Beam power deposition for TFTR is modelled in SNAP by segmenting each beam source into a rectangular grid of individual ``pencil'' beamlets which are then tracked through the plasma along a linear trajectory. These beamlets are assumed to propagate without divergence (in contrast to TRANSP which provides for divergence). The power carried by each beamlet is chosen to yield a Gaussian shape in the horizontal and vertical directions. The Gaussian shape does not extend indefinitely, but rather can be cut off at a selected half-width in each direction.
Thus, the power density of each source in the region (the radial half-width) and (the vertical
half-width) is
Outside of this region, the power is zero.
The TFTR long-pulse ion sources are 12 cm wide by 43 cm tall, so in the absence of beam divergence we would use LR= 6 cm and Lz=21.5 cm, and we would use the Gaussian half-widths and as measured on the beam test stand. But to account for the beam divergence, we assign new values to all of these lengths, to approximate best the actual beam power footprint which would occur in vacuum at the point of closest approach of the beam sightline to the tokamak center, i.e., the tangency point. Historically, the standard values found in many existing SNAP.DAT files are , , LR=20 and Lz=30 cm. Recent analysis has indicated that the current values are , , LR=25 and Lz=52 cm. These values correctly reproduce the beam footprint for a vertical beam divergence of , and a horizontal beam divergence of .