The x-ray ring operates with insertion devices in five of the eight machine long straight sections (Fig. 1)*. These devices generate high power photon beams which are transported by beamlines to their respective experimental stations as far as 40m away. If any of these high power beams were to be accidentally mis-steered, for example by a faulty steering magnet power supply, the energy carried by the beam could cause severe thermal damage to the vacuum chamber. Thus, in each insertion straight, the vacuum chamber must be protected both passively with water cooled absorbers and actively with electronic interlock circuits , .
The x-ray ring active interlock system continuosly monitors the electron beam position both upstream and downstream of each insertion device with active beam position monitors (BPMs)  and "dumps" the stored beam if it moves outside of safe operating limits by interrupting the drive to the machine RF accelerating system. The BPMs derive their signals from dedicated RF pick-up electrodes (PUEs) embedded into the chamber. System response has been adjusted to minimize false trips. A section of the insertion device vacuum chamber with PUEs, absorbers, etc. is shown in Fig. 2 and a block diagram of the interlock system appears in Fig. 3.
With active components in the system, high reliability of operation is achieved by using two completely redundant channels starting from the BPMs all the way through the low-level RF drive interrupt switches, and with all interconnections hard wired. Operation of critical components is continuously monitored in the background by dedicated microcomputers. The overall system is periodically tested with beam at safe levels of beam intensity. A typical cycle of operation is illustrated in Fig. 4.
* A soft x-ray undulator (SXU) at X1, a superconducting wiggler (SCW) at X17, an elliptically polarized wiggler (EPW) at X13, and hybrid wigglers (HBW) at X21 and X25.
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