PRESSURE, HEAT AND CORROSION RESISTANCE
We intend to use a skimmer in a high pressure
environment. Do you have any recommendations or warnings?
Our standard Model 1 or Model 2 skimmers can withstand a 1
atm pressure differential without difficulty. Specifying double thickness will
increase the ability to withstand a higher pressure. Other models, particularly
those with large surface area, do not have the same capacity for pressure
differential.
In addition, models with a sharp corner where the brim
meets the base of the cone (such as a Model 50.0) have a much reduced capacity
for pressure differential. The sharp corner is mechanically weaker and also
results in a thinner plating at the corner. We recommend the addition of a
radius at the corner for any new custom skimmer or caution that extra care must
be taken with the skimmer.
We plan to fill one vacuum chamber through the skimmer
as we bring the other chamber up to air. Would this pressure differential be a
problem?
Yes, it may be. Generally, the chambers on each side of the
skimmer should be vented to air simultaneously through separate valves. Any
other practice may void the warranty on our skimmers. Except for very small
chamber volumes or very large skimmer orifices, venting a chamber through the
skimmer orifice will put a lot of stress on the skimmer mount, and may distort
or otherwise damage the skimmer. See comments above regarding the larger size
skimmers.
The addition of a spring loaded relief valve between the
two vacuum chambers might help protect the skimmer when opening the apparatus to
air so all of the pressure equalization is not done through the skimmer orifice.
A simpler solution may be to arrange the existing “up to air” valve so as to
take a longer time in filling.
What is the heat tolerance of a Beam skimmer?
We use either nickel or copper for our skimmers, which have
melting temperatures of 1453°
C and 1083°
C, respectively. The actual skimmer temperature will depend on the ratio of heat
transferred to the skimmer and the thermal conductivity from the skimmer to the
wall on which it is mounted. This will require thermodynamics modeling by the
user.
Beam can produce double thick skimmers for greater strength
at high temperatures, and can add plating for extra resistance.
Can Beam produce a skimmer from a refractory material?
Beam sourced a supplier who would form tungsten cones for
us, but at a very high cost. In addition, although our machinist has experience
working with tungsten, there is no certainty that sharp enough orifice edges
could be produced. Thus we would undertake the production of a tungsten skimmer
only if the process were fully funded by the customer, despite the results.
We have not and likely will not explore ceramics,
molybdenum or any other refractory materials.
I noticed you can produce double thick skimmers for
additional resistance to heat or corrosive gases. Does this affect the
collimating?
The performance of the skimmer won’t be significantly
degraded by making it twice as thick, as the wall is still quite thin.
Why is chromium plating only offered on a time and
materials basis?
Chromium plating sometimes discolors the skimmer and
occasionally cracks the existing skimmer. Beam is not interested in absorbing
the loss of good skimmers if damaged or cosmetically discolored.
We are using (oxygen/fluorines/chlorines/etc.) in a
molecular beam experiment. Will this present a problem and do you have any
suggestions for a solution.
We offer two suggestions. First, Beam can produce double
thick skimmers and add additional plating for resistance against corrosive
gases.
Second, the user should test the planned gas with a piece
of nickel or copper sheet to see whether corrosion will be a problem under the
experimental conditions. Corrosion
resistance is very specific chemically, and there is no combination of skimmer
and coating material which will be resistant to corrosion under all conditions.