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Project Progress

Co-Investigator Rick Flagan has continued work on the development of the apparatus for measurement of nucleation rates under conditions that simulate the atmosphere of Titan. An impinging jet reactor is being fabricated following analysis of different options for the device. The impinging jet nucleation reactor consists of two stages: (i) a nucleation reactor that is designed to produce homogeneous nucleation under carefully controlled conditions; and (ii) a second stage reactor that will employ a second vapor to grow nuclei to sizes that can be optically detected using an in situ optical particle detection apparatus. In order to minimize the volumes of gas that must be processed at controlled cryogenic temperatures, and to facilitate active temperature control, the sizes of the two reactor stages have been minimized. The challenge has been to design this two-stage reactor so that it will perform over a range of cryogenic temperatures. A clear, easy-to-machine epoxy, stycast 1266, with very low thermal conductivity was selected as the main fabrication material for all parts with stainless steel tubing used as interconnects. The epoxy for the laser particle counter stage had to be made opaque. Through trial and error this was accomplished by the addition of 5% by mass carbon black. The resulting mixture, though opaque, had unknown cryogenic properties. It was tested by using it to make a seal on an open Swagelock stainless steel cap fitting. It was put into liquid nitrogen and allowed to equilibrate with the temperature then plunged into hot water. The thermal shock did not visibly affect the seal or cause a leak measurable by a He leak detector.

The mixing chambers have been fabricated and are in the process of assembly. They each contain several fast, ~10 microsecond response time, type E thermocouples to detect the temperature of the incoming gas phase constituents and the outgoing mixture. Fabrication challenges due to the fine wires of the fast response thermocouples have been resolved by wire wrapping the thermocouple leads to larger diameter wires of the same material; the assembly is then varnished and epoxied into place. A high-speed amplification and compensation circuit was designed and built to be used with these thermocouples.

The laser particle counter used is built into the flow path of this device. The opaque housing has been fabricated. In order to simplify and miniaturize the cryogenic device, the free-space optics of previous versions of the detector have been replaced with fiber optics coupled to the center of a GRIN lens. The brittleness of the bare fiber (with cladding removed) has created assembly challenges that are being addressed using a stabilized stage to facilitate positioning and adding of a liquid for lubrication. Once assembled the particle counter will be characterized against a Faraday cup electrometer sensor that was purchased specifically for this.

Data acquisition for the system is accomplished with the use of a high-speed simultaneous sampling card. A customized liquid nitrogen container, originally designed for medical sample transport, will be used to contain the entire system with a port at the top for input and output connections. This is currently being ordered. The temperature of the apparatus within the cooler will be measured with thermocouples; temperature regulation will be maintained with a PID (proportional-integral-differential) algorithm and resistive heating elements. Controlled gas mixtures for the nucleation experiments will be provided using a calibration gas standard generator.