Accelerator Issues for PREX

Transcription

1 Accelerator Issues for PREX Kent Paschke University of Virginia when name E b target, θ what s hard? Aug 2009 HAPPEX III 3.4 GeV 1 H, 12 o polarimetry Oct 2009 PV DIS 6 GeV 2 H, 12 o backgrounds Jan 2010 PREX 1.05 GeV 208 Pb, 5 o everything

2 Helicity Correlated Beam Asymmetries Qty Run Avg d 24 hr. Avg 1hr Avg ji?er at 30Hz A Q 100 ppb 500 ppb 2.5 ppm 500 ppm Δx 2 nm 10 nm 50 nm 10 μm Δ(σ x ) / σ x ? * * will require feedback Δ(σ x ) / σ x will probably not be con]nuously measured. Upper limits on magnitude and sensi]vity will be set by beam studies! Topics: Source configura]on, alignment, and feedback schemes Injector transmission matching (maximize adiaba]c damping benefit) phase trombone 180 o Wien flip

3 Wien Slow Reversal Goal: upgrade the Wien spin rotator to provide 180 degree spin rotation, relative to the optimized launch angle, with minimal beam optics change. This is crucial for cancelling possible higher-order helicity correlated beam asymmetries, such as spot size or vertical polarization moment distribution Criteria: Transfer matrix mapping position motion from photocathode to position or angle at the Hall A target should change by <50% spin slip should not require a major retuning to restore design match One very a?racfve concept: Two Wien rota]ons, op]mized once then held constant, with +/ 90 degree solenoid rota]on Op]cally neutral (IDEAL!!!) and If a job's worth doing, it's worth doing well. But it requires addi]onal beamline space, and Le mieux est l'ennemi du bien. Voltaire, La Bégueule (1772)

4 Polarized Source Specifics Rapid Flip: electronics noise suppression suggests faster helicity flip, a la Qweak (30 Hz 300 Hz) (Homework required!) Helicity IsolaFon and Grounding Clean up Polarizer: ini]al state imperfec]ons will limit source alignment precision unless clean up is used Improved configurafon control: beam divergence and size are important configura]on parameters. (Homework required!) Improved A Q Feedback: likely a faster cycle with faster repor]ng (Hall A analysis must also be improved.) (Homework required!) PosiFon feedback opfons Slow reversal opfons: downstream waveplate? Image inverter? Possibly useful, but none compares to Wien filter!

5 Transverse polariza]on An allowed (non parity viola]ng) asymmetry arises from transverse polariza]on. This may be an order of magnitude larger than A PV, and present a significant challenge to the experimental accuracy. VerFcal: Should be well controlled in injector. We aim for <1% ver]cal polariza]on... is this possible (and stable) using exis]ng solenoids? Accuracy in the Mo? polarimeter will be crifcal! Horizontal: Will require spin dance, and we hope to design a transverse asymmetry monitor in Hall A. Human mediated feedback, on ]me scale of ~week, may be necessary. Goal: Ver]cal and horizontal transverse polariza]on <1%. A T ( 4 He) predicfon: 10 9 HAPPEX He: A T = ppm ± 1.34 ppm (stat) ± 0.37 ppm (syst) Afanasev Curve for E b =3 GeV

6 Compton polarimeter PREX requires 1% polarimetry Compton polarimetry is much harder at 1 GeV An upgrade is underway to replace the IR laser cavity with a green laser cavity. The green laser is likely to be lower power! HAPPEX II required 100 Hz/ A at 5mm, at 3 GeV PREX requires <100 Hz/ μa at 5 mm, 1 GeV electron analysis works if measurements at 3mm are possible!

7 Beam Modula]on System ~25 seconds driping posi]on adds noise systema]c jump when FFB is restarted Use rapid modula]on for a differen]al measurement Is sine wave compa]ble with Fast Feedback? phase advance and phase space coverage issues may be addressed with a more sophis]cated analysis

8 Beam Modula]on System Amplitude Amplitude from FFT plots vs Freq 1C07 1C11 1C12 From Test Study Sine wave amplitude appears to be asenuated by feedback over a broad range of frequencies (more detailed analysis underway) Freq

9 Low Current Calibra]on Runs The distribu]on hitng the PREX detector will require careful characteriza]on. Precision knowledge of the kinema]cs (~0.7% uncertainty in Q 2 ) is cri]cal. This measurement requires the produc]on target, but 860MHz is too much rate to turn on drip chambers. Opera]on at 50 na, assuming the beam orbit stays the same, will allow characteriza]on of distribu]on in detector using spectrometer tracking package. Rate correc]on will s]ll be an issue. New cavity BPMs appear to provide sufficient resolu]on to verify beam trajectory.

10 PREX in Hall A Pb Target Accelerator topics Helicity correlated beam asymmetries o Wien reversal o configura]on/alignment o matching new beam modula]on system transverse polariza]on Compton Polarimetry 50 na calibra]on beam A clear collaborafon statement on high priority, long lead needs must be made: Wien reversal, Mo? polarimeter...