Ampsaga/Amsaga

Par Paul-Albert Février Deux fleuves marquaient la limite de la province de Maurétanie césarienne : la Mulucha à l’ouest, l’Amsaga à l’est. Particularité que l’on ne retrouve pas entre la Numidie et l’Afrique. Or, l’un des cours d’eau, l’Amsaga, est connu à la fois par des sources littéraires et par des textes épigraphiques. S. Gsell, Atlas archéologique de l’Algérie, Paris, Alger, 1911, feuille 8, n° 5, donne la liste des textes anciens relatifs au fleuve. La plus ancienne mention d’une limi..

Abizar

Par P.-A. Février Localité située sur le versant méridional du massif montagneux côtier de Grande Kabylie, à quelques kilomètres, à vol d’oiseau, au Sud-Est de Tigzirt, où fut découverte, en 1859 une stèle de grand intérêt. Haute de 1,25m et large vers le sommet de 1,10m, elle se présente sous la forme d’une dalle de grès aux contours assez irréguliers, en particulier à droite. Un homme y est représenté à cheval et armé. L’homme dont le visage et le torse sont en position frontale est placé a..

Geometrical frustration effects on charge-driven quantum phase transitions

The interplay of Coulomb repulsion and geometrical frustration on charge-driven quantum phase transitions is explored. The ground state phase diagram of an extended Hubbard model on an anisotropic triangular lattice relevant to quarter-filled layered organic materials contains homogeneous metal, 'pinball' and three-fold charge ordered metallic phases. The stability of the 'pinball' phase occurring for strong Coulomb repulsions is found to be strongly influenced by geometrical frustration. A comparison with a spinless model reproduces the transition from the homogeneous metallic phase to a pinball liquid, which indicates that the spin correlations should play a much smaller role than the charge correlations in the metallic phase close to the charge ordering transition. Spin degeneracy is, however, essential to describe the dependence of the system on geometrical frustration. Based on finite temperature Lanczos diagonalization we find that the effective Fermi temperature scale, T*, of the homogeneous metal vanishes at the quantum phase transition to the ordered metallic phase driven by the Coulomb repulsion. Above this temperature scale 'bad' metallic behavior is found which is robust against geometrical frustration in general. Quantum critical phenomena are not found whenever nesting of the Fermi surface is strong, possibly indicating a first order transition instead. 'Reentrant' behavior in the phase diagram is encountered whenever the 2kF-CDW instability competes with the Coulomb driven three-fold charge order transition. The relevance of our results to the family of quarter-filled materials: theta-(BEDT-TTF)2X is discussed.Comment: 15 pages, 11 figure

Synergetic effects of collisions, turbulence and sawtooth crashes on impurity transport

This paper investigates the interplay of neoclassical, turbulent and MHD processes, which are simultaneously at play when contributing to impurity transport. It is shown that these contributions are not additive, as assumed sometimes. The interaction between turbulence and neoclassical effects leads to less effective thermal screening, i.e. lowers the outward flux due to temperature gradient. This behavior is attributed to poloidal asymmetries of the flow driven by turbulence. Moreover sawtooth crashes play an important role to determine fluxes across the q = 1 surface. It is found that the density profile of a heavy impurity differs significantly in sawtoothing plasmas from the one predicted by neoclassical theory when neglecting MHD events. Sawtooth crashes impede impurity accumulation, but also weaken the impurity outflux due to the temperature gradient when the latter is dominant

Reduction in benefits of total flux expansion on divertor detachment due to parallel flows

The Super-X divertor (SXD) is an alternative divertor configuration leveraging total flux expansion at the outer strike point (OSP). Key features for the attractiveness of the SXD are facilitated detachment access and control, as predicted by the extended 2-point model (2PM). However, parallel flows are not consistently included in the 2PM. In this work, the 2PM is refined to overcome this limitation: the role of total flux expansion on the pressure balance is made explicit, by including the effect of parallel flows. In consequence, the effect of total flux expansion on detachment access and control is weakened, compared to predictions of the 2PM. This new model partially explains discrepancies between the 2PM and experiments performed on TCV, in ohmic L-mode scenarios, where in core density ramps in lower single-null (SN) configuration, the impact of the OSP major radius Rt on the CIII emission front movement in the divertor outer leg - used as a proxy for the plasma temperature - is substantially weaker than 2PM predictions; and in OSP sweeps in lower and upper SN configurations, with a constant core density, the peak parallel particle flux density at the OSP is almost independent of Rt, while the 2PM predicts a linear dependence. Finally, analytical and numerical modelling of parallel flows in the divertor is presented, to support the argument. It is shown that an increase in total flux expansion can favour supersonic flows at the OSP. Parallel flows are also shown to be relevant by analysing SOLPS-ITER simulations of TCV

