Posted on: 25th June 2012
Cleanliness is the key to many aspects of operation of a tokamak like JET, in common with other ultra-high vacuum systems. Since you can not gain access to the inside of the torus, a technique called ‘glow discharge cleaning’ is used in most machines to clean the walls of the vacuum vessel. To understand this technique, imagine a fluorescent light tube in your house. This device produces its light by exciting the gas molecules inside the tube. As a by-product, the excited molecules bombard the inside of the glass tube and have the effect of cleaning it. A similar technique is used in JET, using four high voltage direct current electrodes typically at 400 volts.
When the vacuum is good and the walls are clean enough, machines like JET operate in short pulses, switching the magnetic field on for up to 1 minute. This magnetic field is designed to keep the hot plasma contained, producing fusion reactions. However, future reactors such as the experimental reactor ITER currently under construction in the South of France will use superconducting coils for longer pulses. The magnetic field will stay on nearly all the time. Unfortunately, these permanent magnetic fields will render the standard glow discharge cleaning techniques ineffective.
To get over this, powerful antennae (typically used to heat the plasma to fusion relevant temperatures around a hundred million degrees) can be used at lower power to produce cleaning plasmas while the magnetic field is switched on. Several smaller devices have reported good success in using this radio-frequency cleaning technique. However, none of them have the same wall materials as ITER, and this is where JET’s unique features become important. The new ITER-like wall is built of the materials that are planned for ITER (beryllium and tungsten) and JET has suitable antennae to launch radio frequency waves into the machine.
During recent campaigns, experiments have been performed to prove this cleaning concept for the beryllium and tungsten walls. Identical experiments had been run before the old carbon wall was replaced, and the first indications from the physicists are that this will be an excellent candidate for use in ITER.
Following on from the popular Shutdown Weekly, this series aims to give an insight into day-to-day activities at JET, from an engineering perspective. It aims to explain the technical aspects of operation of the world’s most successful tokamak. JET’s new ITER-Like Wall is being exploited for the benefit of its future successor, ITER.