arXiv subjects

Uwe Niedermayer

Publications and source records attributed to Uwe Niedermayer.

18 recordsLinked to original sources

arXiv2024

Subrelativistic Alternating Phase Focusing Dielectric Laser Accelerators

We demonstrate a silicon-based electron accelerator that uses laser optical near fields to both accelerate and confine electrons over extended distances. Two dielectric laser accelerator (DLA) designs were tested, each consisting of two arrays of silicon pillars pumped symmetrically by pulse front tilted laser beams, designed for average acceleration gradients 35 and 50 MeV/m respectively. The DLAs are designed to act as alternating phase focusing (APF) lattices, where electrons, depending on the electron-laser interaction phase, will alternate between opposing longitudinal and transverse focusing and defocusing forces. By incorporating fractional period drift sections that alter the synchronous phase between $\pm 60^\circ$ off crest, electrons captured in the designed acceleration bucket experience half the peak gradient as average gradient while also experiencing strong confinement forces that enable long interaction lengths. We demonstrate APF accelerators with interaction lengths up to 708 $μ$m and energy gains up to 23.7 $\pm$ 1.07 keV FWHM, a 25$\%$ increase from starting energy, demonstrating the ability to achieve substantial energy gains with subrelativistic DLA.

physics.acc-ph↗

arXiv2023

FemtoTrack: A space-charge tracking tool for few-electron ultrashort bunches

Ultrafast electron experiments usually work with low-emittance few-electron pulsed beams. The structures are usually much larger than the (average) electron pulse size posing challenges to the resolution of simulations. We present the tracking code FemtoTrack, which allows tackling these multi-scale challenges in a simple manner. The computationally most heavy interpolation of the fields is treated either by a moving window or by a supplemental grid, leading to a significant speedup. Space charge is treated by direct particle-particle interaction within each bunch, where however many bunches can be simulated in the same window simultaneously. This allows to obtain statistics similar to what is obtained on the screen in an experiment. FemtoTrack is applied to two examples: An ultrafast nanotip electron source and a length scalable laser-driven electron accelerator on a microchip. In these setups, previous results have been reproduced with tremendous speedup, allowing for parameter scans similar to the tuning of an experiment.

physics.acc-ph↗

arXiv2022

Design Study of a Dielectric Laser Undulator

Dielectric laser acceleration (DLA) achieves remarkable gradients from the optical near fields of a grating structure. Tilting the dielectric grating with respect to the electron beam leads to deflection forces and the DLA structure can be utilized as a microchip undulator. We investigate the beam dynamics in such structures analytically and by numerical simulations. A crucial challenge is to keep the beam focused, especially in direction of the narrow channel. An alternating phase focusing scheme is optimized for this purpose and matched lattice functions are obtained. We distinguish synchronous operation with phase jumps in the grating and asynchronous operation with a strictly periodic grating and well-designed synchronicity mismatch. Especially the asynchronous DLA undulator is a promising approach, since a simple, commercially available grating suffices for the focusing lattice design. We pave the way towards experiments of radiation generation in these structures and provide estimates of the emitted radiation wavelength and power. The analytical models are validated by numerical simulations in the dedicated DLA simulation tool DLAtrack6D and Astra, where the underlying laser fields are computed by CST Studio.

physics.acc-ph↗

arXiv2022

Spatio-Temporal Coupling Controlled Laser for Electron Acceleration

Limited by the difficulty in acceleration synchronization, it has been a long-term challenge for on-chip dielectric laser-based accelerators (DLA) to bridge the gap between non-relativistic and relativistic regimes. Here, we propose a DLA based on a spatio-temporal coupling (STC) controlled laser pulse, which enables the acceleration of a non-relativistic electron to a sub-MeV level in a single acceleration structure (chirped spatial grating). It provides high precision temporal and spatial tuning of the driving laser via the dispersion manipulation, leading to a synchronous acceleration of the velocity increasing electrons over a large energy range. Additionally, the STC scheme is a general method and can be extended to driving fields of other wavelengths such as terahertz pulses. Our results bring new possibilities to MeV-scale portable electron sources and table-top acceleration experiments.

