Reflective research – F4E and partners develop mirrors for ITER diagnostics

Mirror, mirror on the wall, who is the cleanest of them all?This is the question the F4E Diagnostics Project Team, experts at Basel University in Switzerland and ITER IO, have been working together to find out. F4E is to provide several optical Diagnostics, to be located in the ITER port plugs, in order to monitor the plasma necessary for the fusion reactions in ITER to occur. These Diagnostic systems use mirrors in various different shapes and sizes, but typically measuring up to 250 x 200 mm, to reflect light from the plasma towards detectors and cameras. When the data are analysed, they provide key information to help monitor and control the plasma. While the number of mirrors can vary depending on the Diagnostic system, all in all, the ITER optical Diagnostic systems will contain more than 100 mirrors.

The conditions that the mirrors will be subjected to on ITER are not very well known. The inner walls of the ITER machine core, where the fusion reaction will take place, are made up of tungsten and beryllium. These metals are designed to withstand the high temperatures in ITER, but are likely to create deposits on the mirrors when ITER will be operating. These deposits would cause the mirrors to lose their reflectivity, reducing the light that they bring from the plasma to the detectors and thus the decreasing the performance of the Diagnostic.

In order to learn more about the problem and develop ways of mitigating the loss of reflectivity, the University of Basel is carrying out testing on how to clean the mirrors and to determine the best mirror surface to use for the ITER environment. For these tests, small mirror samples, with deposits of tungsten and aluminium, are placed in a small vacuum chamber into which a low pressure gas is added. A radio frequency power source of around 100W connected to the mirror is switched on, creating a changing electromagnetic field in front of the mirrors. This field creates a plasma, in which some of the atoms of the gas are turned into fast-moving negatively charged electrons and positively charged ions. These energetic particles strike the mirror surface and, in doing so, ‘kick-off’ some of the atoms from the tungsten and aluminium deposits on the mirror surface. This process is known as sputtering and is the basis of the ‘radio-frequency (RF) cleaning’ scheme that F4E and ITER IO hope will give the mirrors a long life.

Research is also being carried out to find out what materials make the best mirrors to use in conjunction with an RF cleaning system. The mirrors under investigation consist of stainless steel coated with the highly reflective metal rhodium, aluminium-coated stainless steel with a protective zirconium dioxide layer (a hard material, similar to diamond) as well as the hard metal molybdenum. These three these different types of mirrors are being tested to see which material will perform best after many cycles of RF cleaning. There will always be some small damage to the mirror when it is cleaned, but the amount of damage can vary depending on the material of the mirror. For example, a mirror made up of aluminium has a high reflectivity but gets damaged easily in comparison to other materials (hence the need for a protective coating).

The gas which is added to the vacuum chamber is important for determining the efficiency of the RF cleaning. For example argon, helium and neon all have different cleaning properties – which also need to be tested and optimised. For example, it is easier for the energetic ions produced from heavier gases such as Argon to clean heavy deposits (i.e. with a high atomic mass such as tungsten) but these cause more damage to the mirror surface; and Helium, which is a very light gas, is able to clean deposits of beryllium, which are also light, and damages the mirror surface less but is not as efficient in cleaning heavier tungsten deposits.

Each mirror sample will be tested many times in order to see how long in the real operational conditions on ITER, where cleaning may be required frequently to keep the mirrors shiny and bright. Prior to testing, deposits are made on only one half of each mirror – this is done to see how the RF cleaning affects a clean surface compared to a dirty one. As the process removes the top layer no matter what the material is, it has the potential to damage the pristine mirror surfaces. By doing this test it is possible to measure this effect and at the same time see how uneven deposits will affect the cleaning.

“It is the first-time that such varied (multi-cycling) tests are being carried out. In addition it is the first time special equipment will be used enabling six mirror samples to be tested at once – meaning that all six samples are subjected to the same conditions”, explains Ulrich Walach, F4E’s Technical Officer working with Basel University on this contract.

This first stage testing will continue until the beginning of next year, after which testing will continue on bigger sized mirrors.