Design details
GRE won a UKRI contract (for the UK’s Central Laser Facility, but with the equipment ultimately destined for the XFEL laser research centre in Germany) to design, manufacture, test and install two cryogenic cooling / temperature control systems (cryostats) for a state-of-the-art amplification head, for a high-powered research laser application.
The vacuum insulated, cryostat-based system used liquid nitrogen to lower the temperature of a gaseous helium cooling media, which had to be circulated around a closed loop. The requirements were extremely stringent, and the system had to:
- Ramp down the temperature of the gaseous helium coolant, from ambient to 90K at a stable and steady rate (5K per minute for this application)
- Hold the final temperature to within ± 0.5K of setpoint under varying Thermal load and mass flows (selected and adjusted by the user)
- Be ‘oxygen-clean’, and entirely free of hydrocarbons (oil, grease and even fingerprints!)
- Be able to operate entirely independent of the laser system, with its own dedicated instrumentation and controls system, but also to communicate with an upper-level control system to relay operational parameters and alarms.
Solution
GRE design engineers developed a concept, ran analysis and then designed a state-of-the-art vacuum-insulated cryogenic cooling system. The system used a custom-designed cryogenic fan, to circulate gaseous helium through the system and over the laser lens.
Technical Details – Design
Gaseous helium was circulated through the closed loop, and the volumetric flow rate measured then converted into mass flow. The flow could then be controlled from 0 to 250 g/s to optimise the flow profile over the lens itself. The gaseous helium was also circulated through a heat exchanger, which was cooled by a secondary-circuit of liquid nitrogen. The temperature was controlled by varying the helium flow through the heat exchanger and was better than the ± 0.5K specified (approximately ± 0.1K). Temperature, pressure, and flow instrumentation constantly fed data back to the control system, which enabled the user to monitor and “tweak” the performance of the cooling system in real time. This was made possible via the custom-programmed colour, touch-screen, graphical HMI and a two-way communication link to the clients ‘EPICS’ global control software. All electrical and software design was also undertaken, in-house, by GRE.
Technical Details – Manufacture
Constructed almost entirely in-house, in GRE’s controlled environment assembly area, our skilled welders fabricated and assembled the various sub-assemblies (using machined components made by our own precision engineering workshop, Ashbrook Engineering, and with cryogenic control valves, relief devices and instrumentation supplied by our sister company Gas and Liquid Controls).
Before final assembly all parts and components were cleaned to a standard agreed with our client, thermally cycled, then tested for cleanliness and vacuum tightness, using Residual Gas Analysis and Helium Leak Testing, among other methods. Once cleanliness and vacuum tightness was assured, the internal components of the cryostat were extensively insulated with MLI (multi-layer super insulation) and low out-gassing fasteners, assembled, and then conditioned in our automated vacuum bakeout chamber.
Once fully assembled, the cryostats along with their GRE-made SIVL (vacuum-jacketed) helium transfer lines and dummy amplifier head, were then tested in the presence of the client at GRE’s factory, then were securely packed, delivered to site and installed/commissioned by our team.
Further developments
From GRE’s first laser amplifier head cooling system built back in 2015, we have now completed and deployed 10 other system, all with slightly different attributes – most having transitioned to gas foil (oil-free) bearings, some with a different temperature control philosophy and some having multiple, redundant, circulation fans.
If your organisation has a requirement, no matter how demanding/unique the application, for a similar cooling system, please get in touch, as we would love to discuss it with you.