Transcript The International Polarized Positron Production Collaboration Undulator-Based Positron Production in the Final Focus Test Beam (E-166) K.T.

The International Polarized Positron Production Collaboration
Undulator-Based Positron Production
in the Final Focus Test Beam (E-166)
K.T. McDonald, J.C. Sheppard, Co-Spokespersons
SLAC Experimental Program Advisory Committee
November 20, 2002
E-166 Undulator-Based Production of Polarized Positrons
Overview
– Positron production via e- showers in a thick target is
marginal at a Linear Collider due to target heating.
– Alternative: use 150 GeV e- in an undulator to produce
10 MeV g’s, which are then converted to positrons
(with less heating of target) [TESLA baseline].
– With helical undulator, get polarized g‘s and polarized e+
(Mikhailichenko, 1979).
– Physics is clear, but this scheme has never been tested.
– Can produce 10 MeV g’s via 50 GeV e- in SLAC FFTB
using a 1-mm bore undulator.
– FFTB available only thru 2005 (then LCLS).
– E-166 collaboration proposes to demonstrate the
physics and technology of undulator-based polarized
positron production in the FFTB.
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E-166 Undulator-Based Production of Polarized Positrons
NLC Baseline Positron Production Scheme
– 20 MeV e+ collected from shower max of 6 GeV e- in
a 4 radiation length target; pre-accelerated to 250 MeV
– Energy deposition at shower max exceeds single target
material limit
 Would need multiple targets to meet goal of 1 e+/e-
– Positrons from shower max have no memory of incident epolarization.
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E-166 Undulator-Based Production of Polarized Positrons
Polarized Positrons from Polarized g’s
– High energy E&M interactions are helicity conserving,
 Forward positrons from g -> e+e- remember the
g polarization.
– Only 2 charged particles in the
target per positron,

 Less heating of target.
– e+ polarization is degraded by
Bremsstrahlung in the target,
 Use target < 0.5 rad. len.
– Only upper half of positron
spectrum has good polarization.
- spair(10 MeV) ~ 1/5 spair (1 GeV).
(Olsen & Maximon, 1959)
 Need ~ 100 g’s per useful e+.
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E-166 Undulator-Based Production of Polarized Positrons
Polarized g’s from a Helical Undulator
- g’s per period (with K = eBl / 2pmc2 = 1)  a = 1/137.
- (g intensity ~ K2, but spectrum ragged for K > 1.)
- 100 g’s/positron => ~ 10,000 periods.
- Period l ~ undulator diameter ~ 1 cm  ~ 100 m long.
 Eg  Ee2/l  10 MeV g’s for 150 GeV e-.
- Helical undulator simpler to fabricate than planar.
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E-166 Undulator-Based Production of Polarized Positrons
Linear Collider Polarized Positron System Layout
2 Target assemblies for redundancy
Polarized e- source for system checkout (and e-e-, gg running).
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E-166 Undulator-Based Production of Polarized Positrons
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
3x10
8x10
1x1010
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
MeV
9-25
11
9.6
1st 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
12
12
8.5x10
1.5x10
2x107
N+/pulse
3x1010
8x109
2x107
N+/bunch
%
40-70
40-70
Positron Polarization
† Including the effect of photon collimation at g = 1.414.
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E-166 Undulator-Based Production of Polarized Positrons
Physics Motivation for Polarized Positrons
– Electroweak processes e+e- -> WW, Z, ZH couple only to
e-Le+R or e-Re+L (and not e-Le+L or e-Re+R).
– Slepton and squark produced dominantly via e-Re+L.
 Can double rate using polarized positrons
(or suppress rate if both e- and e+ are polarized).
– Effective polarization enhanced,
and error decreased, in
electroweak asymmetry
measurements,
(NL – NR) / (NL + NR) = Peff ALR,
Peff = (P- - P+) / (1 – P-P+).
– Must have both e+ and epolarization for Giga-Z project.
