Minas Bacharis

Electron emission from the surface of solid particles plays an important role in many dusty plasma phenomena and applications. Examples of such cases include fusion plasmas and dusty plasma systems in our solar system. Electron emission complicates the physics of the plasma-dust interaction. One of the most important aspects of the physics of the dust plasma interaction is the calculation of the particle's floating potential. This is the potential a dust particle acquires when it is in contact with a plasma and it plays a very important role for determining its dynamical behaviour. The orbital motion limited (OML) approach is used in most cases in the literature to model the dust charging physics. However, this approach has severe limitations when the size of the particles is larger than the electron Debye length lambda(De). Addressing this shortcoming for cases without electron emission, a modified version of OML (MOML) was developed for modelling the charging physics of dust g...

ABSTRACT The classical source-collector sheath system describes a plasma that forms between a Maxwellian source and an absorbing wall. The plasma is assumed to be collisionless and without ionization. Two distinct areas are being formed: the collector sheath, an ion-rich region in contact with the absorbing boundary, and the source sheath, which is an electron-rich area near the Maxwellian source. In this work, we study a modified version of the classical source-collector sheath system, where the wall is no longer absorbing but emits electrons. As a result, we have two different types of collector sheath, one where a potential well is formed and one without a potential well. We examine the effect of electron emission for a range of conditions for the plasma and the emitted electrons. In the first part of this work, we study the problem analytically, and in the second, using our kinetic Vlasov code, Yggdrasil. The simulation results are in very good agreement with the predictions of our theoretical model. (C) 2014 AIP Publishing LLC.

ABSTRACT The source–collector sheath system describes a plasma that forms between a Maxwellian source and an absorbing wall. The plasma is assumed to be collisionless and without ionization. Two distinct areas are being formed: the collector sheath, an ion-rich region in contact with the absorbing boundary, and the source sheath, which is an electron-rich area near the Maxwellian source. Our work examines the effect that shifted Maxwellian distributions at the plasma source have on the characteristics of such a system. This is studied for a range of drift velocities and ratios of ion and electron temperatures. We study the problem both analytically and using our kinetic Vlasov code, Yggdrasil. The simulation results are in very good agreement with the predictions of our theoretical model.

ABSTRACT Production of dust particles during tokamak operation is a critical issue for magnetic confinement fusion. Their introduction into the reactor can have serious consequence on its performance and can constitute a safety issue. For these reasons the study of dust particles in tokamaks is crucial. Direct experimental observations of such particles that would give insight into their behaviour are quite challenging. In this context, numerical modelling of the relevant phenomena, plays a key role for better understanding the transport mechanisms of dust in tokamaks. In this work the dust transport code, Dust in tokamaks (DTOKS), is used to investigate how far tungsten and beryllium dust grains can penetrate into the ITER plasma. We simulate W and Be dust grains, with radii rd = 1–100 µm, and injection velocities, vinj = 1–100 ms−1, ejected from three different locations of the ITER vessel. It was found that particles with radius larger than 10 µm, with vinj = 10 m s−1, can survive long enough to reach the separatrix. Furthermore, the important roles of the initial injection velocity and injection location have been highlighted.

In this contribution, we report on experiments performed in MAST to investigate dust creation, transport and influence on plasma performance. The exceptional diagnostic access of MAST allows stereoscopic imaging of dust particles' motion in both the divertor and the main chamber, utilizing fast infrared cameras. This technique allows the 3D trajectory of the particles to be reconstructed. Infrared imaging of dust creation during disruptions revealed an isotropic release of dust particles from the surface with very high velocities (up to 350 m s-1). Stereoscopic imaging has been used to study, for the first time, the mobilization and transport in the divertor plasmas of carbon and tungsten particles with known size distributions, which were introduced into the vessel through a divertor probe. A correlation between the carbon particle size and acceleration by the plasma is observed. Tungsten particles are found to move with lower velocities and experience lower acceleration and are found to be more prone to vertical motion towards the core plasma. In the case of large particles this can lead to early disruptions. Modelling of the dust injection experiments has been conducted using the DTOKS code in an attempt to validate the transport equations employed in the simulation.

Interesting wake effects are found in simulations of dust grains in supersonically flowing plasma. A Mach cone is formed at an angle to the flow determined by the ratio of flow to Bohm speed. The latter is well approximated by [k(Te+γTi)/mi]1/2 with γ=3. For ion temperatures significantly lower than the electron temperature, a second (inner) cone forms due to flow convergence. An “ion vacuum” and stagnation point occur downstream. These latter effects cannot be described by conventional (cold-ion) gas dynamics. Critically, none of the cones observed are shocks but are more akin to weak discontinuities.

Dust produced in tokamaks is an important issue for fusion. Dust particles can introduce health and safety issues when in the same time can have an impact on reactor performance. Apart from the associated problems there are also potential benefits that make the better understanding of their behavior important. In this work the dust transport code Dust in TOKamakS will be used to explore the effect that variations in the plasma background and the physical model, describing the plasma-dust interaction, have on their predicted trajectories.

Dust immersed in plasma quickly charges to a potential where the ion and electron currents to it balance; this is the floating potential. In order to determine dust behaviour the floating potential must be known. The most used theory for determining this is orbital motion limited (OML). The OML floating potential depends on the ion to electron temperature ratio (β) and the plasma ion species (A). In reality the floating potential also depends strongly on the size of the dust grain (ρ = a/λD, where a is the radius of the grain and λD is the Debye length). Using a particle-in-cell code, dust is simulated in a collisionless plasma, the floating potential is investigated and the expressions provided allowing fast and accurate prediction of the floating potential as a function of β, A and ρ.

In this work modifications of the orbital motion limited approach are used to investigate the impact of time varying phenomena on the floating potential of a dust grain. The main interest is focused on different regimes relevant for RF discharges. Three cases are considered. First, the case of the RF sheath. Second, the case of charging in the bulk plasma with a time varying electric field and third, the case when the time varying current in the bulk plasma is carried only by a fraction of the electron population. This last case is relevant to low pressure discharges when stochastic heating is important.

The DTOKS code, which models dust transport through tokamak plasmas, is described. The floating potential and charge of a dust grain in a plasma and the fluxes of energy to and from it are calculated. From this model, the temperature of the dust grain can be estimated. A plasma background is supplied by a standard tokamak edge modelling code (B2SOLPS5.0),