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 *L*_{R}= 6 cm and
*L*_{z}=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 ,
, *L*_{R}=20 and *L*_{z}=30 cm. Recent analysis has indicated
that the current values are , , *L*_{R}=25
and *L*_{z}=52 cm. These values correctly reproduce the beam footprint
for a vertical beam divergence of , and a horizontal beam
divergence of .

Fri Jul 11 15:18:44 EDT 1997