B.M. van Oerle

PHOTOEMISSION FROM K-TE PHOTOCATHODES

In this letter the photoemissive properties of K–Te films produced under ultrahigh vacuum conditions are reported. K–Te photocathodes were fabricated by vapor deposition of Te and K onto a Mo substrate into the preparation chamber of the Free Electron Laser of the University of Twente. The highest quantum efficiency obtained at 259 nm was 11.1%, measured just after evaporation; this value decreased in a few minutes to a stable quantum efficiency of 8.3%. The reported results show that K–Te can be considered a promising material for the use as a photocathode in photoinjectors.

Aspects of accelerator-based photoemission

A high brightness electron beam with low energy spread is essential for a free electron laser intended to operate in the wavelength region beyond the UV. Several hundreds of amperes peak current, an emittance of several π mm mrad or less and an energy spread of a few tenth of a percent are required. At present, photocathode linacs are the only devices that can produce such beams. The success of such a linac is strongly dependent on the possibility to develop a high-efficient, stable photocathode and a high-power, stable drive laser used to illuminate the cathodes. In this paper we present some result on our photocathode research as well as our drive laser developments.

High efficiency photoemission from Cs-K-Te

Cs–K–Te films have been fabricated under ultrahigh vacuum by vapor deposition of Te, K, and Cs onto a Mo substrate into the preparation chamber of the free electron laser of the University of Twente. Their photoemission properties are reported in this letter. The average quantum efficiency at 259 nm measured on 5 evaporated Cs–K–Te photocathodes is 22.5%, whereas the best quantum efficiency obtained at 259 nm is 23.4%. To our knowledge, this is the highest quantum efficiency at 259 nm reported so far, being about twice that of Cs2Te photocathodes. The reported results demonstrate that Cs–K–Te photocathodes are very promising for the use in photoinjectors.

Optimization of the power and control of the shape of amplified trains of laser pulses.

We have performed an optimization study on a train of laser pulses in a Nd:YLF master-oscillator power amplifier chain. Instead of using a flat train of input pulses and proper timing of the input pulses with respect to the pump pulse to keep the output pulse flat, we used a pulse-shaping technique. Then the maximum gain could be used, resulting in a 70% increase in output pulse energy. We constructed a special feed-forward loop to control the temporal shape of the train. We compare the results with a computational model based on the rate equations in the Nd:YLF amplifiers.

Cs–K–Te photo cathodes: a promising electron source for free-electron lasers

The characteristics of Cs–K–Te photo-cathodes when used in a photo-cathode linear accelerator will be presented together with a short review of their photo-emissive properties. The cathodes have been illuminated by light pulses obtained by frequency quadrupling the light of a Nd:YLF laser. The emitted electrons have been accelerated in a 1.3 GHz RF linear accelerator operating at an energy of 6 MeV. The quantum efficiency of the Cs–K–Te cathodes at 259 nm decreases exponentially with operation time, but recovers partially during storage in UHV between successive operating sessions. These two effects tend to compensate for long times, so that saturation of the quantum efficiency value at about 2% is reached after 20 h of operation. A Cs–K–Te cathode, degraded by use in the accelerator, can be rejuvenated to about 60% of its initial quantum efficiency by heating it to 120°C for 1 hour.

The effect of transverse modes in a wave-guide resonator on the resonance condition of a compton FEL

In a free electron laser where the resonator has a wave-guide structure the phase velocity of the light has a significant impact on the wavelength of the output radiation. This effect is well known in Raman-type FELs. It is however less known that this effect can also play an important role in Compton-type FELs. This effect has been experimentally verified in our 6 MeV Compton-type FEL, having a wave-guide resonator with a hole in the outcoupling mirror. Successive transverse modes, which have distinct phase velocities, show up at different wavelengths in the output spectrum. The difference appeared to be as large as 5% of the resonance wavelength. In normal lasers this difference is not more than about 0.1%.

Lasing experiments at TEUFEL

The waveguide structure of the resonator of the TEUFEL system has a significant impact on the resonance condition for lasing. The phase velocity of the modes in the waveguide shifts the resonance wavelength to larger values. Although this effect is known in Raman type FELs, it is less known that this mechanism has a severe impact on the resonance condition for Compton-type FELs. Due to this effect, various modes, that differ significantly in wavelength, can be resonant simultaneously. This paper shows that the mode spacing between adjacent modes can be as large as 5%, whereas, the gain bandwidth is not more than 1%.