Beam Loss Issues of ERL Accelerators

1. CY Yao, L. Emery, M. borland, A. Xiao Advanced Photon Source Beam Loss Issues of ERL Accelerators

2. Acknowledgement Thanks Rod Gerig and Efim Gluskin for their many suggestions and support.

3. Introduction • Basic parameters of proposed APS ERL upgrade • Beam energy: 7 GeV • Max. beam current: 100 mA • Injector energy: 10 MeV • Spent beam energy: 10 MeV • Total number of SRF cavities: 350 • RF frequency: 1.3 Ghz • Micro bunch length: 2 ps • Beam emittance: 0.022 μm-rad • Number of passes: 2 Figure 1: Layout of Proposed APS-ERL upgrade.

4. Table1: Operation modes* * Numbers scaled from G Hoffstaetter’s talk at 06 ERL workshop

5. Consequence of beam loss: Radiation hazards The APS-ERL has 1 MW of beam power of injector beam. Although the linac is power limited, even without energy recovery it can still generate 2.5 MW of beam power. A small fraction of beam loss presents a high radiation hazards and must be controlled. Current APS safety envelope is 308 W, or beam loss rate of 44 nA. This is only allowed to last 1 hour (most credible incident). Argonne/DOE requires below 500 mrem/year for controlled area and 100 mrem/year for uncontrolled area. Current APS beam loss during top-up operations: 21 pA. The radiation dose level is measured below 100 mrem/year, which allows to Re-designate the experimental hall as uncontrolled area. We need to control beam loss in APS and TAA to similar level. It is a challenge.

6. Consequences of beam loss: equipment damage and activation Radiation from beam loss damages undulators Direct beam hit can damage frond-end devices and vacuum chambers. Heat deposit on the SRF cavities can cause quenching and operation downtime. Radiation activates accelerator parts that may impact hands on maintenance. This probably is not a big problem for electron machine.

7. Consequence of beam loss: increase both equipment and operations cost Increase the need for better shielding---high construction budget. Increase the cooling requirement and the cost of the cryogenic system of SRF cavities. A 10 W/cavity heat load requires 3.5 kW of additional cooling capacity. Additional cost: ~$35M. Increase the operational power and cost of the cryogenic system. Total heat load of 3.5 kW at 2ºK requires estimated of 3.5 MW of wall power. Additional operation cost: $1.75M/year. In order to maintain this level of heat load, beam loss at any single point of the SRF linac must be < ~10 nA.

8. Beam loss mechanisms Beam halo formation. Gas scattering. Intra-beam scattering. Emittance growth CSR and ISR effect Wake field and other instability Beam instability Beam break up (BBU) in the linac. Full or partial beam loss due to incidents: Radiation protection. Machine protection. Beam dump due to failure of equipment.

9. Beam Halo Beam halo can be formed in many parts of the accelerator structure. Dark current of the electron gun. Stray laser light can produce dark current in a photo-cathode gun. Space charge effect . Non-linearity of lattice. Field emission at the SRF cavities. Mismatch of the beam transport. Scattered particles that are not lost but form beam halo.

10. Beam halo related R & D work Low dark current low emittance gun development. Field emission study in high gradient superconducting RF cavities. Computer simulation of halo formation in electron gun and periodic focusing beam transport system. Development of beam diagnostics for halo characterization. Study halo formation process with existing APS linac and guns. Development of collimation configurations to eliminate halo particles in the early stage, such as combination of beta function collimation.

11. Gas Scattering Beam loss due to gas scattering is a concern for any accelerator. Beam can be scattered and lost both in energy aperture and transverse aperture. Beam energy, vacuum pressure, gas composition, longitudinal and transverse acceptance are the main factors. At the APS and TAA areas the conditions for gas scattering are similar to current APS. Gas scattering in the current APS ring is very small compared to intra-beam scattering. We don't think gas scattering is a serious problem for APS-ERL. R &D work: Optimize lattice design that maximizes both transverse and energy acceptance in APS and TAA area. Develop simulation tools to include the injector and linac. Assess the effect of gas scattering on the beam emittance and heat load on the SRF cavities of linac.

12. Intra-beam Scattering lntra-beam scattering is the main cause of beam loss of APS storage ring during normal operations. Will this be worse for the ERL? Preliminary estimates with elegant simulation shows that the highest loss rate is ~50 nA for the high flux mode. Further study is need: Lattice optimization. Upgrade elegant to simulate with acceleration. Find detailed distribution of the beam loss around the facility. Assess the impact on beam emittance, the linac SRF cavities and radiation safety.

13. Energy Aperture Optimization Intra-beam/Touschek scattering beam loss rate depends strongly on machine energy aperture. A Method was developed to directly minimize beam loss while varying sextupole settings. Energy aperture for the APS can be increased to 5% with this method, which can reduce beam loss substantially. Need to consider the impact on other parts of the machine: High energy aperture at APS and TAA may increases beam loss in the linac. Need more realistic simulation to include such factors as orbit errors in the sextupoles, acceleration, etc.

