3.6 Internal Wiring and Housekeeping
Semi-flex SMP cables
Low-loss coax cables with SMP connectors are installed at the 20K interface. The dipole assembly/LNAs plug directly into these cables. The RF signal received by the dipole elements is carried out through them to the multi-channel flexible stripline. The semi-flex cables provide a good combination of low loss and shape retention once routed as compared to flexible cables.
Flexible RF Stripline
The flexible striplines in ALPACA carry the RF signal and DC bias current between the room temperature electronics and the LNAs at the 20K stage inside the cryostat. These striplines have been rigorously tested and deployed in the instrumentation for NASA's GUSTO mission. The addition of a bias-tee at the cryogenic LNAs allow us to remove all wiring associated with LNA bias supply, sensing and monitoring from the cryostat.
Figure 2: Top: Showing a 8-channel RF stripline manufactured for the GUSTO mission. These RF striplines have very low loss at low temperature from 1-2 GHz and carry lower heat load to the 20K stage as compared to stainless steel coax cables. The multi channel functionality significantly reduces the complexity and hassle of routing a large number of wires through the cryostat associated with each receiver element. Center: Shows a render of the 4x1 ganged SMP connector we have designed for use in the ALPACA striplines replacing the individual SMP connectors seen above. The ganged connector would be installed using automated pick-n-place machines and the unibody design improves handling with respect to installing the stripline and reduces mechanical stress on the SMP interface while connecting/disconnecting individual coaxial cables. Bottom: Shows a schematic of the RF signal cabling through the ALPACA cryostat.
We expect to use six temperature sensors in each section ("trient") of the cryostat (total three sections)
- One temperature sensor each on the copper thermal links that are directly connected to the two stages of the cold head.
- One temperature sensor each on the base plates of the low temperature stage (20K) and guard stage (90K).
- Two temperature sensors, nominally on a dipole element and on one of the radiation shields.
We will utilize Lakeshore diode sensors to cover the entire expected operating temperature range from room temperature to 20K. These diode sensors are reliable up to 4K. We will use the data from these sensors to verify and revise the current thermal model in lab and continually monitor them to assess the operating health of the cryogenic system. These sensors would also allow us to verify the thermal model by measuring temperatures of different components during lab tests.
We will use one dual-stage pressure gauge that is capable of monitoring through 1000 mbar to 10-7 mbar. This sensor would continually monitor the overall health of the vacuum vessel.
The ALPACA cryostat has a large volume and as such would take a long time to warm-up. We will install a high-power resistor bank inside the cryostat to provide ~100W of additional heating to help increase the pace of warm-up. The banks will be equipped with integrated thermal cutouts to prevent the possibility of over heating the system. This functionality would be remote controlled at the super-user or maintenance level and be useful both in the lab and at the telescope.