Narrow Energy Spread Research Articles

Laser-driven acceleration of a 94.8 MeV uniform proton beam enhanced by a thin axial absorber rod

Laser-based proton accelerators offer promising advantages for many sensitive applications due to their compact size. However, achieving uniform beams with a narrow energy spread remains a challenge. Here, a beamline simulation is performed using the racein and codes to optimize the spectral and spatial characteristics of a laser-driven proton beam. This beamline, comprised of four magnetic quadrupoles and equipped with a thin axial absorber rod in conjunction with an energy selection aperture for narrowing the energy spread, a carbon scatterer foil for beam spot smoothing, and an angular selection aperture for beam collimation, effectively transports a selected proton beam energy within the range of 50–250 MeV. In this work, laser-accelerated protons with a wide energy spread were transported through the proposed beamline, which was optimized for 109 MeV protons. The protons emerged well-collimated with a significantly narrowed energy spread of 94.8±3.9 MeV (a reduction from the intended 109 MeV due to passage through the scatterer) and a homogenized, 11 mm diameter beam spot while maintaining a transmission efficiency above 2.2%. Published by the American Physical Society 2024

Proton acceleration from optically tailored high-density gas jet targets

Laser-driven ion acceleration is well established using solid targets mainly in the target normal sheath acceleration regime. To follow the increasing repetition rate available on high-intensity lasers, the use of high-density gas targets has been explored in the past decade. When interacting with targets reaching densities close to the critical one, the laser pulse can trigger different acceleration mechanisms such as Collisionless Shock Acceleration (CSA) or hole boring. Particle-in-cell simulations using ideal target profiles show that CSA can accelerate a collimated, narrow energy spread and few hundreds of megaelectronvolts ion beam on the laser axis. Nevertheless, in real experiments, the laser will not only interact with an overcritical, thin plasma slab with sharp density gradients, but also with lower density regions surrounding the core of the gas jet, extending to several hundreds of micrometres. The interaction of the laser with these lower density wings will lead to nonlinear effects that will reduce the available energy to drive the shock in the high-density region of the target. Optically tailoring this target could mitigate that issue. Recent experiments conducted on different laser facilities aimed at testing several tailoring configurations. We first tested a scheme with a copropagating picosecond prepulse to create a lower density plasma channel to facilitate the propagation of the main pulse, while the second one was a transverse tailoring driven by nanosecond laser pulses to generate blast waves and form a high-density plasma slab. The main results will be presented here and the methods compared.

Intense laser interaction with micro-bars

Intense laser fields interact very differently with micrometric rough surfaces than with flat objects. The interaction features high laser energy absorption and increased emission of MeV electrons, ions, and of hard x-rays. In this work, we irradiated isolated, translationally-symmetric objects in the form of micrometric Au bars. The interaction resulted in the emission of two forward-directed electron jets having a small opening angle, a narrow energy spread in the MeV range, and a positive angle to energy correlation. Our numerical simulations show that following ionization, those electrons that are pulled into vacuum near the object’s edge, remain in-phase with the laser pulse for long enough so that the Lorentz force they experience drive them around the object’s edge. After these electrons pass the object, they form attosecond duration bunches and interact with the laser field over large distances in vacuum in confined volumes that trap and accelerate them within a narrow range of momentum. The selectivity in energy of the interaction, its directionality, and the preservation of the attosecond duration of the electron bunches over large distances, offer new means for designing future laser-based light sources.

Proton acceleration with multi-peak energy spectra tailored by vortex laser

A novel flying cascaded acceleration mechanism is proposed to generate energetic proton beams with multi-peak energy spectra using a circularly polarized (CP) Laguerre–Gaussian (LG) laser pulse in three-dimensional particle-in-cell simulations. Simulations show that the protons are initially accelerated and compressed into the beam center via the radiation pressure of the CP LG (σz = −1) laser pulse. Then, they are tailored by flying dipolar electric fields in this LG laser, resulting in a multi-peak energy spectrum. Each shaped proton peak exhibits a narrow energy spread of ∼5% and high flux of ∼2 × 108 protons/MeV at giga-electron volts energy. Such a flying cascaded acceleration mechanism extends the energy spectra of proton beams from monoenergetic to multi-peak structure, thereby potentially enhancing the generation efficiency of monoenergetic proton beams for various applications, such as proton-induced spallation reactions, proton radiography, and proton therapy.

