• Y. Otake, N. Hosoda, H. Maesaka, T. Ohshima
  RIKEN/SPring-8, Hyogo
• S. Matsubara
  JASRI/SPring-8, Hyogo-ken

A pump-probe experiment at XFEL/SPring-8 is one of the most prominent parts to extract the future of a coherent short-pulse X-ray laser. A commercial Ti:Sapphire mode-locked laser is presently used as a pump laser, while a probe laser is the XFEL. However, the time jitter of the commercial mode locked laser, as which is caused by the noise of an electrical mode-locking circuit, is around several hundred femto-seconds. This jitter value is not sufficient for a temporal resolution requirement of our pump-probe experiment with a laser pulse width of several ten femto-seconds. To improve this time jitter, the method, using a 770 nm Ti:Sapphire laser amplifiers directly triggered by a 1540 nm master optical oscillator as a time reference signal source for an XFEL accelerator, was devised. This method could eliminate the noise caused by the electrical mode-locking circuit. The basic principle of the method was proved by a preliminary experiment with laser pulse manipulation employing an E/O crystal shutter with a several ten ps response. This presentation describes a basic idea of this pump–probe method, a preliminary experiment set-up to check its feasibility, and experiment results.

• G. Geloni
  European XFEL GmbH, Hamburg
• V. Kocharyan, E. Saldin
  DESY, Hamburg

The magnetic gap of the baseline XFEL undulators can be varied mechanically for wavelength tuning. In particular, the wavelength range 0.1 nm - 0.4 nm can be covered by operating the European XFEL with the SASE2 undulator. The length of the SASE2 undulator (256.2 m) is sufficient to independently generate three pulses of different radiation wavelengths at saturation. Normally, if a SASE FEL operates in saturation, the quality of the electron beam is too bad for generation of SASE radiation in the subsequent part of undulator which is resonant at a few times longer wavelength. The new method of SASE undulator-switching based on the rapid switching of the FEL amplification process proposed in this paper is an attempt to get around this obstacle. Using mechanical SASE shutters installed within short magnetic chicanes in the baseline undulator, it is possible to rapidly switch the FEL photon beam from one wavelength to another, providing simultaneous multi-color capability. Combining this method with a photon-beam distribution system can provide an efficient way to generate a multi-user facility.

• G. Geloni
  European XFEL GmbH, Hamburg
• V. Kocharyan, E. Saldin
  DESY, Hamburg

This paper describes a scheme for pump-probe experiments that can be performed at LCLS and at the European XFEL and determines what additional hardware development will be required to bring these experiments to fruition. It is proposed to derive both pump and probe pulses from the same electron bunch, but from different parts of the tunable-gap baseline undulator. This eliminates the need for synchronization and cancels jitter problems. The method has the further advantage to make a wide frequency range accessible at high peak-power and high repetition-rate. An important feature of the proposed scheme is that the hardware requirement is minimal. Our technique is based in essence on the "fresh" bunch technique. For its implementation it is sufficient to substitute a single undulator module with short magnetic delay line, i.e. a weak magnetic chicane, which delays the electron bunch with respect to the SASE pulse of half of the bunch length in the linear stage of amplification. This installation does not perturb the baseline mode of operation. We present a feasibility study and we make exemplifications with the parameters of the SASE2 line of the European XFEL.

• P. Gessler, M.K. Bock, M. Felber, K.E. Hacker, W. Koprek, F. Ludwig, H. Schlarb, B. Schmidt, S. Schulz
  DESY, Hamburg

Electron bunches at the free electron laser FLASH at DESY have a duration of 10 fs to 150 fs and an arrival-time jitter of about 150 fs (rms). It is anticipated that the newly installed optical synchronisation system will stabilize the seed and pump-probe lasers to within ~10 fs. In order to perform reliable and stable seeding, the electron bunch timing jitter needs to be reduced. Bunch arrival-time monitors measure the arrival-time fluctuations at different locations and are used in a beam-based feedback loop to correct the amplitude of the accelerator RF. In order to provide reliable operation and high availability of the bunch arrival-time feedback, intensive efforts have been undertaken in system automation and exception handling. This will be discussed along with the latest results and limitations on the stability of the arrival-times at FLASH.

• S. Schulz, L.-G. Wißmann
  Uni HH, Hamburg
• M.K. Bock, M. Felber, P. Gessler, K.E. Hacker, T. Lamb, F. Ludwig, H. Schlarb, B. Schmidt
  DESY, Hamburg

The optical synchronization system of the free-electron laser in Hamburg (FLASH) is based on the stabilized pulse-train distribution of a passively mode-locked laser. This master laser oscillator is based on erbium-doped fiber technology and is built in a σ-configuration, enabling passive mode-locking through nonlinear polarization evolution. Recently, a commercial laser system has been installed in addition to the existing laser. Besides maintenance-free operation, this SESAM-based laser shows an even lower timing jitter, enabling a tighter synchronization to the accelerator's RF reference. In this paper we report on the commissioning, the characterization and the long-term stabilty of the new laser system, as well as on the performance of the laser with the existing pulse-train distribution scheme and optical front-ends of the synchronization system in comparison to the old one.

