NLC - The Next Linear Collider Project
Positron Production and Test in the
FFTB of Undulator-Based Concepts
(E-166)
Machine Advisory Committee
Thursday, November 7, 2002
R. Pitthan and J. C. Sheppard
Sheppard /Pitthan
November 7, 2002
NLC - The Next Linear Collider Project
Statement at MAC in May, 2002
Reason was: May-2002Design was possible, but
cumbersome at best. New
directions desired even
without polarization.
November 7, 2002
R. Pitthan /J. C. Sheppard
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NLC - The Next Linear Collider Project
Polarized Positron Collaboration Personnel
http://www-project.slac.stanford.edu/lc/local/PolarizedPositrons/pdfs/E-166TLD.pdf
Since May, we have been able to
form an International
Collaboration – E-166 - to
explore Polarized Positron
Production, which includes:
o Past experts from SLD and
HERMES
o Participation from all major
LC Labs (CERN, DESY,
KEK, SLAC) and JLAB
o The Japanese Collab. which
has done the only
experiments sofar (at
the ATF).
November 7, 2002
R. Pitthan /J. C. Sheppard
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NLC - The Next Linear Collider Project
Why 2 Stages?
Because of the uncertainty in the level of detector
backgrounds, propose two stages. The goals for the
stages are:
o Stage I
flux, spectra and polarization of undulator
gammas
- background measurements for positron
polarimetry
- preliminary positron polarimetry if
backgrounds permit
o Stage II background improvements and positron
polarimetry
November 7, 2002
R. Pitthan /J. C. Sheppard
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NLC - The Next Linear Collider Project
What Does This Collaboration Want to Do?
 Make polarized photons:
 Using a meter-long, short-period,
pulsed helical undulator (lu = 2.4
mm, K=0.17),
 And using the SLAC low emittance
electron beam at 50 GeV,
 To produce polarized photons in
the energy range of a few MeV up
to a cutoff energy of about 10
MeV.
Then make polarized positrons:
 Photons are converted to polarized positrons in a target which is
~ 0.5 radiation lengths in thickness (targets of both Ti and W will
be studied).
 Yield, spectrum, and polarization of the photons and positrons
will be measured and compared to the results of simulations.
November 7, 2002
R. Pitthan /J. C. Sheppard
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NLC - The Next Linear Collider Project
Primary Questions I
• I. We all know we need positrons for e+e-, but what are
polarized positrons good for?
The first question can be
partially answered with
this plot:
Even a modest 50% e+
polarization raises the
effective polarization
from, e.g., 80% to above
92%.
November 7, 2002
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NLC - The Next Linear Collider Project
From: American Linear Collider Working Group,
SLAC-R-570, “The Whitebook”
Long List I (Whitebook):
In general, a polarized positron beam at a LC would be:
– enhancing signal-to-background
– increasing the effective luminosity
– improving asymmetry measurements with increased statistical
precision and reduced systematic errors
– improving sensitivity to non-standard couplings
In particular,
– Suppression of W-pair backgrounds can be improved by a
factor 3 with 60% positron polarization.
– By limiting the running time allotted for LL and RR modes to
10%, the effective luminosity for annihilation processes can be
enhanced by 50%.
November 7, 2002
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NLC - The Next Linear Collider Project
Long List II (Whitebook, continued)
 With 60% e+ polarization, for asymmetry measurements, the
effective polarization is substantially increased (e.g., from
80% to 95%) and the systematic precision is improved by a
factor 3.
 With these features, a polarized positron beam may provide
critical information for clarifying the interpretation of new
physics signals.
 Polarized positrons are needed to realize the full potential
for precision measurements, especially those anticipated for
Giga-Z running at the Z0-pole.
