Abstract Vacuum systems containing superconducting cavities which have to be operated at high gradients need to preserve the cleanliness of the superconducting cavity surfaces. In addition to an adequate preparation of the cavities and the neighbouring vacuum components special care needs to be taken during pump down and venting. Neither should be particles introduced into the vacuum system, nor should particles already present within the system be moved towards critical areas. For the superconducting linear accelerators of FLASH and the European XFEL at DESY a series of measurements have been performed to study the movement of particles in long tubes during pump down and venting. For this purpose an in-situ vacuum particle counter has been used. By reducing and varying the gas flow during these processes, it is possible to perform these actions without moving particles present inside such systems. Based on these measurements a set-up using various filters, flow controllers and a pressure gauge has been developed to avoid introducing particles into the vacuum system as well as moving existing particles. This set-up allows automated pump-down and venting of critical vacuum systems in a reliable and reproducible way, being much faster than the procedures used so far

J Wojtkiewicz

K Zapfe

CiteSeerX

PARTICLE FREE PUMP DOWN AND VENTING OF UHV VACUUM 
SYSTEMS 
K. Zapfe and J. Wojtkiewicz, Deutsches Elektronen Synchrotron DESY, D-22607 Hamburg 
Abstract 
Vacuum systems containing superconducting cavities 
which have to be operated at high gradients need to 
preserve the cleanliness of the superconducting cavity 
surfaces. In addition to an adequate preparation of the 
cavities and the neighbouring vacuum components special 
care needs to be taken during pump down and venting. 
Neither should be particles introduced into the vacuum 
system, nor should particles already present within the 
system be moved towards critical areas. 
For the superconducting linear accelerators of FLASH 
and the European XFEL at DESY a series of 
measurements have been performed to study the 
movement of particles in long tubes during pump down 
and venting. For this purpose an in-situ vacuum particle 
counter has been used. By reducing and varying the gas 
flow during these processes, it is possible to perform 
these actions without moving particles present inside such 
systems. Based on these measurements a set-up using 
various filters, flow controllers and a pressure gauge has 
been developed to avoid introducing particles into the 
vacuum system as well as moving existing particles. This 
set-up allows automated pump-down and venting of 
critical vacuum systems in a reliable and reproducible 
way, being much faster than the procedures used so far. 
INTRODUCTION 
During the past decade the maximum achievable 
gradient for superconducting cavities has been improved 
substantially. One major contribution to this improvement 
is the consequent treatment and preparation of the cavities 
in clean rooms with procedures similar to standards in 
semiconductor industry [1]. Dust particles can act as field 
emitters, degrade the quality factor and thus limit the 
performance of the superconducting cavities. Therefore 
particles on the inner surface of the cavities need to be 
absolutely avoided. As a consequence vacuum 
components next to the superconducting cavities need to 
be treated with similar cleaning procedures [2] and [3]. In 
addition special care needs to be taken during pump down 
and venting of the vacuum systems. Neither should be 
particles introduced into the vacuum system, nor should 
particles already present within the system be moved 
towards critical areas. 
For the superconducting linear accelerator of the SASE 
FEL FLASH at DESY [4] and the just started project of 
the European XFEL with more than 1000 cavities to be 
used [5] it is necessary to substantially improve the 
procedures used so far. While particle filters effectively 
prevent the introduction of particles into vacuum systems, 
it is much less known how to avoid the movement of 
particles within the system, which for example have been 
produced by movable elements like valves or actuators. 
So far pump down and venting in such cases is mainly 
done using manually operated needle valves and 
performing the processes much slower than usual. Typical 
process times are several hours, e.g. 20 h for venting a 
cavity string with a volume of 200 liters. 
Especially for the large scale production of 
superconducting cavities for the European XFEL it is 
important to develop a set-up for particle free pump down 
and venting of vacuum systems with automated, reliable 
and reproducible procedures.  
For this purpose a series of measurements have been 
performed to study systematically the movement of 
particles in long tubes during pump down and venting. 
Several commercial elements have been tested to 
automate the procedures resulting in a compact set-up and 
well defined procedures which will be described in this 
paper. 
PARTICLE MOVEMENT IN LONG TUBES 
For systematic studies of the movement of particles 
within long vacuum tubes and how it could be minimized 
or even prevented during pump down and venting an in-
vacuum particle counter has been applied. 
Experimental Set-up 
The set-up consists of a 10 m long tube of 63 mm 
diameter as shown in Fig. 1. The tube corresponds 
roughly to the length and volume of a cavity string. On 
the right side of the tube both a pump station for pump 
down as well as a nitrogen bottle for venting are 
connected. Both connections have initially been equipped 
with manual needle valves in order to vary the gas flow 
over a wide range. In addition a diffuser has been added 
in the vent line at some stage of the measurements to 
prevent particles to enter the system as well as to 
homogenize the gas flow as described in more detail in 
the following section. 
 
