MEIC Electron Cooler Design
Concept
EC potential impact to colliders
Reaching a high start luminosity
• Very short i-bunches achieved by longitudinal cooling in
combination with SRF (cannot be attained with stochastic cooling!)
make sense to design a super-strong focusing (low beta) at IP
• Short bunches allow one to employ the crab-crossing beams, thus
avoiding the parasitic b-binteractions
• Low transverse emittance + high rep. rate allow one to minimize
charge/bunch
Extending the luminosity lifetime
• EC suppresses beam heating and luminosity loss caused by
multiple and Touschek IBS
HEEC basics
Co-moving “cold” electron beam serves as
thermostat for a hot ion beam
(i – e Coulomb collision exchange)
solenoid
ion bunch
Cooling section
Magnetized e-gun
Injector
SRF linac
Cooling conditions:
Cooling time grows with
Therefore: staged cooling
electron
bunch
Staged EC
Initial electron cooling
Parameter (p/e)
High luminosity colliding beams
Unit
Value
GeV/MeV
20/10
%
1
Particles/bunch
1010
0.2/1
Energy spread**
10-4
3/1
Bunch length**
cm
20/3
Proton emittance, norm**
m
4
Cooling time
min
10
Equilibrium emittance, *
m
1
Equilibrium bunch length*
cm
2
Energy
Cooling length/ circumf.
Laslett’s tune shift
* norm. rms
** max. amplitude
0.1
Unit
Value
Beam energy
GeV
150/7
Energy of cooling beam
MeV
75
Bunch rep rate
GHz
1.5
Particles/bunch
1010
0.2/1
Beam current
A
0.5/2.5
Cooling current
A
2.5
Horizontal emittance*
m
1/100
Vertical emittance*
m
0.01/1
Parameter (p/e)
4
Number of interaction points
0.04/0.16
Total beam-beam tune shift
Laslett’s tune shift in p-beam
0.02
cm-2s-1
2
Cooling/IBS time in p-beam core
min
5
Luminosity Touschek’s lifetime
h
20
Luminosity overall IP (1035)
* norm. rms
Staged Cooling in Ion Collider Ring
• Initial cooling after ions injected into the collider ring for reduction of
3d emittance before acceleration
• After boost & re-bunching, cooling for reaching design values
of beam parameters in colliding mode
• Continuous cooling during collision for suppressing IBS,
maintaining luminosity lifetime
Initial
after boost
Colliding Mode
GeV/MeV
15 / 8.15
60 / 32.67
60 / 32.67
A
0.5 / 1.5
0.5 / 1.5
0.5 / 1.5
Particles/Bunch
1010
0.416 / 2
0.416 / 2
0.416 / 2
Bunch length
mm
(coasted)
10 / 20~30
10 / 20~30
Momentum spread
10-4
10 / 2
5/2
3/2
Hori. & vert. emittance, norm.
µm
4/4
Energy
proton/electron beam current
Laslett’s tune shift (proton)
0.002
0.35 / 0.07
0.005
0.06
High Energy e-Cooler for Collider Ring
Design Requirements:
• up to 10.8 MeV for cooling at injection energy (20 GeV/c)
• up to 54 MeV for cooling top proton energy (100 GeV/c)
• Cooling e-beam current :
• up to 1.5 A CW beam at 750 MHz repetition rate
• About 2 nC bunch charge (possible space charge issue at low energy)
Solution: ERL Based Circulator Cooler (ERL-CCR)
• Must be an SRF Linac for accelerating electron beam
• Must be Energy Recovery (ERL) to solve RF power problem
• Must be Circulator -cooler ring (CCR) for reducing current from source/ERL
ERL-CCR is considered to provide
the required high cooling current while consuming
fairly low RF power and
reasonable current from injector
Conceptual Design of Circulator e-Cooler
(Layout A)
solenoid
ion bunch
electron
bunch
Electron
circulator
ring
Cooling section
Fast beam
kicker
Fast beam
kicker
energy
recovery
path
SRF
Linac
electron
injector
dump
ERL Circulator Electron Cooler
(Layout B)
solenoid
ion bunch
electron
bunch
Cooling section
(Fast) kicker
(Fast) kicker
injector
SRF Linac
dump
Optimized Location of Cooling Channel
(Layout C)
10 m
injector
SRF
Center of
Figure-8
Eliminating a long circulating beam-line could
• cut cooling time by half, or
• reduce the cooling electron current by half, or
dumper
Cooler Design Parameters
• Number of turns in circulator cooler
ring is determined by degradation of
electron beam quality caused by
inter/intra beam heating up and space
charge effect.
