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Design and Test of a Prototype Cavity for a Stern-Gerlach Polarimeter Peter Cameron - BNL

Design and Test of a Prototype Cavity for a Stern-Gerlach Polarimeter Peter Cameron - BNL. Design and Test of a Prototype Cavity for a Stern-Gerlach Polarimeter

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Design and Test of a Prototype Cavity for a Stern-Gerlach Polarimeter Peter Cameron - BNL

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  1. Design and Test of a Prototype Cavity for a Stern-Gerlach PolarimeterPeter Cameron - BNL

  2. Design and Test of a Prototype Cavity for a Stern-Gerlach Polarimeter P. Cameron1, M. Conte4, N. D’Imperio1, W. Franklin6, D.A.Goldberg3, A. Luccio1, M. Palazzi4, M. Pusterla5, R. Rossmanith2, W. MacKay1, T. Zwart6 1Brookhaven National Laboratory, Upton, NY 11973, USA 2Forschungszentrum Karlsruhe GmbH, D-76021 Karlsruhe, Germany 3Lawrence Berkeley National Laboratory, Berkeley, CA 94720 USA 4Universita and Sezione INFN di Genova, 16146 Genova, Italy 5Universita and Sezione INFN di Padova, 35131 Padova, Italy 6MIT-Bates Laboratory, Boston MA 01949 USA

  3. Outline • What this is about • Stern-Gerlach History • Derbenev and the transverse pickup • Conte et al and the longitudinal kicker • Conte et al and the transverse kicker • The Bates polarimeter • The RHIC polarimeter • What next?

  4. What this is about • Polarization measurement as beam instrumentation rather than a scattering experiment • The essence of the problem • Enhance interaction of m with pickup • Dynamic range – accomplish the measurement in the presence of the electric charge background • The approach • Resonant pickup • Magnetic dipole has geometry – take advantage of relativity • Mode suppression and filtering • First electrons, then protons

  5. Brief History 1896 Zeeman splitting - fine structure dE = hdn = mB 1922 Stern/Gerlach splitting - ‘space quantization’ F = grad(mB) kicker 1927 Pauli proposes spin S = sqrt(s(s+1)) = 1.732/2 s = 1/2 observable 1983 Barber & Cabrera and Michigan/AGS - Squid pickup 1985 Niinikoski and Rossmanith- transverse splitting in a synchrotron kicker 1993 Derbenev - RF Resonance Polarimeter - transverse moment pickup 1995 Conte et al - longitudinal spin splitting g? 1/g?kicker 1996 Argonne BIW Cameron et al - Squid Polarimeter - longitudinal pickup 1998 RHIC Note Cameron et al - MIT-Bates Cavity - longitudinal pickup 2000 LANL preprint server - Conte et al - transverse g2pickup 2001 PAC - poster and paper based on MIT-Bates meeting pickup 2002 September - Spin 2002 MIT-Bates, RHIC, LHC,… pickup, kicker

  6. Derbenev - Transverse • Hamiltonian approach with spin motion as described by BMT. • Potential confusion - BMT lives in more than one reference frame • Requires TM cavity mode, which couples strongly to beam charge. • Excitation to move spin away from stable spin direction (spectral separation) also drives the cavity • No gamma dependence - small signals, no advantage to working at high energy • Requires extremely high-Q (~1010) resonant cavity

  7. Conte et al - Longitudinal • Longitudinal magnetic moment transforms as g – Jackson, Hagedorn,… • First proposal for a longitudinal spin splitter • Proposal for polarimeter at MIT-Bates • Nature conspires against observation - contribution due to space and time gradients of magnetic field cancel to order 1/ g • Squid polarimeter should still work for electrons (non-linear device, energy comes not from beam but rather from junction bias) but if it doesn’t work for protons, why bother?

