SINAP, Shanghai, People's Republic of China
In this paper, design studies for the proof-of-principle experiment of the recently proposed phase-merging enhanced harmonic generation (PEHG) mechanism are presented. A dogleg and a new designed transverse gradient undulator should be added in the undulator system of SDUV-FEL to perform the phase-merging effect. With the help of 3D simulation codes, we show the possible performance of PEHG with the realistic parameters of SDUV-FEL.
* H. Deng, C. Feng, Phys. Rev. Lett. 111, 084801.
** C. Feng, H. Deng, D. Wang, Z. Zhao, New J. Phys.,16, 043021.
*** C. Feng, T. Zhang, H. Deng, Z. Zhao, Phys. Rev. ST Accel. Beams 17, 070701.
LBNL, Berkeley, California, USA
SLAC, Menlo Park, California, USA
The initial design for LCLS-II incorporates both SASE and self-seeded configurations. Increased stability and/or coherence than is possible with either configuration may be provided by seeding with external lasers followed by one or more stages of harmonic generation, especially in the soft x-ray regime. External seeding also allows for increased flexibility, for example the ability to quickly vary the pulse duration. Studies of schemes based on high-gain harmonic generation and echo-enabled harmonic generation are presented, including realistic electron distributions based on tracking through the injector and linac.
DELTA, Dortmund, Germany
High Gain Harmonic Generation (HGHG) seeding has been demonstrated in the visible and ultraviolet, but it is limited in performance at high harmonics of the seed by the initial uncorrelated energy spread of the electron beam. A recent proposal from SINAP using a chirped electron beam and a canted pole undulator has suggested a new mechanism for cooling the uncorrelated energy spread of the electron beam in order to improve the performance of HGHG seeding at high harmonics. This note reviews the mechanism, the limitation of the concept and extrapolates to some new concepts using analogous mechanisms derived from transverse gradients of the laser properties. The impact of CSR wakes on the vanishingly short microbunches produced by the methods are also investigated.
[1] H. Deng and C. Feng, Phys. Rev. Lett. 111, 084801 (2013)
DELTA, Dortmund, Germany
This note describes a scheme to seed the FLASH2 FEL over a range of 4-40 nm without impacting SASE capabilities. This scheme combines multiple seeding techniques, builds on current infrastructure and offers a maximized range of performance with higher pulse energies than what are available at lower-peak current facilities. The concept relies on Echo Enabled Harmonic Generation (EEHG), cascaded seeding, and Second Harmonic Afterburners (SHAB) while maintaining the possibility to operate with High Gain Harmonic Generation (HGHG) seeding at >30 nm wavelengths.
DESY, Hamburg, Germany
USTRAT/SUPA, Glasgow, United Kingdom
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom
A review is presented of some of the methods and their origins that have recently been proposed to improve the temporal coherence of SASE output. These methods do not require any external laser seed field, or the use of the so-called self-seeding methods, where the SASE radiation is optically filtered and improved at an early stage of the interaction before re-injection and amplification to saturation. By using methods that introduce an additional relative propagation between the electron beam and the radiation field, the localised collective interaction, which leads to the formation of the ‘spiking’ associated with normal SASE output, is removed. The result is output pulses which are close to the fourier transform limit without the need for any external seeds or intermediate optics.
Stanford University, Stanford, California, USA
SLAC, Menlo Park, California, USA
A 3.2 m adjustable phase Delta undulator* will soon be installed on the last girder of the LCLS undulator line. The Delta undulator will act as an afterburner terminating the 33 undulator line, providing arbitrary polarization control to users. Two important figures of merit for users will be the degree of polarization and the x-ray yield. In anticipation of this installation, machine development time at the LCLS was devoted to maximizing the final undulator x-ray contrast and yield with a standard canted pole undulator acting as a stand in for the Delta undulator. Following the recent suggestion** that a reverse taper (dK/dz > 0) in the main undulator line could suppress linearly polarized light generated before an afterburner while still producing the requisite microbunching, we report on a reverse taper study at the LCLS wherein a yield contrast of 15 was measured along the afterburner. We also present 1D simulations comparing the reverse taper technique to other schemes.
* Nuhn, H.-D., Anderson, S., Bowden, G., Ding, Y., Gassner, G., et al., (2013).
** Schneidmiller, E. A. and Yurkov, M. V., Phys. Rev. ST Accel. Beams 16, 110702 (2013).
HZB, Berlin, Germany
Uppsala University, Uppsala, Sweden
Ariel University, Ariel, Israel
Stony Brook University, Stony Brook, USA
BNL, Upton, Long Island, New York, USA