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FLASH and its Perspectives
Improvements for the users, self-seeding, faster beam switching and more
The main goal at FLASH in the next few years is to extend the possibilities for frontline research that may lead to scientific breakthroughs. Thus, several technological developments are being considered in order to improve the performance of the FEL.
FLASH is the only soft X-ray FEL user facility worldwide until 2009 when the Linear Coherent Light Source LCLS in Stanford will become available. Therefore all efforts are currently made to provide reliable and stable conditions for user experiments. To reach this goal and especially the design specifications for FLASH, extensive studies of the FEL process and improvements of the injector and the linear accelerator have to be performed in alternation with operation for users. Easy and precise tuning of the electron energy is another goal of great importance for the users. The development of diagnostic tools for electron and photon beams will continue to be of key importance for the success of the facility. A wiggler for the production of infrared (IR) radiation installed behind the undulators of FLASH will allow the determination of the longitudinal charge distribution of the electron bunches as well as novel pump-and-probe experiments combining IR with FEL radiation in the spectral range of the vacuum ultraviolet and extreme ultraviolet.
For the years to come most users are mainly interested in intense flashes of X-rays of 10 femtoseconds duration or even shorter, e.g. for pump-and-probe experiments. Here very good synchronization or accurate shot-by-shot measurements of the arrival time of the two pulses are needed.
Measurements of the time differences down to 100 fs have been done successfully at FLASH, these techniques will be further improved and developed to become routine tools. Research and development activities at DESY aim at reducing the jitter in the linac to the 10-femtosecond level. The most promising approach is based on the distribution of laser pulse streams in length-stabilized optical fiber links to all timing-critical locations including the linac cavities and the optical lasers used for pump-and-probe experiments. The laser pulse stream is generated by an erbium-doped fiber laser operating at a wavelength of 1550 nanometers where telecommunication components are inexpensive and readily available.
The timing and synchronization information is encoded in the precise repetition rate of the laser pulses. The system acts like an extremely precise master clock with an exact transmission of the timing signal to the various components affecting the jitter in a pump-and-probe experiment. A scheme for transporting short optical laser pulses over long distances with an accuracy of the order of few femtoseconds has already been demonstrated. In addition split and delay devices are under construction, which are of special importance for pump-and-probe experiments using the fundamental and higher harmonics in the FEL beam.
