In total, JET’s power supply system has an installed capacity approaching 1400 megawatt, a significant proportion of the maximum output of a large power station. However, not all of the installed capacity is necessary when an experiment is run – the different systems rather serve as a portfolio of many options on how we may produce various plasma conditions.
JET, as a large tokamak with pulses extending at times beyond twenty seconds, needs 10 gigajoule of energy per pulse – the peak power requirements exceed 1000 megawatts, but JET is limited to drawing 575 megawatts from the grid. It thus employs two massive flywheel generators to accumulate stored energy during the breaks between pulses.
The rotating part (rotor) of each flywheel generator is 9 metres in diameter and weighs 775 tons, much of which is concentrated on the rim to form a large flywheel. For experts – the total moment of inertia is 13.5 million kg/m2 per flywheel.Before each pulse the flywheel is accelerated by its 8.8 megawatts electric motor (even high-speed trains like Eurostar or TGV have less powerful motors.) Each flywheel can be spun up to 225 rounds per minute (3.75 hertz) so that the edge of the flywheel rotates at the speed of 380 kilometres per hour (236 miles per hour ).
Each flywheel generator is capable of providing 3750 megajoule of energy for the JET pulsed power systems, with a maximum of 400 megawatts power output. One generator supplies the toroidal field coils, another the poloidal field coils. The remaining power required during the pulse – namely part of the toroidal coils’ consumption and all the additional heating – is obtained directly from the national grid.
However, when the UK public’s electricity consumption hits a peak, the National Grid operator can quickly inhibit the operation of JET in order to prevent overload of its power plants. Our scientists, being naturally very curious people, have tried to find out when these periods of “JET blackouts” are likely to occur. To our surprise, the intervals of TV advertising spots that are broadcast in the middle of highly popular programmes (e.g. Coronation Street, football finals etc.) are common causes of delays in JET’s evening operations. Presumably adverts cause millions of viewers to switch on their kettles all at the same time!