Effect of Damping Resistors

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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. - PowerPoint PPT Presentation

Text of Effect of Damping Resistors

LIU DAY

Effect of Damping ResistorsJose 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 TWCVacuum Flanges800MHz TWCThe 1.4GHz vacuum flange peak is comparable to the 800MHz cavity.

Why do we care about damping resistors?200MHz TWCVacuum Flanges800MHz TWCWithout 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 ModelElementEnamelResistor *Num.f [GHz]Z [k]QR/Q []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.YesNo901.2106333152010YesLong391.2804992002495YesShort90% of 83 !1.410722134 **5388YesNo10% of 83 !1.410160268597YesShort90% of 14 !1.410133143 **930YesNo10% of 14 !1.41030285105NoShort90% of 26 ***1.410449242 **1855NoNo10% of 26 ***1.4103771828206YesNo991.57017.455316NoNo201.610588980600YesLong391.62061601016NoNo751.800651881739YesNo991.8901871751070An 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 model4

Comparison: Simulations - Measurements

Preliminary ResultsDamping Resistorfres [GHz]QR/Q []MBA QF

Non enamelledSim.No1.415180082Meas.No1.4011100 5.585 2.5%Meas.Short1.39520081 2.5%MBA MBA

EnamelledSim.No1.41028575Meas.No1.415270 3.579 5% Meas.Short1.4157565 5%Many thanks to:Antoine BoucherieJose A. FerreiraSebastien CalvoEric MontesinosResonance DampingElementEnamResistor *Num.f [GHz]Z [k]QR/Q []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.YesNo901.2106333152010YesLong391.2804992002495YesShort90% of 83 !1.410722134 **5388YesNo10% of 83 !1.410160268597YesShort90% of 14 !1.410133143 **930YesNo10% of 14 !1.41030285105NoShort90% of 26 ***1.410449242 **1855NoNo10% of 26 ***1.4103771828206YesNo991.57017.455316NoNo201.610588980600YesLong391.62061601016NoNo751.800651881739YesNo991.8901871751070Damping 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 QFQSimulation1800Measurement1100Measurement with Vetronite550Vetronite disks enhance RF radiation losses. Therefore, Q.

Can we put vetronite disks whenever a flange is opened?

Preliminary results. Further measurments needed.So farEffect of the damping resistorsThey reduce the impedance and Q of resonances by factor 5.5 / 3.5.Thus, they greatly help for coupled bunch instabilities.

HoweverThe damping resistors do not help for single bunch instabilities since R/Q remains approximately constant.A more drastic measure is needed for thisFirst Thoughts on Impedance ReductionPros:Minimum Impedance (to be checked)Enamel Compatible

MBA / QF BellowsFlanges without additional gapCons:Expensive Bellows.Difficult to implement (if possible) in already installed elements.Option 1: Ad-hoc bellowsProbably 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

ShieldRF fingers

High-pass filter for enamelled flangesFor elements already in the machine ?ConclusionsImpedance modelThe 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 resistorsThey reduce the Q of resonances by factor 5.5 / 3.5.Thus, they greatly help for coupled bunch instabilities.

HoweverThe 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