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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.

- BGV design
- Impedance studies for the LHC
- First studies with 147 mm diameter
- Studies with smaller diameters and various geometries
- Conclusions and next steps

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

- BGV design
- Impedance studies for the LHC
- First studies with 147 mm diameter
- Studies with smaller diameters and various geometries
- Conclusions and next steps

- 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.

- BGV design
- Impedance studies for the LHC
- First studies with 147 mm diameter
- Studies with smaller diameters and various geometries
- Conclusions and next steps

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

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?

- 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

- 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

- 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

- 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

- 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

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

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

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)

- 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

- 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

- 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?).

- 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