Ekaterina Gacheva (IAP RAS)
Photoinjector laser activities in IAP RAS started in collaboration with the High Energy Accelerator Research Organization, KEK institute (Japan) in 2008. At that time under the project of the International Linear Collider (ILC) by means of the Super-conducting Test Facility (STF) functioning in KEK the technology of cryogenic RF accelerator modules production was being refined and demonstrated. As an electron source in its RF electron gun served a photoinjector based on a laser driven CsTe cathode. So that in IAP RAS the prototype of such a laser system was build and delivered to KEK in 2010. Requirement of a 3.2 nC charge per injected electron pulse corresponds to the 8-picosecond UV (λ = 262 nm) laser pulse (micropulse) energy of 1.5 µJ. 2437 micropulses running with the frequency of 2.708 MHz (subharmonic of accelerator klystrons) form an exactly rectangular 900µs-long train (macropulse), repeated in turn with the frequency of 5 Hz. High energy stability both for micropulses inside a macropulse and for macropulses themselves is required. Following the strict specification for the micropulse repetition rate we have chosen an Yb-doped fiber master oscillator (λ = 1047 nm) with self-mode-locked operating regime without Q-switching. Subsequent amplification in the Yb-doped fiber is limited by the cubic nonlinearity effects distorting the radiation spectrum. Thus the final amplification system is embodied as a two pass two cascade Nd:YLF solid state amplifier with flash lamp pump. Pumping pulses of two cascades are shifted in time in order to amplify the macropulse more uniformly. To compensate temporal inhomogeneity of the gain the macropulse is previously modified in the fiber fast acousto-optic modulator (AOM) so that its amplified envelope become exactly rectangular. The time dependent AOM transmittance mask is calculated in the feedback algorithm. The 2nd and the 4th harmonics are generated in KTP and BBO crystals respectively. Utilizing the decreasing segment of the 4th harmonic generation efficiency curve allows reducing the mean power fluctuation to 2.3%. The 2nd stage of the photoinjector devoted collaboration is tightly connected with the Joint Institute for Nuclear Research (JINR) in Dubna. As a linear accelerator project with characteristics similar to those in KEK shortly after started in JINR, the second laser system of the type was made in IAP RAS to drive its photocathode. Comparably cheap and reliable solution for a photocathode laser driver has done well again. Laser system for JINR exceeds the KEK project more than six times in power due to raising micropulse repetition rate up to 10 MHz and macropulse repetition rate up to 10 Hz. Micropulse and macropulse repetition rates, micropulse number within a macropulse tuning options are added. The micropulse energy of 1.85 µJ is achieved. Due to additional feedback stabilization algorithm adjusting the fiber preamplifier current the mean power fluctuation drops down to 0.8%. The laser system for JINR has been transported to Dubna and at the moment is being installed on its final location. At the 3rd stage the most challenging problem was attacked. It was theoretically shown that for the electron beam transverse emittance minimization the photo injector laser driver micropulse must have a 3D ellipsoidal shape. Such an exotic requirements are imposed on a laser system started in 2012 in IAP RAS for the Photo Injector test facility at DESY, Location Zeuthen (PITZ), where the developing of the electron source for the European X-ray Free Electron Laser (XFEL) is being carried out. In the output of the photocathode laser for PITZ 7-picosecond 3D ellipsoidally shaped UV micropulses (3DESP) with the energy of 30 µJ must run with the frequency of 1 MHz inside 300-microsecond long macropulses with rectangular envelope repeated in turn with the frequency of 10 Hz. The project concept consists of four main terms. The 1st task is 3DESP creation by means of two liquid crystal Spatial Light Modulators (SLM) (for amplitude and phase masking) in the mixed spatial-frequency domain. The experiment with one amplitude masking SLM has already been done along this line. Then these wide-band pulses must be amplified without dramatic distortion of the shape. The thin-disk Yb:KGW multi-pass (18 V-passes) imaging amplifier is aligned and tested in small signal and working regimes. Thirdly the second and the fourth harmonics in thin LBO and BBO crystals respectively have to be generated utilizing the angular chirp technique to achieve high efficiency of conversion without shape degradation. And finally for the micropulse shape diagnostics an ultra fast 3D cross-correlation scanner has been developed. The probe micropulse noncollinearly interacting in BBO crystal with the micropulse under study gives the cross-correlation function that contains the information about temporal micropulse distribution. Each scanning slice gives 2D intensity distribution in the corresponding cross-section. Engineering decision for time dependent delay line of the probe channel represents 80 m of single-mode polarization-maintaining fiber coiled up on a piezo ceramic disk. This devise is able to scan the micropulse during one macropulse. The cross-correlation scanner is realized to the 1st harmonic diagnostics. The thorough investigation of its resource is done.
Ekaterina Gacheva (IAP RAS)