JLab-TN-02-025
Preliminary Results from Cavity and Process Particulate Collection
By: John Mammosser and Andy T. Wu
Particulates from the cavity production High Pressure Rinse (HPR) filter and Jl009 cavity after
vertical test were collected and analyzed to determine the source of recent poor cavity RF performances.
Commonalities in particles found from the cavity, trapped in the HPR filter media and the filter media
material, suggest that many of the particulates found in JL009 could have come from the DI water and HPR
system due to maintenance and replacement of this filter prior to processing of JL009, however it is not
known which of these particulates would cause field emission, or if they were the material causing the field
emission in JL009. A method for collecting and analysis of particulates collected for this study, the results
from the particulate analysis and several suggestions to improve procedures are reported in this document.
Background
In preparation for building the SL21 cavity string, cavities must first be qualified for string
assembly by demonstrating that they can operate above design specification at which they will be operated
in the accelerator. The SL21 module cavities are required to operate at a Q-value of 6.5e9 at gradients
above 12.5MV/m. Recent cavity qualification tests were failing due to low Q-values caused by field
emission loading at gradients lower then these design values. The cause of the field emission is believed to
be surface contamination that enters in during processing and assembly stages for these cavities, prior to the
vertical RF tests. The cavity processing procedures were recently modified to address two identified
critical areas in which particulates could come in and to improve chances of their removal. The two areas
identified were ineffective high pressure rinsing (not enough time) and test flange cleaning. The test
flanges were cleaned in a different way and were not high pressure rinsed like the cavity. The decision was
to extend the amount of high pressure rinsing time to four hours in two-hour intervals, the second interval
having all test flanges attached with the exception of the bottom flange (input coupler) to allow the wand a
path for entering the cavity. After several cavities were processed in this way and RF tested in the VTA, it
became evident that a field emission was not reduced by the procedure changes in all tests. Also the cavity
performance seemed to vary even more with few good tests and the rest dominated by field emission, most
likely caused by particulate contamination on cavity interior surfaces. To investigate the root cause of this
contamination, particulate samples were taken from the high-pressure rinse filter, which was removed for
inspection and replaced, and JL009 cavity after the vertical RF tests and after the filter replacement.
Particulate Collection Method
In-order to collect particulate samples and be able to identify them on the Scanning Electron
Microscope (SEM), double sided carbon tapes (manufactured by SPI) were used for collecting particulates
on both the filter and cavity surfaces. These tapes are designed for clean collection and analysis of small
size particulates.
Particulate Collection from HPR Filter
On 6/18 the HPR filter was removed for inspection and a new filter was put in its place. The filter
is a 20-inch long borosilicate glass fibers held together with a polypropylene binder. The filter thickness is
½ inch and is sealed at both ends and designed for filtering liquids to 0.2um. The following procedure
was used to remove the filter and collect the particulates.
 A clean nylon bag was cut to length and sealed at the bottom in the main chemroom and placed in the
pass thru.
 In the cleanroom inside the HPR cabinet he filter housing was unscrewed and the filter was removed
and immediately placed into the nylon bag. The unsealed end was folded back and taped across the
entire opening to the bag itself.
 The filter was transported to the offline cleanroom where the outside of the bag was wiped with a
cleanroom wipe with methanol.
 The laminar flow bench was then wiped down with acetone and then methanol to clean work surfaces.
 The bag was then cut open with scissors below the taped end and the filter was removed from the bag.
JLab-TN-02-025
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Next the scissors were cleaned and outer plastic guard was removed and the filter cut open down the
middle.
Carbon double sided tape was then used to pick samples from the filter using blotting at areas of
visible particulates
Inspection of the filter showed the internal surfaces where the water enters was a brownish color and
after a few layers in the filter media was white and clean. The outside of the filter was inspected and no
visible particles were found. But at the base of the housing there was also a brownish stain.
JL009 History and Performance
JL009 was assembled with the modified procedure mentioned above and RF tested in the VTA on
6/23, see Figure 1. JL009 Vertical Test Results. The processing and assembly of this cavity encountered no
problems and was typical for this type of cavity. The results from this test showed early onset of field
emission at 5.8MV/m and a low Q-value at low gradients of 8E9. The performance of this cavity test was
typical of a performance dominated by heavy field emission and was identified as a good candidate for
collecting particulates from the interior surfaces to gain knowledge on what could be causing the failure.
JL009b Vertical Test 6/23/02
10
10
10
9
1
2
3
4
5
6
7
8
Eacc (MV/m)
Figure 1. JL009 Vertical Test Results.
