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2016

CERN – EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH

POST-MORTEM ANALYSIS AFTER HIGH POWER OPERATION OF

THE TD24_R05 TESTED IN XBOX_1

Alberto Degiovanni, Rolf Wegner, Nerea Mouriz Irazabal, Markus Aicheler, Ana Pérez Fontenla,

Walter Wuensch, Wilfrid Farabolini

CERN/Switzerland

Abstract

The CLIC prototype structure TD24_R05 has been high power tested in Xbox_1 in 2013. This report

summarizes all examinations conducted after the high power test including bead-pull measurements,

structure cutting, metrology and SEM observations. A synthesis of the various results is then made. The

structure developed a hot cell progressively during operation and detuning was observed after the test was

complete. The post mortem examination clearly showed a developed standing wave pattern which was

explained by the physical deformation of one of the coupler iris. An elevated breakdown count through SEM imaging in the suspected hot cell however could not be confirmed. Neither any particular feature

offering an explanation for the observed longitudinal breakdown distribution could be detected.

Geneva, Switzerland

01.08.2016

CLIC – Note – 1070

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Contents 1. Introduction ..................................................................................................................................... 3

2. RF History ......................................................................................................................................... 4

2.1. Bead-pull measurements and tuning ...................................................................................... 4

2.2. RF high power test history ....................................................................................................... 4

3. Sample preparation ......................................................................................................................... 7

3.1. Cutting operations ................................................................................................................... 7

3.2. Wire EDM cutting at CERN....................................................................................................... 8

3.3. Chemical cleaning: Degreasing ..............................................................................................10

4. Results of characterization ............................................................................................................10

4.1. RF measurements before cutting ..........................................................................................10

4.2. Metrology ..............................................................................................................................11

4.3. Simulations ............................................................................................................................13

4.4. Scanning Electron Microscope (SEM) observation ................................................................14

4.4.1. Surface study .................................................................................................................14

4.4.2. BD counting and distribution .........................................................................................15

4.4.3. Assessment of bonding quality ......................................................................................17

4.4.4. Presence of B-field arcs .................................................................................................18

4.4.5. Thermal fatigue considerations .....................................................................................19

5. Discussion ......................................................................................................................................19

5.1. Geometrical deformation and Bead-pull ...............................................................................19

5.2. Break down number considerations .....................................................................................19

5.3. Bonding joint and B-field arcs comparison to T18 and TD18 ................................................20

6. Conclusions ....................................................................................................................................20

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1. Introduction The aim of this document is to summarize the post-mortem analysis of the TD24_R05_N1 structure

tested in X-box1 during the first semester of 2013. This document contains links to presentations,

detailed documents and data that are relevant for this study. It is also intended as a guideline for

defining a common protocol for the post high power operation studies on future structures to be

carried on at CERN and other laboratories to allow direct comparison of results.

The structure is made of 24 regular cells and 2 matching cells (input and output couplers). The cells

are numbered from 1 to 26, as shown in Figure 1. The mechanical drawings of the structure can be

found in https://edms.cern.ch/file/1258457/5/12WDSDVG1.8_R05.pdf .

Figure 1: Cells nomenclature for the TD24 structure.

The objective of the post mortem analysis was to quantify the effect of high-gradient operation on the

structure surface. There was also the specific objective to investigate the origin and characteristics of

a hot cell. The analysis of the data collected during the high power tests showed a concentration of

BDs in cell number 5 (see Figure 4 and Figure 5). The hot-cell emerged after approximately two months

of operation. A three times higher total number of BD spots was expected in this location of the

structure. Furthermore, the results of bead pull measurements performed after high power operation

showed a detuning of the coupling cells, indicating a possible mechanical deformation.

The decision to cut, open and examine the structure was taken in May 2014. The following goals for

the post-mortem were established:

1. metrology measurements (for better understanding the detuning effect)

2. hot cells analysis (BD distribution)

3. B-arc features (EDMS: 1096980, TD18 KEK/SLAC post mortem observations)

4. samples (octant/quadrant) for various types of analysis by collaborators

https://edms.cern.ch/file/1258457/5/12WDSDVG1.8_R05.pdf

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More information about the structure can be found in:

EDMS 1069239 – RF design and parameters (https://edms5.cern.ch/document/1069239/1) EDMS 1258457 – traveller of structure (https://edms.cern.ch/document/1258457/5) EDMS 1239394 – RF tuning (https://edms.cern.ch/document/1239394/1) EDMS 1306038 – first RF analysis after high power op. (https://edms.cern.ch/document/1306038/1)

2. RF History

2.1. Bead-pull measurements and tuning After brazing, the structure underwent low power measurements in the lab and tuning. The upper

plot of Figure 2 shows the results of bead pull measurements performed before and after the tuning

of the structure. The values of the longitudinal electric field (normalized to the average along the

structure) at the centre of each accelerating cell measured before (after) tuning are reported in blue

(red). On the top of them, the results of HFSS simulations are presented (black curve). The red curve

and the black curve are in good agreement, meaning that the tuning was carried out successfully.

