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Fusion science is still in the exploratory phase. While much research focuses on devices in the shape of a torus (doughnut), called tokamaks, spherical torus machines have recently emerged as an innovative example of fusion confinement. Spherical tori, smaller than conventional devices (tokamaks), offer particular advantages. For example, the construction and operating costs are lower and the physics parameters are more easily controlled. This leads to greater understanding of plasma behaviours, in particular with regard to the crucial issue of plasma stability, steady-state, and other aspects. Continued R&D focussing on alternative approaches such as spherical torus devices is needed in order to accelerate knowledge of fusion science and realisation of fusion power.
The aim of the ETI focusing on spherical fusion devices (ST) is to contribute to the scientific and technology knowledge of plasma confinement by strengthening co-operation among spherical torus research programmes and facilities worldwide. This information serves to broaden the understanding of fusion science for all devices worldwide. There are currently three Contracting Parties.
Due to the improved stability of an ST device, as compared to conventional tokamaks, the plasma has a higher pressure relative to the magnetic field pressure. For this reason, parameters for starting and maintaining the plasma will be different than for tokamak devices.
The aim of one current ST collaborative experiment, Science and R&D of Spherical Tori, is to increase understanding of the plasma start-up using the electron Bernstein wave (EBW).
The electron Bernstein wave (EBW) is a quasi-electrostatic wave that is typically used by spherical tori devices to heat the plasma and drive the current. In an ST, the plasma current that is needed to confine the plasma is driven by solenoid induction. Demonstrating ST operation without solenoid induction is one of the most important issues for the future of ST devices. The ST chose the MAST1 device to run collaborative experiments.
The MAST1 device was upgraded to include a high-power gyrotron2 capable of delivering up to 350 kilowatts (kW) of microwave power at 28 gigahertz (GHz) for 300 milliseconds. First tests to inject power into the plasma were limited by arcing between the gyrotron and the MAST device, limiting the pulse lengths.
This difficulty was remedied by adding several transmission line components such as corrugated waveguides and transitions. After installation and final verifications of the gyrotron performance, the EBW start-up experiments on MAST under this collaboration will resume in 2013.
1. Refers to the Mega Amp Spherical Tokamak device (United Kingdom).
2. The gyrotron was provided by the Oakridge National Laboratory (United States).
* Photo courtesy of Culham Centre for Fusion Energy.
For more information: The STI IA is under development.
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