Review on two stream instabilities in accelerators Giovanni Rumolo In TWIICE, Topical Workshop on Instabilities, Impedances and Collective Effects January, 2014, Synchrotron SOLEIL

Outline o Introduction Two-stream effects as a sub-class of multi-particle effects in beam dynamics o Positively charged particle (e.g. positron) machines  Electron cloud Build up in accelerator chambers Electron cloud instabilities Observations, modeling o Negatively charged particle (e.e. electron) machines  Ion effects Trapping and accumulation Fast beam ion instability Observations, modeling o Closing remarks TWIICE, 16 January 20142

What do we define as multi-particle effects? → Multi-particle effects: Class of phenomena in beam dynamics, in which the evolution of a particle in a beam depends on both the external EM fields and the extra EM fields created by the presence of other particles. o How other particles can affect a single particle’s motion: Self-induced EM fields − Space charge from beam particles − EM interaction of whole beam with surrounding environment − EM interaction of whole beam with its own synchrotron radiation Coulomb collisions − Long range and multiple two beam particle encounters  Intra-beam scattering − Short range and single event two beam particle encounters  Touschek effect − Elastic and inelastic scattering against residual gas molecules EM fields from another charge distribution (generated or not by the beam itself), like a second “beam” − Beam-beam in colliders − Trapped ions for electron beams − Electron clouds for positron/hadron beams − Interactions with electron lens or electron cooling system 3TWIICE, 16 January 20143

Types of multi-particle effects → Multi-particle effects important when beam density is very high May exhibit a threshold behaviour Result into a measurable response of the beam to the collective interaction, typically detrimental and leading to beam degradation and loss o Longitudinal and transverse impedance/space charge effects Due to self-induced EM fields E.g. tune shift, emittance growth, energy loss, potential well distortion Instabilities (single or coupled bunch) o Collisional effects Due to Coulomb scattering Depopulate the denser beam core, degrade emittance and lifetime o Two-stream effects Due to the interaction with another set of charged particles Lead to instabilities, energy loss, tune shift, emittance growth and losses 4TWIICE, 16 January 20144

o Each single beam particle moves under the overall effect of: Externally applied EM fields (RF, magnets) EM fields created by the second “beam” → We need to describe detailed evolution (and sometimes generation) of the other system of particles to derive its EM fields Theory: simplified models to include the effect of the second “beam” Simulation: describe numerically the two beams, calculate the fields of each beam to be used as additional driving terms in the equations of motion of the macroparticles representing the other beam z y x Modeling of two-stream effects TWIICE, 16 January 20145

o Electron cloud in positron/proton/ion machines Conditions for electron cloud formation Effects in an accelerator & mitigation o Ion trapping in electron machines Conditions for accumulation Effects in an accelerator 6TWIICE, 16 January 20146

o Electron cloud in positron/proton/ion machines Conditions for electron cloud formation Effects in an accelerator & mitigation o Ion trapping in electron machines Conditions for accumulation Effects in an accelerator TWIICE, 16 January 20147

Electron cloud formation in a vacuum pipe 8TWIICE, 16 January Generation of charged particles inside the vacuum chamber (primary, or seed, electrons) Residual gas ionization Photoelectrons from synchrotron radiation Desorption from the losses on the wall

Example: photoelectrons cross sectional view (x,y) o When the synchrotron radiation hits the beam pipe partly it produces electron emission within a 1/  angle from the point where it impinges partly it is reflected inside the pipe and hits at different locations, too, producing electrons with a more complex azimuthal distribution. view from above (x,s) TWIICE, 16 January 20149

Electron cloud formation in a vacuum pipe 10TWIICE, 16 January Generation of charged particles inside the vacuum chamber (primary, or seed, electrons) Acceleration of primary electrons in the beam field Secondary electron production when hitting the wall

Secondary electron emission o When electrons hit the pipe wall, they do not just disappear….. High energy electrons easily survive and actually multiply through secondary electron emission Low energy electrons tend to survive long because they are likely to be elastically reflected. o Secondary electron emission is governed by the curve below 11 EpEp  secondaries elastically reflected TWIICE, 16 January

Electron cloud formation in a vacuum pipe 12TWIICE, 16 January Generation of charged particles inside the vacuum chamber (primary, or seed, electrons) Acceleration of primary electrons in the beam field Secondary electron production when hitting the wall

Electron cloud formation in a vacuum pipe 13TWIICE, 16 January Generation of charged particles inside the vacuum chamber (primary, or seed, electrons) Acceleration of primary electrons in the beam field Secondary electron production when hitting the wall Avalanche electron multiplication After the passage of several bunches, the electron distribution inside the chamber reaches a stationary state (electron cloud)  Several effects associated

o Electron cloud in positron/proton/ion machines Conditions for electron cloud formation Effects in an accelerator & mitigation o Ion trapping in electron machines Conditions for accumulation Effects in an accelerator 14TWIICE, 16 January