Divertor shaping with neutral baffling as a solution to the tokamak power exhaust challenge

Exhausting power from the hot fusion core to the plasma-facing components is one of the biggest challenges in fusion energy. The MAST Upgrade tokamak uniquely integrates strong containment of neutrals within the exhaust area (divertor) with extreme divertor shaping capability. By systematically altering the divertor shape, this study shows the strongest evidence to date that long-legged divertors with a high magnetic field gradient (total flux expansion) deliver key power exhaust benefits without adversely impacting the hot fusion core. These benefits are already achieved with relatively modest geometry adjustments that are more feasible to integrate in reactor designs. Benefits include reduced target heat loads and improved access to, and stability of, a neutral gas buffer that 'shields' the target and enhances power exhaust (detachment). Analysis and model comparisons shows these benefits are obtained by combining multiple shaping aspects: long-legged divertors have expanded plasma-neutral interaction volume that drive reductions in particle and power loads, while total flux expansion enhances detachment access and stability. Containing the neutrals in the exhaust area with physical structures further augments these shaping benefits. These results demonstrate strategic variation in the divertor geometry and magnetic topology is a potential solution to one of fusion's biggest challenges: power exhaust

Comparison of high density and nitrogen seeded detachment using SOLPS-ITER simulations of the tokamak á configuration variable

First of a kind SOLPS-ITER simulations on tokamak á configuration variable (TCV) that include nitrogen have been performed to model recent nitrogen seeded detachment experiments. Based on spectroscopic measurements, a nitrogen recycling coefficient RNp ≈ 0.3–0.5 on the graphite walls of TCV is estimated. The experimentally observed decrease of core nitrogen density with increasing plasma density is reproduced and linked to a reduction of the ionisation mean free path in the scrape-off layer. Although the influence of sputtered carbon impurities from TCV’s graphite wall cannot be fully eliminated, seeding nitrogen increases control over the total impurity density. This facilitates disentangling the effect of impurities from that of high upstream density on the main characteristics of detachment, namely target power and ion current reductions and the development of a parallel pressure drop. Increasing the density and the seeding rate reduce the power on the divertor targets in a different way: with density, the ion current increases and the target temperature strongly decreases, whereas seeding impurities decreases the ion current and affects less strongly the temperature. The reduction in ion current when seeding nitrogen is due to a lower ionisation source, which is not related to power limitation nor an increased momentum loss, but to a decrease of the ionisation reaction rate. Impurity seeding leads to less volumetric momentum losses (and hence pressure drop) than density ramps, for the same level of energy flux reduction. Additionally, main chamber sputtering of carbon is identified as a possible explanation for the missing target ion current roll-over during density ramps in the simulations

Comparison of detachment in Ohmic plasmas with positive and negative triangularity

Detachment is investigated using core density ramps for lower single null Ohmic L-mode plasmas across a wide range of upper, lower, and total triangularity ($\delta$) in the TCV tokamak. It is universally found that detachment is more difficult to access with negative triangularity (NT) shaping. The outer divertor leg of discharges with $\delta\approx -0.3$ could not be cooled below 5 eV using core density ramps alone. The behavior of the upstream plasma and geometrical divertor effects (e.g. a reduced connection length at negative lower triangularity) do not fully explain the challenges of detaching NT plasmas. Langmuir probe measurements of the target heat flux widths ($\lambda_q$) remained constant within 30% across an upper triangularity scan, while the spreading factor $S$ was found to be lower by up to 50% in NT, indicating a generally lower integral SOL width. An interesting pattern has been observed in the particle balance where the line-averaged core density was typically higher in NT discharges for a given fuelling rate. Conversely, the divertor neutral pressure and integrated particle content were typically lower for the same line-averaged density. This indicates that NT plasmas may be closer to the sheath-limited regime than their PT counterparts, which could explain why NT is more challenging to detach

Divertor detachment and reattachment with mixed impurity seeding on ASDEX Upgrade

Using newly developed spectroscopic models to measure the divertor concentration of Ne and Ar, it is shown that the experimental detachment threshold on ASDEX Upgrade with Ar-only and mixtures of Ar+N or Ne+N scales as expected in comparison with an analytical equation derived by Kallenbach et al (2016 Plasma Phys. Control. Fusion 58 045013). However, it is found that Ar radiates more efficiently and Ne less efficiently in the scrape-off layer than the model predicts. By separately increasing the neutral beam injection power and cutting the impurity gas flow, it is shown that the partially detached and strongly detached X-point radiator scenarios reattach in ≈100 ms and ≈250 ms, respectively. The former timescale is set by the core energy confinement time, whereas the latter has an additional delay caused by the time required for the ionisation front to move from the X-point to the target. A simple equation with scalable geometric terms to predict the ionisation front movement time in future machines is proposed