Studies of transverse multi-bunch beam breakup in E-linac Dobrin Kaltchev, Richard Baartman, Yu-Chiu Chao, Philipp Kolb, Shane Koscielniak, Lia Merminga, Amiya Mitra, Vladimir Zvyagintsev TRIUMF, Vancouver B.C., Canada Introduction This paper presents the results from our simulations of single-pass transverse beam-breakup (BBU) effects in E-linac [1]. For the initial single-pass operation of E-linac, a 300 KeV CW electron beam will be injected into a 9- cell Tesla cavity (maximum gradient 10 MV/m) fol- lowed by a beam transport line – quadrupoles and a magnetic dipole chicane, and then accelerated in the main linac section consisting of four such cavities to a final energy of about 45 MeV. The bunch repetition rate is either 650 MHz (fission beam), or 100-110 MHz (high-brightness beam) for the same average current (10 to 30 mA). Each bunch feels the accumulated effects of the long- range wakefields of all the preceding bunches – the amplitude of oscillation of the bunches increases. The conditions for such orbit growth to occur are injection errors and/or cavity misalignments, combined with the presence of dipole high-order modes (HOMs) in the cavity field with high quality factors Q and shunt impedances R/Q. E-linac optics and BBU tool An optics code that includes standing-wave acceler- ating structures as optics elements was created by in- serting the 9-cell cavity matrix in MadX. The matrix is derived by integrating numerically the equations of motion in a standing-wave cavity as given by E.Chambers [2]. E-linac lattice used in BBU simulations (MadX, Astra, Mathematica) BBU model We assume an injection error at the first cavity en- trance x 0 =1 mm equal to one-third of the r.m.s. beam size. A zero-emittance beam, aligned to the beam el- lipse at this point, is tracked in the presence of the 26 dipole HOMs. The beam consists of 10 6 bunches, equally spaced. We use rms cavity-to-cavity HOM fre- quency spread 0.1 MHz and a bunch-to-bunch charge jitter rms = 5% of the nominal bunch charge. The rms orbit offset is defined as the r.m.s. (over the pulse) of the quantity |x HOM out - x s.s. out |/x out . In E-linac, the steady state deflections (s.s.) turn out to be small. Dipole HOMs (CST Microwave Studio) The blue points have a larger chance to fall in the vicinity of a green line than red. 1.5 2.0 2.5 3.0 0 20 40 60 80 100 f, GHz RdQ red: f b 650 MHz green: f b 100 MHz The 26 R/Q values shown on both the 650-MHz mesh (red) and the more dense 100-MHz mesh (green). Sample tracking-pulse shape at linac exit orbit pattern for a single random seed HOM action only (green); mode frequency spread of 0.1 MHz (black); both frequency spread 0.1 MHz and 5 % charge jitter (red) RMS bunch orbit offsets The Figure shows the rms orbit growth calculated from 20 random machines (10 frequency seeds com- bined with 10 seeds of random charge jitter): average (top) and maximum (bottom).                         5 10 15 20 25 30 0.001 0.002 0.005 0.010 0.020 0.050 0.100 I,mA rms orbit offset at exit relative Average of 20 seeds  fission, with abs.  highbr., with abs.  fission, wo absorber  highbr., wo absorber                         5 10 15 20 25 30 0.001 0.005 0.010 0.050 0.100 0.500 1.000 I,mA rms orbit offset at exit relative Max of 20 seeds  fission, with abs.  highbr., with abs.  fission, wo absorber  highbr., wo absorber rms bunch orbit offsets depending on the average current for 20 random machines Conclusions • The high-brightness beam is more affected by BBU (higher bunch charge and a larger chance for a HOM to get near a machine line). • The emittance dilution is negligible for a cavity with absorbers for both fission and high-brightness beam. • The initial E-linac operation at 10 mA is practi- cally not affected. References [1] L. Merminga, et al, ARIEL: TRIUMF’s Advanced Rare IsotopE Laboratory, these proceedings. [2] E.E. Chambers, Particle Motion in a Standing Wave Linear Accelerator, SLAC report 1967 [3] Schuh, M.; Tckmantel, J.; Biarrotte, J. L.; Kaltchev, D., Code Benchmarking of Higher Order Modes Simulation Codes , CERN-sLHC-Project-Note-0010 - Geneva : CERN, 201 [4] TESLA Technical Design Report, Part 2: The Accelerator, DESY, March 2001. [5] R.L. Gluckstern, R.K. Cooper and P.J. Channell, Cumulative Beam Breakup in RF Linacs, Part Acc, vol 16 pp 125-153. [6] CST PARTICLE STUDIO, www.cst.com [7] D. Jeon , L. Merminga , G. Krafft , B. Yunn, R. Sundelin, J. Delayen, S. Kim, M. Doleans, Cumulative beam break-up study of the spallation neutron source superconducting linac, NIM A 495 (2002) 85-94. [8] Klaus Flttmann, ASTRA, http://www.desy.de/ ˜ mpyflo [9] J. Tckmantel, Beam simulations with initial bunch noise in superconducting rf proton linacs, Phys. Rev. ST Accel. Beams 13, 011001 (2010)