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Collective effects Erik Adli, University of Oslo, August 2015, [email protected], v2.12 Introduction • Particle accelerators are continuously being pushed to new parameter regimes with higher currents, higher power and higher intensity • Performance is usually limited by multiparticle effects, including collective instabilities and collisions • A core element of particle accelerator physics is the study of collective effects. The understanding of collective instabilities has made it possible to overcome limitations and increase performance significantly • We here describe the two most common effects, space charge and wake fields, in some detail. Example of increase in beam intensity in the CERN Proton Synchrotron the last decades Intense beams: example Compressed high energy electron beam (at the “FACET” facility at SLAC) : The beam can be defined by the following parameters : • Charge per bunch: Q = 3 nC (N = 2 x 1010 electrons) • Emittance: enx,y = 100 um • Beam energy: E = 20 GeV (v ≈ c) Focused size at interaction point : • sx = 20 um, sy = 20 um, sz = 20 um (Gaussian) Other key beam characteristics deduced from the above accelerator parameters: • Peak current: Ipeak = Qpeak/t = Qc / √2p sz = 19 kA (cf. IAlfven = 17 kA) • Beam density: ne ≈ N / 4/3p(sx sy sz) = 6 x 1017 / cm3 Plasma description: • Electron transverse energy: Ee = ½ mvx2 = ½ m c2 x’2 ~ 1 meV • Electron (transverse) temperature: Te ~ me / kB c2 x’2 ~ 100 K • Debye length (shielding length): lD = √(e0kBT/e2n0) ~ 1 nm -> lD << sx, sy, sz (collective effects dominate over collisional effects) Space charge A general definition of “Space Charge” : a collection of particles with a net electric charge occupying a region, either in free space or in a device. In accelerators, we usually use the term space charge to describe the collective field/force in a same-charge particle bunch. Consider the bunch from the previous example, at rest. We may do a rough estimate of the space charge field using a two-particle model: ~2sx,y This is a very large field (cf. accelerating fields discussed earlier). Using the two particle model, we may estimate the force, and the time scale for the bunch to blow-up due to space charge forces to sub-picosecond. How can we accelerator dense particles bunches at all? ( Later, we give a more detailed calculations, assuming a Gaussian distribution. ) F + Q/2 + Q/2 F Coordinates – speed of light frame x z = s - vt s The beam travels in the s direction, with speed v. The co-moving coordinate z = s-vt is defined in a frame following the beam travelling with speed v , and gives thus the relative position inside the beam. In plasma wakefield applications, the beam is often travelling with velocity v = c. In this case the frame where z = s- ct is defined is called the speed of light frame. In accelerator physics we usually describe fields and charges in the laboratory frame, however, following the co-moving frame. NB: this is not the same as describing the fields and charges in the rest frame of the beam, as quantities described in the co-moving frame are not Lorenz transformed with respect to the lab frame. Space charge – moving charges Two-particle interaction : xlab x’ x v2 v1 z y z s y' Fields transforms to the lab frame as : We observe beams, fields and forces in the lab frame. The force on particle 1 is F = e(E+v1 x B). Particle 2 generates no magnetic field in its rest frame, which gives the relation (BT – v2/c2 x E) = 0. The total transverse force on particle 1 in the lab frame is thus F = e(E - v1 x (v2/c2 x E)), or for parallel velocities : Fx,y = eEx,y (1±v1v2/c2) For v1 = v2 = v we calculate the relativistic space charge suppression : Fx,y = eEx,y (1-v2/c2) = eEx,y / g2 EM fields from a relativistic particle Fields in particle’s rest frame Fields in lab frame Lorentz Tranformations v=0 Ultra-relativistic limit (v=c) : Spherical symmetrical field Field compression -> “Pancake field” Space Charge – using Gauss’ law Alternate calculation: Gauss law's. Gauss law is valid also for relativistic moving (or accelerating) charges. In the rest frame, the beam sees an electrostatic field. In the lab frame, the moving charges produce a magnetic field. We assume a uniform density beam in shape of a cylinder, with beam charge density r = Ne / pa2L : r Gauss law gives : E r r Ampere’s law gives : Bf F + + F Rest frame FE FB + FB + FE Lab frame 2e 0 r v r 2 2e 0 c Combine terms Fr = e(Er - vBf) : + + This is the direct space charge effect. “Direct” : does not take into account the effects of conducting walls surrounding the beam. c c Lab frame, v = c v Space Charge for Gaussian Beams KSC ~ Ipeak / (bg)3 “beam perveance” • Typical lattice focusing (FODO with ~10 m between magnets): <b> ~ 10 m -> Kb = 1/100 m-2 • For our example 20 GeV electron beam (a few slides ago) : KSC = 5 x 10-5 m-2 << Kb -> space charge completely suppressed at high Lorenz factor • For the same electron beam, but at 20 MeV : KSC = 5 x 104 m-2 >> Kb Summary K. Schindl Linear defocusing. Gives tune shift in rings. Can be