EUROFEL-Report-2007-DS4-095 EUROPEAN FEL Design Study Deliverable N : D 4.3 Deliverable Title: Task: Authors: Generation of 3rd harmonic photons at 90 nm DS-4 see next page Contract N : 011935 Project funded by the European Community under the Structuring the European Research Area Specific Programme Research Infrastructures action
D 4.3. Generation of 3rd harmonic photons at 90 nm Mathias Brandin, Nino Cutic, Filip Lindau, Sara Thorin, Sverker Werin; MAX-lab Lund University, Lund, Sweden J. Bahrdt, A. Gaupp, K. Goldammer, Dmytro Pugachov *),; BESSY, Berlin, Germany The final goal of this deliverable was set to the generation of coherent photons at 90 nm from the optical klystron in seeded mode at the MAX-lab test-fel. The activities are performed as a collaboration between BESSY (Berlin) and MAX-lab (Lund). The project has been progressing over the time of the EUROFEL funding period, including design, simulation, construction, installation and commissioning. The complete set-up has been simulated in start-to end simulations. The undulator systems have been rebuilt, re shimmed, measured in Berlin and installed on site at MAX-lab. The beam line, including diagnostics, has been designed, built and installed (fig 1). A combined gun and seed laser system has been specified, purchased through a call for tender, installed and put into operation. Electrons have been passed through the complete system. All relevant components have been operated and many of them together. Despite the effort put into the project and the milestones achieved it has not been possible to reach the very final goal of producing coherent photons at 90 nm from a seeded electron beam. Fig 1. The beamline at MAX-lab with the two undulators of the optical klystron and the chicane in the middle. In the background the laser hutch for the seed laser system. Two main reasons can be seen for not being able to fully meet the final milestone and deliverable. First the laser system foreseen for the project was intended to build on an existing system already available at a user station at MAX-lab. While more carefully reviewing the design it showed that this was not a suitable solution and that, with complement of national funding, a complete new laser system could be bought. The process of specification, call for tender, evaluation, production and installation of this system has taken the majority of the available three years, and in addition there was a delay in delivery of the laser system at the very end. Secondly the design and production of the beamline at MAX-lab has been delayed due to limited resources in mainly drawing and construction at MAX-lab, combined with the project gaining lower priority than the user operation of the National facility. In addition there have been other smaller delays on parts of the project, which each separately probably would have been possible to accommodate within the project period.
Operations in preparation for seeded coherent photons Most of the experience gained with the system so far is qualitative, meaning that experience and the possibility of transporting the electron beam and laser beams has been approached. The first dedicated experimental period in December 2007 was used to such preparations. Two modes have been explored and the available results are given below. Long pulse mode The injector system at MAX-lab normally operates in a long pulse mode where the RF-gun cathode is used thermionically. This produces a 50 ns pulse train of around 15 pulses, each a few ps in length. This pulse is not suitable for generating harmonic photons at shorter wavelengths due to the poor emittance, but as it carries a larger total charge it is well suited for first operations of the beamline. The long pulse has been transported through the optical klystron of the system with the correct beam position. The functionality of the half chicane allowing seed laser injection has been verified. The chicane for microbunching has been operated and the field balance checked. The two undulators (modulator and radiator) have been operated and spontaneous synchrotron radiation produced (fig 2). The operation of all these system has thus been verified. e-beam undulator radiation Fig 2. Image of electron beam OTR and synchrotron radiation from the radiator undulator in long pulse mode. The OTR is generated on the mirror surface while the synchrotron radiation is reflected on the surface giving rise to the separation. Short pulse mode In short pulse mode the RF-gun is used as a photo cathode gun. The temperature of the cathode is reduced to below thermal emission (around 600 C) and electrons generated by the 10 ps gun laser system. It is also possible to maintain a weak thermal emission to simplify the timing of the system during commissioning. The gun laser system has been synchronised to the 3 GHz RF system of the gun and linacs. Electrons have been generated and transported through the linac system reaching 400 MeV. The final part of the transport still remains to be passed, before reaching the optical klystron.
Control of the timing (phase) between the gun laser pulse and the 3 GHz RF system has been achieved. The phase of the gun laser pulse has been scanned over the 3GHz cycle (fig 3) and the electron beam current and energy measured. The rising edge of the extracted current (left in fig 3) should reflect the gun laser pulse length of 10 ps but still show a slightly longer pulse. The optimal phase is, according to simulations, around 30 deg (~30 ps) when the electron energy starts to drop. This point is clearly identified and reproducible. Measured data 140 120 I(a.u.) E(a.u.) 12700 12600 100 12500 Current (a.u.) 80 60 40 12400 12300 12200 Energy (a.u.) 20 12100 0 0 50 100 150 200 Delay (ps) 12000 Fig 3. Measured current and electron energy as a function of relative phase of the laser pulse on the cathode.
1st turn 2nd turn Fig 4. Electron pulses in short pulse mode (generated by the 10 ps gun laser). Current transformer pulses which do not resolve 10 ps. top: at gun exit, middle: at Linac 1, bottom: at After linac 2. First passage (200 MeV) is saturated, second passage (400 MeV) 120 ns later. (timing between signals is given by both different locations, and cable lenthg differences.) Summary The build up of the facility has been slower than expected but successful. Funding has been made available for a continuation of the project and the work continues with the generation of seeded coherent harmonic generation expected in the coming months. In addition to the results presented here there are results from the design and build-up process plus tests of auxiliary systems. Many of these are available in the report D4.4 Report First stage of Harmonic Generation. This work has been partially funded by the EU under the 6th Framework programme, contract no. 011935 EUROFEL.