Transcript Positron Sources for Linear Colliders* Wei Gai JPOS 2009, Jefferson Lab, March 26, 2009 * Acknowledgement of contributions from the ILC and CLIC e+ collaborations.

Positron Sources for
Linear Colliders*
Wei Gai
JPOS 2009, Jefferson Lab,
March 26, 2009
* Acknowledgement of contributions from the ILC
and CLIC e+ collaborations
Content
Overview
Undulator Based Positron Source
Conventional Positron Source
Compton based Positron Source
JPOS09, JLab, Newport News, VA, March 26, 2009
Overview
JPOS09, JLab, Newport News, VA, March 26, 2009
positron production
Gamma
generation
Conversion
target
e- beam
Gamma ray
(multi MeV – hundreds of GeV)
Capturing
optics
Positrons
Gamma generation schemes
 Planar/Helical Wiggler/Undulator
 Bremsstrahlung /Channeling radiation
 Laser Compton scattering
JPOS09, JLab, Newport News, VA, March 26, 2009
Acceleration
Helical undulator Based Scheme:
requires very high energy drive beam (~100 GeV)
Undulator technology is straightforward. (SC or PM)
i
-i
Supper conducting helix
Can produce circularly polarized
photon, good for polarized e+
source.
Drive beam energy: 150GeV
Proposed: A. Mikhailichenko
K. Flottmann, et al
JPOS09, JLab, Newport News, VA, March 26, 2009
Conventional (Bremsstrahlung)
Drive beam energy can be as low as ~ 100 MeV.
AMD
e
E1
hw=E2-E1
e-
6GeV e-
RF LINAC
e+
+
e E2
4 X0 tungsten target
Here, bremsstrahlung refers to radiation from electrons stopping in matter.
If the incident electron is polarized, the photon produced will be circularly polarized.
And this can give us a possible polarized e+ source using conventional scheme.
JPOS09, JLab, Newport News, VA, March 26, 2009
Channeling radiation
-- Coherent bremsstrahlung (separateγand e+ production)
Enhancement can be as high as
40 comparing with incoherent
bresstrahlung (R. Chehab et al.)
e-
Schematic illustration of channeling
An example of positron source using channeling radiation
JPOS09, JLab, Newport News, VA, March 26, 2009
Laser Compton scattering
Circularly polarized YAG Laser or CO2 Laer
Multi GeV e-
Circularly polarized g
Mr. Omori-San’s favorite drawing
JPOS09, JLab, Newport News, VA, March 26, 2009
Undulator based sources for
ILC and CLIC
JPOS09, JLab, Newport News, VA, March 26, 2009
ILC (500 GeV CM) Positron Source Layout
(undulator based scheme)
JPOS09, JLab, Newport News, VA, March 26, 2009
Beam parameters for different machines
JPOS09, JLab, Newport News, VA, March 26, 2009
Photon Spectrum and Polarization of ILC baseline
undulator
1. Photon energy spectrum and polarization from a ILC “baseline”
undulator (K=1, lu=1cm and Edrive =150GeV) up to the 9th
harmonics.
2. Note photons close to critical energy (also near axis) for each
harmonic have higher polarization. Collimating incoming
photons will result polarized e+.
JPOS09, JLab, Newport News, VA, March 26, 2009
Target Energy Deposition Profile:
Energy deposition profile showing here
is calculated per drive e- bunch
 Energy deposition in target is about
0.5255J per bunch
 Energy deposition : about 1482J per pulse
 Power deposition 1482(J)/0.874e-3(s) ~=
1.696MW per pulse
 Average power deposition: 1482*5=7.4KW
Target has to be rotating at high
speed to survive
Ti target
Rotating the 2m diameter target wheel at
1000rpm was estimated for safe operation of the
target.
JPOS09, JLab, Newport News, VA, March 26, 2009
Energy and polarization distribution e+ source at the target
Large energy spread
JPOS09, JLab, Newport News, VA, March 26, 2009
Transverse phase space distribution at the
target
Large divergence, high emittance beam
JPOS09, JLab, Newport News, VA, March 26, 2009
Positron collection and acceleration:
Adiabatic Matching Device (target immersed in a
solenoid B field)
L-band Standing Wave Accelerator.
AMD field:5T-0.25T in 50cm
Accelerating gradient in
pre-accelerator: 12 MV/m
for first 6 m, 10 MV/m for
next 6 m and 8.9 MV/m for
the rest.
