Fusion UK

High-temperature superconductors (HTS) are increasingly being used, for a wide range of applications, from medical devices and transport systems to particle accelerators and future fusion reactors. This range grows exponentially and the improvement of the HTS reliability is an important task. One of the challenges is the quench detection and prevention of its progression in HTS magnets, which manifests itself as a rapid rise in the temperature of the HTS material, leading to interruption of magnet operation. Capability to monitor fast and in-situ the local temperature variations inside the HTS magnet will enable timely detection of the quenching and its prevention. A technique based on millimeter wave time and frequency domain reflectometry (TFDR) is suggested for the HTS quench monitoring. We use the detection and analysis of millimeter wave electromagnetic signals, propagating in the coolant gas waveguide like channels, to monitor the coolant temperature thus enabling the quench monitoring. The techniques suggested should allow both a real-time quench detection capability, as well as, facilitating the acquisition of valuable information regarding the health of the whole magnet system throughout its operation time.

Please see here for all changes: http://stellar.cct.lsu.edu/files/hpx-0.9.11/html/hpx/whats_new/hpx_0_9_11.html.

The kinetic theory of transport in an impure, partially ionized, edge plasma is developed. It is found that the thermal force between bulk ions and impurities is enhanced by the presence of neutral atoms, but the dynamical friction is not affected by the neutrals. When the neutral viscosity is large, an additional force on the impurities also arises. This force is parallel to the magnetic field, and is proportional to the shear of the parallel plasma velocity and the perpendicular ion density and temperature gradients.

The stability of the ion hybrid wave in a plasma containing two thermal ion species present in comparable proportions and a low density population of superthermal alpha-particles is analyzed. A simple, model distribution function consisting of a ring distribution in the perpendicular velocity and a Maxwellian in the parallel velocity is used for the superthermal alpha-particles. This distribution function is relevant to the core plasma of a tokamak in the immediate post-birth phase before the alpha-particles have had time to relax collisionally and is therefore of interest to the alpha-channelling question. It has also been used to interpret ion cyclotron emission from fusion products in the edge plasma of large tokamaks. An approximate dispersion relation is derived which allows the conditions for instability to be explored and an analytic expression for the growth rate to be obtained. It is found that the ion hybrid wave can be unstable for v⊥0/cA≪1 where v⊥0 is the alpha-particle ring speed and cA is Alfvén speed for a plasma with two ion species. The instability conditions obtained from the analytic approximation are used to guide the solution of the exact dispersion relation. Numerical solutions for the specific cases of deuterium–tritium core and edge plasmas in the Tokamak Fusion Test Reactor (TFTR) [K. M. Young et al., Plasma Phys. Controlled Fusion 26, 11 (1984)] are given.