The unusual effect that adding two errors results in a lower
error for the combined error comes from the algebraic form
for
November 7, 2002
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NLC - The Next Linear Collider Project
Primary Question II
II. Is this FFTB Test any good for NLC needs?
We believe yes: This test is a 1% length scale
demonstration of undulator-based production of
polarized positrons :
 Photons are produced in the same energy range (10
MeV) and polarization characteristics (<100%) as for
the collider;
 The same target geometry and material(s) (slab
geometry of Ti and W) are used as in the linear
collider;
 The polarization of the produced positrons is in the
same range as in the linear collider;
 The simulation tools being used (and developed) to
model the experiment are the same that are being
used to design the polarized positron system for the
NLC.
November 7, 2002
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NLC - The Next Linear Collider Project
Primary Question II - Continued
 This demonstration directly tests the present design
approach to the issues of polarized positron production.
 The test will validate the methodologies and benchmark the
design codes: undulator radiation models for photon
characterization, undulator design codes for undulator
fabrication, POL-EGS4 and POL-GEANT for polarized e+
production, and BEAMPATH (Batygin-RIKEN) for collection
and transport.
 The test will provide confidence that the NLC design is
based on solid, demonstrated principles all working together
at the same time.
 The test develops the photon diagnostics required for a
collider.
November 7, 2002
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NLC - The Next Linear Collider Project
Low Energy Polarimetry Not Needed
for LC Operation, but …
 Polarimetry at the few 1/10% level at the present time is only
possible at the GeV level with Compton backscattering. This is
where Linear Colliders plan to do the precision measurements.
 However, from SLAC's experience with positron production (and
polarized cathodes) the lesson is that it is important to be able to
track the yield (and the polarization) back to the inception.
 When at SLC the e+ yield was not as large as expected, each stage
of the positron system was suspect (and guilty).
 Consequently for NLC, to determine where yield and polarization
are being lost, one must be able to measure these quantities close
to a complicated source (undulator, target and capture in the
polarized positron case), at low energy.
 For this reason, the low energy polarimetry experience gained at E166 in the MeV region could be invaluable for starting up and tuning
a future collider.
November 7, 2002
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What is Not Done Experimentally in the FFTB
 E-166 does not test target thermal hydrodynamics and
radiation damage in the target (done in separate engineering
studies with Livermore), nor capture efficiency (done as
separate accelerator physics design) and high precision
(0.3%) positron polarization diagnostics. The latter is an
improvement on what was achieved at SLD by only a factor
of X2, and is planned for the actual collider at higher
energies.
 The only physics explicitly omitted from the simulations at
the present time is depolarization of positrons in the target
due to multiple scatter and ionization loss. These effects
are estimated to be small in comparison with the dominant
depolarization of about 10%, from bremsstrahlung, which is
included in the simulations.
November 7, 2002
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NLC - The Next Linear Collider Project
Always Good to Actually DO Something
One big problem for SLC was the
combined mechanical (J/g) and
radiation (neutron induced atomic
dislocations, Mrad) damage to the
target.
While working on the Proposal we realized
that with the externally produced gs
we could stay below the onset of
production of the evil Giant Dipole
Resonance (GDR) neutrons.
 Guidance from shell model: while the GDR maximum is at 2 shell spacings
(~80 A-1/3 MeV), neutrons start to come out of the nucleus at one shell
spacing = ~40 A-1/3 MeV.
 When using titanium (Ti), this translates into ~22 MeV, for the GDR, and
~11 MeV for the neutron onset. Ti can only be used with externally produced
gs, it cannot withstand the bremsstahlung cascades from high energy
electrons.
 This conceptual insight may be the first step toward fighting the bane of
high intensity positron production, target damage, but more work is needed.
November 7, 2002
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NLC - The Next Linear Collider Project
NLC Polarized Positron System Layout
2 Target assembles for redundancy
Polarized e- Source for system
checkout, and possibly e-e- , gg running.
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NLC - The Next Linear Collider Project
Required NLC Helical Undulator Length
The length for the cut photon spectrum is Lc:
Ng = 2.6 photons/m/e-, Tc = 50%, Yc = 2.92%, xc=20%
Lc  NgTcYc xc   132 m .
-1
S3 = 59%
Lc is the length for a unity gain system, i.e. no
overhead factors, or enhancements.
