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Beam collimation and control in the high energy injectors N. Catalan Lasheras

Explore scenarios for LHC luminosity upgrade, challenges of beam collimation, machine protection, and loss mechanisms. Learn about collimation principles, acceptance needs, and energy loss mechanisms.

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Beam collimation and control in the high energy injectors N. Catalan Lasheras

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  1. Beam collimation and control in the high energy injectors N. Catalan Lasheras Scenarios for the LHC Luminosity Upgrade Arcidosso, Italy, 31 August–3 September 2005

  2. Scenario for the injectors upgrade Two rings in the SPS tunnel • 25GeV to 150 GeV • normal conducting • 150 GeV to 1 TeV • Superconducting 4.5 T • Bmax = 100 m; D = 4m; en = 7 mm; delta bucket 10-3 • Acceptance 6s • 20kW tolerable beam losses 5W/m compatible with cryo load • Only a scenario but representative of all potential problems

  3. Outline • Why do we have to collimate the beam? • Background for the experiments in colliders (RHIC, Hera, Tevatron) • Activation and hands on maintenance (SNS, high power machines) • Machine protection, quenches (LHC, Hera, Tevatron II) • What exactly do we have to collimate? • Beam halo and Transient losses • Betatron and longitudinal losses • How to do it? • Multi-stage colllimation • Scrapers, targets, crystals, kickers

  4. How much losses can we tolerate? • Heat load in the cryogenic system of the SC machine • 20 KW distributed homogeneously 3 W/m • Activation and residual radiation • 1-10 W/m. Higher for higher energies • Quench protection • 10-50 W/m from LHC magnets Very similar figures based in still rough approximations We may have to clean the beam for all these reasons How much power can we expect to loss?

  5. Other machines. What should we expect? The 2nd Mini-Workshop on Particle Losses, December 9-11, 1996, KEK In the injection loss column the mode of injection (H-:Hinjection, B:Bunch-to-bucket transfer, S:Septum injection) and the injection energy in GeV is listed. In the transition column the transition energy in GeV is listed in parenthesis. In the extraction column the mode of extraction (F:Fast extraction, S:Slow resonant extraction) is listed.

  6. How much losses can we expect? • 15% of the beam is lost from booster to SPS • Injection losses are in PS and SPS at the level of % • Same level of losses expected during slow extraction. • Power quickly exceed the 20 KW • And we still have to make sure it is homogeneously lost

  7. Loss mechanisms • Slow continuous losses (Beam halo) • Space charge (Bunch intensity, energy, emittance, acceptance) • IBS & Touscheck effect (Intensity, energy, transverse and longitudinal emittance, transverse and longitudinal acceptance) • Beam gas scattering (Intensity, vacuum conditions) • Beam-beam (Intensity, emittance, crossing angle, acceptance) • Slow resonant extraction (acceptance) • Transient (still slow) losses • Capture loss and ramping (Bdot, RF voltage) • Transition (Uff!) • Instabilities (…) • Accidental (fast) losses • Misinjection • Kicker failures • They have to be treated separately but kept in mind all the time!!

  8. Acceptance needs • Losses will be caused by multiple mechanisms not all of them known • BUT, for all halo formation mechanisms, an halo remains an halo if no beam tube is present!! • Losses will be quantified by the acceptance of the machine • Both the aperture of the magnets and the optics functions have to be optimized • If losses occur (and we know they will) then collimators are necessary and they will further reduce the aperture of the machine • Need sufficient magnet aperture, small betas and small dispersion • Should be easier than in a collider but space is restricted • Very difficult to fit in an old machine

  9. Collimation principles Ax Transverse collimatorh=0 Longitudinal collimatorh>0 dp/p0 • Collimators are material blocks limiting the aperture of the ring. • Lattice functions determine the resolution and phase space cuts • Three main mechanisms capture the beam • Ionization • Multiple Coulomb scattering • Nuclear interactions (Elastic + inelastic) • Hadronic showers are created after an inelastic interaction and power is deposed along the way • Typically power is spread along one meter

  10. Energy loss by ionization Energy loss or stopping power • Continuous process inside the collimator • Well known (Bethe-Bloch equation) • Lighter materials are more ionizing (but less dense) Closer look to the interesting range 25, 150 and 1000 GeV Practically constant across energies!

