1. Halo Collimation of Protons and Heavy Ions in SIS-100
I. Strašík1,2, E. Mustafin1
1GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, Germany
2Johann Wolfgang Goethe Universität, Frankfurt am Main, Germany

2. Beam halo and beam losses
Various processes related to the beam dynamics can cause particles to enter into unstable orbits with large betatron amplitudes which leads to the beam halo formation.
Beam halo is one of the reason for uncontrolled losses of the beam.
The characteristics of beam halo depend on the mechanism of halo production. Main sources of halo are:
• space charge force,
• mismatched beam,
• nonlinear forces,
• RF noise,
• magnet errors,
• scattering (intra beam, residual gas, stripping foil, screens),
• instabilities and resonances,
• electron clouds.
[1] K. Wittenburg, CERN Accelerator School: Course on Beam Diagnostics.

3. Need for halo collimation
The halo particles and subsequent beam losses can cause:
Vacuum degradation due to desorption of the molecules from the beam-pipe wall.
Superconducting magnets quenches.
Activation of the accelerator structure.
Background in experiments.
Radiation damage and heating of the equipment and devices.
The purpose of the halo-collimation system:
To remove the halo and reduce above mentioned problems.
To provide a well defined storing location for beam losses.
In addition...
Reduced halo → small beam size → minimal geometrical aperture.
Keeping the aperture of the magnets small for a cost reason.
[1] K. Wittenburg, CERN Accelerator School: Course on Beam Diagnostics.

4. Halo collimation in SIS 100
Vacuum degradation due to desorption process.
Essential issue for heavy ions.
Superconducting magnets quenches.
The power of the beam (tenths of kW) and consequently the losses are likely to be low for quenches.
Activation of the accelerator structure.
Important issue for protons, beam power ~ 50 kW, uncontrolled losses (1%) ~ 500 W, along the whole circumference of the SIS100 ~ 0.5 W/m
("hands-on" maintenance limit ~ 1 W/m).
Background in experiments.
Halo particles are supposed to be scraped after extraction.
Radiation damage of the equipment and devices.
Crucial issue for heavy ions, although experiments showed also the damage of insulators irradiated by protons.

5. Two-stage collimation system
- Primary collimator (thin foil) acts as a scatterer of the halo particles.
Secondary collimators (bulky blocks) are necessary to absorb the scattered particles.
- Particles have small impact parameter on primary collimator.
By scattering with the primary collimator, the impact parameter at the secondary is
enlarged → leak of the particles from secondary the collimator is reduced.
[2] M. Seidel, The Proton Collimation System of HERA, DESY Report.
[3] J. B. Jeanneret, Optics of a Two-Stage Collimation System, PhysRewST AB.
[4] K. Yamamoto, Efficiency simulations for the beam collimation system of the JPARC, PhysRewST AB.

6. Normalized betatron oscillation amplitudes
Optimal phase advances:
nP, nS, - normalized apertures of the primary and the secondary collimators
dP, dS, - transverse positions of the primary and the secondary collimators
d – retraction distance
Protons which are scattered at the primary collimator suffer an angular deflection.
To remove scattered protons two collimators are needed:
1. particles scattered outwards from the beam centre.
2. particles scattered towards to the beam centre.
[2] M. Seidel, The Proton Collimation System of HERA, DESY Report.

7. Scattered particles in phase space
Halo particles can only reach the primary collimator if they pass it with maximum amplitude.
The particles hit the primary collimator perpendicular to the surface.
The scattered protons populate
a very selected region of phase space, namely a straight line.
[2] M. Seidel, The Proton Collimation System of HERA, DESY Report.
[3] J. B. Jeanneret, Optics of a Two-Stage Collimation System, PhysRewST AB.

8. x, y and x’, y’ distribution after scattering
The scattering process adds an arbitrary value only to x' and y' (angles) but not to x and y (positions).
Simulation of the scattering process of 2 GeV protons on 1 mm thick tantalum foil.
Simulation codes: FLUKA and SRIM
Statistics: 106 particles

9. Phase space plots at the collimators
outwards
The phase advances are close to the optimum values.
towards
 ~ 30°
 ~ 150°
collimation of the particles scattered towards to the beam centre
collimation of the particles scattered outwards from the beam centre
[2] M. Seidel, The Proton Collimation System of HERA, DESY Report.

