Case history – Bespoke cryogenic cooling

Background

Our client is a prestigious multi-disciplinary science organisation at the forefront of UK research within the physical and life sciences.

Laser research forms a key element of our client’s activities using state-of-the-art technology that enables the study of states of matter, high definition imaging through biochemical and biophysical processes. The diversity of laser sources offers a vast range of potential applications across a wide range of industries including materials processing, medical and pharmaceutical, as well as fundamental science studies.

Challenge

Our client is developing high energy high repetition diode pumped laser systems. Operating the amplifiers for these systems at high repetition rates generates heat in the enclosed laser gain media. The temperature must be tightly controlled by extracting this heat to ensure that the laser operates at optimum efficiency. There was no pre-existing model for a cooling solution of this nature, so a new approach would be required.

The research team required a precision-engineered system to cool the laser amplifier using a 20 bar pressurised helium medium in a turbulent flow regime, with highly accurate temperature and flow control providing precise cooling management. With target cooling temperatures as low as 120K (-153°C) and a tolerance of ±0.5K or better, the system needed to perform beyond the range of conventional techniques.

The solution was also required to:

• Achieve close control of the mass flow rate of helium gas at between 5 and 35 grams/second
• Achieve a controlled ramped cool down rate to protect the optical equipment
• Be scrupulously clean to avoid contamination which could lead to damage to optics in the gas stream
• Achieve high level of leak-tightness
• Operate ‘out of the box’ as a standalone system

Solution

GRE worked closely with our client to develop and refine their technical requirements, used as the basis to create two systems delivering between 1 – 6kW of cooling to achieve the target temperature. GRE also visited the end user location in Germany to ensure that the solution precisely matched both physical and technical requirements of the project.

A key element of the design and manufacturing process was flexibility in identifying modifications required as the project progressed. As there was no precedent for this type of system, it was essential to our client that we were able to provide continuous feedback and respond to changing requirements. For instance, it became apparent that one of the systems would be required to straddle the plant room and lab, for which a single HMI (human machine interface) screen would be insufficient. GRE was able to modify the controls system design and deliver two colour touch screen HMIs: one would be in control at any given time, mirrored by the other with read-only access.

Our system design included a full controls philosophy written and implemented by GRE developers. The cryogenic solution operates as a standalone system, achieving and managing the required helium mass flow rate with a maximum tolerance of ±0.1 gram/second. The controls programme also manages the strict cooling ramp rate of 5°C/minute.

The cleanliness requirements of the system ruled out the use of grease during manufacture, requiring our engineers to develop a new flange face sealing technique without the use of lubricant. Our team applied their creative expertise to introduce an alternative method which achieved extremely high leak-tightness at a rate of 10^-9mbar litres/second, equivalent to less than one part per million per year at atmospheric pressure or – alternatively – more than 1,000 years to lose 1% of the helium inventory.

To achieve the hydrocarbon cleanliness requirements of the system, GRE instigated the creation of a cleaning regime which was approved by the client and carried out prior to build completion. Following cleaning, the system was tested for remaining hydrocarbon content using residual gas analysis techniques. This process is able to identify, at the atomic level, the presence of undesirable compounds or elements which could damage the contained optical equipment. The system passed with flying colours.

Following build completion, the customer attended factory acceptance testing carried out at our facility over two days. This included: testing the ability of both systems to respond to temperature and flow requirements, control the transition to these, and stabilise; verifying the control accuracy to +/-0.1K of setpoint within five minutes; testing the stability of the gradual cooling rate and testing the numerous alarm setpoints within the control system. Following client sign-off, we carried out further successful site acceptance testing at the client’s facility.

Client Comment

“GRE’s tender stood out for its attention to the technical detail of our specification, their proactive approach to our strict requirements, and the team’s track record of getting under the skin of their clients. We knew that Richard and his colleagues fully understood what we needed to achieve and that they have the skills, expertise and – importantly – the commitment to make it happen. The service from the team has been outstanding throughout and GRE has gone above and beyond for us. GRE’s senior project management team even stopped in one Sunday afternoon on their way to the airport to lend their support so there would be no delay to our progress. I have no hesitation in recommending GRE.”