Another way of looking at the data is the phase advance per cell (bottom plot of Figure 2). The

structure was tuned to obtain a constant phase advance per cell of 120° (red curve).

Figure 2: results of bead pull measurements before and after tuning in terms of accelerating field and phase advance at the centre of each cell.

2.2. RF high power test history The structure was installed in CTF2 (CLIC Test Facility 2) in January 2013 and was tested under high

power RF for about 5 months. The full history plot of the high power test is shown in Figure 3.

https://edms5.cern.ch/document/1069239/1https://edms.cern.ch/document/1258457/5https://edms.cern.ch/document/1239394/1https://edms.cern.ch/document/1306038/1

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The average accelerating gradient (green curve) calculated during the 50 ns compressed RF pulse, was

raised smoothly by using a preliminary version of the Xbox1 conditioning algorithm to keep the BDR

constant (for more details http://accelconf.web.cern.ch/accelconf/IPAC2014/papers/wepme016.pdf)

Figure 3: Summary history plot of the test of the TD24_R05_N1 structure.

After the accelerating gradient reached 105 MV/m (light blue curve in Figure 3) the pulse length was

increased while the power was reduced in order to keep a constant BDR. The BDR evaluated on a

sliding window of 1 Million pulses is indicated by the magenta line in Figure 3. The red curve indicates

the cumulated number of interlocks on the structure Reflected power. About 40% of those events

were detected within 30 seconds from the previous breakdown event (cluster). The blue curve

indicates the cumulated number of cluster during the whole run.

Once the conditioning was completed, the structure was tested at constant power level, in order to

measure BDR dependence with respect to accelerating gradient. By looking at the distribution of the

BD location inside the structure obtained by timing methods (described in

https://cds.cern.ch/record/2025952/files/tupp029.pdf) it was noticed that towards the end of the

conditioning period the structure started to develop a hot spot, in the vicinity of cell number 5.

Breakdown distribution plots are shown in Figure 4 and Figure 5.

In total the structure saw about 190 million pulses and accumulated 8000 BDs. From the cell location

study those BD were not equally distributed along the structure but are expected to be concentrated

in the first irises, in particular on the two irises forming cell number 5.

http://accelconf.web.cern.ch/accelconf/IPAC2014/papers/wepme016.pdfhttps://cds.cern.ch/record/2025952/files/tupp029.pdf

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Figure 4: Normalized measured BD distribution in the TD24_R05_N1 structure, with respect to record number (from 1 to 140). Each record corresponds to 8 hours of run at 50 Hz. The very high peak values are an artifact of the normalization (if only 2 BDs during a record these cells will result very active).

Figure 5: BD distribution inside the structure evaluated for the run at different pulse length. The missing bin at cell number 4 is an artefact due the limited time resolution (4 ns) compared to the round trip delay time of the signals in the first cells. Breakdowns physically happening in cell 4 dilute over to the adjacent bins.

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3. Sample preparation

3.1.Cutting operations Different cutting methods were considered and compared, taking into account the type of features to

be observed and the possible problems for analysis after cutting. Further details can be found in the

presentation http://indico.cern.ch/event/281511/contribution/0/material/slides/1.pdf

The following table summarizes the considerations for the different cutting methods.

Table 1: Various cutting methods and their evaluations

Cutting method

Cutting issues Indicated for type…

Example Metrology check

BD distribution on iris

SEM chemical analysis

Detailed SEM exploration

Saw

High deformation High velocity chips and pollution* into the cavity (*The pollution due to cooling liquids could be reduced or even suppressed)

damped TD18 KEK/SLAC

no difficult difficult difficult

2.a Milling- 2.b Turning (and ripping apart)

Very clean method (for undamped only) Possible small deformation

undamped T18 KEK/SLAC

limited yes yes yes

Wire EDM cut (Electrical Discharge Machining)

No deformation Sparks in the cutting plane and possible oxidation; Pollution into the cavity (de-ionized water)

