_applied_  
_sciences_  
_Article_  
Flexible and Coherent Soft X-ray Pulses at High Repetition  
Rate: Current Research and Perspectives  
[PERSON] 1, [PERSON] 1, [PERSON] 1,*, [PERSON] 1,Karolin [PERSON] 1, [PERSON] 1, [PERSON] 1, [PERSON] 1, [PERSON] 2, [PERSON] 1, [PERSON] 1, [PERSON] 1, [PERSON] 1, [PERSON] 2, [PERSON] 1, [PERSON] 2, [PERSON] 1, [PERSON] 1, [PERSON] 1, [PERSON] 1, [PERSON] 1 and [PERSON] 1  
Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany; [PERSON]S] (L.S. ); [PERSON]] (S.A.); [PERSON] (P.A. ); [EMAIL_ADDRESS] (K.B. ); [EMAIL_ADDRESS] (M.B. ); [EMAIL_ADDRESS] (C.G. ); [PERSON]SS] (L.H. ); [EMAIL_ADDRESS] (K.H. ); [EMAIL_ADDRESS] (M.M.K. ); [EMAIL_ADDRESS] (L.H. ); [EMAIL_ADDRESS] (P.N. ); [EMAIL_ADDRESS] (J.R.); [PERSON].S. ); [EMAIL_ADDRESS] (S.S.); [PERSON] (M.T. ); [EMAIL_ADDRESS] (M.V. ); [EMAIL_ADDRESS] (M.V. ); [EMAIL_ADDRESS] (J.Z)  
Institute for Experimental Physics, University of Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany; [EMAIL_ADDRESS] (W.H. ); [PERSON]S] (F.P. ); [PERSON]D.S.) Curent Research and Perspectives. _Article_  
Flexible and Coherent Soft X-ray Pulses at High Repetition  
Since the first FEL pulses became available to users at FLASH (DESY Hamburg, Germany) [4] more than 15 years ago, there has been continuous technological and scientific progress allowing to extend the parameter range accessible by these sources. The use of higher energy accelerators combined with high brightness electron beams has allowed exceeding the 25 keV of photon energy [11]. Moreover, the recognizable progress in the undulator design has made features such as variable polarization available [12; 13]. The latter, in combination with the accurate control of the beam dynamics, supported advanced options for two-colors or two pulses [14; 15] or harmonic lasing [16; 17; 18] that have been proposed and delivered to the users. The control of the coherence of the X-ray pulses has also become attainable due to the seeding techniques relying on external lasers [19; 20; 21] or intermediate monochromatization during the FEL process [22; 23; 24]. Thanks to the availability of fully coherent sources in the XUV and soft X-ray spectral range, coherent control [25] has recently been extended to the this spectral range opening the door to a new field of research that is growing very fast [26; 27; 28; 29]. Coherent control methods developed in the optical regime are now possible at much shorter wavelengths enabling new science by addressing elementally selective core resonances uniquely found at these wavelengths. Full control of the FEL pulse phase has become possible and exploited in a variety of experiments [26; 27; 29; 30; 31; 32]. Each FEL pulse contains a large number of photons but the number of pulses typically produced by FELs is limited to a few tens per second being limited by the repetition rate of the accelerator. This limit is overcome by the use of superconducting accelerating cavities [33] that can allow for much higher repetition rates. Currently, only FLASH [34] and European XFEL [10] are relying on this technology and can produce several thousands of pulses per second. Few more facilities are planning to use the same technology to go even further [35; 36]. At present, these high repetition rate FELs only operate in Self Amplified Spontaneous Emission (SASE) [37; 38] mode and development toward highly coherent FELs at high repetition rate is still ongoing [39]. Very recently European XFEL has demonstrated the possibility to operate the self-seeding setup in multi-bunch mode showing effects produced by the heat load into the monochromatizing crystal [40] that need to be understood and solved before being able to reach the full accelerator repetition rate. To date, no externally seeded FEL exists that provides more than 50 pulses per second, mainly due to the challenge to develop a laser system that is capable of satisfying the stringent requirements for seeding with the high repetition rate of superconducting accelerators.