A C-Band Compact Spherical RF Pulse Compressor for the SXFEL Linac Energy Upgrade

A new compact C-band (5712 MHz) spherical RF pulse compressor has been designed for Shanghai Soft X-ray Free Electron Laser (SXFEL) facility energy upgrading at Shanghai Institute of Applied Physics (SINAP), Chinese Academy of Sciences (CAS). This pulse compressor contains one high Q0 spherical RF resonant cavity which works on two TE113 modes and a novel coupler. As there is only one storage cavity, this pulse compressor can be much smaller than the traditional SLED. With the coupling coefficient 4.9, the average power gain can be as high as 3.8. In this paper, the scheme of the C-band spherical pulse compressor and RF design are presented. INTRODUCTION A compact soft X-ray Free Electron Laser facility has been constructed at SINAP. This facility utilizes C-band RF technology with accelerating gradient of about 40 MV/m to accelerate the electrons to 0.84 GeV by a linac [1]. As an upgrade, there will be more C-band accelerating structures cascaded to increase the electron energy to 1.5 GeV in the future. A new kind of pulse compressor based on only one spherical cavity is proposed and developed for LCLS Xband linac linearizer at SLAC [2]. In this scheme, two polarized spherical TE114 modes exist in same spherical cavity, which makes the resonant frequencies of these two modes easy to tune to same frequency, and keep the same during high power operation to make RF power stable. Meanwhile, only one spherical cavity also makes this pulse compressor small remarkably. Based on this X-band spherical pulse compressor, we have designed a C-band one as an upgrade of our SLED-type pulse compressor [3]. Considering the practical requirements of SXFEL, the two spherical TE113 modes are chosen to carry RF power, which can make the pulse compressor as compact as possible while the energy gain can be still high enough. CALCULATION AND SIMULATION This new compact pulse compressor is composed of one spherical cavity and a combined 3-dB coupler. They are designed separately by 3D simulation software, and then assembled together to get final results. General Parameters Calculation of C-band Pulse Compressor The output power of klystron is 50 MW with pulse width 2.5 μs. Power pulse is designed to reverse at 2.0 μs, and pulse width will be compressed to 0.5 μs by pulse compressor. Eventually the power will be fed into two accelerating structures with attenuation factor of τ=0.6 and filling time of Ta=0.372 μs. By solving wave functions with the boundary conditions that tangential components of E vanish at r=a, the resonant frequency in a spherical cavity of TEmnp mode is: 0 0 2 np f a      (1) where μnp presents the zeros of the spherical Bessel function ��̂ and a is the cavity radius. As f=5712 MHz, μ13=10.904, therefore a=91.083 mm. For a spherical resonant cavity that works on TEmnp mode, the unloaded quality factor Q0 can be calculated as Q=a/δ, where � = 1 √���1�1 ⁄ is the skin depth. The unloaded quality factor then is 1.04×10 as a=91.083 mm. Associated with a traveling wave constant gradient accelerator, the expression of energy multiplication factor M can be obtained [4]: 1 1 (1 ) ( 1) (1 ) a