are as well presented for the RHIC Blue ring. INTRODUCTION The measurements of transverse beam coupling impedance are presented for the AGS and RHIC. In the first section we present the measurements on the AGS whose beam position monitor (BPM) system allowed for a global transverse impedance estimation. In the second section we perform the same measurement for the RHIC Blue and Yellow rings. The impedance of the Blue ring was found to be higher than expected. This motivated a first attempt at impedance localization observing the impedance-induced phase advance beating. A detailed report on the performed measurements and analysis can be found in [1]. AGS MEASUREMENTS In the AGS accelerator due to a limited number of turnby-turn BPMs, the system allowed only for the measurement of the total effective impedance through the observation of the transverse tune shift with intensity. A measurement in the vertical plane could be performed while time constraints didn’t allow the measurement of the horizontal plane. It is however believed to be small due to the elliptical shape of the vacuum chamber. The measurement has been performed at 23.8 GeV total energy injecting a single bunch of σb = (5.8 ± 0.1) ns rms bunch length with transverse emittance of ε95% n = 13.5 μrad on both planes and varying intensity from Nb = 5 · 1010 to Nb = 25 ·1010 ppb. The bunch was excited with a transverse kicker synchronized with a dedicated high-resolution BPM from which 300 turns of coherent betatron oscillation were stored. The total effective transverse impedance Z f f x,y is defined as