Turning Feedforward ON or OFF

Fast feedforward is controlled from the FEED FORWRD EP01 panel. This can be reached from the FEEDBACK SYSTEM INDEX or from the FFBK MAGNET panel.

The brute force way to turn off fast feedforward is to select AMPL 2 INTENS GAIN, enter a VDES of zero and TRIM. This will eliminate any contribution that fast feedforward has been making to the energy. If fast feedback is on, it will increase its energy contribution to keep the energy constant. There are two problems with this approach.

1. Assume that all 3 beams had an intensity of . Then when the gain goes to zero the scavenger beam energy will instantaneously drop by 2 GeV (the amount feedforward was contributing). The scavenger beam will not make the turn into the scavenger line so feedback will not be able to measure and fix the beam energy.
2. Even if feedback could measure the beam energy, it is very likely it would not have enough energy headroom to add the necessary 2 GeV to the scavenger beam energy. It will normally be necessary to reduce the headroom that feedback leaves for feedforward. This is OK since with feedforward off it does not need any headroom.

To overcome these problems there are is a button macro available from a button on the above panel. It is labelled FEED FORWRD OFF. In several steps it decreases the feedforward gain while decreasing the headroom that feedback leaves for feedforward. In between each step it pauses for a few seconds to allow feedback to increase its energy contribution.

To turn fast feedforward on, the above actions must be reversed resulting in the AMPL 2 INTENS GAIN being set to 0.999. The FEED FORWRD ON button does this. Please note that this button macro must be updated if the normal running value of the headroom is changed.