Max/min energy of e-beam
MeV
54/11
Electrons/bunch
1010
1.25
• Space charge effect could be a leading
issue when electron beam energy is
low.
• It is estimated that beam quality (as
well as cooling efficiency) is still good
enough after 100 to 300 turns in
circulator ring.
• This leads directly to a 100 to 300
times saving of electron currents from
the source/injector and ERL.
bunch revolutions in CCR
Current in CCR/ERL
~100
A
1.5/0.015
MHz
750/7.5
CCR circumference
m
~80
Cooling section length
m
15x2
Circulation duration
s
27
RMS Bunch length
cm
1-3
Energy spread
10-4
1-3
T
2
mm
~1
Beta-function
m
0.5
Thermal cyclotron radius
m
2
Beam radius at cathode
mm
3
Solenoid field at cathode
KG
2
Bunch repetition in CCR/ERL
Solenoid field in cooling section
Beam radius in solenoid
Laslett’s tune shift @60 MeV
Longitudinal inter/intra beam heating
0.07
s
200
Issues
Space charge limitations in CCR:
• Coulomb interaction (non-linear Laslett detune)
• CSR
Intra- and Inter-Beam Scattering in CCR
Source/Injector/ERL/CCR beam matching gymnastics
•
•
•
•
Magnetized cathode
Matching with cooling solenoids, straights and arcs
Beam size at cathode and related canonical emittance
Other agendas? (space charge dominated beam in axial optics…)
Fast kicker (beam-beam or other)
And more…
Backup slides
ERL-based EC with circulator ring
Parameter
Max/min energy of e-beam
Electrons/bunch
Number of bunch revolutions in CR
Current in CR/current in ERL
Bunch rep. rate in CR
CR circumference
Cooling section length
Circulation duration
Bunch length
Energy spread
Solenoid field in cooling section
Beam radius in solenoid
Cyclotron beta-function
Thermal cyclotron radius
Beam radius at cathode
Solenoid field at cathode
Laslett’s tune shift in CR at 10 MeV
Time of longitudinal inter/intrabeam heating
Unit
MeV
1010
100
A
GHz
m
m
s
cm
10-4
T
mm
m
m
mm
KG
s
Value
75/10
1
1
2.5/0.025
1.5
60
15
20
1
3-5
2
1
0.6
2
3
2
0.03
200
Technology: Ultra-Fast Kicker
Beam-beam kicker
V. Shiltsev, NIM 1996
F
surface charge
density
• A short (1~ 3 cm) target electron bunch
passes through a long (15 ~ 50 cm)
low-energy flat bunch at a very close
distance, receiving a transverse kick
v≈c
h
D
kicking beam
σc
v0
L
Circulating beam energy
MeV
33
Kicking beam energy
MeV
~0.3
Repetition frequency
MHz
5 -15
Kicking angle
mrad
0.2
Kinking bunch length
cm
15~50
Kinking bunch width
cm
0.5
Bunch charge
nC
2
• The kicking force is F 
e e
(1   0 )
20
integrating it over whole kicking
bunching gives the total transverse
momentum kick
• Proof-of-principle test of this fast kicker
idea can be planned. Simulation
studies will be initiated.
An ultra-fast RF kicker is
also under development.
Estimates for Injector to ERL
Electron source
e-gun V
500 KeV
Pulse duration
0.33 ns
Bunch charge
2 nC
Peak current
0.65 A
Emittance, norm
1 mm.mrad
Rep.rate
15 MHz
Average current
30 mA
1st accellerator cavity
Voltage
2 MV
Frequency
500 MHz
Beam energy
2.5 MeV
1st compressor
Prebuncher frequency
Voltage
Energy gradient after prebuncher
1st drift
Bunch length after 1st compression
Beam radius (assumed value)
Coulomb defocusing length
2nd compressor
Buncher frequency
Energy gradient
2nd drift
Bunch length, final
Beam radius
Coulomb defocusing length
500 MHz
0.2 MV
2x 10%
2m
1 cm
2 mm
30 cm
1.5 GHz
2 x 10%
1.8 m
0.5mm
2 mm
35 cm