FINAL DESIGN FOR THE bERLinPro MAIN LINAC CAVITY

The Berlin Energy Recovery Linac Project (bERLinPro) is designed to develop and demonstrate CWLINAC technology for 100-mA-class ERLs. High-current operation requires an effective damping of higher-order modes (HOMs) of the 1.3 GHz main-linac cavities. We have studied elliptical 7-cell cavities based on a modified Cornell ERL design combined with JLab’s waveguide HOM damping approach. This paper will summarize the final optimization of the end-cell tuning for minimum external Q of the HOMs, coupler kick calculations of the single TTF fundamental power coupler (FPC) as well as multipole expansion analysis of the given modes and a discussion on operational aspects. INTRODUCTION For the bERLinPro ERL [1] a 100mA beam has to be accelerated and decelerated in the main linac of the recirculator from 6.5MeV to 50MeV and vice versa while preserving a low normalized emittance of smaller than 1 mm mrad. Effectively the linac cavities therefore experience the passage of two with respect to the TM010-π mode 180 deg. phase shifted high current beams. Thus strong HOM damping is required in order to avoid beam break-up instability (BBU) [2] by the interaction with dipole-like mode patterns excited by the beam during previous passages. By that the cavity design has to be optimized for low transverse shunt impedance R/Q⊥ and low external quality factors Qext regarding the HOMs, while the fundamental has to feature low peak field ratios and a high shunt impedance. This challenge was addressed by combining Cornell’s mid-cell shape [3] featuring low Epeak/Eacc, thus avoiding field emission, with JLabs concept of waveguide damped cavity structures [4]. The latter has a natural cutoff frequency by the waveguide dimensions and the danger of dust propagating from ferrite-based beam tube absorbers is avoided. Figure 1 shows a sketch of the bERLinPro demonstrator ERL and the main linac cavity design. By two Y-shaped waveguide (WG) end groups rotated by 60 deg. any HOM’s polarization is covered. To even improve the propagation of the HOMs, the beam tube is enlarged from the 72 mm iris to 105 mm via a spline based nose cone transition. Table 1 ∗ Work supported by German Bundesministerium für Bildung und Forschung, Land Berlin, by grants of Helmholtz Association and by Federal Ministry for Research and Education BMBF under contract 05K10HRC and 05K10PEA † [email protected] summarizes the calculated RF performance of the cavity. The structure was developed and optimized within a collaboration of the universities Dortmund and Rostock with HZB. The methods applied and developed for this project are published in [5–9]. Table 1: Calculated RF parameters and operating conditions of the main Linac cavity with a TTF-III FPC and five HOM damping rectangular waveguides. Number of cells 7 R/Q‖ 788 Ω fTM010 − π 1.3 GHz Epeak/Eacc 2.08 Bpeak/Eacc 4.4 mT/MVm -1 Qext TM110 dipole ≤ 8 · 10 3 Beam tube TE01 cutoff 1.596 GHz Waveguide TE10 cutoff 1.576 GHz QL for TM010-π 1 · 10 7 − 1 · 10 Pforward at QL = 5 · 10 (∆ f = 0) 1.4 kW Recently [10] end-cell tuning optimizations using CST MWS’s [11] swarm particle optimizer with the eigenmode solver were performed to achieve best HOM damping results while still keeping the cavity field-flat and within given limits regarding peak field ratios and R/Q‖ . The best candidate of this approach was selected for more detailed studies, Figure 1: Scheme of the bERLinPro ERLwith the main linac section within the recirculating ring. Above the linac cavity structure is shown including the FPC and five waveguides for HOM damping. presented here, in order to achieve a design ready for production. Proceedings of LINAC2014, Geneva, Switzerland MOPP070 03 Technology 3A Superconducting RF ISBN 978-3-95450-142-7 217 C op yr ig ht © 20 14 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s HOM DAMPING CONCEPT AND PERFORMANCE 1.6 1.8 2 2.2 2.4 2.6 10 2 10 3 10 4 10 5 10 6 10 7 10 8 Freq. (GHz) Q e x t berlinpro 7−cell 5 WG+coaxial FPC TE 111 , TM 110 TE 211 TE 011 , TM 210