eRHIC Main Linac Design

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eRHIC Main Linac Design

• E. Pozdeyev eRHIC team
• BNL

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Outline and Design Parameters
Energy, GeV 10
Bunch spacing, ns 71
Bunch intensity, 1011 1.2
Beam current, mA 270
?rms, µm, normalized 80
Rms bunch length, cm 1
Polarization, 80

• Npass 5 up 5 down
• ?dE/ds? 8 8.5 MeV/m
• L 230 m

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Linac Design
quads

• Nfc 6 (per module)
• N3h 2 (per module)
• N modules 18
• ?dE/ds? 8 8.5 MeV/m
• Ef 19.5 MeV/m
• E3h 19.0 MeV/m
• G 340 Gauss/cm
• Lq20 cm
• µ0 90º

Cryomodule
703.75 MHz 5 cell, 1.4 m with dampers
2.1 GHz 5 cell, 0.75 m with dampers
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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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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 fHOM
  distributions
• no specific optimization of beam optics to
  maximize BBU threshold

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Energy Loss / Spreadcaused by the longitudinal
wake
Loss factor (no fundamental wake) k 0.57
V/pC Average energy loss per e dEloss -12.3
MeV Full energy spread dEspread 21.3 MeV
Monopole wake field simulated by
ABCI. Fundamental wake is the convolution of the
cosine wake with the charge distribution. Suppose
dly, the fundamental wake is recovered.
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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
For ?Ei 21.3 MeV and V100 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.
Large d?, fits the RF wave -gt small dE
The optimized phase width can be estimated as
The energy spread compression can be estimated as
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Compensation of the energy spread
? 1.7 m (175 MHz) V 100 MeV m56 -60 m566
-235 (in RF degrees) Energy spread compressed
by 5.8 times (21.3 -gt 3.68 MeV)
Compensated energy spread as a function of RF
frequency. Note 3rd harmonic RF can increase the
suppression ratio.
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Minimum Turn-On time
Assuming the maximum current ramp rate is limited
by the available RF power
Assuming V20 MV, PRF10 kW, Trev13 µs,
Ibeam280 mA, nap5
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RD items

• Strong ions beam cooling (CEC, for example) can
  reduce the required electron current and
  alleviate intensity related effects - RD 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 3rd harmonic in designated
  cryomodules, etc. )
• Increase BBU threshold
• Lower frequency RF to increase the bunch length
  and possibly drop the 3rd harmonic

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