Background Algorithms

When analyzing Thomson scattering data it is common for the desired signal to be sitting on a background signal which must be accounted for. This background tends to come in two flavors, bremsstrahlung background and other Thomson signals. Occasionaly other sources may be observed such as line emission or stray light.

TSADAR only attempts to account for bremsstrahlung background. Other sources of background should be windowed out using the spectral fit range options.

Three algorithms are included in TSADAR and accesible through the data:background:type field. These are shot, pixel, and fit.

Shot background

The shot option for the data:background:type field can be selected if a dedicate background shot was taken with no probe beam. In this case the background shot number should be supplied to the data:background:slice field. This algorithm is quite simple, A small smoothing kernel is applied to the background data and corresponding lineouts are taken in order to provide backgounds for each lineout.

Pixel background

This is the workhorse algorithm for time resolved data which tends to have lower background levels. This algorithm can be called by supplying pixel to the data:background:type field and the pixel location where the background is to be analyzed should be supplied to to the data:background:slice field.

The pixel algorithm takes a group of lineouts centered at the specified location with a width 2* data:dpixel +1 and averages them. This background lineout is then smoothed. The smoothing is done with a moving average whose window size is set with the data:background:bg_smoothing_window. At this point the algorithm divereges for the EPW and IAW. For IAW the spectral range is very small and very little spectral dependence is usualy seen in the background, so the background is averaged again to produce a scalar background value. For the EPW there is far more spectral dependence so the smothed background used as is. Finaly this idealized background is scaled in magnitude to match the data at that lineout using the endges of the data.

Fit background

The fitted background only applies to the EPW data, if applied to IAW data or combined fits the pixel algorithm will be applied to the IAW. When applied to the EPW this algorithm attempts to fit a theoretical model to the edges of the data outside the EPW allowing interpolation of the background into regions with data. This is adventageous for imaging data where the bremstruhlung can change singificantly as a function of space and there may be no location with background and no data. The domain for this fit must not include any Thomson scattering data or contamination of Thomson scattering of other beams.

Multiple fit models are availible for this and specified with the field data:background:bg_alg. The options are rat11, rat21, exp2 and power2. Each of these has 3 or 4 fitting parameters whose inital guesses must be supplied with the data:background:bg_alg_params field. If there are stray sources of light or the fit is seeing the Thomson scattering signal, the domain of the fit can be altered with data:background:bg_alg_domain. This field has 4 values, and the domain is constructed as a linear domain between the first two pixel values and the second two.