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Laboratory for Plasma Physics - ERM/KMS

The Laboratory for Plasma Physics (LPP) was founded in 1961 by Prof. Paul Vandenplas. In 1969 the laboratory and the Département de Physique Statistique, Plasmas & Optique Non-linéaire of the Université Libre de Bruxelles, became the founding members of the Association "EURATOM - Belgian State". The former was specialized in the interaction of electromagnetic waves with charged particles, while the latter was more focussed on the theoretical description of transport processes in plasmas. A few years later, the Studiecentrum voor Kernenergie / Centre d'Etudes Nucléaires, of Mol joined the Association. Equally, experiments on LPP's small size tokamak ERASMUS, the first university tokamak in Europe, were initiated. From 1981 onwards, activity on ERASMUS faded while an intense collaboration with the Institut für Plasmaphysik in Jülich (Germany) was developed around the TEXTOR tokamak. On TEXTOR, the Laboratory for Plasma Physics took the responsibility of designing and exploiting the radio frequency heating system. In 1996 the Trilateral Euregio Cluster (TEC) came to life. This cluster of 3 fusion research laboratories (LPP-ERM/KMS, IPP-FZJ and the Dutch FOM Institute "Rijnhuizen") collaborates on the TEXTOR tokamak.

Right from the start, the Laboratory for Plasma Physics has been focused mainly on the study and exploitation of radio frequency heating. Not only the theoretical aspects of the interaction of electromagnetic waves and charged particles are studied, but also the experimental side has received lots of attention. The laboratory has a long-standing tradition of designing RF antenna, transmission lines as well as matching systems, and of operating RF heating installations. The design of the ITER antennae is e.g. partly in the hands of Belgian researchers.

The laboratory is also involved in the understanding of the influence on tokamak confinement of radiative mantles and edge electric fields, and contributes to developing diagnostics. For some details, take a look at "Made in LPP-ERM/KMS"

Head of Research Unit: R. Weynants

For more info, see http://fusion.rma.ac.be.

Made in LPP-ERM/KMS

Plasmas can be heated by radiating electromagnetic waves into them. The Laboratory for Plasma Physics is specialized in radio frequency or ion cyclotron resonance heating. With the presently existing technology, powerful magnets can be produced which confine ions by forcing them to execute helicoidal trajectories around magnetic field lines. The frequency with which the ions circulate around the field lines is known as the cyclotron frequency. For magnetic fields capable of confining ion particles that are energetic enough to fuse together, this frequency is in the range of a few tens of MHz, which is the domain of frequencies adopted for radio transmission. Exciting an antenna just outside the plasma at this frequency produces waves that penetrate the plasma and that are absorbed by the ions at the position where the antenna frequency matches the cyclotron frequency. The underlying principle rests on the resonant interaction of the wave with charged ion particles. Roughly speaking, this goes as follows. A wave can only accelerate a particle when the force associated with it is parallel to the particle's velocity. Although the wave's force is not constant when the wave is excited by a high-frequency generator, the fact that it oscillates at exactly the same frequency as that with which the particles gyrate around their guiding rail, makes that the particles "see" a constant force on their path which causes them to accelerate (strictly, the total force is composed of a constant and an oscillating part). Because the magnetic field in a tokamak monotonically grows towards its symmetry axis, this heating scheme can indeed be "tuned" to heat the ions at a predetermined position by changing the antenna frequency or the magnetic field strength.

The JET A2 antenna. Click on the image to enlarge.

In the Laboratory for Plasma Physics radio frequency heating is studied both theoretically and experimentally. One of the laboratory's tasks is the design of RF antennae. The left figure shows the antenna presently in use in the JET tokamak, and the right figure below shows the antenna designed for the ITER machine. The actual antenna is hidden under a Faraday screen, which is a set of densely spaced narrow conducting straps. The purpose of such a screen is to avoid the excitation of unwanted waves. By lining up conductors in a plane in one direction, the electrical conductivity in that direction is large while that in the perpendicular direction in the plane of the blades is zero. This allows sealing waves that propagate parallel rather than perpendicular to the confining magnetic field inside the antenna box. As they follow the magnetic field lines near the antennae, these undesirable waves tend to remain close to the machine wall and to accelerate particles there rather than heating the core. Underneath the screen, the large stainless steel straps that constitute the main antenna components are visible. These straps are fed via transmission lines and carry an alternating current which flips sign a few tens of millions of times each second.

