PAC38, Newport News, August 22-26, 2011
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Transcript of PAC38, Newport News, August 22-26, 2011
PAC38, Newport News, August 22-26, 2011
PEPPoPEPPo a Proof-of-Principle Experimenta Proof-of-Principle Experiment
Polarized Electrons for Polarized PositronsPolarized Electrons for Polarized Positrons
A. Adeyemi4, P. Aguilera1, H. Areti1, M. Baylac2, G. Bosson2, A. Camsonne1,
L. Cardman1, S. Covrig1, C. Cuevas1, O. Dadoun3, J. Dumas1,2, E. Fanchini2,
T. Forest5, E. Forman1, A. Freyberger1, S. Golge6, J. Grames1, P. Guèye4,
C. Hyde7, Y. Kim1,5, J.-F. Muraz2, M. Marton2, M. Poelker1, S. Setiniyaz5, J.-S. Réal2, R. Suleiman1, A. Variola3, E. Voutier2
1 Jefferson Lab, Newport News, VA, USA2 LPSC, Grenoble, France
3 LAL, Orsay, France4 Hampton University, Hampton, VA, USA
5 Idaho State University & IAC, Pocatello, ID, USA6 North Carolina University, Durham, NC, USA
7 Old Dominion University, Norfolk, VA, USA
Newport News, August 22-26, 2011
This proposal (PR12-11-105) aims at a demonstration of a new technique for producing polarized positrons, and constitutes a first step towards the development of a polarized positron source for Physics experiments at JLab, and other potential applications.
Objectives
It follows PAC35’s enthusiastic endorsement of LOI-10-010 which noted that
“Any accelerator facility, like JLab, using polarized electrons for its physics program would like an intense beam of polarized positrons. This Letter marks a proof of principle experiment that should become a full
proposal.”
2/17
Why polarized positrons at JLab ?
The investigation of the partonic structure of nucleons and nuclei via Generalized Parton Distributions (GPDs) is a major goal of the JLab and
JLab12 experimental programs.
Parton Imaging
M. Burkardt, PRD 62 (2000) 071503 M.Diehl, EPJC 25 (2002) 223
Newport News, August 22-26, 2011
• The 4 quark-helicity conserving GPDs (Hq, H
q, E
q,
Eq) of the nucleon are extracted from the DVCS
reaction off proton and neutron targets, combining experimental observables from polarized beam and targets.
• These observables involve different linear and bilinear combinations of Compton form factors whose real and imaginary parts are linked to GPDs.
• Other reactions (DVMP, TCS…) are expected to bring additional information.
A large set of different observables is required to unravel the partonic structure of hadronic matter and
extract unambiguously the flavored nucleon GPDs.
~ ~
3/17
(e,e′) Differential Cross Section
M. Diehl at the CLAS12 European Workshop, Genova, February 25-28, 2009
INTlINTlDVCSlDVCSBHeP0 σ~Pσeσ~Pσσσ
Why polarized positrons at JLab ?
Newport News, August 22-26, 2011
γNNγ Ae
γNNγ Am
INTDVCSBH00 σσσσ
INTDVCS00 σ~2σ~2σσ
Electron observables
INT0000 σ2σσ
INT0000 σ~4σσσσ
Electron & positron observables INTlINTlDVCSlDVCSBHl
eP0
ePS σPσ~eσPσ~σPSσσ
Polarized electrons and positrons provide the separation of the different reaction amplitudes of the (e,e') cross section allowing for an experimentally complete determination of the GPDs.
4/17
Why polarized positrons at JLab ?
Two Photons Physics
The availability of a 100 nA e+ beam (unpolarized or polarized) would permit
an order of magnitude improvement in the statistical accuracy of e+e-
comparisons.
Following the very first measurements of polarization transfer observables in electron elastic scattering, the validity of the 1 exchange approximation of the electromagnetic interaction has
been questioned.
P.A.M. Guichon, M. Vanderhaeghen, PRL 91 (2003) 142303
J. Guttmann, N. Kivel, M. Meziane, M. Vanderhaeghen, arXiv:hep-ph:1012.0564
The precision of Coulomb corrections in the study of the EMC effect at JLab energies is also a concern.
5/17
e+ p
) /
(e- p
)
Storage Ring Scheme
Polarized positrons have been obtained in high energy storage ring taking advantage of the Sokolov-Ternov effect which leads
to positrons polarized parallel to the magnetic field.
Why PEPPo ?
Newport News, August 22-26, 2011
5
3
2
22e
γ
ρ
e
cm
35
8τ
Polarization builds up exponentially with a time constant characteristic of the energy and the curvature of the positrons
~ 20 mn @ HERA
6/17Not compatible with CW beam delivery at JLab.
Fixed Target Schemes
The principle of polarization transfer from circular photons to longitudinal positrons has been demonstrated in the context of the ILC project.
T. Omori et al, PRL 96 (2006) 114801
G. Alexander et al, PRL 100 (2008) 210801
P(e+) = 73 ± 15 ± 19 %
Newport News, August 22-26, 2011 7/17
Why PEPPo ?
Compton Backscattering Undulator
1.3 GeV
Require independent ~GeV to multi-GeV electron beam.