compensated by stronger lattice focusing. Non-linear defocusing. Gives tune shift and tune spread in rings. Beam-beam effects Relativistic space charge suppression holds only for equal charge moving at the same velocity, in the same direction (even then, only holds fully in free space). These requirement are often violated. An important example is the Beam-Beam interaction : For v1 and v2 of opposite sign in our previous calculation (5 slides ago) we get : Fx,y = eEx,y (1+v1v2/c2) • • Two colliding beams see the field of each other before collision. They may be strongly attracted, and may deform. Important limitation for collider luminosity. Considered the main challenge for LHC luminosity. We will revisit the topic of beam beam effects in the linear collider lectures. Wake fields Most of the material is G. Rumolo’s slides from CAS Course on wake fields Wake fields (general) Source, q1 Witness, q2 2b z z z L – While source and witness ( qi d(s-ct) ), at a distance z, move centered in a perfectly conducting chamber, the witness does not feel any force (g >> 1) – When the source encounters a discontinuity (e.g., transition, device), it produces an electromagnetic field, which trails behind (wake field) o The source loses energy o The witness feels a net force all along an effective length of the structure, L 13 Wake field characteristics Wake field framework approximations When calculating wake fields in accelerator, the calculations are greatly simplified, allowing for simple descriptions of complex fields, by the following two approximations : 1) Rigid beam approximation: bunch static over length wake field is calculated 2) Impulse approximation: we only care about integrated force over the length wake field is calculated, not the details of the fields in time and space We will define wake function (wake potential) as such integrated quantities. Wake fields (general) Source, q1 Witness, q2 2b z L – Not only geometric discontinuities cause electromagnetic fields trailing behind sources traveling at light speed. – For example, a pipe with finite conductivity causes a delay in the induced currents, which also produces delayed electromagnetic fields o No ringing, only slow decay o The witness feels a net force all along an effective length of the structure, L – In general, also electromagnetic boundary conditions can be the origin of wake fields. 16 dp/p0 z 1. The longitudinal plane Longitudinal wake function: definition Source, q1 Witness, q2 z 2b L 18 Longitudinal wake function: properties – The value of the wake function in 0, W||(0), is related to the energy lost by the source particle in the creation of the wake – W||(0)>0 since DE1<0 – W||(z) is discontinuous in z=0 and it vanishes for all z>0 because of the ultra-relativistic approximation W||(z) z 19 The energy balance What happens to the energy lost by the source? – In the global energy balance, the energy lost by the source splits into o Electromagnetic energy of the modes that remain trapped in the object ⇒ Partly dissipated on or into purposely designed inserts or ⇒ Partly transferred to following particles (or the same particle over successive turns), possibly feeding into an instability o Electromagnetic energy of modes that propagate down the beam chamber (above cut-off), which will be eventually lost on surrounding lossy materials 20 The energy balance What happens to the energy lost by the source? – In the global energy balance, the energy lost by the source splits into o Electromagnetic energy of the modes that remain trapped in the object ⇒ Partly dissipated on or into purposely designed inserts or ⇒ PartlyThe transferred to loss following particles (or the same particle over energy is very important successive turns), possibly feeding into an instability! because o Electromagnetic energy of modes that propagate down the beam chamber ⇒which It causes beam induced heating lossy materials (above cut-off), will be eventually lost on surrounding of the beam environment (damage, outgassing) ⇒ It feeds into both longitudinal and transverse instabilities through the associated EM fields 21 Longitudinal impedance – The wake function of an accelerator component is basically its Green function in time domain (i.e., its response to a pulse excitation) ⇒ Very useful for macroparticle models and simulations, because it can be used to describe the driving terms in the single particle equations of motion! – We can also describe it as a transfer function in frequency domain – This is the definition of longitudinal beam coupling impedance of the element under study [W] [W/s] 22 Longitudinal impedance: resonator W|| Re[Z||] Im[Z||] wr T=2p/wr – The frequency wr is related to the oscillation of Ez, and therefore to the frequency of the mode excited in the object – The decay time depends on how quickly the stored energy is dissipated (quantified by a quality factor Q) 23 Longitudinal impedance: cavity – A more complex example: a simple pill-box cavity with walls having finite conductivity – Several modes can be excited – Below the pipe cut-off frequency the width of the