The Crystal Collimation System of the Relativistic Heavy Ion Collider

1. The Crystal Collimation System of the Relativistic Heavy Ion Collider Ray Fliller III University of Stony Brook Brookhaven National Laboratory

2. BNL Angelika Drees Dave Gassner Lee Hammons Gary McIntyre Steve Peggs Dejan Trbojevic IHEP – Protvino Valery Biryukov Yuriy Chesnokov Viktor Terekhov Collaborators

3. Outline • Brief RHIC Overview • Collimation • Crystal Channeling • RHIC Crystal Collimation System • Channeling Results • Crystal Collimation and Background Reduction • Conclusion

4. Crystal Collimator

5. RHIC Capabilities • Two 3.8 km counter-propagating superconducting rings BLUE (clockwise) and YELLOW (counterclockwise). • Can accelerate anything from polarized protons (250 GeV) to fully stripped gold ions (100 GeV/u), possibility of colliding uneven species. • Six IRs with four experiments (STAR, PHENIX, BRAHMS, PHOBOS). • Typical store each ring contains 110 bunches of 109 gold ions or 1011 polarized protons.

6. Typical RHIC Parameters • 95 % norm. Emittance: e=15 p mm-mrad • rms momentum spread: sp = 0.13 % • Bunch length: sl = 0.19 m • Energy: 100 GeV/u • Store Length: 4 hours • Beam size at collimator: 5.3mm (b*PHENIX=1m)

7. Need for Collimation Various processes cause particles to enter into unstable orbits with large betatron amplitudes, causing beam halo formation. These halo particles cause: The job of the collimation system is to remove the halo and alleviate these problems. In addition, it should provide a well defined location for beam losses in case of equipment failure. • Background in experiments • Excessive radiation in uncontrolled areas of the tunnel • Magnet quenches in superconducting machines • Equipment malfunction and damage

8. Naive Collimation Collimator Beam Naively, all particles that enter the collimator are stopped in the collimator. Most particles hit near edge and scatter out of the collimator forming secondary halo! However, that is usually not the case….

9. Two Stage Collimation Since primary collimator acts as a scatterer, secondary collimators are necessary to increase energy loss and absorb secondary halo particles. The number of secondary collimators grows quickly when background or machine protection requirements are strict and a high collimation efficiency is required (see LHC collimation system!).

10. A simpler way to collimate Use a bent crystal to channel halo away from the beam core, intercept with a scraper downstream. Number of secondary collimators can be greatly reduced.

11. Crystal Channeling Ions properly aligned to the crystal planes are channeled…. …Particles with large incident angles scatter through the crystal

12. Large electron density – particles will get lost. Ec -xc xmax xc Particles with are not channeled. Interplanar Potential Ions “properly aligned” to the crystal planes see an average potential. This potential is skewed by the bending of the crystal. dp Curvature shifts minimum

13. To have a large channeling efficiency, the angular divergence of particles impacting crystal should be less than 2qc. Critical Angle qc The channeling condition gives an angle qc, above which a particle will not be channeled. Using a Si crystal with 100 GeV/u Au or 250 GeV p , qc=11 mrad For 100 GeV p, qc=19 mrad

14. 2F Channeling Efficiency The integral of the incoming particle distribution over the channeling phase space is the channeling efficiency For a beam with uniform divergence: 2F>2qc

15. Scattering from: Impurities Electrons Lattice Defects And sudden curvature changes all cause particles to dechannel. The same processes cause dechanneled particles to become channeled – volume capture. Dechanneling and Volume Capture

16. CATCH Simulation CATCH by Valery Biryukov

17. How to we predict these?? Important Considerations for Crystal Collimation • Crystal alignment to beam halo. • Angular divergence of beam halo hitting crystal.

18. Crystal Collimator Geometry

19. Model of Beam Hitting Crystal Assuming a Gaussian beam distribution of: • J = J(x, x’, d) is the particle amplitude • e is the rms unnormalized emittance • d is the fractional momentum deviation • sp is the rms fractional momentum spread By transforming from {J, d} to {x, x’, d} and integrating over momentum:

20. Angular Alignment Assuming the distribution extends over the entire crystal face, the angle between the beam orbit and particles striking the crystal is • x0 is the distance between crystal and beam center • Dx is width of crystal face • a, b, D, D’ are lattice functions at crystal The crystal planes need to be at this angle relative to the beam orbit! This is proper alignment!

21. Angular Divergence The equation for angular divergence, sx’(x0), is not very illuminating. However, it depends strongly on: • a, D’– large values increasesx’(x0) • sp – large values increasesx’(x0) • b, D – large values decreasesx’(x0) • Dx – large values increasesx’(x0)(assuming particles hit whole crystal face) By optimizing these parameters, the angular spread of beam across the crystal face is minimized. For those who REALLY want to see the equation, read my thesis!