Controlled transition to different proton acceleration regimes: Near-critical-density plasmas driven by circularly polarized few-cycle pulses

A controlled transition between two different ion acceleration mechanisms would pave the way to achieving different ion energies and spectral features within the same experimental set up, depending on the region of operation. Based on numerical simulations conducted over a wide range of experimentally achievable parameter space, reported here is a comprehensive investigation of the different facets of ion acceleration by relativistically intense circularly polarized laser pulses interacting with thin near-critical-density plasma targets. The results show that the plasma thickness, exponential density gradient, and laser frequency chirp can be controlled to switch the interaction from the transparent operating regime to the opaque one, thereby enabling the choice of a Maxwellian-like ion energy distribution with a cutoff energy in the relativistically transparent regime or a quasi-monoenergetic spectrum in the opaque regime. Next, it is established that a multispecies target configuration can be used effectively for optimal generation of quasi-monoenergetic ion bunches of a desired species. Finally, the feasibility is demonstrated for generating monoenergetic proton beams with energy peak at E≈20–40 MeV and a narrow energy spread of ΔE/E≈18%–28.6% confined within a divergence angle of ∼175 mrad at a reasonable laser peak intensity of I0 ≃ 5.4 × 1020 W/cm2.

A novel design of a retarding field electron energy analyzer with a cavity electrode providing extremely high energy resolution

An electron energy analyzer based on the retarding electrostatic field generated by a cavity electrode was prototyped, where the designed energy resolution is smaller than 10 meV at acceleration voltages Vo less than 5 kV and a half angle spread θo less than 20 mrad for the incident electron beams with the scanning voltage range of 3 V. Precise energy resolution, however, has not been recognized experimentally because of the difficulties of equipping such a narrow energy spread electron source. This paper describes the principle of the newly developed energy analyzer and shows the obtained energy spectra from a Schottky emitter ZrO/W(100) with a tip of the curvature radius of 1.5 μm, which could highly reproduce the theoretically expected spectra at various emitter temperatures because of its predominantly thermal emission.

Electron-Beam Source with a Superconducting Niobium Tip

Electron-beam sources are foundational in high-resolution electron microscopy and spectroscopy, but applications have been limited due to their relatively large energy spread. The authors fabricate a monocrystalline niobium nanotip electron field emitter and characterize it in superconducting and normal-conducting regimes. This bright, stable, coherent electron beam source features an exceptionally narrow energy spread. The authors also study the role of xenon adsorption and two-electron correlations. This work may improve aberration-corrected microscopy and electron energy-loss spectroscopy and enable high-resolution vibrational spectroscopy or quantum electron microscopy.

Experimental study of a neutralizer-free gridded ion thruster using radio-frequency self-bias effect

An experimental study on the quasi-neutral beam extracted by a neutralizer-free gridded ion thruster prototype was presented. The prototype was designed using an inductively coupled plasma source terminated by a double-grid accelerator. The beam characteristics were compared when the accelerator was radio-frequency (RF) biased and direct-current (DC) biased. An RF power supply was applied to the screen grid via a blocking capacitor for the RF acceleration, and a DC power supply was directly connected to the screen grid for the DC acceleration. Argon was used as the propellant gas. Furthermore, the characteristics of the plasma beam, such as the floating potential, the spatial distribution of ion flux, and the ion energy distribution function (IEDF) were measured by a four-grid retarding field energy analyzer. The floating potential results showed that the beam space charge is compensated in the case of RF acceleration without a neutralizer, which is similar to the case of classical DC acceleration with a neutralizer. The ion flux of RF acceleration is 1.17 times higher than that of DC acceleration under the same DC component voltage between the double-grid. Moreover, there are significant differences in the beam IEDFs for RF and DC acceleration. The IEDF of RF acceleration has a widened and multi-peaked profile, and the main peak moves toward the high-energy region with increasing the DC self-bias voltage. In addition, by comparing the IEDFs with RF acceleration frequencies of 3.9 and 7.8 MHz, it is found that the IEDF has a more centered main peak and a narrower energy spread at a higher frequency.

