Comparison of breakdown behavior between klystron and beam driven structure W. Farabolini With the support of J. Kovermann, B. Woolley, J. Tagg 1HG may 2013 TriesteW. Farabolini

Contents Main test characteristics of TBTS vs. X-Box 1 BD locations BD precursor research BDR as function of RF power BD distribution within time BD ignition and transmitted RF falling time Structure RF analysis after removal 2HG may 2013 TriesteW. Farabolini

Typical RF signals HG may 2013 TriesteW. Farabolini3 Drive beam generated with PETS Klystron generated with pulse compressor Triangular shape (recirculation) Often instable pulse (and trips) Pulse length and power not really flexible Pre-pulse Quite stable pulse 24/7 Great flexibility in pulse length and power After-pulse in case of BD: reflected power perturbation on RF generator

Stability of the RF power HG may 2013 TriesteW. Farabolini4 X-Box1 : Klystron generated power with pulse compression Two Beam Test Stand : beam generated power with RF recirculation Many beam trips Energy reduction after BD detection

Data production Total number of RF pulses – ACS 1 in TBTSabout 3 millions (0.8 Hz repetition rate) – T24 : over 98 millions (50 Hz repetition rate) – TD24R05 : over 144 millions (4.3 millions per day max ) Total number of BDs – ACS 1 in TBTSabout (?) (10 -2 < BDR < ) – T24 : 3502(BDR = ) – TD24R05 : 7278(BDR = ) Total number of 8 hours data log (about 40 Mbit each) processed – ACS1 in TBTS : few 10’ – T24 : 116 – TD24R05 :228 HG may 2013 TriesteW. Farabolini5

T24 test condition summary HG may 2013 TriesteW. Farabolini6 Power ramping Pulse length to keep BDR around Conditioning not achieved

TD24R05 test condition summary HG may 2013 TriesteW. Farabolini7 Power and pulse length ramping strategy. (limit the available energy in case of BD) Full gradient 100 MV/m and pulse length 220 ns achieved with BDR = 10 -5

BD location determination  t between Reflected rising edge and Transmitted falling edge (BD start) Reflected rising edge Transmitted falling edge  t (correlation) between Input falling edge and Reflected falling edge (BD end) Input falling edge Reflected falling edge 1 st method 2 nd method (echo) Edge detection is always tricky especially for the transmitted signal (BD ignition time) Cross-correlation method is much more robust but possibly biased (needs strong and structured Reflected signal) HG may 2013 Trieste8W. Farabolini time

Delays as function of cell # HG may 2013 TriesteW. Farabolini9 Accuracy : 3.5 ns per cell (RF input side) / 7.5 ns per cell (RF output side) Sampling rate: 1 ns on TBTS, 4 ns on X-Box (log detector), but 1 ns available Effect of tapered cells

HG may 2013 TriesteW. Farabolini10 Hot spot at cell #6 in the 1 st TBTS structure Ref -Trans method Evenness = 0.66 Evenness = 0.33 Ref –In method Evenness = 1 for equally distributed BDs

HG may 2013 TriesteW. Farabolini11 No hot spot in the 2 present TBTS structures Present 2 ACSs in TBTS compilation Evenness = 0.96Evenness = 0.95

T24 BD locations evolution in X-Box1 HG may 2013 TriesteW. Farabolini12 Hot cell(s) from the beginning Nota: possible positions absolute shift due to line delays uncalibrated

TD24R05 BD locations evolution in X-Box1 HG may 2013 TriesteW. Farabolini13 Hot cell has appeared after 2 months

Histogram of all BDs location (X-Box1) HG may 2013 TriesteW. Farabolini14 T24 during 6 weeks Evenness_1 st = 0.77 Evenness_2 nd = 0.78 TD24R05 Feb. & Mar. Evenness_1 st = 0.97 Evenness_2 nd = 0.82 TD24R05 May. & Jun. Evenness_1 st = 0.83 Evenness_2 nd = 0.45 No BD in this cell !

HG may 2013 TriesteW. Farabolini15 During BD cluster a hot cell (# 4 or # 5) appears Blue marks show failures in BD location, often related to no current in FCU (red dots) 2 examples of 8 hours sequences

A proposed diagnostic for BD location HG may 2013 TriesteW. Farabolini16 Franck Peauger – IRFU 2009 RF input RF output Plasma ignited by the breakdown plasm a Additional passive or/and active diagnostics via damping waveguides A. Grudiev Plasma modelling in RF simulations, this WS Segmented PMT rising time < 1 ns Possible to observe plasma oscillation

HG may 2013 TriesteW. Farabolini17 Research of precursors in FCU and Reflected RF peak values Uncalibrated data Faraday cup currents are negative (either dark current or BD burst). -1: saturated. Reflected RF power are positive. Background levels (offset) are suppressed. All these signals are used to detect BDs and the 2 previous pulses are also data logged. Motivation: Y. Ashkenazy, using stochastic theory for RF breakdown analysis, this WS

HG may 2013 TriesteW. Farabolini18 Zoomed data from the 4 th March Still no evidence of any precursor

More subtle data processing to be applied HG may 2013 TriesteW. Farabolini19 Look for power spectral density of the dark current (to be done) Faraday cups signals (zoomed) RF signals Possible BD outside the structure Real BD Dark current onlyBurst of electrons

HG may 2013 TriesteW. Farabolini20 BDR as function of RF Power in TBTS But conditioning is still under progress Date Mean power [MW] sigma power [MW] Pulse number BD ACS up BD ACS down 2012_11_ _11_ _11_ _11_ _12_ _12_ _12_ _12_

HG may 2013 TriesteW. Farabolini21 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 BDR as function of Power (2)

HG may 2013 TriesteW. Farabolini22 Distribution of the number of RF pulses between BDs (clusters problem) BD count evolution shows several period of intense BDs activity: clusters Inside clusters the BD probability becomes very high. Discarding BDs within clusters allows to focus on the stationary BD statistics, well fitted by a Poisson law

HG may 2013 TriesteW. Farabolini23 Ignition and falling edge duration Two categories of BDs : fast/slow ignition time Mean ignition duration 40 ns Mean falling edge 50 ns (for commuting 50MW) Can it be related to neutrals and ions growth as shown by K. Sjobak, this WS ? Ignition Falling

Structure RF analysis after removal HG may 2013 TriesteW. Farabolini24 Jiaru Shi, analysis of T18 HFSS result: Iris deform 10um  ~ 2MHz R. Wegner found identical results for the 1 st structure tested in TBTS However cutting it with wire is delicate since activated (pb. of the TBTS) Great interest in the “internal geometry measurement tool” presented by M. Aicheler, this WS

Conclusion Stand alone test stand provide a incredible capability of massive results production. Fitting on of them with beam capability will be ideal. BD theory / modeling and experimental activities can gain a lot in exchanging ideas and suggestions of tests. 25HG may 2013 TriesteW. Farabolini

HG may 2013 TriesteW. Farabolini26

HG may 2013 TriesteW. Farabolini27 Typical RF signals during BDs in TBTS Ignition Falling Transmitted falling edge and Reflected rising edge supposed to be produced synchronously (ignition only absorbs power, not reflects) Early BDs reflected power disrupts Input power (recirculation process in PETS) Transmitted phase quite stable up to the falling edge (even during ignition) Reflected phase can drift or jump a lot (Input phase disruption and/or BD displacement?) Exposure