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Transcript of Instabilities Part III: Transverse wake fields · PDF file · 2017-09-29Part 3:...
![Page 1: Instabilities Part III: Transverse wake fields · PDF file · 2017-09-29Part 3: Transverse wakefields – their different types and impact on beam dynamics •Transverse wake function](https://reader031.fdocuments.net/reader031/viewer/2022030500/5aabe2487f8b9a2b4c8ca899/html5/thumbnails/1.jpg)
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Instabilities Part III:Transverse wake fields –
impact on beam dynamicsGiovanni Rumolo and Kevin Li
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 2
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Outline
Part 3: Transverse wakefields –
their different types and impact on beam dynamics
• Transverse wake function and impedance
• Numerical implementation, transverse „potential well distortion“ ad headtail instabilities
• Two particle models, transverse mode coupling instability
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 3
We will close in into the description and the impact of transverse wake fields.We will discuss the different types of transverse wake fields, outline how theycan be implemented numerically and then investigate their impact on beamdynamics. We will see some examples of transverse instabilities such as thetransverse mode coupling instability (TMCI) or headtail instabilities.
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Signpost
Part 3: Transverse wakefields –
their different types and impact on beam dynamics
• Transverse wake function and impedance
• Numerical implementation, transverse „potential well distortion“ ad headtail instabilities
• Two particle models, transverse mode coupling instability
07/09/2017 Beam Instabilities II - Giovanni Rumolo and Kevin Li 4
• We have discussed longitudinal wake fields and impedances and their impact onboth the machine as well as the beam.
• We have learned about beam induced heating and how it is related to the beampower spectrum and the machine impedance.
• We have discussed the effects of potential well distortion (stable phase andsynchrotron tune shifts, bunch lengthening and shortening).
• We have seen some examples of longitudinal instabilities (Microwave, Robinson).
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Definition as the integrated force associated to a change in energy:
• In general, for two point-like particles, we have
w is typically expanded in the transverse offsets of source and witness particles. This yields the different types of wake fields (dipole, quadrupole, coupling wakes)
Recap: wake functions in general
06/09/2017 Beam Instabilities I - Giovanni Rumolo and Kevin Li 5
2b
L
z
Dx1 (or Dy1)
Dx2 (or Dy2)
Source, q1
Witness, q2
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Transverse wake functions
• Transverse wake fields
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 6
2bz
L
Dx1 (or Dy1)
Dx2 (or Dy2)
Source, q1
Witness, q2
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Transverse wake functions
• Transverse wake fields
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 7
2bz
L
Dx1 (or Dy1)
Dx2 (or Dy2)
Source, q1
Witness, q2
Transverse deflecting kick of thewitness particle from transverse wakes
![Page 8: Instabilities Part III: Transverse wake fields · PDF file · 2017-09-29Part 3: Transverse wakefields – their different types and impact on beam dynamics •Transverse wake function](https://reader031.fdocuments.net/reader031/viewer/2022030500/5aabe2487f8b9a2b4c8ca899/html5/thumbnails/8.jpg)
Transverse wake functions
• Transverse wake fields
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 8
2bz
L
Dx1 (or Dy1)
Dx2 (or Dy2)
Source, q1
Witness, q2
Transverse deflecting kick of thewitness particle from transverse wakes
![Page 9: Instabilities Part III: Transverse wake fields · PDF file · 2017-09-29Part 3: Transverse wakefields – their different types and impact on beam dynamics •Transverse wake function](https://reader031.fdocuments.net/reader031/viewer/2022030500/5aabe2487f8b9a2b4c8ca899/html5/thumbnails/9.jpg)
Transverse wake functions
• Transverse wake fields
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 9
2bz
L
Dx1 (or Dy1)
Dx2 (or Dy2)
Source, q1
Witness, q2
Zeroth order forasymmetric structures Orbit offset
Dipole wakes –depends on source particle Orbit offset
Quadrupole wakes –depends on witness particle Detuning
![Page 10: Instabilities Part III: Transverse wake fields · PDF file · 2017-09-29Part 3: Transverse wakefields – their different types and impact on beam dynamics •Transverse wake function](https://reader031.fdocuments.net/reader031/viewer/2022030500/5aabe2487f8b9a2b4c8ca899/html5/thumbnails/10.jpg)
Transverse wake functions
• Transverse wake fields
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 10
2bz
L
Dx1 (or Dy1)
Dx2 (or Dy2)
Source, q1
Witness, q2
Zeroth order forasymmetric structures Orbit offset
Dipole wakes –depends on source particle Orbit offset
Quadrupole wakes –depends on witness particle Detuning
We have truncated to the first order, thus neglecting
