Single shot cathode transverse momentum imaging in high brightness photoinjectors

In state of the art photoinjector electron sources, thermal emittance from photoemission dominates the final injector emittance. Therefore, low thermal emittance cathode developments and diagnostics are very important. Conventional thermal emittance measurements for the high gradient gun are time-consuming and thus thermal emittance is not measured as frequently as quantum efficiency during the lifetime of photocathodes, although both are important properties for the photoinjector optimizations. In this paper, a single shot measurement of photoemission transverse momentum, i.e., thermal emittance per rms laser spot size, is proposed for photocathode rf guns. By tuning the gun solenoid focusing, the electrons' transverse momenta at the cathode are imaged to a downstream screen, which enables a single shot measurement of both the rms value and the detailed spectra of the photoelectrons' transverse momenta. Both simulations and proof of principle experiments are reported

Emittance Reduction of RF Photoinjector Generated Electron Beams by Transverse Laser Beam Shaping

Laser pulse shaping is one of the key elements to generate low emittance electron beams with RF photoinjectors. Ultimately high performance can be achieved with ellipsoidal laser pulses, but 3-dimensional shaping is challenging. High beam quality can also be reached by simple transverse pulse shaping, which has demonstrated improved beam emittance compared to a transversely uniform laser in the 'pancake' photoemission regime. In this contribution we present the truncation of a Gaussian laser at a radius of approximately one sigma in the intermediate (electron bunch length directly after emission about the same as radius) photoemission regime with high acceleration gradients (up to 60 MV/m). This type of electron bunch is used e.g. at the European XFEL and FLASH free electron lasers at DESY, Hamburg site and is being investigated in detail at the Photoinjector Test facility at DESY in Zeuthen (PITZ). Here we present ray-tracing simulations and experimental data of a laser beamline upgrade enabling variable transverse truncation. Initial projected emittance measurements taken with help of this setup are shown, as well as supporting beam dynamics simulations. Additional simulations show the potential for substantial reduction of slice emittance at PITZ. © Published under licence by IOP Publishing Ltd

Highly Sensitive Beam Size Monitor for pA Currents at the MLS Electron Storage Ring

For the operation of the Metrology Light Source MLS [1], the electron storage ring of the Physikalisch Technische Bundesanstalt PTB , as a primary radiation source standard all storage ring parameters have to be known absolutely. For the measurement of the electron beam size and the control of the stability of the orbit location a new imaging system has been set up, that operates at very different intensity levels covering more than 11 decades, given by the variation of the electron beam current. The system uses a commercial zoom lens for the achromatic optical imaging of the electron beam source point onto two different camera systems. One camera system is for life imaging of the source point at electron beam currents from 200 mA down to some amp; 956;A. The second system is a cooled CCD camera that allows imaging of the electron beam source size and location at very low currents, down to only one stored electro

Observation of Ion Induced Effects and Their Impact on the Performance of the MLS Electron Storage Ring

The Metrology Light Source MLS [1] is in user operation since 2008 at operating energies ranging from 105 MeV up to 630 MeV and with multi bunch currents up to 200 mA. At the injection energy of 105 MeV as soon as the beam current exceeds a few mA, the beam is strongly blown up in all three spatial dimensions and strong oscillations at very different spectral frequencies can be observed. These effects are caused by the interaction of beam charge with ions present and their strength and characteristic time scales depend on several machine parameters. As ion effects can strongly deteriorate the performance of the MLS, we report on first investigation

Tracing ultrafast dynamics of strong fields at plasma-vacuum interfaces with longitudinal proton probing

If regions of localized strong fields at plasma-vacuum interfaces are probed longitudinally with laser accelerated proton beams their velocity distribution changes sensitively and very fast. Its measured variations provide indirectly a higher temporal resolution as deduced from deflection geometries which rely on the explicit temporal resolution of the proton beam at the position of the object to probe. With help of reasonable models and comparative measurements changes of proton velocity can trace the field dynamics even at femtosecond time scale. In longitudinal probing, the very low longitudinal emittance together with a broad band kinetic energy distribution of laser accelerated protons is the essential prerequisite of the method. With a combination of energy and one-dimensional spatial resolution, we resolve fast field changes down to 100 fs. The used pump probe setup extends previous schemes and allows discriminating simultaneously between electric and magnetic fields in their temporal evolution. VC 2014 AIP Publishing LLC.1571sciescopu

