We discuss the progress in the R&D program for a future upgrade of the European XFEL facility, namely for an operation in the continuous wave (cw) and long pulse (lp) modes, which will allow for significantly more flexibility in the electron and photon beam time structure. Results of cw/lp runs with preseries XFEL cryomodules and status of components needed for the new operation modes are presented here.

A radio-frequency photoinjector is widely used for generating picosecond-femtosecond electron bunches. During particle acceleration, the electron bunch shape varies under the influence of both the acceleration fields and space charge fields. To restore a distorted electron bunch, we propose an injection phase compensation technique for the electrons by illuminating the photocathode with a pulse-front-corrected laser pulse. Using an S-band 1.6-cell photoinjector as an example, our study shows that the proposed technique can generate nearly undistorted electron bunches up to a bunch rate of a few hundred terahertz.

A detailed model of the high-luminosity LHC inner triplet region with new large-aperture ${\mathrm{Nb}}_3\mathrm{Sn}$ magnets, field maps, corrector packages, and segmented tungsten inner absorbers was built and implemented into the fluka and mars15 codes. Detailed simulations have been performed coherently with the codes on the impact of particle debris from the 14-TeV center-of-mass pp-collisions on the short- and long-term stability of the inner triplet magnets. After optimizing the absorber configuration, the peak power density averaged over the magnet inner cable width is found to be safely below the quench limit at the luminosity of $5\times {10}^{34}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$. For the anticipated lifetime integrated luminosity of $3000{\mathrm{fb}}^{-1}$, the peak dose calculated for the innermost magnet insulator ranges from 20 to 35 MGy, a figure close to the commonly accepted limit. Dynamic heat loads to the triplet magnet cold mass are calculated to evaluate the cryogenic capability. fluka and mars results on energy deposition are in very good agreement.

A decade-long effort at the Advanced Photon Source (APS) of Argonne National Laboratory (ANL) on development of superconducting undulators culminated in December 2012 with the installation of the first superconducting undulator “SCU0” into Sector 6 of the APS storage ring. The device was commissioned in January 2013 and has been in user operation since. This paper presents the magnetic and cryogenic design of the SCU0 together with the results of stand-alone cold tests. The initial commissioning and characterization of SCU0 as well as its operating experience in the APS storage ring are described.

We proposed a novel design for a compact superconducting crab cavity with a double quarter wave (DQWCC) shape. After fabrication and surface treatments, this niobium proof-of-principle cavity was tested cryogenically in a vertical cryostat. The cavity is extremely compact yet has a low frequency of 400 MHz, an essential property for service in the Large Hadron Collider luminosity upgrade. The cavity’s electromagnetic properties are well suited for this demanding task. The demonstrated deflecting voltage of 4.6 MV is well above the required 3.34 MV for a crab cavity in the future High Luminosity LHC. In this paper, we present the design, prototyping, and results from testing the DQWCC.

Propagation of a Gaussian x-ray laser beam has been analyzed in collisionless thermal quantum plasma with considering a ramped density profile. In this density profile due to the increase in the plasma density, an earlier and stronger self-focusing effect is noticed where the beam width oscillates with higher frequency and less amplitude. Moreover, the effect of the density profile slope and the initial plasma density on the laser propagation has been studied. It is found that, by increasing the initial density and the ramp slope, the laser beam focuses faster with less oscillation amplitude, smaller laser spot size and more oscillations. Furthermore, a comparison is made among the laser self-focusing in thermal quantum plasma, cold quantum plasma and classical plasma. It is realized that the laser self-focusing in the quantum plasma becomes stronger in comparison with the classical regime.

A novel technique using single-slit x-ray Fresnel diffraction has been developed to resolve $\mu \mathrm{m}$-order electron beam sizes at the insertion devices (IDs) of photon beam lines. The new technique is promising for diagnostics of next-generation light sources, where a tuning of ultralow emittance at insertion devices is essentially important to ensure the absence of the degradation of brilliance and the transverse coherence of radiation at beam lines due to distortion of lattice functions. The validity of the method was experimentally studied at SPring-8, and the achievable resolution is discussed.

This paper presents a comparison between simulation results and a first principles analytical model of electron back-bombardment developed at Colorado State University for single-cell, thermionic-cathode rf guns. While most previous work on back-bombardment has been specific to particular accelerator systems, this work is generalized to a wide variety of guns within the applicable parameter space. The merits and limits of the analytic model will be discussed. This paper identifies the three fundamental parameters that drive the back-bombardment process, and demonstrates relative accuracy in calculating the predicted back-bombardment power of a single-cell thermionic gun.

The drive beam phase stability is one of the critical feasibility issues of the Compact Linear Collider (CLIC) project. This paper presents a step-by-step analysis of the error propagation through the CLIC drive beam complex using realistic rf potential and beam loading amplitude functions for the drive and main beam accelerating structures. The impact of planned stabilization systems for drive beam bunch charge and longitudinal phase is simulated and the optimal specifications for such systems are calculated and discussed.

