The JET tokamak was designed for a toroidal magnetic field strength of 3.45T and a plasma current of ~5MA. Normal operation of JET is limited primarily by electromagnetic stresses in the magnetic field coils, and thermomechanical stresses in the toroidal field coils and the central solenoid (which drives the plasma current) and in the plasma-facing components. The classical off-normal behaviour of tokamaks, ie disruptive cessation of the plasma current, creates further electromagnetic forces and torques on the in-vessel components and the vessel itself. Operational experience and detailed analysis of these stresses has increased the limits to 4.0T and 6MA, with plasma heating powers and energies presently up to ~34MW and ~200MJ. There are many complex interactions between different components, leading to a set of “JET Operating Instructions” aimed at maximising experimental flexibility while minimising life consumption and the risk of damage to the main elements of the machine. The key “JOIs”, how they are evolved, and how they impact the design of new assemblies introduced into JET, will be described.
Any modification to JET or its supporting systems (plasma heating, power supplies, bake-out and cooling, ventilation, remote handling etc) cannot be implemented until the design has undergone assessment, which may include analysis against a set of in-vessel and ex-vessel criteria. These establish the induced forces, torques, accelerations etc that the assembly has to withstand in order to satisfy the JOIs, so as not to constrain the operating limits unnecessarily. The technical control process, part of the UKAEA QA system, identifies the key interfaces - hardware, software, people - and nominates suitably experienced individuals to contribute to the design. This ensures no incompatibilities of the new items of plant with the old. Further checks are provided by “configuration control”. This is a steadily growing 3D CAD model of the machine, pre-empting clashes between pieces of hardware, or between hardware and plasma diagnostic lines of sight. This design control system will also be described, using as examples the new ITER-Like Wall and Neutral Beam Enhancement projects, due for implementation by 2009. |