Low Angular Divergence Research Articles

High-energy quasi-monoenergetic proton beam from micro-tube targets driven by Laguerre–Gaussian lasers

High-energy proton beams with low energy spread and angular divergence are highly valuable in various applications, such as proton therapy, ultrafast science, and nuclear physics. Laser-driven ion sources are being explored as a more compact and cost-effective alternative to conventional accelerators. However, producing high-quality proton beams using lasers has been a significant challenge. In this study, we propose a novel approach to achieve high-energy quasi-monoenergetic proton beams by utilizing a Laguerre–Gaussian (LG) laser to irradiate a solid hollow tube target. Our three-dimensional particle-in-cell simulations demonstrate that the LG laser efficiently pulls out and accelerates electrons. As these electrons exit the rear side of the tube, a charge-separation electric field for proton acceleration is self-established. Meanwhile, a considerable part of the electrons are confined to the laser axis by the ponderomotive force of the LG laser, generating a longitudinal bunching field and transverse focusing field. As a result, protons are accelerated and focused in these fields, producing a high- quality proton beam with high energy (247 MeV) and low energy spread (∼2%) using an LG laser with an intensity of W cm−2 and a tube target with an electron density of 40nc . In comparison to a standard Gaussian laser, the LG laser exhibits superior performance in terms of proton cut-off energy, energy spread, and angular divergence.

Energy-chirp compensation of laser-driven ion beams enabled by structured targets

We show using 3D simulations that the challenge of generating dense mono-energetic laser-driven ion beams with low angular divergence can be overcome by utilizing structured targets with a relativistically transparent channel and an overdense wall. In contrast to a uniform target that produces a chirped ion beam, the target structure facilitates formation of a dense electron bunch whose longitudinal electric field reverses the energy chirp. This approach works in conjunction with existing acceleration mechanisms, augmenting the ion spectra. For example, our 3D simulations predict a significant improvement for a 2 PW laser pulse with a peak intensity of $5 \times 10^{22}$ W/cm$^2$. The simulations show a mono-energetic proton peak in a highly desirable energy range of 200 MeV with an unprecedented charge of several nC and relatively low divergence that is below 10$^{\circ}$.

Theoretical Modeling for the Thermal Stability of Solid Targets in a Positron-Driven Muon Collider

A future multi-TeV muon collider requires new ideas to tackle the problems of muon production, accumulation and acceleration. In the Low EMittance Muon Accelerator concept a 45 GeV positron beam, stored in an accumulation ring with high energy acceptance and low angular divergence, is extracted and driven to a target system in order to produce muon pairs near the kinematic threshold. However, this scheme requires an intensity of the impinging positron beam so high that the energy dissipation and the target maintenance are crucial aspects to be investigated. Both peak temperature rises and thermomechanical shocks are related to the beam spot size at the target for a given material: these aspects are setting a lower bound on the beam spot size itself. The purpose of this paper is to provide a fully theoretical approach to predict the temperature increase, the thermal gradients, and the induced thermomechanical stress on targets, generated by a sequence of 45 GeV positron bunches. A case study is here presented for Beryllium and Graphite targets. We first discuss the Monte Carlo simulations to evaluate the heat deposited on the targets after a single bunch of 3 × 1011 positrons for different beam sizes. Then a theoretical model is developed to simulate the temperature increase of the targets subjected to very fast sequences of positron pulses, over different timescales, from ps regime to hundreds of seconds. Finally a simple approach is provided to estimate the induced thermomechanical stresses in the target, together with simple criteria to be fulfilled (i.e., Christensen safety factor) to prevent the crack formation mechanism.

