Overview

Maintaining a constant energy of the scavenger electron beam at sector 19 where it is extracted into the scavenger extraction line (EP01) is critical to the operation of the SLC. The energy acceptance of the extraction line is only % or MeV. If the beam energy is wrong by more than this, some of it is lost on extraction.

We have two systems working in tandem and sharing sector 17 and 18 phase shifters to maintain the proper beam energy. The system is depicted in figure 2.

• Fast feedback's scavenger energy loop reads BPMs in LI19 and EP01. Using those readings it calculates the error in the energy and corrects it by adjusting the phases of sectors 17 and 18. It does this with solid-state (fast) phase shifters which are in the 476 MHz line just upstream of the sub-booster.
• Fast feedforward reads the beam intensities in the damping rings, calculates the amount of beam loading affecting the scavenger beam (energy reduction due to the longitudinal wake-fields of all three bunches), and compensates for that energy loss by adjusting the phases of sectors 17 and 18.

Why, you may ask, do we need two systems to maintain a single energy? First of all, if a klystron trips off in LI05, fast feedforward would know nothing about the resulting low energy. Hence feedback is needed to correct for this type of energy change.

Fast feedforward is needed to handle the following scenario (and similar but less extreme examples). Assume the accelerator is running smoothly with all beam intensities at . Let the scavenger extraction kicker miss a single pulse so the scavenger beam is not extracted properly and no positrons are produced. Two pulses later (remember the positrons are normally stored in the south damping ring for two pulses) there will be no positrons accelerated down the LINAC. The positrons are the first of the three bunches going down the LINAC and normally their beam loading removes about 600 MeV of energy from the following bunches. Since there are no positrons on this pulse, the scavenger bunch will have an energy that is 600 MeV too high and hence will not go into the extraction line properly. Since the energy is so far off, almost no BPMs in the extraction line have good readings so feedback cannot measure and correct the energy. The result of all of this, is that no positrons will be made and you are stuck in a rut. In this scenario a single missed pulse causes half the following pulses to be lost. It is very difficult to recover from this. Fast feedforward would cure this problem by measuring the positron beam intensity in the damping ring a few milliseconds before it is extracted and (for this example) reduce the energy contributed by sectors 17 and 18 by 600 MeV in order to keep the scavenger beam energy constant.

• Feedback
• Feedforward
• Feedforward and Feedback Working Together