1. Failure modes & Beam loss studies in ILC Bunch compressor and main linac Arun Saini Fermi National Accelerator Laboratory ALCW’15, Tsukuba, KEK, Japan, 20-25 th April,

2. Outline INTRODUCTION FAILURE MODES –Bunch Compressors –Main Linac CONSEQUENCES of FAILURE –Single Bunch Damage –Average Beam Losses MITIGATION and MINIMIZATION STRATEGY SUMMARY LCWS'15, Tsukuba, 04/2015 Arun Saini 2

3. Damping Rings Polarised electron source Polarised positron source Ring to Main Linac (RTML) e- Main Linac (incl. bunch compressor Beam Delivery System (BDS) & physics detectors e+ Main Linac (incl. bunch compressor) Beam dump ILC schematics N. Walker ILC PAC TDR review 3 Total length (500 GeV) 30.5 km SCRF ML (RTML) 22.2 km RTML_BC 2.8 km Positron source 1.1 km BDS / IR 4.5 km Damping Rings 3.2 km RTML+ML+BDS ~ 150/300 bunches in a time LCWS'15, Tsukuba, 04/2015 Arun Saini

4. ILC: Basic Parameters LCWS'15, Tsukuba, 04/2015 Arun Saini 4 TDR Parameter Lists

5. Motivation Dealing with high average beam power and smaller beam size ( < 100  m) results in stringent tolerances on beam losses. Extensive studies are required to investigate each possible scenario that may lead to beam losses. A better understanding of beam losses allows –To quantify them in terms of their severity. –To design a better Machine Protection system (MPS). –To protect the beam line elements from sever damage. –Overall cost of facility. LCWS'15, Tsukuba, 04/2015 Arun Saini 5 Thickness of wall separating service and operational facility is determined by maximum beam losses in tunnel

6. Beam Losses Initiated by Failure Continuous operation of linac in pulse mode put stringent tolerances on beam line elements that increases possibility of failure during operation. Failures can be categorized as: –Temporary Failure: Recoverable using appropriate methods. Fast Failure: happens usually at the time scale of micro seconds, such as RF and phase fluctuation in cavities. –Impacts: among the bunches in a pulse. Slow Failure: happens usually at the time scale of millisecond such as, quenching of superconducting cavities and magnets, power trip, vacuum break-down. –Impact: Among the pulses –Permanent Failure: Non-recoverable, need to replace element, such as failure of RF cavity due to breaking of auxiliary components (power coupler, tuner etc..), breaking of magnet coil. –Impact: All pulses LCWS'15, Tsukuba, 04/2015 Arun Saini 6

7. Outline INTRODUCTION FAILURE MODE –Bunch Compressors Timing System Failure: RF Phase errors Transverse Mismatch of Input Beam Quad Failure Cavity Failure –Main Linac CONSEQUENCES of FAILURE –Single Bunch Damage –Average Beam Losses Mitigation and Minimization Strategy Summary LCWS'15, Tsukuba, 04/2015 Arun Saini 7

8. Bunch Compressor LCWS'15, Tsukuba, 04/2015 Arun Saini 8 2 stage BC TDR design is used for this studies. Bunch compressor Lattice: 3 CM’s with quads in first stage of Bunch compressor ( BC1). 16 RF units in BC2 RF (48 CM’s; 416 cavities) to reduce gradient. Parameter optimization of BC wigglers (S. Seletskiy) New output parameters from DR is used. New treaty point from RTML to ML S. Seletskiy, A.Vivoli Longitudinal phase space at the end of bunch compressor for nominal operatiing mode (f rep = 5 Hz, E cm = 500 GeV).

9. A Note :Sector Magnet in Bunch Compressor LCWS'15, Tsukuba, 04/2015 Arun Saini 9 Sector Magnet Dimension Sector Magnet Schematic In present lattice files, horizontal and Vertical aperture are 25.4 mm that causes huge losses even for nominal operation. Large aperture of 460 mm is used for failure mode studies and a collimator with correct aperture ( 460mm, 25.4 mm) is placed at both ends of magnet.

