We successfully injected polarized protons in both RHIC rings and maintained polarization during acceleration up to 100 GeV per ring using two Siberian snakes in each ring. Each snake consists of four helical superconducting dipoles which rotate the polarization by 180{sup o} about a horizontal axis. This is the first time that polarized protons have been accelerated to 100 GeV

Ahrens, L.

Alessi, J.

Bai, M.

Beebe-Wang, J.

Brennan, J. M.

Brown, K. A.

Bunce, G.

Cameron, P.

Courant, E. D.

Drees, A.

Fischer, W.

Fliller, R. III: Glenn, W.

Huang, H.

Luccio, A. U.

MacKay, W. W.

Makdisi, Y.

Montag, C.

Pilat, F.

Ptitsyn, V.

Roser, T.

Satogata, T.

UNT Digital Library

BNL-71026-2003-CP Submitted to :  Workshop on Increas ing  t h e  AGS Po la r i za t ion  Univers i ty  of Michigan, Nov. 6 - 9 ,  2002 First Polarized Proton Collisions at RHIC' T. Roser', L. h e n s * ,  J. Alessi*, M. Bai*, J. Beebe-Wang*, J.M. Brennan*, K.A. Brown*, G. Bunce*?, P. Cameron*, E.D. Courant*, A. Drees*, W. Fischer*, R. Hiller III*, W. Glenn*, H. Huang*, A.U. Luccio", W.W. MacKay*, Y. Makdisi*, C. Montag*,, E Pilat*, V. Ptitsyn*, T. Satogata*, S. Tepikian*, D. Trbojevic*, N. Tsoupas*, J. van Zeijts*, A. Zelenski*, K. Zeno*, A. Deshpande?, K. Kurila?, K. h e g e r * * ,  H. Spinka**, D. Underwood**, M. Syphers$, I. Alekseev§, D. SviridaS, V. Ranjbary, J. Tojoll, 0. Jinnouchi??, M. Okarnura?? and N. Saitoti-II *BNL, IJpton, NY tRIKEN BNL Research Ceiitel; Uptoil, NY *'ANL, A.rgomie, IL * FNAL, Batavia, IL BITET: Moscow %diaiaa Uiiiv., Bloomiiigton, IN 11 Kyoto Uiiiv., Japan tT RIKEN, Japan Abstract. We successfully injected polarized protons in both RHIC rings and maintained polar- ization during acceleration up to 100 GeV per ring using two Siberian snakes in each ring. Each snake consists of four helical superconducting dipoles which rotate the polarization by 180 O about a horizontal axis. This is the first time that polarized protons have been accelerated to 100 GeV. INTRODUCTION Polarized proton colliders will open up the completely unique physics opportunities of studying spin effects in hadronic reactions at high-luminosity high-energy proton-proton collisions. It will allow study of the spin structure of the proton, in particular the degree of polarization of the gluons and antiquarks, and also verification of the many well- documented expectations of spin effects in perturbative QCD and parity violation in W and Z production. The Brookhaven Relativistic Heavy Ion Collider (RHIC) is the first hadron accelerator and collider consisting of two independent rings. It is designed to operate at high collision luminosity over a wide range of beam energies and with particle species ranging from polarized protons to heavy ions. The RHIC center-of-mass energy range of 200 to 500GeV[l] is ideal in the sense that it is high enough for perturbative QCD to be applicable and low enough so that thlz typical momentum fraction of the valence quarks is about 0.1 or larger. This guarantees significant levels of parton polarization. Work performed under the auspices of the U.S. De:partment of Energy Absolute Polarimeter (k jet). RHIC pC Polarimeters 70% Polarization Partial Siberian SnaE \ Strong AGS Snake 2 x 10" Pol. Protons / Bunch '01. H- Source F E =20 Kmm mrad 200 MeV Polari AGS Internal Polarimeter * -  RfDipoles A- -1, AGS pC Polarimeters FIGURE 1. Layout and design parameters for the Brookhaven polarized proton collider. The eight spin rotators and the absolute polarimeter were not installed for this run. The beam parameters given correspond to the goal luminosity of 2 x lo3' cm-' s-l During the second RHIC run polarized proton beams were successfully accelerated to 100 GeV and stored and collided with a peak luminosity of about 1.5 x lo3' cm-2 s-l. With the two full Siberian Snakes installed in each of the two RHIC rings essentially all beam polarization was preserved during acceleration and beam storage in RHIC. SPIN DYNAMICS, RESONANCES AND SIBERIAN SNAKES Accelerating polarized beams requires the control of both the orbital motion and spin motion. The evolution of the spin direction of a beam of polarized protons in external magnetic fields, such as those existing in a circular accelerator, is governed by the Thomas-BMT equation [2], -=-('> d? [Gy$+(l+G)4]  x ?  