  11. Energy vs. momentum acceptance at 25 GeV • Energy loss is very important for almost any size collimator. • Protons are outside the momentum acceptance with a weak nuclear absorption. Jaws are most effective in spreading the beam. • Primary scrapers could be used with a preference for heavier elements with downstream longer collimators. • Secondary jaws need to be very efficient • One to few turns process. • Local losses around magnetic elements

  12. Energy vs. momentum acceptance at 1 TeV • Energy loss in the collimator is negligible for practical size jaws. • MCs angle is very small. Definitely a multi-turn process • Absorption in a single jaw could be done very efficient. Limited by heat concerns and by out-scattering. • Shorter jaws of lighter materials are more resistant • Collimator length needs to be adapted to the expected level of losses or vice versa. • Very challenging in the extraction channel

  13. Energy vs. momentum acceptance at 150 GeV • Somewhere in the middle. Momentum loss is important but particles stay in the bucket after one passage. • Momentum acceptance needs to be high to allow multi-turn cleaning • Injection & ramping losses in the high energy ring • Assume short injection and fast ramping. Insignificant halo growth (?) • Only momentum cleaning would be necessary. Could use thin scrapers of heavy materials to induce energy loss

  14. Nuclear interaction. Optimal jaw length. • To capture 99.9 % of protons 44 cm of copper are necessary • Includes both inelastic and elastic nuclear interaction (10% Nuclear elastic will be lost in the vicinity of the collimator) • We may not want to absorb that much in a single jaw for the shake of collimator integrity • In practice, the final efficiency is less than that but no significant gain afterwards

  15. Out-scattering • Multiple Coulomb scattering (mCs) as well as ionization is a continuous process. • Proton stays on the beam acceptance but it is deflected by the collimator material • Protons close to the edge are “out-scattered” • Out-scattering due to mCs reduces the effective length of the jaw • One or more secondary collimators located at larger aperture are necessary to capture the out-scattered particles

  16. Maximum efficiency with a single collimator • Efficiency of the jaw depends on out-scattering • Out-scattering depends on the impact parameter (diffusion speed and loss mechanism). • For continuous losses we assume a 6s halo and a normalized emittance en=7mm • Maximum impact parameter estimated from bmax=3/2(rp2Dr2)1/3 using r=0.3 mm estimated from SPS experiments (T. Risselada, JB. Jeanneret) • Beam divergence for 7um emittance and an average b=40m

  17. Out-scattering vs.energy • For a copper block 0.3 m long • At high energy out-scattering is less important and efficiency is higher. • Not strongly dependent on the jaw length. After the out-scattering length no gain for longer jaw • Increasing the efficiency implies catching the secondary halo • We need to have a closer look to their distribution.

  18. Energy vs. one passage efficiency • Out-scattering decreases with energy as well as deflected angle • Absorption decreases with energy for the same collimator length

  19. Multi-stage collimator system ResidualHalo Secondary Halo Primary Halo • Reducing the losses implies catching the secondary halo out-scattered from the jaw • Secondary collimators have to be retracted from primary jaws and located at optimized phase advance. • To increase efficiency the secondary halo has to circulate freely in the machine Collimation efficiency depends of machine acceptance as well!!

  20. What do we find in other machines? Low energy high intensity machines

  21. What do we find in other machines? High energy • Main injector FERMILAB 8 - 150 GeV • Hera p 40 – 820 GeV • Two stage collimation system • Tevatron 150 Gev - 1TeV • Single collimation system. • Two stage system for Tevatron II • SPS 26 - 450GeV • Single collimation/scrapping system • AGS 33 GeV • No collimation system. High activation • RHIC 100 GeV/n Au. • Crystal assisted collimation system • Two stage collimation system from Run 4 Aim is more like 99.99 %efficiency!!

  22. Very first ideas • Momentum cleaning at low energy in the SPS • a heavy material scraper • local collimators at the right phase advance • Protection absorbers in the transfer line • Momentum and betatron cleaning at injection and extraction energy for the SSPS • If possible using the same secondary collimators but different scrapers for different energies to save space • Separate function scrapers betatron/momentum • Common absorbers. Special optics with high dispersion • Could share location with warm injection?

  23. Do collimators work? Dec,5 2003 • Fermilab 980 GeV proton beam. Quench on 2/3 of the machine • Tungsten target 5mm • Stainless steel collimator 1.5m long • Could be considered a success

  24. Collimator damage • Studies are on-going (SPS, FERMILAB) to help predict the damage • Still very difficult to predict the geometry • Collimators in Hera have grooves but still protected the machine

  25. Remaining issues • Jaw damage studies depending on the energy and beam spot size may be necessary. Good progress with the LHC studies • Jaw length definition based on these studies • Study different jaw cooling, Composite materials • The typical length of an hadronic shower is 1m. Losses in SPS will be seen by the SSPS. Cross-talk between machines when in the same tunnel • Injection and extraction lines are too short for anything? • Impedance of the collimators needs to be weighted • Residual radiation and hands on maintenance requirements to be defined • Shielding and maintenance procedures are better considered soon than later

  26. Conclusions • Collimation is necessary for heat load, machine protection and activation concerns • Enough aperture is essential for low losses and high cleaning efficiency. Do not forget it when defining the magnets • Most losses are expected at injection energy. • Collimation system very dependent on the energy • Two stage collimation is necessary at all energies • Collimation system needs to be integrated from the beginning but it is feasible • More difficult to implement in an old machine • A lot to learn from LHC specially for 1 TeV • Either the beam defines the collimation system or the collimation system will define the beam!!

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