Plasma physics

Fusion on earth requires the fusion fuel to be heated to 150 million degrees. In that state, matter forms a plasma. For the operation of a fusion power plant, a thorough understanding is needed of the behaviour of this "fourth state of matter". European research focuses on using strong magnetic fields to keep a hot fusion plasma together, and to isolate it from material walls: so-called magnetic confinement. In the past two decades, significant progress has been made in the understanding of the basic principles of plasmas and magnetic confinement.

Magnetic confinement

At the very high temperatures required for fusion to occur, electrons are knocked of the atoms they belong to, leaving the atoms fully ionized. The resulting mixture of positively charged ions and negatively charged electrons is very different from a normal gas or liquid and is given a special name - plasma.

Figure 1:plasma follows magnetic field lines

Because a plasma is a mixture of charged particles, it can be controlled and influenced by magnetic fields. Charged plasma particles can move easily along magnetic field lines, but not across them. If the magnetic field is given the right shape, plasma particles can be kept together and away from the walls of the plasma vessel, in other words, the plasma can be confined.

Figure 2:plasma without and with magnetic field lines

The confining magnetic fields are generated by several sources: the first is strong superconducting coils located around the plasma vessel, and the second is electrical currents flowing in the plasma itself. By its very nature, plasma is an excellent conductor of electricity.

Although the use of magnetic fields for the confinement of plasma is a logical choice, it still leaves a lot of room for different designs of the plasma vessel. Early fusion experiments used cylindrical designs, flat plates, 8-shaped vessels, magnetic mirrors, etcetera. Although a lot of progress was made in that time, all of the designs suffered from loss of plasma particles and energy along the magnetic field lines and out through the ends of the vessel, and none of the designs could achieve the levels of thermal insulation required for fusion.

If one takes a cylindrical vessel with straight magnetic field lines, and bends it around to join the two ends, the field lines close on themselves, the plasma particles run around in circles, and thermal insulation is improved. One of the closed configurations obtained in this way is in the shape of a doughnut or torus. Several torus-shaped configurations for magnetic confinement devices have been studied - the most advanced of these is called the tokamak. Alternative torus-shaped devices also being studied include the stellarator, the reversed field pinch, and the spherical tokamak, which are all described in one of the next sections.

In Europe, researchers have benefited from the availability of a wide variety of plasma devices to develop their understanding of high temperature plasma physics. The different configurations and capabilities of the various plasma devices are used for studies into many different aspects of plasma behaviour, including how to optimise the confinement, transport and control of impurities in the plasma, control of instabilities in the plasma, diagnostics to measure plasma properties, the control of steady state plasmas, etcetera.

Figure 3:Hot plasma in the TEXTOR
tokamak in Forschungszentrum
Jülich, Germany.

In addition to the experimental topics mentioned above, there is a strong effort in the theoretical understanding of plasmas and the modelling of plasmas in the European fusion Associations. Theoretical understanding and modelling provide the tools to analyze the data from fusion experiments, help in the development of new plasma operation scenarios, and provide a deeper understanding of the complexity of the physical phenomena of plasmas.

Working with plasmas

There are three aspects of a fusion plasma that together determine the "quality" of the plasma: the temperature, the number of particles per volume (density), and the time span during which the plasma particles can be confined by the magnetic field. The product of these three characteristics defines the triple product, which is a figure-of-merit which measures the performance of a fusion plasma.

Figure 4:The progress of fusion research through the years, measured by the triple product, which is an indication of the performance of a fusion plasma. Please note the logarithmic scale on the vertical axis. F or comparison, the development of computer chips is indicated.

To control these conditions, scientists must know a lot about the plasma, for example, how well is it heated, how fast particles are lost from the plasma, how stable the plasma is in the magnetic field, how the plasma particles interact with the magnetic field, and how unwanted particles at the edge can be prevented from travelling back into the plasma. To answer these questions physicists must design specialised diagnostic tools for measurement as well as equipment to heat and control the plasma.

A major challenge in fusion research is to maintain the temperature of the plasma for the required amount of time. The plasma is constantly being cooled by impurities picked up from the reactor vessel wall, and by the leakage of hot plasma to the walls. Therefore, processes by which the walls of the plasma vessel are eroded and how these eroded particles are transported to the core of the plasma - so-called plasma-wall interaction - need to be understood.

A particular challenge with the tokamak principle, is that the plasma can become unstable producing a current in the metal structure that surrounds it. This current reacts with the magnetic field producing high forces on the components. In addition, when the plasma touches the surrounding structure it can erode the surface which can reduce the lifetime of the components.

The instabilities in the plasma that cause these events must be understood and controlled. Increasing computing power makes it possible to model this behaviour and to search for ways to prevent them from happening in future reactors. Physicists are working on several methods for controlling these instabilities, for instance, the injection of microwave energy in a way that stabilises disturbances in the plasma.

Figure 5:Left: the CUTIE model, developed by the British fusion lab in Culham, and applied to the Russian Tokamak T10 by FOM-Rijnhuizen. Right: the GYRO-model, developed by researchers of General Atomics (USA). The models compute characteristics of the plasma, such as density and the strength of electric fields.

An alternative approach to fusion involves inertial confinement. Here ultra-short, high-power laser or particle beam pulses are used to heat small frozen pellets of fusion fuel. The European fusion programme maintains a "keep in touch" activity in this field and monitors developments.