rf break-down studies in the ctf3 tbts

RF Break-Down Studies in the CTF3 TBTS 1

RF break-down studies in the CTF3 TBTS

Accurate measurements on TBTS

29 Jan. 2013 Wilfrid Farabolini


The newly installed structures


Franck Peauger - IRFU

Germana Riddone

• Since September 2012


New setup with 2 accelerating structures

Roger Ruber

2 phase shifters 1 variable splitter

15 RF channels(Diodes and IQ)

7 BPMs on PB

2 screens 1 Flash box

Thermal probes and flow rate

3 PMTs16 WFMs channelsFranck’s talk

1 FCU

Alexey’s talk

Andrea’s talk



Accurate Energy measurement

• It is no longer possible with the energy gained with 2 ACS to track simultaneously on the same spectrum line screen both accelerated and non accelerated beams (dipole strength change is required)

• Califes beam energy fluctuates by +/- 2 MeV with a period around 150 s (temperature ?)• A fit with a sinusoidal function is valid at least for a duration up to 30 minutes

Ener

gy [M

W]

Time


For stabilization see Tobias Persson talk, Wednesday


Procedure to determine the maximum energy gain

• Extrapolated Califes energy is subtracted to measured accelerated beam energy gain.

• During RF power cut magnet is set to measure Califes energy and check extrapolation

• RF powers from couplers is logged as well

• Califes / Drive beam phase is scanned over 360 deg of 12 GHz

• Upstream / downstream phase was previously adjusted to identical phase vs. beam



Upstream / downstream phase optimization

• Inter-structures phase shifter is moved up to the point where no acceleration is measured whatever the Drive Beam / Califes phase. At this phase the 2 structures act oppositely.

• From this point we move the phase by 180 deg in order to place the 2 structures at the same phase vs. the probe beam.

• Due to this phase shifter lack of repeatability no systematic scan was performed after this setting

Califes phase scan with ACS’s phase set in oppositionconstant energy = Califes energy (195 MeV)

Input phases when ACSs in opposition



Energy gain as function of RF power

• Since both the ACS measured power are not equal, an averaged value is computed for the RF power coordinate.

• With this representation the maximum measured acceleration constantly failed to reach its nominal value by 4 MeV approx.

Energy gain versus root mean power during two records of phase scan

Phase scan

Power fluctuations


RF Break-Down Studies in the CTF3 TBTS

Structure tuning frequency check

LO = 11994.2 MHz LO = 11994.2 + 1 MHzLO = 11994.2 - 2 MHz

• Down mixing the RF output signal produced by a short probe beam pulse (6 bunches) allows to measure the ACS resonant frequency. Very well tuned (better than 1 MHz).

• The RF produced last 65 ns (structure filling time)

RF output frequency is now forced by the probe beam pulse frequency • RF output rising time = ACS filling time (65 ns)• RF output rising time + sustain time = pulse length• RF output falling time = ACS filling time (65 ns)

Short pulse: 4 ns

LO = 11894.2 MHz

Long pulse: 150 ns

Long pulse: 194 ns

LO = 11994.2 MHz

Alexandra Andersson

29 Jan. 2013 Wilfrid Farabolini 8


Water temperature method to derive the deposited mean RF power

Upstream Downstream

File Date time start time stopmoy time start

moy time stop Power RF Power Temp Ratio Temp in Temp out Power RF Power Temp Ratio Temp in Temp out

2012_11_29 01 00 07 00 03 00 03 10 6.25 4.4 1.42 29.27 29.07 5.66 5 1.13 29.65 29.49

2012_11_30 17 00 23 00 21 45 22 10 5.89 4.37 1.35 29.23 29.02 5.23 4.91 1.06 29.6 29.45

18 10 18 20 5.84 4.29 1.36 5.21 4.79 1.09

2012_11_30_a 10 00 16 40 11 40 12 10 5.67 4.57 1.24 29.26 29.06 5.11 4.92 1.04 29.65 29.47flow rate change

2012_12_04 19 20 21 40 20 50 21 00 5.99 4.52 1.32 29.25 29.06 5.34 4.99 1.07 29.65 29.48

2012_12_04 19 20 21 40 20 17 20 22 6.4 4.81 1.33 29.24 29.05 5.71 5.28 1.08 29.62 29.46

2012_12_05 14 00 23 00 20 45 20 55 7.59 5.66 1.34 29.24 29.07 5.76 6.41 1.05 29.58 29.45

2012_12_06 00 00 23 00 01 20 01 30 7.35 5.09 1.45 29.21 29.02 6.45 5.94 1.09 29.57 29.42

Average : 1.35 1.08

• From thermal method and averaging on a lot of runs, it appears that the RF power is overestimated by a factor 1.35 for the Upstream ACS and 1.08 for the Downstream ACS



Energy gain vs. power after recalibration

• Applying the correction factor derived by the thermal method allows to plot an acceleration vs. power chart much closer to the nominal ACS performances.

