Agenda:

General kickers analysis

Wang-Tsutsui method for computing impedances

Benchmarks

Conclusions

Bibliography

Acknowledgments: E.Métral, M.Migliorati, A.Mostacci, N.Mounet, B.Salvant, H.Tsutsui, V.Vaccaro, N.Wang, C.Zannini.

Kickers analysis and benchmarkN.Biancacci

General kicker analysis

Kickers are one of the most important contributors to the global value of impedance in accelerator rings.

Constant studies are carried on at CERN in order to correctly evaluate their impedance contribution and, in case, reduce it.

In this direction we want to:

1. compute the impedance for a model as close as possible to the real one,

2. compute the impedance for any value of β (i.e. in PS we have β=0.91 at injection).

3. update our machine models in HEADTAIL simulations.

The inner C-shape magnet has been modeled in many different ways. Mainly we’ll consider Tsutsui’s model (case a) comparing it with a flat geometry model studied by N.Mounet-E.Metràl (case b).

General kicker analysis

Vacuum

Ferrite

PEC

t

b

a

(a) Tsutsui’s model

(b) Flat chamber model

Tsutsui-Wang’s method

F

Method description:

V

A field matching method is applied:1. Divide geometry in ferrite (F) and vacuum (V) subdomains.2. Solve Helmholtz equation in F + boundaries3. Solve Helmholtz equation in vacuum splitting the inner field in Evacuum=Esource+Eresidual.

The residual field can be expressed in terms of waveguides modes (HOMogeneus Helmholtz equation in vacuum).

Hom.Helmholtz Eferrite

+ Hom.Helmholtz Eresidual

“free space+plates” Esource

Approximation: the source field is approximated as being in free space limited by two vertical parallel plates.Avantages: 1) the impedance will be computed only using the homogeneus solution, directly separating direct SC due to the beam itself, and indirect SC due to horizontal image currents. 2) Avails the following Fourier development for matching on ferrite-vacuum layer.

Tsutsui-Wang’s method

Method description:

4. Set matching condition for Ez, Hz, Ex, Dy at the ferrite-vacuum boundary.5. The system coming out from matching procedure is a 4x4 system solvable symbolically.

Some symmetry consideration around source field leads to further semplifications in the final unknowns expression.

6. Impedance calculation: Basically integrating Eresidual along the paths shown in the pictures (X cross = path; green spot = Beam position).

x x x

Zlong Zdriv Zdet

beta=1 H.Tsutsui H.Tsutsui B.Salvant

beta<=1 N.Wang N.Wang N.Biancacci

Beta and models:

Zlongitudinal ZxDipolar ZxQuadrupolar

xx

ZyDipolar ZyQuadrupolar

Technical Note: Direct and indirect SC effects have been directly separated at the beginning splitting the vacuum field as sum of Evacuum=Esource+Eresidual. In N.Mounet-E.Metral method this is done at the end, separating the impedance contributions.

Wang-Tsutsui Impedances

PS SPS LHCLinacPSB

machine Kinetic energy (Extraction)

β at extraction

LINAC 50 MeV 0.314

PSB 1.4 GeV 0.916

PS 26 GeV 0.9993

SPS 450GeV 0.999998

LHC 3.5 TeV 0.999999991

We choose three values of β in Wang-Tsutsui impedance calculation: 0.85, 0.9, 0.99999

xxx

Relativistic β starts to be significantly different from 1 in PSB and PS at injection.

β=0.99999

β=0.9

β=0.85

Wang-Tsutsui Impedances

β=0.99999

β=0.9

β=0.85

Wang-Tsutsui Impedances

β=0.99999

β=0.9

β=0.85

Wang-Tsutsui Impedances

β=0.99999β=0.9

β=0.85

Wang-Tsutsui Impedances

Wang-Tsutsui Impedances

2- Tsutsui-Wang Vs CST

The same structure is implemented in CST. Beta less than one simulations should agree with N.Wang theory. Tsutsui β=1 already benchmarked in the past.

1- Tsutsui-Wang Vs Mounet-Metral

N.Mounet and E.Metràl developed the analysis for a two infinite parallel multilayer flat chamber, for any β. Taking Tsutsui–Wang's theory in the limit a → ∞ we should have a convergence between these two models.

a

Benchmarks

a → ∞

1- Theory Vs Theory

Good agreement between the two theories!

Longitudinal impedance for N.Mounet-E.Metral model and N.Wang-H.Tsutsui one.

Ferrite Model

Im(Z) decrease with β

Re(Z) increase with β

1- Tsutsui-Wang Vs Mounet-Metral

a

ferrite

1- Tsutsui-Wang Vs Mounet-Metral

a

ferrite

Im(Z) decrease with β

Re(Z) decrease with β

!

!

From theory, the imaginary part of transverse propagation constant in ferrite becomes negative (damping modes). -1/Ky~2cm < t = 6cm

One more check...

2 layers 1 layer

Ky ( f )

1- Tsutsui-Wang Vs Mounet-Metral

Eliminating PECs and extending ferrite to infinity we expect the beam “doesn't see” the boundaries from ~10MHz.

≈

f >10MHz

1- Tsutsui-Wang Vs Mounet-Metral

Graphite

Re(Z) increase with β

Im(Z) decrease with β

1- Tsutsui-Wang Vs Mounet-Metral

Graphite

Re(Z) increase with β

Im(Z) decrease with β

A first rough model for MKP was studied in CST and compared with Wang’s impedances. The real part of Zlong shows a good agreement for different values of β. On the contray the imaginary part shows a strong discrepancy probably dued to code artefacts in the ports setup and SC effects inclusion.

β=1

β=0.95

2- Tsutsui-Wang Vs CST

2- Benchmarking

● N.Wang’s formulas were benchmarked with Tsutsui’s ones in the limit β1 with success.

● N.Wang formulas were benchmarked with N,Mounet-E.Metral flat chamber showing basically a good agreement. Simulations for ferrite and graphite were performed.

● N.Wang formulas were benchmarked also with CST code without success. Probably a problem in the ports setup.

1- Tsutsui-Wang model

● Tsutsui-Wang model for kicker was studied in dedail understanding procedure and main assumptions

● Longitudinal, dipolar impedance was derived implementing N.Wang new formulas for β<=1. Also the quadrupolar component has been derived.

● Imaginary part of the impedance is mainly decreasing for high frequencies (above 1GHz), the real part is instead increasing.

Conclusions

"Coupling impedance and collective effects in the RCS ring of the China spallation neutron source" N. Wang, PhD thesis

"Longitudinal wakefields and impedance in the CSNS/RCS" N. Wang, Q. Qin, EPAC 2008

"Transverse Coupling Impedance of a Simplified Ferrite Kicker Magnet Model", H. Tsutsui

"Some Simplified Models of Ferrite Kicker Magnet for Calculation of Longitudinal Coupling Impedance", H. Tsutsui, CERN-SL-2000-004-AP, 2000

Impedances of an Infinitely Long and Axisymmetric Multilayer Beam Pipe: Matrix Formalism and Multimode Analysis / Mounet, N (EPFL, Lausanne) ; Metral, E (CERN)

Bibliography