As the information from the bolometers is a key part of the control and safety of ITER, they must continue to function throughout the twenty years of expected operation, whilst sitting in a uniquely hostile environment, bathed in ultra-fast subatomic particles and mind-bogglingly high temperatures. This is difficult to guarantee - there is no environment that can adequately reproduce the conditions in ITER, and so we must test individual aspects of our diagnostic and build in redundancy to guard against failure of some of the bolometers.

For example, one aspect of the ITER environment is the neutrons. These subatomic particles have no electrical charge, so they can escape the strong magnetic fields which confine the rest of the plasma. Travelling at a reasonable fraction of the speed of light, they act like a bowling ball, colliding with the atoms in solid materials and rearranging them, leaving voids and weakening the impacted material. Even when hidden behind the blanket modules, our bolometers will be constantly bombarded by neutrons, which will cause them to weaken and fatigue. Even worse, neutrons can transmute elements - one traditional material used in bolometers is thin gold resistors, which can be turned into mercury by the neutrons. As mercury is a liquid, this is bad news - our bolometer could literally melt and drip away!

We can attempt to mimic the neutron damage expected in ITER using a nuclear fission reactor. We can place a sample inside a fission reactor and expose it to the neutrons there, and then study the damage. Although the conditions are not the same as in ITER, we can try to scale up the damage to understand how likely a given material is to survive. One possible solution is to use platinum instead of gold, which should be more resilient to the neutrons.

As well as these nuclear fission tests, we subject our bolometers to high temperatures under vacuum, compress and stress them to mimic the forces expected during ITER operation, blast them with electromagnetic noise to see how the accuracy of the signal is affected, and spray them with jets of steam to mimic the rupturing of water coolant pipes. These tests feed into our constantly evolving design.