⇒ First order coupling terms between x and y planes
⇒ All higher order terms in the wake expansion (including mixed higher order terms with products of the dipolar/quadrupolar offsets)
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Transverse dipole wake function
• The value of the transverse dipolar wake function in z=0 vanishes because sourceand witness particles are traveling parallel and they can only – mutually – interactthrough space charge, which is not included in this framework
• WDx(0–)<0 since trailing particles are deflected toward the source particle (Δx1and Δx’2 have the same sign)
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 11
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Transverse dipole wake function
• The value of the transverse dipolar wake function in z=0 vanishes because sourceand witness particles are traveling parallel and they can only – mutually – interactthrough space charge, which is not included in this framework
• WDx(0–)<0 since trailing particles are deflected toward the source particle (Δx1and Δx’2 have the same sign)
• WDx(z) has a discontinuous derivative in z=0 and it vanishes for all z>0 because ofthe ultra-relativistic approximation
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 12
Wx,y(z)
z
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Transverse quadrupole wake function
• The value of the transverse quadrupolar wake function in z=0 vanishes becausesource and witness particles are traveling parallel and they can only – mutually –interact through space charge, which is not included in this framework
• WQx(0–)<0 can be of either sign since trailing particles can be either attracted ordeflected yet further off axis (depending on geometry and boundary conditions)
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 13
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Transverse quadrupole wake function
• The value of the transverse quadrupolar wake function in z=0 vanishes becausesource and witness particles are traveling parallel and they can only – mutually –interact through space charge, which is not included in this framework
• WQx(0–)<0 can be of either sign since trailing particles can be either attracted ordeflected yet further off axis (depending on geometry and boundary conditions)
• WQx(z) has a discontinuous derivative in z=0 and it vanishes for all z>0 because ofthe ultra-relativistic approximation
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 14
WQx,y(z)
z
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Transverse impedance
• The wake function of an accelerator component is basically its Green function intime domain (i.e., its response to a pulse excitation)
→ Very useful for macroparticle models and simulations, because it can be used todescribe 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 transverse beam coupling impedance of the element understudy
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 15
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Examples of wakes/impedances
• Resistive wall of beam chamber
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 16
o The case of a conductive pipe with an arbitrarynumber of layers with specified EM properties can besolved semi-analytically
o Layers sometimes required for impedance, but alsofor other reasons (e.g. coating against electron cloudor for good vacuum)
b
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Examples of wakes/impedances
• Resistive wall of beam chamber
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 17
r1
=r1
o The equations for thecoefficients of the azimuthalmodes of Es must be solved inall the media and theconservation of the tangentialcomponents of the fields isapplied at the boundariesbetween different layers
o E.g. ImpedanceWake2D codecalculates impedances and thenwakes. It can also deal with flatstructures
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• Resistive wall of beam chamber
b
t
Examples of wakes/impedances
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 18
o An example: a 1 m long Cu pipe with radius b=2 cmand thickness t = 4 mm in vacuum
• Highlighted region shows thetypical ω-1/2 scaling
• Another scaling is with respect tob where:• Longitudinal impedance ~ b-1
• Transverse imepdance ~b-3
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Signpost
Part 3: Transverse wakefields –
their different types and impact on beam dynamics
• Transverse wake function and impedance
• Numerical implementation, transverse „potential well distortion“ ad headtail instabilities
• Two particle models, transverse mode coupling instability
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 19
• We have seen the definition of transverse wake fields and how they can be classifiedinto constant, dipolar and quadrupolar wake fields.
• We have discussed the basic features of each of the different types of transversewake fields.
• We will now look into how the impact of wake fields onto charged particle beamscan be modeled numerically to prepare for investigating the different types ofcoherent instabilities further along.