10. Proton collimation in SIS-100
SIS-100 lattice, sector 1, straight section SIS-100 → SIS-300 transfer.
Transverse acceptance (horizontal/vertical) [mm·mrad]
SIS-100 lattice [5]
Primary collimator
Secondary collimators
203/53
150/37
182/45
[5] SIS-100 working group, FAIR Technical Design Report SIS-100, December 2008
Efficiency of the collimation system
Nt - total number of the particles scattered on the primary collimator
NS - number of the scattered particles intercepted by the secondary collimators

11. Parameters affecting the collimation efficiency
Phase advances between the primary and the secondary collimators.
Position of the collimation system in the SIS-100 lattice.
Retraction distance of the secondary collimators with respect to the primary one.
Number of the secondary collimators.
Scattering on the primary collimator.
Using a bent crystal as the primary collimator.
Transverse acceptance of the SIS-100 lattice.

12. Position of the collimation system
Scattering at the primary collimator: FLUKA.
(2 GeV protons on 1 mm thick tantalum foil)
Transport of the scattered particles: MAD-X.

13. Phase space plots at the collimators
P – primary collimator
S1 – 1st secondary collimator
S2 – 2nd secondary collimator

14. Retraction distance and number of the SC
Retraction distance of the secondary collimators
Normalized apertures ratio
Retraction distance
Collimation efficiency [%]
nP/nS = 0.9
d = 0.11 62
nP/nS = 0.95
d = 0.05 74
Number of the secondary collimators
(a) primary collimator
(b) 1st secondary collimator
(c) 3rd secondary collimator
(d) 5th secondary collimator
[4] M. Yamamoto, Efficiency simulations for the beam collimation system of JPARC RCS, PhyRew ST AB.

15. k = RMS for the thickness 1 mm
Scattering of the halo particles on the PC
RMS (1σ) of the angular distribution
2 GeV protons
k = RMS for the thickness 1 mm
RMS [mrad]
Thickness [mm] k
2.46
1.0
2k
4.92
3.5
3k
7.38
7.1
2 GeV protons scattered on the tantalum foil of different thickness simulated by FLUKA.
Collimation efficiency

16. Bent-crystal collimation
Crystal channelling
CERN, BNL, IHEP or FNAL
Advantage of using the crystal primary-collimator

17. Bent-crystal collimation
Efficiency of the channeling
[5] V.M. Biryukov, Crystal Channeling in Accelerators, EPAC2006.
Collimation efficiency with crystal primary collimator
1 GeV protons channeled through 1 mm thick Si crystal
Primary collimator
Collimation efficiency [%]
d = 0.11
d = 0.05
1 mm tantalum (conventional) 62 74
1 mm silicon (crystal) 79 87
[6] V.M. Biryukov, Possibility of Crystal Extraction and Collimation in the Sub-GeV Range, PhyRew ST AB.

18. Halo collimation of heavy ions in SIS-100
Charge exchange of the halo particles on the primary collimator and deflect to the
secondary collimator by a bending magnet.
Primary beam: U28+
Injection energy: 200 MeV/u
Fully ionized U92+ ions end in the middle of the dipole.
Consider charge exchange by only a few charge states.

19. Halo collimation of heavy ions in SIS-100
Thick foil placed in the slow-extraction area for U28+ → U92+.
Fraction of losses in the Slow extraction area of the SIS100
Most of the U92+ would end at the warm quadrupole doublet and collimator in the slow extraction area of the SIS100.
[6] A. Smolyakov et al., Radiation Damage Studies for the Slow Extraction from SIS100, EPAC08.

20. Halo collimation of heavy ions in SIS-100
Very thin foil for U28+ → U29+.
For a good mechanical stability to use a foil having the knife-blade shape.
U29+ can be intercepted by a combined collimation/pumping system which minimize the desorbed gas entering the beam pipe.
Collimators are placed in the arc sections of the SIS-100 lattice.
[7] J. Stadlmann et al., Collimation and Material Science Studies (ColMat) at GSI, IPAC’10.
[8] L. Bozyk et al., Development of a Cryocatcher Prototype for SIS100, IPAC’10.

21. Summary Proton collimation Heavy-ion collimation
Several halo collimation concepts were considered.
Dependence of the collimation efficiency on various parameters was studied.
Collimation efficiency > 90 % by conventional two-stage system → small retraction distance and strong scattering on the primary collimator.
Problems: interception by secondary collimators, hot spots generation, precision requirements.
Bent-crystal collimator.
Heavy-ion collimation
Crucial issue – charge exchange.
The most convenient is to use very thin foil and existing designed collimator/pumping system.
Experiment.

22. Thank you for your attention