The ITER antenna. Click on the image to enlarge.

Transmission lines are coaxial lines via which the radio frequency power is carried from the generator to the antennae. The picture below shows a part of the transmission line of the TEXTOR heating system, a detail of the automatic tuning system and a picture of the antennae. Two antennae are sitting side-by side. A phase difference is applied between the antennae to ensure optimal damping of the waves. The reason is that the complete resonance condition does not just require the wave frequency to coincide with the cyclotron frequency: Just like the sound of an approaching ambulance differs from that of a receding one because the wave fronts (maxima and minima of the sound wave of the ambulance's siren) hit you with smaller time intervals than they were sent when the ambulance comes towards you (the later waves having to cover a smaller distance to reach you than the earlier ones) and hit you with larger intervals than sent in the case the ambulance drives away (the later waves then having to cover an even longer distance than the earlier ones), the frequency has to be corrected by a so-called Doppler-shift to account for the fact that the wave and the particle move with respect to one another. One consequence is that the power deposition is somewhat spread out, the location where the resonance condition is satisfied being shifted a bit when the relative velocity is a bit different.

Part of the transmission lines of the TEXTOR RF heating system. Click on the image to enlarge.

Aside from its main expertise of radio frequency heating, the Laboratory for Plasma Physics contributes to other research topics. After having been tested on the TEXTOR tokamak, the potential of the "radiative improved" tokamak operation mode was examined on various other machines, including JET. The radiative improved or RI mode has the potential to become the operation mode of the future fusion power station since it combines a good confinement of the plasma (and thus of the fuels that need to be contained in the vessel long enough for them to fuse) with relaxed requirements for the wall, in particular the divertor. The "divertor" concept was originally developed on the ASDEX tokamak in Garching. It gave rise to a significant improvement of the confinement (a regime henceforth labeled as "H" or "high confinement" mode) but required the development of materials capable of withstanding large heat and particle fluxes. The idea of the divertor is to divert particles away from the main plasma by changing the magnetic topology of the plasma edge, creating guiding rails for the charged particles that intersect the divertor plates (as opposed to the helicoidal guiding rails in the plasma core which never intersect the wall). The RI-mode relaxes the thermal stress on the divertor by adopting traces of impurities that form a mantle near the plasma edge in which a fraction of the power that is otherwise carried by diverted particles is radiated away.

Automatic tuning module of the TEXTOR RF heating system. Click on the image to enlarge.

A pair of TEXTOR RF antennae. Click on the image to enlarge.

Another point of interest of the laboratory is related to confinement: ever since the H-mode has been discovered on ASDEX, there is a wealth of data available that makes a link between the confinement properties of the tokamak and the toroidal and poloidal rotation that is observed in the outermost plasma layers. Both theoretical and experimental efforts are devoted to understanding this intricate link. Because this rotation is associated with the formation of a (static) radial electric field, "biasing" experiments have been carried out that aimed at producing such an electric field using edge probes and studying the impact the field has on the plasma dynamics. In TEXTOR, the DED (dynamic ergodic divertor) has been installed both to excite low frequency rotating electromagnetic waves in the plasma edge and to perturb the magnetic field. The aim of the former is to transfer momentum to the charged edge particles and cause the edge to rotate, while that of the latter is to modify the edge turbulence (thought to be controlled by the plasma constituents themselves on account of the various moving charged particles interacting with one another directly via Coulomb collisions or indirectly by setting up electric fields resulting from local transient charge separations). Both are impacting on the confinement quality of the discharge.