Summary Workshop Polarized Electron Sources and Polarimeters PESP-2004

Summary Workshop Polarized Electron Sources and Polarimeters PESP-2004 October 7 -9 2004 presented by Kurt Aulenbacher (IKP, Mainz)

PESP-2004 Hosted by: Institut für Kernphysik der Universität Mainz, Germany Sponsored by: Institut für Kernphysik, University of Mainz, Committee for Spin Physics Symposia, Deutsche Forschungsgemeinschaft Statistics: 34 participants from 16 different institutions 8 sessions, 24 talks Poster session Round table discussion: Polarized source requirements for the ILC

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 semicoductors Basic idea: Polarisation by helicity transfer: Photabsorbtion withhin the bandstructure of suitable semiconductor 3 -step procedure: Photoabsorbtion Transport to the surface Emission through NEA-surface: CB s+ Problem: Find the best compromise Towards Polarization and QE: VB Best structure/lowest transport losses NEA-losses?

Parameters of strain-compensated SLs Gerchikov (Theory), Mamaev(exp) Composition Thickness Doping As cover Ga. As QW 60 A 1 1019 cm-3 Zn Ga. As 0. 75 P 0. 25 40 A SL 4 1017 cm-3 Zn In 0. 16 Al 0. 14 Ga 0. 7 As 40 A Al 0. 3 Ga 0. 7 As Buffer 0. 5 mm 6 1018 cm-3 Zn p-Ga. As substrate, Zn doped

Fit to Data with Parameters VB-scattering/smearing. . . (Gerchikov, SPTU) Matrix elements, splitting, QSE: theory Probematic: transport/emission depol/surface-states Ga. As 0. 83 P 0. 17/Al 0. 1 In 0. 18 Ga 0. 72 As (4 x 5 nm)x 20

SL‘s with P > 80% ; 1% QE, low activation temperature! (MAMAEV, SPTU) (In. Al. Ga. As, Ga. As)

Promising option: Ga. As/Ga. As. P • 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 60 ns (suitable for NLC). Charge relaxation time constant is of order <10 ns (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 highest Polarisation 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(Sam ple)) • ELSA/Bonn

Highlights of c. w. operation: Very high reliability/availability Polarisation 80+ Average currents up to 200 Mikroamps (Poelker JLAB) Current stability on target DI/I<10^-3 HC-I- asymmetry <1 ppm, Energy stability DE/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) M. Frakondeh, MIT-Bates • Highly automated ring fill and BLAST data taking based on EPICS controls system. 6 -8 K Coulombs per day on tape

Polarimeters • Compton backscattering polarimeter with 850 Me. V beam integrated in lasercavity (J. Imai, Mainz) • Ultracompact spin analyzer for low energy electrons based on transmission of magnetic thin films (D. Lamine, Ecole. Polytechnique, Palaiseau) • High accuracy Mott-polarimeter at 3. 5 Me. V, 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), • 31 MHz and 499 Mhz rep-rate synchro-Lasers (Titanium-sapphire) with 70 pikosecond pulse length commercially available (Poelker, JLAB) • 2. 5 GHz rep rate 40 ps 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; 170 MV/m at 1 n. A (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

Test experiments with bulk-Ga. As 200 ke. V (Yamamoto, Nagoya) Gun at Nagoya 350 ke. V (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 Ga. As source at Heidelberg: (talk by D. Orlov): transverse energy distribution <1 me. V Thermal conductivity optimized to 20 deg/Watt: Would ‚thermally‘ allow to produce >7 m. A average current from SL -Kathode (high polarization) Mask activation (Grames, JLAB) offers reduction of transmission Losses, and ion backbombardment Large emittance beams (2 mm dia at Cathode) can be transported with losses <10^-5 and high extractabe charge (i=1 m. A, C=200 Coulomb, Mainz), guns with extreme pumping speed (JLAB, Nagoya) and reduction of outgassing by NEG coating (Mainz) are in prepartion 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.