| Paper | Title | Page |
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| TUPMS036 | Characterization of Orbital Angular Momentum Modes in FEL Radiation | 1263 |
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| Optical guiding of the radiation pulse through the source electron bunch in a free-electron laser is a well known phenomena that suppresses diffraction of the output radiation, and thus enhances the gain. The resulting radiation can be described by an expansion of orthogonal modes that are also composed of eigenstates of orbital angular momentum (OAM). In the VISA-FEL experiment at the ATF-BNL, gain guiding has been observed under self-amplified spontaneous emission conditions at 840 nm with a strongly chirped input electron beam. The resulting far-field transverse radiation profiles are observed to contain multiple modes in the angular intensity spectrum, and exhibit both hollow and spiral structures characteristic of single or multiply interfering OAM modes. Current efforts to characterize the transverse radiation profile both experimentally and through start-to-end simulations are presented. |