Dark Current Studies at Relativistic Electron Gun for Atomic Exploration – REGAE

Electron diffraction is a tool for exploring structural dynamics of matter. The scattering cross section is orders of magnitude higher for electrons than for X-rays so that only a small number of electrons is required to achieve comparable results. However, the required electron beam quality is extraordinary. To study e. g. proteins a coherence length of 30 nm is required which translates into a transverse emittance of 5 nm at a spot size of 0.4mm. In addition short bunch lengths down to 10 fs and a temporal stability of the same order are required in order to study chemical reactions or phase transitions in pump probe type experiments. These are challenging parameters for an electron source, which demand improvements at many frontiers. Dark current degrades contrast of diffraction patterns in all experiments. Understanding dark-current generation and propagation can lead to better methods to decrease it. In this paper dark current studies that are performed at REGAE will be presented

Dark Current Studies at Relativistic Electron Gun for Atomic Exploration – REGAE

Electron diffraction is a tool for exploring structural dynamics of matter. The scattering cross section is orders of magnitude higher for electrons than for X-rays so that only a small number of electrons is required to achieve comparable results. However, the required electron beam quality is extraordinary. To study e. g. proteins a coherence length of 30 nm is required which translates into a transverse emittance of 5 nm at a spot size of 0.4mm. In addition short bunch lengths down to 10 fs and a temporal stability of the same order are required in order to study chemical reactions or phase transitions in pump probe type experiments. These are challenging parameters for an electron source, which demand improvements at many frontiers. Dark current degrades contrast of diffraction patterns in all experiments. Understanding dark-current generation and propagation can lead to better methods to decrease it. In this paper dark current studies that are performed at REGAE will be presented

Transverse Emittance Measurement at REGAE

A new linac, named REGAE (Relativistic Electron Gun for Atomic Exploration) has been built at DESY and operates as an electron source for ultra-fast electron diffraction. An RF photocathode gun provides electron bunches of high coherence, sub-pC charge and energies of 2-5 MeV. In order to film time-resolved structural changes of excited specimens, bunch lengths of several femtoseconds need to be created. Taking into account these critical parameters, beam diagnostics at REGAE is very challenging. The existing diagnostics consists of energy, energy spread, beam profile, beam charge and emittance measurements. For transversal diagnostics, specific approaches have to be considered to overcome complications associated with the low charge and to carry out the beam diagnostics in single shot. In this paper, the contribution of the transversal diagnostics to the measurement of the transverse emittance is presented

A Phase Matching, Adiabatic Accelerator

Tabletop accelerators are a thing of the future. Reducing their size will require scaling down electromagnetic wavelengths; however, without correspondingly high field gradients, particles will be more susceptible to phase-slippage – especially at low energy. We investigate how an adiabatically-tapered dielectric-lined waveguide could maintain phase-matching between the accelerating mode and electron bunch. We benchmark our simple model with CST and implement it into ASTRA; finally we provide a first glimpse into the beam dynamics in a phase-matching accelerator

A Transverse Deflection Structure with Dielectric-Lined Waveguides in the Sub-THz Regime

Longitudinal bunch measurements are typically done with rf-powered transverse deflection structures with operating frequencies 1-12 GHz. We explore the use of mm-scale,THz-driven, dielectric-lined cylindrical waveguides as transverse deflectors by driving the fundamental deflecting mode of the structure, the HEM$_{11}$. We give a brief overview ofthe physics, history, and provide an example with a 5 MeV beam using$\tiny{ASTRA}$ andCST-MWS

Dielectric Loaded THz Waveguide Experimentally Optimized by Dispersion Measurements

Emerging high power THz sources pave the road for THz-driven acceleration of ultra-short bunches, and enable theirmanipulation for diagnostic purposes. Due to the smallfeature sizes of THz-guiding devices new methods are nec-essary for their electromagnetic characterization. A newtechnique has recently been developed which characterizesTHz waveguides with respect to their dispersion relationsand attenuation. Here, the method is applied to circularwaveguides, partially filled with polymer capillaries of dif-ferent thicknesses, to find a suitable size for THz drivenstreaking at 287 GHz. Further, rough 3d-printed metallicwaveguides are measured to study the effect of roughnesson attenuation and phase constant. In general, additive man-ufacturing techniques show promise for advanced integrateddesigns of THz driven structures

Preparation of a Prototype Plasma Lens as an Optical Matching Device for the ILC e⁺ Source

In recent years, high-gradient, symmetric focusing with active plasma lenses has regained significant interest due to the potential advantages in compactness and beam dynamics compared to conventional focusing elements. One potential application is the optical matching of highly divergent positrons from the undulator-based ILC positron source into the downstream accelerating structures. A collaboration between University Hamburg and DESY Hamburg has been established to develop a prototype design for this application. Here, we discuss beam dynamics simulation results, preliminary parameters of the lens prototype, and the current status of the prototype design

A Versatile THz Source for High-Repetition Rate XFELs

The development of high-repetition rate XFELs brings an exciting time for novel fundamental science exploration via pump-probe interactions. Laser-based pump sources can provide a wide range of wavelengths (200-10000~nm) via various gain media. These sources can also be extended with optical parametric amplifiers to cover a largely versatile spectral and bandwidth range. However beyond 10~μm, toward the THz regime, there exists no suitable gain media, and optical-to-THz efficiencies are limited below 1\%. In this paper we discuss the use of Cherenkov-based radiators with conventional electron bunches to generate high-power THz radiation over a wide range of parameters for existing and future XFEL facilities

Thermal Simulation of an Energy Feedback Normal Conducting RF Cavity

Thermal simulation has been performed for an energy feedback normal conducting RF cavity. The cavity is going to be used as a fast actuator to regulate the arrival time of the electron bunches in fs level in FLASH. By measuring the arrival time jitter of one bunch in a bunch train, the designed cavity apply a correcting accelerating or decelerating voltage to the next bunches. The input power of the cavity is provided by a solid state amplifier and will be coupled to the cavity via a loop on the body. To achieve the fs level precision of the arrival time, the cavity should be able to provide accurate accelerating voltage with a precision of 300 eV. We performed thermal simulation to find out the temperature distribution of the cavity and make sure that heating will not affect its voltage precision. The simulation results show that by using two input loops the coupling constant will vary from 4.11 to 4.13 during the operation of the cavity which effect on the bunchs' arrival time would be less than 0.25 fs. While using just one input loop can lead to an error of about 1 fs

Plasma Lenses: Possible alternative OMD at the ILC

In the baseline design of the International Linear Collider (ILC) an undulator-based source is foreseen for the positron source in order to match the physics requirements. The recently chosen first energy stage with sqrt(s)=250 GeV requires high luminosity and imposes an effort for all positron source designs at high-energy colliders. In this paper we perform a simulation study and adopt the new technology of plasma lenses to capture the positrons generated by the undulator photons and to create the required high luminosity positron beam