Target normal sheath acceleration with a large laser focal diameter

The dependence of the laser-driven ion acceleration from thin titanium foils in the Target Normal Sheath Acceleration (TNSA) regime on target and laser parameters is explored using two dimensional particle-in-cell simulations. The oblique incidence (θL=45°) and large focal spot size (w0=40μm) are chosen to take an advantage of quasi one-dimensional geometry of sheath fields and effective electron heating. This interaction setup also reveals low and achromatic angular divergence of a proton beam. It is shown that the hot electron temperature deviates from the ponderomotive scaling for short laser pulses and small pre-plasmas. This deviation is mainly due to the laser sweeping, as the short duration laser pulse each moment in time effectively heats only a fraction of a focal spot on the foil. This instantaneous partial heating results in an electron temperature deviation from the ponderomotive scaling and, thus, lower maximum proton energies than it could have been expected from the TNSA theory.

Prospects of application of the focusing crystals at the large hadron colliders

Ideas of use the particle channeling in bent crystals for steer the beams have been checked up and advanced in many experiments. However, until now, this method of beam formation has limitations in application, since the channeling process involves beam particles with low angular divergence, limited by the angle of Lindhard. Here we describe the focusing crystals, which expand the boundaries of application of bent crystals at accelerators.

Near‐3‐MeV protons from target‐normal‐sheath‐acceleration femtosecond laser irradiating advanced targets

Advanced targets based on graphene oxide and gold thin film were irradiated at high laser intensity (1018–1019 W/cm2) with 50‐fs laser pulses and high contrast (108) to investigate ion acceleration in the target‐normal‐sheath‐acceleration regime. Time‐of‐flight technique was employed with SiC semiconductor detectors and ion collectors in order to measure the ion kinetic energy and to control the properties of the generated plasma. It was found that, at the optimized laser focus position with respect to the target, maximum proton acceleration up to about 3 MeV energy and low angular divergence could be generated. The high proton energy is explained as due to the high electrical and thermal conductivity of the reduced graphene oxide structure. Dependence of the maximum proton energy on the target focal position and thickness is presented and discussed.

X-ray investigation of strained epitaxial layer systems by reflections in skew geometry

Four different SiGe/Si layer structures, pseudomorphically grown and (partially) relaxed, are used as examples to demonstrate that reflections in symmetric skew geometry can successfully be used to realize a complex analysis of these systems. Taking the intensity exactly along the truncation rod of a reciprocal lattice point, it is possible to simulate this diffraction curve and determine the layer parameter in the projection according to the netplane tilt relative to the surface. The main precondition for this technique and for performing reciprocal space mapping with sufficiently high resolution is a low angular divergence of the incident and detected beams perpendicular to the diffraction plane, which can also be achieved by suitable optical elements on laboratory-based diffractometers.

High-resolution characterization of the forbidden Si 200 and Si 222 reflections

The occurrence of the basis-forbidden Si 200 and Si 222 reflections in specular X-ray diffraction ω-2Θ scans is investigated in detail as a function of the in-plane sample orientation Φ. This is done for two different diffractometer types with low and high angular divergence perpendicular to the diffraction plane. It is shown that the reflections appear for well defined conditions as a result of multiple diffraction, and not only do the obtained peaks vary in intensity but additional features like shoulders or even subpeaks may occur within a 2Θ range of about ±2.5°. This has important consequences for the detection and verification of layer peaks in the corresponding angular range.

Vectorial velocity filter for ultracold neutrons based on a surface-disordered mirror system.

We perform classical three-dimensional Monte Carlo simulations of ultracold neutrons scattering through an absorbing-reflecting mirror system in the Earth's gravitational field. We show that the underlying mixed phase space of regular skipping motion and random motion due to disorder scattering can be exploited to realize a vectorial velocity filter for ultracold neutrons. The absorbing-reflecting mirror system proposed allows beams of ultracold neutrons with low angular divergence to be formed. The range of velocity components can be controlled by adjusting the geometric parameters of the system. First experimental tests of its performance are presented. One potential future application is the investigation of transport and scattering dynamics in confined systems downstream of the filter.

Scanning X-ray microanalysis of bottom sediments using synchrotron radiation from the BINP VEPP-3 storage ring

Combining the features of synchrotron radiation (low angular divergence and continuous spectrum, natural polarization, and high intensity) with the use of modern X-ray optics (polycapillary lenses) and registering devices results in a new quality of analysis: measuring picogram concentrations of a broad set of elements with micron spatial resolution. Using a dedicated experimental station and developed methodologies, numerous scanning microanalyses of bottom sediments is performed to obtain a time series of geochemical indicators of paleoclimate in the Central Asian region. The detection limit under optimum conditions is 5 × 10−5% with a spatial resolution of ∼100 μm.