Polarized Bremsstrahlung
E.G. Bessonov, A.A. Mikhailichenko, EPAC (1996) A.P. Potylitsin, NIM A398 (1997) 395
Sustainable polarized electron intensities up to 4 mA have been demonstrated from a superlattice photocathode.
R. Suleiman et al., PAC’11, New York (NJ, USA), March 28 – April 1, 2011
PEPPo intends to demonstrate the feasibility of using bremsstrahlung of polarized electrons for the production of
polarized positrons.
Newport News, August 22-26, 2011 8/17
e- → → e+
Why PEPPo ?
The PEPPo experiment @ JLab
Experiment Layout
PEPPo is proposed to run at the injector, taking advantage of the existing experimental capabilities that uniquely define with precision the CEBAF
polarized electron beam at creation.
Newport News, August 22-26, 2011
Polarization controls
Variable energy 2-7.5 MeV
Polarization measurement (1.5%)
Energy measurement (10-2)
PEPPo
9/17
Laser
Intensity controls
Production TargetT1 (0.1-1 mm W)
The PEPPo experiment @ JLab
Newport News, August 22-26, 2011 10/17
Beam position monitors
Viewers
Positron Selection Device
Dipoles
Solenoid
Viewer
Collimator
Corrector magnets
Solenoid
Faraday Cup
Annihilation detector
Beam Profiler
Viewers
Reconversion TargetT2 (2 mm W)
Polarized Analyzing Target
(7.5 cm Fe)
Compton Transmission Polarimeter
Corrector magnets
Quadrupoles
ELEGANT beam optics
PEPPo branch jonction
G4PEPPo experiment model
The PEPPo experiment @ JLab
T1
T2
S1
S2
D
D
Pt
Pe-
Newport News, August 22-26, 2011
Calorimeter
11/17
e+
e-
PEPPo
Principle of Operation
PEPPo is proposing to measure the polarization transfer from longitudinal electrons to longitudinal positrons in the 2-5 MeV/c
momentum range.
o Longitudinal e- (Pe-) produce elliptical whose circular (P) component is proportional to
Pe-.
o P transfers to e+ into longitudinal (Pe+) and
transverse (Pt) polarization components. On the average
Pt=0.
Ie = 1 µAT1 = 1 mm
The PEPPo experiment @ JLab
Compton Transmission Polarimetry
12/17
Integrated Method Semi-Integrated Method
PEPPo polarimetry relies on the sensitivity of the Compton process to the polarization of the photons generated in the T2 target by the interaction of
incoming electrons/positrons.
eTeT APPA
EE
EEAT
jj
jjjT
NN
NNA
Data acquisition based on the JLab FADC250 allows implementation of 2 data taking modes.
Expected experimental asymmetries are small (1-8х10-3) and we will take advantage of the current JLab practices
for the control of helicity correlated systematics.
Newport News, August 22-26, 2011
Detector non-linearities, polarized background PMT gain, Crystal response, pile-up
The PEPPo experiment @ JLab
Proposed Measurements
Electron Measurements Positron Measurements
p(e-)(MeV/c)
AT
(10-
3)
A(e-)(%)
Pe-/Pe-
(%)
2 1.0 1.6 3.4
3 3.5 5.9 3.0
4 6.1 10.4 3.0
5 7.1 12.0 3.0
6 7.8 13.2 3.0
p(e+)(MeV/c)
T1
(mm)
A(e+)(%)
AT
(10-3)
P(e+
)(%)
Pe+/Pe+
(%)
21.0
4.61.6 50 5.9
0.1 1.3 40 16.2
31.0
8.74.0 67 4.1
0.1 3.5 58 5.7
41.0
12.36.4 75 3.9
0.1 6.2 73 4.3
51.0
13.87.7 81 4.8
0.1 7.5 79 5.2
Up to 4 different production (T1) and 2 reconversion (T2) targets configurations will be measured. Background data will be acquired for each configuration. Measurements will be done for + and - analyzing magnet polarities.
%3/PδP%1.5/PδP%85P TTeee
1 h/e- setting
2 h/e+ setting
Newport News, August 22-26, 2011 13/17
Use measured polarization of the e- beam to calibrate the
polarimeter.
The PEPPo experiment @ JLab
Beam Time Request
p(e-)(MeV/c)
I(e-)P(e-)(%)
Time(h)
Commissioning 2 – 6 1 nA – 1 µA ≥ 85 105
Mott Measurements
6 1 – 5 µA ≥ 85 10
e- Calibration 2 – 630 pA – 10
nA≥ 85 85
DetectorMeasurements
6.8 1 – 4 µA ≥ 85 36
e+ Measurements
6.8 1 – 4 µA ≥ 85 100
Total 336
The PEPPo experiment is asking for 2 weeks of beam time, including the commissioning of the new beam line, detector calibration, and
measurements of the polarization transfer to positrons.
PEPPo operation requires the injector and the PSS north linac locked.
Newport News, August 22-26, 2011 14/17
The Collaboration is in a great position to finish PEPPo installation before the end of the 6MSD.
