Energy Spread Research Articles

Nanosecond Photoemission near the Potential Barrier of a Schottky Emitter

Nanosecond electron pulses are appealing for ultrafast imaging and electron-gating applications, where tunable currents and narrow energy spreads are desirable. Here, we demonstrate photoemission from a Schottky emitter triggered by nanosecond laser pulses. Using photon energies optimally tuned to the emission potential barrier, we generate pulses containing over ${10}^{5}$ electrons with energy spreads below 1 eV by means of a prompt single-photon photoemission process. These results are consistent with a theoretical model of laser-triggered electron emission and energetic broadening during propagation and can be widely implemented.

High-Quality Laser-Accelerated Ion Beams from Structured Targets

In this work, we reviewed our results on the prospect of increasing the quality of ion acceleration driven by high-intensity laser pulses using low-Z structured targets. It is shown that the radiation pressure acceleration mechanism dominates over target normal sheath acceleration for assumed laser target parameters when the laser intensity is high enough. The target thickness is optimized for this regime and double-layer structure is investigated. When a corrugation is fabricated on the interface of such a target, a relativistic instability with Rayleigh–Taylor and Richtmyer–Meshkov like features can be driven by the target interaction with a high intensity laser pulse. The proper development of this instability leads to the generation of a collimated quasi-monoenergetic ion beam with lower emittance, divergence, and energy spread compared to a single and double-layer target with planar interface. A steep-front laser pulse is used in our simulations to mitigate other type of instabilities arising at the target surface from the laser–target interaction. We discuss the use of a plasma shutter to generate the required pulse profile, which also locally increases intensity. The obtained shape improves the ion acceleration, including higher maximal energy and lower beam divergence, in our simulation of a high-Z target.

Three-dimensional acoustic monitoring of laser-accelerated protons in the focus of a pulsed-power solenoid lens

Abstract The acoustic pulse emitted from the Bragg peak of a laser-accelerated proton bunch focused into water has recently enabled the reconstruction of the bunch energy distribution. By adding three ultrasonic transducers and implementing a fast data analysis of the filtered raw signals, I-BEAT (Ion-Bunch Energy Acoustic Tracing) 3D now provides the mean bunch energy and absolute lateral bunch position in real-time and for individual bunches. Relative changes in energy spread and lateral bunch size can also be monitored. Our experiments at DRACO with proton bunch energies between 10 and 30 MeV reveal sub-MeV and sub-mm resolution. In addition to this 3D bunch information, the signal strength correlates also with the absolute bunch particle number.

Simulation of Supercell Defect Structure and Transfer Phenomena in TlInTe2

The local environment of atoms in a semiconductor compound TlInTe2 with tetragonal syngony is studied by the density functional theory (DFT). The introduction of a point defect (indium vacancies) into the TlInTe2 lattice is modeled using supercells. The DFT electronic properties (total and local partial densities of states (PDOS) of electrons) are modeled for the primitive TlInTe2 cell (16 atoms per unit cell) and for the defective TlInTe2 cell (where is the vacancy In) consisting of 32 atoms. The DFT-GGA calculations of the TlInTe2 band structure show that the band gap ( ) is = 1.21 eV. This value is significantly dif-ferent from the experimental value. The Hubbard model is used to correct the interaction of particles in the lattice. The DFT-GGA + U (U is the Hubbard potential) calculated by the TlInTe2 band gap is 0.97 eV. For the TlInTe2 supercell, the energies of the formation of a vacancy, the chemical potential of indium, and the standard enthalpy of the formation of TlInTe2 are calculated. When explaining the effect of various factors on the transport phenomena in TlInTe2, their thermal and electrical conductivity, both the DFT-calculated data and experimental data, are used. Taking into account the experimental data for the p-TlInTe2 crystals, the mechanism of conduction in the direction of structural chains (c axis of the crystal) is established. From the experimental data in the temperature range = 148–430 K, the band gap = 0.94 eV and the activation energy of impurity conduction = 0.1 eV (at 210–300 K) are estimated. At temperatures of ≤ 210 K, DC hopping conduction takes place in the p-TlInTe2 crystals. With this in mind, the following physical parameters are calculated for p-TlInTe2: the density of states localized near the Fermi level, their energy spread, and the average hopping distance.

