De-convolution of complex residual gas spectra at JET

Spectral analysis of the vacuum conditions within the JET tokamak provides unique challenges due to the complex interactions of Hydrogen, Deuterium and Tritium with the large amounts of Carbon forming the plasma facing first wall. Overlapping masses and low resolution of the quadrupole Residual Gas Analyzer (qRGA) means discrimination of individual species is not possible operating in a conventional mode. Baking the vessel to temperatures in excess of 300 degrees Celsius further complicates an already difficult task. Here, we present and demonstrate a complimentary technique operating the qRGA in a mode where the energy of the electrons emitted within the ionization source is variable. Different elements have defined ionization energies required to remove an orbiting electron, this energy is dependent on the electron orbital, i.e. outer shell electrons generally have weaker ionization energies due to the greater distance and lower electrostatic forces from the nucleus. Using this technique known as "Soft Ionization" de-convolution of the mass spectra obtained at JET could lead the way to providing a better understanding of the chemistry within the vacuum vessel as well as providing invaluable diagnostic information during vessel conditioning. Initial experimental data has proved encouraging demonstrating ionization energy discrimination of D₂ and ⁴He at 4amu and D₂O /Ne /Ar4⁺⁺ at 20amu. De-convolution of hydrocarbon and deuterated hydrocarbon molecules in the mass spectrum is also currently ongoing, with initial results revealing clear discrimination of CH₄ and CD₂ molecular ions at 16amu. Using electron ionization cross-section theory we also demonstrate the generation of algorithms within the qRGA software to automatically discriminate quantitatively between overlapping peaks in the mass spectra. This technique can be applied to historical as well as real time data. 2010 brings the installation of t- he Beryllium plasma facing wall at JET and an opportunity to accurately determine the vessel chemistry during commissioning. The results from this phase of JET operations and further applications of the technique including leak detection and cryogenic pumping regeneration inventory determination could prove valuable for ITER commissioning and operations.