'Fixgrid'

Sometimes, the x-values at which a given signal is sampled vary with time.

Often when this is the case, for example with TRANSP, an x-vector with two dimensions is provided to describe it -- one dimension represents the time, and the other one the value of x at which the corresponding index in the data vector the sampling was performed.

Many processing and visualisation packages (such as JETDSP and the PPF system) cannot handle a time-dependent x-vector like this, and a new time-independent x-vector will need to be generated, followed by interpolation of the main signal values on to it. This is what the 'fixgrid' function is for.

Some JET diagnostics generate data like this that need to be fixgridded, but record it with the data values under one signal name, and the x-vector's time variation under another (e.g. KG10/NE and KG10/R respectively). These can also be combined together and fixgridded in one action by using the 'Combine and fixgrid' processed signal function. Note that the nominal 'x-vectors' of both original signals provided in this case are discarded, as they match exactly by index number and thus the values are meaningless.

When using fixgrid, be careful of edge effects. The algorithm uses the IDL default of linear interpolation, and if at a given t-value the x-vector does not span from its all-time minimum to all-time maximum then data 'interpolated' at the edges of this t-slice will increase in magnitude without bound.