Accelerating Polarized Protons to High Energy Mei Bai
Accelerating Polarized Protons to High Energy Mei Bai Collider Accelerator Department Brookhaven National Laboratory
Outline ¢ General introduction of l l accelerator physics spin dynamics ¢ Accelerating polarized protons to high energy ¢ RHIC: the first polarized proton collider l l ¢ brief history of RHIC pp program achieved performance of RHIC pp Summary
Synchrotron Rf cavity ¢ The acceleration comes from the electric field with an oscillating frequency synchronized with the particle’s revolution frequency ¢ Alternating gradient l A proper combination of focusing and defocusing quadrupoles yields a net focusing force in both horizontal and vertical planes QF ¢ FODO cell: most popular building block for synchrotrons QF QD L L
Particle motion in a synchrotron ¢ Transverse l Betatron oscillation: Betatron tune: number of betatron oscillations in one orbital revolution l Beta function: the envelope of the particle’s trajectory along the machine l
Spin motion in a circular accelerator y ¢ Thomas BMT equation z x l l l beam direction In a perfect accelerator, spin vector precesses around its guiding field, i. e. vertical Spin tune Qs: number of precessions in one orbital revolution. In general, horizontal field kicks the spin vector away from its stable direction, i. e. vertical, and can lead to Ø Depolarization resonance when the spin vector gets kicked at a frequency close to the frequency it precesses
Depolarizing spin resonances Horizontal magnetic fields Dipole errors, steering dipoles, quadrupole misalignments Imperfection resonance: Focusing field due to betatron oscillation Other multipole fields Intrinsic resonance: high order resonance Resonance strength: Size of vertical closed orbit distortion Resonance strength: Size of vertical betatron oscillation For protons, imperfection spin resonances are spaced by 523 Me. V
Challenge in accelerating polarized protons to high energy: preserve beam polarization q Break the resonance condition: l Full Siberian snake o Ø Rotates spin vector by 180 Ø Cancels the kicks on the spin vector in between the snakes Ø Keeps the spin tune independent of energy l Partial snake l Tune jump o Ø rotates spin vector by an angle of <180 Ø Keeps the spin tune away from integer Ø Jump the betatron tune as quickly as possible Ø Intrinsic resonance only and can cause emittance blow up
Challenge in accelerating polarized protons to high energy: preserve beam polarization q Change the resonance strength l Minimize the resonance strength Ø Harmonic close orbit correction ü Imperfection resonance ü Operationally difficult for high energy accelerators l Enhance the resonance strength to achieve full spin flip with normal resonance crossing rate Ø Induce full spin flip using an ac dipole ü Can only be applied to strong intrinsic spin resonances ü not ideal for coupling resonances
BRAHMS(p) Absolute Polarimeter (H jet) RHIC p. C Polarimeters Siberian Snakes Spin flipper PHENIX (p) STAR (p) Spin Rotators (longitudinal polarization) Spin Rotators Solenoid Partial Siberian Snake (longitudinal polarization) LINAC Pol. H Source 200 Me. V Polarimeter BOOSTER AGS Helical Partial Siberian Snake AGS Polarimeters Strong AGS Snake
Polarized proton acceleration complex at BNL q AGS (Alternating Gradient Synchrotron) l l Energy: 2. 3 Ge. V ~ 23. 8 Ge. V A total of 41 imperfection resonances and 7 intrinsic resonances from injection to extraction Ø One 5. 9% partial snake plus one 10~15% partial snake E 20 5. 9% A 20 10~15%
Courtesy of T. Roser Spin tune with two partial snakes Spin tune Extraction Vertical component of stable spin Vertical betatron tune 36+Qy intrinsic resonance G
Tune scan with the AGS dual snake setup Snake resonances Pr el im in ar y • 15% cold snake +5. 9% warm snake Operating working point Courtesy of H. Huang