  8. Outline • What this is about • Stern-Gerlach History • Derbenev and the transverse pickup • Conte et al and the longitudinal kicker • Conte et al and the transverse kicker • The Bates polarimeter • The RHIC polarimeter • Conclusions

  9. Conte et al - Transverse • Reference – LANL preprint 0003069 • Transverse magnetic moment is invariant • BUT - interaction of moment with appropriate TE cavity mode goes as g2 • analogousto inverse Compton scattering, FELs,???… • Second proposal for a longitudinal spin splitter – kick ~ g2 • Second proposal for polarimeter at MIT-Bates - signal ~ g4 • Cheap, fast, accurate, non-destructive polarimeter • Possibility of calibration from first principles (straightforward EM calculations, comparison with signal from charge) • We learn a lesson - the Italians (Waldo MacKay is an honorary Genoese) are both smart and tenacious

  10. TE011 on-axis Fields B E

  11. TE011 Fields 0 0 0 1 1 bkg problem 1 0 0 0 m=0 n=1 p=1

  12. SG Force in lab frame also Heinemann

  13. Bates S/N • TE011 mode • Signal strength is good • Schottky ~ -150dBm • Charge background requires alignment at the level of a few mrad • First choice is motion control, cheapest is beam steering -50dBm Bates signal -60dBm bkg -80dBm 0 mrad 25

  14. Prototype Cavity • Refine frequency calculations to include beampipe perturbation • Determine probe length for optimal coupling • Determine optimal coupling for TM mode dampers • Investigate need for tuners

  15. S21 – Closed and Beampipe TM121 4.348GHz TE011 2.735GHz TM111 3.211GHz new modes with beampipe? TE031 5.496GHz TM131 5.762GHz blue – closed box red – with beampipe

  16. S21 – with and w/o short TM111 TM121 TE011 modes attenuated by short TM131 blue – with beampipe red – with short

  17. Mode Strengths

  18. Ratio of m to q Power 100dB - Bates 200dB - RHIC

  19. More Mode Strengths • Contamination by finite Q – TM mode damper? • TM111 mode ~ 500MHz away • Lorentzian lineshape down by ~ 120dB at this distance • Ratio - amplitude ~ 100dB above TE011 signal (g4 helps)! • 20dB margin is not comfortable, argues for damper • Beam Stability – TM mode damper? • Conclusion – effect of TM damper on TE modes is weak, to be conservative we will add damper to next iteration.

  20. Block Diagram Cavity FFT Box Mix beam D Filter Filter TM Mode Coupler

  21. What about RHIC? • Ideally one would avoid a superconducting cavity. Signal strengths appear to permit this. • Signal power can be enhanced by high frequency, stimulating coherence with longitudinal kicker. • Problem is charge background (foreground?). • Impossible alignment tolerances? Signal mode • Contamination due to finite Q – TM modes • Measure only when spin is away from stable spin direction? Dynamic range (excitation of TM) • Another is implementation of the 1/g2 suppression of charge interaction. Careful study is necessary.

  22. RHIC S/N • Bates cavity in RHIC • 1% longitudinal bunching at 2.7GHz to provide coherence • Signal strength is adequate • Charge background requires alignment at sub-nanoradian level • We need (at least) one more good idea • free precession? • 1/g2 suppression of charge? • else? -110 RHIC signal -130dBm bkg -145 0 25 mrad x 104

  23. What next? • Add mode dampers to prototype cavity • Cut to frequency and measure • Decide whether or not to add tuners • Design filter • Design Review – at Bates? November? • Build vacuum compatible cavity and measure • Follow on with 1/g2 suppression? • RHIC polarimeter • Polarization at full energy – LHC?

  24. TE201 Fields • Advantages • longitudinal moment transforms as g –Jackson,… • 2nd order (position and angle) cancellation of electric charge interaction due to geometry • BUT • contribution due to space and time gradients of magnetic field cancel to order 1/ g

  25. TE201 Fields 1 0 0 0 1 No bkg problem 0 0 0 1 m=2 n=0 p=1

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