Update on bgv impedance studies
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Update on BGV impedance studies. Alexej Grudiev, Berengere Luthi, Benoit Salvant for the impedance team Many thanks to Bernd Dehning, Massimiliano Ferro-Luzzi, Plamen Hopchev, Nicolas Mounet, Elena Shaposhnikova. Agenda. BGV design Impedance studies for the LHC

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Update on BGV impedance studies

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Update on BGV impedance studies

Alexej Grudiev, Berengere Luthi, Benoit Salvant for the impedance team

Many thanks to Bernd Dehning, Massimiliano Ferro-Luzzi, Plamen Hopchev, Nicolas Mounet, Elena Shaposhnikova.


Agenda

  • BGV design

  • Impedance studies for the LHC

  • First studies with 147 mm diameter

  • Studies with smaller diameters and various geometries

  • Conclusions and next steps


Design of new LHC BGV (Beam Gas Vertex detector)to be installed in LS1

  • request by Plamen, Bernd (BE-BI) and Massimiliano (LHCb)


Agenda

  • BGV design

  • Impedance studies for the LHC

  • First studies with 147 mm diameter

  • Studies with smaller diameters and various geometries

  • Conclusions and next steps


Impedance studies in LHC

  • We study the electromagnetic fields generated by the LHC beam when passing through the BGV.

  • These fields perturb the guiding fields, and can lead to

    • Beam instabilities (longitudinal and transverse)  beam losses and/or emittance growth (many occurrence of transverse instabilities in 2012)

    • Beam induced heating of the surrounding  loss of performance, outgassing, deformation, or destruction of the equipment(many examples in 2012: TDI, BSRT, ALFA, MKI, TOTEM, vacuum bellows)

  • In view of higher brightness after LS1, we need to carefully study all planned installation and modifications of LHC hardware.


Agenda

  • BGV design

  • Impedance studies for the LHC

  • First studies with 147 mm diameter

  • Studies with smaller diameters and various geometries

  • Conclusions and next steps


First studies with CST (with initial radius of 147 mm)

Scan over Angle 2

Time domain wakefield simulations

Angle 1=15 degrees

Longitudinal impedance in Ohm (underestimated)

Frequency in GHz

 Many longitudinal resonances whatever the angle from 800 MHz onwards.


Angle IN: 10 degrees

Angle Out: 10 degrees

Angle IN

Angle OUT

With eigenmode solver:

Largest longitudinal mode at ~1 GHz: R~1 MOhm, Q= 40,000

Angle IN: 30 degrees

Angle Out: 10 degrees

Angle IN

Angle OUT

With eigenmode solver:

Largest longitudinal mode at ~1 GHz: R~0.8 MOhm, Q= 65,000

 Very large resonances, despite the longer taper


New geometry (smaller radius requested by Plamen : 130 mm) : taper IN : 6 degrees and taper OUT: 30 degrees

Re(Zlong)

Frequency (GHz)

Shunt impedance

With eigenmode solver:

Many longitudinal modes after 900 MHz: R~0.07 MOhm, Q between 40,000 and 65,000

Mode number

Still quite large, but factor 10 reduction.

 What is the acceptable limit?


What is the acceptable limit (1/2)

  • Limit for longitudinal instabilities

    • Limit from design report in 400 MHz RF system: 200 kOhmfor ultimate intensity, 2.5 eVs longitudinal emittance at 7TeV (E. Shaposhnikova BE/RF-BR).

    • Hard limit below 500 MHz. In principle, less critical above 500 MHz.

    • However, much safer to stay below 200 kOhm for all frequency range


What is the acceptable limit (2/2)

  • Limit for beam induced heating:

    • The cooling system should be dimensioned to cope with the power lost in the device

    • Ex: 70 kOhm at 900 MHz with 50 ns beam at 1.6e11 p/b Ploss~ 700 W

    • Ex: 70 kOhm at 1100 MHz with 50 ns beam at 1.6e11 p/b Ploss~ 100 W

    • Ex: 70 kOhm at 1200 MHz with 50 ns beam at 1.6e11 p/b Ploss~ 5 W

 It is critical for both limits to:

 push the mode frequencies as high as possible

 reduce the shunt impedance below 200 kOhm


Agenda

  • BGV design

  • Impedance studies for the LHC

  • First studies with 147 mm diameter

  • Studies with smaller diameters and various geometries

    • Impact of cavity length

    • Impact of taper length

  • Conclusions and next steps


Impact of cavity length on shunt impedance of the highest mode

  • 106 mm radius (smaller radius push frequencies higher)

  • Copper coating (increases shunt impedance by a factor ~ 6 for 316LN)

Cavity length

  • Not monotonic

  • The length of the cavity should not be too small

  • Frequency of the modes is not plotted, but is also important to assess their effects


Agenda

  • BGV design

  • Impedance studies for the LHC

  • First studies with 147 mm diameter

  • Studies with smaller diameters and various geometries

    • Impact of cavity length

    • Impact of taper length

  • Conclusions and next steps


L = 0.5 m

Importance of taper (L=0.5m)

l

L

l

  • 106 mm radius (smaller radius push frequencies higher)

  • Copper coating (increases shunt impedance by a factor ~ 6 for 316LN)

 The longer taper, the better


L = 1 m

Importance of taper (L=1m)

l

L

  • 106 mm radius (smaller radius push frequencies higher)

  • Copper coating (increases shunt impedance by a factor ~ 6 for 316LN)

l

 The longer taper, the better


L = 1.5 m

Importance of taper (L=1.5m)

l

L

  • 106 mm radius (smaller radius push frequencies higher)

  • Copper coating (increases shunt impedance by a factor ~ 6 for 316LN)

l

  • The longer taper, the better!

  • The longer cavity length, the better (at least above , complete study ongoing)


Agenda

  • BGV design

  • Impedance studies for the LHC

  • First studies with 147 mm diameter

  • Studies with smaller diameters and various geometries

    • Impact of cavity length

    • Impact of taper length

    • What is the best if total length= 2m?

  • Conclusions and next steps


A more realistic geometry

  • 106 mm radius (smaller radius push frequencies higher)

  • Copper coating (increases shunt impedance by a factor ~ 6 for 316LN)

  • Full length of about 2 m (taper included)

l

L

l

L+2l = 2 m

Cavity length increases

 Taper length decreases


Zoom below the limit

  • The longer the taper, the better (for the symmetric case)

  • Even with copper coating, well below the limit below 1.5 m of flat length (with Ploss of 40 W  is it acceptable from mechanical point of view?).


Conclusions and next steps

  • There is hope with 106 mm radius!

  • Can the system take ~ 50 W of power loss?

  • Actual mechanical constraints to be added to the next round of simulations  What is feasible?

  • Checks of the transverse impedance


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