10. Aperture limitation in Bunch Compressor LCWS'15, Tsukuba, 04/2015 Arun Saini 10 ApertureCavityQuad 1Quad 2SBENCollimator x (mm)3937.5102305 y (mm)3937.51012.75 Horizontal planeVertical plane List of half aperture of some elements in Bunch compressor

11. Failure of timing system: RF Phase Errors Master clock provides the reference timing signal. Offset or failure of this system results in timing error between beam arrival and RF fields in cavity. Failure of LLRF phase amplitude control system leads to coherent phase shift. In order to understand sensitivity of Bunch compressor against phase errors, studies are performed for BC1 and BC2. LCWS'15, Tsukuba, 04/2015 Arun Saini 11 Nominal Parameters in Bunch Compressor Initial beamBC1 parameters Beam after BC1BC2 parameters Final beam dp/p, % σ z, mm E, GeV Grd/-φ, MeV/ deg R 56, mm dp/p, % σ z, mm E, GeV Grd/-φ, MeV/ deg R 56, mm dp/p, % σ z, mm E, GeV 0.116518.67/ -115 3721.420..894.8025.48/ -24.0 551.10.3214.83

12. RF Phase Acceptance in BC1 LCWS'15, Tsukuba, 04/2015 Arun Saini 12 BC1 BC2 Phase scanning in BC1 Phase Acceptance Procedure in BC1: RF Phase scan is performed for all cavities in BC1. Cavities in BC2 are operated at nominal RF phase. Beam is tracked up to end of BC2. Scanning is performed for the range of -180 0 to 180 0. All elements are aligned perfectly. Beam losses are observed for the phase range of -90 0 to 120 0. Beam in Beam out

13. Beam Distribution at BC2 exit Transverse Beam SizeLongitudinal Phase Space LCWS'15, Tsukuba, 04/2015 Arun Saini 13 Beam distribution at the end of BC2 for the case when all cavities in BC1 are operated at phase 0 0. Particles energy vary from 2 GeV to 14 GeV and bunch length 60 mm to 10 mm. Transverse size is also large and most number of losses are in horizontal direction. Hollow beam implies low energy density.

14. RF Phase acceptance in BC2 LCWS'15, Tsukuba, 04/2015 Arun Saini 14 RF Phase scanning is performed for the range of -115 0 to 115 0 for all cavities in BC2. BC1 cavities are operated at nominal RF Phase.

15. Collective RF Phase Acceptance of BC LCWS'15, Tsukuba, 04/2015 Arun Saini 15 Nominal Regime Phase Acceptance of BC Collective Acceptance of BC : RF Phase are varied uniformly in all cavities in Bunch compressor for the range of -90 0 to 90 0. All elements are aligned perfectly. Beam losses are observed when cavities are offset by 50 0 from it nominal phases. Result confirms that BC is robust enough to see phase variation of -90 0 to 40 0

16. Initial Transverse mismatch of Beam LCWS'15, Tsukuba, 04/2015 Arun Saini 16 What will happen if Beam is mismatched at entrance of Bunch compressor ? No Beal Losses are observed. Vertical emittance is increased almost linearly with mismatch

17. Quadrupole Failure Potential Reasons of Quad failure are : –Breaking of magnetic coils. –Power supply failure –Quenching of superconducting quads. Failure of quadrupoles alters the transverse focusing period, resulting in a mismatch of beam with subsequent sections. –It may lead to significant beam losses Quadrupole Failure is studied in BC1 and BC2 separately for three cases –Nominal Operation with fail quads –With Misalignments –After applying one to one correction algorithm. Set up of study: –Location of Failure is randomized. –50 machine, 10k macro particles are used. LCWS'15, Tsukuba, 04/2015 Arun Saini 17

18. Quadrupole Failure in BC1 LCWS'15, Tsukuba, 04/2015 Arun Saini 18 Case study: 5 quads are failed in BC1 Most of seed show very high beam losses. What will happen if all linacs are misaligned ?

19. Alignment tolerances in ILC Linac Error SourceRMS errorUnit Cavity misalignment300 mm Quadrupole Misalignment300 mm Quadrupole Tilt300  rad Cavity Pitch/Tilt300  rad Cryomodule Offset200 mm Cryomodule Tilt35 mm BPM offset300 mm BPM Resolution1 mm LCWS'15, Tsukuba, 04/2015 Arun Saini 19 (a)Longitudinal (b)Pure Transverse (c)Pitch/ Tilt. Misalignment is applied only in ‘Y’ plane. Same magnitude is applied for warm and cold components.

20. Quadrupole Failure in BC1 LCWS'15, Tsukuba, 04/2015 Arun Saini 20 Case study: 5 quads are failed in BC1 After misalignment inclusion Most of seed show 100 % beam losses. What is energy range and where do they get lost in beam line ?