The PEPPo experiment @ JLab
Instrumentation Status
The management of JLab Accelerator Division (Letter from A. Hutton in appendix) and Collaborating Institutions supported the development of
the PEPPo project up to its today status.Polarimeter calorimeter and table
PEPPo branch
S1 Solenoid
DD Spectrometer
Analyzing magnet
DAQ, Power Supplies….
Newport News, August 22-26, 2011 15/17
Our pathway on going forward depends on the recommendation of PAC38.
Towards a CEBAF Polarized Positron Beam
Beyond PEPPo…
16/17Newport News, August 22-26, 2011
PEPPo will demonstrate the principle of a polarized positron source based on the bremsstrahlung of low energy (5-100 MeV) polarized electrons.
PEPPo will provide quantitative information that will further our understanding of polarization transfer in the pair creation process, and that will help to optimize the design of a polarized positron source for JLab physics.
Information will also be provided for potential applications in a low energy source for solid state physics.
o A natural “next step” after PEPPo would study the optimization of a positron collection system and a scheme to define the phase space of the positron beam for CEBAF acceleration.
R&D efforts developed within the context of PhDs told that e+ currents up to 1 µA and polarizations larger than 50% may be expected.
S. Golge, PhD, ODU, Norfolk (2010) J. Dumas, PhD, UJF, Grenoble (2011)
Details of future efforts will be guided by JLab management and the accelerator R&D required to complete a full design.
Summary
Proposal PR12-11-105
There is a strong interest from the User Community to use positrons for nuclear physics
experiments. The PEPPo Collaboration gathers a group of physicists willing to develop a polarized positron source.
The PEPPo experiment proposes to measure the polarization transfer from longitudinally polarized electrons to longitudinally polarized positrons as a proof-of-principle for a new technique for polarized positron source.
Newport News, August 22-26, 2011 17/17
2 weeks of beam data taking at the injector are requested to complete PEPPo.
Possible customers : JLab12, SuperB, EIC, ENC, ILC(?), Solid State Physics…
Newport News, August 22-26, 2011
Proposal PR12-11-105
Why PEPPo ?
Newport News, August 22-26, 2011
Polarized Electron Source
200 kV Inverted Gun + SSL GaAs/GaAsP + RF-Fiber Laser 4 mA
Test Parameter Value
Laser Rep Rate 1500 MHz
Laser Pulselength 50 ps
Laser Wavelength 780 nm
Laser Spot Size 350 µm FWHM
High Polarization Photocathode
SSL GaAs/GaAsP
Gun Voltage 200kV DC
CW Beam Current
4 mA
Run Duration 1.4 hr
Extracted Charge 20 C
1/e Charge Lifetime
85 C
R. Suleiman et al., PAC’11, New York (NJ, USA), March 28 – April 1, 2011
QE(q) = QE0 * e–(q / 84.8)
• High QE ~ 1.5% ( ~6 mA/W/%).
• Current-limited by available laser power.
• Higher 200kV voltage => supersede 1mA demo (PAC’07).
• Pushes technology in support of Electron Ion Colliders > 50 mA, High-P e- Drivers.
Annihilation Detector
Newport News, August 22-26, 2011
The PEPPo experiment @ JLab
Signal integral
Co60
Signal integral
Na22
Two NaI detectors are used to measure the back-to-back photons emitted by the annihilation of positrons in an insertable target.
Radioactive source measurements with the PEPPo DAQ system have been worked out to determine optimum operating conditions and set a
coincidence trigger.
Fiber Array Detector
Newport News, August 22-26, 2011
The PEPPo experiment @ JLab
Detector (FIU) used for the TPE experiment
The beam profiler at T2 consists of a 2 layers fiber array detector read by a multi-anode PMT.
The fibers are currently changed to recover a linear detector
response.
90Sr response profile of the present fiber array showing radiation damages after the TPE run.
Polarimeter Calorimeter
Newport News, August 22-26, 2011
The PEPPo experiment @ JLab
The original E166 calorimeter has been modified for PMT readout for an optimum energy resolution.
New measurements of the energy resolution with the PEPPo DAQ are on-
going.
e+ Beam Concept
Geometric Emittance < 5 mm-mrad Absolute Energy Spread < 1 MeV
Beam Current > 50 nA Bunch Length < 2 ps Duty Factor = 100 % Frequency = 1497 MHz
Beyond PEPPo…
Newport News, August 22-26, 2011
e+ Chicane
Accelerator magnetsMost magnet power supplies are reversible except the arc dipoles which requires a manual action.
Beam diagnosticsBeam position monitors and viewers will work as long as the e+ current is ≥ 50 nA.
Beam modesThe tune mode based on a pulsed beam about tens of µA and 250 µs long and used for beam steering will need to be redefined because of the small e+ current.
RF systemAll passes in the linac are adjusted in phase with each other via the adjustement of their pathlength with the arc dogleg sections. The diagnostic that measures the phase difference between passes require an upgrade to the system.
A. Freyberger, Proc. of the International Workshop on Positrons at Jefferson Lab, Newport News (VA, USA), March 25-27, 2009
Beyond PEPPo…
Newport News, August 22-26, 2011