Secondary electron emission and vacuum electronics

Secondary electron emission serves as the foundation for a broad range of vacuum electronic devices and instrumentation, from particle detectors and multipliers to high-power amplifiers. While secondary yields of at least 3–4 are required in practical applications, the emitter stability can be compromised by surface dynamics during operation. As a result, the range of practical emitter materials is limited. The development of new emitter materials with high yield and robust operation would advance the state-of-the-art and enable new device concepts and applications. In this Perspective article, I first present an analysis of the secondary emission process, with an emphasis on the influence of material properties. From this analysis, ultra-wide bandgap (UWBG) semiconductors and oxides emerge as superior emitter candidates owing to exceptional surface and transport properties that enable a very high yield of low-energy electrons with narrow energy spread. Importantly, exciting advances are being made in the development of promising UWBG semiconductors such as diamond, cubic boron nitride (c-BN), and aluminum nitride (AlN), as well as UWBG oxides with improved conductivity and crystallinity. These advances are enabled by epitaxial growth techniques that provide control over the electronic properties critical to secondary electron emission, while advanced theoretical tools provide guidance to optimize these properties. Presently, H-terminated diamond offers the greatest opportunity because of its thermally stable negative electron affinity (NEA). In fact, an electron amplifier under development exploits the high yield from this NEA surface, while more robust NEA diamond surfaces are demonstrated with potential for high yields in a range of device applications. Although c-BN and AlN are less mature, they provide opportunities to design novel heterostructures that can enhance the yield further.

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.

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.

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.

High efficiency uniform positron beam loading in a hollow channel plasma wakefield accelerator

We propose a novel positron beam loading regime in a hollow plasma channel that can efficiently accelerate $e^+$ beam with high gradient and narrow energy spread. In this regime, the $e^+$ beam coincides with the drive $e^-$ beam in time and space and their net current distribution determines the plasma wakefields. By precisely shaping the beam current profile and loading phase according to explicit expressions, three-dimensional Particle-in-Cell (PIC) simulations show that the acceleration for $e^+$ beam of $\sim$nC charge with $\sim$GV/m gradient, $\lesssim$0.5% induced energy spread and $\sim$50% energy transfer efficiency can be achieved simultaneously. Besides, only tailoring the current profile of the more tunable $e^-$ beam instead of the $e^+$ beam is enough to obtain such favorable results. A theoretical analysis considering both linear and nonlinear plasma responses in hollow plasma channels is proposed to quantify the beam loading effects. This theory agrees very well with the simulation results and verifies the robustness of this beam loading regime over a wide range of parameters.

Quasimonochromatic Bright Gamma-ray Generation from Synchronized Compton Scattering via Azimuthal Spatial-Temporal Coupling

High energy photons can be generated via inverse Compton scattering (ICS) in the collision between energetic electrons and intense laser pulse. The development of laser plasma accelerators promises compact and all-optical gamma-ray sources by colliding the electrons from laser wakefield accelerators to its high-power driving pulse reflected by a plasma mirror. However, the law of optical focusing hinders realization of both high photon yield and monochromatic spectrum in this scenario. We propose an azimuthal spatial-temporal convertor that decouples the focal field strength from laser spot size using helical parabolic geometry. It decomposes the driving laser beam into a pulse train of almost identical divergence angle and focal depth, creating synchronized ICS in the optimized linear regime. The scheme resolves the dilemma between high efficiency and narrow energy spread, facilitating the generation of monochromatic gamma-ray using high power lasers beyond relativistic field strengths.

High Efficiency Uniform Wakefield Acceleration of a Positron Beam Using Stable Asymmetric Mode in a Hollow Channel Plasma.

Plasma wakefield acceleration in the blowout regime is particularly promising for high-energy acceleration of electron beams because of its potential to simultaneously provide large acceleration gradients and high energy transfer efficiency while maintaining excellent beam quality. However, no equivalent regime for positron acceleration in plasma wakes has been discovered to date. We show that after a short propagation distance, an asymmetric electron beam drives a stable wakefield in a hollow plasma channel that can be both accelerating and focusing for a positron beam. A high charge positron bunch placed at a suitable distance behind the drive bunch can beam-load or flatten the longitudinal wakefield and enhance the transverse focusing force, leading to high efficiency and narrow energy spread acceleration of the positrons. Three-dimensional quasistatic particle-in-cell simulations show that an over 30% energy extraction efficiency from the wake to the positrons and a 1% level energy spread can be simultaneously obtained. Further optimization is feasible.

Improvement of electron beam quality in laser wakefield acceleration by a circularly-polarized laser pulse

Electron acceleration driven by laser wakefield using a laser pulse with circular polarization is studied to generate better-quality electron bunches. In order to inject electrons into the accelerating phase of the plasma wave, a density transition shaped bump is employed. Using particle-in-cell simulations, we demonstrate the influence of laser polarization on the quality of electron bunch. The simulation results (using experimentally achievable parameters) show that the electron bunch with higher peak-energy, narrow energy-spread, and low emittance can be obtained by using a laser pulse with circular polarization as compared to the usually employed linear polarization in a mm-scale length plasma. The results show the improved quality of the electron bunch generated from laser wakefield accelerations. The predicted electron bunch quality may be crucial in developing plasma-based electromagnetic radiation sources.

