An Experimental Study of an FEL Oscillator with a Linear Taper

(WE-1-04) Presenting Author: Stephen Benson

S. Benson, J. Gubeli, and G. R. Neil  

TJNAF, Newport News, VA

The first work on tapered FELs involved models of FEL amplifiers. Early experiments were carried out with simple tapered FEL oscillators. More recent work has looked at the behavior of a step tapered oscillator(1). The latter showed that an inverse step taper performed better in an oscillator than a positive step taper. No experimental study has been done to date of the relative performance of a positive and negative linear taper. Using the work of Saldin, Schneidmiller and Yurkin as a starting point(2), we have measured the detuning curve widths, spectral characteristics, efficiency, and energy spread as a function of the taper for low and high Q resonators in the IR Demo FEL at Jefferson Lab. Both positive and negative tapers were used. Several predictions of a simple one-dimensional single mode theory were observed while other measurements differed markedly from the predictions of this model. For a low Q resonator, for instance, the highest efficiency was found for an untapered system. The gain vs. taper and the observation that a negative taper has better performance than a positive taper agreed with the 1-D single-mode theory. The deviations between the theory and experimental results are presumed to be due to the short-pulse nature of the IR Demo FEL. This is supported by the observation that a high Q resonator brings the results much closer to the 1-D single-mode theory predictions.

1 D. A. Jaroszynski et al., Nucl. Inst. And Meth. In Phys. Res. A358 (1995) 228.

2 E. L. Saldin, E. A. Schneidmiller, and M. V. Yurkov, Nucl. Inst. And Meth. In Phys. Res. A375 (1996) 336.