MPS Specifications for the IR Demo Wiggler

Stephen Benson, January 13, 1997

A wiggler such as the IR Demo wiggler, which uses NdFeB as a permanent magnet material, can show some degradation in field quality with as little as107 Rad and a dose of 108 Rad will probably lead to unacceptable wiggler performance. If we use NdFeB we need to be very careful with beam loss around the wiggler. G. Neil has suggested that an integrating radiation detector be placed near the wiggler so that the dose could be monitored. If the dose gets near allowable limits, the beam loss must be reduced somehow.

Radiation damage calculation:

How much beam loss can produce a dose of 107 Rad? A quick calculation indicates that the degradation in the wiggler from a single high current beam loss would be negligible (the dose from one 100 µsec., 5 mA loss would be one kRad). A more serious hazard is a low current loss over a very long time. We might tolerate as much as 500 nA of beam loss for longer than one minute in one place ( this is 20W of beam power loss and is equivalent to the tune-up beam current). At 42 MeV the radiation in the forward direction is 106 Rad/mA/min at 1 meter from the source [1]. If we say that the beam is lost 10 cm upstream of the wiggler (where the chamber necks down) this much loss would produce a dose rate in the wiggler of 3 MRad/hr and might lead to damage of the wiggler in three hours. Note that the dose rate at 90° to the beam loss would be approximately 90 Rad/hr at one meter from the beam loss (This figure must be confirmed by Geoffrey Stapleton. He is working on it with P. Kloeppel). If we place two tenth-value layers of high-Z shielding (this is four inches of lead) between the neck-down point and the wiggler we could increase the time before damage up to 300 hours of operation. If the wiggler is damaged at this point, it could be repaired with about two to three months of downtime.

Proposed MPS Design:

The upstream shielding must be complemented by at least one integrating ionization chamber near the wiggler entrance. Two chamber are preferred for redundancy, placed on either side of the wiggler entrance. If the trip point of a chamber 30 cm from the neck-down point were set to allow 100 Rad/hr. averaged over all operations, the wiggler should survive for at least 3000 hours. A dose rate averaged over some reasonable time (e.g. 10 minutes) can probably be ten times this average dose rate and still allow operations without tripping the integrated dose limit. The trip point on integrated dose should be 2400 Rad/day. The integrated dose in the monitor should be zeroed each day at midnight. The dose rate trip level should therefore be approximately 1000 Rad/hr and the dose trip level, rezeroed daily, should be approximately 2400 Rad. Note that one could set a higher dose limit in the detector if it were closer but that would imply a very small detector head. For a 30 cm long detector, the distance should be no less than 30 cm. Finally, note that these numbers are ballpark numbers right now and we will need the results from Peter Kloeppel's EGS runs to be sure of them. His results will also indicate a good position for the detector.


1. NCRP Report No. 51, "Radiation Protection Design Guidelines for 0.1-100 MeV particle accelerator facilities", National Council on Radiation Protection and Measurements, Washington D.C., 1977.