Marek Vysinka

Dust as a Gas Carrier

Dust in space can collect particles from surrounding plasma and transport them over long distances. Release of the implanted particles can then change the mass composition in a particular place of the space. The depth of ion penetration into the dust body strongly depends on an initial mutual energy and differs with ion species as well as with the grain composition. The same holds for diffusion constant of implanted ions (already neutralized) exiting back to the free space. We have used our measurements of the release of Ar ions implanted into glassy carbon dust grains for determination of the diffusion coefficient. Our calculations provide the limits for the amount of gas that can be dissolved in the grain as well as its release rate. We discuss the influence of the dust sputtering and dust temperature on the aforementioned quantities.

The influence of secondary electron emission on the floating potential of tokamak-born dust

Dust production and its transport into the core plasma is an important issue for magnetic confinement fusion. Dust grains are charged by various processes, such as the collection of plasma particles and electron emissions, and their charge influences the dynamics of the dust. This paper presents the results of calculations of the surface potential of dust grains in a Maxwellian plasma. Our calculations include the charging balance of a secondary electron emission (SEE) from the dust. The numerical model that we have used accounts for the influence of backscattered electrons and takes into account the effects of grain size, material, and it is also able to handle both spherical and non-spherical grains. We discuss the role of the SEE under tokamak conditions and show that the SEE is a leading process for the grains crossing the scrape-off layer from the edge to core plasma. The results of our calculations are relevant for materials related to fusion experiments in ITER.

Relation of Charging History to Field Ion Emission From Gold and Carbon Dust

A study of dust-grain charging plays a very important role in the understanding of complex (dusty) plasma. The dust grains are charged by several different processes (e.g., electron and ion attachments, secondary electron emission, photoemission, and field electron and ion emissions), and their charge significantly influences the surrounding plasma. Our laboratory experiment based on an electrodynamic quadrupole trap allows us to investigate some of these processes on a single isolated dust grain which can be trapped and influenced with different agents for a very long time (days). In this paper, we focused on the determination of the relation between charging conditions and the field-emission mechanism because this emission limits positive charges that dust grains can acquire due to photoemission, secondary emission, or ion attachment. The field-ion-emission process is based on the field ionization of the atoms that crosses a critical distance from the grain surface. We have found that the sources of these atoms are either the surrounding gas or the ions implanted into the grain and leaving it due to diffusion. The diffusion can be described by two time constants differing by an order of magnitude. We used two sets of dust grains: gold and amorphous carbon. The experimental results are confirmed by a simple model.

Sputtering of Spherical SiO 2 Samples

Dust grains in the interplanetary environment can be basically found in two locations—floating in the free space or attached to a surface of asteroids, comets, or moons. They are sputtered by the impacts of energetic ions, and this process supplies the interplanetary space with heavy elements. The sputtering yield is generally estimated on the basis of laboratory investigations of planar samples. We use silica micrometer-sized spherical grains as a prototype of a space-borne dust, bombard them by 2-keV Ar ions, and monitor the influences of simultaneous application of the electron beam as well as the electric field at the dust surface on the sputtering yield. We found that the increase in the sputtering yield due to the electron impact is much larger than expected and it can enhance the sputtering yield by a factor of 1.6 in a comparison with the sole ion bombardment. On the other hand, the influence of the electric field is not so strong (if any) and it is masked by electron impacts in our experiment. Sputtering of the grains fixed at a surface by 30-keV Ga ions revealed that the angular profile of the yield is flatter than that frequently used for a description of the sputtering process. Finally, we compare these results with the published sputtering yield values.

Composition And Electrical Properties Of Dust From Tokamak Compass

In spite of the fact that fusion is a subject of the study for many years, there are still a lot of open questions. One of the interesting topics in fusion research is a presence of dust grains in reactors [1]. In the paper, dust grains born in tokamak Compass are studied and compared with samples of a spherical geometry and well known composition. A unique experimental setup [2, 3] was used for investigations of charging properties of such grains and the SEM and EDX spectroscopy was applied for a study of grain composition. We focus on the secondary emission because this process plays a prominent role when a portion of energetic electrons is present in surroundings of a particular grain. It was shown that depending on the grain size and material, energetic electrons charge the grains to positive potentials comparable with the energy of impinging electrons.