Tunable fluid-loaded free-electron laser in the low-electron-energy and long-wavelength extreme

A tunable fluid-loaded free-electron laser (FEL) oscillator is demonstrated . This FEL-type experiment employs low-energy electrons, down to 0.4 keV. T herefore, it interacts with extremely long radio waves, in the VHF range (l ambda>1 m). The device consists of a folded-foil planar wiggler (zeta(w) = 4 cm), and a double-stripline cavity. The variable dielectric lending is im plemented by distilled water in glass pipes situated on both sides of the s tripline structure. Coherent oscillations are observed at the fundamental c avity mode, around 270 MHz. By varying the fluid dielectric loading, the op erating: frequency is tuned in a range of 10 MHz. This paper presents (a) t he effect of a variable dielectric loading on the FEL tunability in general , and (b) an extremely low-electron-energy and long radiation wavelength, i n the lowest end of the known FEL operating spectrum. [S1063-651X(99)03703- 4].