Statistics: 34 participants from 16 different institutions 8 sessions, 24 talks Poster session
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Transcript of Statistics: 34 participants from 16 different institutions 8 sessions, 24 talks Poster session
Summary
Workshop Polarized Electron Sources and Polarimeters PESP-2004
October 7-9 2004presented by
Kurt Aulenbacher (IKP, Mainz)
Statistics: 34 participants from 16 different institutions8 sessions, 24 talksPoster session Round table discussion:Polarized source requirements for the ILC
PESP-2004
Hosted by: Institut für Kernphysik der Universität Mainz Mainz, Germany
Sponsored by: Institut für Kernphysik,University of Mainz,Committee for Spin Physics Symposia,Deutsche Forschungsgemeinschaft
Grouping together important subjects
• Photocathode/Photoemission (basic) research (9 talks)
• Source system performance (7 talks)
• Subsystems (6 talks)
• Future requirements (3 talks, round table)
Photoemission from semicoductorsBasic idea:Polarisation by helicity transfer:Photabsorbtion withhin the bandstructure of suitable semiconductor
VB
+
CB
3-step procedure:PhotoabsorbtionTransport to the surface Emission through NEA-surface:
Problem: Find the best compromise Towards Polarization and QE:
Best structure/lowest transport lossesNEA-losses?
p-GaAs substrate, Zn doped
61018 cm-3 Zn0.5 mBufferAl0.3Ga0.7As
40 AIn0.16Al0.14Ga0.7As41017 cm-3 Zn
40 ASL
GaAs0.75P0.25
11019 cm-3 Zn60 AGaAs QW
As cover
DopingThicknessComposition
Parameters of strain-compensated SLs
Gerchikov (Theory), Mamaev(exp)
Layer Compo sition Width NumberSample In Al P b,nm w,nm of periods Pmax, % Ehh-lh, meV
1 0.16 0.12 0.08 4 4 30 73 271a 0.16 0.12 0.08 4 4 20 77 272a 0.18 0.12 0.17 5 4 20 76 532b 0.18 0.12 0.17 4 4 20 83 472c 0.18 0.14 0.17 5 4 20 83 502d 0.18 0.14 0.17 4 4 20 73 493a 0.16 0.12 0.18 4 4 8 74 463b 0.16 0.12 0.18 4 4 12 74 46
1.4 1.5 1.6 1.7 1.8 1.9
10
20
30
40
50
60
70
80
90
hh1-e1
lh1-e1hh2-e2
Tth = 540 C
Tth = 570 C
Tth = 600 C
Pol
ariz
atio
n, %
Excitation energy, eV
Calculations: =7 meV =11 meV, =15 meV + BBR
10-4
10-3
10-2
10-1
100
101
102
Qua
ntum
Yie
ld,
%
Fit to Data with Parameters VB-scattering/smearing...(Gerchikov, SPTU)Matrix elements, splitting,QSE:theoryProbematic: transport/emission depol/surface-states
GaAs0.83P0.17/Al0.1In0.18Ga0.72As
(4x5nm)x20
550 600 650 700 750 800 850 90010-5
10-4
10-3
10-2
10-1
100
101
0
20
40
60
80
QE
, %
Wavelength, nm
QE-1, SL 5-773, Tht=450C, T=300K, 07.06.2004 QE-2, SL 5-773, Tht=500C, T=300K, 08.06.2004 QE-3, SL 5-773, Tht=540C, T=300K, 09.06.2004 QE-4, SL 5-773, Tht=570C, T=300K, 10.06.2004
P-1, SL 5-773, Tht=450C, T=300K, 07.06.2004 P-2, SL 5-773, Tht=500C, T=300K, 08.06.2004 P-3, SL 5-773, Tht=540C, T=300K, 09.06.2004 P-4, SL 5-773, Tht=570C, T=300K, 10.06.2004
Pol
ariz
atio
n,
%
SL‘s with P > 80% ; 1% QE, low activation temperature!(MAMAEV, SPTU)(InAlGaAs, GaAs)
Promising option: GaAs/GaAsP• Achieves high QE (1%), high P
(86%) and low Anisotropy (<2%) (Maruyama, SLAC)
• Experimental observation of P and QE Spectra gives tool to identifiy if structure is in agreement with predictions (Kuwahara, Nagoya)
• Nagoya: P=92%+-6 observed at 0.3% QE
• SLAC: Photovoltage effects are well under control: 10^12 electrons in 60ns (suitable for NLC). Charge relaxation time constant is of order <10ns (emittance ??)
Polarimeter accuuracy is limiting factor in comparison of ‚record‘ polarisations!!!!
