A. P. Smirnov

Absorption of lower hybrid waves in the scrape off layer of a diverted tokamak

The goal of the Lower Hybrid Current Drive (LHCD) system on the Alcator C-Mod tokamak [Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] is to investigate current profile control under plasma conditions relevant to future tokamak experiments. Experimental observations of a LHCD “density limit” for C-Mod are presented in this paper. Bremsstrahlung emission from relativistic fast electrons in the core plasma drops suddenly above line averaged densities of 1020 m−3 (ω/ωLH∼3–4), well below the density limit previously observed on other experiments (ω/ωLH∼2). Electric currents flowing through the scrape off layer (SOL) between the inner and outer divertors increase dramatically across the same density range that the core bremsstrahlung emission drops precipitously. These experimental x-ray data are compared to both conventional modeling, which gives poor agreement with experiment above the density limit and a model including collisional absorption in the SOL, which dramatically improves agreement with experimen...

Measurements of secondary electron emission effects in the Hall thruster discharge

The dependence of the maximum electron temperature on the discharge voltage is studied for two Hall thruster configurations, in which a collisionless plasma is bounded by channel walls made of materials with different secondary electron emission (SEE) properties. The linear growth of the temperature with the discharge voltage, observed in the channel with a low SEE yield, suggests that SEE is responsible for the electron temperature saturation in the thruster configuration with the channel walls having a higher SEE yield. The fact that the values of the electron temperature at saturation are rather high may indirectly support the recently predicted kinetic regime of the space charge saturation of the near-wall sheath in the thruster discharge. A correlation between the effects of the channel wall material on the electron temperature and the electron cross-field current was also observed.

Benchmarking of codes for electron cyclotron heating and electron cyclotron current drive under ITER conditions

Optimal design and use of electron cyclotron heating requires that accurate and relatively quick computer codes be available for prediction of wave coupling, propagation, damping and current drive at realistic levels of EC power. To this end, a number of codes have been developed in laboratories worldwide. A detailed comparison of these codes is desirable since they use a variety of methods for modelling the behaviour and effects of the waves. The approach used in this benchmarking study is to apply these codes to a small number of representative cases. Following minor remedial work on some codes, the agreement between codes for off-axis application is excellent. The largest systematic differences are found between codes with weakly relativistic and fully relativistic evaluation of the resonance condition, but even there the differences amount to less than 0.02 in normalized minor radius. For some other cases, for example for central current drive, the code results may differ significantly due to differences in the physics models used.

Enhanced Performance of Cylindrical Hall Thrusters

The cylindrical thruster differs significantly in its underlying physical mechanisms from the conventional annular Hall thruster. It features high ionization efficiency, quiet operation, ion acceleration in a large volume-to-surface ratio channel, and performance comparable with the state-of-the-art conventional Hall thrusters. Very significant plume narrowing, accompanied by the increase of the energetic ion fraction and improvement of ion focusing, led to 50%–60% increase of the thruster anode efficiency. These improvements were achieved by overrunning the discharge current in the magnetized thruster plasma.

Lower hybrid current drive at high density in Alcator C-Mod

Experimental observations of lower hybrid current drive (LHCD) at high density on the Alcator C-Mod tokamak are presented in this paper. Bremsstrahlung emission from relativistic fast electrons in the core plasma drops suddenly above line-averaged densities of 1020 m−3 (ω/ωLH ~ 3) in single null discharges with large (≥8 mm) inner gaps, well below the density limit previously observed on limited tokamaks (ω/ωLH ~ 2). Modelling and experimental evidence suggest that the absence of LHCD driven fast electrons at high density may be due to parasitic collisional absorption in the scrape-off layer (SOL). Experiments show that the population of fast electrons produced by LHCD at high density ( 10^{20}\,{\rm m}^{-3} SRC=http://ej.iop.org/images/0029-5515/51/8/083032/nf381190in001.gif/>) can be increased by operating with an inner gap of less than ~5 mm with the strongest non-thermal emission in inner wall limited plasmas. A change in plasma topology from single to double null produces a modest increase in non-thermal emission at high density. Increasing the electron temperature in the periphery of the plasma (0.8 > r/a > 1.0) also results in a modest increase in non-thermal electron emission above the density limit. Ray tracing/Fokker–Planck simulations of these discharges predict the observed sensitivity to plasma position when the effects of collisional absorption in the SOL are included in the model.

