First Ultraviolet High-Gain Harmonic-Generation Free-Electron Laser

We report the first experimental results on a high-gain harmonic-generation (HGHG) free-electron laser (FEL) operating in the ultraviolet. An 800 nm seed from a Ti:sapphire laser has been used to produce saturated amplified radiation at the 266 nm third harmonic. The results confirm the predictions for HGHG FEL operation: stable central wavelength, narrow bandwidth, and small pulse-energy fluctuation.

Figure 1

The NSLS DUV FEL layout. 1: gun and seed laser system; 2: rf gun; 3: linac tanks; 4: focusing triplets; 5: magnetic chicane; 6: spectrometer dipoles; 7: seed laser mirror; 8: modulator; 9: dispersive section; 10: radiator (NISUS); 11: beam dumps; 12: FEL radiation measurements area.

Figure 2

Pulse energy versus distance in the radiator for two values of the seed laser input power: (a) 1.8 MW and (b) 30 MW. The solid curves are simulation results by the TDA code.

Figure 3

Single shot HGHG spectrum for 30 MW seed power, exhibiting a 0.1% FWHM bandwidth. The gray line is the single shot SASE spectrum far from saturation when the 30 MW seed was removed. This spectrum serves as the background of the HGHG output. The average spacing between spikes is used to estimate the pulse length. The HGHG spectral brightness is $2\times {10}^5$ times larger than the SASE, because NISUS is too short to achieve SASE saturation. So this is not an appropriate comparison. If the NISUS length was doubled to 20 m, the SASE would reach saturation, but because of its broader bandwidth it would still have an order of magnitude lower brightness than the HGHG, as calculated by the code GENESIS [20].

Figure 4

Histogram of HGHG output pulse energy with 30 MW seed power.