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Quick summary of steps for solving numerically
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 20
• Tracking one full turn including the interaction with wake fields:
Wakefield
1. Initialise a macroparticle distribution with a given emittance
2. Update transverse coordinates and momenta according to the linear periodic transfer map – adjust the individual phase advance according to chromaticity and detuning with amplitude
3. Update the longitudinal coordinates and momenta according to the leap-frog integration scheme
4. Update momenta only (apply kicks) according to wake field generated kicks
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Particle jexcites
Particle ireceives kick
• The wake functions are obtained externally from electromagnetic codes such as ACE3P, CST, GdfidL, HFSS…
• In the tracking code, the wake fields at a given point need to update the particle/macroparticle momenta (i.e. they provide a kick)
• The kick on to a particle/macroparticle‘i’ generated by all particles/macroparticles ‘j’ via the wake fields is:
Numerical implementation of wakefields
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 21
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• The wake functions are obtained externally from electromagnetic codes such as ACE3P, CST, GdfidL, HFSS…
• In the tracking code, the wake fields at a given point need to update the particle/macroparticle momenta (i.e. they provide a kick)
• The kick on to a particle/macroparticle‘i’ generated by all particles/macroparticles ‘j’ via the wake fields is:
Numerical implementation of wakefields
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 22
• To be numerically more efficient, the beam is longitudinally sliced into a set of slices
• Provided the slices are thin enough to sample the wake fields, the wakes can be assumed constant within a single slice
• The kick on to the set of macroparticlesin slice ‘i’ generated by the set of macroparticles in slice ‘j’ via the wake fields now becomes:
slice jexcites
slice ireceives kick
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Numerical implementation of wakefields
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 23
• To be numerically more efficient, the beam is longitudinally sliced into a set of slices
• Provided the slices are thin enough to sample the wake fields, the wakes can be assumed constant within a single slice
• The kick on to the set of macroparticlesin slice ‘i’ generated by the set of macroparticles in slice ‘j’ via the wake fields now becomes
23
• N[i]: number of macroparticles in slice ‘i’ can be pre-computed and stored in memory
• W[i]: wake function pre-computed and stored in memory for all differences i-j
Count 0 1 2 3 4 5 6
N[i] … … … … … … …
W[i] … … … … … … …
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Numerical implementation of wakefields
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 24
n i 0Slice index
A. Bin macroparticlesinto discrete set of slices – binning needs to be fine enough as to sample the wake function
B. Compute number of macroparticles per slice
C. Perform convolution with wake function to obtain wake kicks
D. Apply wake kicks (momentum update)
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Numerical implementation of wakefields
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 25
n i 0Slice index
A. Bin macroparticlesinto discrete set of slices – binning needs to be fine enough as to sample the wake function
B. Compute number of macroparticles per slice
C. Perform convolution with wake function to obtain wake kicks
D. Apply wake kicks (momentum update)
![Page 26: Instabilities Part III: Transverse wake fields · PDF file · 2017-09-29Part 3: Transverse wakefields – their different types and impact on beam dynamics •Transverse wake function](https://reader031.fdocuments.net/reader031/viewer/2022030500/5aabe2487f8b9a2b4c8ca899/html5/thumbnails/26.jpg)
Quick summary of steps for solving numerically
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 26
• Tracking one full turn including the interaction with wake fields:
Wakefield
1. Initialise a macroparticle distribution with a given emittance
2. Update transverse coordinates and momenta according to the linear periodic transfer map – adjust the individual phase advance according to chromaticity and detuning with amplitude
3. Update the longitudinal coordinates and momenta according to the leap-frog integration scheme
4. Update momenta only (apply kicks) according to wake field generated kicks
5. Repeat turn-by-turn…
![Page 27: Instabilities Part III: Transverse wake fields · PDF file · 2017-09-29Part 3: Transverse wakefields – their different types and impact on beam dynamics •Transverse wake function](https://reader031.fdocuments.net/reader031/viewer/2022030500/5aabe2487f8b9a2b4c8ca899/html5/thumbnails/27.jpg)
Examples – constant wakes
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 27
Dipolar term orbit kickSlice dependent change of closed orbit(if line density does not change)
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Closed orbit distortion
