Firstly: Produce plasma

Plasma and its heating

Fusion devices need a plasma with a temperature of hundreds of millions of degrees Celsius. “Plasma” is composed of nuclei and electrons moving independently from each other.

Obtaining these extraordinary high temperatures requires extraordinarily powerful heating. This is done by Neutral Beam Injection Heating (NBI) and Radio Frequency Heating (RF). The total input power to these systems can be up to 250 megawatts. The installed output power is 55 megawatts of the radio frequency power. Powerful heating is also needed to sustain this temperature, otherwise the plasma would rapidly cool down due to inevitable heat losses via radiation and heat convection or conduction.

The working gas for the plasma experiment – usually deuterium (heavy hydrogen isotope), occasionally protium (common light hydrogen isotope), helium and, in special campaigns, tritium – are puffed in just before and during the plasma discharge in accordance with the real-time plasma control requirements.

Secondly: Feed the coils

JET is a magnetic confinement device. It uses magnetic fields to keep the charged particles of the plasma away from the vessel wall and to protect it from the hot plasma. The temperature gradient from the vessel wall to the plasma centre amounts to about one million degrees per centimetre. If the plasma was not confined and well insulated by a magnetic field, it would loose a lot of energy due to this high gradient.

The coils producing the strong magnetic fields need a significant amount of power. With the high currents normally required, the electrical resistance of the coils causes significant losses of energy in form of heat, which is why they need to be water-cooled. The energy to do so is mostly dissipated to the atmosphere via special cooling towers. Some fusion experiments therefore use superconducting coils that avoid energy losses at the expense of running them at very low temperatures, around -270 degree Celsius, using liquid helium. These experiments will run with higher energy efficiency.

Thirdly: Produce pulses

diagram of JET's power loads

JET's power loads

Every individual experiment at JET – called “pulse” – lasts several tens of seconds. During experimental campaigns there are some 30 experiments daily. Running a JET pulse requires around 500 megawatts of power, of which more than 300 is fed to the coils. The rest runs the additional heating sources.
Most of the JET power consumption is concentrated in short bursts, which is quite demanding on the electricity grid and on electrical engineering in general. Moreover, even during a single pulse, the power requirements are not constant – the start-up needs more power than the “plateau”, the sustaining phase. The plasma is always in the move and after it has been created, its position and shape is feedback-controlled by a subset of coils. The magnetic field is continuously measured, and additional power is supplied to the vertical and horizontal poloidal field amplifiers according to plasma behaviour.

The energy conversion efficiencies of all heating systems limit the power the plasma receives. However, in most JET pulses only part of these installed capacities is exploited, depending on experimental scenarios. Last but not least, the plasma also gets a few megawatts of power from ohmic heating. Ohmic heating means electric current induced in the plasma by the inner poloidal coils. In total, JET plasmas usually consume a few tens of megawatts and accumulate only a fraction of the consumed energy. The difference between input and output disappear via radiation, heat conduction and particle losses.