#3205 Summary 6th Nov 2012• Studying beam instabilities along bunch train• 3 observables

– INJ-BPM-01 fast bunch electronics– INJ FCUP-01– Laser pulse power.

• Laser pulse power is measured via a photodiode + splitter located downstream of the pockels cells (for macro pulse selection + burst generator), and the frequency doubler, but UPSTREAM of the attenuator.

• Vary the laser attenuation to see how each observable changes (this will not affect the laser pulse train).

• Change rep rate to 1 Hz to get simultaneous observables from a single train. • Studying transients vs solenoid, corrector strengths, laser spot position. • Key finding: The 6 MHz seen in October ‘12 data is not present now. This is the first shift

since the commissioning break when the PI laser was adjusted to produce higher pulse power. • NB. Calculation of BPM y position in the software was still initially incorrect on this shift in

the saved BPM files in the root shift folder. – The data has been reprocessed to correct this and the corrected data is at

\\Dlfiles03\alice\Work\2012\11\06\Shift 2\newdata3762 – The data on these slides uses the corrected data.

INJ-BPM-01 fast bunch electronics RAW DATA

Note significant droop in all 3 observables

Small transient at start of train

x y charge15 pC

21 pC

30 pC

43 pC

60 pC

Frequency Content, Pre-Processing

• Take bunches 100 bunches to 1000 to avoid early transient and later droop

• As always, subtract mean from data.

• For the CHARGE observable subtract the mean AND normalise by the mean, so that it can be compared the fcup/PI laser traces

BPM frequency content, 0 – 1 MHz

Strong 300 kHz

100 kHz not obviously apparent

Norrmalised the x,y DFT so that the amplitudes are in mm

BPM frequency content, 0 – 8 MHz

NO 6MHz

Faraday Cup Fourier Analysis• FCUP taken at 15 pC, 21 pC, 30 pC, 43

pC, 60 pC, simultaneously with the BPM shots on previous slides (use rep rate 1 Hz)

• Scope records at 10 Gs/sec = 0.1 ns data spacing

• Take 1 in every 10 data points effectively 1 Gs/sec = 1 ns data spacing

• Take the same portion of the train 100 1000 bunches == 6 60 μs

• Subtract the ‘background’• Subtract the mean FCUP voltage and

normalise on the mean• Take DFT

FCUP 60 pC examplefcup after background subtraction (volts vs time)

6-60 μs

(y – <y>)/<y>

• After pre-processing described on previous slide, then compute Fourier

DFT for different frequency ranges

16 MHz + harmonics = bunch frequency300 kHz not seen

lowest frequency is probably slope of data (slope still present even with background subtraction)

FCUP 60 pC example

F-cup Fourier15 pC

21 pC

30 pC

43 pC

60 pC

PI laser trace• PI laser trace taken at 15 pC, 21 pC, 30 pC, 43 pC, 60 pC,

simultaneously with the BPM shots on previous slides (use rep rate 1 Hz)

• Take ALL data points (do not do 1/10 sampling like for FCUP). 10 Gsamples/sec = 0.1 ns data spacing

• No other filtering/binning performed i.e. maximum information retained.

• Once more take 6-60 μs and compute fourier of (y-<y>)/<y>• Remember PI laser power is measured downstream of

frequency doubler (green laser), but upstream of attenuation

PI Laser Fourier 15 pC

21 pC

30 pC

43 pC

60 pC

F-cup Fourier

BPM sumV frequency content, 0 – 1 MHz