Cylindrical Model of RWM in RFP Plasmas and Application on RFX-mod

The RWM instabilities have been demonstrated to be successfully controlled by the feedback equipment implemented in the experiments of RFX-mod Apart from some similar behaviour in both RFP and Tokamak configurations, the RWMs in RFP plasmas are current driven instabilities; while in Tokamak the RWMs are normally driven by the plasma pressure. Furthermore, in RFPs the external kink instabilities, having their rational surfaces outside the plasma, exist as so called externally non-resonant modes (ENRM),which have their rational surfaces located at q < q(a) <0 (q(a) is the safety factor at the plasma edge); and internally non-resonant modes (INRM) with rational surfaces corresponding to q > q(0) > 0. This fact leads to a more severe condition on the stabilization by plasma rotation and dissipation. In addition, the current driven RWM instability in a RFP, where the strong poloidal magnetic field leads to a "bad curvature" dominant along the entire poloidal angle, has much weaker ballooning structure than in a tokamak. In the present work, we study RWM instabilities in cylindrical RFP plasmas by MHD theory, in which the effects of the plasma pressure, compressibility, plasma inertia, longitudinal rotation and parallel viscosity (tensor) have been taken into account. The resistive wall is modeled with a finite thickness, which allows to treat a large equilibrium flow in the plasma and ωτ b >>1 (ω is the mode frequency and τ b is the wall penetration time scale length). In the RFP configuration the poloidal asymmetry in the equilibrium magnetic field is much weaker than in a tokamak, the existing studies in toroidal geometry of RFPs have shown a weak influence of the toroidal coupling effects on the growth rate of the RWM modes I. RFP equilibrium and eigenmode equation. Considering a cylindrical plasma with minor radius r=a, surrounded by a resistive wall at r=b with thickness h and conductivity σ, an

A Key to Improved Ion Core Confinement in the JET Tokamak: Ion Stiffness Mitigation due to Combined Plasma Rotation and Low Magnetic Shear

New transport experiments on JET indicate that ion stiffness mitigation in the core of a rotating plasma, as described by Mantica et al. Phys. Rev. Lett. 102 175002 (2009)] results from the combined effect of high rotational shear and low magnetic shear. The observations have important implications for the understanding of improved ion core confinement in advanced tokamak scenarios. Simulations using quasilinear fluid and gyrofluid models show features of stiffness mitigation, while nonlinear gyrokinetic simulations do not. The JET experiments indicate that advanced tokamak scenarios in future devices will require sufficient rotational shear and the capability of q profile manipulation. © 2011 American Physical Societ

Statistical assessment of ELM triggering by pellets on JET

© 2021 IAEA, Vienna. This article investigates the triggering of ELMs on JET by injection of frozen pellets of isotopes of Hydrogen. A method is established to determine the probability that a specific pellet triggers a particular ELM. This method allows clear distinction between pellet-ELM pairs that are very likely to represent triggering events and pairs that are very unlikely to represent such an event. Based on this, the pellet parameters that are most likely to affect the ability of pellets to trigger ELMs have been investigated. It has been found that the injection location is very important, with injection from the vertical high field side showing a much higher triggering efficiency than low field side (LFS) injection. The dependence on parameters such as pellet speed and size and the time since the last ELM is also seen to be much stronger for LFS injection. Finally, the paper illustrates how improvements to the pellet injection system by streamlining the pellet flight lines and slightly increasing the pellet size has resulted in a significantly improved ability to deliver pellets to the plasma and trigger ELMs.s

Experimental determination of the energy dependence of the rate of the muon transfer reaction from muonic hydrogen to oxygen for collision energies up to 0.1 eV

We report the first experimental determination of the collision-energy dependence of the muon transfer rate from the ground state of muonic hydrogen to oxygen at near-thermal energies. A sharp increase by nearly an order of magnitude in the energy range 0 - 70 meV was found that is not observed in other gases. The results set a reliable reference for quantum-mechanical calculations of low-energy processes with exotic atoms, and provide firm ground for the measurement of the hyperfine splitting in muonic hydrogen and the determination of the Zemach radius of the proton by the FAMU collaboration.Comment: 30 pages, 10 figure