Damped/ undamped

T18 CERN yes yes no could be affected by pollution

Abrasive saw slow cutting

Slow rotation speed implies long time (~days) «clean» method No deformation

Damped/ undamped

Never done before Aquistion of suitable machine...

yes yes maybe maybe

Water jet cutting

No deformation Pollution into the cavity

Damped/ undamped

Never done before

yes yes no could be affected by pollution

The Wire EDM cutting technique was chosen because of its availability at CERN and because it does

not affect the geometry of the structure. Cutting forces are extremely low, deformation is only

expected from the release of internal stresses. Furthermore, this method was already used

successfully for other structures in the past.

http://indico.cern.ch/event/281511/contribution/0/material/slides/1.pdf

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3.2.Wire EDM cutting at CERN The scheme sketched in Figure 6 was proposed and then used for the cutting. In total 9 cuts were

needed, 2 on the longitudinal plane and 7 on the transverse plane.

Figure 6: Cutting scheme. The numbers indicate the cuts executed.

Figure 7 shows the structure after the first transversal cut. The wire has a thickness of 0.35 mm and

the cutting speed is of the order of 0.1 mm per minute. The structure was brought to the CERN main

workshop on the 20th of May 2014. The cutting was completed in three days.

Figure 7: Setup during EDM wire cutting.

The distances of the cutting planes positions are shown in Figure 8. The values in the top drawing refer

to the centre of the dimple tuners. A correction of 2.65 mm was applied in order to avoid cutting

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through the dimple tuner. The longitudinal cuts were performed on the two perpendicular planes at

the distances shown in the bottom right drawing.

Figure 8: Position of the cuts. A correction of 2.65mm was applied to the number shown in order to avoid cutting through the dimple tuners.

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3.3.Chemical cleaning: Degreasing After cutting, the different parts of the structure were degreased using the CERN standard process.

The procedure was completed on 13th of June 2014. Figure 9 shows some pieces of the structure after

degreasing.

Figure 9: Pieces of the structure after degreasing.

4. Results of characterization

4.1.RF measurements before cutting In order to track the changes due to the RF test the structure underwent low power measurements in

the lab after high power operations.

The upper plot of Figure 10 shows the accelerating field value at the centre of each cell (normalized

to the average along the structure) measured after tuning and after High Power Operation (HPO). On

the top of these two curves, the results from the simulations with deformation of 10 um and 25 um

are super-imposed.

The same comparison was done for the phase advance per cell. The phase advance per cell after high

power operation shows a strong standing wave pattern (green curve on bottom plot of Figure 10). The

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same pattern in phase advance can be obtained in simulation by adding a 10 um tilt deformation in

the last iris (black).

Figure 10: measured field profile and phase advance compared to simulations results.

4.2.Metrology Metrology measurements were performed on one of the two halves of both the input and output

coupler. Five sections were measured on each iris face. The data obtained from the measurements

can be found in EDMs (https://edms.cern.ch/document/1391907/3).

Figure 11 shows the reference axis used for the measurement of the output coupler and the position

of the five sections on the first iris surface (section A1 to A5).

Figure 11: definition of axis and of measured section for the output coupler piece.

https://edms.cern.ch/document/1391907/3

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The plots of Figure 12 and Figure 13 summarize the results of the measurements on the output and

input coupler pieces. The maximum deviation is found in section 1 and section 5 (the ones closer to

the cutting plane, respectively at 10° and 170°) and for the first iris (named disk A). As shown in Figure

12, the output iris tilted towards the main body of the structure with a maximum deflection at the tip

of 0.2 mm, which corresponds to an angular deviation with respect to the vertical plane of about 35

mrad.

Figure 12: Output coupler deformation.

Figure 13: Input coupler deformation.

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4.3.Simulations In order to evaluate the effects of the deformations determined by metrology on the RF properties of

the structure, simulations were performed with HFSS. The nominal design

[https://edms5.cern.ch/document/1069239/1] has been modified by introducing a tilt in the last iris

on the output coupler as shown in Figure 14. The tilt is obtained by rotating the iris around an axis

parallel to the x axis and passing at a height of the bore radius at the centre of the last iris. It measures

the difference in position of the tip of the iris along the z direction. A tilt of 100 micron is shown as an

example in Figure 14.

Figure 14: Geometry used for RF simulations of the deformed structure.