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E-166 Undulator-Based Production of Polarized Positrons
The E-166 Collaboration
http://www-project.slac.stanford.edu/lc/local/PolarizedPositrons/pdfs/E-166TLD.pdf
The E-166 Collaboration
includes:
– Participation from all major
Linear Collider Labs (CERN,
DESY, KEK, SLAC) and JLAB.
– Participation from several
universities with past FFTB
experience.
-
– e polarization experts from
SLD and HERMES.
– e+ polarization experience via
the Japanese groups.
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E-166 Undulator-Based Production of Polarized Positrons
Scope of E-166
 Make polarized photons (Stage 1):
 Use a 1-m-long, short-period, pulsed helical undulator (lu = 2.4 mm,
K = 0.17) in the 50-GeV Final Focus Test Beam  Egmax ~ 10 MeV.
 Characterize the g polarization with a transmission polarimeter.
Then make polarized positrons (Stage 2):
 g’s are converted to polarized positrons in a < 0.5 radiation length
target (Ti and/or W).
 Characterize the positron polarization by converting e+ back into
g’s and using a transmission polarimeter.
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E-166 Undulator-Based Production of Polarized Positrons
Undulator Design
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 20, 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
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E-166 Undulator-Based Production of Polarized Positrons
Electron Beam Requirements
–
–
–
–
50 GeV desired (48 GeV OK, but Eg  Ee2, so not lower).
30 Hz (parasitic operation OK).
1010 electrons per pulse.
Electrons need not be polarized (the undulator provides the
needed g beam polarization).
– Spot size of 40 x 40 microns (12 x 3 microns achieved in E-150
at same location).
• Corresponds to ex,y = 2•10-5 m-rad in both planes (up from 1.65•10-5
m-rad calculated from SLC) , with b*x,y  7.5 m.
• Very low current in E-166 (only 1/60 of E-158).
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E-166 Undulator-Based Production of Polarized Positrons
Double Undulator Scheme
•
•
•
•
Sign of e- polarization can be chosen randomly on a
pulse-by-pulse basis at the GaAs photoemission source.
To control e+ polarization on a pulse-by-pulse basis, use
2 undulators of opposite helicity, pulsing only one.
Price is factor of 2 in g rate (0.2 g/e- in 50 cm).
[At the Collider, better to use a single undulator (still
can be pulsed) + pulsed spin rotator at a few GeV.]
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E-166 Undulator-Based Production of Polarized Positrons
Polarized Positron Yield at the FFTB
•
e+/g = 0.005 in 0.5 r.l. Ti target.
•
N+ = N- (g/e-) (e+/g) =
(2e10)(0.2)(0.005) = 2•107 / pulse.
•
Longitudinal polarization, Pave, of
the positrons is 54%, averaged
over the full spectrum
•
[For 0.5 r.l. W converter, yield is
double and Pave is 51%.]
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E-166 Undulator-Based Production of Polarized Positrons
Layout of E-166 in the FFTB
50 GeV, low emittance e- beam.
2.4 mm period, K = 0.17,
helical undulator.
10 MeV polarized g’s.
0.5 r.l. converter target.
51%-54% e+ polarization.
Moffeit/Woods
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E-166 Undulator-Based Production of Polarized Positrons
g Polarimetry via Transmission Thru an Iron Block
-
Measure only g’s transmitted thru
a block of magnetized iron.
- sCompton depends on both Pg and Pe.
- 3% transmission thru 15 cm iron
for Eg > 5 MeV.
- d = (s++ - s+-) / (s++ + s+-) = 0.05 for
10 MeV g’s in 15 cm Fe.
- sd/d ~ 1/(2 d N) ~ 0.02 per pulse.
- Transmission polarimetry is a kind
of Compton polarimetry.
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E-166 Undulator-Based Production of Polarized Positrons
Transmission Polarimeter + g Detector
To avoid soft backgrounds, convert g’s to e+e- and detect
Cerenkov light.
