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Description
The RIKEN SCRIT facility involving an electron storage ring (SR2) is an RI-electron scattering dedicated facility. In the SCRIT system, the target RI ions are confined at high density on the beam axis by the focusing force provided by the electron beam itself. Therefore, electron beam stability is essential to make the luminosity higher and maintain it for a sufficient time during scattering experiments. Since the current RF cavity induces beam instabilities due to higher order modes (HOMs), a new HOM damped cavity is being designed and constructed and will soon be replaced. Here, we adopted the TM020 type harmonic cavity, which has recently been developed and first practically used at the NanoTerasu synchrotron light source. The cavity has coaxial slots at both the front and rear ends with ferrite absorbers inside. It is a very simple and compact design, yet it efficiently damps most resonant modes except for the TM020 mode. The design frequency of the TM020 is 956 MHz, which is the fifth harmonic of the current drive frequency 191 MHz, and accordingly the harmonic number is changed from 14 to 70. As revealing from simulation studies, an increase in the harmonic number makes the trapping lifetime of the target RI ions longer, and this effect, coupled with the stabilization due to the disappearance of HOM, is expected to significantly improve the luminosity performance in RI electron scattering.
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