 
Figure 1: Experimental set up for measuring particle 
movement in long tubes. 
For particle measurements an in-situ vacuum particle 
counter equipped with KF-flanges was used. For these 
measurements metal seals have be used for all UHV 
connections including the particle counter. The in-vacuum 
counter could be installed at three different locations at 
both ends and in the middle of the vacuum tube as 
Proceedings of SRF2007, Peking Univ., Beijing, China WEP74
WEP: Poster Session II 681
indicated in Fig. 1. As the sensitive volume of the particle 
counter is quite small and thus does not cover the 
complete cross section of the vacuum tube, the measured 
particle numbers are no absolute values but rather indicate 
a tendency.  
For these investigations neither have the tube and other 
components been made particle free by special cleaning 
procedures as e.g. described in [3] and [4] nor has the 
assembly been done within a clean room. Instead particles 
had been introduced on purpose at three locations close to 
the ports for the in-vacuum particle counter as indicated 
in Fig. 1. All operations on the needle valves have been 
done manually. Therefore each tested procedure has been 
repeated at least five times to get some statistics of the 
results. 
Results 
The outcome of the investigations can be summarized 
as follows: 
Movement of particles is mainly observed at the 
position next to the pumping and venting ports. 
Sometimes moving particles could be observed at the 
middle position, but very rarely at the opposite position 
from the pump or vent port. Thus the long tube 
homogenizes the gas flow and reduces turbulences. As a 
consequence the set-up for pump down and venting of 
critical systems should be connected as far as possible 
from components being sensitive to particles. 
 
