PhD-candidate Gert Witvoet has developed smart control strategies for the so-called sawtooth instability, an always present 'hiccup' in the hot plasma of a nuclear fusion reactor. The frequency of the hiccups influences the reactor's efficiency. Witvoet, a PhD student in the Control Systems Technology Group, developed a model and control system for the sawtooth together with researchers from the Dutch Institute for Plasma Physics FOM-Rijnhuizen. He tested his microwave system succesfully with experiments at three different European fusion reactors and published five articles in leading fusion journals. Gert Witvoet will defend his PhD thesis today, 21 December 2011.

Optimal strategy
The sawtooth instability is a complex interplay of forces in the plasma, but Gert Witvoet managed to describe the plasma physics in a relatively simple model. His model simulates the sawtooth behaviour and Witvoet used an engineer's approach to characterise the sawtooth. This lead to insights on ways to control the sawtooth period with microwave radiation. Witvoet's analyses show the hurdles in this task and helped determine how to jump those hurdles with advanced control techniques. The end result is a way to quickly tune the sawtooth frequency exactly to a desired value.

New approach – injection locking
The dynamical analyses Gert Witvoet performed during his PhD lead to a new approach for control of the sawtooth frequency in a fusion plasma. This technique, called injection locking, massages the plasma with pulsed electromagnetic waves. The bombardement makes the sawtooth hiccups occur in the same frequency as the electromagnetic waves. This method is a novelty in sawtooth research and has been demonstrated on an experimental tokamak in Switzerland.

Gert Witvoet's PhD research has led to five publications in leading scientific journals on nuclear fusion. The expectation is that his results and insights will be of use to the design and use of ITER. This advanced experimental fusion reactor has been designed to produce ten times the power from fusion than the machine consumes. This will prove the technical feasibility of fusion as a clean, safe and sustainable energy source. ITER is being built by a worldwide collaboration of science and industry at Cadarache in southern France. The experiments will start in 2020.

Fusion as an energy source
Fusion is the energy source of the sun and the stars. Under high enough temperatures, light atomic nuclei will fuse together and produce energy in the form of heat. On earth, the reaction occurs in a plasma and is performed in a magnetic cage. A magnetic confinement reactor or tokamak is regarded as the most likely path to fusion energy. It fuses together hydrogen isotopes into helium at temperatures of 150 million degrees. The energy released is then carried away by cooling systems and converted into electricity via turbines.