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.