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A Mini-Primer for Parity Quality Beam (as seen from the Accelerator)

This mini-primer provides an overview of polarized beam experiments for studying particle spin interactions, focusing on the challenges and sources of problems in achieving parity quality beam. It discusses the importance of polarized beams and targets, measurement techniques, and controlling systematics in order to achieve accurate measurements for parity experiments.

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A Mini-Primer for Parity Quality Beam (as seen from the Accelerator)

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  1. A Mini-Primer for Parity Quality Beam (as seen from the Accelerator) • Outline: • Polarized Beam Experiments • Parity Experiments (the bar lowers) • The Imperfect World • Sources of Problems • Measurement, Controls & Feedback • Summary PQB Meeting April 08, 2004 J. Grames

  2. Why Polarized Beams and Targets? To learn the significance of particle spin in the nuclear interactions studied at JLab we take advantage of preparing the beam and/or target electrons to be polarized. Either (beam or target) is polarized if there is a net difference in the number of spin states along some physical direction, e.g., (N+ - N-) Polarization = (N+ + N-) (9 - 1) = 80% (9 + 1)

  3. Polarization Experiments The common technique you’ll find for learning the spin physics interaction is to reverse the sign of the beam (or target) polarization and measure the relative difference in detected signal: (R+ - R-) Aexp = = Aphysics • Pbeam • Ptarget (R+ + R-) Flip one or other… • For most experiments the z-component is important. This explains why: • Experiments need longitudinal beam polarization. • The word helicity is used (spin parallel/anti-parallel momentum).

  4. (R+ - R-) Aexp = (R+ + R-) Parity Experiments Here’s the catch. For parity experiments the experimental asymmetry is very small. Experiments like G0 and HAPPEX-2 are interested in measuring asymmetries of order 1-10 ppm. One of the presently approved experiments in Hall A would seek something less than 1 ppm. Further down the road, it becomes even smaller. The challenge for these experiments is generally controlling the systematics, as opposed to making the measurement (spectrometer/detectors/electronics).

  5. The Imperfect World So, if R+ or R- changes because of anything other than the spin physics of the interaction, it is a false asymmetry. This results in the seemingly unattainable, golden rule for parity experiments: No beam property other than the beam polarization should change when the beam polarization reverses sign. • But, beam properties do change: • Intensity (first order) • Position (second order) • Energy (second order) • These come in different ways: • Laser light • Photocathode • Accelerator These happen before the electrons are even a beam…

  6. Parity Violation Experiments at CEBAF Parity violation experiments continue to set the standard for Polarized Source performance.

  7. The Polarized Electron Source • Electrons are produced via photoemission, using a laser beam. • The sign and degree of the electron beam polarization is determined by the sign and degree of circular polarization of the laser beam. • The lasers produce linearly polarized light. With the application of high voltage (few kiloVolts) Pockels cells (electro-optic devices) convert the linearly polarized laser beam into a circularly polarized laser beam. • By reversing the Pockels cell voltage the helicity of the laser beam, and thus the electron beam, is reversed. • This is the “Helicity” reversal. Anything that changes with this reversal is said to be “Helicity Correlated”.

  8. Steering (Position) Pockels Cell PITA (Intensity) Pockels Cell Pockels Cell Intervening Optics HV Lensing What can defy the golden rule? Cathode

  9. Quality of Laser Polarization & the Photocathode Photocathode Purity: Even 99.99% circular light is 1.4% linear. When circular light reverses sign linear light rotates by 90 degrees. High-P photocathodes have a QE anisotropy, meaning they emit a different number of electrons in orthogonal directions defined by the material, so voila, the intensity can vary by the percent of linear light. More QE Less QE Uniformity: The profile uniformity of the laser polarization depends on the Pockels cell crystal material and cell design. Poor uniformity can result in the centroid of the transverse charge distribution moving, producing a measured position difference. 99.92% 99.93% 99.95% 99.90% Laser profile

  10. Accelerator • A HC position difference on ANY aperture results in a HC intensity asymmetry. (Note we use absolute difference for position and relative asymmetry for intensity). • Apertures (Profile & Position): • Emittance/Spatial Filters (A1-A4) • Temporal Filter (RF chopping apertures) • Beam scraping monitors. • Any piece of beampipe! • The small apertures and tight spots (separation?) Adiabatic damping of the beam emittance may gain factors of 10-20 because of the reduction in amplitude of the beam envelope. Poor optics can reduce this gain by 10x. Poor optics stability can vary response between source and user.

  11. Diagnostics for Measuring HC Beam Properties BCM’s (intensity) and BPM’s (position) are the main diagnostics used. Dedicated parity DAQ’s for both G0 and HAPPEX-2 exist in the injector and in the experimental halls. The beam properties each period of the helicity reversal (~33 ms). We integrate 10,000 samples to get a statistically meaningful result. Although it is intellectually satisfying to measure the parity beam quality at the injector the diagnostics measure all beams simultaneously.

  12. The 3-User Laser Table All beams have common path

  13. Laser Beam Controls (common to all lasers) 30 Hz PZT (optics) Pockels cell (makes circular light) Steering Lens (positions laser on photocathode) Insertable halfwaveplate (flips sign of polarization) Rotatable half waveplate (nulls analyzing power)

  14. Laser Beam Controls (independent feedback knobs) PZT Mirror for Position IA cell for Intensity Laser output linearly polarized Add non-HC elliptical polarization Analyze light Add HC elliptical polarization Commissioning: Helicity Correction Magnets for Position

  15. Injector Helicity Magnets Installation (0L01-0L03) January 5-6, 2004 MHE0L03V, MHE0L03H MHE0L02H MHE0L01V 110 VAC Isolation Transformer Grounded cage containing electrically isolated helicity magnet controls (VME) Tube protecting Litz magnet wire

  16. HC Software Controls The parity experiments want to null the HC effects at the hall (or further upstream). The parity users implement their own feedback algorithm using the HC knobs of the injector.

  17. Summary & Outlook Parity experiments are different than most experiments done at Jlab because the experiment includes the accelerator performance. From the first Jlab parity experiment (HAPPEX-I) the EGG and users have worked together on parity issues concerning the polarized source. Recently, with G0 and looking forward to more difficult parity experiments broader involvement of the accelerator division (CASA) has become critical. Other electron accelerators have a longer history of parity violation experiments, e.g. SLAC or MIT-Bates, however Jlab is poised to confront some of the most difficult proposed. We need to continue being more comprehensive for the present and future parity experiments to be successful.

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