Comparison of breakdown behavior between klystron and beam driven structure

W. Farabolini 1

Comparison of breakdown behavior between klystron and beam driven

structureW. Farabolini

With the support of J. Kovermann, B. Woolley, J. Tagg

HG2013 3-6 may 2013 Trieste

W. Farabolini 2

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

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Typical RF signals

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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

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Stability of the RF power

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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

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Data production

• Total number of RF pulses – ACS 1 in TBTS about 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 TBTS about 10000 (?) (10-2 < BDR < 10-3)– T24 : 3502 (BDR = 3.6 10-5)– TD24R05 : 7278 (BDR = 5.0 10-5)

• Total number of 8 hours data log (about 40 Mbit each) processed– ACS1 in TBTS : few 10’– T24 : 116– TD24R05 : 228

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T24 test condition summary

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Power ramping

Pulse length to keep BDR around 10-5

Conditioning not achieved

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TD24R05 test condition summary

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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

8

BD location determination

Dt between Reflected rising edge and Transmitted falling edge

(BD start)

Reflected rising edge

Transmitted falling edge

Dt (correlation) between Input falling edge and Reflected falling edge

(BD end)

Input falling edge

Reflected falling edge

1st method

2nd 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)HG2013 3-6 may 2013 Trieste W. Farabolini

time time

time

time

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Delays as function of cell #

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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

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Hot spot at cell #6 in the 1st TBTS structure

Ref -Trans method

Evenness = 0.66 Evenness = 0.33

Ref –In method

Evenness = 1 for equally distributed BDs

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No hot spot in the 2 present TBTS structures

Present 2 ACSs in TBTS compilation

Evenness = 0.96 Evenness = 0.95

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T24 BD locations evolution in X-Box1

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Hot cell(s) from the beginningNota: possible positions absolute shift due to line delays uncalibrated

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TD24R05 BD locations evolution in X-Box1

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Hot cell has appeared after 2 months

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Histogram of all BDs location (X-Box1)

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T24 during 6 weeksEvenness_1st = 0.77Evenness_2nd = 0.78

TD24R05 Feb. & Mar.Evenness_1st = 0.97Evenness_2nd = 0.82

TD24R05 May. & Jun.Evenness_1st = 0.83Evenness_2nd = 0.45

No BD in this cell !

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•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

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A proposed diagnostic for BD location

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Franck Peauger – IRFU 2009

RF input

RF output

Plasma ignited by the breakdown

plasma

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

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Research of precursors in FCU and Reflected RF peak values

Unc

alib

rate

d da

ta

•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

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Zoomed data from the 4th March

Still no evidence of any precursor

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More subtle data processing to be applied

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• 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 only Burst of electrons

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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_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

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• 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)

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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

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Ignition and falling edge duration

Two categories of BDs : fast/slow ignition timeMean ignition duration 40 ns

Mean falling edge 50 ns(for commuting 50MW)Can it be related to

neutrals and ions growth as shown byK. Sjobak , this WS ?

Ignition

Falling

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Structure RF analysis after removal

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Jiaru Shi, analysis of T18

HFSS result: Iris deform 10um ~ 2MHz

• R. Wegner found identical results for the 1st 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

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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.

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Typical RF signals during BDs in TBTSIgnition

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