EIC Collaboration Meeting BNL Oct 1012 2017 Christoph Montag eRHIC electron storage ring To be installed in existing RHIC tunnel 38km circumference 381m tunnel curvature radius 2 eAu and ep Energy Combinations ID: 744333
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Slide1
Electron Storage Ring Design
EIC Collaboration Meeting
BNL, Oct.
10-12, 2017
Christoph MontagSlide2
eRHIC electron storage ring
To be installed in existing RHIC tunnel
3.8km circumference
381m tunnel curvature radius
2Slide3
e-Au and e-p Energy Combinations
Large center-of-mass energy range
Discrete number of individual beam energies (
γ’s)
3Slide4
Synchrotron radiation damping
eRHIC
requires a
large beam-beam parameter of ξ=0.1 to reach high luminosity over the entire energy
rangeThis beam-beam parameter was achieved in KEKB, with a transverse synchrotron radiation
damping decrement of
δ
=2*(
δ
E/turn)/E=1/4000
Damping decrement in
eRHIC should be at least as large as in KEKB to achieve the same beam-beam tune shift at all energies
4Slide5
Superbends
Short
, sharp bends to increase damping decrement
at low energies, thus allowing high electron beam-beam parameter
ξ
=0.1
5Slide6
Required emittances vs. energy
Hadron beam size at IP is energy dependent due to constant normalized emittance (no cooling) and aperture limit in low-
β
quadrupolesCross sections of both beams have to be matched to reduce detrimental beam-beam effects
Parameter optimization yields nearly constant (within factor 2)
electron beam emittances over entire energy range
, from 5 to 18 GeV
6Slide7
Required
electron emittances at 18 GeV are lower than at 5 GeV – opposite of what would happen in a fixed lattice
Storage ring
lattice needs to be highly tunable
to provide those
emittances
7Slide8
How to achieve required emittances?
90
degree FODO cell phase advance to achieve 24nm at
18GeV
At 10GeV, 60 degrees phase advance for 27nmFor 48nm at 5GeV, tight bending radii in
superbends
, and
60
degrees phase
advance
Emittance fine tuning by radial shift (modification of damping partition numbers)
Vertical emittances controlled by long vertical dispersion bumps8Slide9
Polarization
Ramping would destroy electron polarization
Electrons self-polarize at store due to synchrotron
radiation (
Sokolov-Ternov effect):
Self-polarization is not viable except at highest energies
Need a full-energy polarized injector
9Slide10
Fully flexible electron spin patterns
Electron spin patterns with alternating polarization (as in RHIC proton fills) are required for single-spin physics
Such fill patterns can
be
generated by a full-energy polarized injectorBunches with the “wrong” (unnatural) polarization direction will slowly flip into the “right” (natural) orientation. Time scale given by Sokolov-Ternov
self-polarization time
Bunch-by-bunch replacement at 1Hz (330 bunches in 5.5 minutes) yields sufficient polarization even at full energy where
t
= 26min
Requires good intensity lifetime
(> 1h) to minimize the beam-beam effect of electron bunch replacement on proton bunches, and fast kickers that only effect one bunch in the fill(
talk by F.
Meot
)
10Slide11
RF requirements
At maximum energy (18GeV), energy loss per turn is
37MeV
Required RF voltage: ~70MVSelf-imposed RF power limit:
10 MWWith each SRF cavity providing 2 MV/cell,
need
34 cells in total (no overhead) for either single and multi-cell cavities.
RF cavities will be installed in two locations on opposite sides of the ring
(
Irs
4 and 10) to
reduce energy variation due to synchrotron radiation, and preserve symmetry( Talk by W. Xu)
11Slide12
Overall layout
Electron storage ring to be installed next to hadron ring (same plane)
Rings intersect in each straight to keep circumferences equal, similar to present RHIC
Main
eRHIC detector in IR6, with a possible second detector in IR8
Two locations for RF on opposite sides of the ring, IR’s 4 and 10
12Slide13
Lattice and optics
FODO
lattice with high dipole fill factor to reduce synchrotron radiation
18 GeV lattice is most challenging due to small emittance and highest energy, requiring highest fields. Therefore, lattice development has so far focused on this case
90 degrees from IP to first sextupole (in 18GeV 90 degree lattice)
Low-
β
squeeze achieved by
β
-wave through adjacent arcs (“ATS lattice
”)
Spin matching ( Talk by F. Méot)Coupling compensation of spin rotator solenoids
13Slide14
Spin rotator concept
Electron spin rotators will be based on solenoids with subsequent horizontal
bends:
Proper setting of the two solenoids provides
longitudinal polarization at all energies, 5 – 18 GeVMaximum integrated solenoid field:105Tm
Maximum solenoid field: 7T
14
electrons
rot1
j
1
rot
3
j
2
rot
4
j
1
rot2
j
2
bend1
y
1
bend2
y
2
bend1
y
2
bend1
y
1
sold
j
dSlide15
IR straight lattice
15Slide16
Dynamic Aperture
Optimized
sextupole
configuration using NSLS-II approach
No lattice errorsRF cavities off (fixed momentum offset)
=1.6m,
=0.4m (intermediate squeeze; final squeeze to be accomplished by ATS scheme)
16Slide17
18
σ
dynamic aperture at
dp/p=0.5%Need to re-check after application of final squeeze
17Slide18
Summary
Storage ring lattice is very demanding due to required high flexibility
Highest energy (
18 GeV) configuration is most challenging and has been developed first. Almost ready
Tracking studies now need to confirm viability of this lattice with low-β squeeze, while solutions for other energies are being developed simultaneously
18