Paper  Title  Page 

TUPMF068  Beam Dynamics on a Coupling Resonance at PETRA III  1417 


Working on a coupling resonance is a usual way of producing round beams in a synchrotron. The beam dynamics in this regime is however more complicated, and the emittance is sensitive to the working point, coupling correction, and bunch current drop with time, which complicates the operation. We present experience with optics setup for working on a coupling resonance in PETRA III, including linear and nonlinear beam optics characteristics, and the measurement of the horizontal and vertical beam emittances with a 2D interferometer. Beam dynamics on a coupling resonance for PETRA IV, the MBA upgrade of PETRA III currently under consideration, is also presented.  
DOI •  reference for this paper ※ https://doi.org/10.18429/JACoWIPAC2018TUPMF068  
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TUPMF069  Low Gain FEL Oscillator Option for PETRA IV  1420 


Next generation synchrotron storage rings will have electron beam density approaching that necessary for driving an XFEL. It falls short of the quality required for the highgain xray regime above 1 keV, mainly due to the large energy spread and small peak current, bit is sufficient to reach lowgain regime. Here we explore the parameter space of a low gain XFEL oscillator, to establish the feasibility range of such a device for the Petra upgrade project.  
DOI •  reference for this paper ※ https://doi.org/10.18429/JACoWIPAC2018TUPMF069  
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WEPAF044  Automatic Tuning of PETRA, its Injector Complex, and Prospects of Autonomous Operation of PETRA IV  1912 


We present the progress in tuning automation of the PETRA injection complex. The OCELOT optimizer has been ported to the PETRA control system and proofofprinciple tests of transmission efficiency optimization done. We further argue that the next steps in tuning and automation are impossible without rethinking the architecture of the high level contol system. A possible approach to the new system is then sketched.  
DOI •  reference for this paper ※ https://doi.org/10.18429/JACoWIPAC2018WEPAF044  
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TUPMF071  Status of Impedance Modeling for the PETRA IV  1423 


The diffraction limited synchrotron light source envisioned for the PETRA IV project will require strong focusing to produce the small emittances in both planes. The large natural chromaticity together with small dispersion will require very strong sextupoles. In order to cope with high gradient magnets the radius of vacuum chamber tends to be in the range of 10 mm, which is very small compared to the current 40mm wide elliptic chamber. The impedance element in the PETRA III was scaled down to fit into the smaller aperture so that the short range wakepotential can be computed numerically. For instance the beam position monitor (BPM) was reduced to 60% in dimension so that it can be used in PETRA IV. Even if the actual design of hardware does not exist yet, we assume that generic feature of PETRA III model is still valid. In this paper we report the uptodate information on impedance model of PETRA IV together with the preliminary impedance budget based on the analytical formula. We also report the specific studies carried out to understand the kickfactor scaling with the chamber aperture whose radius is in the range of 812 mm.  
DOI •  reference for this paper ※ https://doi.org/10.18429/JACoWIPAC2018TUPMF071  
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TUPMF072  Microwave Instability and Energy Spread Measurement via Vertical Dispersion Bump in PETRA III  1427 


The recent measurement of bunch length versus current indicated that the longitudinal impedance (Z/n) is 0.15 Ω in close agreement with the impedance model*. Naive application of KeilSchnell criteria predicts the threshold of microwave instability at 0.25 mA. Since the single bunch intensity is in the range of 0.22.5 mA depending on the fillpattern of PETRA III, we expect to observe the fillpattern dependent energy spread according to the theory. However, the 3rd generation light sources comparable to PETRA III often reported the observation which was much greater than the theoretical one. In order to induce the beam size variation we had used skew quadrupoles to generate the dispersion in vertical plane. In particular we made dispersion bump at the undulator sector so that we were able to use the Xray optics for the precise determination of small vertical beam size. In this paper we report the experimental setup and measurement data with the estimate on the instability threshold. We also report the vertical emittance and energy spread based on the Xray beam size measurement as well as the RF signal which was excited by the beam at the longitudinal feedback cavity.
* K. Balewski, R. Wanzenberg, "OBSERVATION OF INTENSITY DEPENDENT SINGLE BUNCH EFFECTS AT THE SYNCHROTRON LIGHT SOURCE PETRA III", Proc. of IPAC2011, p. 730. 

DOI •  reference for this paper ※ https://doi.org/10.18429/JACoWIPAC2018TUPMF072  
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TUPMF073  Impedance Optimization of Small Gap Chambers for the High Single Bunch Current Operation at the Undulator Based Light Sources  1430 


In the undulator based light sources the intensity limit of single bunch is often determined by the strong vertical instability caused by the wakefield in the ring, where the undulator itself is large impedance source. The optimization of transition from the large aperture to undulator's smallgap chamber is ongoing research topic in an effort to reduce the vertical impedance; at the same time, the demand on singlebunch current is high from the timingmode xray user community. In this paper, after showing the results obtained by exploring the parameter space guided by Stupakov's formula, we propose the linearlysegmented transition which can reduce the impedance down to 60% or less of the original linear taper. The reduction can be utilized either to increase the bunch current substantially or to install a smaller gap chamber without impacting the bunch current limit. For the definite result we considered the transition between two ellipses, namely, (a, b) = (42 mm, 21 mm) and (18 mm, 4 mm) over the length 1530 cm in beam direction.  
DOI •  reference for this paper ※ https://doi.org/10.18429/JACoWIPAC2018TUPMF073  
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TUPMF074  Control of Collective Effects by Active Harmonic Cavity in an MBAbased Light Source with Application to the PETRA Upgrade  1433 


Based on the reference lattice for PETRA IV* we investigated collective effects with nonzero current. Out of many possibilities we firstly computed the intrabeamscattering (IBS) effects on the emittance as well as lifetime as a function of current. The result indicated that PETRA IV would benefit from the reduced peak current when the harmonic cavity lengthens the bunch. The operating point of harmonic cavity was explored by tracking simulations as well as analytic formula. In order to compute the energy spread and bunch length we had used the known impedance function of the APS**. In this way more realistic estimation of IBS effects was expected. However, because of the complex nature of PETRA IV lattice, which includes achromatic cells for undulators, arc cells of octants and straight sectors for damping wigglers, we simplify the longitudinal dynamics by assuming the ring made of 92 multibendachromat (MBA) cells. The optics is approximated as a linearchromatic transfer map enabling fast tracking and the ring impedance is concatenated into the one location. The detailed collective effects with and without harmonic cavities are presented in the paper.
* J. Keil, "A PETRA IV Lattice Based on Hybrid Seven Bend Achromats", these proceedings. ** Y.C. Chae and Y. Wang, "Impedance Database II for the Advanced Photon Source Storage Ring", Proc. PAC2007. 

DOI •  reference for this paper ※ https://doi.org/10.18429/JACoWIPAC2018TUPMF074  
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