A pacemaker for nuclear fusion plasmas.

By Menno Lauret (in collaboration with Gert Witvoet, Federico Felici, Tim Goodman, Oliver Sauter, Gerd Vandersteen, Marco de Baar and the TCV team)

Increasing energy demands and depleting resources is one of the major challenges that humanity has to face. Moreover, the waste products of energy plants like CO2 or long term radioactive material lays a heavy burden on future generations.

Among the many alternatives for coal and nuclear fission plants, nuclear fusion is a unique approach to generate clean and reliable energy. Essentially, fusion energy uses the same physical mechanism that makes the sun scorching hot. If small and very hot sun-like plasma’s (ionized gas, with a temperature of around 100 million degrees Kelvin) could be reliable made  on earth, this could be used for the  generation of clean and reliable energy.

Physicists working on fusion energy have made great progress in the last 50 years. However, there are still some processes in the plasma that need to be solved before energy can be produced using this method.

The Control Systems Technology group at the Eindhoven University of Technology and the Dutch plasma fusion institute FOM DIFFER are actively involved in develloping controllers for magnetohydrodynamic modes in the plasma. One of these is the sawtooth instability, that leads to a periodic reorganistation of the core of the fusion plasma, and could affect the stability of the plasma as a whole.

The sawtooth period can be interpreted as the plasma’s heartbeat, and is considered essential for a lot of processes in the plasma. In current fusion experiments (so-called Tokamaks) the sawtooth beats around 100 times per second. To ensure the functionality of future fusion energy plants, it will be necessary to be able to regulate this heartbeat: to make it go faster or slower.

2D Measurements of the sawtooth cycle in the TEXTOR tokamak compared with theoretical predictions. The pictures in the middle two columns depict the temperature variations during the sawtooth instability as calculated with one of the theoretical models. The pictures in the columns a and b correspond to the left and the right hand sides of the plasma column, respectively. The pictures in the outermost columns depict the measurements for the corresponding situations in the development of the sawtooth. The area of measurement is indicated by the rectangles in the middle two columns.
Courtesy Dr. N.C. Luhmann, Prof. Dr. A. Donné, Dr. I. Classen and Dr. H.K. Park.

A surprisingly simple and new concept for controlling the sawtooth period mimics the way a pacemaker regulates the human heart beat. A traditional pacemaker gives a periodic electrical pulse to the heart, to which the heart synchronizes. To translate this idea to a Tokamak experiment, we could not use small electrical pulses, but a microwave source of several megawatts was available.

Joint experiments at the TCV experiment in Lausanne, Switzerland (in collaboration with EPFL, CRPP and VUB) showed that this simple idea works better than we could have imagined. The sawtooth period almost instantaneously changes to the period of the power pulse and we were able to make the sawtooth period four times slower. These experiments and simulations confirm that there is a surprisingly simple method to control the heartbeat of the plasma. Nuclear fusion plasma’s have very complex behavior, just as many biological systems, and biology can be a source of inspiration to make progress in fusion and hopefully to solve other related plasma problems.

Experimental demonstration of ‘sawtooth period locking.’ The sawtooth cycle in the plasma sychronizes quickly and robustly with the externally applied beat from our 1 MegaWatt pacemaker.

The work is explained in detail in two articles, Demonstration of sawtooth period locking with power modulation in TCV plasmas and  Numerical demonstration of injection locking of the sawtooth period by means of modulated EC current drive.