21. Losses Distribution Location of lost particles Energy of lost particles LCWS'15, Tsukuba, 04/2015 Arun Saini 21 Case study: 5 quads are failed in BC1 After misalignment Several seeds shows most concerned single bunch loss scenario All particles in bunch are lost at same location. Which is limiting aperture, Horizontal or Vertical ?

22. Quadrupole Failure in BC1 Horizontal planeVertical plane LCWS'15, Tsukuba, 04/2015 Arun Saini 22 Case study: 5 quads are failed in BC1 After inclusion of misalignment Most of the Beam losses in this case occurs in BC1 and BC2 wiggler section.

23. Quadrupole failure in BC1 LCWS'15, Tsukuba, 04/2015 Arun Saini 23 Case study:5 quads are failed in BC1 Beam Losses Distribution Average Beam Losses are ~32 % 90 % Beam Losses are 81 %

24. Quadrupole Failure in BC1 LCWS'15, Tsukuba, 04/2015 Arun Saini 24 Failure of one quad alone does not lead to beam losses. Failure in misaligned linac causes significant beam losses. One to one steering helps to reduce beam losses but becomes less effective if number of failure incidents are large. Average Losses versus Quads Failure 90 % Beam Losses versus quad failure

25. Quadrupole Failure in BC2 LCWS'15, Tsukuba, 04/2015 Arun Saini 25 Failure of one quad alone does not lead to beam losses. Failure in misaligned linac causes significant beam losses. One to one steering helps to reduce beam losses but becomes less effective if number of failure incidents are large. Average Losses versus Quads Failure 90 % Beam Losses versus quad failure

26. Cavity Failure Potential Source that affects cavity functionality are –High Level RF: Trip in klystron / modulator/ drive amplifier (fast) Timing/phase synchronization system failed –Low Level RF: Phase / amplitude control failed, coherent phase shift (worst case) –Failure of auxiliary components such as couplers, Tuner etc. – Quench (~0.5ms) – Field Emission/Dark current. Cavity Failure results in energy mismatch with subsequent sections. LCWS'15, Tsukuba, 04/2015 Arun Saini 26

27. Cavity failure in BC1 LCWS'15, Tsukuba, 04/2015 Arun Saini 27 Case Study: 15 cavities are failed in BC1 Only Failure of Cavities in BC1 does not result in huge losses

28. Cavity Failure in BC1 Average Losses 90% Losses LCWS'15, Tsukuba, 04/2015 Arun Saini 28 Beam losses increases with number of failed cavity in presence of misalignments. One to One steering brings the losses down to negligible.

29. Cavity Failure in BC2 Average Losses 90% Beam Losses LCWS'15, Tsukuba, 04/2015 Arun Saini 29 Beam losses increases with number of failed cavity in presence of misalignments. One to One steering brings the losses down to negligible.

30. Outline INTRODUCTION FAILURE MODE –Bunch Compressors – Main Linac Quadrupole Failure Cavity Failure CONSEQUENCES of FAILURE –Single Bunch Damage –Average Beam Losses Mitigation and Minimization Strategy Summary LCWS'15, Tsukuba, 04/2015 Arun Saini 30

31. ILC Main Linac Main Linac is composed with 285 RF Units One RF Unit in Main includes 3 Cryomodule One Quadrupoles per RF unit Acceleration Range is 15 – 250 GeV Same approach is used for Failure studies in main linac. –Perfectly aligned linac –Misaligned linac In y plane only –Beam Bases correction : One to one steering. LCWS'15, Tsukuba, 04/2015 Arun Saini 31

32. Quad Failure in Main Linac Average losses 90% Beam Losses LCWS'15, Tsukuba, 04/2015 Arun Saini 32 Failure of quads with misaligned components results in significant beam losses. One to one steering correction can deal with only 5 quads failure.

33. Quad Failure in Main Linac LCWS'15, Tsukuba, 04/2015 Arun Saini 33 Case study:18 quads failure Lost Particles Energy Lost Particles Locations Losses are distributed all over linacs Particle energy varies from 50 GeV to 250 GeV

34. Quad failure in Main Linac LCWS'15, Tsukuba, 04/2015 Arun Saini 34 Case study:18 quads failure Vertical Trajectory for all seeds Failure of quads result in transverse beam oscillation with large amplitude. It helps to reduce beam density at impact location. Charge density is about three order smaller (~ 1e-03) than critical charge density.