The ILC Collaboration Meeting, IHEP, Beijing, Jan 31 – Feb 2, 2007
Comparison of positron yield from different
undulators
High K Devices
Low K Devices
BCD
UK I
UK II
UK III
Cornell I
Cornell II
Cornell III
Period (mm)
10.0
11.5
11.0
10.5
10.0
12.0
7
K
1.00
0.92
0.79
0.64
0.42
0.72
0.3
Field on Axis (T)
1.07
0.86
0.77
0.65
0.45
0.64
0.46
Not
Defined
5.85
5.85
5.85
8.00
8.00
First Harmonic Energy
(MeV)
10.7
10.1
12.0
14.4
18.2
11.7
28
Yield(Low Pol, 10m drift)
~2.4
~1.37
~1.12
~0.86
~0.39
~0.75
~0.54
Yield(Low Pol, 500m drift,
25%)
~2.13
~1.28
~1.08
~0.83
~0.39
~0.7
~0.54
Yield (Pol. 60%)
~1.1
~0.7
~0.66
~0.53
~0.32
~0.49
~0.44
Beam aperture (mm)
Target: 1.42cm thick Titanium
JPOS09, JLab, Newport News, VA, March 26, 2009
Proposed ILC target geometry and simulation of the
target rotating in magnetic fields.
Solenoid
positioned
at 0.95m
1.4cm
The model is checked against known
experiments.
JPOS09, JLab, Newport News, VA, March 26, 2009
Power vs RPMs for the ILC Target
hoursepower
kWatts
Simulation with the magnet, 5T on the solid disk
1400
1000
1200
800
1000
600
800
σ=59.99e6 (copper)
σ=20e6
σ=10e6
σ=5e6
σ=1.8e6 (titanium)
600
400
200
400
200
RPM
0
0
200
400
600
0
800
1000
JPOS09, JLab, Newport News, VA, March 26, 2009
1200
1400
Thousands
1600
Cockroft institute prototype experiment simulation
Technical drawing provided by I.Bailey
z0
Simulation, Induced field,
z-component, 2000RPM
D – 1m, rim width – 30mm, rim thickness
– 14mm, distance between magnet poles
is 5cm, field – 1.5Tesla
JPOS09, JLab, Newport News, VA, March 26, 2009
JPOS09, JLab, Newport News, VA, March 26, 2009
Another proposed solution:
A pulsed flux concentrator
 Pulsing the exterior coil enhances the
magnetic field in the center.
– Needs ~ 1ms pulse width flattop
– Similar device built 40 years ago.
Cryogenic nitrogen cooling of the
concentrator plates.
– ANL and LLNL did initial rough
electromagnetic simulations. Not
impossible but an engineering
challenge.
– No real engineering done so far.
JPOS09, JLab, Newport News, VA, March 26, 2009
Advanced Solution: Lithium lens
 Lithium Lens
– Will lithium cavitate under pulsed heating?
• window erosion
– Will lithium flow adequately cool the windows?
– Increased heating and radiation load in the
capture section
– Needs R&D to demonstrate the technology.
A. Mikhailichenko
A. Mikhailichenko et al.
P.G. Hurh & Z. Tang
JPOS09, JLab, Newport News, VA, March 26, 2009
What if every capturing magnet technology
fails, a safe solution: ¼ wave solenoid
 Low field, 1 Tesla on axis,
tapers down to ¼ T.
ANL ¼ wave solenoid simulations
 Capture efficiency is only 25%
less than flux concentrator
 Low field at the target reduces
eddy currents
 This is probably easier to
engineer than flux concentrator
 SC, NC or pulsed NC?