peaks is only determined by the finite conductivity of the walls – Above, losses also come from propagation in the chamber Re[Z||] Im[Z||] 24 Single bunch effects W|| Im[Z||] Re[Z||] 25 Single bunch effects W|| 26 Multi bunch effects Re[Z||] Im[Z||] 27 Multi bunch effects W|| 28 Multi bunch effects Dz 29 Example: the Robinson instability – To illustrate the Robinson instability we will use some simplifications: ⇒ The bunch is point-like and feels an external linear force (i.e. it would execute linear synchrotron oscillations in absence of the wake forces) ⇒ The bunch additionally feels the effect of a multi-turn wake dp/p0 z Unperturbed: the bunch executes synchrotron oscillations at ws 30 The Robinson instability – To illustrate the Robinson instability we will use some simplifications: ⇒ The bunch is point-like and feels an external linear force (i.e. it would execute linear synchrotron oscillations in absence of the wake forces) ⇒ The bunch additionally feels the effect of a multi-turn wake dp/p0 z The perturbation also changes the oscillation amplitude Unstable motion The perturbation changes ws 31 The Robinson instability – To illustrate the Robinson instability we will use some simplifications: ⇒ The bunch is point-like and feels an external linear force (i.e. it would execute linear synchrotron oscillations in absence of the wake forces) ⇒ The bunch additionally feels the effect of a multi-turn wake dp/p0 z The perturbation also changes the oscillation amplitude Damped motion 32 2. The transverse plane Transverse wake function: definition Source, q1 Witness, q2 z 2b L – In an axisymmetric structure (or simply with a top-bottom and left-right symmetry) a source particle traveling on axis cannot induce net transverse forces on a witness particle also following on axis – At the zero-th order, there is no transverse effect – We need to introduce a breaking of the symmetry to drive transverse effect, but at the first order there are two possibilities, i.e. offset the source or the witness 34 Transverse dipolar wake function: definition Source, q1 Witness, q2 Dx1 (or Dy1) z 2b L 35 Transverse dipolar wake function – The value of the transverse dipolar wake functions in 0, Wx,y(0), vanishes because source and witness particles are traveling parallel and they can only – mutually – interact through space charge, which is not included in this framework – Wx,y(0--)<0 since trailing particles are deflected toward the source particle (Dx1 and Dx’2 have the same sign) – Wx,y(z) has a discontinuous derivative in z=0 and it vanishes for all z>0 because of the ultra-relativistic approximation Wx,y(z) z 36 Discrete approximation of cavity transverse impedance Impedances in frequency domain calculated using electromagnetic solvers. Impedance can be approximated using a number of discrete modes (above 9 modes used). From CLIC decelerator design Dipole wake instabilities in linacs Single bunch: head drives tail resonantly -> banana shape, beam-break up Two-bunches: one bunch drives the second resonantely Linac Beam breakup growth factor: ~NWs/kb N: charge W: dipole wake s: distance kb: betatron k Rings: A glance into the head-tail modes • Different transverse head-tail modes correspond to different parts of the bunch oscillating with relative phase differences. E.g. – Mode 0 is a rigid bunch mode – Mode 1 has head and tail oscillating in counter-phase – Mode 2 has head and tail oscillating in phase and the bunch center in opposition 39 Calculation of coherent modes seen at a wide-band pick-up (BPM) h • The patterns of the head-tail modes (m,l) depend on chromaticity Q’=0 Q’≠0 m=1 and l=0 m=1 and l=1 m=1 and l=2 40 Coherent modes measured at a wide-band pick-up (BPM) m=1 and l=0 1 Instabilities are a good diagnostics tool to identify and quantify the main impedance sources in a machine. 41 Wake field (impedances) in accelerator ring design The full ring is usually modeled with a so called total impedance made of three main components: • Resistive wall impedance • Several narrow-band resonators at lower frequencies than the pipe cutoff frequency c/b (b beam pipe radius) • One broad band resonator at wr~c/b modeling the rest of the ring (pipe discontinuities, tapers, other non-resonant structures like pick-ups, kickers bellows, etc.) The total impedance is allocated to the single ring elements by means of off-line calculation prior to construction/installation Total impedance designed such that the nominal intensity is stable We will talk more about wake fields in the lecture about linear colliders (tomorrow). For more details on wake fields, derived from first physical principles see the excellent book “Physics of Collective Beam Instabilities in High Energy Accelerators”, A. W. Chao (freely available; see course web pages). Part II Overview of multi-particle effects Adapted from G. Rumolo’s slides from USPAS Course on collective effects General definition of multi-particle processes in an accelerator or storage ring Class of phenomena in which the evolution of the particle beam cannot be studied as if the beam was a single particle (as is done in beam optics), but depends on the combination of external fields and interaction between particles. Particles can interact between them through • Self generated fields: Direct space charge fields Electromagnetic interaction of the beam with the surrounding environment through the beam‘s own images and the wake fields (impedances) Interaction with the beam‘s own synchrotron radiation • Long- and short-range Coulomb collisions, associated to intra-beam scattering and Touschek effect, respectively • Interaction of electron beams with trapped ions, proton/positron/ion beams with electron clouds, beam-beam in a collider ring, electron cooling for ions Multi-particle processes are detrimental for the beam (degradation and loss, see next slides) Several names to describe these effects ‚Multi-particle‘ is the most generic attribute. ‚High-current‘, ‚high-intensity‘, ‚high brightness‘ are also used because these effects are important when the beam has a high density in phase space (many particles in little volume) Other labels are also used to refer to different subclasses • Collective effects (coherent): The beam resonantly responds to a self-induced electromagnetic excitation Are fast and visible in the beam centroid motion (tune shift, instability) • Collective effects (incoherent): Excitation moves with the beam, spreads the frequencies of particle motion. Lead to particle diffusion in phase space and slow emittance growth • Collisional effects (incoherent): Isolated two-particle encounters have a global effect on the beam dynamics (diffusion and emittance growth, lifetime) • Two-stream phenomena (coherent or incoherent): Two component plasmas needed (beam-beam, pbeam-ecloud, ebeam-ions) and the beam reacts to an excitation caused by another „beam“ The performance of an accelerator is usually limited by a multi-particle effect. When the beam current in a machine is pushed above a certain limit (intensity threshold), intolerable losses or beam quality degradation appear due to these phenomena Direct space charge forces (as discussed in more details earlier) x Transverse space charge • Force decays like 1/g2 • It is always repulsive y Longitudinal space charge dp/p0 z • Force decays like 1/g2 • it can be attractive above transition Wake fields, impedances (as discussed in more details earlier) z W0(z) L e q s Model: A rigid beam with charge q going through a device of length L leaves behind an oscillating field and a probe charge e at distance z feels a force as a result. The integral of this force over the device defines the wake field and its Fourier transform is called the impedance of the device of length L. Single bunch versus multi-bunch effets • Single or multi-bunch behavior depends on the range of action of the wake fields Single bunch effects are usually caused by short range wake fields (broad-band impedances) Multi bunch or multi-turn effects are usually associated to long range wake fields (narrow-band impedances) Wake decays over a bunch length Single bunch collective interaction Wake decays over many bunches Coupled bunch collective interaction W W(z-z‘)l(z‘)dz‘ W(kd)N(kd) z s d s W Electron cloud Principle of electron multipacting: Example of LHC Electron multiplication is made possible by: Electron generation due to photoemission, but also residual gas ionization Electron acceleration in the field of the passing bunches Secondary emission with efficiency larger than one, when the electrons hit the inner pipe walls with high enough energy Example of coherent vs. incoherent effects Coherent: coherent synchrotron radiation (CSR) • Previous calculations of synchrotron radiation in this course assumed each particle radiates independently Prad N • • • Incoherent: intra-beam scattering • Particles within a bunch can collide with each other as they perform betatron and synchrotron oscillations. The collisions lead to a redistribution of the momenta within the bunch, and hence to a change in the emittances. If particles are close with respect to the radiation wavelengths, the particles will radiate coherently (as one macro particle), Prad N2 Significant effect for short bunch lengths, low energy beams, with large number of particles per bunch May lead to instabilities • This is effect is called IBS, intra-beam scattering. If there is a large transfer of momentum into the longidunal plane. It is called Touchek scattering. • May lead to emittance growth and reduced beam life time. More examples Bunch in the CERN SPS synchrotron BNL-RHIC, Au-Au operation, Run-4 (2004) 16h Coherent effects: When the bunch current exceeds a certain limit (current threshold), the centroid of the beam, e.g. as seen by a BPM, exhibits an exponential growth (instability) and the beam is lost within few milliseconds Multi-particle effects Collective Transverse Longitudinal Ecloud/trapped ions Multi- bunch Microwave/tur bulent Head-tail Incoherent/ collisional Two-stream TMCI Instability/beam loss Potential well distortion Coherent tune shift Beam/beam IBS/Touschek Incoherent tune spread Beam quality degradation/ emittance growth