Taper Shape Effects on the HOM Damping of the PLS-II SRF Cavities

PLS-II storage ring employs three cryomodules to accelerate an electron beam. Two commercial cryomodules will be installed in a long-straight section during the upgrade phase; one will be in a short-straight section in the future. The available length of a long straight section is not enough to install two commercial cryomodules with its supplementary parts. In order to install two cryomodules in the section, we need to modify the tapers for reducing their total length. The main concern for this modification is the effects on the beam instability due to the HOM damping efficiency change. In this paper, the HOM damping effects of different taper shapes has been studied. INTRODUCTION The Pohang Light Source (PLS) of the Pohang Accelerator Laboratory (PAL) is under upgrade. A superconducting RF (SRF) system has been chosen for the PLS-II [1-3]. Three cryomodules will be use in the PLS-II storage ring. Two commercial cryomodules will be installed into one long-straight section during upgrade stage, and one will be in a short-straight section of PLS-II storage ring in the future. The available length for the cryomodules of the long-straight section is 6.28 m. In this space, two beam-pipe transitions from elliptical section of vacuum valves to circle cross section of cryomodules, two commercial cryomodules, three bellows, three or four vacuum valves need to be installed. Two commercial cryomodules, either of CESR-type cryomodules by RI or KEKB-type cryomodules by Mitsubishi Electric Corporation, are too long to be installed into this section. In order to install two cryomodules in this section, we need to reduce the total length of the two cryomodules. The most simple and economic method is to shorten the cryomodules warm tapers or replace two adjacent warm tapers by a beam-pipes with bellows. The main concern for this modification is the beam instability, which is evaluated by higher-order mode (HOM) damping effects. Both CSER-type and KEKB-type cryomodules use the coaxial ferrite HOM dampers to absorb the HOM-induced power [4, 5]. Here we only shorten the tapers, but two HOM damper positions are not changed. Shorting cavity length could induce the HOM standing wave point position movement. The movement will lead the variation of the HOM power absorbability on ferrite HOM absorbers, that means that the Qext or Qload of the HOM or HOM damping effects are changed. In this paper, 3 plans to install the two cryomodules in a long-straight section of PLS-II storage ring have been presented, and the HOM spectrum and impedance have been calculated by using SuperLans [7]. After evaluating the HOM damping effects for different taper shapes, the reasonable plans are presented. TAPER ANGLE EFFECTS ON THE HOM DAMPING OF THE CSER-TYPE CRYOMODULE A CESR-type cryomodule, such as used in SSRF, if we change its taper angle to 11.31, two cryomodules can be installed in a long-straight section (see Fig. 1). Figure 1: Two-cryomodule installation in a long-straight section of the PLS-II storage ring. Figure 2 shows the PLS-II storage ring’s longitudinal impedance thresholds [4, 7] and impedance of the CESRtype cryomodules with different taper angle. We find that there are no monopoles with sharp impedance change for different taper angle from original angle to 11.31; and no monopoles, whose impedance is over the impedance threshold. This means the beam is stable in longitudinal direction. 0.5 1.0 1.5 2.0 2.5 3.0 3.5 10 10 10 10 10 10 R L ( k ) an d R L t hr es h ( k ) 9.0, 10.0 SSRF, 11.0 11.31, 12.0 RL thresh