Plasma engineering R&D

In a fusion power plant, the plasma needs to be monitored and controlled for a sustained period of time. To achieve this end, the knowledge on the behaviour of plasmas gained in the multitude of different plasma experiments, needs to be translated in engineering solutions for the optimisation of the fusion conditions in the plasma. This involves a close collaboration of engineers and plasma physicists in the fields of heating systems, diagnostic development and plasma wall interactions studies.

Plasma heating

Plasma heating systems are an essential part of high temperature plasma engineering research as the fusion reaction would not continue if the plasma was not heated by an external source. In addition, the heating systems have become an essential tool for the control of instabilities in the plasma that can cause it to touch the surrounding structures, cooling the plasma and damaging the surfaces.

Figure 1:Radio-frequency
heating antenna © Image:ENEA

Three main types of heating systems are required in fusion experiments. With Ion Cyclotron Resonance Heating (ICRH), ions in the plasma are heated by a high-intensity beam of electromagnetic radiation with a frequency of 30 to 50MHz. The frequency of the beam is such that the ions in the plasma can absorb the energy.

With Electron Cyclotron Resonance Heating (ECRH), the electrons in the plasma are heated by a high-intensity beam with a frequency of 100 to 200MHz, the frequency being such that the electrons can absorb it. This system is also used to deposit heat in very specific places in the plasma, as a mechanism to minimize the build-up of certain instabilities that lead to cooling of the plasma. ECRH has the advantage that the beam can be transmitted through air which simplifies the design and allows the source to be far from the plasma, which makes maintenance simpler.

Figure 2:A gyrotron, which produces
one MW of continuous electromagnetic
power. © Image: IPP Garching

With Neutral Beam Injection a small amount of plasma is generated outside the fusion device. The plasma is then accelerated to a very high speed and neutralised so that the high energy particles can pass through the magnetic field and enter the fusion plasma. The fast particles transfer their energy to the plasma, with the result that the temperature of the plasma increases.

All these heating systems are capable of driving an electrical current in the plasma, which will ultimately be used to make a fusion power plant steady state.

Diagnostics

In a fusion reactor, instruments that measure a variety of parameters are needed to keep the plasma in an optimal condition, including temperature, density and the type of impurities present. Some plasma parameters are measured by multiple instruments using different methods.

Figure 3:ITER diagnostics

The most reliable way of measuring the temperature is by shining a very powerful laser beam into the plasma. The photons in the laser beam are scattered by the electrons and this scattered light can be measured.

The moving electrons change the properties of the photons (by Doppler shift), which gives a direct measurement of the speed and hence the temperature of the electron. The intensity of the reflected light gives the density of the plasma.

One method of measuring the level of impurities is to take measurements of the ultraviolet radiation emitted by the particles. Particles with different masses will radiate different wavelengths of ultraviolet since they have different excitation energies. Knowing the ultraviolet spectrum of the plasma therefore reveals the nature and amount of impurities present.

These are only two examples of the many diagnostic techniques developed in fusion research. Many of the techniques have found applications in fields outside fusion.

Plasma wall interaction studies

In order to remove the waste of the fusion process (helium) and impurities from the plasma, the plasma is allowed to touch the wall of the plasma vessel in a controlled way. This is achieved by shaping the magnetic field lines in such a way that they strike a special system designed to withstand a very high heat load - up to 20MW/m² - which is called the divertor system. The geometry of the magnetic field is such that the plasma near the vessel wall, which is prone to higher impurity levels from the walls, is removed via the divertor.

Figure 5:In the white (hot) regions, the
plasma interacts with the wall in JET.

Elsewhere, when the plasma makes contact with the vessel wall, the high energy of the particles will erode the surface. Studying and controlling this erosion process is a very important subject, as any impurities entering the core of the plasma will cool it and stop the fusion reaction. It is also very important to limit this erosion to maximise the lifetime of reactor components.

The material currently used as the target for the divertor is carbon, reinforced with carbon fibre. In addition to this critical part of the divertor design, it is also important to design the components in such a way that they can withstand the high mechanical loads experienced in the reactor chamber, allowing high vacuum pumping to remove the helium "ash" from the plasma and still perform after long exposure to neutron radiation which makes materials brittle and reduces their ability to conduct heat.