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Effect of Damping Resistors

Effect of Damping Resistors. Jose E. Varela on behalf of the BE-RF-BR team With input from J.A. Ferreira, B. Salvant and C. Zannini. The SPS Longitudinal Impedance Model. 200MHz TWC. 8 00MHz TWC. Vacuum Flanges. The 1.4GHz vacuum flange peak is comparable to the 800MHz cavity.

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Effect of Damping Resistors

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  1. Effect of Damping Resistors Jose E. Varela on behalf of the BE-RF-BR team With input from J.A. Ferreira, B. Salvant and C. Zannini

  2. The SPS Longitudinal Impedance Model 200MHz TWC 800MHz TWC Vacuum Flanges The 1.4GHz vacuum flange peak is comparable to the 800MHz cavity. * More details on the current SPS longitudinal impedance model can be found in [LIU-SPS BD WG meeting 27/03/2014]

  3. Why do we care about damping resistors? 200MHz TWC Vacuum Flanges Without damping resistors the 1.4GHz vacuum flange peak would be twice as big. High impedance + High Q Bad situation for instabilities. 800MHz TWC

  4. Flange Contribution to the Impedance Model An accurate impedance model is necessary for beam dynamics simulations. Currently, beam dynamics simulations do not reproduce measurements To some extent, due to an incomplete impedance model

  5. Comparison: Simulations - Measurements Many thanks to: Antoine Boucherie Jose A. Ferreira Sebastien Calvo Eric Montesinos

  6. Resonance Damping Damping Resistors greatly reduce the impedance and Q of resonances. With damping resistors -> Q < 300. However, what about the non-enamelled flanges where the damping resistor does not fit? Last week I measured: Vetronite disks enhance RF radiation losses. Therefore, Q↓. Can we put vetronite disks whenever a flange is opened? Preliminary results. Further measurments needed.

  7. So far… • Effect of the damping resistors • They reduce the impedance and Q of resonances by factor ≈5.5 / ≈3.5. • Thus, they greatly help for coupled bunch instabilities. • However… • The damping resistors do not help for single bunch instabilities since R/Q remains approximately constant. • A more drastic measure is needed for this…

  8. First Thoughts on Impedance Reduction Option 1: Ad-hoc bellows • Cons: • Expensive Bellows. • Difficult to implement (if possible) in already installed elements. • Pros: • Minimum Impedance (to be checked) • Enamel Compatible Flanges without additional gap MBA / QF Bellows Probably good solution for new elements going into the machine ?

  9. First Thoughts on Impedance Reduction Option 2: Bellow Shielding • Cons: • Special RF fingers for enamelled flanges. • Installation procedure. • Pros: • Chosen solution for the pumping port shielding campaign. RF fingers High-pass filter for enamelled flanges Shield For elements already in the machine ?

  10. Conclusions • Impedance model • The current model does not totally explain beam measurements. • Therefore, the current impedance model is incomplete . • The impedance model is being constantly updated. • Effect of the damping resistors • They reduce the Q of resonances by factor ≈5.5 / ≈3.5. • Thus, they greatly help for coupled bunch instabilities. • However… • The damping resistors do not help for single bunch instabilities since R/Q remains approximately constant. • A more drastic measure is needed for this… ‘First Thoughts on Impedance Reduction’ • However, at this point we can not assure that vacuum flanges are the only reason for single bunch instabilities in the SPS.

  11. Thanks for your attention

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