physics.optics↗

arXiv2022

High Gradient Silicon Carbide Immersion Lens Ultrafast Electron Sources

We present two compact ultrafast electron injector designs with integrated focusing that provide high peak brightness of up to $1.9*10^{12} A/m^2Sr^2$ with 10s of electrons per laser pulse using silicon carbide electrodes and silicon nanotip emitters. We demonstrate a few centimeter scale 96 keV immersion lens electron source and a 57 keV immersion lens electron source with a 19 kV/mm average acceleration gradient, nearly double the typical 10 kV/mm used in DC electron sources. The brightness of the electron sources is measured alongside start-to-end simulations including space charge effects. These sources are suitable for dielectric laser accelerator experiments, ultrafast electron diffraction, and other applications where a compact high brightness electron source is required.

physics.acc-ph↗

arXiv2021

Electron phase space control in photonic chip-based particle acceleration

Particle accelerators are essential tools in science, hospitals and industry. Yet, their costs and large footprint, ranging in length from meters to several kilometres, limit their use. The recently demonstrated nanophotonics-based acceleration of charged particles can reduce the cost and size of these accelerators by orders of magnitude. In this approach, a carefully designed nano-structure transfers energy from laser light to the particles in a phase-synchronous manner, thereby accelerating them. However, so far, confinement of the particle beam in the structure over extended distance has been elusive; it requires complex control of the electron beam phase space and is mandatory to accelerate particles to the MeV range and beyond with minimal particle loss. Here, we demonstrate complex electron phase space control at optical frequencies in the 225 nanometre narrow channel of a record-long photonic nanostructure. In particular, we experimentally show alternating phase focusing, a particle propagation scheme for, in principle, arbitrarily long minimal-loss transport. We expect this work to enable MeV electron beam generation on a photonic chip, with direct ramification for new forms of radiotherapy and compact light sources, and other forms of electron phase space control resulting in narrow energy or zeptosecond-bunched beams, for instance.

physics.acc-ph↗

arXiv2020

Low Energy Spread Attosecond Bunching and Coherent Electron Acceleration in Dielectric Nanostructures

We demonstrate a compact technique to compress electron pulses to attosecond length, while keeping the energy spread reasonably small. The technique is based on Dielectric Laser Acceleration (DLA) in nanophotonic silicon structures. Unlike previous ballistic optical microbunching demonstrations, we use a modulator-demodulator scheme to compress phase space in the time and energy coordinates. With a second stage, we show that these pulses can be coherently accelerated, producing a net energy gain of $1.5\pm0.1$ keV, which is significantly larger than the remaining energy spread of $0.88 \,_{-0.2}^{+0.0}$ keV FWHM. We show that by linearly sweeping the phase between the two stages, the energy spectrum can be coherently moved in a periodic manner, while keeping the energy spread roughly constant. After leaving the buncher, the electron pulse is also transversely focused, and can be matched into a following accelerator lattice. Thus, this setup is the prototype injector into a scalable DLA based on Alternating Phase Focusing (APF).

physics.acc-ph↗

arXiv2020

Threedimensional Alternating-Phase Focusing for Dielectric-Laser Electron Accelerators

The concept of dielectric-laser acceleration (DLA) provides the highest gradients among breakdown-limited (nonplasma) particle accelerators and thus the potential of miniaturization. The implementation of a fully scalable electron accelerator on a microchip by twodimensional alternating phase focusing (APF), which relies on homogeneous laser fields and external magnetic focusing in the third direction, was recently proposed. In this Letter, we generalize the APF for DLA scheme to 3D, such that stable beam transport and acceleration is attained without any external equipment, while the structures can still be fabricated by entirely twodimensional lithographic techniques. In the new scheme, we obtain signifcantly higher accelerating gradients at given incident laser field by additionally exploiting the new horizontal edge. This enables ultra-low injection energies of about 2.5 keV (beta = 0.1) and bulky high voltage equipment as used in previous DLA experiments can be omitted. DLAs have applications in ultrafast time-resolved electron microscopy and -diffraction. Our fndings are crucial for the miniaturization of the entire setup and pave the way towards integration of DLAs in optical fiber driven endoscopes, e.g., for medical purposes.

