Magnetic fields and electron trajectories at the end of a helical undulator
Craun, Daniel E.
Colson, William B.
Burkirk, Fred Ramon
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An undulator is a periodic magnetic field device that is an integral part of a Free Electron Laser (FEL). The key to FEL performance is the undulator design. This thesis models the undulator magnetic fields by using the Biot- Savart law, a current element integration technique. Applying this technique to different winding schemes for a bifilar helical compact undulator, the field structure is computed for positions outside the entrance to the undulator. These stray fields can have a focus or defocusing effect on the incoming electron beam. This disturbs the critical matching of the electron velocity vectors with the co-propagating laser radiation. The matching of velocity vectors controls the bunching of the electrons. This bunching action controls the gain of the FEL, and thus, the ultimate performance of the entire device. By choosing a better design for the undulator, this unwanted effect is reduced. The field structure found from integration determines the electron trajectories. The best design for the undulator is then determined by the imposed input conditions on the electron beam for entry into the cavity. An additional benefit of this technique is a potential application to finding coil winding tolerances for undulator construction.
RightsThis publication is a work of the U.S. Government as defined in Title 17, United States Code, Section 101. Copyright protection is not available for this work in the United States.
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