Optimization on target material, thickness, undulator
characteristics, photon cut, beam energy, etc. will be
made in the future.
November 7, 2002
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NLC - The Next Linear Collider Project
Back to the FFTB Experiment (E-166)
The experiment we are proposing uses a meter-long, short-period
pulsed helical undulator, and the SLAC low emittance electron
beam at 50 GeV, to produce circularly polarized photons in the
energy range of a few MeV up to a cutoff energy of about 10 MeV.
Those photons are converted to polarized positrons in targets of
varying radiation lengths.
We plan to study targets of titanium (Ti) and tungsten (W), which
are both candidates for collider positron targets.
The goal of the experiment is to measure the yield, spectrum, and
polarization of the photons and positrons, and to compare the
results to simulations.
November 7, 2002
R. Pitthan /J. C. Sheppard
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NLC - The Next Linear Collider Project
Back to the FFTB Experiment (E-166)
This test is a 1% length scale demonstration of undulator-based
production of polarized positrons for linear colliders:
 Photons are produced in the same energy range and
polarization characteristics as for the collider;
 The same target thickness and material are used as in the
linear collider;
 The polarization of the produced positrons is expected to be
in the same range as in the linear collider;
 The simulation tools being used to model the experiment are
the same as those being used to design the polarized
positron system for a next linear collider.
November 7, 2002
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NLC - The Next Linear Collider Project
FFTB Schematic
50 GeV, Low Emittance
Electron Beam
2.4 mm period, K=0.17, helical
undulator
10 MeV, polarized photons
0.5 r.l. converter target
51%-54% positron
polarization
Moffeit/Woods
November 7, 2002
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g Detector, Transmission Polarimetry
Moffeit/Woods
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NLC - The Next Linear Collider Project
TESLA, NLC, and FFTB Positron Production
Table 1: TESLA, NLC, FFTB Polarized Positron Parameters
Parameter
Units
TESLA
NLC
FFTB
GeV
150-250
150
50
Beam Energy, Ee
10
9
10
3x10
8x10
1x10
Ne/bunch
2820
190
1
Nbunch/pulse
Hz
5
120
30
Pulses/s
planar
helical
helical
Undulator Type
1
1
0.17
Undulator Parameter, K
cm
1.4
1.0
0.24
Undulator Periodl u
st
MeV
9-25
11
9.6
1 Harmonic Cutoff, Ec10
photons/m/e
1
2.6
0.37
dNg/dL
m
135
132
1
Undulator Length, L
Ti-alloy Ti-alloy Ti-alloy, W
Target Material
r.l.
0.4
0.5
0.5
Target Thickness
%
1-5
1.8†
0.5
Yield
%
25
20
Capture Efficiency
8.5x1012 1.5x1012
2x107
N+/pulse
3x1010
8x109
2x107
N+/bunch
%
40-70
40-70
Positron Polarization
† Including the effect of photon collimation at g = 1.414.
Table 2: FFTB Beam Parameters
November 7, 2002
Ee
Ne
GeV
50
e1x1010
gx=gy
xy
m-rad
m
-5
1.5x10
10,10
x,y
x',y'
D
m
rad


m
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R. Pitthan /J. C. Sheppard
 σ 2x
2 
 2 + σ x' 
D

rad

1
2
1
γ
rad

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NLC - The Next Linear Collider Project
Double Undulator Scheme, Mikhailichenko
One of the traditional concepts
in polarimetry is to have a
random change in the sign of
the polarization. In lieu of a
better scheme, which we should
develop for the collider, we
will randomly pulse one or the
other of 2 otherwise identical
undulators with the opposite screw sense.
For the collider we could think of either a pulsed spin rotator at the
2-GeV scale, or a pulsed beam splitter magnet with two spin rotators,
or a variety of other ideas, all not well developed as of yet.
November 7, 2002
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NLC - The Next Linear Collider Project
Undulator Design, Mikhailichenko
PULSED HELICAL UNDULATOR FOR TEST AT
SLAC THE POLARIZED POSITRON PRODUCTION
SCHEME. BASIC DESCRIPTION.
