The focusing of the electron beam and the desired consequent pinching of the optical beam at the output end of the wiggler could be effected by application of an externally generated magnetic field or by utilizing the betatron oscillation of the electron-beam envelope. Pinching of the optical beam would reduce its Rayleigh length, thereby making it possible to put the first relay mirror closer to the wiggler without exceeding the damage threshold. The mirror would be oriented for grazing incidence to reduce the intensity at incidence and thereby increase the damage threshold. The combination of pinching the optical beam and grazing incidence would enable placement of the first relay mirror at or perhaps closer than a distance of 3μm from the output end of the wiggler.

The FEL efficiency could be increased by tapering the wiggler. Alternatively, if using a uniform wiggler, enhanced efficiency could be obtained through frequency detuning of the input oscillator signal. Electron-pulse slippage, which could be large enough to limit the FEL interaction length in a low-gain design, has been shown to be small enough not to be of concern in the present high-gain design conceptual design. In one example design calculation, it was found that for an electron-beam current of 1 kA at an electron kinetic energy of 81 MeV, a wiggler magnetic flux density of 5 kG (0.5 T), frequency detuning of -1.5 percent, and linear-accelerator duty factor of 1.5×10-3, the FEL power-gain length would be 14 cm, the intrinsic efficiency would be about 1.2 percent, the peak optical output power would be 1 GW, and the average optical output would be 1.5 MW.

This work was done by Phillip Sprangle and Joseph Peñano of the Naval Research Laboratory, and Bahman Hafizi of Icarus Research, Inc.

NRL-0025

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