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)

Many longitudinal resonances whatever the angle from 800 MHz onwards.

Angle 1=15 degrees

Scan over Angle 2

Time domain wakefield simulations

Frequency in GHzLong

itudi

nal i

mpe

danc

e in

Ohm

(und

eres

timat

ed)

Angle IN: 10 degreesAngle Out: 10 degrees

Angle IN: 30 degreesAngle Out: 10 degrees

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

With eigenmode solver:Largest longitudinal mode at ~1 GHz: R~0.8 MOhm, Q= 65,000 Very large resonances, despite the longer taper

Angle IN Angle OUT

Angle IN Angle OUT

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

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

Still quite large, but factor 10 reduction. What is the acceptable limit?

Frequency (GHz)

Re(Z

long

)

Shun

t im

peda

nce

Mode number

What is the acceptable limit (1/2)• Limit for longitudinal instabilities

– Limit from design report in 400 MHz RF system: 200 kOhm for 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

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

Cavity length

• 106 mm radius (smaller radius push frequencies higher)• Copper coating (increases shunt impedance by a factor ~ 6 for 316LN)

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

Importance of taper (L=0.5m)Ll

lL = 0.5 m

The longer taper, the better

• 106 mm radius (smaller radius push frequencies higher)• Copper coating (increases shunt impedance by a factor ~ 6 for 316LN)

Importance of taper (L=1m)Ll

lL = 1 m

The longer taper, the better

• 106 mm radius (smaller radius push frequencies higher)• Copper coating (increases shunt impedance by a factor ~ 6 for 316LN)

Importance of taper (L=1.5m)Ll

lL = 1.5 m

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

• 106 mm radius (smaller radius push frequencies higher)• Copper coating (increases shunt impedance by a factor ~ 6 for 316LN)

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)

Cavity length increases Taper length decreases

Lll

L+2l = 2 m

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