1. Ion effects in low emittance rings Giovanni Rumolo Thanks to R. Nagaoka, A. Oeftiger In CLIC Workshop 3-8 February, 2014, CERN

2. Outline o Introduction Ion trapping Beam instabilitiy o Ongoing studies Experimental observations of fast beam ion effects Numerical simulations o Conclusions and outlook CLIC Workshop, 6 February, 20142

3. Ion accumulation in a vacuum pipe 33 Generation of charged particles inside the vacuum chamber (in particular, ions) Residual gas ionization Ion emission from synchrotron radiation Desorption from the losses on the wall CLIC Workshop, 6 February, 2014

4. Residual gas ionization 44 The number of electron/ion pairs created per unit length ( =dN ion /ds = dN el /ds) Scattering ionization (depends on cross section of ionization process) Field ionization, first bunch (only when beam electric field is above threshold) CLIC Workshop, 6 February, 2014

5. Ion accumulation in a vacuum pipe 55 Generation of charged particles inside the vacuum chamber (in particular, ions) Ion motion in the beam field Possible trapping around the beam depending on ion mass CLIC Workshop, 6 February, 2014

6. Trapping condition (Gaussian beams) Section i Section i+1 T b = L sep /c Ion of mass A 6CLIC Workshop, 6 February, 2014

7. Trapping condition Example: CLIC Damping Rings 7 CO, N 2 H2OH2O CLIC Workshop, 6 February, 2014

8. Ion accumulation in a vacuum pipe 88 Generation of charged particles inside the vacuum chamber (in particular, ions) Ion motion in the beam field Possible trapping around the beam depending on ion mass After the passage of several bunches, ion density can affect beam motion  Tune shift along the train & coherent beam instability CLIC Workshop, 6 February, 2014

9. Transverse Fast Beam Ion Instability 9 → The ions accumulate along one bunch train → Positive tune shift towards the tail of the train → Head and tail of the train are coupled through the ions (both in linear and circular machines). 9CLIC Workshop, 6 February, 2014

10. 10 → The ions keep memory of the offset of the generating bunch and transfer this information to the following bunches. → The driven oscillation is expected to be at a main frequency related to the ion oscillation frequency. 10 Transverse Fast Beam Ion Instability CLIC Workshop, 6 February, 2014

11. 11 Transverse Fast Beam Ion Instability Example: CLIC Damping Rings CLIC Workshop, 6 February, 2014 Assuming to have 1 nTorr of trapped species Model of ion-beam coupled oscillators, as described in “Fast beam-ion instability. I. Linear theory and simulations”, T.O. Raubenheimer, F. Zimmermann, Phys. Rev. E 52, 5, 5487

12. Observations in existing machines 12 o Fast Beam Ion Instabilities observed in APS (with He injection), PLS (with H 2 injection), SOLEIL, SSRF, BESSY II, ELETTRA, ALBA … o Usually not observed in regular operation, but: Artificially induced by injecting gas into the vacuum chamber and raising the pressure by more than one order of magnitude (for studies, see above) During commissioning/start up (chamber not yet conditioned, bad vacuum, feedback system not yet operational) Because of some local pressure rise (e.g., directly connected to impedance induced heating with high current operation, see next slides) o Seems to be less severe than predictions, probably stabilizing effects not included in existing models ? o Quantitative comparison between theoretical predictions, simulations and measurements yet to be made Experiment planned at Cesr-TA (April 2014) CLIC Workshop, 6 February, 2014

13. Observations (II) 13CLIC Workshop, 6 February, 2014 Ions enhanced by local heating (outgassing) seem to trigger some recently observed high current instabilities @ SSRF and SOLEIL

14. 14CLIC Workshop, 6 February, 2014 mbtrack simulations suggest that SOLEIL instability results from an intriguing interplay between resistive wall, ion effect and transverse feedback Observations (III)

15. Numerical simulations: the FASTION code 15 Focus on a section of the accelerator with given  functions CLIC Workshop, 6 February, 2014 Generate the ions (macroparticles) from residual gas ionization during electron bunch passage (macroparticles) 1.Calculate ion and bunch E-field 2.Apply mutual kicks from E-fields to ions and electrons 3.Track ions to next bunch N bunches

16. Numerical simulations: the FASTION code 16 Focus on a section of the accelerator with given  functions CLIC Workshop, 6 February, 2014 Generate the ions (macroparticles) from residual gas ionization during electron bunch passage (macroparticles) 1.Calculate ion and bunch E-field 2.Apply mutual kicks from E-fields to ions and electrons 3.Track ions to next bunch N bunches 1.Check for scattering ionization vs. field ionization 2.Create ions distributed over the bunch cross section and with species densities proportional to partial pressures

17. Numerical simulations: the FASTION code 17 Focus on a section of the accelerator with given  functions CLIC Workshop, 6 February, 2014 Generate the ions (macroparticles) from residual gas ionization during electron bunch passage (macroparticles) 1.Calculate ion and bunch E-field 2.Apply mutual kicks from E-fields to ions and electrons 3.Track ions to next bunch N bunches PIC solver

18. Numerical simulations: the FASTION code 18 Focus on a section of the accelerator with given  functions CLIC Workshop, 6 February, 2014 Generate the ions (macroparticles) from residual gas ionization during electron bunch passage (macroparticles) Track the electrons to the next section with the transfer matrix from Twiss file N bunches N steps = N lattice (x N turns ) 1.Calculate ion and bunch E-field 2.Apply mutual kicks from E-fields to ions and electrons 3.Track ions to next bunch

19. Simulations: the CLIC Main Linac 19 o Along the 20 km, a coherent instability develops due to 20 nTorr of H 2 O o A characteristic frequency of 250 MHz can be identified CLIC Workshop, 6 February, 2014

20. Simulations: the CLIC Main Linac 20 o Along the 20 km, a coherent instability develops due to 20 nTorr of H 2 O o A characteristic frequency of 250 MHz can be identified CLIC Workshop, 6 February, 2014

21. Simulations: the CLIC Main Linac 21 o Not only level of vacuum is important, but also its composition o Usually H 2 is not trapped and ions are lost due to overfocusing, therefore it does not actively contribute to the instability CLIC Workshop, 6 February, 2014

22. Conclusions and plans TWIICE, 16 January 201422 Ion effects are important in electron machines and can determine the performance of future low emittance accelerators or damping rings − Theories developed  formulae typically used to predict behavior − Observations in running machines usually made in presence of vacuum degradation During commissioning Caused by impedance heating Deliberately for study purposes − Numerical model available (FASTION) Strong-strong simulation tool including several ingredients Applied to linear machines (e.g. CLIC Main Linac and long transfer line) Optimization and generalization to rings underway − Important for future electron machines Vacuum specifications Low emittance design  Low-gap chambers and high intensity short bunches  Outgassing