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Update on structure optimization procedure, input and results. CLIC reference structure.

Detailed update on the optimization procedure of the CLIC reference structure, highlighting the new design for the CLIC accelerating structure, including RF performance parameters and challenges faced in previous structures.

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Update on structure optimization procedure, input and results. CLIC reference structure.

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  1. Update on structure optimization procedure, input and results. CLIC reference structure. 16.01.2008 Alexej Grudiev

  2. Introduction Optimization procedure: reminder and an update New design of CLIC accelerating structure Outline

  3. CLIC main linac optimization model taking into account complex interplay between beam dynamics and rf performance has been developed over the past few years In 2006, new experimental data both at 30 GHz and at X-band have been obtained CLIC total cost parametric model has become available Optimization of CLIC frequency and gradient has been done which (together with some other considerations) resulted in major change of CLIC parameters from 150MV/m at 30GHz to 100MV/m at 12GHz 2007: Design of X-band CLIC accelerating structure: from CLIC_A to CLIC_G Optimum frequency and gradient

  4. Parameters of previous structure CLIC_C

  5. Bunch population reduction from 5.8x109 to 4x109 due to beam dynamics reasons made that the structure became non-optimum. Maximum surface electric filed of almost 300 MV/m at more than 200 ns pulse length is alarming and certainly never was demonstrated before with BDR~10-6. A bug was found in pulsed surface heating temperature rise calculation for non-rectangular pulse. Correct value of 71 K is too high (compared to the limit: 56 K) What’s wrong with CLIC_C ?

  6. WDS cell geometry Waveguide Damped Structure (WDS) 2 cells • Minimize E-field • Minimize H-field • Provide good HOM damping • Provide good vacuum pumping

  7. Optimization procedure <Ea>, f, ∆φ, <a>, da, d1, d2 BD Bunch population Cell parameters N Q, R/Q, vg, Es/Ea, Hs/Ea Q1, A1, f1 Structure parameters Bunch separation BD Ns Ls, Nb η, Pin, Esmax, ∆Tmax rf constraints Cost function minimization YES NO

  8. Optimization constraints • Beam dynamics (BD) constraints based on the simulation of the main linac, BDS and beam-beam collision at the IP: • N – bunch population depends on <a>/λ, Δa/<a>, f and <Ea> because of short-range wakes (reduced since last ACE) • Ns – bunch separation depends on the long-range dipole wake and is determined by the condition: • Wt,2 · N / Ea= 10 V/pC/mm/m · 4x109 / 150 MV/m • RF breakdown and pulsed surface heating (rf) constraints: • ΔTmax(Hsurfmax, tp) < 56 K • Esurfmax < 250 MV/m (it was 380 MV/m before) • Pin/Cin·(tpP)1/3 = 18 MW·ns1/3/mm

  9. 250 MV/m 18 MW/mm·ns1/3 RF breakdown constraints for Cu 100ns, BDR=10-6 BDR=10-6

  10. Pulse shape dependences Pin/Pinload= 0.9 pl=Poutload/Poutunload tr tf tf tb tr η: tp = tb + tf + tr P/C*(tPp)1/3: tPp = time when Pin/Pinload> 0.9 ∆T~(tTp)1/2: tTp = tp-[tf●(1-pl)/2+ tr●(1-pl/2)] <-corrected

  11. Optimizing Figure of Merit Luminosity per linac input power: Collision energy is constant Figure of Merit (FoM = ηLbx/N) in [a.u.] = [1e34/bx/m2•%/1e9]

  12. Total cost model Total cost = Investment cost + Electricity cost for 10 years Ct = Ci + Ce Ci = Excel{fr; Ep; tp; Ea ; Ls ; f; Δφ} Repetition frequency; Pulse energy; Pulse length; Accelerating gradient; Structure length (couplers included); Operating frequency; rf phase advance per cell Ce = (0.032 + 2.4/FoM)

  13. Parameters of new structure CLIC_G

  14. Parameters of new structure CLIC_G Previous structure: CLIC_C New structure: CLIC_G

  15. Transverse impedances and wakes in cells Blue – first cell Red – middle cell Black – last cell

  16. Transverse long-range wakes in CLIC_G First dipole band Limit at 2nd bunch Tapered structure

  17. New design of CLIC accelerating structure (CLIC_G) has been done taking into account: Modified beam dynamics constraint on the bunch charge Reduced maximum surface electric field Reduced maximum pulsed surface heating temperature rise In the new structure, long-range wake field damping has been validated taking into account higher-order dipole modes. Nevertheless, there are still few issues to be addressed in the next CLIC accelerating structure rf design: Pulsed surface heating seems to be too high for annealed Cu structures. Surface electric field has not been demonstrated yet in an accelerating structure at the required pulse length and BDR. Summary

  18. CLIC performance and cost versus gradient Ecms = 3 TeV L(1%) = 2.0 1034 cm-2s-1 Performance Cost Previous Previous New New Optimum • Performance increases with lower accelerating gradient (mainly due to higher efficiency) • Flat cost variation in 100 to 130 MV/m with a minimum around 120 MV/m

  19. CLIC performance and cost versus frequency Ecms = 3 TeV L(1%) = 2.0 1034 cm-2s-1 Performance Cost New Optimum Previous Previous New Optimum • Maximum Performance around 14 GHz • Flat cost variation in 12 to 16 GHz frequency range with a minimum around 14 GHz

  20. FoM =Lbx/N · η RF BD Interplay between BD and RF Lbx/N BD optimum aperture: <a> = 2.6 mm Why X-band ? Crossing gives optimum frequency RF optimum aperture: <a>/λ = 0.1 ÷ 0.12

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