Time resolved studies Reveal: • not all superlattices Have fast response with low depolarisation• ‚first‘ electrons have highestPolarisation P=91+-4.5%(Mainz data)even higher P Is possible
Emission from surface States always contributes, Can be taken as ‚quality check‘ (Terekhov Novosibirsk)
Theoretica understanding of Cs-O covered NEA surface Is under way, but not yet complete(Kulkova,Tomsk)
Operating sources for high energy exp.
• c.w. regime:• JLAB• MAMI/Mainz
• Pulsed regime:• SLAC• MIT/Bates
(Storage (BLAST)/LINAC(Sample))
• ELSA/Bonn
Highlights of c.w. operation:
Very high reliability/availabilityPolarisation 80+Average currents up to 200 Mikroamps (Poelker JLAB)Current stability on target I/I<10^-3 HC-I- asymmetry <1ppm, Energy stability E/E =10^-6,HC-E-asymmetry <3*10^-8(Maas, IKP-Mainz),
Present day PV-experiments are limited by statistics, rather than HC-systematic effects
Pulsed operation (storage ring)
• Highly automated ring fill and BLAST data taking based on EPICS controls system.
6-8 K Coulombs per day on tape
M. Frakondeh, MIT-Bates
Polarimeters
• Compton backscattering polarimeter with 850 MeV beam integrated in lasercavity (J. Imai, Mainz)
• Ultracompact spin analyzer for low energy electrons based on transmission of magnetic thin films (D. Lamine, EcolePolytechnique, Palaiseau)
• High accuracy Mott-polarimeter at 3.5 MeV, with double focussing spectrometers (V. Tioukine, Mainz)
Experimental techniques• Hydrogen cleaning: reduces activation temperature of
photocathodes from typ. 580 to 450 °C (Maruyama, SLAC)• Very reliable q-switched lasers for pulsed operation (Brachmann,
SLAC), • 31MHz and 499 Mhz rep-rate synchro-Lasers (Titanium-sapphire)
with 70 pikosecond pulse length commercially available (Poelker, JLAB)
• 2.5 GHz rep rate 40ps semiconductor synchro-laser with rms stability <10^-3 (Mainz)
• Field emission ‚fundamental‘ studies at Nagoya:Very high static field gradients possible with Mo/Ti Kathode/Anode
Combination; 170MV/m at 1nA (but low gap separation)
Photocathode lifetime:
• Lifetime well sufficient for present day accelerators.Extractable charges in one lifetime several hundert C.
• ELIC-type accelerators could require extractable charges of 10^4 Coulomb (talk by M.Farkondeh), depending on accelerator design.
• High c.w. current + low emittance + good lifetime + high polarization is problematic, the simultaneous tasks cause interacting problems
BUT:It‘s worthwhile
200 keV (Yamamoto, Nagoya) Gun at Nagoya350 keV (JLAB):Both are making good progress: low emittance, high current density Vacuum lifetime of photocathodes is considerably smaller than ‚standard‘ sources. Field emission? Vacuum problems?
Ultracold GaAs source at Heidelberg: (talk by D. Orlov): transverse energy distribution <1meV Thermal conductivity optimized to 20deg/Watt: Would ‚thermally‘ allow to produce >7mA average current from SL-Kathode (high polarization) Mask activation (Grames, JLAB) offers reduction of transmission Losses, and ion backbombardment
Large emittance beams (2mm dia at Cathode) can be transported with losses <10^-5 and high extractabe charge (i=1mA, C=200 Coulomb, Mainz), guns with extreme pumping speed (JLAB, Nagoya) and reduction of outgassing by NEG coating (Mainz) are in prepartion
Test experiments with bulk-GaAs
TEST OF ‚nonlinear‘ current induced lifetime effects necessary!
ILC-round table• S-RF design: low frequency, large acceptance loosens restrictions
towards emittance & bunch length: Conservative HV-design possible, but again: low emittance high gradient high potential, desirable but must not compromise availability
• Long bunch train not yet demonstrated (should be no problem)• >90% beam polarization desirable: +1% in P +2% higher
‚statistical ROI‘ of collider investment. • International Photocathode research should be cordinated to find
comparable testing conditions • Polarized positron sources are well under way, two approaches in
cirular gamma ray production: Helical ondulator (Leihem, DESY) and Compton backscattering (Omori, KEK)
Summary of Summary
• Existing sources work well.• 90% Polarization barrier is about to be
broken• Great potential of Photoemission source
for higher c.w. currents.• may be necessary to realize it for future
accelerators. • PESP-2004 proceedings will be published
togehter with this conference.