dt '/m where the polarization vector P is expressed in the frame that moves and rotates with the particle's velocity. This simple precession equation is very similar to the Lorentz force equation: dt Comparison of these two equations readily shows that, in a purely vertical field, the spin rotates Gy times faster than the orbital motion. Here G = 1.7928 is the anomalous magnetic moment of the proton and y = E /m. Gy gives the number of full spin preces- sions for every revolution and is also called the spin tune vsp. At top RHIC energies vsp reaches about 450. The acceleration of polarized beams in circular accelerators is complicated by the presence of numerous depolarizing spin resonances. During acceleration, a spin res- onance is crossed whenever the spin precession frequency equals the frequency with which spin-perturbing magnetic fields are encountered. There are two main types of spin resonances corresponding to the possible sources of such fields: imperfection reso- nances, which are driven by magnet errors and misalignments, and intrinsic resonances, driven by the focusing fields. The strengths of both types of resonances increases with beam energy. The resonance condition for imperfection depolarizing resonances arise when vsp = Gy = II, where n is an integer. Imperfection resonances are therefore separated by only 523 MeV energy steps. The condition for iintrinsic resonances is vsp =. kP f v,,, where k is an integer, v,, is the vertical betatron tune and P is the superperiodicity. With a localized spin rotator that rotates the spin by the angle 6 about a horizontal axis the spin tune is given by COS ( xvsP) = COS (nGy) COS (6/2) . The spin tune can never reach an integer for any non-zero 6 and therefore all imper- fection resonances are avoided. A 'full Siberian snake'[3], which is a 180" spin rotator, will make the spin tune a half-integer and. energy independent. Therefore, neither im- perfection nor intrinsic resonance conditions can ever be met. In the presence of strong resonances the spin rotation of the snake has to be much larger than the total spin rotation from the resonances. FIGURE 2. spin tracking shows the reversal of the vertical polarization. Orbit and spin tracking through the fiiur helical magnets of a Siberian Snake at y = 25. The 1 0.8 in averaged over beam 0.6 Ah 0-4 m V 0.2 - - 0 -  Polarization for 2.50 particle -0.2 - -0.4 ' I I , I I 370 380 390 400 41 0 420 430 GY FIGURE 3. Result of spin tracking through the rlsgion of the strongest resonance in RHIC. The closed orbit was corrected with residuals of less than 0.2 nun. ACCELERATING POLARIZED PROTONS IN AGS AND RHIC Fig. 1 shows the lay-out of the Brookhaven accelerator complex highlighting the com- ponents required for polarized beam acceleration. The new 'Optically Pumped Polar- ized Ion Source' (OPPIS)[4] produced 10'" polarized protons per pulse. A single source pulse is captured into a single bunch, which is ample beam intensity to reach the nominal RHIC bunch intensity of 2 x 10' ' polarized protons. In the AGS a 5% solenoidal partial snake that rotates the spin by 9" is sufficient to avoid depolarization from imperfection resonances up to the required RHIC transfer en- ergy of about 25 GeV[5]. Full spin flip at the four strong intrinsic resonances can be achieved with a strong artificial rf spin resonance excited coherently for the whole beam by driving large coherent vertical betatron oscillations[6]. The remaining polarization loss in the AGS is caused by coupling resonances and weak intrinsic resonances. Faster acceleration rate and a future, much stronger partial Snake should