Particulate Collection from the Cavity
The following procedure was used to collect particulates from the cavity interior surfaces:



First the cavity was moved to the cleanroom on the test stand, cavity in the vertical position and was
back filled with filtered nitrogen to 1 ATM so it could be opened.
Several clean nylon bags were cut to length and sealed at one end in the chemroom.
The bottom flange hardware was removed and then the test flange was separated from the cavity
flange.
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Next the surface between the vacuum port opening and the input probe port was blotted with the
carbon tape and the tape was immediately placed into the nylon bag and closed.
The cavity interior beam tube, FPC end was also blotted with tape and added to the nylon bag.
Finally the field port test flange was removed in the same way (cavity top) and the flange and the
HOM beam tube end were blotted with the tape.
The nylon bag opening was folded back and taped across the entire opening to the bag itself.
Cleaning and preparing samples for the SEM
The following procedure was used to prepare the SEM chamber:
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Prior to starting the analysis, the SEM chamber was vented and cleaned by wiping the interior surfaces
with a cleanroom wipe and methanol.
Next the stage was removed and brought to the laminar flow bench where it was cleaned by wiping
with methanol.
Next a new package of silicon wafers was opened and a single wafer was removed and wiped off with
methanol.
Next the nylon bag was opened, by cutting with a clean scissors and the carbon tape was pealed from
the nylon bag and the backside protection strip was removed.
Finally the tape was adhered to the silicon wafer. All samples from the cavity were mounted to this
wafer and the wafer was placed into the stage and clipped into place for a good electrical contact.
Next the stage was carried to the chamber and placed onto the stage holder and the door was closed.
The chamber was then evacuated and identification and analysis was ready to start.
Particulate Identification
The two sample tapes were collected from the HPR filter and these showed many very large size
particles with at least one side having a length of greater then 100m and many smaller sizes less then
100m in maximum length. The type particle could also be categorized as a solid element with some
additional trace elements or consisting of a composite of many different elements forming the particle. The
picture John04 shows a typical smaller size solid element that consists of the following makeup:
Element
Si
S
Fe
%
97.56
0.81
1.63
Picture John04 -A small solid element mostly Silicon, collected from the HPR filter.
JLab-TN-02-025
The picture John02 shows a large particulate that is a composite and has the following makeup:
Element
S
Mo
Fe
Cr
%
63.1
34.12
2.18
0.58
These pictures show clearly the filter media glass fibers with their polypropylene binder and a clear texture
to the particles that were removed from the filter. The small solid element John04, has a smooth texture
and the large composite John02, a course texture. Both pictures show a rectangular box locating the
position on the particle where the electron beam was focused for analysis. For smaller size particles a spot
marked by a
Picture John02 – A large composite particle collected from the HPR filter.
cross hairs was used to reduce background from entering into the collection process. The identification of
the particle makeup was performed with the SEM X-ray analysis (EDAX) system. This system has a lower
energy cutoff of about 0.5keV so it cannot detect elements with atomic numbers below fluorine and this is
why carbon tape makes for an excellent collection media. The carbon tape background was analyzed to
determine if any energy peaks were identified and is provided at the end of this document with the picture
locating the electron beam spot. For large composite particles, the particle makeup can be dependent on
where on the sample the beam is focused and no attempt was made to fully characterize each particle.
The picture John03 shows a large composite particle that has a very different texture that that of
John02. At the rectangle, smaller coarser particles are evident and the makeup consists of 6 different
elements as follows:
Element
S
%
38.09
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F
Fe
Si
Mg
Cr
34.22
12.78
6.76
4.24
3.92
Picture John03 – A large composite particulate collected from the HPR filter.
The summary of ten particles analyzed shows that eleven different elements were found and that
some of these elements are consistent with the type of suspended matter that can be found in high purity
water (complex colloids). For example, silica is a common colloid that can contain heavy metal and
organic ions that collect and form complex particulates and are suspended in the water and pass through
most ion exchange beds but should be filter out with ultra filtration stages1, see Figure 2. Results from
analysis of particles trapped in the HPR filter.
Particulates Collected From The HPR Filter
200
150
100
50
0
S
Al Fe
Si Mo Ti
F
Cr Ni Mg Cl Ca
P Cu Au Na
K
Figure 2. Results from analysis of particles trapped in the HPR filter.
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Additionally, the filter media was analyzed to compare to the elements found in the cavity, see figure 3.