35. Cavity failure in Main Linac LCWS'15, Tsukuba, 04/2015 Arun Saini 35 Main linac is robust enough to deal with significant number of cavity failure. No Beam losses are observed even after inclusion of component misalignments Linear decrease in average beam energy with cavity failure.

36. Outline INTRODUCTION FAILURE MODE –Bunch Compressors –Main Linac CONSEQUENCES of FAILURE –Single Bunch Damage –Average Beam Losses Mitigation and Minimization Strategy Summary LCWS'15, Tsukuba, 04/2015 Arun Saini 36

37. Consequences of Beam Losses LCWS'15, Tsukuba, 04/2015 Arun Saini 37 Single Bunch Losses: Loss of a bunch or complete train to particular element in beam line. These are most concerned case as : –They can do severe thermal damage especially in high energy section where beam power density is relatively higher. Critical beam area is ~ 50  m 2 for bunch charge of 3.2 nC. –Some cases of failure modes exhibits single bunch losses scenario. Average Beam Losses: Fractional losses of several bunches over a period of time comes under this category. These are crucial in terms of –Radiation level, environment concerns, hand on maintenances –life span and protection of beam line components. –Additional heating load to cryogenic system. Beam Losses Classification:

38. Single Bunch Damage Critical density ~1pC/µm 2 ~10 13 /mm 2. (CLIC use ~0.4 pC/µm 2 for Cu). Grazing incidence ( ∼ 1 mrad) will help. Is the beam size of missteered bunch is small enough damage cavity wall? At what accident scenario it is possible? SLC: single bunch damage in 1.4mm Cu (courtesy M.Ross) Yield Temp: LCWS'15, Tsukuba, 04/2015

39. Bunch Charge Density in Main Linac LCWS'15, Tsukuba, 04/2015 Arun Saini 39 Bunch Charge Density is higher than critical density in main linac. Single Bunch losses may result in significant damage in main linac.

40. Strategy : Mitigation of single bunch losses LCWS'15, Tsukuba, 04/2015 Arun Saini 40 Checking the preparedness of system before extracting pulses from damping ring. –Check Fields levels and availability of components Pilot Bunch leading nominal bunch train: The Pilot bunch must traverse the machine properly before nominal train is allowed to pass otherwise abort system is triggered to abort the beam. 10  sec ahead of train with 1% of nominal bunch charge( 3.2nC). – Drawback : BPM’s must have resolution and systematic offsets not more than 10 times worse at the low end of the intensity range 2e9 ppb to 2e10 ppb. (Availability and failed reading rates must also be very good). »Simulation can be performed to understand BPM Resolution sensitivity with bunch intensity in order to determine threshold. – Ignoring single bunch intensity dependent effects such as wake fields. Nominal Bunch charge with larger emittance. – More reliable measurement and reduced impact of damage due to large emittance. Dedicated abort system: –Composed with spoiler/collimator/absorber block –Provides flexibility to dump the beam if anything happens after its extraction form damping ring.

41. Strategy : Minimization of Average Beam Losses Potential source of average beam losses are: –Spread in design parameters due to rapid fluctuation of field levels. –Halo particles –Dark Currents Detection of Average beam losses is performed using: –Radiation, thermal and beam intensity sensor Average beam losses can be controlled using: –Controlling the rapid changes in fields of some critical components. –Applying appropriate beam based correction and feed back mechanism. Machine protection system should be capable to stop the operation if average losses exceeds the threshold limits. LCWS'15, Tsukuba, 04/2015 Arun Saini 41 LCLS-II DOE Status Review, Sept. 30 - Oct. 2, 2014 Potential high dose to components, e.g. quads @10 nA (reqs 50 times lower)  (courtesy N. Solyak)

42. Summary Failure modes studies are performed for Bunch Compressor and Main Linac. Quads failure result in significant beam losses both in BC and main linac. –Beam charge density is below than critical bunch charge density at impact time Bunch compressor and Main Linac is not as sensitive to cavities failure as they are to quad failures. Understanding of these losses scenario with material interaction codes such as FLUKA, MARS and estimation of radiation level in order to precise estimation of shielding wall thickness. LCWS'15, Tsukuba, 04/2015 Arun Saini 42 Next Step

43. Thank you LCWS'15, Tsukuba, 04/2015 Arun Saini 43