Conceptual Design of the Clic Damping Ring Rf System

In order to achieve high luminosity in CLIC, ultra-low emittance bunches have to be generated in both electron and positron damping rings. To achieve this goal, big energy loss per turn in the wigglers has to be compensated by the RF system. This results in very strong beam loading transients affecting the longitudinal bunch position and bunch length. In this paper, the conceptual design of the RF system for the CLIC damping ring (DR) is presented. Baseline for the CLIC conceptual design report (CDR) is discussed and the corresponding requirements for the cavities and the RF power sources are presented in order to meet stringent tolerances on the bunch-to-bunch phase and bunch length variations. INTRODUCTION Table 1: CLIC DR baseline parameters RF frequency: f [GHz] 1 Circumference: C [m] 427.5 Energy : E [GeV] 2.86 Momentum compaction: αp 1.28 x 10 Bunch population: Ne 4.1 x 10 Number of bunches: NB 312 Number of trains: NT 2 Energy loss per turn: eVA [MeV] 3.98 Energy spread in the bunch: σE/E [%] 0.12 Bunch length: σZ [mm] 1.8 RF voltage: VC [MV] 5.1 Harmonic number: h 1426 Synchronous phase : φ [] 38.7 Synchrotron frequency: fs [kHz] 3.45 Energy acceptance: ΔE/E [%] 2.34 Bucket length 2ΔZ [mm] 70 In Table 1, parameters of the CLIC DR are presented [1]. The bunch spacing in the CLIC main linac is 0.5 ns which correspond to the bunch repetition frequency of 2 GHz. The parameters set presented in Table 1 correspond to a 1 GHz RF system where the bunch spacing is 1 ns. In this case, 2 trains of 156 bunches circulating in the DR symmetrically must be interleaved after extraction in order to provide nominal bunch train structure. A possible alternative to this baseline is based on a 2 GHz RF system. It has only one train of 312 bunches with nominal spacing. No train interleaving is required after the DR in this case. The main disadvantage of this alternative is that the peak current is twice higher than in the baseline case. This has strong implications for several subsystems, one of which is the DR RF system. In this paper, the baseline at 1 GHz with two bunch trains circulating in the DR is described. Several alternative solutions both at 1 and at 2 GHz are described elsewhere [2]. In addition, tight specifications from the Ring-ToMain-Linac (RTML) line and the main linac are set in order to provide nominal bunch parameters and bunch train structure in the main linac. Two specifications are of paramount importance for the DR RF system: 0.1° at 1 GHz (280 fs) RMS spread in the bunch spacing from the nominal value of 0.5 ns, and 2% RMS bunch length spread along the bunch train [3].