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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
The International Polarized Positron Production The International Polarized Positron Production CollaborationCollaboration
K.T. McDonaldNovember 20, 2002 2
E-166 Undulator-Based Production of Polarized Positrons
Overview
– Classical positron production schemes are marginal due to target heating and radiation damage.
– Alternative: use 150 GeV e- in an undulator to produce 10 MeV ’s, which are then converted to positrons (with less heating of target) [TESLA baseline].
– With helical undulator, get polarized ‘s and polarized e+ (Balakin&Mikhailichenko, 1979).
– Physics is clear, but this scheme has never been tested.
– Can produce 10 MeV ’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.
K.T. McDonaldNovember 20, 2002 3
E-166 Undulator-Based Production of Polarized Positrons
– 25 MeV e+ from shower max of 6 GeV e- in a 4-6 radiation length target.
– Heating at shower max many times that from useful positrons alone.
Need multiple targets to meet goal of 1 e+/e-.
– Positrons from shower max have no memory of incident e- polarization.
NLC Baseline Positron Production Scheme
K.T. McDonaldNovember 20, 2002 4
E-166 Undulator-Based Production of Polarized Positrons
Polarized Positrons from Polarized ’s
– High energy E&M interactions are helicity conserving Forward positrons from -> e+e- remember the 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.
-pair(10 MeV) ~ 1/5 pair (1 GeV). Need ~ 100 ’s per useful e+.
(Olsen & Maximon, 1959)
K.T. McDonaldNovember 20, 2002 5
E-166 Undulator-Based Production of Polarized Positrons
Polarized ’s from a Helical Undulator
- ’s per period (with K = eB / 2mc2 = 1) (1/137)
- ( intensity ~ K2, but spectrum ragged for K > 1.)- 100 ’s/positron => ~ 10,000 periods.- Period ~ undulator diameter ~ 1 cm ~ 100 m long. First-order cutoff at ~ 10 MeV ’s for 150 GeV e-.- Helical undulator simpler to fabricate than planar.
K.T. McDonaldNovember 20, 2002 6
E-166 Undulator-Based Production of Polarized Positrons
Table 1: TESLA, NLC, FFTB Polarized Positron Parameters
Parameter Units TESLA NLC FFTB Beam Energy, Ee GeV 150-250 150 50 Ne/bunch - 3x1010 8x109 1x1010
Nbunch/pulse - 2820 190 1 Pulses/s Hz 5 120 30 Undulator Type - planar helical helical Undulator Parameter, K - 1 1 0.17 Undulator Period
u cm 1.4 1.0 0.24 1st Harmonic Cutoff, Ec10 MeV 9-25 11 9.6 dN/dL photons/m/e- 1 2.6 0.37 Undulator Length, L m 135 132 1 Target Material - Ti-alloy Ti-alloy Ti-alloy, W Target Thickness r.l. 0.4 0.5 0.5 Yield % 1-5 1.8† 0.5 Capture Efficiency % 25 20 - N+/pulse - 8.5x1012 1.5x1012 2x107
N+/bunch - 3x1010 8x109 2x107
Positron Polarization % - 40-70 40-70 † Including the effect of photon collimation at = 1.414.
TESLA, NLC, and FFTB Positron Production
K.T. McDonaldNovember 20, 2002 7
E-166 Undulator-Based Production of Polarized Positrons
Sample Collider Polarized Positron System Layout
2 Target assemblies for redundancy
Polarized e- source for system checkout (and ee, running).
K.T. McDonaldNovember 20, 2002 8
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 production 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+).
K.T. McDonaldNovember 20, 2002 9
E-166 Undulator-Based Production of Polarized Positrons
The E-166 Collaboration
The E-166 Collaboration includes:
– Participation from all major LC Labs (CERN, DESY,
KEK, SLAC) and JLAB.
– Participation from several universities.
– e- polarization experts from SLD and HERMES.
– e+ polarization experience via the Japanese groups.
K.T. McDonaldNovember 20, 2002 10
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 (u = 2.4 mm, K = 0.17) in the 50-GeV Final Focus Test Beam Emax ~ 10 MeV.
Characterize the polarization with a transmission polarimeter.
Then make polarized positrons (Stage 2): ’s are converted to polarized positrons in a < 0.5 radiation length target (of both Ti and W).
Characterize the positron polarization by converting them back into ’s and using a transmission polarimeter.