14. Collimation strategies Collimation has been applied successfully to many high energy accelerators. It removes halo particles and protects downstream equipment. Locations of the collimators are determined by: location of beam halo source. equipment that needs protection. Lattice function (betatron collimation, energy collimation ). For APS ERL the main halo source is the injector, at the merger and the end of energy recovery. The areas that need protection are APS, TAA beamline areas and SRF cavities of the linac.

15. A possible collimation scheme • Preliminary simulation with EGS4 indicates that a 10 cm lead collimator located at the entry of linac can reduce the energy deposit on the downstream linac structure to 9%. • R&D work: • More realistic Monte Carlo simulation, possibly coupled with a tracking program such as elegant. Figure 2: a possible collimation scheme. *courtesy of L. Emery

16. Beam Abort System A beam abort system is needed to protect the APS and TAA areas and the linac SRF system. Loss of stored beam: At APS storage ring 100 mA beam is safely dumped by simply shutting off RF power. The APS-ERL has the same stored energy per length of accelerator as APS now. Not a problem for radiation safety. For the SRF cavity the estimated heat deposit: ~6 J/cavity. Injector beam: Beam power: up to 2.5 MW. Additional heat deposit ~5 J/cavity, assuming a 1 ms abort system reaction time. The beam loss can further reduced by combination of kickers and beam dumps.

17. Beam Loss Monitoring • For radiation protection, the commercially available Gamma and Neutron monitors are adequate. • For the protection of SRF cavities of the linac: • 500 μs detection time. • Sensitivity < 10 nA of beam loss. • Ion chambers, PMT based detectors or Cerekov detectors. • Preliminary simulation indicates installing a monitor every 5 meters along the linac is sufficient. • Another possibility is to directly measure beam current. But can it meet the required sensitivity and reliability?

18. A proposed abort system • One kicker is located at the linac entrance to deflect the injector beam to a beam dump. • The second kicker is located before TAA area, which directs the accelerating beam to the spent beam dump. • The third kicker is located at the entrance of APS area. • Kicker requirement: • strength: 1 mRad • rise time: 100 to 200 ns. Figure 3:A proposed abort/dump plan. *Courtesy of L. Emery

19. Shielding Consideration In the APS and TAA beamline area: The current shielding with some modification is adequate for incidental beam loss. Option of shielding improvement should be considered if the continuous beam loss can not be brought down to satisfactory level with lattice design and collimation. Linac and injector tunnel: Enhanced shielding is required to handle the high beam power. Extra shielding is required for some local areas such as beam dumps and collimators. R & D work: Monte Carlo simulation with EGS4, MARS or other programs.

20. Conclusion Beam loss of the proposed APS-ERL upgrade presents a challenge for accelerator design. Radiation dose in the APS/TAA area and the energy deposit on the linac SRF cavities due to continuous beam loss are the two main concerns. R&D work should be carried out in these areas: Research and understanding the various beam loss mechanisms in an ERL environment. Optimize lattice design for both high performance and low beam loss rate. Development effective collimation configuration. Development of a fast beam abort system.

21. References [1] M. Borland, “Optimization of ERL Energy and Undulator Parameters,” OAG-TN-2007-021, http://www.aps4.anl.gov/operations/ops_www/APSOnly/oagTechnicalReports.shtml. [2] G. Hoffstaetter, “Status of the Cornell ERL Project,” FLS 2006 Workshop, working group2, http://adweb.desy.de/mpy/FLS2006/proceedings/HTML/SESSION.HTM. [3] “Advanced Photon Source Safety Assessment Document,” APS-3.1.2.1.0, June 1996. [4] A. Nassiri, “ERL cost update,” APS upgrade presentations, http://www.aps4.anl.gov/operations/ops_www/APSOnly/APS_Upgrade.html. [5] C. Chen, “Halo Formation in Intense Linacs,” Proc. Of LINAC1998, P. 729-733, 1998. [6] Y. Shimosaki, K. Takayama, “Nonlinear-resonance Analysis of Halo-Formation Excited By Beam-Core Oscillation,” Proc. of EPAC 2000, Vienna, Austria, P. 1330-1332, 2000. [7] A. Xiao, “Estimate of Beam Loss Rate from Touschek Effect for APS-ERL Lattice,” OAG-TN-2006-048, http://www.aps4.anl.gov/operations/ops_www/APSOnly/oagTechnicalReports.shtml. [8] L. Emery, “Beam Simulation and Radiation Dose Calculation at the Advanced Photon Source with shower, an Interface Program to the EGS4 Code System,” Proc. of PAC 1995, P. 2309-2311.