Photo Credit © ITER Organization,
Photo Credit © ITER Organization,


ITER, the next step on the road to provide fusion power as a virtually limitless energy source, is a tokamak experimental fusion reactor. Here, strong axisymmetric magnetic fields confine the hot plasma, with temperatures of about 150 million degrees or 10 times the temperature in the centre of the sun.

The tokamak configuration consists of strong poloidal and toroidal fields. External poloidal coils create the toroidal field while a toroidal current creates the poloidal field. In the basic configuration of the tokamak this current is inductively driven by a transformer which results in pulsed operation. A future fusion reactor will most like be operated in a steady-state configuration where the toroidal current will be sustained non-inductively. However, this so called advanced tokamak operation gives rise to a set of global mode instabilities which must be controlled.

One possible way of controlling these global modes is by adding a weak, non-symmetric 3D magnetic field through a set of external coils. ITER will be supplied with such coils but there is still a lot to be learned about these modes and how to control them best.

The EXTRAP-T2R, a medium-sized fusion experiment of the reversed-field pinch configuration and located at the Royal Institute of Technology, gives excellent opportunities to study global mode instabilities. The reversed-field pinch is inherently more unstable than the tokamak and hosts a range of so called resistive wall modes, a type of global modes. The experiment is equipped with 4 times 32 sensor and control coils in poloidal and toroidal direction, respectively, to give full coverage of the torus. EXTRAP-T2R was the first fusion experiment to show that global modes can be controlled by applying a weak 3D magnetic field.

The purpose of the project is to further study the physics of the resistive wall modes and to optimize the active control system. The work will be initiated at EXTRAP-T2R, utilizing the extensive coil coverage and feedback system, before tested at the ASDEX Upgrade fusion experiment, a tokamak designed to study and prepare for advanced operation scenarios at ITER.

The interaction between the plasma and the applied magnetic field is complicated and not fully understood. The interaction affects the plasma flow velocity as well as the radial electric field, quantities that are important for the global mode instabilities. The aim is to measure these quantities in such a way that they may be added to the feedback control system. Doppler Spectroscopy is the common method to measure plasma flow velocity. However, this method includes data processing such as spectral line fitting and must therefore be adapted to create an online signal suitable for input to an active feedback system. The radial electric field is usually measured with probes but this method is limited to the plasma edge and also disturbs the plasma. Another possibility is to measure the electric field without disturbing the plasma by means of Stark Spectroscopy. The method is not limited to the plasma edge and can in principle give the radial profile as well as the direction of the electric field.

Possible degree projects include

Investigation and implementation of a fast Doppler spectroscopy plasma flow velocity measurement system suitable for active feedback control

Investigation of Stark Spectroscopy for radial electric field measurements