Statement_of_Work.docx

SOW - Statement of Work - AQV Cryogenic System for a Sapphire Optical Cavity

BACKGROUND:

The Ion Storage Group is developing aluminum ion optical clocks, which require extremely stable laser local oscillators. The reference for these ultrastable lasers will be a Fabry-Perot optical cavity with high mechanical stability made of sapphire and cooled to cryogenic temperatures.

PURPOSE:

The Ion Storage Group at NIST requires a closed-cycle cryogenic cooler system capable of cooling a Fabry-Perot optical cavity and the associated vacuum chamber and heat shields to temperatures approaching 4 K. The system should be designed to operate continuously and achieve high temperature stability at temperatures between 4 K and 40 K, while maintaining minimum levels of vibration. Of particular importance is the isolation of the cavity mounting structure from the vibrations of the cold head. The design of this vibration isolation system should be based on complete mechanical decoupling via a helium-gas heat exchange chamber.

DELIVERABLES:

(1) Custom closed-cycle cryostat system including:

a. Pulse-tube cryocooler with associated compressor, electronics, gas lines (>20 ft) hardware for running the cooler.

b. Vibration-isolated thermal design that mechanically separates the cold sample volume from the vibrating cooler via a helium-gas heat exchange system, while maintaining sufficient cooling power to reach the operating temperature.

c. Two radiation shields: one at the operating temperature (isothermal shield) and one at an intermediate temperature between the operating temperature and room temperature.

d. Vacuum system enclosing all components: cryogenic cooler, thermal connections, heat shields and inner vacuum system.

e. Temperature control system: 3 temperature sensors (including 1 Cernox sensor), cartridge heater for maintaining a temperature between 4 K and 80 K in the sample region, temperature controller.

(2) Thermal and mechanical analysis/test data including measured cool-down curves, leak testing and mechanical analysis relevant for vibration performance to be decided during contract

TECHNICAL SPECIFICATIONS (see attached drawing):

1. Sample volume (Approximately cylindrical: 11 in OD x 13 in height) The “sample” in this case is an interior vacuum chamber housing the optical cavity, which is pumped out and sealed via a pinch-off tube at room temperature then placed in the cryostat. The dimensions of the cylindrical inner vacuum chamber are included in the attached Vacuum Assembly sketch.

2. Mounting to baseplate The sample will be mounted via three steel posts to a temperature-controlled baseplate as depicted in the attached Vacuum Assembly sketch. It is desirable to not move the inner vacuum chamber after it is assembled, so heat shields and vacuum enclosures that can be constructed around the sample are preferred. Since we want to run this system for long periods in a steady state condition, a longer setup time is acceptable.

3. Optical access (Vertical access at both the top and bottom of the vacuum chamber) We require optical access for a laser beam at 1069 nm through windows at both the top and bottom of the inner vacuum chamber. Optical access into the cryostat should accomplished through windows at the top and bottom with care taken to avoid back reflections (use wedged, tilted windows), birefringence (limit mechanical stress in mounting), and transmission of energy due to room temperature black body radiation.

4. Cool down time ( < 1 week) Due to the large cold mass and stringent temperature stability requirements, we expect to have a relatively long cool down time for our sample. We would like to keep this time to less than 1 week, which will be limited by the thermal time constants designed into the inner vacuum chamber.

5. Temperature stability (T < 10 mK at 1 s, T < 1 mK at timescales greater than 10 s) We plan on actively stabilizing the temperature of the baseplate via a temperature sensor mounted on the baseplate and a heater. Based on modelling the thermal time constants of our vacuum chamber we estimate that our application requires temperature stability at the baseplate better than 1 mK over time scales longer than 10 seconds.

6. Vibration Isolation (< 10 nm RMS, acceleration PSD < 1 x 10-9 (m/s2)2/Hz – white noise model) The optical cavity length stability is affected by vibrations in the mounting structure, which deforms the cavity via accelerations. Therefore, we are interested in minimizing the accelerations induced by the cryocooler and building vibrations etc. Based on modelling of the mechanical deformation of the cavity we have estimated our vibration sensitivity to require a vibrational spectrum at the level of 1 x 10-9 (m/s2)2/Hz assuming a white noise spectrum.. This acceleration specification will be met on a best-effort basis, while the displacement specification of < 10 nm rms being a fixed specification. If possible, NIST will work with the vendor to help measure and minimize the vibrations.

TECHNICAL CONSIDERATIONS:

Due to the stringent reliability requirements of the project the supplier must have successfully designed and built closed-cycle cryogenic systems with temperature stability at the 1 mK level. The supplier must also have experience with mechanical design of these systems for ultralow vibration levels.

GOVERNMENT FURNISHED PROPERTY OR INFORMATION: Price should reflect the cost of the deliverables described above. Specifically excluded from this list are:

1. Vibration isolation legs for the optical cavity table

2. Vacuum pumps for preparing and maintaining vacuum pressure in the outer vacuum chamber

DELIVERABLE SCHEDULE:

We must have delivery by December 10, 2015

INSPECTION AND ACCEPTANCE: The system delivered will be inspected for accordance with all mechanical and geometrical specifications described above. The temperature stability will be measured under operation at NIST to ensure that the bare system (without the sample mounted inside) achieves the levels specified. The vibration levels will be measured interferometrically on three axes to ensure that it meets our minimum requirements.

Attachments: Inner Vacuum Assembly Sketch.pdf

TYPE OF CONTRACT: Firm fixed price