A high-gain harmonic-generation free-electron laser is demonstrated. Our approach uses a laser-seeded free-electron laser to produce amplified, longitudinally coherent, Fourier transform-limited output at a harmonic of the seed laser. A seed carbon dioxide laser at a wavelength of 10.6 micrometers produced saturated, amplified free-electron laser output at the second-harmonic wavelength, 5.3 micrometers. The experiment verifies the theoretical foundation for the technique and prepares the way for the application of this technique in the vacuum ultraviolet region of the spectrum, with the ultimate goal of extending the approach to provide an intense, highly coherent source of hard x-rays.

We report on an experimental investigation characterizing the output of a high-gain harmonic-generation (HGHG) free-electron laser (FEL) at saturation. A seed CO2 laser at a wavelength of 10.6 microm was used to generate amplified FEL output at 5.3 microm. Measurement of the frequency spectrum, pulse duration, and correlation length of the 5.3 microm output verified that the light is longitudinally coherent. Investigation of the electron energy distribution and output harmonic energies provides evidence for saturated HGHG FEL operation.

The longitudinal accelerating field E, has been measured as a function of azimuthal angle in the full cell of the cold test model for the 1.6 cell BNL/SLAC/UCLA #3 S-band RF Gun using a needle rotation / frequency perturbation technique [l]. These measurements were conducted before and after symmetrizing the full cell with a vacuum pump out port and an adjustable short. Two different waveguide to full cell coupling schemes were studied. The dipole mode of the full cell is an order of magnitude less severe before symmetrization for the &coupling scheme. The multi-pole contribution to the longitudinal field asymmetry are calculated using standard Fourier series techniques. The Panofsky-Wenzel theorem [2] is used in estimating the transverse emittance due to the multipole components of E t .

In this article, we report the investigation of sputtered magnesium as a suitable photoelectron emitter in a rf injector. The measurements were conducted on a 20 m thick Mg layer, ion sputtered on a copper substrate. UV laser induced damage threshold measurements on this sample indicate that laser energy densities of up to 100 J/mm 2 at 266 nm, the irradiated surface is indistinguishable from the unirradiated surface. For laser energy densities in the range of 100-300 J/mm 2 , slight roughening of the surface was evident. At 600 J/mm 2 , damage is visible, with the depth estimated to be ϳ1 m. This damage threshold is six times larger than that of the bulk magnesium and approaches that of copper. Systematic laser cleaning has improved the quantum efficiency of the surface from 2ϫ10 Ϫ5 to 2ϫ10 Ϫ3 . This combination of high quantum efficiency and high damage threshold of sputtered magnesium produces a record current density of 25 kA/mm 2 for a metal cathode. The effect of laser cleaning on sputtered Mg, the sample preparation technique for reliable performance, and the parametric dependence of quantum efficiency are reported as well.

The symmetrized 1.6 cell S-band photocathode gun developed by the BNL/SLAC/UCLA collaboration is in operation at the Brookhaven Accelerator Test Facility(ATF). A novel emittance compensation solenoid magnet has also been designed, built and is in operation at the ATE These two subsystems form an emittance compensated photoinjector used for beam dynamics, advanced acceleration and free electron laser experiments at the ATF. The highest acceleration field achieved on the copper cathode is 150 e, and the guns normal operating field is 130 s. The maximum rf pulse length is 3ps. The transverse emittance of the photoelectron beam were measured for various injection parameters. The 1 nC emittance results are presented along with electron bunch length measurements that indicated that at above the 400 pC, space charge bunch lengthening is occurring. The thermal emittance, eo, of the copper cathode has been measured.