Imaging and passive spectroscopy
The most straightforward optical techniques that can be used for plasma diagnostics are optical imaging and emission spectroscopy. They both use the radiation that is emitted by the plasma itself to gather information about conditions and processes within the plasma. In the group EPG, a variety of techniques is used for both imaging and spectroscopy.
With passive imaging we can observe the shape and intensity profile of a plasma. This is simple, but often vital information that tells a lot about processes inside the plasma. For example, the rate of constriction of an arc in an HID lamp reveals a lot about the heat conduction processes in the plasma.
Many of our imaging is done time- or phase-resolved. This is achieved by means of fast intensified CCD cameras, which can deliver detailed imagery with sub-nanosecond resolution. Such techniques are applied to fast transient discharges like streamer and spark discharges or the ignition of HID lamps. The resulting image sequences show the evolution of the discharge.
sequences show the evolution of the discharge.
The cameras that are employed in our group are sensitive from the near UV (~250 nm) to the near infrared (~800 nm). Occasionally we use wavelength filters to study emissions in only a small band of the EM spectrum to get information about the distribution of specific species in the plasma.
A disadvantage of simple passive imaging is that it only shows the 2D projection of a 3D plasma. For plasmas with complex 3D patterns, like a streamer discharge, this can lead to interpretation difficulties. For example, when two streamer paths seem to cross in a 2D image, it cannot be determined whether this is really the case. To avoid such problems we employ a stereoscopic technique were we use some optics to get two images of the discharge (from different observation angles) onto a single ICCD camera. From these two images we can reconstruct the real 3D structure of the original discharge and, for example, get insight into the interaction between the different channels.