 
Figure 2: Particle counts per min during fast pump down 
without any measures to avoid particle movement. 
During pump down the number of particles decreased 
with decreasing pressure, indicating a reduction of 
turbulences. As seen in the measurement in Fig. 2, where 
no special measure where taken during pump down, 
particles could be measured down to a pressure of about 
100 mbar, with the initial peak originating from particles 
introduced at the port next to the pump and the second 
one due to particles introduced at the middle port. 
However, if turbulences are again introduced by sudden 
changes of the throughput of e.g. a needle valve, particles 
had been detected down to pressures of about 1 mbar. 
Only for pressures < 1 mbar no more particle movement 
has been observed. As a consequence manual valves are 
not well suited to adjust the gas flow for particle free 
pump down and venting. For venting a system this means 
that the pressure difference between the vacuum vessel 
and atmosphere should be below 1 mbar before opening 
the system. 
In vented systems movement of particles has been 
observed after some hours. This has not been studied in 
further detail, most likely the movement is due to thermal 
changes and/or mechanical vibrations. As a consequence 
the time the vacuum system is kept vented should be 
minimized. 
The results described above have been confirmed for a 
tube with smaller diameter of 16 mm. 
SET-UP FOR PARTICLE FREE PUMP 
DOWN AND VENTING 
Based on the measurements described above, a compact 
set-up based on commercially available components has 
been developed. The key components are mass flow 
controllers and diffusers. 
A mass flow controller keeps a given gas flow constant 
using a variable diaphragm. For the applications 
described in this paper, flow controllers applying a soft 
start, i.e. increasing the aperture to its initial value slowly 
and uniformly, have been chosen, thus avoiding 
turbulences by sudden changes of the open cross section. 
The mass flow controllers have an intrinsic particle filter 
on the side of the incoming flux. 
Diffusers are widely used in semi-conductor industries 
for fast and particle free venting of large vacuum systems. 
The diffuser combines a stainless steel filter to prevent 
particles larger than 3 nm entering the system and a 
diffuser membrane that allows gas flow in 360°. Thus 
turbulences in the gas flow are largely reduced and large 
amounts of gas can flow through in a uniform manner, 
limiting the disturbance of particles in the system. For the 
described application a commercially available diffuser 
has been adopted to our needs. The KF flange has been 
cut and the remaining body has been welded into a tube 
with sufficiently large diameter with a ConflatTM flange 
on the other side. 
Concerning pressure both the information of the 
absolute pressure, especially in the region around 1 mbar, 
as well as the differential pressure against air is required. 
Both functionalities are provided by a single sensor, a 
Loadlock Transducer. It combines a Piezo for gas 
independent differential pressure measurement against air 
pressure and MicroPiraniTM sensor technologies for 
absolute pressure measurement between 1000 mbar down 
to 10-5 mbar. Automated as well as manual valves 
complete the set-up.  
It turned out that separate components are necessary for 
pump down and venting as shown schematically in the 
left part of Fig. 3. In the vent line a nitrogen gas bottle or 
dewar is connected via a manual valve (V3) to a mass 
flow controller (1) at an inlet pressure of 2-3 bars. In case 
of using a gas bottle special attention must be given to the 
cleanliness of the fittings, eventually special filters (e.g. 
charcoal) should be added. The vent line continues with a 
diffuser and an automated valve (V1). Using the same set-
WEP74 Proceedings of SRF2007, Peking Univ., Beijing, China
682 WEP: Poster Session II
up for pump down the diffuser would increase the pump 
out time from atmosphere to 1 mbar by a factor of 4 from 
30 min to 120 min. Thus for pump down a second mass 
flow controller (2) and a bypass line with an automated 
valve (V2) are installed. The Loadlock Transducer is 
located next to the flange which is connected to the 
manual valve (V0) of a vacuum vessel. This compact set-
up shown in the photograph of Fig. 4 (right) could be 
easily connected to a pump station. 
Although the set-up is currently not yet completely run 
by computer control, the goal is to have the complete 
automation of the whole procedure in the near future 
using a programmable logic controller (PLC).  
PROCEDURES FOR PARTICLE FREE 
PUMP DOWN AND VENTING 
For practical applications the set-up has to be connected 
to the manual valve of a vacuum vessel (V0 in Fig. 3), a 
gas bottle (e.g. nitrogen) in the vent line and a pump 
station in the pump line.  
In case of venting, it is advisable to remove particles in 
the vent line between the diffuser, valve V2, the mass 
flow controller 2 and the manual valve V0. Performing 
pump and purge, e.g. fast pump down followed by fast 
flushing with nitrogen several times, most particles will 
be moved out towards the pumps. For pump down this 
procedure is not needed as particles will be moved 
towards the pumps. 
Pump down procedure 
To start pumping, the valves to the vent and bypass line 
V1 and V2, need to be closed. The pumping will start 
through the mass flow controller 2, which is set to a mass 
flow of 50 mbar l/s. The controller in use increases the 
aperture to its initial value slowly and uniformly, so 
called soft start, thus avoiding turbulences. With 
decreasing pressure, the aperture is further increased to 
keep the flow constant. However once the maximum 
opening is reached, the achievable gas flow will decrease. 
The flow controller with its intrinsic particle filter will 
always limit the pump speed to much smaller values than 
without it and thus increase the further pump down time 
to unacceptable values. Therefore a bypass line is 
foreseen. The valve V2 is safely opened without 
introducing further particle movement, once the Loadlock 
Transducer indicates  a pressure of 1 mbar. 
Using this procedure no particles are registered by the 
in-vacuum counter in the upper graph in Fig. 4 during the 
pump down  
 
 
 
Figure 4: Particle counts per min during pump down (top) 
and venting (bottom). 
V2
Pump Station
Mass Flow
Controller 2Mass Flow
Controller 1
V3
Nitrogen
 