• P. Gessler, M.K. Bock, M. Felber, K.E. Hacker, F. Ludwig, H. Schlarb, B. Schmidt, S. Schulz
  DESY, Hamburg
• J. Szewinski
  Warsaw University of Technology, Institute of Electronic Systems, Warsaw

Bunch arrival-time monitors measure the arrival-time of each bunch in the electron bunch train at several locations at FLASH. The temporal reference for the monitors is provided by the optical synchronization system which distributes laser pulses with a repetition rate of 216 MHz and a length of around 200 fs FWHM. The pulses are delivered to the monitors with an arrival-time stability of about 10 fs. The bunch arrival-time is encoded as an amplitude modulation of a laser pulse from the optical synchronization system. These laser pulse amplitudes need to be sampled and processed together with additional input parameters. Because the arrival-time information is used in a feedback loop to adjust the accelerator fields, the signal processing, calibration and transmission of the bunch arrival-time information via a low-latency, high-speed link to an accelerator RF control station is needed. The most challenging problems of the signal processing are the synchronisation of several clock domains, regeneration and conversion of optical laser pulses, on-line calibration, and exception handling.

• M. Aiba, H.-H. Braun, M. Böge, C. Calvi, T. Garvey, B. Keil, S. Reiche, V. Schlott, T. Schmidt
  PSI, Villigen

Transverse beam trajectory control is of great importance for SwissFEL as the lasing strategy is based on a relatively low energy and low emittance beam compared with other X-FEL facilities, thus aiming at a reasonable construction cost and size of the facility. A study of beam based alignment and orbit feedback has been performed, and a trajectory correction scenario, which would fulfill the beam requirements as well as the hardware constraints, has been set up. The beam based alignment will be discussed for the linac and the undulator section separately because of the much tighter tolerance in the latter. Several correction algorithms are examined using numerical simulations. BPM requirements and orbit feedback concept will be discussed, with reference to some available data on dynamic disturbances such as ground motion at the PSI site, e.g. at the SwissFEL injector test facility currently under commissioning.

• W.K. Fullagar, D.M. Paganin
  Monash University, Faculty of Science, Victoria
• C.J. Hall
  ASCo, Clayton, Victoria

From a detection perspective, short wavelength phase information is lost when event sizes exceed radiation wavelengths, making conventional holography impossible above a material-dependent quantum energy limit. Despite this, and prior to the invention of lasers or holography, Bragg's X-ray microscope* opened the door to optical computation in short-wavelength studies using spatially coherent visible light, including phase retrieval methods. This optical approach lost ground to semiconductor detection and digital computing in the 1960s. Since then, visible optics such as spatial light modulators, array detectors and femtosecond lasers have become widely available, routinely allowing versatile and computer-interfaced imposition of optical phase, detection, and molecular coherent control in pump-probe studies. Today, FELs begin to offer opportunities for atomic resolution and ultrafast studies. Thus we investigate an overlooked aspect of Bragg's X-ray microscope: the short-wavelength to visible-wavelength, incoherent to coherent conversion that is a necessary prerequisite for coherent optical computations. Some potential approaches, techniques and opportunities are outlined.

* W.L. Bragg, A new type of 'X-Ray microscope', Nature 143, 678 (1939)

• B. Maraghechi, M.H. Rouhani, E. Salehi
  AUT, Tehran

The dynamical stability of electron trajectories in a free-electron laser with planar wiggler is studied. The analysis is based on the numerical simulation of Kolmogorov entropy to investigate how the separation of the trajectories of two neighboring electrons in the six-dimensional phase space evolves along the undulator. Self-electric and self-magnetic fields are taken into account and an adiabatically tapered wiggler magnetic field is used in order to inject the electrons into the wiggler. A considerable decrease in the dynamical stability of electron trajectories was found near the resonance region. It was found that self-fields decrease the dynamical stability of electron trajectories in group I orbits and increase it in group II orbits. Furthermore, the electromagnetic radiation weakens the dynamical stability of electrons as it grows exponentially and become very intense near the saturation point.

• B. Maraghechi, M. Olumi, M.H. Rouhani
  AUT, Tehran

The relativistic cold fluid model is used to study the propagation of the nonlinear traveling wave in a free electron laser (FEL) with electromagnetic wiggler. It is convenient to transform the relevant equations to the frame of reference rotating with the wiggler. The traveling-wave ansatz is employed to obtain three coupled, nonlinear ordinary differential equations that describe the nonlinear propagation of the coupled wave. Saturation and solitary waves in FELs with electromagnetic wiggler may be investigated using these equations. In the small signal limit, the wave equations are linearized and the dispersion relation for the traveling wave is obtained. The numerical solution of the traveling-wave dispersion relation reveals the range of parameters for its unstable solutions. Instability curves with two peaks are found, for which the phase velocity is smaller and larger than the beam velocity.