The field profiles and phase advances of the simulations on the deformed design have been compared

to the results of the nominal structure, giving the results shown in Figure 15. The upper plot shows

the normalized value of the magnitude of the axial electric field for 4 different values of tilt of the last

iris (respectively 0 um, 10 um, 25 um and 50 um). One can notice the peaks in correspondence of the

centre of the cells. The bottom plot of Figure 15 shows only the values of the peaks.

The presence of the tilt induces a standing wave pattern which is more evident with the increase in

deformation.

https://edms5.cern.ch/document/1069239/1

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Figure 15: field profiles obtained from HFSS simulations for different values of tilt of the last iris.

4.4.Scanning Electron Microscope (SEM) observation SEM observations were made of the samples after cutting. Special focus was put on surface

modification in the areas affected by breakdown (BD). One aspect of the observation was to look for

previously observed structures like craters, worm-like features (WLF), B-field arcs, but another was to

look for new features.

4.4.1. Surface study Cells surface presented a large variety of craters morphologies consistent with clusters of sparks or

isolated craters observed previously in post-mortem analysis of other RF structures and DC-spark

samples. Apart from craters, surface modifications related with breakdown activities were detected

as brunches of molten material (that could be a prelude of crater or a form of very soft breakdown)

and WLF. Exemplary SEM images (SE2) of the observed features are included in Figure 16

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Figure 16: SEM (SE2) images of sites affected by BD activities at cell #4

4.4.2. BD counting and distribution In order to compare the hot cell aspect with a “normal” cell, global images composed by thousands of

high resolution SEM images acquired and assembled with Smart Stitch were performed in various

neighbouring cells. Affected BD sites were marked manually on the global image highlighting areas

with higher BD density. Additionally, the number of marks were estimated by image analysis software

AxioVision. This procedure results in a BD number estimation lower than the real number because of

close proximity of craters, their overlapping and various other not classified features. An example of

overview is shown in Figure 17.

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Figure 17: BD activity affected sites overview at cell #4 (upstream) and estimated total number (460 sites)

The estimated number of BD sites in cells #4 and #5 with this procedure was 750 and 790 sites

respectively. Breakdown affected regions were randomly distributed on the cells and there was not a

clear correspondence between facing irises in terms of position and number. For cells #22 and #23 the

extrapolated number (only one cell side counted) is 340 and 390 respectively. Figure 18 shows the BD

distribution as in Figure 5 overlaid with the above mentioned numbers from the SEM BD counting.

Figure 18 BD distribution inside the structure similar to Figure 5 but overlaid with SEM breakdown counting (green circles).

In addition, an analysis of the BD radial distribution was performed as shown in Figure 19. As expected

from field simulations, no clear relation to max E-area nor to max Sc-area could be established, since

geometrically by design those two areas are very close. It can however be confirmed that in most

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cases the breakdown density is diminishing with increasing distance from the iris. More statistics might

reveal a finer substructure in radial distribution. However, at this point the analysis has been

completed for the given accelerating structure.

Figure 19: study of the BD radial distribution in cell #4 and cell #5

4.4.3. Assessment of bonding quality The quality of bonding joints was investigated. Additionally, disk number 22 was cut into quadrants,

allowing for a direct view with the microscope into the area under a better angle (perpendicular to

the cell wall).

Non-uniformity in the bonding joint meniscus can be seen in cell 22 as shown in Figure 20. There is

less non-uniformity than in previous structures (T18 and TD18), possibly due to improvements in the

bonding design (most notable, the reduced edge radius of 50um). The maximum lengths of areas

lacking material are 10um as compared to >25um in TD18).

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Figure 20: SEM (SE2) images of bonding contact at cell #4 and cell #22

4.4.4. Presence of B-field arcs In previous structures (T18 and TD18) the presence of features associated to B-field arcs were

observed. B-field arcs are called features emerging at the former joint region (meniscus) in areas

where the magnetic fields are highest and hence electrical currents are highest, too. Those features

form and grow as seen in Figure 21 and might evolve to a state where electrical breakdown is triggered

in and around those sites, although electric fields are not as high here as in other regions.

Figure 21: SEM (SE2) images of B-field sites at the bonding plane in cell #23

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Initial features of B-field arcs were also found in the current structure TD24 R-05 as shown in Figure

21, but only in the opened cells at the end of the structure (cell #22 and cell #23), whereas here no

extensive arcing was associated to these sites.

More information concerning the SEM observation on structure TD24_R05_N1 can be found in the

EDMS document 1450676.

