1 MA plasma current

The commissioning of the Equilibrium Field (EF) coils and Central Solenoid (CS) modules was completed up to 3 kA in October. Besides confirming that the each coil could be safely discharged by the quench protection circuits from plus or minus 3 kA, tests were also made on each coil to confirm the rapid response of the coils when high voltages were applied.

Next the number of coils operated at the same time was gradually increased. After all ten EF and CS coils had been operated at the same time in a similar way to how they would be run during plasma operations, then similar combination tests were performed with the Toroidal Field (TF) magnet also energized. Finally fully integrated tokamak operation to produce tokamak plasma began.

Electron Cyclotron Resonance Heating (ECRH) is used to assist the initiation of the plasma, not least because to avoid any unnecessary risk operation is starting with limited voltage applied to the EF and CS coils. After a small quantity of helium gas is admitted into the plasma vessel, gigahertz frequency microwave radiation produced by powerful gyrotrons is used to help ionise it.

The first tokamak plasma was unambiguously produced on 23 October, with a plasma current of about 130 kA. Since then numerous tests have been made to optimise the plasma breakdown, to increase the plasma current and to optimise control of the plasma current, shape and position.

In early November “diverted” plasmas were produced. In this configuration the magnetic field is arranged with a characteristic “X-point” null with the effect that particles escaping the magnetic confinement are “diverted” into a specific region. This contributes to higher plasma purity, more effective confinement and ultimately higher performance.

By increasing the EF and CS coil currents to 5 kA it was possible to further increase the plasma current and its duration. During the Inauguration Ceremony on 01 December a divertor plasma with one million amps of current was demonstrated. See if you can spot the X-point in the movie!