As described above and illustrated in the SCAVENGER ENERGY SUMMARY DISPLAY, feedback provides some energy from sectors 17-18 and fast feedforward adds to that. Unfortunately the total energy gain of the two sectors is limited. It is thus possible that feedback and feedforward could desire to add more energy than is available. This is known as running out of headroom.
The feedforward hardware (which is analog) is fairly dumb and has no logic to tell if it has run out of headroom. If it does run out of headroom it will just move the phase shifters past zero degrees (the phase for maximum energy gain) resulting in a decrease instead of an increase of energy.
To prevent this, feedback always leaves a certain amount of headroom
for feedforward. The amount of headroom it leaves is indicated on the
summary display and can be changed with the FEED FORWRD HEADRM button
on the FEED FORWRD EP01 panel. This can be reached from the FEEDBACK
SYSTEM INDEX or from the FFBK MAGNET panel. One must leave a headroom
corresponding to the maximum beam intensity that is expected. A
headroom of 2.0 GeV is needed if all 3 beam intensities may reach
. Similarly if all 3 beam intensities are limited to
then a headroom of 
Gev is
needed.
When you get this error message from feedback:
CTL Task - Need energy gain above 2145 MeV - Add klystronsit is telling you that it cannot add enough energy to the beam while leaving enough headroom for fast feedforward. You should probably do as it says and add klystrons. If you are running (and intend to continue running) at beam intensities much lower than that for which the headroom provides, then a second alternative is to lower the headroom parameter.