Results of AGS dual snake setup q The dual snake setup in the AGS was successfully commissioned. With the four new trim quadrupoles located at the entrance and exit of the cold and the warm snakes for compensating the optics distortion from the focusing fields from the snakes, 1. 5 x 1011 protons with a beam polarization of 65% were accelerated with the vertical tune close to 9. H. Huang’s talk: “Polarized Proton Acceleration in the AGS with Two Helical Partial Snakes”, Oct. 2, 17: 00, Hall II
Polarized proton acceleration complex at BNL q RHIC: l l Energy: 23. 8 Ge. V ~ 250 Ge. V (maximum store energy) A total of 146 imperfection resonances and about 10 strong intrinsic resonances from injection to 100 Ge. V. Ø Two full Siberian snakes
snake depolarization resonance q even order resonance l When m is an even number l Disappears in the two snake case like RHIC if the closed orbit is perfect q odd order resonance l When m is an odd number l Driven by the intrinsic spin resonances 5/8 Ramp working pt. Store working pt. q Condition 5/6 3/4 7/8
Snake resonance observed in RHIC ¼ snake resonance Maximum vertical tune Coupled 3/14 snake resonance |Horizontal tune – 3/14|
Snake resonance observed in RHIC 7/10 snake resonance polarized protons were accelerated to an energy of G =63, a location of a strong intrinsic spin resonance
How to avoid a snake resonance q Keep the spin tune as close to 0. 5 as possible l snake current setting q Keep the vertical closed orbit as flat as possible Flat orbit: Sum of kicks on the spin vector from quads as well as the dipole correctors = 0
How to avoid a snake resonance q Keep the spin tune as close to 0. 5 as possible l snake current setting q Keep the vertical closed orbit as flat as possible q Keep the betatron tunes away from snake resonance locations l Precise tune control
Polarized proton collisions in RHIC
Design parameters for RHIC pp Parameter Unit p-p relativistic , injection … 25. 9 relativistic , store … 266. 5 no of bunches, nb … 112 ions per bunch, Nb 1011 2. 0 emittance e. N x, y 95% mm-mrad 20 average luminosity 1030 cm-2 s-1 150 polarization, store % 70
Milestones of RHIC pp development § 2000 § § 2002 2003 2004 New polarized proton source (OPPIS) commissioned One snake was installed in Blue ring and commissioned CNI polarimeter in Blue installed and commissioned § All snakes for both rings installed and commissioned CNI polarimeter in Yellow installed and commissioned § Spin rotators installed and commissioned § RHIC absolute polarimeter using H Jet target installed and commissioned AGS 5% helical warm snake installed and commissioned § § New super-conducting solenoid was installed in the OPPIS source § Polarized protons in RHIC were accelerated to 205 Ge. V with about 30% polarization at top energy § AGS strong super-conducting helical snake installed and commissioned § 2005 The AGS dual snake configuration was commissioned in the AGS. This configuration (10% cold snake + 5. 9% warm snake) yields a polarization of 65% at the AGS extraction with 1. 5 x 1011 protons per bunch § Polarized protons were accelerated to 250 Ge. V with a polarization of 45% § 2006
RHIC pp achieved performance Ptitsyn’s talk: “RHIC Performance with Polarized Protons in Run-6”, Oct. 2, 16: 45, Hall II Parameter Unit 2002 2003 2004 2005 2006 No. of bunches -- 55 55 56 106 111 bunch intensity 1011 0. 7 0. 9 1. 3 store energy Ge. V 100 100 * m 3 1 1 1 1 peak luminosity 1030 cm-2 s-1 2 6 6 10 35 average luminosity 1030 cm-2 s-1 1 4 4 6 20 Collision points -- 4 4 4 3 2 Time in store % 30 41 38 56 46 average polarization, store % 15 35 46 47 60~65
RHIC pp performance: delivered luminosity Ø one week shutdown Ø FY 05: Provided a total of 12. 6 pb-1 luminosity with longitudinal polarization at STAR and PHENIX FY 06: faster setup, higher luminosity and higher polarization
RHIC pp performance: average store polarization FY 2005 Blue average polarization: 49. 5% Yellow average polarization: 44. 5% FY 2006 Goal!