Deconvolve g
spectrum via
use of several
Cerenkov
radiators with
graded energy
thresholds.
Moffeit/Woods
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E-166 Undulator-Based Production of Polarized Positrons
e+ Polarimeter Using Transmission Polarimetry
 Convert the e+ back to g’s.
 54% e+ pol.  44% g pol.
 dCompton ~2.5% asymmetry.
 ~ 10 e-ge+ge- per pulse.
 Need 1 hour for sd/d = 0.1

Can it work close to g beam?
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E-166 Undulator-Based Production of Polarized Positrons
Option to Bring e+ Outside the FFTB
Use a 20-90º bend + solenoid channel to transport
positrons thru FFTB wall to the polarimeter.
Moffeit/Woods
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E-166 Undulator-Based Production of Polarized Positrons
Compton/Annihilation Polarimeter for g’s and e+
V. Gharibyan, K.P. Schuler: Use variants on
Møller scattering in a magnetized iron foil.
For g’s, can use Compton scattering
(+ sweep magnet SM):
•Asymmetry d = Pg• Pfoil • AC,
AC() up to 0.7  dmax ~ 0.05 Pg.
•Scattering polarimeter is “noninvasive”.
•For e+, can use annihilation polarimetry
• e+e-  gg in the foil.
• Asymmetry d = Pe+• Pfoil • AA,
Pfoi ~ 1/13, AA ~ 1,  dmax ~ 0.08 Pe+.
• Problem is low rate (high background?).
• (If use e+e- scattering, A ~ 7/9.)
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E-166 Undulator-Based Production of Polarized Positrons
E-166 as Linear Collider R&D
– E-166 is a proof-of-principle demonstration of undulator based
production of polarized positrons for a linear collider.
• This technique is much less demanding on target performance than
conventional positron production with e- on a thick target.
• A helical undulator is simpler than a planar one -- and provides polarized
positrons.
• [LCLS explores large-scale, long-term implementation of an undulator.]
– The pulsed helical undulator is a scale model, 1% in length,
~ 20% in diameter, of that appropriate for a collider.
– The g and e+ polarimeters are prototypes of those appropriate for
low-energy diagnostics at a collider.
[High energy polarimetry is also needed at a collider,
but is well demonstrated at present e- and e+e- facilities.]
– The hardware and software expertise developed for E-166 will be the
basis for implementation of polarized positrons at a linear collider.
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E-166 Undulator-Based Production of Polarized Positrons
The MAC was supportive of the E-166 scientific
goals and experimental plan.
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E-166 Undulator-Based Production of Polarized Positrons
E-166 Beam Request
– E-166 is to be performed in the FFTB, with the undulator
just upstream of the e- dump magnets, and polarimeters
downstream of these.
– E-166 needs initial background studies [1-2 weeks in FY03]
– followed by Stage 1 running with the g polarimeter(s)
[2-3 weeks in FY04]
– Stage 2 running with the e+ polarimeter(s) [2-3 weeks in
FY05]
– All before the FFTB is dedicated to the LCLS (~ 2005).
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E-166 Undulator-Based Production of Polarized Positrons
Summary
– Undulator-based production of positrons offers relief on target
stress issues at a linear collider, and improved physics
opportunities via positron polarization.
– This test will provide confidence that the design proposed for the
next generation of linear colliders is based on solid, experimentally
demonstrated principles, all working together at the same time.
– [Parallel efforts should continue to study collider target
operational issues.]
– With 50 GeV e-, the FFTB at SLAC is the only existing facility
suitable to demonstrate this experimentally untested concept.
– The E-166 collaboration is strong, with experts in polarimetry at
DESY, KEK, JLAB, and SLAC (SLD), and accelerator physicists
responsible for E-158.
– E-166 can be run interleaved with PEP-II with 2-3 weeks of beam
time over each of the next 3 years.
– The E-166 budget of ~ $1M is comparable to that of other recent
efforts in FFTB.
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