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eRHIC: an Efficient Multi-Pass ERL based on FFAG Return Arcs
June 9, 2015 Stephen Brooks, ERL 2015 1
On behalf of the eRHIC design team
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eRHIC Schematic
June 9, 2015 Stephen Brooks, ERL 2015 2
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Cost Savings for a ~20GeV ERL EICParameter Non-FFAG Design FFAG Design Reduction
Linac energy per turn 3.33 GeV 1.32 GeV 2.52x
Turns until collision 6 16 (2.67x increase)
Beamline loops built 6 2 3x
Synchrotron power loss for I=20mA
2.18 MW 9.87 MW 4.5x increase
June 9, 2015 Stephen Brooks, ERL 2015 3
• eRHIC is ERL-based to achieve high luminosity– Electron beam only interacts once so can have an
extremely high beam-beam tune shift of ~20– Would have to be kept stable if in a storage ring
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FFAG Cell Orbits
• FFAG1 limited by energy range 4-5x• FFAG2 optimised for low synchrotron power
June 9, 2015 Stephen Brooks, ERL 2015 4
10mm
3.59mOrbits transversely exaggerated x100
FFAG1BD, QF, BD, QF
FFAG2BD, QF
2.7GeV
6.6GeV
1.3GeV
21.2GeV
7.9GeV
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FFAG Cells Parameter Tables
June 9, 2015 Stephen Brooks, ERL 2015 5
Parameter FFAG1 FFAG2
Energy range 1.334 – 6.622 GeV 7.944 – 21.164 GeV
Energy ratio 4.96× 2.66×
Number of turns 5 11
Cell length 1.795m 3.591m
BD, QF lengths 0.425m, 0.471m 1.272m, 1.809m
Drift lengths 45cm, 45cm 6cm, 45cm
BD, QF gradients 25 T/m, -25 T/m 29.256 T/m, -25 T/m
Maximum orbit span 16.6mm 21.5mm
Tune per cell range 0.033 – 0.408 0.034 – 0.410
TOF variation 31.0ppm (6.6cm/ring) 38.0ppm (8.1cm/ring)
Maximum field on orbit 0.266 T 0.416 T
Synchrotron power loss 0.34MW, I=50mA
* Just one option, there is also a 4+12 turn scheme
9.87MW, I=20mA, 21.2GeV9.48MW, I=50mA, 15.9GeV2.79MW, I=50mA, 10.6GeV
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eRHIC Tunes per Cell vs. Energy
June 9, 2015 Stephen Brooks, ERL 2015 6
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eRHIC TOF Variation with Energy
June 9, 2015 Stephen Brooks, ERL 2015 7
100fs per FFAG2 cell = 1.8cm per turn (c*t)Path length is corrected in splitters
8.1cm
6.6cm
Increases to 14.5cm with extraction section (later)
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eRHIC Synchrotron Power per Turn
June 9, 2015 Stephen Brooks, ERL 2015 8
Blue filled bars = 50mA taken to 15.9GeVGreen bars = 20mA taken to 21.2GeVBoth total ~10MW for eRHIC arcs
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eRHIC FFAG Straight Sections
June 9, 2015 Stephen Brooks, ERL 2015 9
Orbits exaggerated transversely x1000
Quadrupole offsets and curvature adiabatically removed over 17 transition cells.
As dipole component disappears, all orbits move to straight centre line with small errors:± 0.436 mm in low-energy ring± 0.066 mm in high-energy ring…that can be corrected with fine adjustments
High energy FFAG
Low energy FFAG
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eRHIC FFAG Rings in Perspective
June 9, 2015 Stephen Brooks, ERL 2015 10
Orbits exaggerated transversely x5000, shape of hexagonal RHIC is evident
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Detector Bypasses: a Flexible FFAG
June 9, 2015 Stephen Brooks, ERL 2015 11
Orbits exaggerated 2000x, beamline to scale
Curvature
Bypass straight
Normal straight
2Angle
Displacement
3.08m
76 cells
3 99 17 99 317 90.3m
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3D Bypass Layout in RHIC Tunnel
June 9, 2015 Stephen Brooks, ERL 2015 12
Enough clearance for an R=4.16m (STAR-sized) detector if it is displaced inwards on existing rails by 1.76m. Clearances would be >34cm from beamline centre to walls/detector
Orbits exaggerated transversely x5000
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Extraction Scheme
• Adiabatically expand cells in 5 & 9 o’clock arcs• Cells increase in length by factor e ~= 2.718
– Orbits separate by factor e2 ~= 7.389
• In centre, high-energy orbits separated by 2cm– Use 0.7T/1.1m massless septum, BD, QF 8cm
June 9, 2015 Stephen Brooks, ERL 2015 13
Transverse orbit exaggeration x256
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• eRHIC path length must ~= RHIC hadron ring– Can’t stay on inside or outside all the way around
• Space under 4 and 10 o’clock no-cryo pipe
Tunnel Crossover under RHIC Pipe
June 9, 2015 Stephen Brooks, ERL 2015 14
Adiabtic scheme similar to bypass gives 3.36m total horizontal displacement
30m central warm section with 1.81m displacement
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All FFAG Special Sections
June 9, 2015 Stephen Brooks, ERL 2015 15
Linac
Splitter/Combiner
Arc
Matcher
Straight
Crossover straight
Extraction arc
Bypass straight
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Orbit Error Correction is Important
• Even energy spread ~ 10-3 in such a long channel can cause phase differences– Of the order of 1000 cells around RHIC tunnel– Natural chromaticity + errors emittance growth– Must correct orbits to within less than beam size
June 9, 2015 Stephen Brooks, ERL 2015 16
x’
x
x’
x
x’
x
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Field Error Sources and Mitigations
June 9, 2015 Stephen Brooks, ERL 2015 17
Field error source
Relative size
Equivalent displacement
Time scale
Linear / Nonlinear
Mitigation
Material magnetisation
2e-2 200µm Constant Both Magnet tuning on bench
Magnet assembly
2e-3 20µm Constant Both Magnet tuning on bench
Alignment on girder
1e-2 100µm Constant Linear 1% (50G) linear corrector coils
Radiation damage
<1e-3 <10µm Years Unknown 1% (50G) linear corrector coils
Temperature coefficient
1e-3/K 10µm/K Minutes - hours
Linear (?) Orbit feedback + corrector coils
Slow vibrations
<1e-3 <10µm < few Hz Linear Orbit feedback + corrector coils
Fast vibrations
<1e-5 <0.1µm > few Hz Linear Not corrected (small enough)
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Permanent Magnet Prototyping
June 9, 2015 Stephen Brooks, ERL 2015 18
Open-midplane eRHIC Halbach concept by Nick TsoupasMeasured on magnet division rotating coil
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NdFeB Irradiation Test
June 9, 2015 Stephen Brooks, ERL 2015 19
Test magnets of 5 material grades placed in hottest parts of RHIC tunnel~500Gy irradiation so far, eRHIC 20 year estimate 1kGyNo change in 4 of 5 materialsCheapest material with opposing fields shows field decay
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C Prototype Facility at Cornell
• See talks on Wednesday!
June 9, 2015 Stephen Brooks, ERL 2015 20
See other talks at this meetinge.g. Chris Mayes on Wednesday pm