Most of the electrical power consumed by JET is transferred into heat. The main reason for this is that all of JET’s massive coils which produce the strong, plasma confining magnetic fields are made from copper – and although copper is a very good conductor, it still has a small resistance to the electric current. At the very high electric currents needed to achieve the strong magnetic fields, this resistance causes significant heating in all of JET’s coils. They must be continuously cooled down to prevent overheating of the facility.
During the plasma discharge, which typically lasts for 20 seconds, the temperature of the coils increases sharply. The cooling system has been designed so that after each discharge the facility can be cooled down in 15 minutes, to match the similar time intervals required to spin up the flywheel generators and to download and save all data acquired from the JET diagnostic systems.
Most of JET’s coils are cooled by water. The cooling water needs ongoing effective demineralisation and deionisation in order to keep its conductivity very low, and therefore a special water treatment facility has been installed at JET. However, in the event of leaks, the water becomes re-ionised and conductive after a short period, so that it can cause electrical short-circuits. That is why the most vulnerable areas – the toroidal and divertor coils – use a special cooling liquid (Galden), a non-flammable heat transfer fluid that maintains its high resistance under all conditions.
Tens of powerful pumps that force the circulation of the liquids can be found just below the JET Torus Hall, in its basement area.
Deionised water and Galden circulate in closed loops and exchange their heat with the main water circuit in heat exchangers that are situated next to the pumps and look like large engine radiators. The main water circuit then carries the excess heat to JET’s four cooling towers, each with a two speed fan. Although these towers are very small in comparison to the cooling towers of nearby Didcot Power Plant, they still have a significant capacity of 4 x 35 MW corresponding to 4 x 1000 m3 of water per hour cooled down from 50 °C to approximately 20 °C.
Next to the towers are five large pumps which drive the water circulation in the main circuit – one per tower, the fifth is spare – each with 200 kilowatts power and operating on 3300 Volts. An additional booster pump supports the water flow at the far end of the main circuit.
To further boost the performance of JET – whenever high magnetic fields or long discharges are required – two massive 3 megawatt chillers (large refrigerators) have been installed on site. When operational, these chillers are connected to the heat exchanger of the Galden-cooled units (toroidal field coils and divertor coils), replacing the main water circuit. The chillers can push the temperature of the Galden fluid down to 12 °C.
The JET magnetic field coils are not the only system in need for cooling. Other ‘customers’ are the Neutral Beam Injectors, principally for their ion dumps and deflection magnets and the JET flywheel generators. In addition, many minor systems need to be connected to the cooling pipework, including the air conditioning plant, cryogenic plant and individual plasma diagnostics.