A clean, eco-friendly and virtually inexhaustible source of energy: using nuclear fusion to produce electricity has been the dream of countless scientists for many years. However, until recently it has not been possible to construct a fusion reactor robust enough to withstand the enormous heat flux in the fusion plasma. To investigate this problem, a unique measuring device has been developed at the Institute of Applied Physics at Vienna University of Technology: One of the world’s most accurate scales.

The quest for nuclear fusion

An international fusion reactor is currently being built in Cadarache, France. This is a vital step on the way towards commercially successful fusion power plants, but problems well known from earlier fusion tests are bound to arise: to sustain fusion inside a reactor, temperatures of several hundred million degrees Celsius have to be maintained. At these temperatures, charged particles (ions) from the fusion plasma can hit the reactor’s walls with such power that they will be destroyed very quickly. “The interaction of particles and the reactor surface has to be studied very accurately, if we are to ever build stable fusion reactors”, Professor Aumayr, from the Institute of Applied Physics at the Vienna University of Technology explains.

Testing surfaces with a microscale

Professor Aumayr’s research group for atomic and plasma physics reproduces the physical conditions on the fusion reactor’s walls in their experiments. In the laboratory, the interaction between high-energy ions and solid surfaces can be studied much more precisely than it ever could inside an actual fusion reactor. A key tool for this research is the quartz crystal microbalance, which was developed by Professor Michael Schmid, at the Vienna University of Technology. A small piece of the surface material, which is supposed to be used in the fusion reactor, is irradiated with high-energy particle beams, and tiny changes of its weight are measured with great accuracy. This way, one can determine whether the particle bombardment knocks atoms out of the surface, reducing the mass of the specimen, or whether the incident particles are instead implanted into the material, thereby increasing its mass.
 

Record-braking precision

The microscale developed at VUT is one of the world’s most accurate scales. “Mass changes of as little as one billionth of a gram can be measured”, says Katharina Dobes, research assistant at the Institute of Applied Physics. Even if the particle bombardment only removes one single atomic layer from the surface, the resulting change in mass can still be evaluated.
The fundamental idea behind this incredible precision is rather simple: a quartz crystal is vibrated and its resonance frequency is measured. This frequency depends very sensitively on the crystal’s mass. If the crystal surface is coated with the material under investigation and then hit by particles, the changing resonance frequency of the crystal can be translated in a mass change of the material on top of it. That way it is possible to determine the effect the particle bombardment has on the surface.

The application of this measuring device is not restricted to fusion research. “In particle-surface interactions, there are many quantum mechanical phenomena which play a crucial role. In this field, many interesting fundamental questions are yet to be answered”, Professor Aumayr believes.

The Vienna quartz-crystal-balance – a high precision tool for surface research
Once nuclear fusion is achieved – how can the walls of the reactor be made strong enough to widhstand the extreme energy flux of the plasma? Ion-surface interaction is one of the crucial fields in fusion research today.

Professor Aumayr’s research group for atomic and plasma physics at Vienna University of Technology reproduces the physical conditions on the fusion reactor’s walls in their experiments. A key tool for this research is the quartz crystal microbalance. A small piece of the surface material, which is supposed to be used in the fusion reactor, is irradiated with high-energy particle beams, and tiny changes of its weight are measured with great accuracy.

This microscale is one of the world’s most accurate scales. Mass changes of as little as one billionth of a gram can be measured. Even if the particle bombardment only removes one single atomic layer from the surface, the resulting change in mass can still be evaluated.
The fundamental idea behind this incredible precision is rather simple: a quartz crystal is vibrated and its resonance frequency is measured. This frequency depends very sensitively on the crystal’s mass. If the crystal surface is coated with the material under investigation and then hit by particles, the changing resonance frequency of the crystal can be translated in a mass change of the material on top of it.

The application of this measuring device is not restricted to fusion research. “In particle-surface interactions, there are many quantum mechanical phenomena which play a crucial role. In this field, many interesting fundamental questions are yet to be answered”, Professor Aumayr believes.