Exclusive Interview with Bart Verlaat about CO₂ cooling for particle detectors

 

Bart Verlaat: At CERN and in the high-energy physics world we use CO₂ cooling for particle physics detectors. We used CO₂ because in detectors it is highly important to have very low mass structures, whilst also needing low temperatures. CO₂ is used in small channels, which is perfect for our applications. 

 

We have built two systems now, one for the alpha magnetic spectrometer and one for the LHCb detector at CERN. The alpha magnetic spectrometer is on the International Space Station. Initially we developed CO₂ cooling for this latter application, so we had to use technology from satellite cooling, which we combined with standard technology from the refrigeration sector, eventually developing a new cooling concept. This worked rather well, and has proved very stable. We then applied the same principle for the detector at CERN, which also works really well. For all future detectors we will follow the work that we did for the AMS and LHCb. 

 

Bart Verlaat: The cooling systems we need are quite different to what you might find in commercial applications because we are not interested in the highest COP. Of course, we want to have an efficient system but that is not the driving factor. Our goal is to have stable temperatures. This means that in a two-phase system we need to have very good control over the pressure and temperature. Moreover, the cooling is applied inside the detectors, which are really big, making access difficult. The controller equipment must therefore be far way from the evaporators. As a result we have developed a special pumped and pressure control system.

 

 

Bart Verlaat: The key benefits are very small channels. Since we have a spread of evaporators over a large volume all needing to operate at the same temperature, pressure and temperature uniformity must be very high. CO₂ is ideal for these requirements because it has a high pressure. So, pressure drop is not really an issue, which in turn does not translate into temperature gradients. Thus we can have small lines and very homogeneous temperatures in our detector. Cooling channels with CO₂ can be made much smaller than fluorocarbon channels, gaining at least a factor of 5 in applied cooling systems mass.

 

 

Bart Verlaat: We started developing these systems 12 years ago when we discovered that CO₂ is a very good coolant for our applications. As we were not connected to the refrigeration world we did not know that the commercial sector was working with CO₂. At that time it was very hard for us to get components – they did not exist, so most of the components for the LHC detector we made ourselves. For the AMS detector the components were specially developed anyway because it was for aerospace.

 

Now we are developing new systems for future detectors we still we have problems getting standard equipment. It exists on the market but most of it is only available for large suppliers. It would be good if we could also have access to these products.

 

Bart Verlaat: It has benefited us to a certain extent because there are more products than before but lots of the products we would like to use are difficult to get, and require special agreements with suppliers. This is a pity because I think that we there could be an exchange of ideas and technology. The commercial refrigeration sector could learn from the things that we do differently.

 

Bart Verlaat: For the system in space there is no way to do anything more to it once it is up there. It has been running very nicely since May and has to run for the next 15 years. That is a long time so we have redundancies in the system. The only active component is the pump, which is the only component where there could be a problem. Each system therefore has two pumps, once for back up. For the critical part of the pump we also have three redundant systems.

 

For the other systems the challenges for the future are the scaling. The system we developed for the AMS is very small. It is only 200 Watts. For the LHCb we scaled this system up to 2 kW. What we are now doing in the short-term for the CMS detector is getting it up to 10 kW. In the future we have to scale up to 100 kW. Scaling is one of our main challenges.

 

 

Bart Verlaat: We work within a radiation environment therefore the cooling fluid for detectors must be radiation hard and CO₂ is a very good candidate for such applications. Also, at CERN we always have safety issues that have to be dealt with, since everything is under the ground. We could use ammonia, but it is not advisable in a confined space underground.

 

 

Bart Verlaat: At the moment we are working on two small-scale CO₂ cooling systems to support the particle physics laboratories in their research and development of future particle detectors. Specifically, we are working on the upgrade programs of the inner detectors of ATLAS and CMS, which have already been damaged by the radiation, so we have to replace them. For the ATLAS detector we are making a system for the Insertable B-Layer (IBL), which is in the order of the 2 kW, which is similar to the LHCb system, the only challenge is that we have to achieve temperatures of -40°C instead of -30°C. For the CMS we are working on the pixel upgrade, which will be installed in 3 years time from now, which is a out 9 kW. 

 

In 10 years from now the whole inner detector will need to be replaced by a new one, and that will require a new 100-200 kW cooling system.