K.T. McDonaldNovember 20, 2002 11
E-166 Undulator-Based Production of Polarized Positrons
Table 3: FFTB Helical Undulator System Parameters
Parameter Units Value Number of Undulators - 2 Length m 0.5 Inner Diameter mm 0.889 Period mm 2.4 Field kG 7.6 Undulator Parameter, K - 0.17 Current Amps 1800 Pulse Width s 30 Inductance H 1.8x10-6 Wire Type - Cu Wire Diameter mm 0.6 Resistance ohms 0.125 Repetition Rate Hz 30 Power Dissipation W 225 T/pulse
0C 4
Undulator Design
PULSED HELICAL UNDULATOR FOR TEST AT SLAC THE POLARIZED POSITRON PRODUCTION
SCHEME. BASIC DESCRIPTION.
Alexander A. Mikhailichenko
CBN 02-10, LCC-106
K.T. McDonaldNovember 20, 2002 12
E-166 Undulator-Based Production of Polarized Positrons
Double Undulator Scheme
• Price is factor of 2 in rate (0.2 /e- in 50 cm).
• At the Collider, need better scheme: e.g., single undulator + pulsed spin rotator at ~ GeV. Or other.
• Sign of e- polarization can be chosen randomly on a pulse-by-pulse basis at the GaAs photoemission source.
• In lieu of better scheme, use 2 undulators of opposite helicity, pulsing only one.
K.T. McDonaldNovember 20, 2002 13
E-166 Undulator-Based Production of Polarized Positrons
Polarized Positron Yield at the FFTB
• e+/ = 0.005 in 0.5 r.l. Ti target.
• N+ = N- (/e-) (e+/) = (2e10)
(0.2)(0.005) = 2e7 / 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%.)
K.T. McDonaldNovember 20, 2002 14
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 ’s.
0.5 r.l. converter target.
51%-54% e+ polarization.
Moffeit/Woods
K.T. McDonaldNovember 20, 2002 15
E-166 Undulator-Based Production of Polarized Positrons
Polarimetry via Transmission Thru an Iron Block
- Measure only ’s transmitted thru a block of magnetized iron.
- Compton depends on both P and Pe.
- = (++ - +-) / (++ + +-) = 0.05 for 10 MeV ’s in 15 cm Fe.
- Transmission polarimetry is a “1-Compton” polarimetry.
K.T. McDonaldNovember 20, 2002 16
E-166 Undulator-Based Production of Polarized Positrons
Transmission Polarimeter + Detector
Moffeit/Woods
To avoid soft backgrounds, convert ’s to e+e- and detect Cerenkov light.
Deconvolve spectrumvia use ofseveralCerenkovradiatorswith gradedenergythresholds.
K.T. McDonaldNovember 20, 2002 17
E-166 Undulator-Based Production of Polarized Positrons
e+ Polarimeter Using Transmission Polarimetry
Convert the e+ back to ’s.
54% e+ pol. 8.4% pol.
Compton 2.5% asymmetry.
Will be statistics limited.
Can it work close to beam?
K.T. McDonaldNovember 20, 2002 18
E-166 Undulator-Based Production of Polarized Positrons
Compton/Annihilation Polarimeter for ’s and e+
Annihilation polarimetry for e+:
• e+e- in a thin magnetized iron foil.
• Asymmetry = Pe+• Pfoil • AA, Pfoi ~ 1/13, AA ~ 1 at 90 in CM frame, max ~ 0.08 Pe+.
• Problem is low rate (high background?).
• (If use e+e- scattering, A ~ 7/9.)
For ’s, can use Compton scattering in the polarized foil (+ sweep magnet SM):
•Asymmetry = P• Pfoil • AC, AC() up to 0.7 max ~ 0.05 P
•Scattering polarimeter is “noninvasive”.
K.T. McDonaldNovember 20, 2002 19
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.
– The pulsed helical undulator is a scale model, 1% in length, ~ 20% in diameter, of that appropriate for a collider.
– The 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.
K.T. McDonaldNovember 20, 2002 20
E-166 Undulator-Based Production of Polarized Positrons
Endorsement by the NLC Machine Advisory Committee
• Well, I hope that we have an endorsement to mention here!
K.T. McDonaldNovember 20, 2002 21
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 (parasitic T-experiment)
– followed by Stage 1 running with the polarimeter(s),
– and Stage 2 running with the e+ polarimeter(s) – all before the FFTB is dedicated to the LCLS (~ 2005).
K.T. McDonaldNovember 20, 2002 22
E-166 Undulator-Based Production of Polarized Positrons
Time Line
– We plan to start construction of equipment in FY03, finish construction and installation in the first 6 month of FY04, and do the experiments in the second half.
– In case background problems require additional measures, or we find unexplained fact, we will request additional time in FY05.
K.T. McDonaldNovember 20, 2002 23
E-166 Undulator-Based Production of Polarized Positrons
E-166 Budget Estimate (1)