Alexander A. Mikhailichenko
CBN 02-10, LCC-106
Table 3: FFTB Helical Undulator System Parameters
Parameter
Number of Undulators
Length
Inner Diameter
Period
Field
Undulator Parameter, K
Current
Pulse Width
Inductance
Wire Type
Wire Diameter
Resistance
Repetition Rate
Power Dissipation
T/pulse
November 7, 2002
Units
m
mm
mm
kG
Amps
s
H
mm
ohms
Hz
W
0
C
Value
2
0.5
0.889
2.4
7.6
0.17
1800
30
1.8x10-6
Cu
0.6
0.125
30
225
4
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NLC - The Next Linear Collider Project
Photon Intensity, Angular Dist., Number,Polarization
November 7, 2002
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NLC - The Next Linear Collider Project
Polarized Positron Production in the FFTB
Convert polarized photons (from
the undulator) to polarized
positrons in a 0.5 r.l. (1.79 cm)
thick target of Ti-alloy (yield is
about 0.5% )
Longitudinal polarization of the
positrons is 54%, averaged over
the full spectrum
Note: for 0.5 r.l. W converter,
the yield is about 1% and the
average polarization is 51%.
November 7, 2002
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NLC - The Next Linear Collider Project
Photon Transmission Polarimetry: Iron Block
Use polarization dependent Compton cross section in magnetized iron
(an old idea, Goldhaber 1957, currently being used by Fukuda et al. at
KEK-ATF; also at Bates, MIT).
Measure transmission asymmetry of polarized photons through a 15 cm
thick, iron block for parallel and anti-parallel magnetization.
Advantage: due to the tight collimation, “one-scatter-and-you-areout”, reduces background).
   0  fPe e
November 7, 2002
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NLC - The Next Linear Collider Project
Photon Transmission Polarimetry II
Clearly the flux goes
down with increased
length, but the
asymmetry increases
with length.
For 10 MeV gammas an
optimum would be about
10 cm.
   0  fPe e
November 7, 2002
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NLC - The Next Linear Collider Project
Preliminary Positron Polarimetry: Iron Block
Use the photon transmission concept:
convert the positrons back to gammas.
54% Positron Polarization converts to
8.4% Photon Polarization.
This results in a ~2.5% measurement
asymmetry. High flux (relatively,
compared to ATF) in FFTB important for
statistics.
November 7, 2002
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NLC - The Next Linear Collider Project
Scattering Compton Polarimeter, DESY Collabs.
Concept similar to Møller
polarimetry: scatter off thin
magnetized iron foil, change
polarization of foil.
Since scattered photons are
measured, charged particle
back ground is suppressed.
In this case asymmetry is a
function of the scattering
angle.
Plotted both for sum of single
photons and calometric signal
(sum of energies).
November 7, 2002
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NLC - The Next Linear Collider Project
Present State of Affairs
Equipment to produce polarized positrons in
FFTB is “standard” (including not so
“standard” pulsed undulator).
Ongoing discussion of photon polarimetry; this
is not viewed as a problem, mainly a question
of style.
Preliminary positron polarimetry has been
modeled.
Ongoing discussion of positron polarimetry;
this is seen as key to making a demonstration.
Positron polarimetry is very doable in a low
background environment.
November 7, 2002
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NLC - The Next Linear Collider Project
Issues
Approval Process:
Seek support from MAC
As E-166 before SLAC’s EPAC:
Funding:
NLC
SLAC Research Division
Cornell, DESY
University Consortium LC (DOE)
LC R & D (NSF)
November 7, 2002
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NLC - The Next Linear Collider Project
Issues, cont’d
Question of Detector Backgrounds that
may adversely affect e+ polarization
measurement, may be okay
Straightforward cure for stage 2: bring
positrons outside of FFTB enclosure
Rest of measurements look to be very
doable
November 7, 2002
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NLC - The Next Linear Collider Project
The FFTB Experiment, E-166
This test is a 1% length scale demonstration of undulator-based
production of polarized positrons for linear colliders:
 Photons are produced in the same energy range and
polarization characteristics as for the collider;
 The same target thickness and material are used as in the
linear collider;
 The polarization of the produced positrons is expected to be
in the same range as in the linear collider;
 The simulation tools being used to model the experiment are
the same as those being used to design the polarized
positron system for a next linear collider.