eliminate depolariza- tion in the AGS [7]. The full Siberian snakes (two for each ring) and the spin rotators (four provide longitudinal polarization for an experiment) for RHIC each consist of four 2.4m long, 4 T  helical dipole magnet modules each having a full 360" helical twist [8][9]. The 9 cm diameter bore of the helical magneis can accommodate 3 cm orbit excursions at injection. Fig. 2 shows the orbit and :spin trajectory through a RHIC snake. The superconducting helical dipoles were constructed at BNL using thin cable placed into 90 80 70 - 60 .3 + cd N 'C 50 - 90 - 80 - 70 - 60 - 50 - 40 - 30 - 20 - 10 i, Experiment data (1997) Simulation (1997) Experiment data (2000) Simulation (2000) Experiment data (2002) Simulation (2002) - --- - I 10 - 2ol 0 0 1,,,,I,,,,I,,,,Ii ' ' 1  ' I ' 1  ' 1  ' 1  1 ' ' 1  ' 1  ' 1  I ' 1  " 1 5 10 15 20 25 30 35 40 45 50 FIGURE 4. Beam polarization measured at the A.GS extraction energy for recent running periods. Also shown are simulations of the polarization evolution during the acceleration ramp taking into account all intrinsic depolarizing resonances, the ramp rate, and coupling from the solenoidal partial Siberian snake. helical grooves that have been milled into iI thick-walled aluminum cylinder. The acceleration through the energy region of the strongest resonance was simulated in great detail including a 1 m rms misalignment of the quadrupoles, and sextupoles as well as the corrector dipoles used to correct the closed orbit. The result is shown in Fig. 3 for a beam with a normalized '95% emittance of 20nmm mrad. Although there is a significant decrease of the polarization at the energy of the strong resonance, the polarization of the full beam is restored after accelerating completely through the resonance region. The simulation also shows that there is significant polarization loss at 60 40 20 0 0.004 0.003 0.002 0.001 Beam Currents [x101* protons] -25 % 1 - Measured Asvmmetrv 0 5 10 15 20 0.0001' " " ' " ' " " ' " " ' " " ' 1 '  Time [hours] FIGURE 5. Circulating beam and measured asymmetry in the blue and yellow RHIC ring (blue(dark) and yellow(light) lines and symbols, respectively) for two typical stores. the edge of the beam. In addition to maintaining polarization, the accurate measurement of the beam po- larization is of great importance. Veiy sm.all angle elastic scattering in the Coulomb- Nuclear interference region offers the possibility for an analyzing reaction with a high figure-of-merit which is not expected to be strongly energy dependent[lO]. For polar- ized beam commissioning in RHIC an ultra-thin carbon ribbon was used as an internal target, and the recoil carbon nuclei were (detected to measure both vertical and radial polarization components. The detection of the recoil carbon with silicon detectors using both energy and time-of-flight information showed excellent particle identification. It was demonstrated that this polarimeter can be used to monitor polarization of high en- ergy proton beams in an almost non-destructive manner and that the carbon fiber target could be scanned through the circulating beam to measure polarization profiles. FIRST RHIC POLAIRIZED PROTON RUN The first polarized proton collider run in RHIC took place from Dec. 2001 to Jan. 2002. Polarized beams were successfully accelerilted to 100 GeV and stored and collided with -51 -10 FIGURE 6. Typical vertical closed beam orbit in RHIC during the polarized proton acceleration cycle. FIGURE 7. Vertical misalignments of the :RHIC beam position monitors and quadrupoles a peak luminosity of about 1.8 x 1030 cm-2 s-'. The beam polarization at the AGS was only about 30% mainly due to the fact that a back-up AGS main power supply had to be used with a much reduced ramp rate that amplified the effect of the weak intrinsic and the coupling depolarizing resonances. Fig. 4 shows the measured beam polarization at AGS extraction energy achieved at recent polarized proton runs. Also shown are