Photocathode performance characterisation of ultra-thin MgO films on polycrystalline copper

The performance expected from the next generation of electron accelerators is driving research into photocathode technology as this fundamentally limits the achievable beam quality. The performance characteristics of a photocathode are most notably: normalised emittance, brightness and energy spread. Ultra–thin oxide films on metal substrates have been shown to lower the work function (WF) of the surface, enhancing commonly utilised metal photocathodes, potentially improving lifetime and performance characteristics.We present the characterisation of two MgO/Cu photocathodes grown at Daresbury. The surface properties such as: surface roughness, elemental composition and WF, have been studied using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The photoemissive properties have been characterised with quantum efficiency (QE) measurements at 266 nm. Additionally, we measure the Transverse Energy Distribution Curves (TEDC) for these photocathodes under illumination at various wavelengths using ASTeC’s Transverse Energy Spread Spectrometer (TESS) and extract the Mean Transverse Energy (MTE).

Benchmarking ionization potentials using the simple pCCD model.

The electron-detachment energy is measured by the ionization potential (IP). As a result, it is a fundamental, observable and important molecular electronic signature in photoelectron spectroscopy. A precise theoretical prediction of electron-detachment energies or ionization potentials is essential for organic optoelectronic systems like transistors, solar cells, or light-emitting diodes. In this work, we benchmark the performance of the recently presented IP variant of the equation-of-motion pair coupled cluster doubles (IP-EOM-pCCD) model to determine IPs. Specifically, the predicted ionization energies are compared to experimental results and higher-order coupled cluster theories based on statistically assessing 201 electron-detached states of 41 organic molecules for three different molecular orbital basis sets and two sets of particle-hole operators. While IP-EOM-pCCD features a reasonable spread and skewness of ionization energies, its mean error and standard deviation differ by up to 1.5 eV from reference data. Our study, thus, highlights the importance of dynamical correlation to reliably predict IPs from a pCCD reference function in small organic molecules.

A tunable photo-electric co-excited point electron source: low-intensity excitation emission and structure-modulated spectrum-selection.

The development of a point electron source requires an efficient excitation mode with low energy consumption, flexible tunability, and high performance. In particular for traditional electron emission cathode materials, it is necessary to expand the function of this aspect to meet application demands in many emerging fields. In this study, we propose a photo-electric co-excited scheme to drive a tungsten (W) needle nano-cold-cathode. The developed W needle cathode has been demonstrated to show electron emission performance with a narrow energy spread of 0.76 eV and a high brightness of 4 × 109 A m-2 sr-1 V-1. This could be achieved through low-intensity co-excitation, including an electrostatic field below ∼0.5 V μm-1 and a laser intensity at ∼10 W cm-2 level. Based on this co-excitation, the electron emission further exhibited a tunable property relative to the intrinsic properties of the incident light, such as optical frequency and polarization, which is shown to be directly modulated by the structure of the W needle nano-cold-cathode. This work provides a choice to achieve tunable, miniaturized and integrated vacuum micro- and nano-electronic devices.

Voltage waveform tailoring for high aspect ratio plasma etching of SiO2 using Ar/CF4/O2 mixtures: Consequences of ion and electron distributions on etch profiles

The quality of high aspect ratio (HAR) features etched into dielectrics for microelectronics fabrication using halogen containing low temperature plasmas strongly depends on the energy and angular distribution of the incident ions (IEAD) onto the wafer, as well as potentially that of the electrons (EEAD). Positive ions, accelerated to high energies by the sheath electric field, have narrow angular spreads and can penetrate deeply into HAR features. Electrons typically arrive at the wafer with nearly thermal energy and isotropic angular distributions and so do not directly penetrate deeply into features. These differences can lead to positive charging of the insides of the features that can slow etching rates and produce geometric defects such as twisting. In this work, we computationally investigated the plasma etching of HAR features into SiO2 using tailored voltage waveforms in a geometrically asymmetric capacitively coupled plasma sustained in an Ar/CF4/O2 mixture at 40 mTorr. The tailored waveform consisted of a sinusoidal wave and its higher harmonics with a fundamental frequency of 1 MHz. We found that some degree of control of the IEADs and EEADs is possible by adjusting the phase of higher harmonics φ through the resulting generation of electrical asymmetry and electric field reversal. However, the IEADs and EEADs cannot easily be separately controlled. The control of IEADs and EEADs is inherently linked. The highest quality feature was obtained with a phase angle φ = 0° as this value generated the largest (most negative) DC self-bias and largest electric field reversal for accelerating electrons into the feature. That said, the consequences of voltage waveform tailoring (VWT) on etched features are dominated by the change in the IEADs. Although VWT does produce EEADs with higher energy and narrower angular spread, the effect of these electrons on the feature compared to thermal electrons is not large. This smaller impact of VWT produced EEADs is attributed to thermal electrons being accelerated into the feature by electric fields produced by the positive in-feature charging.