Examples – constant wakes
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 28
Injected beam
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Examples – constant wakes
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Examples – constant wakes
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Examples – dipole wakes
• Without synchrotron motion:kicks accumulate turn after turn – the beam is unstable beam break-up in linacs
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 31
Offset dependent orbit kick kicks can accumulateDipolar term orbit kick
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Examples – dipole wakes
• Without synchrotron motion:kicks accumulate turn after turn – the beam is unstable beam break-up in linacs
• With synchrotron motion:• Chromaticity ≠ 0
• Headtail modes beam is unstable (can be very weak and often damped by non-linearities)
• Chromaticity = 0• Synchrotron sidebands are well separated beam is stable
• Synchrotron sidebands couple (transverse) mode coupling instability
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 32
With synchrotron motion we can get into a feedback loop
Offset dependent orbit kick kicks can accumulateDipolar term orbit kick
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Examples – dipole wakes
• Without synchrotron motion:kicks accumulate turn after turn – the beam is unstable beam break-up in linacs
• With synchrotron motion:• Chromaticity ≠ 0
• Headtail modes beam is unstable (can be very weak and often damped by non-linearities)
• Chromaticity = 0• Synchrotron sidebands are well separated beam is stable
• Synchrotron sidebands couple (transverse) mode coupling instability
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 33
With synchrotron motion we can get into a feedback loop
Offset dependent orbit kick kicks can accumulateDipolar term orbit kick
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Dipole wakes – headtail modes
• As soon as chromaticity is non-zero, a ‘resonant’ condition can be met as particles now can ‘synchronize’ their synchrotron amplitude dependent betatron motion with the action of the wake fields.
• Headtail modes arise – the order of the respective mode depends on the chromaticity together with the impedance and bunch spectrum
• Different transverse head-tail modes correspond to different parts of the bunch oscillating with relative phase differences, for example:• 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
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Dipole wakes – headtail modes
• As soon as chromaticity is non-zero, a ‘resonant’ condition can be met as particles now can ‘synchronize’ their synchrotron amplitude dependent betatron motion with the action of the wake fields.
• Headtail modes arise – the order of the respective mode depends on the chromaticity together with the impedance and bunch spectrum
• Different transverse head-tail modes correspond to different parts of the bunch oscillating with relative phase differences, for example:• 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
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Remark:
Due to this ‚synchronicity‘, below transition (η<0):
• Mode 0 is unstable if Q‘>0.
• Higher order modes tend to be unstable if Q‘<0 (though at lower growth rates).
The situation is reversed when a machine is operated above transition.
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Dipole wakes – headtail modes
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Example: Headtail modes in the LHC
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Lower chroma – mode 2 Higher chroma – mode 3
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Signpost
Part 3: Transverse wakefields –
their different types and impact on beam dynamics
• Transverse wake function and impedance
• Numerical implementation, transverse „potential well distortion“ ad headtail instabilities
• Two particle models, transverse mode coupling instability
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 38
• We have seen how the impact of wake fields on charged particle beams can beimplemented numerically in an efficient manner via the longitudinal discretizationof bunches.
• We have used the simulation models to show orbit effects and headtail instabilitiesfrom transverse wake fields.
• We will now derive another fundamentally limiting effect using analytical models.One very simple but already quite powerful tool are two-particle models.
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The Strong Head Tail Instability
39
We will:• Calculate a stability condition (threshold) for the transverse motion• Have a look at the excited oscillation modes of the centroid
• Aka the Transverse Mode Coupling Instability:o To illustrate TMCI we will need to make use of some simplifications:
The bunch is represented through two particles carrying half the total bunch chargeand placed in opposite phase in the longitudinal phase space
They both feel external linear focusing in all three directions (i.e. linear betatronfocusing + linear synchrotron focusing).
