Posted on: 3rd September 2012
In the story of “The Princess and the Pea”, as told by Hans Christian Andersen, a princess is able to detect a pea in the bed she is lying on, despite a large pile of mattresses and feather quilts on top of it. However physicist Katharina Dobes can do much better: she can detect single layers of atoms, weighing a billionth of a milligram – around 10-12 of a pea. She does this not by lying on them, but by using a device known as a quartz crystal microbalance (QCM).
Ms Dobes is doing a PhD at the Institute of Applied Physics at the Vienna University of Technology, and the layers that she is studying are the surfaces of plasma facing materials, such as tungsten, carbon and beryllium. In her experiments these materials are bombarded with ions, which knocks off atoms, in a process known as sputtering. This process happens to plasma-facing materials in a fusion vessel – with the QCM Katharina can measure exactly how much sputtering occurs with each combination of wall material and plasma ion. This in turn gives vital information about how much of the wall material will contaminate the plasma.
Recent experiments have explored bombarding a tungsten wall with argon or nitrogen, because these have been used in JET to cool the edge of the plasma. These gases are much heavier than deuterium and so were expected to cause more sputtering of tungsten, which these measurements confirmed. “It was not a particularly disappointing result for fusion,” says Head of the Atomic and Plasma Physics Research Group, Professor Friedrich Aumayr. “but you don’t need bad surprises!”
The quartz crystal microbalance consists of a quartz crystal on to which layers are deposited. A vibration is then electronically induced in the crystal and the resonant frequency of the crystal measured. When atoms are sputtered off the crystal the resonant frequency changes, as it gets lighter. Although QCMs are not uncommon in the thin film manufacturing industry – for measuring deposition, not erosion – this particular experiment has achieved outstanding results. “QCMs can usually measure less than a micrometer,” says Professor Aumayr, “but thanks to a carefully controlled lab environment and specially built electronics we can detect less than one thousandth of a monolayer – fractions of a nanometer”
The next set of measurements will use beryllium as the wall material, however the Vienna University of Technology does not have a beryllium handling facility. This means, in keeping with the fairytale theme, to complete her quest Katharina Dobes will need pack the QCM up into a truck and journey to a faraway kingdom – the Institute for Plasma Physics in Garching, Germany.
The Institute of Applied Physics at the Vienna University of Technology is a signatory to EFDA through the Austrian Fusion Research Programme ÖAW-EURATOM. The Max-Planck-Institute for Plasma Physics in Garching is one of three German signatories to EFDA.