Design and Analysis of a High-Power Electron Beam Source $(>\hbox{50}\ \hbox{KW/cm}^{2})$ Confined in a Highly Uniform Magnetic Field Region

A directly heated line source emitter with beam power (>; 50 KW/cm2) is developed. The emitter assembly design parameters and beam characteristics are analyzed using a highly uniform magnetically confined region. Circular cross-sectional tungsten cathodes of diameters 0.9 mm and 1.5 mm were used, and their results are compared. The developed emitter configuration ensures low angular divergence, millisecond evaporation time for large samples, operational reproducibility, and beam stability. The emitter source has been tested by experimentally developing extremely homogeneous magnetic field in large cylindrical region of more than one-half of the coil's diameter.

Development of Focused Neutral Beams with Small Angular Divergence for Plasma Heating and Diagnostics

A series of neutral beam injectors for plasma heating and diagnostics in modern magnetic fusion devices has been developed in the Budker Institute of Nuclear Physics. In ion sources of these injectors arc discharge or RF plasma boxes are used. Ion optical systems are optimized to produce ion beams with a low enough angular divergence. In order to provide beam focusing, the grids are formed as spherical segments. Such ballistically focused beams are further neutralized in a gas target and subsequently are used to heat or diagnose plasma. Obtained diagnostic neutral beams with precise focusing are widely used to measure plasma parameters by beam emission spectroscopy methods in tokamaks, stellarators, reversed field pinches and open traps. High power focused beams with small divergence are also necessary for heating of localized regions of plasma and in the devices with narrow access ports through which only small size, high power density beams can be transported. Transition to steady state operation regime of the injectors is discussed.

Long-pulse arc-discharge plasma source with cold cathode for diagnostic neutral beam injector

Long-pulse cold cathode arc-discharge plasma generators have been successfully used as high-quality hydrogen ion sources for plasma diagnostic neutral beams. One of the main advantages of this type of plasma source is a high proton fraction (80%-90%). However, the lifetime of the plasma source is limited due to intensive electrode's erosion, especially at the cathode region. An optimized design of the cathode and the nearest electrodes is found which reduces the erosion and allows us to increase the pulse length. The plasma source produces the extracted ion current up to 3 A at a low angular divergence, and at the pulse duration up to 2 s.

An evaluation of grazing-incidence optics for neutron imaging

The focusing capabilities of neutron imaging optics based on the Wolter-1 geometry have been successfully demonstrated with a beam of long wavelength neutrons with low angular divergence. A test mirror was fabricated using an electroformed nickel replication process at Marshall Space Flight Center. The neutron current density gain at the focal spot of the mirror is found to be at least 8 for neutron wavelengths in the range from 6 to 20Å. Possible applications of the optics are briefly discussed.

Design and characterization of an atomic beam source with narrow angular divergence for alkali-earth targets

A new atomic beam source for use in laser-based experiments is described and characterized. The physically collimated source which has been tested using calcium, produces an atomic beam with low angular divergence and narrow transverse Doppler profile, as measured using fluorescence techniques with a near resonant laser beam. The low angular divergence of the source is essential for new experiments which study super-elastic scattering of electrons from laser prepared targets within a magnetic field, since the atomic beam must pass through a narrow gap between the coil windings. The beam density for calcium is calculated to be 2.8 × 109 atoms cm−3 when the source is operated at a temperature of 1090 K. This is sufficient to produce good electron energy loss and super-elastic scattering signals, while ensuring negligible radiation trapping within the atomic beam.

Femtosecond laser source of nanolocalized directed photoelectrons

A nanolocalized electron beam has been formed by electrons passing through a quartz microcapillary. The authors have demonstrated that such electrons are capable of producing the image of a dielectric nanoaperture. Using the capillary and a metal tip illuminated by femtosecond laser pulses, the authors have developed a photoelectron source based on nanolocalized photoelectron bunches with low angular divergence. It allows the observation of femtosecond-laser-induced processes on a surface with high temporal and spatial resolutions.