Zero chromaticity (Q’x,y=0)
Constant transverse wake left behind by the leading particle
Smooth approximation constant focusing + distributed wake
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• During the first half of the synchrotron motion, particle 1 is leading and executes freebetatron oscillations, while particle 2 is trailing and feels the defocusing wake ofparticle 1
z
dp/p0 Particle 1 (+Ne/2)
Particle 2 (+Ne/2)
T < Ts/2
The Strong Head Tail Instability
z
Wy
-W0
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• During the first half of the synchrotron motion, particle 1 is leading and executes freebetatron oscillations, while particle 2 is trailing and feels the defocusing wake ofparticle 1
• During the second half of the synchrotron period, the situation is reversed:
z
dp/p0 Particle 1 (+Ne/2)
Particle 2 (+Ne/2)
T < Ts/2
The Strong Head Tail Instability
T > Ts/2
z
Wy
-W0
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The Strong Head Tail Instability• We solve with respect to the complex variables defined below during the first half of
synchrotron period
• y1(s) is a free betatron oscillation
• y2(s) is the sum of a free betatron oscillation plus a driven oscillation with y1(s) beingits driving term
• Second term in RHS equation for y2(s) negligible if ωs << ωβ
• We can now transform these equations into linear mapping across half synchrotronperiod
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The Strong Head Tail Instability• We can now transform these equations into linear mapping across half synchrotron
period
• In the second half of synchrotron period, particles 1 and 2 exchange their roles – wecan therefore find the transfer matrix over the full synchrotron period for bothparticles. We can analyze the eigenvalues of the two particle system
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Strong Head Tail Instability – stability condition
• Since the product of the eigenvalues is 1, the only condition for stability is that theyboth be purely imaginary exponentials
• From the second equation for the eigenvalues, it is clear that this is true only whensin(ϕ/2)<1
• This translates into a stability condition on the beam/wake parameters
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Strong Head Tail Instability – stability condition
• Since the product of the eigenvalues is 1, the only condition for stability is that theyboth be purely imaginary exponentials
• From the second equation for the eigenvalues, it is clear that this is true only whensin(ϕ/2)<1
• This translates into a stability condition on the beam/wake parameters
• Proportional to p0 bunches with higher energy tend to be morestable
• Proportional to ωs the quicker is the longitudinal motion withinthe bunch, the more stable is the bunch
• Inversely proportional to βy the effect of the impedance isenhanced if the kick is given at a location with large beta function
• Inversely proportional to the wake per unit length along the ring, W0/C a large integrated wake (impedance) lowers the instability threshold
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Strong Head Tail Instability – mode frequencies• The evolution of the eigenstates follows:
Eigenfrequencies:They shift with increasing intensity
l = 0
l = -1
Unstable region
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Strong Head Tail Instability – mode frequencies• The evolution of the eigenstates follows:
Eigenfrequencies:They shift with increasing intensity
l = 0
l = -1
Unstable region
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Strong Head Tail Instability – why TMCI?• For a real bunch, modes exhibit a more complicated shift pattern
• The shift of the modes can be calculated via Vlasov equation or can be found through macroparticle simulations
Simplified calculation for a short bunch
Full calculation for a relatively long SPS bunch (red lines) + macroparticle simulation (white traces)
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Quick summary of steps for solving numerically
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 49
• Tracking one full turn including the interaction with wake fields:
Wakefield
1. Initialise a macroparticle distribution with a given emittance
2. Update transverse coordinates and momenta according to the linear periodic transfer map – adjust the individual phase advance according to chromaticity and detuning with amplitude
3. Update the longitudinal coordinates and momenta according to the leap-frog integration scheme
4. Update momenta only (apply kicks) according to wake field generated kicks
5. Repeat turn-by-turn…
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Examples – dipole wakes
• Without synchrotron motion:kicks accumulate turn after turn – the beam is unstable beam break-up in linacs
• With synchrotron motion:• Chromaticity ≠ 0
• Headtail modes beam is unstable (can be very weak and often damped by non-linearities)
• Chromaticity = 0• Synchrotron sidebands are well separated beam is stable
• Synchrotron sidebands couple (transverse) mode coupling instability
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 50
With synchrotron motion we can get into a feedback loop
Offset dependent orbit kick kicks can accumulateDipolar term orbit kick
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Dipole wakes – beam break-up
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Dipole wakes – beam break-up
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Dipole wakes – TMCI below threshold
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Dipole wakes – TMCI below threshold
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 54
Mode A Mode B
As the intensity increases the coherent modes shift – here, modes A and B are
approaching each other
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When the two modes merge a fast coherent instability arises – the transverse
mode coupling instability (TMCI) which often is a hard intensity limit in many
machines
Dipole wakes – TMCI above threshold
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 55
Mode A Mode B
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Dipole wakes – TMCI above threshold
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Signpost
Part 3: Transverse wakefields –
their different types and impact on beam dynamics
• Transverse wake function and impedance
• Numerical implementation, transverse „potential well distortion“ ad headtail instabilities
• Two particle models, transverse mode coupling instability
07/09/2017 Beam Instabilities II - Giovanni Rumolo and Kevin Li 57
• We have discussed transverse wake fields and impedances, their classification intodifferent types along with their impact on the beam dynamics.