Toward monochromated sub-nanometer UEM and femtosecond UED

A preliminary design of a mega-electron-volt (MeV) monochromator with 10−5 energy spread for ultrafast electron diffraction (UED) and ultrafast electron microscopy (UEM) is presented. Such a narrow energy spread is advantageous in both the single shot mode, where the momentum resolution in diffraction is improved, and the accumulation mode, where shot-to-shot energy jitter is reduced. In the single-shot mode, we numerically optimized the monochromator efficiency up to 13% achieving 1.3 million electrons per pulse. In the accumulation mode, to mitigate the efficiency degradation caused by the shot-to-shot energy jitter, an optimized gun phase yields only a mild reduction of the single-shot efficiency, therefore the number of accumulated electrons nearly proportional to the repetition rate. Inspired by the recent work of Qi et al. (Phys Rev Lett 124:134803, 2020), a novel concept of applying reverse bending magnets to adjust the energy-dependent path length difference has been successfully realized in designing a MeV monochromator to achieve the minimum energy-dependent path length difference between cathode and sample. Thanks to the achromat design, the pulse length of the electron bunches and the energy-dependent timing jitter can be greatly reduced to the 10 fs level. The introduction of such a monochromator provides a major step forward, towards constructing a UEM with sub-nm resolution and a UED with ten-femtosecond temporal resolution. The one-to-one mapping between the electron beam parameter and the diffraction peak broadening enables a real-time nondestructive diagnosis of the beam energy spread and divergence. The tunable electric–magnetic monochromator allows the scanning of the electron beam energy with a 10−5 precision, enabling online energy matching for the UEM, on-momentum flux maximizing for the UED and real-time energy measuring for energy-loss spectroscopy. A combination of the monochromator and a downstream chicane enables “two-color” double pulses with femtosecond duration and the tunable delay in the range of 10 to 160 fs, which can potentially provide an unprecedented femtosecond time resolution for time resolved UED.

Laser-driven collisionless shock acceleration of ions from near-critical plasmas

This paper overviews experimental and numerical results on acceleration of narrow energy spread ion beams by an electrostatic collisionless shockwave driven by 1 μm (Omega EP) and 10 μm (UCLA Neptune Laboratory) lasers in near critical density CH and He plasmas, respectively. Shock waves in CH targets produced high-energy ∼50 MeV protons (ΔE/E of ≤30%) and 314 MeV C6+ ions (ΔE/E of ≤10%). Observation of acceleration of both protons and carbon ions to similar velocities is consistent with reflection of particles off the moving potential of a shock front. For shocks driven by CO2 laser in a gas jet, ∼30 MeV peak in He ion spectrum was detected. Particle-in-cell simulations indicate that regardless of the target further control over its density profile is needed for optimization of accelerated ion beams in part of energy spread, yield and maximum kinetic energy.

Physics to plan AWAKE Run 2

We briefly describe the basic physics principles considered for planning of the AWAKE Run 2 experiment. These principles are based on experimental results obtained during Run 1 and knowledge obtained from numerical simulation results and other experiments. The goal of Run 2 is to accelerate an electron bunch with a narrow relative energy spread and an emittance sufficiently low for applications. The experiment will use two plasmas, electron bunch seeding for the SM process, on-axis external injection of an electron bunch and electron bunch parameters to reach plasma blow-out, beam loading and beam matching.

Preliminary study: Measurement of ion beam energy spreads produced by a Penning ionization gauge-type ion source using electromagnets for a mega-electron volt compact ion microbeam system.

The energy spreads of ion beams generated from a penning ionization gauge-type ion source with electromagnets were measured using a parallel electrostatic analyzer. The ion source was developed to be installed in a mega-electron volt (MeV) compact ion microbeam system. A gaseous ion beam of expectedly high brightness and narrow energy spread was generated from the ion source to form a microbeam. To produce such an ion beam, a high-density plasma with a small volume was generated using a strong magnetic field in the ion source. The beam energy spread width was of particular importance because it forms an ion microbeam by reducing the chromatic aberration at a focusing lens. In this report, the energy spread was investigated by changing the parameters of the ion source, e.g., extraction voltage, excitation current of electromagnets, vacuum, and anode voltage. The investigation showed that spread widths are influenced by the extraction voltage, vacuum, and anode voltage. The minimum width of ∼5.0 ± 0.1 eV was obtained at a beam energy of 200 eV. This value is acceptable for the MeV compact ion microbeam system.