BY| Allyson Whitaker
One of the most important aspects of the data collection in CATE is the ability to accurately capture and track the movement of the particles. Along with selecting a proper camera, selection of the proper material for the chamber (in our case the tube) is also important. In the current design, we have elected to capture the movements from outside the evacuated tubes. This requires the use of transparent materials, which eliminated metals as the primary material. The remaining options were either plastics or glass.
When deciding upon which material to use inside the chamber, the following chemical and mechanical properties were used to eliminate potential material candidates: optical distortion, outgassing rates, and reactivity with the aggregates inside the tube. Oftentimes when looking through a transparent material that has also been manufactured into something other than a flat sheet, the object seen through the material appears slightly distorted due to the distortion in the field of view. Some common examples of this include radial distortions such as “pin-cushion” or “barrel” distortions, as illustrated below.
No distortion view Pin-cushion distortion Barrel distortion
Since we are capturing the movements from outside the evacuated tubes, these distortions can cause a potential problem with accurate tracking of the particles. This would require additional computer software to correct. In both glass and plastic, comparable radial distortions occur. Therefore, other criteria were required to help determine which material would best suit our needs.
Another factor we considered was how the material of the tube reacts with the dust to be used in the experiment. In the experiment, CATE will launch an accretion mass through a cloud of aggregate and measure the amount of aggregate collected on the accretion mass. Additionally, CATE will measure how the particles move in relation to the accretion mass. In order to have the most amount of aggregate available for the experiment, it is important that the cloud of aggregate not cling to the tube walls. The static charges in a vacuum for polyethylene, polycarbonates, acrylics, and fluoropolymers for plastics, and borosilicate for glass, were compared. Through this comparison, it was determined that polycarbonates and polyethylene plastics had a much greater tendency to attract particles to them, which is unfavorable for CATE’s results. For this reason, these materials were given less priority for use in the CATE design, but would still be under consideration should they have greater strengths in other categories.
By giving priority to fluoropolymers, acrylics and borosilicate glass as the stronger candidates for use in the experiment design, their respective outgassing properties could now be examined. Because CATE occurs in a vacuum, it is essential that the material being used to hold the vacuum does not leak (outgas), or allow the chamber to return to ambient atmospheric pressure. By returning the inside of the chamber to ambient atmospheric conditions, the parameters of the experiment would no longer be accurate and change the reactions occurring inside the chamber, thus rendering the data unusable. Because acrylics have a high outgassing rate and borosilicate glass and fluoropolymers have low outgassing rates, acrylics were stricken from the list of possible tube materials. The final comparison came down to borosilicate glass and fluoropolymers.
Although fluoropolymers and borosilicate glass are similar in each category, the decision was made to go with borosilicate glass as the material of choice to manufacture the tubes for CATE at this time. Borosilicate glass suffers less from the effects static electricity, has a very low outgassing rate, and can be machined to join with metals, eliminating the need to seek custom parts to hold other sections of the vacuum. With the ease to fit to regularly available metals already manufactured for vacuum chambers, there is room to expand design ideas and explore other options should there be an adjustment in design. Despite all these benefits, however, the major disadvantage to borosilicate glass is the cost to machine it into the chamber shape and design needed for CATE. Borosilicate glass is expensive to produce at the parameters needed for CATE, but it was decided that the benefits of using borosilicate glass outweighed the cost of having it made.