16–21 Sept 2018
Giardini Naxos
Europe/Rome timezone

P2.120 The adhesion of tungsten dust on rough tungsten surfaces

18 Sept 2018, 11:00
2h
Pantelleria Hall - Terrace - ATA Hotel Naxos Beach Resort (Giardini Naxos)

Pantelleria Hall - Terrace - ATA Hotel Naxos Beach Resort

Giardini Naxos

Via Recanati, 26 Giadini Naxos, Messina - Sicily (Italy)
P2

Speaker

Marco De Angeli (Istituto di Fisica del Plasma CNR)

Description

Adhesion plays a pivotal role in numerous aspects of tokamak-generated dust such as in-situ removal techniques, post-mortem collection activities, resuspension during loss-of-vacuum accidents and in-plasma remobilization. Due to insurmountable difficulties in the theoretical treatment of the interaction between technical (rough, polycrystalline, adsorbate covered) surfaces, adhesive or pull-off force measurements for reactor relevant materials are imperative.
Recent measurements of the adhesion of spherical micron W dust (R_d≃5-15μm) on planar W surfaces (R_q≃10-100nm) revealed the importance of surface morphology. Due to nano-roughness: (i) Adhesion is not dominated by metallic bonding interactions but by London dispersion forces, implying that the Van der Waals expression can provide a zero-order estimate of the pull-off-force. (ii) Adhesion is not deterministic but acquires a statistical nature and is better described by cumulative distribution functions. In particular, the pull-off force behaves as a log-normally distributed random variable.
However, in situ tokamak roughness is characterized by much higher root-mean-square values. Surface roughness of the order of the contact radius (R_q∼1μm) should lead to a respectable decrease of the mean value of the measured pull-off force and increase of its relative standard deviation. To quantify adhesion for tokamak-relevant surfaces, we have performed systematic pull-off force measurements for W dust deposited in a controlled manner on rough planar W surfaces (R_q∼1μm). Monodisperse spherical W dust (R_d=9,15μm) was adhered to rough W samples through impacts below the sticking threshold. The samples were then inserted in an electrical mobilization setup that allows the measurement of the pull-off force by exerting an electrostatic force of well-known magnitude. The measurements revealed a ∼50% reduction in the mean value and a ∼100% increase in the relative standard deviation of the pull-off force for rough surfaces (R_q≃1μm) compared to smooth surfaces (R_q≃10nm). Despite these differences, the pull-off force still follows the log-normal distribution.

Presentation materials

There are no materials yet.