Update on BGV impedance

August 1st 2013

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

Many thanks to Bernd Dehning, Massimiliano Ferro-Luzzi,

Plamen Hopchev, Nicolas Mounet, Elena Shaposhnikova, Ray Veness.

• Longitudinal modes for the asymmetric case• Low frequency longitudinal impedance• Low frequency transverse impedance• Starting to check transverse modes

Longitudinal impedance at high frequency Total length: 2 m Copper and Steel structure

Angle 2=5 degrees

Scan over Angle 1

The lower the tapering angle, the better (also for power loss) Under the limit even with angle close to 90°

Copper

Stainless Steel 316LNCopper

Largest mode: 60 W

Largest mode: 6 W

Longitudinal impedance at high frequency Total length: 2 m Angle 2=5 degrees

Scan over Angle 1

Copper

Stainless Steel 316LNCopper

Longitudinal impedance at low frequency

Angle in degrees

Impedance (Z/n) in mOhm

Impedance Z/nin % of total LHC

10 0.20 0.2220 0.32 0.3630 0.39 0.4340 0.44 0.4950 0.52 0.5760 0.60 0.6670 0.67 0.7580 0.76 0.84

89.5 0.84 0.94

Long

itudi

nal i

mpe

danc

e Z

(in O

hm)

Frequency in GHz

Should be minimized to avoid longitudinal instabilities (currently 90 mOhm) Total length: 2 m

Angle 2=6 degrees

Scan over Angle 1

One single BGV is not negligible compared to the rest of the machine. The lower the tapering angle, the better (factor ~5 between 10 and 90 degrees)

Transverse impedance at low frequency Should not be increased to avoid transverse instabilities (currently 2 MOhm/m at injection) Total length: 2 m

Angle 1=6 degrees

Scan over Angle 2

Tran

sver

se im

peda

nce

Z (in

Ohm

/5m

m d

ispl

acem

ent)

Frequency in GHz

Angle in degrees

Transverse Impedance in kOhm/m

Transverse Impedance

in % of total LHC10 0.86 0.1020 1.18 0.1430 1.46 0.1740 1.66 0.2050 1.84 0.2260 2.04 0.2470 2.20 0.2680 2.36 0.28

89.5 2.28 0.27

Limited impact on low frequency transverse impedance

Transverse impedance at high frequency Total length: 2 m Angle 2=5 degrees

Scan over Angle 1

Copper

Stainless Steel 316LNCopper

Beta functions at the location of the BGV: 167 m (from Plamen). These modes need to be multiplied by beta/<beta>=2.4 (i.e. a range of 1 MOhm/m) Need to check the impact on beam dynamics

Some peaks can be fake due to large noise being checked

Conclusions so far

• Taper clearly helps also in the asymmetric case• Longitudinal and transverse low frequency impedance are

between 0.1% and 1% of the full LHC impedance.• Effect of transverse modes for the final structure should be

checked by beam dynamics studies