physics.acc-ph↗

arXiv2019

Analytical energy spectra and wake effects for relativistic dielectric laser accelerators

Dielectric laser acceleration (DLA) is one of the advanced concepts for more compact accelerators. DLA gratings have apertures and period lengths within the range of optical wavelengths. Phase stability and wakefield effects are thus crucial for upcoming experiments with relativistic electrons. For this, we present a method to analytically calculate energy spectra for comparison with measurements in order to reconstruct the phase of injection into the DLA grating structure. Knowing the injection phase is important for both alignment and interpretation of measured data. Furthermore, we estimate the effects of wakefields on bunches which are coherently accelerated in a DLA. We are calculating the energy spectrum affected by the longitudinal wake by evaluating the analytical description numerically and give estimates for the transverse kicks of an off-centered injected electron bunch.

physics.acc-ph↗

arXiv2019

Tracking with wakefields in dielectric laser acceleration grating structures

Due to the tiny apertures of dielectric laser acceleration grating structures within the range of the optical wavelength, wakefields limit the bunch charge for relativistic electrons to a few femtocoulomb. In this paper, we present a wakefield upgrade of our six-dimensional tracking scheme DLAtrack6D in order to analyze these limitations. Simulations with CST Studio Suite provide the wake functions to calculate the kicks within each tracking step. Scaling laws and the dependency of the wake on geometrical changes are calculated. The tracking with wakefields is applied to beam and structure parameters following recently performed and planned experiments. We compare the results to analytical models and identify intensity limits due to the transverse beam breakup and strong head-tail instability. Furthermore, we reconstruct phase advance spectrograms and use them to analyze possible stabilization mechanisms.

physics.acc-ph↗

arXiv2019

Generation and Characterization of Attosecond Micro-Bunched Electron Pulse Trains via Dielectric Laser Acceleration

Dielectric laser acceleration is a versatile scheme to accelerate and control electrons with the help of femtosecond laser pulses in nanophotonic structures. We demonstrate here the generation of a train of electron pulses with individual pulse durations as short as $270\pm80$ attoseconds(FWHM), measured in an indirect fashion, based on two subsequent dielectric laser interaction regions connected by a free-space electron drift section, all on a single photonic chip. In the first interaction region (the modulator), an energy modulation is imprinted on the electron pulse. During free propagation, this energy modulation evolves into a charge density modulation, which we probe in the second interaction region (the analyzer). These results will lead to new ways of probing ultrafast dynamics in matter and are essential for future laser-based particle accelerators on a photonic chip.

physics.acc-ph↗

arXiv2019

Dielectric-laser electron acceleration in a dual pillar grating with a distributed Bragg reflector

We report on the efficacy of a novel design for dielectric laser accelerators by adding a distributed Bragg reflector (DBR) to a dual pillar grating accelerating structure. This mimics a double-sided laser illumination, resulting in an enhanced longitudinal electric field while reducing the deflecting transverse effects, when compared to single-sided illumination. We improve the coupling efficiency of the incident electric field into the accelerating mode by 57 percent. The 12 $μ$m long structures accelerate sub-relativistic 28 keV electrons with gradients of up to 200 MeV/m in theory and 133 MeV/m in practice. Our work shows how lithographically produced nano-structures help to make novel laser accelerators more efficient.

physics.acc-ph↗

arXiv2019

Laser-Driven Electron Lensing in Silicon Microstructures

We demonstrate a laser-driven, tunable electron lens fabricated in monolithic silicon. The lens consists of an array of silicon pillars pumped symmetrically by two 300 fs, 1.95 $μ$m wavelength, nJ-class laser pulses from an optical parametric amplifier. The optical near-field of the pillar structure focuses electrons in the plane perpendicular to the pillar axes. With 100 $\pm$ 10 MV/m incident laser fields, the lens focal length is measured to be 50 $\pm$ 4 $μ$m, which corresponds to an equivalent quadrupole focusing gradient $B'$ of 1.4 $\pm$ 0.1 MT/m. By varying the incident laser field strength, the lens can be tuned from a 21 $\pm$ 2 $μ$m focal length ($B'>3.3$ MT/m) to focal lengths on the cm-scale.

