Impedance of SPS travelling wave cavities (200 MHz) A. Grudiev, E. Métral, B. Salvant, E. Shaposhnikova, B. Spataro Acknowledgments: Erk Jensen, Eric Montesinos, Rolf Wegner Impedance meeting – March 3 rd 2010

This is a work in progress…

Agenda Context EM Simulations to get the transverse wakes –Particle Studio –Try with ABCI Effect of these transverse wakes on beam dynamics Future plans

Context: SPS impedance model A potential way to improve the LHC performance is to increase the intensity in the SPS Fast vertical instability is one of the limitations to increase the intensity in the SPS  many arguments point towards a Transverse Mode Coupling Instability Need to identify the sources of transverse impedance in the SPS to be able to find more solutions to increase the threshold of this instability current impedance model (kickers, beam pipe, BPHs, BPVs) explains ~60% of the tune shift with intensity and instability thresholds when used in HEADTAIL beam dynamics simulations. (direct space charge is not included in these simulations). Other potential sources?  RF cavities  septa  pumping ports

Context: SPS 200 MHz TW cavities 4 cavities –ACTA: 4*11 cells (  x ~ 31m,  y ~ 76 m) –ACTB: 4*11 cells (  x ~84 m,  y ~27 m) –ACTC: 5*11 cells (  x ~25 m,  y ~91 m) –ACTD: 5*11 cells (  x ~85 m,  y ~27 m) Example: ACTB 4 m 18.4 m For HEADTAIL, to lump all these impedances in the same point with  x =  y =40 m, need to apply a factor 24 to the wake of a single cell

Context: previous studies Resonant impedances in the SPS (T. Linnecar and E. Shaposhnikova) SL-Note RF and G. Dome (CERN-SPS/ARF/77-11)  Longitudinal and transverse parameters of known impedances in the SPS  SW and TW cavities, kickers, septa, pumping ports included Longitudinal Transverse (dipolar most likely) From SL-Note RF

Context: previous studies Mafia Electromagnetic simulations by Bruno Spataro –Simulations of 9 cells instead of 11 (very large structure) and checked linearity with cells of the low frequency part

Simulations with CST Particle Studio (1) input file of Bruno was translated. (2) Sharp edges were Blended (small impact) (3) the couplers on each side were added (4) cavity was rotated by 90 degrees around z (to be checked with Bruno) (5) bunch length=2 cm rms (~6 million mesh cells)

Transverse wakes (displacement=1 cm) s (cm) Wy (V/pC) Wx (V/pC) s (cm) Wdip (V/pC) Wquad (V/pC)

Transverse impedance (displacement=1 cm) s (cm) Re(Zy) (Ohm) f (GHz) Im(Zy) (Ohm) Re(Zx) (Ohm) Im(Zx) (Ohm) f (GHz)

Remarks Very similar results than Bruno for the vertical dipolar impedance. Quadrupolar peaks are located at the same frequencies as the longitudinal resonances (200 MHz and 630 MHz)  noise or true signal? Im(Zx)=Im(Zy)~200  at 1 cm  Im(Z trans dipolar ) ~ 0.5 M  /m for all the cavities Source bunch length is large  DFT/iDFT to get wake function is more tricky than for kickers  use of wake potential directly  widens artificially the wake imported into HEADTAIL  need to think of other solutions: 2D codes or GdfidL  first try with ABCI (2D code)

First try with ABCI Rotational symmetry implies that the rods cannot be accounted for

First try with ABCI 200 MHz??? Bunch length=10 cm (rms)

Back to Particle Studio… Same structure as ABCI Original structure from Bruno

Back to Particle Studio… s (cm) Wz (V/pC) s (cm) Zlong (Ohm) Longitudinal wake Longitudinal impedance (1) CST PS and ABCI are giving very similar results (hopefully!!!) (2) Maybe we should stick to 3D simulations in this case…

Effect on beam dynamics Wakes to be input into Headtail s (m) Wx dipolar (V/pCmm) Wy dipolar (V/pCmm) Wx quad (V/pCmm) Without cavities With cavities 2 cm rms bunch length is probably too large  need to find tricks or change code

Mode spectrum with HEADTAIL accounting for the current model Threshold seen at p/b Seems to be between modes -1 and -2 More to be done…

Future plans Get the low currents to obtain the tune shift Reduce the bunch length to obtain a better wake over 1 m –Try with GdfidL Include other RF cavities, septa and pumping ports in the model Refine existing models (in particular the kickers)