It's Skylon SSTO project. They were in the need of heat exchangers to rapidly cool incoming hot air from 1000C to -140C for their SABRE engine to work. So they were able to develop required technology. I quote:
The precooler is also the most aggressive and difficult part of the whole SABRE design. The mass of this heat exchanger is an order of magnitude better than has been achieved previously; however, experimental work has proved that this can be achieved. The experimental heat exchanger has achieved heat exchange of almost 1 GW/m³, believed to be a world record. Small sections of a real precooler now exist.
So, why not using their solutions for Polywell?These heat exchangers have a mass of 1250 kg and are designed to transfer about 400 MW of heat at Mach 5.
http://www.reactionengines.co.uk/ - company site.
They also answer emails, as I already wrote them asking some questions about their project.