| Paper |
Title |
Page |
| MOPPH044 |
Optical Beam Quality in Free-Electron Lasers
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134 |
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- P. Sprangle, J. Penano
NRL, Washington, DC
- H. Freund
SAIC, McLean
- B. Hafizi
Icarus Research, Inc., Bethesda, Maryland
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The quality of the FEL optical beam is an important consideration for many applications. The quantity M-squared is a single parameter that is used to quantify the higher-order transverse mode content of the beam. For steady state propagation in the paraxial limit, equations for the axial variation of the laser spot size and M-squared are derived. The quantity M-squared for the output of an FEL can also be determined by making measurements of the spot size at three locations and making use of the parabolic propagation law. We consider the optical beam quality for a MW-class amplifier. In this configuration the radiation is optically guided, maintaining a constant spot size through the wiggler, and is pinched at the wiggler exit. This leads to a relatively good optical beam quality, short growth length, short wiggler length, and good efficiency. Diffractive spreading of the FEL output beam can be sufficiently large to allow the first relay mirror to be close to the exit of the wiggler without exceeding the mirror damage intensity threshold, particularly in a grazing incidence configuration. The minimum distance to the relay mirror is shown to be inversely proportional to M-squared.
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