Public awareness of hydrogen energy: A comprehensive evaluation based on statistical approach

Climate change and environmental problems, increasing global energy demand, and uncertainties in energy supply have brought the production and use of sustainable energy resources to be crucial. Hydrogen, which is an important energy carrier, is being produced and used globally within the perspective of sustainable energy and environmental technologies. Public awareness is an important step toward the spread of hydrogen energy (HE). In the literature, while public awareness studies focus on renewable energy sources, no study that examines public awareness in the field of HE in a way that addresses different perspectives has come across. This pioneering study aims to explore public awareness of HE by analyzing survey data collected from individuals via basic statistical techniques. Findings indicate that individuals' awareness of hydrogen energy mainly varies according to their knowledge level, education level, and age group. The results of the study will provide an important perspective for all stakeholders on hydrogen energy. Especially the policymakers should consider these results to constitute the frameworks of policy without compromising any perspective of public awareness.

X-ray free electron laser linear accelerator without a laser heater

Linear accelerator based X-ray free electron laser (FEL) light sources provide an important tool for scientific discoveries. Most of these light source facilities employ a laser heater to increase the electron beam’s uncorrelated energy spread to suppress microbunching instability through the linear accelerators. In this paper, we first studied the microbunching instability in an X-ray FEL linear accelerator with lower initial peak current (∼10 A) and moderate final peak currents (1−2 kA). In this regime, the microbunching instability can be substantially mitigated with modest initial uncorrelated energy spread. We then suggested a less expensive method to mitigate the microbunching instability using a section of low beta FODO lattice instead of the laser heater. With the use of the high brightness electron beam from a photoinjector, the intrabeam scattering effect inside the beam through the FODO lattice can generate sufficient uncorrelated energy spread to mitigate the microbunching instability. At last, we demonstrated the feasibility of this method with self-consistent solution of the Fokker–Planck equation through the X-ray FEL linear accelerator.

Theoretical analysis of the influence of electron beam parameters on the harmonic powers in free electron lasers

We theoretically analyse the influence of the electron beam parameters on the radiation in free electron lasers (FELs). We consider three FEL installations, SPARC, LEUTL and SwissFEL, where several experiments with different values of the beam emittance, energy spread and current were conducted and documented in details. Based on the available data and on our analytical approach to the FEL harmonic power computation, we distinguish the key factors, affecting the harmonic generation, and compare our theoretical results with the available experimental data. Alternative theoretical approaches were also employed for the harmonic power estimation. At the reviewed installations and experiments the variation of the emittance, energy spread and electron current occurred; we model these experiments with account for all variations, and analyze and compare the results with each other for different sets of parameters; we compare the results with the available experimental data and measurements. We demonstrate as the emittance, the energy spread and the beam current density values interplay in their influence on the radiation characteristics of a single pass FEL and we show as individual variations of these parameters can mutually compensate each others influence to some degree.

Rayleigh-Taylor instability of radiation pressure driven foils: 2D effects

Rayleigh–Taylor instability of radiation pressure accelerated ultra-thin foils of different thickness profiles and initial curvature is investigated in two dimensions using numerical simulations. The convex curvature of the foil (when viewed from the rear side of the foil) provides radially inward motion to the off-axis ions countering the radial divergence due to the Gaussian intensity distribution of the laser. Nonuniform foils, having maximum thickness on laser axis have similar effect. When a small ripple is superimposed on the foil on the scale of laser wavelength, the radiation pressure acts nonuniformly on the microscopic scale and the perturbation grows as the foil moves. After a certain distance of travel, the foil crests turn into cusps and the phase of quasi mono-energy ion acceleration terminates. The planar foil with uniform laser has a strong growth of Rayleigh–Taylor instability (RTI). The foils with curvature and nonuniform thickness have marginally lower growth rate. However, under all circumstances, RTI is found to induce additional energy spread, in radiation pressure acceleration (RPA) of thin foils.

