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DDS limits and perspectives

DDS limits and perspectives. Alessandro D’Elia on behalf of UMAN Collaboration. Damped and detuned design. Detuning: A smooth variation in the iris radii spreads the dipole frequencies. This spread does not allow wake to add in phase

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DDS limits and perspectives

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  1. DDS limits and perspectives Alessandro D’Elia on behalf of UMAN Collaboration

  2. Damped and detuned design • Detuning: A smooth variation in the iris radii spreads the dipole frequencies. This spread does not allow wake to add in phase • Error function distribution to the iris radii variation results in a rapid decay of wakefield. • Due to limited number of cells in a structure wakefieldrecoheres. • Damping: The recoherence of the wakefield is suppressed by means of a damping waveguide like structure (manifold). • Interleaving neighbouring structure frequencies help enhance the wake suppression

  3. VDL

  4. Why a Detuned Damped Structure (DDS) for CLIC • Huge reduction of the absorbing loads: just 4x2 loads per structure • Inbuilt Wakefield Monitors, Beam Position Monitors that can be used as remote measurements of cell alignments • Huge reduction of the outer diameter of the machined disks

  5. CLIC_DDS_A: regular cell optimization The choice of the cell geometry is crucial to meet at the same time: Wakefield suppression Surface fields in the specs DDS1_C DDS2_E Consequences on wake function Cell shape optimization for fields

  6. RF Properties of CLIC_DDS_A in comparison with CLIC_G * 312 bunches, only first dipole band ** 120 bunches, quarter structure GdfidL wake

  7. A new approach: a Hybrid Structure for CLIC_DDS_B Hybrid Structure WGD_Structure DDS_Structure + =

  8. Study of the wake function The problem 571MHz; F=2GHZ  F Question: How big must be F in order to have acceptable wake damping starting from 0.5ns?

  9. Study of the wake function F=2GHZ F=2.5GHZ Wt16-7V/[pC mm m], considering that W(0)170-180V/[pC mm m], the maximum acceptable bump must be 4% F=2.9GHZ F2.9GHz and 0.830GHz

  10. What about a “Sinc” wake? Wake uncoupled Real(Zx) Wake coupled 2Kdn/df This is the wakefield considering only the first dipole band

  11. What about a “Sinc” wake? GdfidL “Full Wake” 1st Dipole wake from GdfidL The presence of the higher order bands makes the scenario even less comfortable Conclusion: It is not possible to control the position of the zeros along the wake, a smooth function of the impedance is needed

  12. Can other types of distributions improve the wake decay? 906MHz F=2.9GHZ 830MHz

  13. Can other types of distributions improve the wake decay? 967MHz F=2.9GHZ 1.036GHz

  14. Can other types of distributions improve the wake decay? F=2.5GHZ =1GHz 926MHz

  15. What about 0.67ns? F=2GHZ

  16. How big is the bandwidth we may achieve? We must consider that 400-500<Av. Cross.<800-900 in order to get Qs in the order of 500-600 which will preserve the fsyn distribution Assuming SlotW constant throughout the full structure NB: The BW has been evaluated considering the difference between 1st Reg. Cell and Last Reg. Cell, i.e. Cell#27, but the total number of the cells is 26 (26 cells  27 irises); then the real BW will slightly decrease in the real structure

  17. Bandwidth coupled and uncoupled • - Uncoupled 27 cells: F= 2.685GHz • Uncoupled 26 cells (not shown): F= 2.47GHz • Coupled (GdfidL): F= 2.363GHz Av. Cross~600MHz From theoretical distribution to real structure one must take into account a reduction of ~200MHz in the BW

  18. What is the bandwidth of the real coupled structure? GdfidL Uncoupled wake with 25 peaks (F=2.314GHz) Reconstructed wake (only 1st Dipole band) The uncoupled wake with 25 frequencies (black dashed curve, F=2.314GHz) falls faster than the 1st dipole band reconstructed wake from GdfidL (red dashed curve): is there any strange effect from uncoupled to coupled that further reduce the bandwidth?

  19. Non Linear Fit to improve wake reconstruction The procedure: • I take GdfidL wake as “objective” function of my non linear regression • I use reconstruction formula as my fitting function • Fsyn are considered as given from Lorentzian fit of the impedance peaks while Qdip and Kicks are the parameters to be optimized • Initial guess for Qdip and kicks are from Lorentzian fit

  20. Results (1) The agreement with GdfidL is quite good and, as expected, the new procedure produces a major correction at the beginning of the curve while for the rest there are no appreciable variation with the wake reconstructed using the data from Lorentzian fit. =94 =67 <Qdip>=312 <Qdip>=512 It is clear that the wake is reconstructed from unphysical values of kicks and Qdip. Constraints on the parameters are needed.

  21. Results (2) =94 =67 <Qdip>=312 <Qdip>=337 With same constraints and an appropriate length of the wake, kicks and Qdip starts to converge.

  22. First results for sech1.5 Very preliminary 2Kdn/df Very sharp deep, before 0.15m Need to finalize the simulation to finalize the analysis

  23. Conclusions • With conventional DDS (DDS_A) it seems very difficult to meet beam dynamics criteria • With hybrid DDS, using Gaussian distribution, it seems non realistic to get damping within 6 RF cycles • With different distribution (in particular sech1.5) it is possible to relax the constraint on the BW and this could allow to stay in the 0.5ns bunch spacing • Play with Kdn/df would be interesting to see what happen and especially whether it is possible to increase the bandwidth by distributing differently the frequencies • However the requirement of 0.5ns is quite tricky and we have not yet considered surface fields… • I would not close totally the door to 8 RF cycles

  24. THANKS Igor

  25. Additional slides

  26. Physical interpretation of the result • Constraints: • First and last three peaks in the impedance are well separated then their Qdip and kicks are considered fixed • The rest of the kicks must be positive and spanning in a range from zero to roughly 10 • The rest of the Qdip can span from zero to a maximum of 1500 =94 =67 <Qdip>=312 <Qdip>=576 Wake is still well approximated but kicks and especially Qdip do not seem correct. The constraints I gave are still not enough.

  27. Extrapolation for longer wake If I extrapolate for a longer wake it is clear that Qdip and kicks evaluated from Non Linear Fit are not correct. I need more wake to improve Qdip calculation

  28. Increasing the length of the wake: 10m =67 =67 <Qdip>=315 <Qdip>=312 This makes me much more confident on the wake reconstruction

  29. Going back to the beginning Question was: can I evaluate the bandwidth reduction from uncoupled? 2Kdn/df Uncoupled 27 Cells Uncoupled 26 Cells Uncoupled 25 Cells From GdfidL Uncoupled 25 Cells Answer: It seems Yes, with some minor approximation. In particular in this case it is clear that the major reduction comes from one peak which is missed. Then I estimate a reduction of ~230MHz and not of 322MHz  if I choose ~2.75GHz, I should stay around 2.5GHz which is the minimum required for sech1.5 distribution.

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