4.4.5. Thermal fatigue considerations No features of thermal fatigue have been observed in the structure. The highest ∆T according to the

RF design was 40.5K at an average unloaded gradient of 100MV/m. This result confirms the RF

optimisation choice of lowering the max ∆T below 50K (empirically determined from the TD18 post

mortem observation) in order to avoid any deterioration due to accumulative thermal cycling effects.

5. Discussion

5.1.Geometrical deformation and Bead-pull The bead-pull measurements after high power operation revealed anomalies as presented in the

results in form of reflections at the output coupler. Simulations indicate that even small deformations

of the geometry of the output coupler (tilts of 10-20 um) could generate measurable effects on the

field profile and on the cell-to-cell phase advance.

Unfortunately it was not possible to find a consistency amongst RF measurements, metrology and

corresponding simulations. Also information is missing from intermediate steps to determine at which

stage the changes occurred. Making all of the necessary measurements must be balanced against the

risk of causing new problems - contamination, scratches, etc. – which could compromise high gradient

performance. In addition the design of the CLIC baseline ‘compact coupler’ does not have the coupling

iris which deformed, so this may not be a long term issue.

5.2.Break down number considerations The seemingly good agreement of counted numbers of breakdowns in cell number 5 and recorded

from RF signal (within 10% in Figure 18) does not hold true anymore after closer examination. As

mentioned in the BD distribution plot in Figure 5 the bin of cell number 4 is supposed to be distributed

to the adjacent bins. Hence the initial < 10% agreement cannot be true. Unfortunately, it is at this

point impossible make a better estimation. However, the situation towards the end of the structure

(cell #22 and #23) shows a clear underestimation of the recorded number of breakdowns with respect

to the counted ones by a factor of 2-3.

The discrepancy found between the recorded number of breakdowns during high power operation

and the observed and counted number of breakdowns by microscopy confirms again the difficulty in

defining exactly what an individual breakdown event looks like: potentially multiple sites affected by

one recorded breakdown, surface changes without triggering the breakdown recorder or other

unknown effects. Understanding these processes are object to current research and are key in

understanding and operating the accelerating structures better.

https://edms5.cern.ch/document/1450676/1

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5.3.Bonding joint and B-field arcs comparison to T18 and TD18 The bonding joint in the current structure shows an improved aspect and a smaller meniscus with

respect to the previously observed T18 (EDMS: 1211776) and TD18 (EDMS: 1096980) thanks to the

improvement in the bonding design. The radius at the bonding edge was reduced to 50um and

resulted in local areas of lack of material smaller than 10um.

However, the current structure also shows a similar trend in developing features in the bonding joint

as the formerly observed T18 and TD18. Early stages of features are observed which in the case of the

TD18 led to features which could be clearly associated with significant electrical breakdown activity.

However, in the current structure, as well as in the older T18, those features do not show any kind of

breakdown activity due to lower local magnetic fields and hence less pronounced trigger features.

6. Conclusions The analysis reported in this document is the last part of a long process that starts from the RF design

and the manufacturing of the accelerating cells involving several steps that are summarized in Figure

22.

Figure 22: Schematic summary of lifecycle of a CLIC prototype accelerating structure

The problems that are addressed by the proposed analysis can be summarized in two main questions:

1- Is the geometry of the disks preserved during the bonding?

2- Are the dimensions of the disks influenced by high power operations?

https://edms5.cern.ch/document/1211776/1https://edms5.cern.ch/document/1096980/2

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The use of non-destructive measurements (such as bead-pull) taken before and after high power

operation could help in answering such question. Those results, complemented with the destructive

analysis and microscopy of the inner part of the structure can provide with important feed-back for

the RF design and the whole manufacturing and handling process, towards a better defined procedure

better suited for industrialization.

CLICcoverpage_CLIC_Note_1070Post mortem TD24_R05_N1 --- V71. Introduction2. RF History2.1. Bead-pull measurements and tuning2.2. RF high power test history

3. Sample preparation3.1. Cutting operations3.2. Wire EDM cutting at CERN3.3. Chemical cleaning: Degreasing

4. Results of characterization4.1. RF measurements before cutting4.2. Metrology4.3. Simulations4.4. Scanning Electron Microscope (SEM) observation4.4.1. Surface study4.4.2. BD counting and distribution4.4.3. Assessment of bonding quality4.4.4. Presence of B-field arcs4.4.5. Thermal fatigue considerations

5. Discussion5.1. Geometrical deformation and Bead-pull5.2. Break down number considerations5.3. Bonding joint and B-field arcs comparison to T18 and TD18

6. Conclusions