MODIFIED LATTICE OF THE COMPACT STORAGE RING IN THE cSTART PROJECT AT KARLSRUHE INSTITUTE OF TECHNOLOGY A. I. Papash † , E. Bründermann, B. Haerer, A.S. Müller, R. Ruprech, J. Shäfer, M. Schuh, M. Schwarz Karlsruhe Institute of Technology, Karlsruhe, Germany Abstract The compact storage ring project for accelerator research and technology (cSTART) is realized at the Institute for Beam Physics and Technology (IBPT) of the Karlsruhe In- stitute of Technology (KIT). Combination of a storage ring and a laser wake-field accelerator (LWFA) might be the ba- sis for future compact light sources and advancing user fa- cilities. Meanwhile the post-LWFA beam should be adapted for storage and accumulation in a dedicated circu- lar accelerator. Modified geometry of compact ring oper- ating at 50 MeV energy range has been studied and main features of a new model are described here. The new de- sign, based on 45° bending magnets, is suitable to store a wide momentum spread beam as well as ultra-short elec- tron bunches in the fs range from the Plasma cell as well as from the ferninfrarot linac- und test- experiment (FLUTE). The DBA lattice with different settings and relaxed param- eters, split elements and higher order optics of tolerable strength allows improving the dynamic aperture and mo- mentum acceptance to acceptable level. This contribution discusses the lattice features in details and different possi- ble operation schemes of a ring. INTRODUCTION A dedicated storage ring with adapted features should be built and tested in order to provide experimental "proof of principles" on possible operation with beams after a laser wake-field accelerator 1. Stable rotation of ultra-short (fs) bunches, storage of a wide-momentum spread beam in a circular ring would be a solid ground for further R&D on LWFA facilities. Meanwhile the post-LWFA beam is not directly suitable for storage and accumulation in conventional storage rings. New generation rings with adapted features are required. One important goal of the project is to demonstrate injec- tion and storage of a post-LWFA beam in a storage ring. As a first stage the compact linear accelerator FLUTE will serve as an injector of 50 MeV bunches to test the ring’s performance 2. The cSTART ring will be used also for accelerator R&D studies. Different geometries, lattices and operation modes of a ring to store electron beam at 50 MeV energy range have been extensively investigated and results of prelimi- nary studies were reported at 3-5. Ring dimensions are strongly limited by the compact size of the FLUTE Bunker 2 while maximum value of dispersion function should be limited in order to accommo- date wide momentum spread beam. A highly non-linear DBA lattice with 22.5 gradient dipoles, split quadrupoles and two-meter-long straight sections has been chosen as a best candidate for further detailed studies 4, 5. Later it was realized that short straight sections cannot accommo- date all necessary equipment and proposed set up is not flexible to manage different operation modes. Also bend- ing magnets with high gradient can’t guarantee the re- quired parameters in specific case of a wide momentum spread beam. STORAGE RING Final setup of the cSTART ring includes flat (no gradi- ent) 45sector bends, split quadrupoles with relaxed pa- rameters, sextupole and octupoles lenses, diagnostic equip- ment. Four-meter-long straight sections accommodate ele- ments of injection, equipment for dedicated diagnostics and physical experiments. An OPA code and new ring ge- ometry were used for computer simulations 6. The ring layout is shown in Fig. 1. Figure 1: Layout of the cSTART ring. A special transfer line delivers short bunches from the FLUTE linac to the ring and includes focusing elements, energy analyzer and bunch compressor that provides phase space “gymnastics” of the ultra-short bunches 7. The ring should accommodate large momentum spread beam and allow circulation of ultra-short bunches at non- equilibrium conditions:  Momentum acceptance is increased to about 6%.  Dynamic aperture (DA) is opened to 20 mm at posi- tion of maximum value of horizontal betatron function  x .  Strength of sextupoles is limited to open the DA. -6 -4 -2 0 2 4 6 X[m] -12 -10 -8 -6 -4 -2 0 Y[m] ____________________________________________ † alexander.papash@kit.edu 12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW Publishing ISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-MOPAB035 MC2: Photon Sources and Electron Accelerators A04 Circular Accelerators MOPAB035 159 Content from this work may be used under the terms of the CC BY 3.0 licence (© 2021). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI