Impact of impurity seeding on the electron energy distribution function in the COMPASS divertor region
PLASMA PHYSICS AND CONTROLLED FUSION
Authors: Dimitrova, M.; Popov, Tsv K.; Kovacic, J.; Dejarnac, R.; Gunn, J. P.; Ivanova, P.; Imrisek, M.; Stockel, J.; Vondracek, P.; Hron, M.; Panek, R.
In the COMPASS tokamak, series of experiments were performed aimed at studying the impact of nitrogen, neon, and argon impurity seeding on the electron energy distribution function (EEDF) in the divertor region. The experiments were conducted in D-shaped, L-mode, deuterium plasmas. In order to obtain the radial distribution of the floating potential, ion saturation current, electron temperatures, and densities, the current-voltage characteristics were measured by Langmuir probes embedded in the COMPASS tokamak divertor. The properties of the plasma in the divertor region were measured before and during impurity seeding. Before the N-2 seeding, the EEDF was bi-Maxwellian with a low-energy electron fraction with temperatures 3.5-5 eV, and a higher-energy one with temperatures in the range of 10 eV to 23 eV. During seeding with an increasing number of molecules per second, the EEDF changed from bi-Maxwellian to Maxwellian and the electron temperature decreased. The time-evolution was studied of the change in the EEDF during N-2 seeding. When the seeding was carried out by a valve in the private flux region, the duration of the transition from a bi-Maxwellian to a Maxwellian EEDF was about 10-15 ms. When the N-2 seeding took place through a low-field side valve, the transition from a bi-Maxwellian to a Maxwellian EEDF took longer -25-45 ms. The temporal evolution was also analyzed of the plasma parameters' radial profiles when neon and argon were puffed using a valve in the divertor low-field side. The application is discussed of the probe measurements' results to calculating the parallel heat-flux densities in the divertor region of the COMPASS tokamak.
Influence of pressure on phase transition, electronic and thermoelectric properties of SnSe
JOURNAL OF ALLOYS AND COMPOUNDS
Authors: Yang, Lin Tai; Ding, Li-Ping; Shao, Peng; Tiandong, Yun Hao; Zhao, Zi Li; Zhang, Fang-Hui; Lu, Cheng
Single-crystalline Tin Selenide (SnSe) is a good thermoelectric material with record-breaking figure of merit. Here, we reveal the influence of pressure on structures, electronic and thermoelectric properties of SnSe crystals by combining CALYPSO and first-principle calculation. In our results, the experimental synthetic phase Pnma- SnSe is the most stable structure at ambient pressure. However, a phase transition from Pnma to Cmcm occurs at about 22 GPa. More than 22 GPa, some new crystal structures (Pm (3) over barm-, C2/m- and Cmmm-SnSe) are revealed. The calculated phonon spectrum indicates their dynamic stabilities. From the energy band structures and density of states, it is found that only Pnma-SnSe phase is a semiconductor with bandgap of 0.79 eV which is benefit to suppress thermal activation of electrons in the conduction band, while the other phases are metallic. Moreover, our calculated Seebeck coefficient, conductivity versus time ratio and thermal conductivity indicate that SnSe in Pnma and Cmcm phases possess good thermoelectric properties with the ZT values of 0.9 and 0.8, respectively. (C) 2020 Elsevier B.V. All rights reserved.