Single-Bunch Effects from SPX Deflecting Cavities

Transcription

1 Single-Bunch Effects from SPX Deflecting Cavities Yong-Chul Chae and Louis Emery Accelerator Operation Group Accelerator System Division Measurements March 13, 2013

2 Introduction The single bunch current is limited by the impedance in the ring and the chromaticity of the lattice The proposed long straight sections (LSS) for SPX and for one other straight section are new impedance sources, which have to be mitigated Main concern is preservation of present operations with powered cavities or un-powered cavities. Can we maintain the 16-mA hybrid bunch pattern operation with un-powered cavities? (Yes) Could the impedance increase the vertical emittance with powered cavities? (No) Looked at SPX straight section impedance in detail (ID chamber, SPX chamber transitions) Looked at SPX cavity impedance 2

3 Impedance and Wakefield Definition A bunch passing through structure will leave behind fields which act on itself or other bunches, E(r,t), B(r,t) Wake potentials are defined for three planes and are the wake fields on or near the beam axis normalized to charge and displacement Can be calculated from 3d models of chambers and cavities with electromagnetic codes We can usually avoid large wake fields by making smooth transitions between accelerator parts Resistivity of smooth walls contribute to wake potentials Strong dependence with aperture dimensions 3

4 Impedance and Wakefield Definition Impedance (Z) is the Fourier transform of the wake potential (normalized by bunch spectrum). In general discussions we often interchange the word impedance and wake potential, e.g. this chamber has too much impedance! Tracking programs uses wake potentials in three planes to predict bunch charge limits thresholds The threshold is usually limited by the wake potential in one of three planes Cavities have very long range wake fields for both the main modes and higher-order modes, and are normally described in frequency domain by a sum of resonator impedances 4

5 Impedance Effect on the Single Bunch Current Shows the single bunch current as function of vertical-plane impedance in the current APS ring. Zt=1 is nominal APS. Vertical plane impedance is the limiting impedance in light sources with small-gap insertion devices and we will assume for the remainder that we are speaking of vertical plane Can predict the current with the hypothetical increase and decrease of impedance in the APS-U 5

6 Impedance Component in the ring Use the kick factor to quantify the impedance effect on the beam Kick factor is product of wake potential and charge distribution along the bunch For transverse plane kick factor is weighted by beta function Found that ID impedance contributes 64% of total ring impedance ID (Geometric) 37%, ID (Resistive) 27%*. ID impedance is mainly controlled by the gap and transition. *The kick factors were evaluated at chromaticity 10. 6

7 Impedance Issue with Long Straight Section (LSS) Length increased from 5 m to 8 m approximately. ID geometric impedance increased by 8/5 due to the increased βy at the taper ID resistive impedance increased by >8/5 due to longer length and increased βy If uncorrected, the 3-LSS (two for SPX and one for SCU) will decrease the single bunch current. Restore ID geometric impedance by creating new taper transition Restore ID resistive impedance by applying Cu coating on Al (require R&D) However, in SPX straight sections, resistive impedance is halved because about half of straight section has superconducting cavities. 7

8 Long Straight Section (2) Studied various ways of reducing the taper impedance Found that the long taper is simple and effective solution for the APS-U. Necessary transition length was ~50 cm for 8-mm gap chamber. AES-MED group designed new chamber for the APS-U. reference Courtesy of L. Morrison, M. Givens 8

9 Schematic Layout of LSS for SPX Sector ID Vacuum Chamber Assembly Available space for SCRF cavities including bellows & valves Beam S4 Girder 5 V B T1 T2 B V V B V S6 Girder 1 P P Bellows & valves included in cryostat B T1 T2 Bellows (90 mm) End Box Transition (200 mm) (SR elliptical aperture to ID chamber aperture) End Box Transition (200 mm) (ID chamber aperture to cavity aperture) SR Long Taper Transition (332 mm) Taper Transition (100 mm) (Cavity aperture to SR aperture) Undulator System P Vacuum Pump V Cryostat Valves V Valve (85 mm) B Cryostat Bellows

10 Model of Chamber and Cavity HOM resonator parameters calculated in Microwave Studio Multi-bunch effects computed from these resonator modes Cavity wakefields modeled with GdfidL Single Bunch effects computed from these wakefields 10

11 Vertical Wake Potentials of SPX Sector SPX chamber with its transition is as good as LSS chamber, thus can be treated as one of the LSS chambers without increasing the impedance of present APS SPX cavities add a significant amount of vertical kick to the beam. The total impedance effect of ring with SPX is equivalent to the one of present APS plus SPX cavities. 11

12 Bunch Lengthening due to SPX Cavity (2 MV, N=8) Longitudinal impedance effects of SPX make longer bunch, thus reduces the peak current Increase the single bunch current 12

13 SPX Effect on Single Bunch Current Limit Simulation of Injection Losses Transverse impedance alone (i.e. no longitudinal impedance) reduces the single bunch current limit as expected. With longitudinal impedance included, we could store more current with SPX cavities due to the reduced peak current by bunch lengthening the increased phase mixing by bunch widening above microwave instability threshold. 13

14 14 Emittance Degradation Due to Impedance Track particles in a 33-ps bunch with/without the impedance effect The transverse impedances were added to the tracking simulation in three stages to assess the effect: Impedance of two sets of SPX cavity, each consisting of 10 deflecting cavities at Sector 7 and 8 (old configuration), respectively, and each cavity impedance kicks the beam in x and y plane Chamber impedance of arcs of Sector 7 and Sector 8 was lumped at S7A:Q1 and S7B:Q1 for Sector 7 and at S8A:Q1 and S8B:Q1 for Sector 8 Chamber impedance of sectors outside SPX was lumped at the center each straight section. Inside the SPX, the bunch will get an impedance kick proportional to the slice offset in the deflecting plane y Outside the SPX there could be an impedance kick to the beam if there are any residual oscillation in transverse plane

15 5 SPX Effect on Vertical Emittance (4 MV case) Studied a standard 24-singlet mode with 4-mA in 33-ps rms length per bunch. SPX cavity impedance effect was small. SPX sector impedance would increase the emittance by 1 pm

16 Summary and Conclusion SPX sector has two impedance sources Chamber with transitions Deflecting cavities, which dominates as source in all three planes Transverse impedance with the effect of longitudinal plane preserves the single-bunch current limit in the hybrid-fill pattern Little effect of impedance on the vertical emittance 16