The presence of an electron cloud inside an accelerator ring is revealed by several typical signatures Fast pressure rise, outgassing Additional heat load Baseline shift of the pick-up electrode signal Tune shift along the bunch train Coherent instability  Single bunch effect affecting the last bunches of a train  Coupled bunch effect Beam size blow-up and emittance growth Luminosity loss in colliders Energy loss measured through the synchronous phase shift Active monitoring: signal on dedicated electron detectors (e.g. strip monitors) and retarding field analysers 15 Machine observables Beam observables Effects of the electron cloud TWIICE, 16 January

Transverse beam instability Single bunch mechanism o A beam going through an electron cloud focuses the electrons (pinch), so that the central density of electrons changes along the bunch o Since electrons are drawn toward the bunch local centroid, this is the mechanism that can couple head and tail of a bunch →While the bunch is perfectly centered on the pipe axis, the pinch also happens symmetrically and no coherent kick is generated along the bunch 16 s y TWIICE, 16 January

Transverse beam instability Single bunch mechanism 17 → If the head of the bunch is slightly displaced by an amount  y head, an asymmetric pinch will take place, resulting into a net kick felt by the bunch tail  y’ tail TWIICE, 16 January

Transverse beam instability Single bunch mechanism → After several turns (passages through the electron cloud), the “perturbation” in the head motion transfers to the bunch tail, and its amplitude may grow under some conditions TWIICE, 16 January

Transverse beam instability Single bunch mechanism →After a number of turns much larger than the synchrotron period, the unstable coherent motion has propagated to the whole bunch s y Intra-bunch motion Emittance blow up TWIICE, 16 January

Observations (I) Blow up at KEK-LER The electron cloud causes beam size blow up (through instability and incoherent effects) that manifests itself at the tail of the bunch train Vertical beam size blow up observed with a streak camera Train head Train tail TWIICE, 16 January From K. Ohmi, K. Oide, F. Zimmermann, et al.

o Horizontal and vertical tune shifts along a 46 bunch train in Cesr-TA (Cornell facility presently used for electron cloud studies) taken during a positron run o Dependence on the beam current is shown, clearly pointing to stronger electron cloud for higher currents. 21 Observations (II) Tune shift at Cesr-TA From M. Palmer, J. Crittenden, G. Dugan, et al. TWIICE, 16 January

22 Observations (III) Instabilities in Da  ne From T. Demma (LER Workshop 2010) TWIICE, 16 January o Coupled bunch instability data from DAFNE (only positron ring) have been compared with PEI-M simulations o Very good agreement found, it confirms that the observed horizontal instability is caused by electron cloud Horizontal instability on mode -1

23 Observations (IV) Instabilities in LHC From H. Bartosik, et al., ECLOUD12 TWIICE, 16 January Some motion only for last bunches … up to ±5mm ~ bunch 25 is the first unstable 48b injection test (26/08/11) Headtail silation 1 Headtail 1 48x PyECLOUD e - distribution (  max =2.1) bunch 48 48x HEADTAIL simulations reveal the onset of instability

Possible Solutions Clearing electrodes installed along the vacuum chambers (only local, impedance) Coating with thin films with an intrinsically low SEY. Rendering the surface rough enough to block secondary electrons. … or both combined By machiningBy chemical or electrochemical methods By coating No need of heating once in vacuum (a-C) Lower activation temperature (NEG) Solenoids (only applicable in field-free regions) Tolerate e-cloud but damp the instability: feedback system Rely on machine scrubbing during operation (but reachable SEY) Electron cloud mitigation TWIICE, 16 January Outgassing, impedance !!

o Electron cloud in positron/proton/ion machines Conditions for electron cloud formation Effects in an accelerator & mitigation o Ion trapping in electron machines Conditions for accumulation Effects in an accelerator 25TWIICE, 16 January

Ion accumulation in a vacuum pipe 26TWIICE, 16 January 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

Example: Residual gas ionization 27TWIICE, 16 January 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)

Ion accumulation in a vacuum pipe 28TWIICE, 16 January 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

Trapping condition (Gaussian beams) Section i Ion of mass A Kick from the passing bunch TWIICE, 16 January

Trapping condition (Gaussian beams) Section i Transport through the drift space between bunches Section i+1 TbTb Ion of mass A TWIICE, 16 January

