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From a report submitted by Dr. Luke Alsebrook, The University of Kent. The primary objective of this project was to produce a new target holder, the Ice Target Holder (ITH), for use with the university of Kent’s two stage light gas gun. Prior to this, all experiments impacts into ice targets were performed with the ice exposed to the environmental conditions of the main target chamber (room temperature, ~0.5mBar pressure). Once a target is set up and the gun evacuation is initiated, there is an ~40 minute wait before the shot can take place, during which the target will begin to warm up and melt/sublime. As a consequence, we have historically been limited in the size, mass, shot conditions and type of ices which could reasonably be investigated. An ideal setup would allow for the ice to remain unchanged whilst the target chamber is evacuated. This required an active cooling system and thin film mount. In addition recent collaborators have expressed an interest in looking at ices at low (<50°C) temperatures which could only be achieved via direct cooling with LN2. As a result several key aims were identified at the outset: 1) Build a mount that enables ices at a range of temperatures to be actively cooled whist in the chamber environment, 2) Provide a method for sealing the ice against the low vacuum pressures and 3) Provide a method for achieving ultra-low temperature cooling to allow clathrate ices to be shot. To this end, the ITH was designed and constructed. The ice target holder consists of a stainless steel (STST) body, with a two part STST breech in which cooling channels are milled. The heavy STST body provides good thermal inertia which allows for significant pre-cooling via the Impact Facilities controlled temperature freezer which has a minimum temperature of -150˚C. This greatly reduces the active cooling time. The two part breech is design to be deconstructed to allow for the cooling channels to be cleaned and dried when swapping between coolant. The breech is sealed by a 2 mm thick PTFE gasket which is clamped between the segments. Initial testing of the target holder (with pre-cooling in the ultralow temperature freezer, but without active cooling by LN2) has shown that it is capable of achieving the required temperature for standard ice shots in normal atmospheric conditions (1 bar and up to 32 degrees external temperature), and exceeds our requirements under the standard target chamber conditions for a shot. Testing with the LN2 supply has, however, been delayed due to reasons outside of the laboratory control but should take place within the next few months. The better than expected results with only pre-cooling has enabled a complete re-write of the standard operating procedures for standard water ice shots and enabled the facility to offer a significantly improved service to external users. The issues facing the LN2 connection are likely to be resolved allowing for testing to be carried out over the next few months. The design and build are currently being written up and when these final tests are completed we will look to submit a technical paper for publication. and will result in a number of interdisciplinary projects being undertaken over the next two years. A number of researchers have already expressed interest in using the ITH, each with the potential to result in further publications over the next few years.
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