Photocathode Laser System

Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has been working on the development of a superconducting electron source (SRF gun) for about 20 years. An integral part of this electron source is a photocathode laser which, in combination with a photocathode, uses the external photoelectric effect to generate electron pulses. These pulses are then accelerated by the electric field of the superconducting RF resonator. The required laser wavelength for this purpose is about 260 nm, while the aimed pulse energy at this wavelength is 5 μJ. Detailed requirements in the table below (2. Performance quote) are specified separately for the fundamental wavelength in infrared (IR) and the 4th harmonic in ultraviolet (UV). Pulse repetition rate shall be flexible and reach 2 MHz. The average power in UV has to reach at least 1 W.

Laser pulse duration is a key optimization parameter that needs to be variable in the range of 0.3-10 ps FWHM. Moreover, the temporal (longitudinal) profile of the laser pulse overall is important for properties of the generated electron pulse and thus also needs to be adjustable. For this, a spatial light modulator for modifying the spectrum needs to be integrated in the offered system. It is desirable that the manufacturer finds settings of the spectral shaper that result in a flat-top temporal shape of the UV pulse. Given that the temporal shape of a chirped pulse is critical in the planned application, spectral smoothness, stability and absence of long wings or shoulders are of high importance. Spectrum distortions caused by nonlinear effects shall be insignificant even at the maximum required pulse energy.

Ability to synchronize the oscillators' repetition rate to a radio-frequency (RF) reference signal is necessary. Achievable integrated time jitter in the range between 10 Hz and 1 MHz of the laser with respect to the reference shall not exceed 100 fs. However, synchronization electronics are not part of the current tender and will be obtained from another source. The oscillator needs to be prepared for repetition rate locking, i.e. contain a short-range and a long-range actuators for fine adjustment of the cavity length. Another aspect of synchronization is the ability to control the repetition rate and the emission time of the laser by an external trigger signal. This ability needs to be available. Reliability and permanent availability (24/7) are of particular importance. The system will be located in an optical laboratory with ±1°C temperature stability.

Beside the fulfilment of the criteria described in annex 01A, the scope of deliveries and services includes, that means are to be offered in the quotation: • Factory Acceptance Test (FAT), with proof of fulfilment of the performance requirements • Delivery according to DDP (Incoterms 2010) to the place of usage • Installation und commissioning including On-Site Acceptance Test (OSAT), with proof of fulfilment of the performance requirements • 12 months warranty from OSAT • CE marking • complete manual in English • suitable "water-to-water chiller"