Directional emission from photonic crystal waveguides

We propose a novel photonic crystal waveguide design with defects at the termination to provoke directional emission. Because the resulting optical field distribution at the waveguide exit resembles a triple point source, the light can emit with low angular divergence. This phenomenon appears both in photonic crystal structures formed by dielectric rods as well as by air holes.

Structural studies on ion-implanted semiconductors using X-ray synchrotron radiation: Strain evolution and growth of nanocrystals

X-ray synchrotron radiation is a very powerful tool in structural analysis of solids. Due to its extremely high intensity and low angular divergence, synchrotron radiation allows for the study of structures that otherwise is impossible. Here, the typical studies on different semiconductor materials using various methods of synchrotron X-ray scattering are presented. Ion implantation in compound semiconductors is a quite complicated process leading to some usually detrimental effects like buildup of damage and strain. In the case of InP, produced strain is rather low and is hardly accessible by the conventional X-ray analysis. InP single crystals implanted with different doses of 1.2 MeV As + ions were studied. It was found that implanted layers are under tensile stress, which relaxes upon prolonged storage at RT. Another important issue is the analysis of nanocrystals. Diamond–SiC heterostructures produced by ion beam synthesis are considered as a promising technique for production of novel devices for high-power applications. Synthesized by ion bombardment nanocrystals that grow in the single crystalline matrix are highly oriented. As revealed by X-ray scattering, size and shape of nanocrystals, as well as incorporated strain depend on the ion implantation parameters.

Diagnostic Neutral Beam Injector for Active Plasma Spectroscopy at Gas-Dynamic Trap

A diagnostic beam system has been developed for the GDT. This injector is the modification of the diagnostic injector RFX-DNBI. The system is primarily used for magnetic field measurements via motional Stark effect (MSE). The ion source provides 50keV, 5A hydrogen beam. Ions are extracted from a plasma created by an arc-discharge source and, after accelerating and focusing, are neutralized in a gas target. A plasma emitter, which is formed by collisionless expansion of a plasma jet on to the grids, has low perpendicular ion temperature. These results in rather low (0.01 rad) angular divergence of the extracted ion beam. In the accelerator, there is a set of four nested grids with 421 circular apertures of 4 mm diameter configured in a hexagonal pattern. The geometry of the elementary cell was optimized by using 2D computer code PBGUNS to obtain small angular divergence of the beam. The grids of ion optical system are spherically curved providing geometric focusing of the beam at a distance 1.5 m. Arc-discharge plasma box provides highly ionized plasma, so that the extracted beam has about 90% of full energy specie. The injector provides up to 4 ms duration pulse.

Neutral beam injector for active plasma spectroscopy

A diagnostic beam system has been developed for the RFX reversed field pinch, Padova, Italy. Currently the system is loaned to Alcator C-mod, MIT, Boston. The system is primarily used for measurement of the ion temperature by charge-exchange recombination spectroscopy and for magnetic field measurements via motional Stark effect. The system comprises an ion source, beam duct equipped with vacuum pumps and various diagnostics of the beam. The ion source provides 50 keV, 5 A hydrogen beam. Ions are extracted from a plasma created by an arc-discharge source and, after accelerating and focusing, are neutralized in a gas target. A plasma emitter, which is formed by collisionless expansion of a plasma jet on to the grids, has low perpendicular ion temperature. These results are in rather low (0.01 rad) angular divergence of the extracted ion beam. The grids of ion optical system are spherically curved providing geometric focusing of the beam at a distance 4 m. Current density at the focal plane reaches 100 mA/cm2. Arc-discharge plasma box provides highly ionized plasma, so that the extracted beam has about 80% of full energy specie. The injector provides 50 ms duration pulse each 5 min. In order to increase the signal to noise ratio the beam can be modulated with a frequency variable up to 250 Hz.