physics.acc-ph↗

arXiv2018

Silicon dual pillar structure with a distributed Bragg reflector for dielectric laser accelerators: Design and fabrication

Dielectric laser accelerators (DLAs) have proven to be good candidates for miniaturized particle accelerators. They rely on micro-fabricated dielectrics which are able to modulate the kinetic energy of the incoming electron beam under a proper laser illumination. In this paper we demonstrate a dual pillar structure with a distributed Bragg reflector to mimic a double sided illumination to the electron path. The structure is fabricated by an electron beam lithography technique followed by a cryogenic reactive ion etching process. Such a structure can accelerate the injected 28 keV electrons by a gradient of approximately 150 MeV/m which can be further optimized towards the GeV/m regime.

physics.acc-ph↗

arXiv2018

Alternating Phase Focusing for Dielectric Laser Acceleration

The concept of Dielectric Laser Acceleration (DLA) provides highest gradients among non-plasma particle accelerators. However, stable beam transport and staging have not been shown experimentally yet. We present a scheme that confines the beam longitudinally and in one transverse direction. Confinement in the other direction is obtained by a single conventional quadrupole magnet. Within the small aperture of 420 nm we find the matched distributions, which allow an optimized injection into pure transport, bunching, and accelerating structures. The combination of these resembles the photonics analogue of the Radio Frequency Quadrupole (RFQ), but since our setup is entirely twodimensional, it can be manufactured on a microchip by lithographic techniques. This is a crucial step towards relativistic electrons in the MeV range from low-cost, handheld devices.

physics.acc-ph↗

arXiv2018

Bench Measurements and Simulations of Beam Coupling Impedance

After a general introduction, the basic principles of wake-field and beamcoupling- impedance computations are explained. This includes time domain, frequency domain, and methods that do not include excitations by means of a particle beam. The second part of this paper deals with radio frequency bench measurements of beam coupling impedances. The general procedure of the wire measurement is explained, and its features and limitations are discussed.

physics.acc-ph↗

arXiv2017

Beam Dynamics Analysis of Dielectric Laser Acceleration using a Fast 6D Tracking Scheme

A six-dimensional symplectic tracking approach exploiting the periodicity properties of Dielectric Laser Acceleration (DLA) gratings is presented. The longitudinal kick is obtained from the spatial Fourier harmonics of the laser field within the structure, and the transverse kicks are obtained using the Panofsky-Wenzel theorem. Additionally to the usual, strictly longitudinally periodic gratings, our approach is also applicable to periodicity chirped (sub-relativistic) and tilted (deflection) gratings. In the limit of small kicks and short periods we obtain the 6D Hamiltonian, which allows, for example, to obtain matched beam distributions in DLAs. The scheme is applied to beam and grating parameters similar to recently performed experiments. The paper concludes with an outlook to laser based focusing schemes, which are promising to overcome fundamental interaction length limitations, in order to build an entire microchip-sized laser driven accelerator.

physics.acc-ph↗

arXiv2014

Analytic Modeling, Simulation and Interpretation of Broadband Beam Coupling Impedance Bench Measurements

In the first part of the paper a generalized theoretical approach towards beam coupling impedances and stretched-wire measurements is introduced. Applied to a circular symmetric setup, this approach allows to estimate the systematic measurement error due to the presence of the wire. Further, the interaction of the beam or the TEM wave, respectively, with dispersive material such as ferrite is discussed. The dependence of the obtained impedances on the relativistic velocity $β$ is investigated and found as material property dependent. The conversion formulas for the TEM scattering parameters from measurements to impedances are compared with each other and the analytical impedance solution. In the second part of the paper the measurements are compared to numerical simulations of wakefields and scattering parameters. In practice, the measurements have been performed for the circularly symmetric example setup. The optimization of the measurement process is discussed. The paper concludes with a summary of systematic and statistic error sources for impedance bench measurements and their diminishment strategy.

physics.acc-ph↗