RF break-down studies in the CTF3 TBTS Accurate measurements on TBTS RF Break-Down Studies in the CTF3 TBTS 29 Jan Wilfrid Farabolini 1

The newly installed structures RF Break-Down Studies in the CTF3 TBTS 29 Jan Wilfrid Farabolini 2 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 PMTs 16 WFMs channels Franck’s talk 1 FCU Alexey’s talk Andrea’s talk RF Break-Down Studies in the CTF3 TBTS 29 Jan Wilfrid Farabolini 3

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 Energy [MW] Time RF Break-Down Studies in the CTF3 TBTS 29 Jan Wilfrid Farabolini 4 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 RF Break-Down Studies in the CTF3 TBTS 29 Jan Wilfrid Farabolini 5

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 opposition constant energy = Califes energy (195 MeV) Input phases when ACSs in opposition RF Break-Down Studies in the CTF3 TBTS 29 Jan Wilfrid Farabolini 6

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 29 Jan Wilfrid Farabolini 7

Structure tuning frequency check LO = MHz LO = MHz LO = 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 = MHz Long pulse: 150 ns Long pulse: 194 ns LO = MHz Alexandra Andersson RF Break-Down Studies in the CTF3 TBTS 29 Jan Wilfrid Farabolini 8

Water temperature method to derive the deposited mean RF power Upstream Downstrea m File Datetime starttime stop moy time start moy time stopPower RF Power TempRatioTemp inTemp outPower RF Power TempRatioTemp inTemp out 2012_11_ _11_ _11_30_a flow rate change 2012_12_ _12_ _12_ _12_ Average : 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 29 Jan Wilfrid Farabolini 9

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 RF Break-Down Studies in the CTF3 TBTS 29 Jan Wilfrid Farabolini 10

Energy spread vs. accelerating phase Energy spread (  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 29 Jan Wilfrid Farabolini 11

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.  ES min = 1.1 MeV,  ES max = 9.05 MeV,  = 8.98 MeV ->  length = Asin(  E gain max) = 16.2 deg ->  length = 3.7 ps -> FWHM gauss = 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 T 3GHZ for energy gain and 1/8 T 3GHZ for energy spread. Phase shifter linearity seems poor on its lower range. RF Break-Down Studies in the CTF3 TBTS 29 Jan Wilfrid Farabolini 12

RF and BDs detection signal monitoring Pulses main parameters are continuously data logged 6 hours RF Break-Down Studies in the CTF3 TBTS 29 Jan Wilfrid Farabolini 13

Accurate BD detection Two criteria used: Reflected Power and Missing Energy Miss = Ener in – Ener out x attenuation Data are post processed with adapted thresholds. Thresholds = mean  [ P Gauss (X>3.72  ) = ] Compromise between Detection prob. and False Alarm prob. RF Break-Down Studies in the CTF3 TBTS 29 Jan Wilfrid Farabolini 14

BD count evolution and BD rate What to do with the periods of high activity ? (clusters) RF Break-Down Studies in the CTF3 TBTS 29 Jan Wilfrid Farabolini 15

BDR as function of RF Power But conditioning is still under progress previous structure:  RF pulses theses structures:  RF pulses ? 1 day of Stand alone Test Stand:  RF pulses RF Break-Down Studies in the CTF3 TBTS 29 Jan Wilfrid Farabolini 16 Date Mean power [MW] sigma power [MW] Pulse number BD ACS up BD ACS down 2012_11_ _11_ _11_ _11_ _12_ _12_ _12_ _12_

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 RF Break-Down Studies in the CTF3 TBTS 29 Jan Wilfrid Farabolini 17

BD location inside the structures Reflected rising edge Transmitted falling edge 1 st method (transmission): looking at BD position when BD strikes Input falling edge Reflected falling edge 4 th method (echo): looking at BD position when RF pulse stops Reflected rising edge 2 nd and 3rd methods (combining previous signals with FCU): Transmitted falling edge FCU edge RF Break-Down Studies in the CTF3 TBTS 29 Jan Wilfrid Farabolini 18 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 29 Jan Wilfrid Farabolini RF Break-Down Studies in the CTF3 TBTS19 Previous ACS compilation

No hot spot in the 2 present structures RF Break-Down Studies in the CTF3 TBTS 29 Jan Wilfrid Farabolini 20 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. 29 Jan Wilfrid Farabolini RF Break-Down Studies in the CTF3 TBTS21

Back-up slides 29 Jan Wilfrid Farabolini RF Break-Down Studies in the CTF3 TBTS22

FCU and PM FCU 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. CTF3 days - 11 October W. Farabolini FCU max output [V] PM on FCU max output [V] Max Transmitted Power [MW] Max Reflected Power [MW] Max Transmitted Power [MW]

Coupled BDs 29 Jan Wilfrid Farabolini RF Break-Down Studies in the CTF3 TBTS24 Reflected power Transmitted power Input power