• We have seen how the wake field interaction with a charged particle beam can becarried out numerically in an efficient manner.
• We have seen some examples of the effects of transverse wake fields on the beamsuch as orbit distortion or headtail instabilities.
• We have discussed the two-particle model and an analytically solvable problemwhich led to the description of the transverse mode coupling instability (TMCI).
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End part 3
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Raising the TMCI threshold – SPS Q20 optics
• In simulations we have the possibility to perform scans of variables, e.g. we can run 100 simulations in parallel changing the beam intensity
• We can then perform a spectral analysis of each simulation…
• … and stack all obtained plot behind one another to obtain…
• … the typical visualization plots of TMCI
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 59
Mode A Mode B
Q 26
TMCI threshold
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Raising the TMCI threshold – SPS Q20 optics
• In simulations we have the possibility to perform scans of variables, e.g. we can run 100 simulations in parallel changing the beam intensity
• We can then perform a spectral analysis of each simulation…
• … and stack all obtained plot behind one another to obtain…
• … the typical visualization plots of TMCI
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 60
TMCI threshold
Q 26
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TMCI threshold
Raising the TMCI threshold – SPS Q20 optics
• In simulations we have the possibility to perform scans of variables, e.g. we can run 100 simulations in parallel changing the beam intensity
• We can then perform a spectral analysis of each simulation…
• … and stack all obtained plot behind one another to obtain…
• … the typical visualization plots of TMCI
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 61
TMCI threshold
Q 26 Q 20
Qs = 0.0059 Qs = 0.017
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Backup
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Wakefields – rough formalism
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Wakefields – rough formalism
• Expansion
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Wakefields – rough formalism
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 65
n m type
0 0, 1
1 0
Constant transverse wake (n=0, m=0)
Dipole transverse wake (n=0, m=1)
Quadrupole transverse wake (n=1, m=0)
• Expansion – up to second order:Quadrupole term (n=2) change of tune
Dipole term (n=1) change of orbit
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Examples – constant wakes
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 66
Dipolar term orbit kick
Slice dependent change of closed orbit(if line density does not change)
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Closed orbit distortion
Examples – constant wakes
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Examples – quadrupole wakes
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 68
Quadrupole term tune kick
Slice dependent change of tune(if line density does not change)
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Detuning
Examples – quadrupole wakes
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Detuning
Examples – quadrupole wakes
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Examples – dipole wakes
• Without synchrotron motion:kicks accumulate turn after turn – the beam is unstable beam break-up in linacs
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 71
Dipolar term orbit kick
Offset dependent orbit kick kicks can accumulate
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Examples – dipole wakes
• Without synchrotron motion:kicks accumulate turn after turn – the beam is unstable beam break-up in linacs
• With synchrotron motion:• Chromaticity = 0
• Synchrotron sidebands are well separated beam is stable
• Synchrotron sidebands couple (transverse) mode coupling instability
• Chromaticity ≠ 0• Headtail modes beam is unstable (can be very weak and often damped by non-linearities)
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 72
With synchrotron motion we can get into a feedback loop
Dipolar term orbit kick
Offset dependent orbit kick kicks can accumulate
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Dipole wakes – TMCI below threshold
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 73
As the intensity increases the coherent modes shift – here, modes A and B are
approaching each other
Mode A Mode B
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When the two modes merge a fast coherent instability arises – the transverse
mode coupling instability (TMCI) which often is a hard intensity limit in many
machines
Dipole wakes – TMCI above threshold
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 74
Mode A Mode B
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Backup - wakefields
