1. Two beam instabilities in low emittance rings Lotta Mether, G.Rumolo, G.Iadarola, H.Bartosik Low Emittance Rings Workshop INFN-LNF, Frascati September 17 th, 2014

2. Two beam instabilities o Collective effects induced by electromagnetic field of second charge distribution, in addition to primary beam Beam-beam effects in colliders Electron cloud in positron machines Trapped ions in electron machines o Second “beam” may be produced by primary beam Synchrotron radiation on wall: electrons, ions Residual gas ionization: electrons, ions Desorption from wall due to losses: electrons, ions, neutrals o May cause instabilities, tune shift, emittance growth, beam and energy losses 17 September 2014 Low emittance rings 2014, L. Mether2

3. Outline o Introduction o Electron cloud in positron machines Electron cloud formation Effect on beam Observations Modelling o Ion effects in electron machines Trapping of ions Effect on beam Observations Modelling 17 September 2014 3LER2014, L. Mether

4. Electron cloud formation 1. Primary (seed) electrons are generated inside beam chamber 17 September 2014 Low emittance rings 2014, L. Mether4 Ionization of residual gas Photoelectrons from synchrotron radiation Desorption due to losses on wall

5. Electron cloud formation 1. Primary (seed) electrons are generated inside beam chamber 2. Seed electrons are accelerated by beam field, and may produce secondary electrons when hitting the wall 17 September 2014 Low emittance rings 2014, L. Mether5

6. Electron cloud formation 1. Primary (seed) electrons are generated inside beam chamber 2. Seed electrons are accelerated by beam field, and may produce secondary electrons when hitting the wall Under suitable conditions, avalanche electron multiplication (multipacting) occurs 17 September 2014 Low emittance rings 2014, L. Mether6

7. Electron cloud formation 1. Primary (seed) electrons are generated inside beam chamber 2. Seed electrons are accelerated by beam field, and produces secondary electrons when hitting the wall 3. Eventually a stationary state - the electron cloud - is reached, when space charge limits further growth of electron density 17 September 2014 Low emittance rings 2014, L. Mether7 CLIC-DR wiggler CLIC-DR quad

8. Electron cloud induced instability o The electron density gives rise to a single bunch head-tail instability, due to the beam focusing (pinching) the electron distribution If e.g. the head of the bunch is displaced, an asymmetric pinch will take place, resulting into a net kick felt by the bunch tail After several turns, the offset in head motion can be transferred to the tail After a sufficient number of turns, the unstable coherent motion has propagated to the whole bunch 17 September 2014 Low emittance rings 2014, L. Mether8

9. Electron cloud effects & observations o Beam degradation Coherent instability Single bunch, affecting the last bunches of a train Coupled bunch Beam size blow-up and emittance growth Tune shift along the bunch train Energy loss measured through synchronous phase shift o Machine observables Fast pressure rise, outgassing Additional heat load o Observed in several machines KEK-LER, Da  ne, CesrTA (see following presentation)… 17 September 2014 Low emittance rings 2014, L. Mether9

10. Electron cloud simulations o Several numerical codes for modelling electron cloud formation and/or instabilities exist o At CERN, partly coupled simulation tools for modelling electron cloud formation and instability PyECLOUD Macroparticle code for simulation of electron cloud build-up (Py)HEADTAIL Macroparticle code for simulation of single bunch instability, based on electron distribution from PyECLOUD 17 September 2014 Low emittance rings 2014, L. Mether10

11. Electron cloud simulations o Electron build-up and single bunch instability in CLIC-DR wigglers Electron cloud builds up for SEY > 1.4 Threshold electron density for instability ~ 1.2 x 10 13 / m 3 Emittance growth rate fast compared to damping times ~ 2 ms 17 September 2014 Low emittance rings 2014, L. Mether11 PyECLOUD rise time τ ≈ 0.7 ms τ ≈ 0.5 ms τ ≈ 0.4 ms Vertical emittance (Py)HEADTAIL

12. Maximum central density along train Electron cloud simulations o Electron build-up and single bunch instability in CLIC-DR wigglers Electron cloud builds up for SEY > 1.4 Threshold electron density for instability ≈ 1.2 x 10 13 / m 3  Beam is unstable for all SEY values above build-up threshold! 17 September 2014 Low emittance rings 2014, L. Mether12 PyECLOUD (Py)HEADTAIL