Trapping condition (Gaussian beams) TWIICE, 16 January

Trapping condition Example: CLIC Damping Rings TWIICE, 16 January CO, N 2 H2OH2O

Ion accumulation in a vacuum pipe 33TWIICE, 16 January 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

o Electron cloud in positron/proton/ion machines Conditions for electron cloud formation Effects in an accelerator & mitigation o Ion trapping in electron machines Conditions for accumulation Effects in an accelerator 34TWIICE, 16 January

Transverse Fast Beam Ion Instability 35 → The ions accumulate along one bunch train → Head and tail of the train are coupled through the ions (both in linear and circular machines). TWIICE, 16 January

36 → 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. TWIICE, 16 January Transverse Fast Beam Ion Instability

o In circular machines two possible regimes exist: Bunches are uniformly distributed around the machines  no clearing gap, classical beam ion instability Bunches are distributed in one (or more) train(s) with a long enough gap for ion clearing  The instability develops over one train length, fast beam ion instability 37 Transverse Beam Ion Instability

Theory and simulations TWIICE, 16 January o Detailed theory of fast beam ion instability in several references, e.g. “Fast beam-ion instability. I. Linear theory and simulations”, T.O. Raubenheimer, F. Zimmermann, Phys. Rev. E 52, 5, 5487 “Fast beam-ion instability. II. Effect of ion decoherence”, G. V. Stupakov, T.O. Raubenheimer, F. Zimmermann, Phys. Rev. E 52, 5, 5499 A. Chao, Notes on “Fast Ion Instabilities”, in USPAS lectures on Advanced Concepts in Accelerator Physics o Macroparticle simulation tools developed from 1995 onwards Weak-strong  E.g. “Simulation study of Fast Beam Ion Instability”, X. L. Zhang, et al., Proc. of APAC98, “Fast Beam Ion Instability simulations in the TESLA electron damping ring and the FEL beam transfer line”, C. Montag, Proc. of PAC01, “Simulation of the Beam-Ion Instability in the electron damping ring of ILC”, L. Wang et al., Proc. of PAC07 Strong-strong with analytical field calculations  “Fast beam-ion instability. I. Linear theory and simulations”, T.O. Raubenheimer, F. Zimmermann, Phys. Rev. E 52, 5, 5487 Self-consistent strong-strong model with PIC, acceleration, tunneling ionization, multi-species  FASTION code, “Fast Ion Instability in the CLIC transfer line and main Linac”, G. Rumolo et al., Proc. of EPAC08

Observations (I) Blow up at ALS Under injection of additional He, the fast beam ion instability affects later bunches in the train and causes emittance growth TWIICE, 16 January From J. Byrd, A. Chao, S. Heifets, et al. Phys. Rev. Lett. 79 (1997), 79-82

Observations (II) TWIICE, 16 January o Similar instabilities observed in PLS (especially H 2 injection), SOLEIL (see talk by R. Nagaoka), BESSY II, ELETTRA, ALBA o Usually the fast beam ion instability has been observed in electron rings 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) Artificially induced by injecting gas into the vacuum chamber and raising the pressure by more than one order of magnitude (for studies) o It seems to be less severe than predictions, as if it naturally benefits from some 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)

Simulations: the CLIC Main Linac TWIICE, 16 January 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

Simulations: the CLIC Main Linac TWIICE, 16 January 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

Simulations: the CLIC Main Linac TWIICE, 16 January 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 contribute to the instability

Two-stream phenomena Ion effects in positron rings o Ions from gas ionization can also cause trouble in the positron DRs o When lost to the chamber walls, they produce more molecules according to their energy and the wall desorption yield o Consequently, more ions are produced and the process can lead to an ion induced pressure instability From O. Malyshev, LER2010 TWIICE, 16 January

Two-stream phenomena Electrons in electron machines o There is experimental evidence of electron cloud formation also in rings running with electrons An anomalous heat load was observed in the ANKA superconducting wiggler, possibly ascribed to electron cloud (?) Both tune shift and RFA measurements taken at Cesr-TA with electrons circulating in the machine, demonstrate the existence of electron accumulation. Measurements from RFAs in 3 different test chambers TWIICE, 16 January

To summarize and conclude TWIICE, 16 January Two-stream effects are important and often determine the performance of running accelerators → Electron cloud formation and instabilities − Detailed modeling available for both processes − Plenty of observations in running machines and reliable extrapolations to future projects → Promising results from ongoing research on techniques for mitigation or suppression (coating, clearing electrodes, scrubbing) to be applied to future machines → Ion accumulation and instabilities − Theories developed and detailed modeling available − Observations in running machines usually made in presence of vacuum degradation, new experiments planned − Important for vacuum specifications of future electron machines