Dust characterization in FTU tokamak

Pre-plasma remobilization of ferromagnetic dust in FTU and possible interference with tokamak operations

“…In order to experimentally verify that the magnetic moment force is responsible for the observed phenomena, exposures of calibrated adhered ferromagnetic and non-magnetic dust to pure magnetic field discharges have been carried out. Dust collection activities in FTU have revealed that the dust size distribution ranges from the sub-micron range to the millimeter range [39]. Our exposures will focus on dust sizes of several microns for a number of reasons (i) larger grains are much easier to remobilize (see section 3.1), which could lead to problems during sample transportation and mounting, (ii) the sticking velocity of larger grains is very small [24], which could lead to problems during sample preparation, (iii) smaller grains are hardly remobilizable and cannot interfere with tokamak start-up unless their density is unrealistically high.…”

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

“…During the shutdown of FTU at the beginning of 2013, as already reported in a previous paper [16], the vacuum vessel was thoroughly cleaned and deposited dust was collected. Since the FTU wall is made of stainless steel (SS) AISI 304LN grade and the tiles for poloidal and toroidal limiters are made of TZM 6 , the collected dust was predictably mainly composed of SS elements and Mo based grains.…”

“…Dust collection methods, dust size and classification are presented in more details in ref. [16]. The dust collected from the bottom of the machine by gross vacuuming was later divided in two different batches, comprising grains either smaller or larger than 210 μm (value chosen on the basis of meshes availability around the value of 200 μm); this value was chosen to distinguish the two batches because dust larger than few 100's μm are unlikely to be mobilized by plasma [12,[16][17][18][19].…”

“…Figure 8 shows some grains collected around the LLL; they are composed of whitish powder, small and large grains (up to several millimeters in diameter), mixed with metal dust similar to the dust collected in the rest of the vessel. XRD analysis on some large whitish grains have revealed a high concentration of lithium carbonate (Li 2 CO 3 ) and lithium hydroxide (LiOH) [16]; a typical XRD spectrum of a whitish grain is shown in figure 9 as an example. In order to get an overview of the composition of the whole dust found around the LLL, ND analysis have been carried out on dust collected around the LLL and on particulate collected after a deep mechanical scratch of the vessel walls around the LLL location (see table 3 and figure 10).…”

“…In this work we present the result of a systematic analysis of the dust collected in FTU in 2013, during a shutdown period, by gross vacuuming after several years of operations. In particular we focus our attention on the relation between particles composition and their morphology, namely the spherical shaped dust; the poorly investigated ferromagnetic dust; and the lithiated grains produced by the presence of the LLL in FTU [16].…”