Operation of an Energy Recovery Linac with an Internal Target

Because an Energy Recovery Linac (ERL) decelerates and dumps the beam after at most a few passes, beam degradation in the interaction region that is orders of magnitude larger than in a storage ring is tolerable in such a device. As a consequence, some new types of Nuclear Physics experiments can be carried out. In 2016, an experiment (the DarkLight experiment) was installed in the Jefferson Lab FEL Driver ERL to test out this idea. The ERL had previously been used for FEL applications where high peak current and a large growth in energy spread was present. For the internal target, the machine setup required very small energy spread, and had to deal with large transverse emittance growth in the target. Additionally, the addition of a strong solenoid in the transport complicated the details of energy recovery. This presentation will describe how these new machine physics challenges were addressed.

Phase-Shimming of the BESSY II in-Vacuum APPLE II Undulator IVUE32 with Transverse Slides

The phase shimming of an in-vacuum APPLE II undulator requests a precise slit adjustment between neighbouring magnet rows besides the conventional gap tuning such as in an in-vacuum undulator. In an out-of-vacuum APPLE II, the slit between the magnet rows is precisely defined via a stiff needle bearing, which is located closely to the critical volume. In the APPLE II design, the slit adjustment must be done from outside of the vacuum chamber far away from the electron beam. In this article, we discuss the tolerances of systematic phase errors of various types and their relaxation with emittance and energy spread. We developed specific transverse slides for the slit adjustment to achieve full flexibility in phase tuning. We discuss their capability in slit adjustment precision in the readily assembled and evacuated system. The discussions are based on spectral and FEM-simulations.

A Laser Heater Undulator for FLASH2020+

Space charge induced micro bunch instabilities, amplified in cascaded bunch compression stages, spoil the phase space of compressed bunches and lead to unstable seeding. To “heat” up the bunch with an external laser and induce an uncorrelated energy spread to suppresses the micro bunching instabilities, a short undulator will be installed in front of the first compression chicane. The undulator has a planar hybrid magnet structure with 43 mm period length, 11 full periods, 22 mm minimum gap and a deflection parameter K of 1.52 (K = (eBλ u / (2πme c, with B - max. magnetic field amplitude, λ u - period length) to match the 534 nm laser wavelength at 146 MeV beam energy. The paper describes the undulator’s compact design, its control system and results of magnetic measurements and tuning.

Chromatic transverse dynamics in a nonlinear plasma accelerator

We present a theoretical investigation of the chromatic dynamics of the witness beam within a plasma based accelerator. We derive the single particle motion of an electron in an ion column within a nonlinear, blowout wake including adiabatic dampening and adiabatic variations in plasma density. Using this, we calculate the evolution of the beam moments and emittance for an electron beam. Our model can handle near arbitrary longitudinal phase space distributions. We include the effects of energy change in the beam, imperfect wake loading, initial transverse offsets of the beam, and mismatch between the beam and plasma. We use our model to derive analytic saturation lengths for the projected, longitudinal slice, and energy slice emittance under different beam loading conditions. Further, we show that the centroid oscillations and spot sizes vary between the slices and the variation depends strongly on the beam loading. Next, we show how a beam evolves in a full plasma source with density ramps and show that the integral of the plasma density along the ramp determines the impact on the beam. Finally, we derive several simple scaling laws that show how to design a plasma based injector to produce a target beam energy and energy spread.