W. Liu
JPOS09, JLab, Newport News, VA, March 26, 2009
Summary of Capture Efficiency for Different AMD
AMD
Capture efficiency
Immersed target
(6T-0.5T in 20 cm)
~30%
Non-immersed target
(0-6T in 2cm, 6T-0.5T 20cm)
~21%
Quarter wave transformer
(1T, 2cm)
~15%
0.5T Back ground solenoid only
~10%
Lithium lens
~29%
JPOS09, JLab, Newport News, VA, March 26, 2009
Undulator based e+ for CLIC (3 TeV)
J. Sheppard
L. Rinolfi,
W. Gai
JPOS09, JLab, Newport News, VA, March 26, 2009
A possible CLIC scheme for polarized e+
To the IP
e- beam
Cleaning chicane
Ti
alloy
e+
250
GeV
e+
2.2 GeV
NC Linac
450 m
Pre-Injector Linac
Undulator
Injector Linac
G = 12 MV/m
G = 17 MV/m
K = 0.75
E = 200 MeV
E = 2.424 GeV
λu = 1.5 cm
fRF = 1.5 GHz
f RF = 1.5 GHz
L = 100 m
B = 0.5 T
f rep= 50 Hz
JPOS09, JLab, Newport News, VA, March 26, 2009
A possible CLIC complex layout with undulator based e+
source
Booster linac
Following the tunnel
back to e+ injector
e+
ee- main linac
e+ main linac
undulator
undulator
e+ capturing optics
and preaccelerator
Bending assemblies, 20 of them, each one bends the
electron beam by 1/20 of the angle between axis of
undulator and the axis of the rest of electron main linac
>2m
target
JPOS09, JLab, Newport News, VA, March 26, 2009
e+ capturing etc
Numerical Simulation on the effect of undulator parameter
and accelerating gradient
 Drive e- beam energy: 250GeV
 Undulator parameters: K = 0.5 - 0.75, λ= 1.3 - 1.5cm, L= 100 m
 Drift to target: 450m
 Accelerator L-band Linac, AMD: 7T - 0.5T in 20cm;
 Target material: 0.4 rl Titanium,
 Positron capture is calculated by numerical cut using damping ring
acceptance window: +/-7.5 degrees of RF(1.3GHz),
ex+ey<0.09p.m.rad,1% energy spread with beam energy ~2.4GeV
Yield and polarization for the CLIC undulator based source
Yield is calculated as Ne+ captured/Ne- in drive beam
Bottom line:
It works
Conventional e+ for LCs
The original ILC conventional source schematic layout
Target
AMD
PPA
Superconducting linacs
With quadropole focusing
5 GeV e+
e~ 120 MeV
•Target
Material
W23Re
Length
4.5 RL
•Electron Beam
Energy
0.25 - 6 GeV
Transverse size, σx = σy
2 mm
Longitudinal size, σt
1.5 ps
Polarized electron →polarized positron (?)
To damping
ring
After sweeping through the parameter space, this original scheme seems to be not
viable for ILC due to the excessive energy deposition in target.
Courtesy of M.Kuriki
Courtesy of M.Kuriki
Liquid metal target (BINP design)
Liquid metal target development
Lead flow
Cog-wheel pump test bench (BINP)
Temperature distribution using ILC beam time
structure: 600MeV drive beam, 1mm spot size,
AMD immersed target (130 and 260 bunches)
x
x
z
z
130 bunches
260 bunches
Too hot to handle!!!!!!!!! Ways to improve: higher energy, larger
spot size and increasing flow rate
Need 30m/s pumping speed to keep the liquid from boiling.
Time structure of 300Hz conventional source
Courtesy of T.Omori
Output timing
structure from
DR per ILC
specs
Advantage:
Only deal with 132
pulse each time
Low speed target
Temperature in target after 2 triplets
Target is moving at 10m/s
JPOS09, JLab, Newport News, VA, March 26, 2009
JPOS09, JLab, Newport News, VA, March 26, 2009
Compton Based Scheme
JPOS09, JLab, Newport News, VA, March 26, 2009
JPOS09, JLab, Newport News, VA, March 26, 2009
Photon spectrums a CO2 laser compton scattering with 3
different drive beam energy
Photon number is high
but the interaction time is
short. Total number of
photon produced is small.
Stacking is needed.
JPOS09, JLab, Newport News, VA, March 26, 2009
JPOS09, JLab, Newport News, VA, March 26, 2009
JPOS09, JLab, Newport News, VA, March 26, 2009
F. Zimmerman et al.
JPOS09, JLab, Newport News, VA, March 26, 2009
JPOS09, JLab, Newport News, VA, March 26, 2009
JPOS09, JLab, Newport News, VA, March 26, 2009
Summary
 Three schemes discussed here, each scheme has
pluses and minuses,
 Due to the designed pulse structure, the ILC source is
the most difficult one. Intensive R&D are on going,
there will be solutions.
 Seems no fundamental issues with the CLIC scheme.
 Looking forward to build a linear collider in my life time.
JPOS09, JLab, Newport News, VA, March 26, 2009
Heat transfer simulation up to 2650 bunches,
700MeV, 4mm rms spot size, 30m/s pumping speed,
Lithium lens
Temperature distribution
Interpolated temperature on line
(z=1.2cm,x=0) at different time
Hot spot temperature
changing with time
With 4mm rms spot size, when using lithium lens, the
yield is about 0.27. Using the yield of about 0.31 when
using AMD with 1mm rms spot size and energy
deposition of about 2.27J per bunch, the deposited
energy 4mm spot with lithium lens can be estimated as
2.626J per bunch. With the scaled energy deposition
profile and deposited energy, heat transfer simulation for
700MeV, 4mm rms spot, 30m/s pumping speed and
optimized lithium lens shows the temperature after 2650
bunches is still bellow 1600K.