22. And the angular spread increases! Phase Space at Crystal When crystal is moved into beam, it needs to be realigned

23. Angular Width – Model Optics Critical Angle b*PHENIX = 2 m model

24. Angular Width – Measured Optics • and D affect ellipse orientation and shape Critical Angle b*PHENIX = 2 m measured (FY2001)

25. Caveat Emptor! There are a few holes in the model: • Particle distribution – Gaussian in the tails?? • Assumption that particles strike across the whole face of crystal. • Does not take into account multiple turns. • Not useful for volume capture predictions. However, this model gives us a starting place….

26. Placement of the Crystal • Crystal should be placed at a location that has low a and D’ and a maximun of b so that: • xp’ is independent of x0 • sx’(x0) is reduced • Channeling efficiency is increased • Operation of crystal collimator is easier However, in RHIC all warm spaces have large a!

27. RHIC Collimation System Changed after FY2003 STAR Scraper can move horizontally, vertically and rotate in horizontal plane Downstream PIN Diodes Upstream PIN Diodes Hodoscope courtesy of Y. Chesnokov and V.Terekhov

28. Vessel Cutaway Pivot Inchworm Crystal Moveable Stage

29. Crystal Vessel Crystal Crystal Motion Beam

30. Crystal

31. Measuring Crystal Angle By measuring the deflection of the laser beam, the crystal angle is measured • Crystal can rotate approx: 6 mrad • Measurement Resolution: 20 mrad • Angular Step Size: 30 nrad

32. Crystal Collimator Scraper PHENIX Lattice Functions b*PHENIX = 2 m FY2003

33. Synopsis of Data

34. qb A “Typical” Crystal Scan sx’(x0) Crystal Aligned Volume Capture x’p Crystal Channeling November 12, 2001 Au beam at store.

35. Hodoscope Signal Very noisy compared to PIN diodes. Coincidence rate is almost useless. Limited use in analysis.

36. Comparison to Simulation • Model Optics: • Location wrong • dip width too narrow • efficiency too large Design optics do not agree well with data. However, measured optics agrees better. Simulation used CATCH and one turn matrix.

37. Comparison to Simulation Volume capture region strongly affected by number of turns in simulation.

38. sxx’/sx2 is independent of b*PHENIX. Measurements during other runs indicate 36 2 mrad/mm. Other datasets agree with this number as well. Channeling Angle vs. Position b*=1m at PHENIX Design: mrad/mm Measured Optics: mrad/mm Data: mrad/mm

39. Beam Divergence Even using the correct optics, the predicted angular spread is too small. Multiple turns are not in the theory! Assumed Gaussian halo distribution!

40. Channeling Efficiency Channeling Efficiency does not match predictions from the theory. This is because the beam divergence on the crystal does not match theory. Using the measured beam divergence (from sx’(x0) ) the efficiency agrees well for most cases.

41. Channeling Results • RHIC optics did not match model, so initial predictions overestimated crystal performance • Simple theory overestimates channeling efficiency – lacking multiple turns, model of halo distribution too simple. • Simulation agrees with data well. • Channeling efficiency is understood once optics and beam halo distribution are understood. • Accurate knowledge of lattice functions and halo distribution VERY IMPORTANT! Will low channeling efficiency result in too much scattering and hurt collimation?

42. STAR Background 4 crystal scans with different scraper positions - xs Crystal not moved.

43. Other Experiment Backgrounds Only BRAHMS see significant effect

44. Placing the Scraper Scattering from scraper Scattering from crystal By using both sets of PIN diodes, we can know when the scraper becomes the primary aperture!

45. STAR Background Reduction Scraper only “Raw” Background Crystal collimation does not do better than scraper alone!

46. Crystal Collimation vs. Raw Background Scraper moves closer to beam Crystal Collimation reduces Background to uncollimated rate Au beam, d-Au run, crystal collimation not always effective in reducing background.

47. Crystal Collimation Results • Crystal can cause background in experiments. • Scraper position very important. • Because of low channeling efficiency, crystal collimation was not successful. • Scraper alone collimated the best. • Crystal Collimator removed from RHIC. Traditional two stage collimation system installed for FY2004 run.

48. Summary • Bent Crystals were used for collimation in RHIC • Crystal Channeling worked as expected once lattice functions and halo distribution were understood. • Collimation was unsuccessful because lattice was not optimized in area of collimator. • Crystal caused background. • Tevatron is going to install our vessel (and I’ll be following it there!) Questions??

49. Single Stage Collimation During d-Au run, backgrounds were reduced by as much as a factor of 5. Fill 03094 d-Au run Vertical Collimator Closer to beam Horizontal Collimator Partially retracting the vertical collimator increases backgrounds

50. Upgraded Collimation System PIN Diodes downstream of V1 and H1 collimators are not shown for clarity • Crystal Collimator removed • Primary is the same collimator as previous runs, moved to location reserved for the Crystal Collimator • Secondary collimators are based on design of primary • Controls software upgraded to include manual/automatic control of collimators