GRAVARC experiment set-up before integration in one of the LDC gondolas.
The GRAVARC experiment is masterminded by two PhD students from Masaryk University, Czech Republic. It studies the behaviour of a gliding arc electrical discharge under different levels of gravity.
The experiment was performed on the Large Diameter Centrifuge, located at ESA’s ESTEC facility in the Netherlands. The 8-metre-wide centrifuge is capable of simulating anything up to 20 times the pull of Earth’s gravity.
In the GRAVARC apparatus, the gliding electrical arc is formed between two diverging electrodes. It starts at the smallest distance between the electrodes and glides along towards the wider part. When the maximum lengths of the gliding arc is reached, the discharge will be quenched and the cycle repeated. The purpose of the team’s experiment was to study the effect of gravity on the shape, intensity, colour and emission spectra of the discharge.
GRAVARC students recovering their carbon nano structures.
They also wanted to study the effect of the carbon nano material that was produced in the discharge. “This was totally curiosity-driven research,” says team member Jiri Sperka, “There has been a lot of research into electrical arcs but not so much into gliding arcs.”
They discovered that the arc glided faster in conditions of higher levels of gravity. Arcing can occur in technological systems such as those found on aeroplanes and spacecraft. Knowing the conditions that start the arc, and how it will develop once it has started could be a very important safety concern.
Also, during launches, there is so much acceleration required to lift a spacecraft into orbit that different gravitational conditions prevail. Experiments such as GRAVARC can show how gliding electrical arcs behave in such situations.
The team is now working on computer models to describe the behaviour of the gliding arc. This is a challenging task because the plasma from the arc mingles with the surrounding gas, creating a complex mixture.
GRAVARC student operating their hypergravity experiment.
The second part of the experiment was to study the nano structures built in the gas surrounding the arc. This happens because the arc heats up the gas, turning it into a plasma in which electron particles are removed from the atoms. By embedding the electrodes in methane-rich gas this promotes the formation of carbon nano structures.
Such methods are widespread in the synthetic production of carbon nano materials. The team investigated whether higher gravity affected the process. They found that there was a distinct change in the structures that were built at 1g , 6g and 15g. Both surface growth and volume growth were observed at the higher gravity levels.
The team has published these results in a paper in Materials Research Bulletin and are now incorporating them into their PhD theses.
“It was a very special opportunity to perform an experiment at such an amazing facility,” says Sperka.