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eRHIC Main Linac Design E. Pozdeyev + eRHIC team BNL

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May 19, 2008E. Pozdeyev, BNL2 Outline and Design Parameters PHENIX STAR e-ion detector eRHIC Four recirculation passes Main ERL (1.9 GeV) Low energy recirculation pass Beam dump Electron source Possible locations for additional e-ion detectors Energy, GeV10 Bunch spacing, ns71 Bunch intensity, 10 11 1.2 Beam current, mA270 rms, μm, normalized 80 Rms bunch length, cm1 Polarization, %80 N pass = 5 up + 5 down dE/ds = 8 – 8.5 MeV/m L = 230 m

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May 19, 2008E. Pozdeyev, BNL3 Linac Design cryomodule DFD 11.9 m1m DFFF N fc = 6 (per module) N 3h = 2 (per module) N modules = 18 dE/ds = 8 – 8.5 MeV/m E f = 19.5 MeV/m E 3h = 19.0 MeV/m G = 340 Gauss/cm L q =20 cm μ 0 = 90º 703.75 MHz 5 cell, 1.4 m with dampers 2.1 GHz 5 cell, 0.75 m with dampers quads Cryomodule

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May 19, 2008E. Pozdeyev, BNL4 Optical Functions and Beam Size -functions in the linac (m) 5 up + 5 down, unity recirculations (not shown) Beam size in the linac (mm) 5 passes up unity recirculations (not shown)

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May 19, 2008E. Pozdeyev, BNL5 Multipass transverse BBU Beam Breakup as a function of the HOM frequency spread 72 modes per cavity simulated and measured modes in copper model with HOM absorbers 5 random seeds x 2 HOM orientations = 62 f HOM distributions no specific optimization of beam optics to maximize BBU threshold I=270 mA

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May 19, 2008E. Pozdeyev, BNL6 Energy Loss / Spread caused by the longitudinal wake Monopole wake field simulated by ABCI. Fundamental wake is the convolution of the cosine wake with the charge distribution. Supposedly, the fundamental wake is recovered. Loss factor (no fundamental wake): k || = 0.57 V/pC Average energy loss per e: dE loss = -12.3 MeV Full energy spread: dE spread = 21.3 MeV

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May 19, 2008E. Pozdeyev, BNL7 Compensation of the energy spread Energy loss (12.3 MeV) can be compensated only by off-phasing or by an additional cavity without recovery. The energy spread can be reduced if the beam phase width is increased and beam is matched to the RF wave. Smal d , Large dE Large d , fits the RF wave -> small dE The optimized phase width can be estimated as The energy spread compression can be estimated as For E i = 21.3 MeV and V=100 MeV estimated f ~ 38º. The initial bunch phase width for the fundamental RF is ~ 35º. Longer wavelength RF is required to reduce the energy spread. dE compensation has to be done at lower energies (~100 MeV). The low frequency RF can be used up to E ~ 100 MeV.

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May 19, 2008E. Pozdeyev, BNL8 Compensation of the energy spread λ = 1.7 m (~175 MHz) V = 100 MeV m 56 = -60 m 566 = -235 (in RF degrees) Energy spread compressed by 5.8 times (21.3 -> 3.68 MeV) Compensated energy spread as a function of RF frequency. Note 3 rd harmonic RF can increase the suppression ratio.

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May 19, 2008E. Pozdeyev, BNL9 Minimum Turn-On time Assuming the maximum current ramp rate is limited by the available RF power Assuming V=20 MV, P RF =10 kW, T rev =13 μs, I beam =280 mA, n ap =5

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May 19, 2008E. Pozdeyev, BNL10 R&D items Strong ions beam cooling (CEC, for example) can reduce the required electron current and alleviate intensity related effects - R&D on the ion beam that can benefit e-linac design Compact, multi-cell cryomodule without sacrificing HOM damping efficiency Other linac optics options (smaller -function, “concentrated” 3 rd harmonic in designated cryomodules, etc. ) Increase BBU threshold Lower frequency RF to increase the bunch length and possibly drop the 3 rd harmonic

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May 19, 2008E. Pozdeyev, BNL11 Other Linac Setup scenarios 2 x 200 m SRF linac 10-12.5 MeV/m 4-5 GeV per pass 5 (6) vertically separated passes eSTAR ePHENIX New tunnel construction can be expensive. Linac can be constructed in RHIC tunnel.

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