Comparison of breakdown behavior between klystron and beam

Comparison of breakdown behavior between klystron and beam driven structure W. Farabolini With the support of J. Kovermann, B. Woolley, J. Tagg HG 2013 3 -6 may 2013 Trieste W. Farabolini 1

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 HG 2013 3 -6 may 2013 Trieste W. Farabolini 2

Typical RF signals Drive beam generated with PETS Klystron generated with pulse compressor • Pre-pulse • Triangular shape (recirculation) • Quite stable pulse 24/7 • Often instable pulse (and trips) • Great flexibility in pulse • Pulse length and power not really length and power flexible After-pulse in case of BD: reflected power perturbation on RF generator HG 2013 3 -6 may 2013 Trieste W. Farabolini 3

Stability of the RF power Many beam trips Two Beam Test Stand : beam generated power with RF recirculation Energy reduction after BD detection X-Box 1 : Klystron generated power with pulse compression HG 2013 3 -6 may 2013 Trieste W. Farabolini 4

Data production • Total number of RF pulses – ACS 1 in TBTS about 3 millions (0. 8 Hz repetition rate) – T 24 : over 98 millions (50 Hz repetition rate) – TD 24 R 05 : over 144 millions (4. 3 millions per day max ) • Total number of BDs – ACS 1 in TBTS about 10000 (? ) (10 -2 < BDR < 10 -3) – T 24 : 3502 (BDR = 3. 6 10 -5) – TD 24 R 05 : 7278 (BDR = 5. 0 10 -5) • Total number of 8 hours data log (about 40 Mbit each) processed – ACS 1 in TBTS : few 10’ – T 24 : 116 – TD 24 R 05 : 228 HG 2013 3 -6 may 2013 Trieste W. Farabolini 5

T 24 test condition summary Power ramping Pulse length to keep BDR around 10 -5 Conditioning not achieved HG 2013 3 -6 may 2013 Trieste W. Farabolini 6

TD 24 R 05 test condition summary 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 HG 2013 3 -6 may 2013 Trieste W. Farabolini 7

BD location determination Reflected rising edge time Transmitted falling edge time 1 st method Input falling edge Dt between Reflected rising edge and Transmitted falling edge (BD start) time Reflected falling edge time 2 nd method (echo) Dt (correlation) between Input falling edge and Reflected falling edge (BD end) 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 HG 2013 3 -6 may 2013 Trieste W. Farabolini 8 and structured Reflected signal) •

Delays as function of cell # Effect of tapered cells 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 HG 2013 3 -6 may 2013 Trieste W. Farabolini 9

Hot spot at cell #6 in the 1 st TBTS structure Ref -Trans method Ref –In method Evenness = 1 for equally distributed BDs Evenness = 0. 66 HG 2013 3 -6 may 2013 Trieste Evenness = 0. 33 W. Farabolini 10

No hot spot in the 2 present TBTS structures Present 2 ACSs in TBTS compilation Evenness = 0. 96 HG 2013 3 -6 may 2013 Trieste Evenness = 0. 95 W. Farabolini 11

T 24 BD locations evolution in X-Box 1 Hot cell(s) from the beginning Nota: possible positions absolute shift due to line delays uncalibrated HG 2013 3 -6 may 2013 Trieste W. Farabolini 12

TD 24 R 05 BD locations evolution in X-Box 1 Hot cell has appeared after 2 months HG 2013 3 -6 may 2013 Trieste W. Farabolini 13

Histogram of all BDs location (X-Box 1) No BD in this cell ! T 24 during 6 weeks Evenness_1 st = 0. 77 Evenness_2 nd = 0. 78 HG 2013 3 -6 may 2013 Trieste TD 24 R 05 Feb. & Mar. Evenness_1 st = 0. 97 Evenness_2 nd = 0. 82 W. Farabolini TD 24 R 05 May. & Jun. Evenness_1 st = 0. 83 Evenness_2 nd = 0. 45 14

2 examples of 8 hours sequences • 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) HG 2013 3 -6 may 2013 Trieste W. Farabolini 15

A proposed diagnostic for BD location Franck Peauger – IRFU 2009 Segmented PMT rising time < 1 ns A. Grudiev Plasma modelling in RF simulations, this WS RF output RF input Additional passive or/and active diagnostics via damping waveguides • Possible to observe plasma oscillation ma plas Plasma ignited by the breakdown HG 2013 3 -6 may 2013 Trieste W. Farabolini 16

Research of precursors in FCU and Reflected RF peak values Uncalibrated data Motivation: Y. Ashkenazy, using stochastic theory for RF breakdown analysis, this WS • • 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. HG 2013 3 -6 may 2013 Trieste W. Farabolini 17

Zoomed data from the 4 th March Still no evidence of any precursor HG 2013 3 -6 may 2013 Trieste W. Farabolini 18

More subtle data processing to be applied RF signals Real BD Possible BD outside the structure Faraday cups signals (zoomed) Dark current only Burst of electrons • Look for power spectral density of the dark current (to be done) HG 2013 3 -6 may 2013 Trieste W. Farabolini 19

BDR as function of RF Power in TBTS But conditioning is still under progress Date 2012_11_16 2012_11_19 2012_11_23 2012_11_29 2012_12_04 2012_12_05 2012_12_06 2012_12_07 HG 2013 3 -6 may 2013 Trieste W. Farabolini Mean sigma power Pulse BD ACS [MW] number up down 29. 2 2. 2 14807 3 2 30. 3 1 36955 5 15 29 2. 1 10932 1 1 37. 2 2. 6 45535 102 60 38. 4 2. 9 10174 12 14 46. 1 1. 8 13394 16 20 46. 5 2. 1 21622 27 8 36. 2 3 9311 3 6 20

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. HG 2013 3 -6 may 2013 Trieste W. Farabolini 21

Distribution of the number of RF pulses between BDs (clusters problem) BD count evolution shows several period of intense BDs activity: clusters HG 2013 3 -6 may 2013 Trieste • 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 W. Farabolini 22

Ignition and falling edge duration Ignition Falling Can it be related to neutrals and ions growth as shown by K. Sjobak , this WS ? HG 2013 3 -6 may 2013 Trieste Two categories of Mean falling edge 50 ns BDs : fast/slow (for commuting 50 MW) ignition time Mean ignition duration 40 ns W. Farabolini 23

Structure RF analysis after removal Jiaru Shi, analysis of T 18 • 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) HFSS result: Iris deform 10 um ~ 2 MHz • Great interest in the “internal geometry measurement tool” presented by M. Aicheler, this WS HG 2013 3 -6 may 2013 Trieste W. Farabolini 24

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. HG 2013 3 -6 may 2013 Trieste W. Farabolini 25

HG 2013 3 -6 may 2013 Trieste W. Farabolini 26

Typical RF signals during BDs in TBTS Ignition Exposure 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? ) HG 2013 3 -6 may 2013 Trieste W. Farabolini 27