Spectrograph Captures Data in Free Fall

Daniel C. McCarthy

Aerial stunts are not typically associated with pure physics research. However, a group of physicists studying fullerenes -- the nanoscale cagelike structures constructed of carbon molecules -- have participated in several parabolic aircraft flights organized by the European Space Agency.

On the downward pitch of the parabolic trajectory, zero gravity is created in the aircraft for about 20 seconds. During this time, the scientists performed spectroscopic analyses to understand gravity's effect on a carbon arc plasma in which fullerenes are formed. The unique nature of the experiments led the researchers to use a multichannel spectrograph from Multichannel Instruments AB.

The study extends to formation of other carbon nanostructures, such as nanotubes and diamond, according to Jean-Michel Beuken, a researcher on material sciences and processes at the Université Catholique de Louvain. But the main focus is the carbon arc plasma fabrication technique in which the structures form. In this process, the arc discharge produced between two carbon electrodes placed 1 to 2 mm apart can reach temperatures of 3700 °C. The plasma of this arc forms cylindrical deposits on the surface of the cathode in a process that is poorly understood and, as described by Beuken, almost magical.

The plasma is a mixture of neutral helium atoms, ionized carbon ions and molecules, and neutral carbon species. Gravity may influence the main physical parameters of the plasma, including its temperature distribution, density, stability and especially its shape. To chart this influence, the researchers measure the emission spectrum of plasma during the 20-second absence of gravity while the plane is diving. That requires analysis of as many spectra as possible at high resolution.

Multichannel Instruments' high-speed spectrometer captures the plasma's emission spectrum from 200 to 1100 nm. Under specific assumptions and using mathematical processes, the spectral measurements indicate plasma temperatures and compositions through the analysis of specific emission peaks.

The instrument delivers 18,000 data points for each spectrum with a rate of one spectrum for each second. That means 20 spectra will be captured during one zero gravity event. Another important feature is that the captured spectra are continuous, without gaps or overlapping wavelengths.

Beuken added that the instrument's absence of moving parts makes it especially apropos for microgravity environments.