Effects of enhanced cathode electron emission on Hall thruster operation

Interesting discharge phenomena are observed that have to do with the interaction between the magnetized Hall thruster plasma and the neutralizing cathode. The steady-state parameters of a highly ionized thruster discharge are strongly influenced by the electron supply from the cathode. The enhancement of the cathode electron emission above its self-sustained level affects the discharge current and leads to a dramatic reduction in the plasma divergence and a suppression of large amplitude, low frequency discharge current oscillations usually related to an ionization instability. These effects correlate strongly with the reduction in the voltage drop in the region with the fringing magnetic field between the thruster channel and the cathode. The measured changes in the plasma properties suggest that the electron emission affects the electron cross-field transport in the thruster discharge. These trends are generalized for Hall thrusters of various configurations.

Mutagenic analysis of Potato Virus X movement protein (TGBp1) and the coat protein (CP): in vitro TGBp1–CP binding and viral RNA translation activation

Previously, we have shown that encapsidated Potato virus X (PVX) RNA was non-translatable in vitro, but could be converted into a translatable form by binding of the PVX movement protein TGBp1 to one end of the virion or by coat protein (CP) phosphorylation. Here, a mutagenic analysis of PVX CP and TGBp1 was used to identify the regions involved in TGBp1-CP binding and translational activation of PVX RNA by TGBp1. It was found that the C-terminal (C-ter) 10/18 amino acids region was not essential for virus-like particle (VP) assembly from CP and RNA. However, the VPs assembled from the CP lacking C-ter 10/18 amino acids were incapable of TGBp1 binding and being translationally activated. It was suggested that the 10-amino-acid C-ter regions of protein subunits located at one end of a polar helical PVX particle contain a domain accessible to TGBp1 binding and PVX remodelling. The non-translatable particles assembled from the C-ter mutant CP could be converted into a translatable form by CP phosphorylation. The TGBp1-CP binding activity was preserved unless a conservative motif IV was removed from TGBp1. By contrast, TGBp1-dependent activation of PVX RNA translation was abolished by deletions of various NTPase/helicase conservative motifs and their combinations. The motif IV might be essential for TGBp1-CP binding, but insufficient for PVX RNA translation activation. The evidence to discriminate between these two events, i.e. TGBp1 binding to the CP-helix and TGBp1-dependent RNA translation activation, is discussed.

Off-midplane launch of electron Bernstein waves for current drive in overdense plasmas

Numerical modeling shows that localized, efficient current drive is possible in overdense toroidal plasmas (such as reversed field pinches and spherical tokamaks) using perpendicular launch of electron Bernstein waves. The wave directionality required for driving current can be obtained by launching the waves above or below the midplane of the torus and is a geometric effect related to the poloidal magnetic field. Wave absorption is strong, a result of the electrostatic nature of the waves, giving efficient suprathermal tail formation and current drive.

Electron Bernstein wave emission from an overdense reversed field pinch plasma

Blackbody levels of emission in the electron cyclotron range of frequencies have been observed from an overdense (ωpe∼3ωce) Madison Symmetric Torus [Dexter et al., Fusion Technol. 19, 131 (1991)] reversed field pinch plasma, a result of electrostatic electron Bernstein waves emitted from the core and mode converted into electromagnetic waves at the extreme plasma edge. Comparison of the measured radiation temperature with profiles measured by Thomson scattering indicates that the mode conversion efficiency can be as high as ∼75%. Emission is preferentially in the X-mode polarization, and is strongly dependent upon the density and magnetic field profiles at the mode conversion point.

Recommendation system for tourist attraction information service

The paper proposes a description of information decision support system in the tourism domain and a set of methods and algorithms for generating recommendations for a user that allow significant increase of the system usability. The system generates for the user recommendations which attractions at the moment are better to attend based on the user preferences and the current situation in the location area. The system also allows showing the user information about interesting attraction in more detail, which is based on analyzing information evaluations made by other users.