Beam DynamicsMolecular Beam Skimmers

 

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.

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