Shaoheng Wang

Longitudinal Coupled Bunch Instability in JLEIC

The high luminosity performance of JLEIC requires maximum number of bunches in the collider rings with moderate single bunch charges. This makes the coupled bunch instability an important issue for the JLEIC beam stability. For the ion beam, the fast growth rate of the longitudinal coupled-bunch instability (LCBI) sets unprecedented demands for the fast bunch-by-bunch feedback system. In this paper, we identify the most offensive mode in the HOMs for both the JLEIC electron and ion rings. This will assist the RF cavity design in improving the HOM damping and in reducing the severity of LCB

JLEIC SRF cavity RF Design

The initial design of a low higher order modes (HOM) impedance superconducting RF (SRF) cavity is presented in this paper. The design of this SRF cavity is for the proposed Jefferson Lab Electron Ion Collider (JLEIC). The electron ring of JLEIC will operate with electrons of 3 to 10 GeV energy. The ion ring of JLEIC will operate with protons of up to 100 GeV energy. The bunch lengths in both rings are ~12 mm (RMS). In order to maintain the short bunch length in the ion ring, SRF cavities are adopted to provide large enough gradient. In the first phase of JLEIC, the PEP II RF cavities will be reused in the electron ring to lower the initial cost. The frequency of the SRF cavities is chosen to be the second harmonic of PEP II cavities, 952.6 MHz. In the second phase of JLEIC, the same frequency SRF cavities may replace the normal conducting PEP II cavities to achieve higher luminosity at high energy. At low energies, the synchrotron radiation damping effect is quite weak, to avoid the coupled bunch instability caused by the intense closely-spaced electron bunches, low HOM impedance of the SRF cavities combined with longitudinal feedback system will be necessary. INTRODUCTION In order to achieve high luminosity in the electron ion collider, shorter bunches and higher beam current are needed. In the ion collider ring of JLEIC [1], after a series of acceleration, the last step of manipulation before colliding is bunching. Since there is no significant synchrotron radiation the ion bunches are located at the zero crossing of RF voltage. The resulting bunch length is inversely proportional to the RF cavity frequency and square root of RF voltage. So, higher frequency is preferred and 952.6 MHz was chosen as mentioned above. With this frequency, the required total RF peak voltage can be derived to be around 43 MV to obtain a bunch length of 12 mm, so SRF cavities are preferred for the ion collider ring. In the electron ring of JLEIC, the synchrotron radiation power loss becomes more significant when beam energy is higher. CEBAF is used as injector for the electron collider ring. Initial energy at injection will be up to 10 GeV with an option to upgrade to 12 GeV in the future. The number of normal conducting PEP II RF cavities available won’t be sufficient at this higher energy, so SRF cavities of the same type as used in ion ring may be added to the electron ring in the second phase of JLEIC. Eventually the PEP-II NC cavities may be phased out and replaced by 952.6 MHz SRF cavities, enabling a higher bunch rate in both collider rings. In the electron collider ring the beam current at the high energy end will be limited by the RF power supply available, so the beam current at high energy end could be increased in the future by adding more klystrons if desired. At the low energy end, the synchrotron radiation damping effect becomes too weak to suppress the coupled bunch instability of the nominal 3 A beam current without proper HOM impedance control. Two strategies are used to ensure the beam stability: low HOM impedance cavities and bunch-to-bunch feedback systems. LOW HOM IMPEDANCE CAVITY In order to lower HOM impedance, it is proposed to use three on-cell dampers to extract HOM power, just as in the PEP-II RF cavity [2]. A one third model of the present concept with electric and magnetic field on surface is shown in Fig. 1. In ion ring application, the SRF cavity will see the max gradient, about 8 MV/m, the corresponding Bmax is 71 mT. Figure 1: One-third model of the cavity. Left: electric field; right: magnetic field. All three damper waveguides are located on one side of the cavity and symmetrically located around the central axis of the cavity. We compared two, three and four damper waveguides in different configurations; this is the one that gives lowest overall impedance. The cut-off frequency of the damper waveguide is chosen to be half way between the fundamental mode and the lowest HOM, ~1 GHz. In this way HOMs can be extracted well without too much leakage of fundamental mode. Good coupling between the damper waveguides and cavity is essential for the low impedance design, however too large of an opening may cause unacceptable field concentration around the aperture. A ridged waveguide iris with a dumbbell shaped cross-section is added be___________________________________________ * Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177 and DE-AC02-06CH11357. The U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce this manuscript for U.S. Government purposes. † email address: wang@jlab.org WEPMW039 Proceedings of IPAC2016, Busan, Korea ISBN 978-3-95450-147-2 2522 C op yr ig ht