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