Diffuser
Vacuum System
Loadlock
Transducer
(p and    p)
V1
 
V0
Figure 3. Set-up for particle free pump down and venting: schematics (left) and photograph (right). 
Proceedings of SRF2007, Peking Univ., Beijing, China WEP74
WEP: Poster Session II 683
Venting procedure 
For venting nitrogen with 2-3 bar is filled into the line 
from the gas bottle through the manual valve V3 up the 
mass flow controller 1, which is closed. The remaining 
vent line is initially pumped out using the pump station 
with the valves V1 and V2 open. Once a pressure of at 
least 0.1 mbar is reached the manual valve to the vacuum 
vessel V0 is opened and the pump line is shut off by 
closing V2 and mass flow controller 2. Venting of the 
vessel starts by setting mass flow controller 1 to a 
throughput of 50 mbar l/s, which slowly and uniformly 
adjusts its aperture. The venting is stopped by closing 
valve V1 if the differential pressure to atmosphere at the 
Loadlock Transducer is 0. Only then the vacuum vessel 
should be opened. As the air pressure usually varies, there 
should be not much time difference between venting and 
opening of the system. Otherwise one should make sure, 
that there is no pressure difference between the vented 
and still closed system and the ambient air.  
The lower graph in Fig. 4 shows an example without 
any particle detection for venting a long tube. 
Practical Applications 
The new set-up and procedures described above have 
been applied for pump down and venting of a full string 
of 8 TESLA cavities. For quality control the in-situ 
particle counter had been installed in between the set-up 
and the manual valve of the cavity string. No particles had 
been detected during the complete pump and vent cycle. 
So far these procedures have been performed according 
to best knowledge, but without systematic studies as 
described in this paper using particle filters and manual 
needle valves as well as doing the process very slowly.  
For pump down the process time to reach a pressure of 
10-6 mbar has been reduced by a factor of 3 from 10 h to 
3.5 h. The reduction in time for venting is more than a 
factor of 10 from 24 h to 1.5 h as shown in Fig. 5. This is 
a substantial reduction of the process time.  
 
 
 
Figure 5: Pressure versus time during venting of a cavity 
string using the old procedure () and much faster new 
procedure (). 
SUMMARY 
Systematic studies in a long vacuum tube have shown 
that pump down and venting of UHV vacuum systems is 
possible without introduction and/or movement of 
particles within the system. Based on these measurements 
a set-up using various commercially available filters, flow 
controllers and a pressure gauge has been developed. This 
set-up allows automated pump-down and venting of 
critical vacuum systems like superconducting cavities in a 
reliable and reproducible way. The process times have 
been significantly reduced. The next step is the full 
automation of the procedures using a PLC to be well 
prepared for industrial production of the XFEL cryo 
modules. 
Nevertheless, the time where such sensitive systems are 
kept at air pressure should be minimized as particles 
might start to migrate due to thermal changes or 
mechanical vibrations. 
ACKNOWLEDGEMENT 
The authors would like to thank W. Griebele, Y. 
Brzelinski and M. Lengkeit for their contribution to the 
measurements described in this paper within the 
framework of their final exam and S. Holm for the 
technical support. 
REFERENCES 
[1]  D. Reschke, “Final Cleaning and Assembly”; Proc. of 
the 10th Workshop on RF Superconductivity, 
Tsukuba, 2001, http://conference.kek.jp/srf2001/. 
[2] K. Zapfe, U. Hahn, M. Hesse and H. Remde, “A 
Cleaning Facility to prepare Particle free UHV-
Components, Proc. 11th Workshop on RF 
Superconductivity, Travemünde, 2003, 
http://srf2003.desy.de/fap/. 
[3]  U. Hahn, M. Hesse, H. Remde and K. Zapfe, Vacuum 
73 (2004) 231. 
[4]  E. Vogel, “FLASH Progress Report”, this workshop. 
[5]  L. Lilje, “XFEL: Plans for 101 Cryo Modules”, this 
workshop. 
WEP74 Proceedings of SRF2007, Peking Univ., Beijing, China
684 WEP: Poster Session II


Particle Free Pump Down and Venting of UHV Vacuum Systems

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Particle Free Pump Down and Venting of UHV Vacuum Systems

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