For many new accelerator applications, superconducting radio frequency systems are the enabling technology. In particular for CW applications, much effort is being expended to minimize the power dissipation (surface resistance) of niobium cavities. Starting in 2009, we suggested a means of reducing the residual resistance by performing a thermal cycle [O. Kugeler et al., in Proceedings of the 14th International Conference on RF Superconductivity (2009), p. 352], a procedure of warming up a cavity after initial cooldown to about 20 K and cooling it down again. In subsequent studies [J. M. Vogt, O. Kugeler, and J. Knobloch, Phys. Rev. ST Accel. Beams 16, 102002 (2013)], this technique was used to manipulate the residual resistance by more than a factor of 2. It was postulated that thermocurrents during cooldown generate additional trapped magnetic flux that impacts the cavity quality factor. Here, we present a more extensive study that includes measurements of two additional passband modes and that confirms the effect. In this paper, we also discuss simulations that support the claim. While the layout of the cavity LHe tank system is cylindrically symmetric, we show that the temperature dependence of the material parameters results in a nonsymmetric current distribution. Hence a significant amount of magnetic flux can be generated at the rf surface.

The CERN Large Hadron Collider (LHC) is designed to accelerate and bring into collision high-energy protons as well as heavy ions. Accidents involving direct beam impacts on collimators can happen in both cases. The LHC collimation system is designed to handle the demanding requirements of high-intensity proton beams. Although proton beams have 100 times higher beam power than the nominal LHC lead ion beams, specific problems might arise in case of ion losses due to different particle-collimator interaction mechanisms when compared to protons. This paper investigates and compares direct ion and proton beam impacts on collimators, in particular tertiary collimators (TCTs), made of the tungsten heavy alloy INERMET 180. Recent measurements of the mechanical behavior of this alloy under static and dynamic loading conditions at different temperatures have been done and used for realistic estimates of the collimator response to beam impact. Using these new measurements, a numerical finite element method (FEM) approach is presented in this paper. Sequential fast-transient thermostructural analyses are performed in the elastic-plastic domain in order to evaluate and compare the thermomechanical response of TCTs in case of critical beam load cases involving proton and heavy ion beam impacts.

Radio-frequency (rf) photoinjectors are commonly used to generate intense bright electron beams for a wide range of applications, most notably as drivers for X-ray Free-Electron Lasers. The photocathode, mounted inside an rf gun and illuminated by a suitable laser, thereby plays a crucial role as the source of the electrons. The intrinsic emittance and the quantum efficiency of the electron source are determined by the properties of the photocathode’s surface material. We present measurements of the intrinsic emittance and the quantum efficiency performed with copper and cesium telluride cathodes in the same rf photoinjector, thus comparing, for the first time, the performance of metal and semiconductor cathodes under the same conditions. Our results are consistent with theoretical expectations and show that the difference in intrinsic emittance for the two types of material is not significant in view of accelerator applications. We conclude that cesium telluride photocathodes provide a much higher quantum efficiency at essentially negligible degradation in beam emittance.

Dynamics of shielding of a moving charged particle in a confined plasma, which represents a realistic model of an electron beam in accelerators, is studied. This is a longstanding problem in plasma physics with important applications in accelerator physics. However, only solutions for an infinite unrealistic plasma are available. We developed a novel method to solve this problem, which consists of transformation of the Vlasov-Poisson differential equations to an integral equation for the Laplace image of the electron density perturbation created by an external charge. The integral equation is then solved numerically via the piecewise polynomial collocation method and the fast Fourier transform. We present thorough analysis of the results obtained and their physical interpretation.

To meet the Facility for Antiproton and Ion Research science requirements higher beam intensity has to be achieved in the present GSI-accelerator complex. For this an advanced upgrade program for the UNILAC is ongoing. Stripping is a key technology for all heavy ion accelerators. For this an extensive research and development program was carried out to optimize for high brilliance heavy ion operation. After upgrade of the supersonic ${\mathrm{N}}_2$-gas jet (2007), implementation of high current foil stripping (2011) and preliminary investigation of ${\mathrm{H}}_2$-gas jet operation (2012), recently (2014) a new ${\mathrm{H}}_2$-gas cell using a pulsed gas regime synchronized with arrival of the beam pulse has been developed. An obviously enhanced stripper gas density as well as a simultaneously reduced gas load for the pumping system result in an increased stripping efficiency, while the beam emittance remains the same. A new record intensity (7.8 emA) for ${{}^{238}\mathrm{U}}^{28+}$ beams at $1.4\mathrm{MeV}/\mathrm{u}$ has been achieved applying the pulsed high density ${\mathrm{H}}_2$ stripper target to a high intensity ${{}^{238}\mathrm{U}}^{4+}$ beam from the VARIS ion source with a newly developed extraction system. The experimental results are presented in detail.