X-BAND CRAB CAVITIES FOR THE CLIC BEAM DELIVERY SYSTEM

1. X-BAND CRAB CAVITIES FOR THE CLIC BEAM DELIVERY SYSTEM Dr G Burt Cockcroft Institute / Lancaster Universtity P.K. Ambattu, A.C. Dexter, T. Abram - Lancaster V. Dolgashev, S. Tantawi - SLAC R. M. Jones - Manchester X-Band Workshop, CI, Dec 2008

2. Crab Cavity Function Crab cavities are required for ILC, LHC upgrade and CLIC The crab cavity is a deflection cavity operated with a 90o phase shift. A particle at the centre of the bunch gets no transverse momentum kick and hence no deflection at the IP. A particle at the front gets a transverse momentum that is equal and opposite to a particle at the back. The quadrupoles change the rate of rotation of the bunch. X-Band Workshop, CI, Dec 2008

3. Transverse magnetic and electric field components of the TM110 dipole mode combine to give the overall transverse momentum kick. The net transverse momentum kick is phase dependent. If the beam has an offset it can be accelerated or retarded by the longitudinal electric field and hence delivers or extracts power from the cavity. TM110 Dipole mode Electric Field Beam vertical horizontal Magnetic field cross section X-Band Workshop, CI, Dec 2008

4. ILC Crab cavity Based on FNAL 3.9 GHz CKM cavity 3.9GHz : compact longitudinally and transversely 3.9GHz cavity achieved 7.5 MV/m (FNAL) Input coupler LOM coupler HOM coupler SOM coupler To minimise wakefields for the short time structure of the ILC bunches, the number of cells must be optimised against overall length. Crab cavity needs extraction of LOM (avoid unwanted energy spread), SOMs and HOMs. X-Band Workshop, CI, Dec 2008

5. Transverse Kick for 3 TeV CM To minimise required cavity kick R12 needs to be large hence put the cavity close to IP (25 metres suggested) At 20 MV/m transverse gradient this is only 12 cm which is 10-30 cells depending on the cavity design and gradient. This is about 3 MW RF for a SW design probably more for TW. X-Band Workshop, CI, Dec 2008

6. electron bunch Δx positron bunch Interaction point Phase synchronisation requirement for no more than 2% luminosity loss Crabbed crossing angle with phase jitter Luminosity reduction factor S is given as and X-Band Workshop, CI, Dec 2008

7. t The cavity cell structure D Cell length t Iris thickness b Equator radius a Iris radius b Ri D Periodic boundary conditions Four independent cell parameters – but one fixed by frequency and one fixed by phase advance hence investigative plots only vary iris thickness and iris radius X-Band Workshop, CI, Dec 2008

8. Beam-loading Issues • Beam-loading is typically large and depends on bunch offset • Beam-loading might at worst vary randomly • Estimating voltage induced in crab cavity from one offset bunch • rb ~ 0.5 mm (hopefully not this bad) • R/Q = 4000 • = 0 q = 0.6 nC w = 2p 12 GHz Electric Field Beam horizontal Magnetic field dV = 190 V V = 2.4 MV / cells ~ 160 kV hence 16 offset bunches could shift amplitude by 2% X-Band Workshop, CI, Dec 2008

9. Group vel. vs iris radius and thickness Lines labelled in key correspond to differing iris thicknesses, 2 - 3 mm is preferred x-axis gives iris radius, 4 mm or above is preferred X-Band Workshop, CI, Dec 2008

10. Cavity Optimisation Lines labelled in key correspond to differing iris thicknesses, 2 - 3 mm is preferred x-axis gives iris radius, 4 - 5 mm is preferred X-Band Workshop, CI, Dec 2008

11. Short Wakes Short Range Wakes were calculated for several iris radii in ECHO 2D. Assuming 30 cells, a 0.25 mm horizontal beam offset and a 0.5 TeV energy beam the luminosity loss is 2% for a 5 mm iris. Longitudinal wake would be ~0.6 MV for 30 cells. X-Band Workshop, CI, Dec 2008

12. Cavity Design X-Band Workshop, CI, Dec 2008

13. Wakefields in Crab cavities TM010 accelerating mode Higher order modes TM110v Same order mode Need to extract the fundamental mode TM011 frequency Extraction of the lower order mode and the higher order modes is essential to minimise disruption of the beam. The cavity design should allow for as much LOM/SOM/HOM damping as possible. TE111h TM110h crabbing mode TE111v X-Band Workshop, CI, Dec 2008

14. Ways of polarising • Elliptical cells (requires CNC diamond tipped milling machine) • Squash cells • Coupling slots • Stubs (problems at high fields) • Couplers (need to avoid coupling to operating mode) X-Band Workshop, CI, Dec 2008

15. Mode Damping abs H field plots SOM, 10.947 GHz Q=33 Crab, 11.994 GHz Dipole: Q5729 LOM: 239 HOM: 1900 Dipole, phi=120 deg, 11.994 GHz, Q=5140 LOM, 9.12 GHz, Q=71 LOM, 8.11 GHz, Q=130 X-Band Workshop, CI, Dec 2008

16. Elliptical Damped Detuned structures (EDDS) Ellipticity of each cell is altered to detune the SOM throughout the structure. SOM detuning X-Band Workshop, CI, Dec 2008

17. Structure Design In order to have low fields in the matching cells we use the TE111 mode in the matching cells and the TM110 in the middle cells only. TE111 TM110 X-Band Workshop, CI, Dec 2008

18. Travelling wave simulation The structure has a peak electric field of 110 MV/m and a peak magnetic field of 350 kA/m and a transverse gradient of 37 MV/m for 20 MW input power. X-Band Workshop, CI, Dec 2008

19. Coupler Design for CTF3 tests H-field E-field X-Band Workshop, CI, Dec 2008