Towards Experimental Investigation of Hosing Instability Mitigation at the PITZ Facility

Beam-driven plasma wakefield acceleration (PWFA) allows for high gradient acceleration of electron beams and hence is a promising candidate for compact and cost-efficient drivers of applications demanding high brightness beams. One of the main challenges in these accelerators is to control beam-plasma instabilities with rapid growth rates which are induced by the strong transverse components of the wakefields. The hosing instability, a growing transverse oscillation of the beam centroid caused by coherent coupling between bunch slice centroids and transverse wakefields, was predicted to set severe limits on the possible acceleration distance in PWFAs. Several methods have been proposed to damp or even suppress the hosing of the beam, prevent beam-breakup and thus allow stable operation. Here, we present preparations and simulation studies aiming at the experimental investigation of hosing suppression mechanisms at the PITZ facility

Budgeting the Emittance of Photoemitted Electron Beams in a Space-Charge affected Emission Regime for Free-Electron Laser Applications

Free-electron laser based x-ray facilities require high-brightness photoinjectors to provide low emittance electron beams at a fixed bunch charge. The emittance optimization in the injector determines the lowest achievable emittance. Based on experimental emittance optimization at the photoinjector test facility at DESY in Zeuthen, a space-charge affected emission regime is identified, in which the optimum transverse beam emittance is achieved and thus, the injector is routinely operated in this regime. An advanced modeling approach is proposed to consider a dynamic emission process in the simulation of injector beam dynamics, meanwhile allowing detailed studies of the impact of strong space-charge fields during emission on the slice formation of the emitted electron bunch at the cathode. As an application, the proposed approach is used to analyze the budget of the optimized transverse beam emittance. An interplay, taking place in the identified emission regime, between intrinsic cathode emittance and space-charge induced emittance is demonstrated. The resolved behavior by simulation is consistent with the corresponding measurement under practical operation conditions of interest. The obtained results are reported

Dark Current Studies of an L-band Normal Conducting RF Gun

L-band normal conducting RF guns operating at long pulse mode and high gradient are installed at the European X-ray Free-Electron Laser (European XFEL) and the Free-electron LASer in Hamburg (FLASH) as high brightness electron sources. The dark current generated from the gun may cause severe issues, such as component activation, superconducting radio frequency (SRF) cavity quench and undulator degradation. The dark current is one of the parameters limiting the gun accelerating gradient and performance. In this paper, we report the dark current tracking simulations, including the detailed RF field distribution in the gap between cathode plug and copper hole in the backplane. The transmission ratios of the field emission from cathode regions were simulated for different copper hole geometries and different plug insertions. The dark current imaging measurements were conducted in an L-band normal conducting RF gun (gun 4.2) at the Photo Injector Test facility at DESY in Zeuthen (PITZ). The dark current imaging comparison before and after a 180 degree cathode plug rotation shows the main dark current emissions come from the copper hole corner and Cs$_2$Te film edge, which is consistent with the dark current tracking simulations

Overview and Prospects of Plasma Wakefield Acceleration Experiments at PITZ

The Photo Injector Test Facility at DESY in Zeuthen (PITZ) carries out studies of beam-driven plasma wakefield acceleration (PWFA). The facility possesses a flexible photocathode laser beam shaping system and a variety of diagnostics including a high-resolution dipole spectrometer and an rf deflector which enables the observation of the longitudinal phase space of electron beams after their passage through a plasma. Two plasma sources are available: a gas discharge plasma cell and a photoionized lithium vapor plasma cell. Studies at PITZ include investigations of the self-modulation instability of long electron beams and the high transformer ratio, i.e., the ratio between the maximum accelerating field behind the drive beam and the decelerating field within the beam. This overview includes the experimental results and plans for future experiments