No Arabic abstract
Recently, it has been shown that a vertical displacement event (VDE) can occur in ITER even when the walls are perfect conductors, as a consequence of the current quench [A. H. Boozer, Physics of Plasmas 26 114501 (2019)]. We used the extended-MHD code M3D-C1 with an ITER-like equilibrium and induced a current quench to explore cold VDEs in the limit of perfectly conducting walls, using different wall geometries. In the particular case of a rectangular wall with the side walls far away from the plasma, we obtained very good agreement with the analytical model developed by Boozer that considers a top/bottom flat-plates wall. We show that the solution in which the plasma stays at the initial equilibrium position is improved when bringing the side walls closer to the plasma. When using the ITER first wall in the limit of a perfect conductor, the plasma stays stable at the initial equilibrium position far beyond the value predicted by the flat-plates wall limit. On the other hand, when considering the limit in which the inner shell of the ITER vacuum vessel is acting as a perfect conductor, the plasma is displaced during the current quench but the edge safety factor stays above $2$ longer in the current decay compared to the flat-plates wall limit. In all the simulated cases, the vertical displacement is found to be strongly dependent on the plasma current, in agreement with a similar finding in the flat-plates wall limit, showing an important difference with usual VDEs in which the current quench is not a necessary condition.
This paper investigates the effect of the ITER-like wall (ILW) on runaway electron (RE) generation through a comparative study of similar slow argon injection JET disruptions, performed with different wall materials. In the carbon wall case, a runaway electron plateau is observed, while in the ITER-like wall case, the current quench is slower and the runaway current is negligibly small. The aim of the paper is to shed light on the reason for these differences by detailed numerical modelling to study which factors affected the RE formation. The post-disruption current profile is calculated by a one-dimensional model of electric field, temperature and runaway current taking into account the impurity injection. Scans of various impurity contents are performed and agreement with the experimental scenarios is obtained for reasonable argon- and wall impurity contents. Our modelling shows that the reason for the changed RE dynamics is a complex, combined effect of the differences in plasma parameter profiles, the radiation characteristics of beryllium and carbon, and the difference of the injected argon amount. These together lead to a significantly higher Dreicer generation rate in the carbon wall case, which is less prone to be suppressed by RE loss mechanisms. The results indicate that the differences are greatly reduced above ~50% argon content, suggesting that significant RE current is expected in future massive gas injection experiments on both JET and ITER.
The replacement of the JET carbon wall (C-wall) by a Be/W ITER-like wall (ILW) has affected the plasma energy confinement. To investigate this, experiments have been performed with both the C-wall and ILW to vary the heating power over a wide range for plasmas with different shapes.
A direct numerical simulation of many interacting ions in a Penning trap with a rotating wall is presented. The ion dynamics is modelled classically. Both axial and planar Doppler laser cooling are modeled using stochastic momentum impulses based on two-level atomic scattering rates. The plasmas being modeled are ultra-cold two-dimensional crystals made up of 100s of ions. We compare Doppler cooled results directly to a previous linear eigenmodes analysis. Agreement in both frequency and mode structure are obtained. Additionally, when Doppler laser cooling is applied, the laser cooled steady state plasma axial temperature agrees with the Doppler cooling limit. Numerical simulations using the approach described and benchmarked here will provide insights into the dynamics of large trapped-ion crystals, improving their performance as a platform for quantum simulation and sensing.
Values for the vacuum energy of scalar fields under Dirichlet and Neuman boundary conditions on an infinite clylindrical surface are found, and they happen to be of opposite signs. In contrast with classical works, a complete zeta function regularization scheme is here applied. These fields are regarded as interesting both by themselves and as the key to describing the electromagnetic (e.m.) case. With their help, the figure for the e.m. Casimir effect in the presence of this surface, found by De Raad and Milton, is now confirmed.
We report on the impact of anisotropy to tokamak plasma configuration and stability. Our focus is on analysis of the impact of anisotropy on ITER pre-fusion power operation 5~MA, $B=1.8$~T ICRH scenarios. To model ITER scenarios remapping tools are developed to distinguish the impact of pressure anisotropy from the change in magnetic geometry caused by an anisotropy-modified current profile. The remappings iterate the anisotropy-modified current profile to produce the same $q$ profile with matched thermal energy. The analysis is a step toward equilibria that are kinetically self-consistent for a prescribed scenario. We find characteristic detachment of flux surfaces from pressure surfaces, and an outboard (inboard) shift of peak density for $T_{parallel}>T_perp$ ( $T_{parallel}<T_perp$). Differences in the poloidal current profile are evident, albeit not as pronounced as for the spherical tokamak. We find that the incompressional continuum is largely unchanged in the presence of anisotropy, and the mode structure of gap modes is largely unchanged. The compressional branch however exhibits significant differences in the continuum. We report on the implication of these modifications.