• However, an accurate recalibration of the RF couplers lines as well as the diodes crates has been done during this winter shutdown and sensitivity has been improved.

• The correction factors computed by integrating power cannot reveal the diode calibration linearity default (automatic calibration procedure installed)

Energy gain versus root mean corrected power



Energy spread vs. accelerating phase

• Energy spread (s of Gaussian fit and FWHM) is maximum when energy gain is null

• And is minimum when energy gain is extreme (pos. or neg.)



Energy spread and bunch length measurement• An efficient method of deriving bunch

length and even slice energy…• 12 GHz = 83.3 ps: fast slope and high

accelerating field• Ex. sESmin = 1.1 MeV, sESmax = 9.05 MeV, Ds = 8.98 MeV

-> slength = Asin( /Ds Egain max) = 16.2 deg -> slength = 3.7 ps -> FWHMgauss = 8.7 ps

Resolution approx. 0.8 ps FWHM • A model should be developed taking into

account the energy and charge distribution within the bunch

• Sinus fit period should be ¼ T3GHZ for energy gain and 1/8 T3GHZ for energy spread. Phase shifter linearity seems poor on its lower range.



RF and BDs detection signal monitoring

Pulses main parameters are continuously data logged6 hours



Accurate BD detection• Two criteria used: Reflected Power

and Missing Energy Miss = Enerin – Enerout x attenuation

• Data are post processed with adapted thresholds.Thresholds = mean + 3.72 s

[ PGauss(X>3.72s) = 10-4]

• Compromise between Detection prob. and False Alarm prob.



BD count evolution and BD rate

What to do with the periods of high activity ?(clusters)



BDR as function of RF PowerBut conditioning is still under progress• previous structure: 3 106 RF pulses

• theses structures:6 105 RF pulses ?

• 1 day of Stand alone Test Stand:4.3 106 RF pulses


Date Mean power [MW]

sigma power [MW]

Pulse number

BD ACS up

BD ACS down

2012_11_16 29.2 2.2 14807 3 22012_11_19 30.3 1 36955 5 152012_11_23 29 2.1 10932 1 12012_11_29 37.2 2.6 45535 102 602012_12_04 38.4 2.9 10174 12 142012_12_05 46.1 1.8 13394 16 202012_12_06 46.5 2.1 21622 27 82012_12_07 36.2 3 9311 3 6


BDR as function of Power (2)

• Fitting the Power distribution when BD by a power law of the power distribution of all pulses provide an exponent between 12 and 18.

RF power density of Probability of all RF pulses (blue), of RF pulse with BD (red) and power law fit of BD probability (green)

Previous ACS Upstream new ACS



BD location inside the structuresReflected

rising edgeTransmitted falling edge

1st method (transmission): looking at BD position when BD strikes

Input falling edge

Reflected falling edge

4th method (echo): looking at BD position when RF pulse stops

Reflected rising edge

2nd and 3rd methods (combining previous signals

with FCU):

Transmitted falling edge

FCU edge


First 3 methods give consistent results, method 4 seems to show a BD drift toward the structure input coupler

BD with precursor

BD w/o precursor


Hot spot at cell #6 in the previous structure


Previous ACS compilation


No hot spot in the 2 present structures


Present ACSs compilation


Summary

• New ACSs are still in conditioning (not yet at 100 MV/m).• Accurate procedures have been developed to assess their

characteristics (energy gain, RF power, BD detection).• Energy spread at zero crossing allows to measure the bunch

length.• BDR are presently on the same curve than for the previous

structure.• BD locations show no hot spot for whatever structures.



Back-up slides


CTF3 days - 11 October 2012 23

FCU and PMFCU signals reliability

• When RF transmitted power is low (early BD), BD produced electrons are not likely to reach the FCU (not accelerated towards the FCU)

• Also when RF reflected power is low FCU signal is often weak (why ?)

OTR light seen on FCU mirror surface is current and energy dependent but not saturated. (Blue dots correspond to low reflected power)

Alexandra A.

W. Farabolini

FCU

max

out

put [

V]

PM o

n FC

U m

ax o

utpu

t [V]

Max Transmitted Power [MW]

Max Reflected Power [MW]Max Transmitted Power [MW]


Coupled BDs


Reflected power

Transmitted power

Input power

rf break-down studies in the ctf3 tbts

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