Setting up randomisation of a cathode
The databuilder allows the randomisation ton be the same each time (that is, a repeatable randomisation, always with the same sequence), or to be a different, uncontrolled randomisation each time the program is run.
Yet another option is non-random, with the rays starting normal to the surface of the cathode with an energy kT (but note that this is non-physical)
For field emission the velocity components are randomised even if kT is zero. The energy spread is characterised by the acceleration energy 'd' gained over a distance lambda/(4*pi), where lambda is the de Broglie wavelength corresponding to the energy eW and W is the work function (see Hawkes and Kasper). In general d is an order of magnitude larger than kT at room temperature.