Analysis of the HPR filter media. The filter media is important because it is the final filtration and the bulk
volume of high-pressure water must interface with the media.
Filter Media Make-up
500
400
300
200
100
0
Si
Ca
Al
Na
K
Mg
Fe
Figure 3. Analysis of the HPR filter media.
Observations
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Many very large particles were present on the input side of the filter media, which should have
been filtered out, in the polishing loops of the DI water plant. This system has two polishing loops
each with final 0.1micron ultra filters.
 Filter looked clean outside and no visible particles were identified.
 Filter interior was brown as well as the bottom of the filter-holder, this should be analyzed to find
out what it is.
Discussion:
 These particles could have come into the filter during modification to the DI water system,
installation of the filter or from the HPR components itself.
 More effort to collect particulates from the outside of the filter could have revealed smaller size
particulates that might have traveled to the cavity.
 Can these large particles breakup and travel through the filter media and end up in the cavity.
 Does the filter media glass fibers field emit subjected to high electric fields. During the analysis
several of the glass fibers were charged by the electron bean in the SEM and stood straight up on
the carbon tape while scanning the surface. Tests should be carried out to see if the filter media
causes field emission.
JLab-TN-02-025
Results From Cavity Particulates Identification
Twenty different particles collected from cavity JL009 surfaces (after vertical test), were analyzed
to determine their elemental makeup and characteristic sizes. The particles varied in size from 1m to
75 m in maximum length and elemental makeup with 16 different elements identified. These
particles were typically smaller in size then the particles found in the HPR filter (see Figure 4. Particle
Sizes from both JL009 and HPR Filter.).
Particulate Sizes (um)
250
200
150
100
50
0
F2 F3 C2 F4 C10 C3 C5 C9 C11C20 C4 C8 C6 C18C12C17C19 C7
Particle Number (F-filter,C-cavity)
Figure 4. Particle Sizes from both JL009 and HPR Filter
Analysis of the particles showed that most were a composite of at least two elements. Only four
of the twenty particles were solid elements such as Picture JL009-7, which was pure copper and 1 by 2m
in size. The sizes of the smallest of the particles were from 1 to 8m in maximum length and most were
metals such as gold, copper and aluminum. Also it is important to note that all but three of the elements
found in the cavity were present in the filter and filter media. These three elements were copper, gold and
phosphorus.
Picture JL009-7 Solid Copper Particle.
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The Picture JL009-8, shows a typical small composite particle found on the in the cavity JL009
along with its energy dispersive X-ray data that clearly shows seven different elements. This collection of
elements has metals such as iron and titanium as well as the typical dominant element silicon.
Although many particles were present on these samples not all were analyzed but the majority of
the particles, especially the larger sizes, contained silicon as seen in Figure 5. Particulates collected from
JL009 cavity surfaces.
Particulates Collected from JL009 Cavity Surfaces
600
500
400
300
200
100
0
Si S Fe Cl Al Ca Cu Au Ti Mg F Na K Cr P Ni Mo
Elements
Figure 5. Particulates collected from JL009 cavity surfaces.
The cavity particulates also varied in texture as did the ones collected from the filter and is shown
in Picture JL009-9 were six different particulates are in the field of view.
JLab-TN-02-025
Picture JL009-9.
In the picture JL009-9, there are particles present that have elements representing most the
elements found and large composites (upper left) as well as smaller solids (lower center).
Observations
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Discussion
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Silicon seems to be the most dominant large element found in JL009 but the small metals
such as aluminum and copper are also present.
These samples only represent a small sample of the particulates from JL009 and it is not
known if these are a good representation of the interior cell surfaces.
All but three of the elements in the cavity were found in the water system.
The particulate collection method described in this paper was simple and effective in
collecting particle of sizes ranging from sub-micron to few tenths of micron from SRF
cavity surfaces.
Many particulates were collected from the cavity and test flanges. These particulates
need to be reduced through improved assemble and process procedures. More attention
to system operation and maintenance could reduce the number of particulates entering
these components.
It would be important to clearly identify where process improvements need to take place
by analyzing critical process performance such as the HPR system, DI water quality,
vacuum system performance and test-stand contamination.
We would like to thank Ralph Afanador and Danny Forehand for their contributions with
collecting samples.
Appendix
1
Metzer, Theodor H. “High-Purity Water Preparation for The Semiconductor, Pharmaceutical,
and Power Industries”, Tall Oaks Publishing, Inc., pg 9-10, 1997.
JLab-TN-02-025
Background analysis of the carbon tape.