simulation results that include the effect of the ramp rate and the coupling from the sollenoidal partial Siberian snake on the strength of the intrinsic resonances in the AGS. The simulations are in reasonable agreement with the achieved polarizations. The simulatioin also predicts more than 50% polarization when the fast ramp-rate AGS main power supply is back in operation again. In RHIC, essentially all beam polarization was preserved during acceleration and beam storage. Fig. 5 shows circulating beam current and measured asymmetries of two typical stores. The analyzing power at 100 lGeV for the RHIC polarimeters i s  not known but expected to be similar to the value at injection energy. Under this assumption the polarization at store was about 25 %. To preserve beam polarization in RHIC during acceleration and storage the vertical betatron tune had to be maintained between 0.220 and 0.235 and the orbit had to be corrected to better than 1 mm rms. This is in good agreement with the predictions from spin tracking calculations described above. A typical vertical beam orbit in RHIC is shown in Fig. 6. The beam excursions at the interaction regions serve to avoid beam collisions during acceleration. The remaining part of the orbit was adjusted to minimize vertical orbit excursions in the laboratory frame taking into account the absolute position of the RHIC beam position monitors. A new survey of the vertical alignment of the :RHC beam position monitors was completed after the end of the run and revealed quite large offsets up to about 5 mm as shown in Fig. 7. The effect of this offset on the RHIC beam polarization is still being analyzed, but it is clear that careful vertical realignn-tent of the RHIC magnets and beam position monitors is necessary before acceleration of polarized beam to 250 GeV is attempted. Beam-based orbit flattening could also be successful using vertical dispersion as an indication of vertical orbit errors. More than 20 years after Y. Derbenev and A. Kodratenko made their proposal to use local spin rotators to stabilize polarized beams in high energy rings, it has now been demonstrated that their concept is working flawlessly even in the presence of strong spin resonances at high energy. Tests, verifying the Siberian Snake concept at low energy, were performed at IUCF[ 111. All spin rotators will be installed for the 2003 run with the possibility to go to full collision energy of 4 = 500 GeV with longitudinal polarization at STAR and PHENIX. Finally a polarized gas jet will be installed for 2004 as an internal target for small angle proton-proton scattering which will allow the absolute calibration of the beam polarization to better than 5 %. REFERENCES 1. Design Manual - Polarized Proton Collider at RHIC, Brookhaven National Laboratory, July 1998, 2. L.H. Thomas, Phil. Mag. 3, 1 (1927); V. Bargmann, L. Michel, V.L. Telegdi, Phys. Rev. Lett. 2,435 3. Ya.S. Derbenev et al., Part. Accel. 8, 115 (19713). 4. A.N. Zelenski et al., 'Optically-Pumped Polarized H- ION Sources for RHIC and E R A  Colliders', 5. H. Huang et al., Phys. Rev. Lett. 73,2982 (1994) 6. M. Bai et al., Phys. Rev. Lett. 80,4673 (1998); M. Bai, these proceedings. 7. H. Huang, these proceedings. 8. V.LPtitsyn and Yu.M.Shatunov, NLM A398 (1997) 126 9. E. Willen et al., 'Construction of helical magnets for RHIC', Proc. of .PAC99, N. Y., .3161(1999). 10. J. Tojo et al., Phys.Rev.Lett. 89,052302 (200;!); 0. Jinnouchi et al., Spin2002 proceedings (BNL), 11. A. D. Krisch et al., Phys. Rev. Lett. 63, 1137 (1989); J. E. Goodwin et al., Phys. Rev. Lett. 64,2779 http://www.rhichome.bnl.gov/RKIC/spin/design . (1959). Proc. of PAC99, N. Y., 106 (1999); A.N. Zelenski, these proceedings. to be published by AIP; G. Bunce, these proceedings. (1990) 

First Polarized Proton Collisions at RHIC.

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First Polarized Proton Collisions at RHIC.

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