RHIC pp performance: polarization transmission efficiency
First look of beam polarization at 250 Ge. V 45%
Intrinsic spin resonance Qx=28. 73, Qy=29. 72, emit= 10 RHIC intrinsic spin resonance strength achieved
Polarization 250 Ge. V ramp measurement Resonance around 138 Ge. V
Outlook of RHIC polarized protons ¢ Luminosity: Ø Goal: § At 100 Ge. V: 60 x 1030 cm-2 s-1 20 x 1030 cm-2 s-1 (x 3) ü Improve bunch intensity/emittance: B Injector Improvement B improve beam transfer efficiency B Eliminate beam emittance growth during ramp ü Improve luminosity lifetime: B 10 Hz feedback loop to fight against the 10 Hz orbit jitter: work-in-progress B minimize non-linear triplet errors to improve the tolerance to beam-beam effect § At 250 Ge. V: 150 x 1030 cm-2 s-1 (x 2. 5 compared w. 100 Ge. V)
Outlook of RHIC polarized protons ¢ Polarization l Goal: 70% 65% l AGS: operating the AGS with both betatron tunes placed in the spin tune gap that dual snakes provide.
Outlook of RHIC polarized protons ¢ Polarization l Goal: 70% 65% l AGS: operating the AGS with both betatron tunes placed in the spin tune gap that dual snakes provide l RHIC: Snake current setting is critical to make sure the spin precession tune is very close to 0. 5 • Multiple techniques of measuring spin precession tune in RHIC are under development. l Precise tune control • Tune+decoupling feedback system comissioned in FY 06 l Precise orbit control • Re-alignment of the machine • Improve the quality as well as the robustness of the BPM system to achieve the rms value of the orbit distortion below 0. 3 mm
Summary q Over the past 5 years, all the essential hardware and diagnostic apparatus for polarized beam were put in place and successfully commissioned. RHIC has successfully accelerated polarized protons to 100 Ge. V with no polarization loss. q The performance of RHIC pp in Run 2006 was greatly improved due to the great success of the AGS dual snake setup as well as the improvement of RHIC systems. q The 500 Ge. V development demonstrated a beam polarization of 45% at 250 Ge. V and also identified the location of depolarizing resonances. q With the success in the AGS and improvements in RHIC, the RHIC 2006 has achieved a faster setup as well as higher luminosity.
Acknowledgement L. Ahrens, I. G. Alekseev, J. Alessi, J. Beebe-Wang, M. Blaskiewicz, A. Bravar, J. M. Brennan, D. Bruno, G. Bunce, J. Butler, P. Cameron, R. Connolly, J. Delong, T. D’Ottavio, A. Drees, W. Fischer, G. Ganetis, C. Gardner, J. Glenn, T. Hayes, H-C. Hseuh. H. Huang, P. Ingrassia, U. Iriso-Ariz, O. Jinnouchi, J. Laster, R. Lee, A. Luccio, Y. Luo, W. W. Mac. Kay, Y. Makdisi, G. Marr, A. Marusic, G. Mc. Intyre, R. Michnoff, C. Montag, J. Morris, A. Nicoletti, P. Oddo, B. Oerter, J. Piacentino, F. Pilat, V. Ptitsyn, T. Roser, T. Satogata, K. Smith, D. N. Svirida, S. Tepikian, R. Tomas, D. Trbojevic, N. Tsoupas, J. Tuozzolo, K. Vetter, M. Milinski. A. Zaltsman, A. Zelinski, K. Zeno, S. Y. Zhang.
Results of AGS dual snake setup Beam intensity AGS bending magnet field 9. 0 8. 8 Vertical betatron tune (8. 98) Horizontal betatron tune 8. 6 Courtesy of L. Ahrens and K. Brown
How to avoid a snake resonance q Keep the spin tune as close to 0. 5 as possible l snake current setting • set the vertical tune to 0. 745 • measure the beam polarization with different snake current • expect no depolarization if the corresponding spin tune is very close to 0. 5
AGS Horizontal resonance 10% Cold snake 5. 9% Warm snake P P 15% Cold snake 5. 9% Warm snake Target position The tilted stable spin direction also causes about ~5% depolarization at the locations of horizontal tunes Courtesy of F. Lin: “Exploration of Horizontal Intrinsic Spin Resonance in the AGS”, April 22, C 14. 00005
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