The importance of beam breakup (BBU) due to higher order modes (HOMs) in the Jlab 10kW FEL has recently been demonstrated, but has been predicted for some time. However, these predictions did not include the effect of the momentum spread placed on the beam by operation of the laser.

To include this effect into the existing TDBBU simulation program required a journey deep into mysteries of the code and a major change in its algorithms. The techniques used to work with old software should be of general interest to anyone who inherits and wishes to change old code.


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