November 7, 2002
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NLC - The Next Linear Collider Project
Recent Positron System Notes
LCC-0079, "Energy Deposition Using EGS4," J. C.
Sheppard, April 2002
LCC-0082, "NLC Positron Target Heating," D. C.
Schultz, Y. K. Batygin, V. K. Bharadwaj, J. C.
Sheppard, June 2002.
LCC-0085, "Planar Undulator Considerations," J. C.
Sheppard, July 2002.
LCC-0086, "Energy Loss and Enrgy Spread Growth in a
Planar Undulator," J. C. Sheppard, July 2002.
LCC-0087, " NLC Polarized Positron Photon Beam
Target Thermal Structural Modeling," Werner Stein,
John C. Sheppard, July 2002
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NLC - The Next Linear Collider Project
Recent Positron System Notes, II
LCC-0088, "Thermal Stress Analyses for the NLC
Positron Target," W. Stein, A. Sunwoo, J. C.
Sheppard, V. Bharadwaj, D. C. Schultz, July 2002.
LCC-0089, "Structural Modeling of Tesla TDR Positron
Target," Werner Stein, John C. Sheppard, July 2002.
LCC-0090, "Thermal Stress Analyses for a Thermal
Multislug Beam NLC Positron Target," W. Stein, A.
Sunwoo, J. C. Sheppard, V. Bharadwaj, D. C. Schultz
LCC-0092, "Positron Yield as a Function of Drive Beam
Energy for a K=1, Planar Undulator-Based Source"
J.C. Sheppard, July 2002
November 7, 2002
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NLC - The Next Linear Collider Project
Recent Positron System Notes, III
LCC-0093, "Radiation damage induced by GeV
electrons in W-Re targets for next generation linear
colliders", M.-J. Caturla, S. Roesler, V. K. Bharadwaj,
D. C. Schultz, J. C. Sheppard, J. Marian, B. D. Wirth,
W. Stein and A. Sunwoo, July 2002
LCC-0095, "Helical Undulator Radiation," J. C.
Sheppard, July 2002
LCC-0098, "First Test of Short Period Helical SC
Undulator Prototype," A. Mikhailichenko. T. Moore,
August 2002.
LCC-0102,"Thermal Stress Analyses for an NLC
Positron Target with a 3 mm Spot Radius Beam," W.
Stein, A. Sunwoo, J. Sheppard, V. Bharadwaj, D.
Schultz, September 2002.
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NLC - The Next Linear Collider Project
Recent Positron System Notes, IV
LCC-0103,"Characterization of W-26% Re Target
Material," A. J. Sunwoo, D.C. Freeman, W. Stein, V.
K. Bharadwaj, D. C. Schultz, J.C. Sheppard,
September 2002.
LCC-0106, "Pulsed Helical Undulator for Test at SLAC
Polarized Production Scheme," Alexander A.
Mikhailichenko, October 2002.
DESY LC-DET-2002-011, “The MeV Gamma Compton
Polarimeter for the SLAC FFTB Undulator Test,” V.
Gharibyan and K.-P. Schuler, October 2002.
SLAC-Proposal-E166, “A Two Stage Proposal to Test
Production of Polarized Positrons with the SLAC 50GeV Beam in the FFTB,” G. Alexander et al., October
2002.
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NLC - The Next Linear Collider Project
Positron System Notes, in prep.
LCC-01xx,“Report on Radiation Damage effects
in a Ti Target under Photon Irradiation," M.
Caturla et al., November 2002.
LCC-01xx, “Design Studies for SLAC-Proposal
E-166," M. Woods, K. Moffeit, and J. C.
Sheppard, November 2002.
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