Notes on ultra-high vacuum technique

Materials & methods
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ConFlat

The mechanics of conflat are discussed in Lutkiewicz and Rathjen 2008

Flanges made from aluminium are also available, using hardened coatings on the knife-edges, which are exclusively usable with aluminum gaskets. This is sometimes used for the greater thermal capacity. Unless there is a seriously compelling reason to, I would suggest avoiding aluminum gaskets.

In the case of aluminum . Copper gaskets have a work-hardening property that makes it retain a good seal preload even in the case of temperature fluctuations and baking.

In some areas at our facility, equipment (cameras, lights, etc) may be mounted using brackets to the outside of flanges using the flange circle’s bolts or studs. This is convenient as it does not require. However, for the reasons discussed below, great care must be taken when modifying this kind of mount. The distance . groove grabber.

Many . Metal-to-metal. leave some gap between the faces so any leaks can be re-sealed.
dd

gasket stays in the non-rounded punch face. ca

The forces involved in bringing some flanges metal-to-metal may exceed the yield strength of stainless steel bolts. See () for more information on this topic.

Reluctance to use anti-galling compounds for fear of contamination, seizing stainless fasteners is very common.

Copper gasket clips

gasket removal tool

Suppliers and manufacturers

kimball physics

To add

Vacuum hackers warning to samy

Bolts that show signs of deformation should be discarded. Some vacuum workers do not attempt to reuse 1.33" conflat flange screws because a significant number are distorted after one use.

Books
The Physical Basis of Ultrahigh Vacuum,
P.A. Redhead, J.P. Hobson, E.V. Kornelsen,
American Vacuum Society Classics,
American Institute of Physics, 1993
Foundations of Vacuum Science and Technology,
Edited J.M. Lafferty, John Wiley & Sons, 1998
Handbook of Accelerator Physics and Engineering, A. W.
Chao, M. Tigner, World Scientific, 1998
CAS CERN Accelerator School : Vacuum Technology,
Edited : S. Turner. CERN 99-05, 19 August 1999
Handbuch Vakuumtechnik, M. Wutz et. al, Vieweg,
Braunschweig/Wiesbaden, 2000
Journals:
VACUUM
Journal of Vacuum Science and Technology (A)
Nuclear Instruments and Methods (Section A)

http://www.ece.ualberta.ca/~schmaus/vacf/index.html Vacuum basics.
http://www.ece.ualberta.ca/~schmaus/vacf/vacsys.html How to operate a diffusion pump.
http://www.vacuumlab.com/articles.htm More advanced vacuum information.
http://shop.store.yahoo.com/vacuumshopper/oils.html Rotary vane and diffusion pump oils.

Definition Pressure Boundaries
[mbar]
Low Vacuum LV 103 − 1
Medium Vacuum MV 1 − 10−3
High Vacuum HV 10−3 − 10−9
Ultra High Vacuum UHV 10−9 − 10−12
Extreme Vacuum XHV < 10−12
2.1 Basic notions
2.1.1 Ideal Gas Law
A rarefied gas which obeys to the ideal gas state equation can be defined as ideal.
Table 2.1 Degrees of vacuum and their pressure boundaries from Lafferty

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This event is
displayed in Figure 4.16. Shortly after the second sudden increase the QCM failed.
QCM’s are known to fail once too much mass is on the deposition surface. This was
expected to happen eventually. QCM failure is marked by a constant reading (flat
line). The behavior of the QCM before failure is consistent with the film fracturing.
As the film fractures, it delaminates from the surface of the QCM and causes sudden
jumps in the frequency reading (Westley et al. (1998); Pugh (2003)).

https://trace.tennessee.edu/cgi/viewcontent.cgi?article=2364&context=utk_graddiss

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Establish a bag stickering procedure to ensure that non-vacuum-cleaned parts do not get mixed in with vacuum cleaned parts.

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At the 1st International Vacuum Congress (IVC) in 1947 Nottingham suggested that the limit to
the lowest measurable pressure was not caused by the pumps, but by an X-ray effect in the IG: he
proposed that soft X-rays, produced by electrons impinging on the anode, released photoelectrons
from the ion collector; this photocurrent was indistinguishable in the measuring circuit from the
current due to positive ions arriving at the ion collector. This hypothesis was soon confirmed by
Bayard and Alpert [3] who reduced the size of the ion collector from a large cylinder surrounding the
other electrodes to a fine wire on the axis of a grid anode. This elegant solution reduced the lowest
measurable pressure by a factor of 100 and is still the most common design in today’s commercial IG:
the Bayard–Alpert gauge or just BA gauge.

When the X-ray limit was pushed down in this manner, another component to the background
current became evident. Electrons hitting the anode may ionize molecules adsorbed on the surface
with a subsequent release (Fig. 12). Ions generated in this manner cannot easily be distinguished from
those generated in the gas phase. Since a grid structure of a BA gauge has a surface area of about
10 cm2
the amount of adsorbed molecules can be rather high (1016). Therefore it is important that the
grid structure be very clean. Two measures are usually taken to cure this problem: the grid is cleaned
by electron bombardment after the gauge had been exposed to high pressures or the atmosphere, and
the electron current to the anode should not be too small during operation so that the gauge is
continuously ‘self-cleaning’.

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This isn’t the standard fare for this channel, but I’ve just come across a paper that’s really tickled me - A timeshared foreline and roughing vacuum system (Conference) | OSTI.GOV

This paper presents a timeshared rough vacuum system. I don’t know if it would be practical to implement this anywhere around here, and probably folks around here are already aware that this is possible, but I found it a pretty nifty concept.The key is that modern turbopumps can tolerate a much higher foreline pressure than the base pressure that a good scroll pump can achieve (~10 torr >> ~100 mTorr).In this paper, they connect the forelines of 4 sample load-lock chambers and 5 beamline turbopumps to a single scroll pump via two valves. They also add a “ballast tank” made out of a big chunk of PVC pipe. They add a TCG to the ballast tank with the setpoints configured such that, essentially, the scroll pump only turns on when the pressure rises above 1.5 Torr.The results are that they only actually need to run the pump at total of 4 hours per month.

“The large ballast volume, combined with the low leakage rate of the beamlines and sample chambers and the high foreline pressure tolerance, permitted only occasional pumping of the ballast volume, greatly reducing the operating time of the scroll pump”

The advantages are:

  • No vibration 99.4% of the time.
  • The scroll pump essentially never needs the otherwise annual tip seal service.Of course, lots of disadvantages; a few obvious ones might be loss of redundancy and separation of systems, contamination from one region could propagate into another. The foreline pressures are allowed to rise, so I’d guess the base pressures in the turbopumped regions would be higher. Also, the vacuum load / leak rate from all the chambers needs to be sufficiently low for this to even be achievable.

coworkers raised some important operational concerns; how often are coupled chambers vented

if there’s a leak, could be catastrophic across many systems - would want lots of automatic isolation valves in lots of places

Two such systems in use here at work, both on upstream / mono tanks - big foreline tanks. Say scrolls have lasted decades.

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Scroll pumps produce a lot of Teflon tip seal dust. When there is no purge gas and the system is not vented often, this can accumulate in the scroll. This dust can also be found a few tens of centimeters up the foreline. Counterintuitively, there can also be a fair amount of moisture in the centimeters after the foreline.

I’ve seen a filter kit (add mcmaster-carr unit #) used to prevent this dust from