We found one
problem with having the focus offset at the LEH plane for beam pointings 200 and 400 microns into the halfraum.
A bump in the spectrum occurred at high energies (Figure 7) because the laser hot spot size was too small (Figure 8)
By moving the focus offset to 500 microns outside the LEH plane, the hot spots became larger resulting in less energy in a given area (Figure 10), and a lower peak temperature, removing the bump in the spectrum (Figure 9).
The latest version of VisRad has a more realistic beam spot size, which will tend to make the hard spectral bump from the hot spot less severe.
Figure 7: Witness
Plate spectrum with focus offset on the LEH plane
Figure 9: Witness
Plate spectrum with focus off set 500 microns out of the LEH plane