For nearly 30 years atomic physicists have supported fusion scientists to interpret the spectral emissions of a plasma. The recent move to tungsten walls makes this collaboration even more important.

picture of ADAS spectrum

Tungsten spectrum recorded on JET with the KT3b diagnostic run by Andy Meigs, CCFE

When JET started up in 1983, theoretical atomic physicist Hugh Summers, Professor at the University of Strathclyde, Scotland, was asked to join the project for a period of two years. The JET diagnostic team needed atomic models that helped them to interpret the spectral emissions of the plasma. Nearly 30 years later, and, in his own words, “overdue for retirement”, Hugh still shares his time between Strathclyde and JET, working on the ADAS project which originates from those early days.

ADAS stands for Atomic Data and Analysis Structure. The project provides atomic data and computer codes to model the radiation properties of plasma ions. ADAS grew from being a JET project to a global service financed by its participants. These include many European Associates, all ITER partners, the ITER project itself, as well as several astrophysics groups which use ADAS for their spectroscopic measurements. Two years ago, the EU recognised the importance of ADAS for ITER and provided four years of extra funding. ADAS took the opportunity to install experts at some participant’s sites and now finances three post-doctoral fellowships at IPP Garching, CEA Cadarache and JET.

One use of ADAS is, to monitor plasma impurities such as atoms dislodged from the wall. When these impurities enter the hot plasma, they are excited and emit light. ADAS connects the observed spectral lines to the number of incoming atoms. The recently installed tungsten wall tiles are a challenge for ADAS and the spectroscopists: Tungsten has 74 electrons and takes on many different states. The electrons transit between energetically close states and emit a large variety of very weak signals. The extra EU funding boosted the tungsten modelling capabilities of ADAS – at the end of January, the team accomplished an important milestone, the first theoretical estimate for the relation between the observed plasma lines and the number of tungsten atoms coming in from the wall.

An atom emits light with a specific wavelength if one of its electrons moves to a less excited state by filling a vacant orbit closer to the nucleus. As there are only a finite number of orbits in any element, the pattern of colours, or spectrum, that it emits is unique. Thus the spectral emission of a hot gas tells you which chemical elements it contains and their concentration. In a fusion plasma, emission processes are triggered by collisions between plasma particles, by exciting the particles with microwaves, or when the hydrogen atoms from the neutral heating beam pass their electron to a plasma ion. These different events all contribute to any given line of the plasma emission spectrum. To retrieve information about which and how many atoms are contained in the plasma, one needs to identify the individual processes contributing to the spectrum. To do so, spectroscopists use atomic models to calculate the emission spectra of all plasma atoms for all the various processes they experience.