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
* More details on the current SPS longitudinal impedance
model can be found in [LIU-SPS BD WG meeting
27/03/2014]
200MHz TWC
Vacuum Flanges800MHz TWC
The 1.4GHz vacuum flange peak is
comparable to the 800MHz cavity.
Why do we care about damping resistors?200MHz TWC Vacuum Flanges
800MHz TWC
Without damping resistors the 1.4GHz vacuum flange
peak would be twice as big.
High impedance + High Q
Bad situation for instabilities.
Flange Contribution to the Impedance ModelElement Enamel Resistor * Num. f [GHz] Z [kΩ] Q R/Q
[Ω]
Flanges[ Simulation Table ]
* Damping Resistors have not been included in Simulations. This column states whether or not the
flange SHOULD have a damping resistor inside (and its type).
** The damping resistors are estimated to halve the Q of enamelled flanges. For non-enamelled flanges, the damping resistors are estimated to lower the Q by a
factor of 10.
*** Based on survey carried out by Jose A. Somoza.
! Damping resistor presence percentage is assumed to be identical to the *** case.
Yes No 90 1.210 633 315 2010
Yes Long 39 1.280 499 200 2495
Yes Short 90% of 83 ! 1.410 722 134 ** 5388
Yes No 10% of 83 ! 1.410 160 268 597
Yes Short 90% of 14 ! 1.410 133 143 ** 930
Yes No 10% of 14 ! 1.410 30 285 105
No Short 90% of 26 *** 1.410 449 242 ** 1855
No No 10% of 26 *** 1.410 377 1828 206
Yes No 99 1.570 17.4 55 316
No No 20 1.610 588 980 600
Yes Long 39 1.620 61 60 1016
No No 75 1.800 651 881 739
Yes No 99 1.890 187 175 1070
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
Comparison: Simulations - Measurements
Preliminary Results
Damping Resistor
fres [GHz]
Q R/Q [Ω]
MBA – QF
Non enamelled
Sim. No 1.415 1800 82
Meas. No 1.401 1100≈ 5.5
85 ± 2.5%
Meas. Short 1.395 200 81 ± 2.5%
MBA – MBA
Enamelled
Sim. No 1.410 285 75
Meas. No 1.415 270≈ 3.5
79 ± 5%
Meas. Short 1.415 75 65 ± 5%
Many thanks to:Antoine Boucherie
Jose A. FerreiraSebastien CalvoEric Montesinos
Resonance Damping
Element Enam Resistor * Num. f
[GHz] Z [kΩ] Q R/Q [Ω]
Flanges[ Simulation Table ]
* Damping Resistors have not been included in Simulations. This column states whether or not the
flange SHOULD have a damping resistor inside (and its type).
** The damping resistors are estimated to halve the Q of enamelled flanges. For non-enamelled flanges, the damping resistors are estimated to lower the Q by a
factor of 10.
*** Based on survey carried out by Jose A. Somoza.
! Damping resistor presence percentage is assumed to be identical to the *** case.
Yes No 90 1.210 633 315 2010
Yes Long 39 1.280 499 200 2495
Yes Short 90% of 83 ! 1.410 722 134 ** 5388
Yes No 10% of 83 ! 1.410 160 268 597
Yes Short 90% of 14 ! 1.410 133 143 ** 930
Yes No 10% of 14 ! 1.410 30 285 105
No Short 90% of 26 *** 1.410 449 242 ** 1855
No No 10% of 26 *** 1.410 377 1828 206
Yes No 99 1.570 17.4 55 316
No No 20 1.610 588 980 600
Yes Long 39 1.620 61 60 1016
No No 75 1.800 651 881 739
Yes No 99 1.890 187 175 1070
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:Non-enamelled MBA – QF Q
Simulation 1800
Measurement 1100
Measurement with Vetronite 550
Vetronite disks enhance RF radiation losses. Therefore, Q↓.
Can we put vetronite disks whenever a flange is opened?
Preliminary results. Further measurments needed.
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…
First Thoughts on Impedance Reduction
Pros:• Minimum Impedance (to be checked)• Enamel Compatible
MBA / QF Bellows
Flanges without
additional gap
Cons:• Expensive Bellows.• Difficult to implement (if possible) in
already installed elements.
Option 1: Ad-hoc bellows
Probably good solution for new elements
going into the machine ?
First Thoughts on Impedance Reduction
Pros:• Chosen solution for the pumping port
shielding campaign.
Cons:• Special RF fingers for enamelled flanges.• Installation procedure.
Option 2: Bellow Shielding
Shield
RF fingers
High-pass filter for
enamelled flanges
For elements already in the machine ?
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.
Thanks for your attention