RF break-down studies in the CTF3 TBTS

1. RF break-down studies in the CTF3 TBTS Accurate measurements on TBTS RF Break-Down Studies in the CTF3 TBTS

2. The newly installed structures Germana Riddone • Since September 2012 Franck Peauger - IRFU RF Break-Down Studies in the CTF3 TBTS

3. New setup with 2 accelerating structures 2 phase shifters 1 variable splitter 7 BPMs on PB 1 FCU Andrea’s talk 1 Flash box 2 screens 3 PMTs Alexey’s talk 16 WFMs channels Franck’s talk Roger Ruber 15 RF channels (Diodes and IQ) Thermal probes and flow rate RF Break-Down Studies in the CTF3 TBTS

4. Accurate Energy measurement Energy [MW] Time • 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 For stabilization see Tobias Persson talk, Wednesday RF Break-Down Studies in the CTF3 TBTS

5. 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 RF Break-Down Studies in the CTF3 TBTS

6. Upstream / downstream phase optimization Input phases when ACSs in opposition Califes phase scan with ACS’s phase set in opposition constant energy = Califes energy (195 MeV) • 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 RF Break-Down Studies in the CTF3 TBTS

7. Energy gain as function of RF power Power fluctuations Phase scan Energy gain versus root mean power during two records of phase scan • 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. RF Break-Down Studies in the CTF3 TBTS

8. Structure tuning frequency check Short pulse: 4 ns LO = 11994.2 + 1 MHz LO = 11994.2 MHz LO = 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) Long pulse: 194 ns Long pulse: 150 ns LO = 11994.2 MHz LO = 11894.2 MHz • 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) Alexandra Andersson RF Break-Down Studies in the CTF3 TBTS

9. Water temperature method to derive the deposited mean RF power • 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 RF Break-Down Studies in the CTF3 TBTS

10. Energy gain vs. power after recalibration Energy gain versus root mean corrected power • 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) RF Break-Down Studies in the CTF3 TBTS

11. 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.) RF Break-Down Studies in the CTF3 TBTS

12. 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 Break-Down Studies in the CTF3 TBTS

13. RF and BDs detection signal monitoring 6 hours Pulses main parameters are continuously data logged RF Break-Down Studies in the CTF3 TBTS

14. 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. RF Break-Down Studies in the CTF3 TBTS

15. BD count evolution and BD rate What to do with the periods of high activity ? (clusters) RF Break-Down Studies in the CTF3 TBTS

16. BDR as function of RF Power • But 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 RF Break-Down Studies in the CTF3 TBTS

17. BDR as function of Power (2) Upstream new ACS Previous ACS RF power density of Probability of all RF pulses (blue), of RF pulse with BD (red) and power law fit of BD probability (green) • 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 Break-Down Studies in the CTF3 TBTS

18. BD location inside the structures First 3 methods give consistent results, method 4 seems to show a BD drift toward the structure input coupler Reflected rising edge Reflected rising edge Transmitted falling edge Transmitted falling edge FCU edge 2nd and 3rd methods (combining previous signals with FCU): 1st method (transmission): looking at BD position when BD strikes BD with precursor Reflected falling edge Input falling edge 4th method (echo): looking at BD position when RF pulse stops BD w/o precursor RF Break-Down Studies in the CTF3 TBTS

19. Hot spot at cell #6 in the previous structure Previous ACS compilation RF Break-Down Studies in the CTF3 TBTS

20. No hot spot in the 2 present structures Present ACSs compilation RF Break-Down Studies in the CTF3 TBTS

21. 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. RF Break-Down Studies in the CTF3 TBTS

22. Back-up slides RF Break-Down Studies in the CTF3 TBTS

23. FCU and PMFCU signals reliability FCU max output [V] PM on FCU max output [V] Max Transmitted Power [MW] Max Transmitted Power [MW] Max Reflected Power [MW] • OTR light seen on FCU mirror surface is current and energy dependent but not saturated. • (Blue dots correspond to low reflected power) Alexandra A. • 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 ?) CTF3 days - 11 October 2012

24. Coupled BDs Input power Transmitted power Reflected power RF Break-Down Studies in the CTF3 TBTS