13. Outline o Introduction o Electron cloud in positron machines Electron cloud formation Effect on beam Observations Modelling o Ion effects in electron machines Trapping of ions Effect on beam Observations Modelling 17 September 2014 13LER2014, L. Mether

14. Fast beam ion instability mechanism 1. Generation of ions inside beam chamber Scattering / field ionization of residual gas 17 September 2014 Low emittance rings 2014, L. Mether14

15. Fast beam ion instability mechanism 1. Generation of ions inside beam chamber 2. Ions are accelerated by beam field, and possibly trapped, depending on ion mass 17 September 2014 Low emittance rings 2014, L. Mether15 T b = L sep /c

16. Fast beam ion instability mechanism 1. Generation of ions inside beam chamber 2. Ions are accelerated by beam field, and possibly trapped, depending on ion mass 17 September 2014 Low emittance rings 2014, L. Mether16 CO, N 2 H2OH2O H2H2 CLIC-DR

17. Fast beam ion instability mechanism 1. Generation of ions inside beam chamber 2. Ions are accelerated by beam field, and possibly trapped, depending on ion mass 3. Ions accumulate along bunch train, coupling train head and tail 17 September 2014 Low emittance rings 2014, L. Mether17

18. Fast beam ion instability mechanism 1. Generation of ions inside beam chamber 2. Ions are accelerated by beam field, and possibly trapped, depending on ion mass 3. Ions accumulate along bunch train, coupling train head and tail 4. Offset of each bunch is recorded into generated ion distribution, and transferred to the following bunches  coupled oscillations between electrons and ions 17 September 2014 Low emittance rings 2014, L. Mether18

19. Ion instability & observations o Ion trapping can be seen as Coherent multi-bunch instability Phase shift over bunch train Beam size blow-up & emittance growth o Observations in running machines usually under vacuum degradation During commissioning Caused by impedance heating Deliberately, with injected gas, for study purposes o Fast Beam Ion Instabilities have been observed in several machines APS (with He injection), PLS (with H2 injection) SOLEIL, SSRF, BESSY II, ELETTRA, ALBA … Measurements at CesrTA (Dec. 2013, Apr 2014) Varying ion species and pressure, bunch charge, train structure, feedback etc. 17 September 2014 Low emittance rings 2014, L. Mether19

20. Observations at CesrTA 17 September 2014 Low emittance rings 2014, L. Mether20 Vertical beam offset as function of bunch number at varying pressure, with vertical feedback on (dark blue) and off. From A. Chatterjee et al. IPAC 2014

21. Observations at CesrTA 17 September 2014 Low emittance rings 2014, L. Mether21 Vertical beam size as function of bunch number at varying pressure, with vertical feedback on (dark blue) and off. From A. Chatterjee et al. IPAC 2014

22. FBII Models o Analytical model Characteristic ion frequency Instability rise time Limitations: linear regime, assumes bunch train as uniform line charge, assumes ion distribution trapped within bunch distribution o Numerical model available (FASTION) Macroparticle simulation tool, including several ingredients Used for CLIC Main Linac, transfer lines, modified version used at CesrTA Optimization for CLIC-DR ongoing o Instability typically seems to be less severe than predictions, probably stabilizing effects not included in existing models? Quantitative comparison between theoretical predictions, simulations and measurements (CesrTA) in progress 17 September 2014 Low emittance rings 2014, L. Mether22 Raubenheimer et al. Phys. Rev. E 52, 5, 5487, Stupakov et al. Phys. Rev. E 52, 5, 5499

23. FASTION simulations for CesrTA o Simulations of vertical beam offset and beam size as function of bunch number at varying pressure, with vertical feedback on (dark blue) and off 17 September 2014 Low emittance rings 2014, L. Mether23 From A. Chatterjee et al. IPAC 2014

24. FASTION simulations for CesrTA o Comparison of vertical beam offset and beam size 17 September 2014 Low emittance rings 2014, L. Mether24 From A. Chatterjee et al. IPAC 2014 Simulations Measurements

25. FASTION simulations for CesrTA o Comparison of vertical beam offset and beam size 17 September 2014 Low emittance rings 2014, L. Mether25 From A. Chatterjee et al. IPAC 2014 Simulations Measurements

