SKEDSOFT

Absolute intensities of spectral lines

An example of a passive diagnostic technique is the absolute measurement of intensities of spectral lines. If the intensity of radiation of a certain wavelength is known, the density of the excited state that emits this radiation can be determined. The experimental setup for these measurements is very simple. The plasma is imaged onto the entrance slit of a spectrograph that selects the desired spectral line. The intensity is measured with a Photo Multiplier Tube (PMT) or, if time resolution is not important, a CCD camera which offers the possibility of measurement at several positions in the plasma at the same time. In order to make measurements absolute, the system is calibrated with a known light source: a Tungsten ribbon lamp.

Experimental setup used for Absolute Line Intensity (ALI) measurements

Absolute line intensity measurements can be used to measure the Atomic State Distribution function (ASDF). In general, the occupation of excited levels depends on their energies, and the electron density and temperature. Therefore, from the ASDF the electron density and temperature can in principle be derived. This is, however, only true in plasmas which are not too far from (Boltzmann and Saha) equilibrium. When applying simple passive diagnostic methods like this to small scale plasmas, the need for assumptions on equilibrium departure is always a serious limitation in the measurement of electron density and temperature. However, when these quantities are known from other experimental techniques, like for instance Thomson Scattering, ALI measurements can provide information about equilibrium departure, excitation kinetics and transport phenomena in the plasma and is in that sense a valuable diagnostic tool.

One disadvantage of this kind of measurements is that not only the radiation emitted at one position in the plasma (the focal point of imaging optics) is detected, but in fact all the atoms in the cone from the lenses to the focal point, and beyond, will contribute. Therefore, such measurements are commonly reffered to as "line of sight" measurements. In order to be able to say something about the emission coefficient at a certain position in the plasma, one has to measure line of sight intensities at different positions and in different directions. The local distribution is then to be reconstructed with tomographical methods. In case of a cylindrical symmetry of the plasma, which is true in most of our plasmas, the local emission coeffients can be determined in a simplified tomographical procedure known as "Abel Inversion".

Sketch of the technique of Abel Inversion. When I(y) is measured for all values of y, I(r) can be calculated.