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Break
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• Resistive wall of beam chamber
b
t
Examples of wakes/impedances
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 77
o An interesting example: a 1 m long Cu pipe withradius b=2 cm and thickness t = 4 mm in vacuum
3 frequency regions of interest:
1. Below 0.1 kHz, impedance isbasically purely imaginary,EM field is shielded by imagecharges Indirect spacecharge
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• Resistive wall of beam chamber
b
t
Examples of wakes/impedances
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 78
o An interesting example: a 1 m long Cu pipe withradius b=2 cm and thickness t = 4 mm in vacuum
3 frequency regions of interest:
2. Between 10 kHz and 1 THz,the EM field is fullyattenuated in the Cu layerand the impedance is like theone calculated assuminginfinitely thick wall
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• Resistive wall of beam chamber
b
t
Examples of wakes/impedances
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 79
o An interesting example: a 1 m long Cu pipe withradius b=2 cm and thickness t = 4 mm in vacuum
3 frequency regions of interest:
3. Above 1 THz, there is aresonance (100 THz region).In this region also acconductivity should be takeninto account
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• Resistive wall of beam chamber
b
t
Examples of wakes/impedances
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 80
o Correspondingly, in time domain, the wake exhibits differentbehaviours in short and long range
In the range of tenths of ns up to fractionsof ms (e.g. bunch length to several turns forthe SPS) monotonically decaying wake
Medium-long range From behind the source to ~1ps the wakehas an oscillatory behaviour, associated tothe high frequency resonance
Short range
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Beam deflection kick
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x0
x
y
Off-axis traversal of symmetric chamber
x
y
Traversal of asymmetric chamber
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Beam deflection kick
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x0
x
y
Off-axis traversal of symmetric chamber
x
y
Traversal of asymmetric chamber
The beam deflection kicks
⇒ Are responsible for intensity dependent orbitvariations
⇒ Cause z-dependent orbits and can determinetilted equilibrium bunch distributions for longbunches
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Some hints for energy loss estimations
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 83
1) With Z||(w) = Z||Res(w) from slide 77 in the two limiting
cases
Need to expand Re[Z||(w)] for small w
Need to assume |l(w)| constant over Re[Z||(w)]
2) With Z||(w) = Z||RW(w) from slide 64
can be calculated
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A glance into the head-tail modes(as seen at a wide-band BPM)
84
l=0
Q’=0 Q’≠0
l=1
l=2
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A glance into the head-tail modes(experimental observations)
85
• The mode that gets first excited in the machine depends on• The spectrum of the exciting impedance• The chromaticity setting
• Head-tail instabilities are a good diagnostics tool to identify and quantify the main impedance sources in a machine
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• Nonrelativistic wakes can also affect particles ahead of the source particle
• Finite values extend from (-L, L)or “tail –head” & “head – tail”• L: bunch length
Relativistic vs. non-relativistic wakes
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 86
• Relativistic wakes only affect trailing particles following the source particle
• Finite values range for negative distances, i.e. (-L, 0) or “tail – head”• L: bunch length
Source
Witnesstrailing
Relativistic wake
Non-relativistic wake
Witnessahead
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Example non-relativistic wakes
• PS injection oscillations show intra-bunch modulations
• These can only be reproduced when adding non-relativistic wakes caused by indirect space charge fields
A. Huschauer et al
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 87
Measurements
Non-relativistic wake
Source
Witnesstrailing
Witnessahead
![Page 88: Instabilities Part III: Transverse wake fields · PDF file · 2017-09-29Part 3: Transverse wakefields – their different types and impact on beam dynamics •Transverse wake function](https://reader031.fdocuments.net/reader031/viewer/2022030500/5aabe2487f8b9a2b4c8ca899/html5/thumbnails/88.jpg)
Example non-relativistic wakes
• PS injection oscillations show intra-bunch modulations
• These can only be reproduced when adding non-relativistic wakes caused by indirect space charge fields
A. Huschauer et al
08/09/2017 Beam Instabilities III - Giovanni Rumolo and Kevin Li 88
Non-relativistic wake
Measurements
Macroparticle simulations