Feasibility Study of Upgrades to the PLS-II Storage Ring
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Feasibility Study of Upgrades to the PLS-II Storage Ring B.-H. Oh, T. Ha, D.-E. Kim, J.-H. Lim, H.-H. Lee, J.-H. Kim, S. Shin and J. Lee∗ Pohang Accelerator Laboratory, Pohang University of Science and Technology, Pohang 37673, Korea (Received 8 January 2020; revised 26 February 2020; accepted 26 February 2020) The PLS-II is a third-generation light source; it has been operating since 2011. It has a doublebend structure with non-zero dispersion along the straight section. Here, we assess three proposed upgrades to the PLS-II. First, we evaluate use of a canted insertion device beamline to increase the number of beamlines. Second, we evaluate the consequences of upgrading the bending magnet to a superbend magnet; the change doubles the strength of the bending field and increases the critical energy from 8.7 keV to 17 keV. Finally, a short-bunch mode for time-resolved experiments is studied; the goal is to achieve a bunch length of 5 ps rms by using a low-alpha mode and increasing the RF gap voltage. Keywords: Storage ring, Canted ID, Superbend, Low alpha DOI: 10.3938/jkps.77.354
I. INTRODUCTION The Pohang Light Source II (PLS-II) [1] is a thirdgeneration storage ring that works with an electron beam that has 3 GeV energy, 400 mA current, and 5.8 nm emittance. PLS-II has been operated since March 2012. In 2019, it was operating close to its full capacity. A 400mA top-up mode was achieved [2,3], and a feedback control has been used to increase the stabilities of electron beams and photon beams [4, 5]. More than 7000 users have conducted experiments at 32 beamlines (13 bending beamlines and 19 insertion device (ID) beamlines) and have presented high-quality papers. Every straight section is now fully filled with IDs, and competition for beam time is increasing. Under these circumstances, we have kept working to upgrade the capacity and the operating parameters of PLS-II. This paper reports on an evaluation of three proposed upgrades of PLS-II. Section II describes the ‘canted ID’ scheme that accommodates two IDs in one long straight section to increase the number of beamlines. Section III introduces the ‘superbend’, which replaces the bending radiation source to increase the brightness in the high X-ray region. Section IV describes a study to achieve a short-bunch operation mode. Section V summarizes the results.
small bending magnets bump the orbit to tilt the radiations from the two IDs so that they have sufficient angle to be distinguished at the beamline hutch [7]. The long straight section of PLS-II is 6.5 m long, and its in-vacuum undulator (IVU) is about 2 m long. Therefore, we put three steering bending magnets and two IVUs in one long straight section (Fig. 1). The angle between the radiations from the two IDs is 11 mrad, which yields a separation of 190 mm at the end of the frontend. This insertion of three extra steering dipoles breaks the 12-fold symmetry of the storage ring lattice. Consequently, its dynamic aperture and lifetime may decrease. The long straight section of the PLS-II is not dispersionfree, so e
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