Investigation on FTU dust and on the origin of ferromagnetic and lithiated grains

Abstract

A comprehensive analysis of composition and morphology of metallic micrometric particles collected in FTU during the 2013 shut-down is presented. The data-set analyzed is the result of years of experimental activity in FTU which is a full metal machine since the beginning of operation and with a liquid lithium limiter (LLL) from 2005. It was found that the metallic population, consisting of flakes, smashed and spheroidally shaped particles from plasma facing components (mainly SS and Mo), exhibits an unexpectedly high, up to 70 wt%, fraction of magnetic grains. The change of crystalline phase from γ to α/δ of the iron component contained in the particles coming from stainless steel is suggested as the mechanism responsible for magnetic dust production. This phase change can be ascribed to the particle temperature quench and/or to the presence of a strong magnetic field during the re-solidification of molten stainless steel droplets. In connection to the dust collected close to the LLL, consisting mainly of lithium carbonate spherical-like particles up to a few mm, the mechanism of Li 2 CO 3 formation and its role as a source of C and O impurities are discussed.

Cross machine investigation of magnetic tokamak dust: Morphological and elemental analysis

“…It has been suggested that ferromagnetic dust can be generated in tokamaks by a non-equilibrium austenite-to-ferrite phase transformation that is induced by rapid temperature quenching in the presence of strong external magnetic fields [8]. Moreover, there is strong experimental evidence of pre-plasma remobilization of ferromagnetic dust in FTU that has been further supported by theoretical estimates [16].…”

“…(a) FTU. The dust was collected by gross vacuuming around the vessel floor and was meshed out into two different size groups [8]. The batch with sizes m was considered in the present investigation.…”

“…It should be noted that dust from all the tokamak devices could be covered by boron as a result of boronisation activities. In addition, only for FTU, dust could be covered by lithium compounds courtesy of the liquid lithium limiter [8,17]. Both elements are highly reactive with oxygen (O) and are difficult to detect by EDX analysis either due to the low sensitivity at low energies or due to partial overlapping with other elements.…”

“…The presence of dust in fusion devices constitutes an important issue with safety and operational implications [1][2][3][4]. Despite the fact that ferromagnetic dust has been detected in a number of machines [5][6][7][8], magnetic dust has never been systematically investigated probably owing to the obvious absence of bulk magnetic materials in the vessel components. In particular, comprehensive dust collection activities in most tokamaks have never attempted a characterization of the magnetic properties of the sampled dust population [9][10][11][12][13][14][15].…”

Overview of the FTU results

“…cryogenic walls of FTU. The presence of ferromagnetic particles in tokamaks could pose new problems due to their interaction with the magnetic field; in fact magnetic grains could be liftedup during the ramping-up phase of the magnetic field due to ∇B force, and interfere with the start-up phase of plasma pulse [28].…”