26. Summary & conclusions Two-beam effects are relevant for the performance of both running and future low-ε accelerators or damping rings o Electron cloud formation and instabilities Detailed models available for both processes Observed frequently in running machines  reliable estimates for future Ongoing research on techniques for mitigation or suppression (coating, clearing electrodes, scrubbing), to be applied to future machines o Ion accumulation and instabilities Theories developed  formulae typically used to predict behavior Detailed numerical model available  improvement & optimization ongoing Observations usually in presence of vacuum degradation Important for vacuum specifications of future low-ε electron machines  more sensitive to FBII  Low-gap chambers and high intensity short bunches  More outgassing May be controlled through feedback, but feedback can also be trigger 17 September 2014 Low emittance rings 2014, L. Mether26 For more details, see session “Two-Stream Instabilities” at TWIICE 2014

27. FASTION code development 17 September 2014 Low emittance rings 2014, L. Mether27

28. Electron cloud formation 1. Primary (seed) electrons are generated inside beam chamber 2. Seed electrons are accelerated by beam field 3. Produce secondary electrons when hitting the wall 4. Avalanche electron multiplication (multipacting) 5. Eventually a stationary state - the electron cloud - is reached, when space charge limits further growth of electron density 6. Electron cloud density may be very high around beam location 17 September 2014 Low emittance rings 2014, L. Mether28

29. Electron cloud formation Primary (seed) electrons are generated inside beam chamber 17 September 2014 Low emittance rings 2014, L. Mether29 Ionization of residual gas Photoelectrons from synchrotron radiation Desorption due to losses on wall

30. Electron cloud formation Primary (seed) electrons are generated inside beam chamber Seed electrons are accelerated by beam field 17 September 2014 Low emittance rings 2014, L. Mether30

31. Secondary electron production Electrons hitting the chamber wall at low energies are reflected at higher energies produce secondary 17 September 2014 Low emittance rings 2014, L. Mether31

32. Ion trapping by electron beam 1. Generation of ions inside beam chamber 2. Ions are accelerated by beam field, and possibly trapped, depending on ion mass 3. Trapped ions oscillate around beam with characteristic frequency 4. After the passage of several bunches, ion density may grow sufficiently to affect beam 17 September 2014 Low emittance rings 2014, L. Mether32

33. Ion trapping (Gaussian beam) 17 September 2014 Low emittance rings 2014, L. Mether33 Section i Section i+1 T b = L sep /c Ion of mass A

34. Ion trapping in CLIC Damping Ring 17 September 2014 Low emittance rings 2014, L. Mether34 CO, N 2 H2OH2O H2H2

35. Ion trapping (Gaussian beam) 17 September 2014 Low emittance rings 2014, L. Mether35 Section i Section i+1 T b = L sep /c Ion of mass A

36. Fast beam ion instability mechanism 1. Generation of ions inside beam chamber 2. Ions are accelerated by beam field, and possibly trapped, depending on ion mass 3. Ions accumulate along bunch train, coupling train head and tail 4. After several bunches, the ion distribution can affect the beam  coupled oscillations between electrons and ions additional phase shift over bunch train beam size blow-up & emittance growth 17 September 2014 Low emittance rings 2014, L. Mether36

37. Fast Beam Ion Instability o Ions accumulate along a train of bunches, coupling head and tail of train o Offset of each bunch is reflected in generated ion distribution, and thus transferred to the following bunches 17 September 2014 Low emittance rings 2014, L. Mether37

38. Fast Beam Ion Instability o Ions accumulate along a train of bunches, coupling head and tail of train o Offset of each bunch is reflected in generated ion distribution, and thus transferred to the following bunches 17 September 2014 Low emittance rings 2014, L. Mether38 Coherent multi-bunch instability Oscillation expected to be at a main frequency related to the ion oscillation frequency Tune shift towards end of bunch train

39. FBII observations o Signs of trapped ions / FBII Coherent beam instability: beam motion & blow up Additional phase shift over bunch train o Observations in running machines usually made in presence of vacuum degradation During commissioning Caused by impedance heating Deliberately, with injected gas, for study purposes o Fast Beam Ion Instabilities have been observed in several machines APS (with He injection), PLS (with H2 injection) SOLEIL, SSRF, BESSY II, ELETTRA, ALBA … Measurements at CESR-TA (Dec. 2013, Apr 2014) Varying ion species and pressure, bunch charge, train structure, feedback etc. 17 September 2014 Low emittance rings 2014, L. Mether39