Halo Collimation of Protons and Heavy Ions in SIS-100

Beam halo and beam losses  Various processes related to the beam dynamics cause particles to enter into unstable orbits with large betatron amplitudes, causing beam halo formation.  Beam halo is one of the reason for uncontrolled losses of the beam.  The characteristics of beam halo depends on the mechanism of halo production. Some sources of halo: space charge force mismatched beam nonlinear forces RF noise scattering (intra beam, elastic and inelastic scattering, electron clouds) instabilities and resonances [1] K. Wittenburg, CERN Accelerator School: Course on Beam Diagnostics.

Need for halo collimation The halo particles and consequent beam losses cause: Desorption of the molecules from the beam pipe and consequently the vacuum degradation ("beam loss" induced vacuum degradation). Superconducting magnets quenches. Activation of the accelerator structure. Background in experiments. Radiation damage of the equipment and devices. The purpose of the 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 cost reason.

Need for halo collimation in SIS 100  Desorption of the molecules from the beam pipe and the vacuum degradation ("beam loss" induced vacuum degradation). - Essential for heavy ions.  Superconducting magnets quenches. - The power of the beam and consequently the losses are likely to be low for quenches.  Activation of the accelerator structure. - Important 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 will be scraped after extraction.  Radiation damage of the equipment and devices. - Crucial for heavy ions, but experiments showed also the damage of insulators irradiated by protons.

Two stage collimation system for protons - Primary collimator (thin foil) acts as a scatterer of the halo particles. - Secondary collimators (bulky blocks) are necessary to absorb the scattered particles - secondary halo 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 Vol. 1. (1998). [4] K. Yamamoto, Efficiency simulations for the beam collimation system of the JPARC, PhysRewST AB Vol. 11. (2008).

Normalized betatron oscillation amplitudes Optimal phase advances: 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 d PC, d SC1, - transverse positions of the primary and secondary collimator  – retraction distance n PC, n SC1, - normalized apertures of the primary and secondary collimator

Optimal position of the collimators [2] M. Seidel, The Proton Collimation System of HERA, DESY Report Required scattering angle at the primary collimator for a hit the secondary collimator as a function of the phase difference between the primary and the secondary collimators. Two curves belong to different retraction distances  of the secondary collimators (inner curve  = 0.07, outer curve  = 0.14).

Scattered particles in phase space [2] M. Seidel, The Proton Collimation System of HERA, DESY Report [3] J. B. Jeanneret, Optics of a Two-Stage Collimation System, PhysRewST AB Vol. 1. (1998). The scattered protons populate a very selected region of phase space, namely a straight line. Protons can only reach the primary collimator if they pass it with maximum amplitude.

x, y and x’, y’ distribution after scattering  The scattering process adds an arbitrary value only to x' and y' but NOT to x and y.  Halo particle hit the primary collimator perpendicular to the surface at maximum amplitude. Simulation of the scattering process of 2 GeV protons on 1 mm thick tantalum foil. Simulation codes: FLUKA and SRIM Statistics: 10 6 particles

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

Proton collimation for SIS-100 SIS-100 lattice, sector 1, straight section SIS-100 → SIS-300 transfer. SIS-100 lattice, sector 1, arc section.

SIS-100 lattice SIS-100 lattice, sector 1. Collimation efficiency for various positions of the primary collimator. Positions of the primary collimator Transverse acceptance (h/v) [mm·mrad] SIS-100 latticePrimary collimatorSecondary collimators 203/53150/37182/45

Phase space plot at the primary collimator Primary collimator Scattering at the primary collimator: FLUKA. (2 GeV protons on 1 mm thick tantalum foil) Transport of the scattered particles: MAD-X.

Phase space plots at the secondary collimators 1 st secondary collimator 2 nd secondary collimator Collimation efficiency = 61%.

Efficiency of the collimation

Parameters affecting the collimation efficiency 1)Position of the collimators in the lattice. 2)Phase advance between the primary and the secondary collimators. 3)Normalized apertures of the primary and secondary collimator. 4)Scattering on the primary collimator. 5)Transverse acceptance of the SIS )Number of the secondary collimators. The collimation efficiency is affected by:

Collimation system of the JPARC RCS [5] M. Yamamoto, Efficiency simulations for the beam collimation system of JPARC RCS, PhyRew ST AB, Vol 11, (2008). (a) primary collimator (b) 1 st secondary collimator (c) 3 rd secondary collimator (d) 5 th secondary collimator primary collimator – 1mm thick tungsten foil five secondary collimators – 20 cm thick copper blocks Simulations with STRUCT Code

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: U 28+ Injection energy: 200 MeV/u Consider charge exchange by only a few charge state. Fully ionized U 92+ ions end in the middle of the dipole.

Most of the U 92+ would end at the warm quadrupole doublet and collimator in the slow extraction area of the SIS100. Fraction of losses in the Slow extraction area of the SIS100 Thick foil placed in the slow-extraction area for U 28+ → U 92+. [6] A. Smolyakov et al., Radiation Damage Studies for the Slow Extraction from SIS100, EPAC08. Halo collimation of heavy ions in SIS-100

Very thin foil for U 28+ → U 29+. U 29+ would be absorbed by the collimators designed in the SIS-100 lattice for catching of the beam particles (U 28+ ) which interact with the residual gas molecules and lost one electron. Collimators are placed in the arc sections of the SIS-100 lattice. For a good mechanical stability use a foil having the knife-blade shape. Halo collimation of heavy ions in SIS-100

Summary  Proton collimation Appropriate positions of the collimators in the lattice. Parameters of the primary collimator and secondary collimators (size, material…). Angular distribution of the halo particles at primary collimator (FLUKA, SRIM). Transport of the scattered particles (MADX). Maximum efficiency of the collimation system.  Heavy-ion collimation Appropriate type of the collimator for heavy ions. Experiment.

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