Experimental analysis of the particle transport in the magnetized plasma column with an application to the helicon discharge

International audienceDifferent transport mechanisms in a magnetized radio frequency plasmadischarge in the IShTAR device are compared. The total cross fieldparticle transport systematically shows the best agreement with theturbulent diffusion. Also, the ion mobility dominated transport couldsubstantially contribute to radial losses. The relative role of paralleland perpendicular losses in the overall particle confinement is alsocompared. The total perpendicular particle losses are comparable or evenlarger than the parallel ones, imposing a practical limitation onachieving high density plasma simply combining several helicon antennaesequentially arranged along the glass tube. For the same reason,reducing the diameter of a source tube, in an attempt to achieve ahigher plasma density, could be not very efficient when perpendicularparticle loss is dominant

Comparison of Edge and Internal Transport Barriers in Drift Wave Predictive Simulations

We have simulated the formation of an internal transport barrier on JET including a self-consistent treatment of ion and electron temperatures and poloidal and toroidal momentum. Similar simulations of edge transport barriers, including the L-H transition have also been made. However, here only polodal momentum and the temperatures were simulated. The internal barrier included an anomalous spinup of poloidal momentum similar to that in the experiment. Also the edge barrier was accompanied by a spinup of poloidal momentum. The experimental density (with no barrier) was used and kept fixed for the internal barrier. For the edge barrier the edge density was varied and it turned out that a lower edge density gave a stronger barrier. Electromagnetic and nonlocal effects were important for both types of barriers

Preliminary reports on the 2016-2017 excavation of the Neolithic ossuary and terrace

This article reports on the first three systematic excavation seasons at the Neolithic ossuary of Grotte de La Faucille, Belgium. The site was dated on human bone to 4266 ± 40 14C BP (3011-2702 cal BC; 2 sigma), corresponding to the transition from the late to the final Neolithic. The area excavated to date is clearly reworked and the individuals are distributed across the site. Further excavation will focus on the inferior levels at the entrance and inside the cave. This report presents the preliminary analysis of the anthropological and archaeological evidence recorded to date. Five archaeological artefacts were discovered made on bone, tooth and flint. The site has produced skeletal and dental remains of at least 12 humans (MNI 6 juveniles and 6 adults) as well as a number of bone and lithic artefacts. The skeletal remains are fragmentary and some elements, such as the cranium, are highly underrepresented given the number of individuals. The potential to find the remainder of at least 12 individuals is promising and continued excavation may result in one of the largest recent excavation of a multiple Neolithic burial site of the 21st century. The results presented here and ongoing analysis have the potential to significantly expand our understanding of the mortuary behaviours, or variation in behaviours, of the Belgian Neolithic and contribute further to the lively debate on the spread of the Neolithic

Observation of confined current ribbon in JET plasmas

we report the identification of a localised current structure inside the JET plasma. It is a field aligned closed helical ribbon, carrying current in the same direction as the background current profile (co-current), rotating toroidally with the ion velocity (co-rotating). It appears to be located at a flat spot in the plasma pressure profile, at the top of the pedestal. The structure appears spontaneously in low density, high rotation plasmas, and can last up to 1.4 s, a time comparable to a local resistive time. It considerably delays the appearance of the first ELM.Comment: 10 pages, 6 figure

Minority and mode conversion heating in (3He)-H JET plasma

Radio frequency (RF) heating experiments have recently been conducted in JET (He-3)-H plasmas. This type of plasmas will be used in ITER's non-activated operation phase. Whereas a companion paper in this same PPCF issue will discuss the RF heating scenario's at half the nominal magnetic field, this paper documents the heating performance in (He-3)-H plasmas at full field, with fundamental cyclotron heating of He-3 as the only possible ion heating scheme in view of the foreseen ITER antenna frequency bandwidth. Dominant electron heating with global heating efficiencies between 30% and 70% depending on the He-3 concentration were observed and mode conversion (MC) heating proved to be as efficient as He-3 minority heating. The unwanted presence of both He-4 and D in the discharges gave rise to 2 MC layers rather than a single one. This together with the fact that the location of the high-field side fast wave (FW) cutoff is a sensitive function of the parallel wave number and that one of the locations of the wave confluences critically depends on the He-3 concentration made the interpretation of the results, although more complex, very interesting: three regimes could be distinguished as a function of X[He-3]: (i) a regime at low concentration (X[He-3] < 1.8%) at which ion cyclotron resonance frequency (ICRF) heating is efficient, (ii) a regime at intermediate concentrations (1.8 < X[He-3] < 5%) in which the RF performance is degrading and ultimately becoming very poor, and finally (iii) a good heating regime at He-3 concentrations beyond 6%. In this latter regime, the heating efficiency did not critically depend on the actual concentration while at lower concentrations (X[He-3] < 4%) a bigger excursion in heating efficiency is observed and the estimates differ somewhat from shot to shot, also depending on whether local or global signals are chosen for the analysis. The different dynamics at the various concentrations can be traced back to the presence of 2 MC layers and their associated FW cutoffs residing inside the plasma at low He-3 concentration. One of these layers is approaching and crossing the low-field side plasma edge when 1.8 < X[He-3] < 5%. Adopting a minimization procedure to correlate the MC positions with the plasma composition reveals that the different behaviors observed are due to contamination of the plasma. Wave modeling not only supports this interpretation but also shows that moderate concentrations of D-like species significantly alter the overall wave behavior in He-3-H plasmas. Whereas numerical modeling yields quantitative information on the heating efficiency, analytical work gives a good description of the dominant underlying wave interaction physics