2 4.2 Cryogenic Design
Requirements Array temperature: Needs to be in the 100mK regime -Engineering margin on achieving background limited NEPs for large numbers of pixels - Power handling requirements Optics/filters: Need to be a cold as is practical - Must minimize extraneous background power on arrays Ease/cost of operation: Extensive use at observatory site - Instrument could be used up to 20hrs a day -Preferably cost efficient (not using vast amounts of LHe!) - Easy to maintain... Temperature stability: a major concern?
0.06 Total power Pmax 0.04 Resistance (Ohms) Bias power Bias power 0.02 Sky power Sky power 0 95.8 96 96.2 Temperature (mK) Good sky Poor sky Power Handling • The sharp n-s transition means that there is a ~ fixed power level that a TES can handle • This is the power required to warm the device from the base to transition temperature • It is the sum of the bias and sky power • In a practical device it is important to have some headroom in the power handling to allow for a miss-estimation of background levels • To achieve more power headroom the base temperature has to be decreased
Pixel Heater Total power Pmax Bias power Bias power Heater power Heater power Sky power Sky power Good sky Poor sky • Keep • average bias power constant • responsivity constant • bias power low
Baseline design Dilution refrigerator: > aiming for cryo-free system using pulse tube cooler > will do design study with Leiden Cryogenics > 100 to 200W cooling power at 120mK Cryostat: > use a second pulse tube cooler for the optics, filters and radiation shields > LN can for pre-cooling to ensure rapid cooldown Array holder and 1K mount for SQUID arrays are key to design
The Challenges: >Chip holder to allow electrical and thermal connection 60mK temperature with mechanical link to silicon wafer > Mounting arrays fixed with respect to optics with high stiffness > Mounting 20 SQUID array modules at 1K within a few inches of the arrays > Controlling stray lightribbon cables and mechanical structures and desired photons penetrate cold shield > Magnetic shielding > Mechanism for cold shutter possibly using a simple solenoid - cheap and reliable - but concern over magnetic pulse from activating solenoid?