On the control of electron heating for optimal laser radiation pressure ion acceleration

We study the onset of electron heating in intense laser–solid interactions and its impact on the spectral quality of radiation pressure accelerated ions in both hole boring and light sail regimes. Two- and three-dimensional particle-in-cell (PIC) simulations are performed over a wide range of laser and target parameters and reveal how the pulse duration, profile, polarization and target surface stability control the electron heating, the dominant ion acceleration mechanisms and the ion spectra. We find that the onset of strong electron heating is associated with the growth of the Rayleigh–Taylor-like instability at the front surface and must be controlled to produce high-quality ion beams, even when circularly polarized lasers are employed. We define a threshold condition for the maximum duration of the laser pulse that allows mitigation of electron heating and radiation pressure acceleration of narrow energy spread ion beams. The model is validated by three-dimensional PIC simulations, and the few experimental studies that reported low energy spread radiation pressure accelerated ion beams appear to meet the derived criteria. The understanding provided by our work will be important in guiding future experimental developments, for example for the ultrashort laser pulses becoming available at state-of-the-art laser facilities, for which we predict that proton beams with $\sim$ 150–250 MeV, $\sim$ 30% energy spread, and a total laser-to-proton conversion efficiency of $\sim$ 20% can be produced.

The impact of the break-up of molecules in the stripper on the AMS performance

The break-up of molecules in the stripper of an AMS facility induces an energy and angular spread in the beam. This can cause beam losses, thereby reducing the performance of an AMS facility. With an appropriate design, these effects can be minimised and the efficiency enhanced. As an example, the break-up of BeO molecules is discussed here. First the basic physics is described and corresponding models and computer codes are explained. Then experimental data are shown and compared with the models. For gas stripping, beam profiles are available for small and large facilities. All show tails which can be attributed to the dissociation of the molecules. The size of the effect depends on various parameters such as beam energy, selected charge state, stripping gas type and ion optics. Foil stripping leads to energy distributions which are much wider, and show typically a double peak.

Stable and High-Quality Electron Beams from Staged Laser and Plasma Wakefield Accelerators

We present experimental results on a plasma wakefield accelerator (PWFA) driven by high-current electron beams from a laser wakefield accelerator (LWFA). In this staged setup stable and high quality (low divergence and low energy spread) electron beams are generated at an optically-generated hydrodynamic shock in the PWFA. The energy stability of the beams produced by that arrangement in the PWFA stage is comparable to both single-stage laser accelerators and plasma wakefield accelerators driven by conventional accelerators. Simulations support that the intrinsic insensitivity of PWFAs to driver energy fluctuations can be exploited to overcome stability limitations of state-of-the-art laser wakefield accelerators when adding a PWFA stage. Furthermore, we demonstrate the generation of electron bunches with energy spread and divergence superior to single-stage LW-FAs, resulting in bunches with dense phase space and an angular-spectral charge density beyond the initial drive beam parameters. These results unambiguously show that staged LWFA-PWFA can help to tailor the electron-beam quality for certain applications and to reduce the influence of fluctuating laser drivers on the electron-beam stability. This encourages further development of this new class of staged wakefield acceleration as a viable scheme towards compact, high-quality electron beam sources.

Experimental Analysis of the Space Ratio Influence on the Excitation Frequencies of One and Two Cylinders Free to Vibrate in Tandem Arrangement

The present study aims to investigate the dominant frequency ranges of a cylinder free to vibrate transversally to the flow positioned in the first, the second or in both positions of the tandem assembly for L/D = 1.26, 1.4, 1.6, and 3.52 with the increase in the flow velocity. Accelerometers and hot wire anemometers were the experimental tools applied in this study. The range of study encompassed the reduced velocity with values from 6 to 72 and Reynolds number from 7.1 × 103 to 2.4 × 104. Fourier transform, continuous wavelet transform, magnitude-square coherence, and wavelet coherence were applied to analyze the cylinder acceleration results for all L/D and wake velocity values studied. The results show that the amplitudes of vibration are below 1.5% of the diameter for all the cases, except for the lower L/D, where the amplitude increases. The first cylinder free to vibrate presents the highest amplitudes observed. Fourier and continuous wavelet analysis showed high energy associated with the two natural frequencies of the system and a third frequency, which may be associated with the flow excitation. In the second cylinder free to vibrate, energy spreads across the monitored spectrum, justifying the smaller amplitudes but the energy level increases with increasing L/D and may be associated with wake-induced vibration. The cases with both cylinders free to vibrate show that the relation between the assembly parameters of each cylinder is relevant to the vibration response and the excitation frequency range. The results showed that even with a clear excitation in a higher frequency, the main energy in the vibration signals is in the natural frequency range.