Toroidal field magnet energized

The toroidal field (TF) magnet provides the primary confinement for the JT-60SA plasma. The 18 D-shaped coils are designed to operate at 25.7kA and produce together a field of 2.25 Tesla at the toroidal axis running around the tokamak. The corresponding energy stored in the toroidal magnetic field reaches 1.06 GJ.

After completing the vacuum vessel bake at about 200°C and returning the vacuum vessel to its operating temperature of 50°C on 02 August (and confirming it is still leak-free!), and after performing high voltage tests on the magnets under their operating vacuum condition, TF energization began on 07 August.

The first step was to “balance”, or adjust, the quench detectors during repeated ramps up to 3kA. The voltage across each of the 18 individual TF coils is continually compared to that across its neighbour in order to detect any loss of superconductivity (known as a quench). Each measurement is made using a sensitive bridge circuit which should detect any change in resistance but should not react to the inductive voltages arising when the current in the coils is changed.

Next a comprehensive functional test of the quench detectors was made. This was achieved by deliberately warming up some of the coils just enough for them to lose superconductivity and become resistive. Then the current applied to the magnet was gradually increased. As soon as the threshold for quench detection was reached, the quench protection circuits (QPC) were activated. This shuts down the magnet power supply and opens bypass switches forcing the TF current into large resistors to rapidly release the huge magnetic energy. This test was repeated until each of the 9 pairs of quench detectors had triggered correctly (each pair of coils has 2 detectors for redundancy).

Then the current in the magnet could be progressively increased, with test operation of the QPC at each step. The energy in the magnet increases with the square of the current, and it is important to confirm that the energy released during QPC operation can be safely handled. The QPC is so important to protect the magnet that each bypass switch has its own backup pyrobreaker: an explosive charge that can intervene to interrupt the current in case the bypass switch fails. Operation of a pyrobreaker was deliberately tested during QPC operation at 15 kA on 21 August.

QPC operation (or to a greater extent an actual magnet quench) is also disruptive for the cryogenic system due to the sudden additional heat load generated. Hence in preparation for QPC operation at higher currents tests have also been made to trial temporarily disconnecting the TF magnet cooling loop from the cryoplant, and to check that when the pressure in the loop increases the automatic valves relieve the pressure correctly by directing helium to the quench tank.

Confirming the correct operation of all the processes designed to protect the tokamak is a key part of its commissioning.