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Extreme-ultraviolet to x-ray free-electron lasers (FELs) in operation for scientific applications are up to now single-user facilities. While most FELs generate around 100 photon pulses per second, FLASH at DESY can deliver almost two orders of magnitude more pulses in this time span due to its superconducting accelerator technology. This makes the facility a prime candidate to realize the next step in FELs-dividing the electron pulse trains into several FEL lines and delivering photon pulses to several users at the same time. Hence, FLASH has been extended with a second undulator line and self-amplified spontaneous emission (SASE) is demonstrated in both FELs simultaneously. FLASH can now deliver MHz pulse trains to two user experiments in parallel with individually selected photon beam characteristics. First results of the capabilities of this extension are shown with emphasis on independent variation of wavelength, repetition rate, and photon pulse length.

High brightness electron sources for linac based free-electron lasers (FELs) are being developed at the Photo Injector Test facility at DESY, Zeuthen site (PITZ). Production of electron bunches with extremely small transverse emittance is the focus of the PITZ scientific program. The photoinjector optimization in 2008-2009 for a bunch charge of 1, 0.5, 0.25, and 0.1 nC resulted in measured emittance values which are beyond the requirements of the European XFEL [S. Rimjaem et al., Nucl. Instrum. Methods Phys. Res., Sect. A 671, 62 (2012)]. Several essential modifications were commissioned in 2010-2011 at PITZ, resulting in further improvement of the photoinjector performance. Significant improvement of the rf gun phase stability is a major contribution in the reduction of the measured transverse emittance. The old TESLA prototype booster was replaced by a new cut disk structure cavity. This allows acceleration of the electron beam to higher energies and supports much higher flexibility for stable booster operation as well as for longer rf pulses which is of vital importance especially for the emittance optimization of low charge bunches. The transverse phase space of the electron beam was optimized at PITZ for bunch charges in the range between 0.02 and 2 nC, where the quality of the beam measurements was preserved by utilizing long pulse train operation. The experimental optimization yielded worldwide unprecedented low normalized emittance beams in the whole charge range studied.

The photoinjector test facility at DESY, Zeuthen site (PITZ), was built to develop and optimize photoelectron sources for superconducting linacs for high-brilliance, short-wavelength free-electron laser (FEL) applications like the free-electron laser in Hamburg (FLASH) and the European x-ray free-electron laser (XFEL). In this paper, the detailed characterization of two laser-driven rf guns with different operating conditions is described. One experimental optimization of the beam parameters was performed at an accelerating gradient of about 43 MV=m at the photocathode and the other at about 60 MV=m. In both cases, electron beams with very high phase-space density have been demonstrated at a bunch charge of 1 nC and are compared with corresponding simulations. The rf gun optimized for the lower gradient has surpassed all the FLASH requirements on beam quality and rf parameters (gradient, rf pulse length, repetition rate) and serves as a spare gun for this facility. The rf gun studied with increased accelerating gradient at the cathode produced beams with even higher brightness, yielding the first demonstration of the beam quality required for driving the European XFEL: The geometric mean of the normalized projected rms emittance in the two transverse directions was measured to be 1:26 ` 0:13 mm mrad for a 1-nC electron bunch. When a 10% charge cut is applied excluding electrons from those phase-space regions where the measured phase-space density is below a certain level and which are not expected to contribute to the lasing process, the normalized projected rms emittance is about 0.9 mm mrad

Coincident measurements of projectile energy loss and kinetic electron emission yield for grazing scattering of 150 eV/amu to some keV/amu neutral hydrogen and helium atoms from an atomically clean and flat Al͑111͒ surface allow us to correlate electron emission and inelastic interaction mechanisms at a metal surface. Our data show evidence for a threshold behavior of kinetic electron emission which is interpreted by energy transfer in binary encounters of projectiles in the electron selvage of a quasi-free electron gas. Contributions of electron emission to projectile energy loss are found to be negligibly small.Ionization of atoms, molecules, and complex matter by impact of particles ͑electrons, atoms, and ions͒ is of fundamental interest and relevant for many practical applications. Since first treatments of electron-impact ionization of atoms by Thomson, 1 numerous authors have considered the ionization process in a basically classical way, e.g., Thomas, 2 Gryzinsky, 3 or Kingston. 4 This led to semiempirical ionization formulas used in atomic and plasma physics. 5,6 For ionization of solids, one distinguishes between two mechanisms ͑1͒ kinetic electron emission ͑KE͒ mediated by the kinetic energy of the projectile and ͑2͒ potential electron emission ͑PE͒ induced by the internal energy of excited or ionized projectiles. 7,8 In reference to gas-phase collisions mentioned above, it is tempting to ask to what extent the KE process may be described by classical concepts, apart from the KE threshold that corresponds to the minimum-energy transfer of projectiles to electrons in a solid to reach vacuum. Such a classical treatment is probably most appropriate for metals that can be described as a free-electron system ͑jellium͒. 9 Here the threshold of projectile velocity v th for KE is derived by assuming maximum-energy transfer of atomic projectiles to free electrons of the metal with Fermi energy E F ͑velocity v F ) in order to overcome the surfaceExperimental studies on the threshold behavior of KE for impact of light ions were not conclusive with respect to v th so far. 8 Aside from uncertainties inherent in the separation of contributions from PE to electron yields by using ionized projectiles, KE may be caused by several other mechanisms, 8,10 in particular, for heavier atomic projectiles by electron promotion in close collisions with target atom cores. 11 A specific technical problem for a reliable determination of v th concerns small electron yields as low as ␥ р10 Ϫ3 electrons/projectile which are extremely difficult to obtain from measurements of ion and electron currents. 12 Furthermore, KE induced by impact of atomic projectiles is accompanied by ͑electronic͒ excitation of the target which cannot be elucidated by KE measurements only.In this paper, we report on studies on the threshold behavior of KE for grazing incidence scattering of fast neutral hydrogen and helium atoms from an Al͑111͒ surface. For this specific collision geometry, scattering of projectiles proceeds in the regime of planar surface cha...