Beam Brightness Research Articles

Structural and electronic optimization of ring-graphene cathodes and their field emission properties

Cold field emission sources are capable of providing highly coherent and bright electron beams. However, they suffer from some well-known practical difficulties that have prevented their widespread use: unmanageably stringent ultrahigh vacuum requirements, relatively large current instabilities, and rapid emission decays in periods as short as 1–2 h, requiring regular flashing (Joule heating). This paper presents the microfabrication and a successful emission test of a micron-sized concentric ring-graphene cathode cold field emitter, without any observable wall collapse. The cathode is designed to have a nanometer size ring-edge, while its radius can measure several microns or larger, providing stable electron emission under high vacuum conditions. The turn-on electric field of a ring-cathode source can be dramatically lowered by introducing a thin layer of nickel nanoparticles by an in-situ focused ion beam process. The dependence of field enhancement on the anode-cathode distance and the ring-cathode radius has been systematically studied.

Influence of the Frequency Detuning to Electrodynamics Parameters of an Electron Linac

It is very important task to define the tolerances necessary for correct operation of an electron linac. The detuning of linac main parameters as frequency, RF field amplitude and phase can sufficiently influence to the beam parameters. It is highly important for linacs with high brightness beams which are necessary for light sources and colliders. It is also sufficient for industrial linacs. A linac section consists of regular and bunching parts. The influence of one cell detuning on electrodynamics parameters of the whole section was studied. Results of such simulation will discuss in this paper.

RF design of APEX2 two-cell continuous-wave normal conducting photoelectron gun cavity based on multi-objective genetic algorithm

High brightness, high repetition rate electron beams are key components for optimizing the performance of next generation scientific instruments, such as MHz-class X-ray Free Electron Laser (XFEL) and Ultra-fast Electron Diffraction/Microscopy (UED/UEM). In the Advanced Photo-injector EXperiment (APEX) at Berkeley Lab, a photoelectron gun based on a 185.7 MHz normal conducting re-entrant RF cavity, has been proven to be a feasible solution to provide high brightness, high repetition rate electron beam for both XFEL and UED/UEM. Based on the success of APEX, a new electron gun system, named APEX2, has been under development to further improve the electron beam brightness. For APEX2, we have designed a new 162.5 MHz two-cell photoelectron gun and achieved a significant increase on the cathode launching field and the beam exit energy. For a fixed charge per bunch, these improvements will allow for the emittance reduction and hence an increased beam brightness. The design of APEX2 gun cavity is a complex problem with multiple design goals and restrictions, some even competing each other. For a systematic and comprehensive search for the optimized cavity geometry, we have developed and implemented a novel optimization method based on the Multi-Objective Genetic Algorithm (MOGA).

Formation of multi-spot focal arrays by square-shaped optical vortex lattices

In this work, we present numerical simulations and experimental evidence for the creation of controllable multi-spot focal arrays composed of bright beams with flat phase profiles. The input phase structures sent to spatial light modulators were square-shaped optical vortex (OV) lattices containing hundreds of vortices. In order to stabilize each of these lattices in space, all used OVs were singly charged and their topological charges varied alternatively across the structures. It is proven that the OV lattice node spacing can be used as a control parameter for reshaping the generated multi-spot focal arrays. Each peak of these arrays is shown to be able to additionally host a singular beam.

Space-charge distortion of transverse profiles measured by electron-based ionization profile monitors and correction methods

Measurements of transverse profiles using Ionization Profile Monitors (IPMs) for high brightness beams are affected by the electromagnetic field of the beam. This interaction may cause a distortion of the measured profile shape despite strong external magnetic field applied to impose limits on the transverse movement of electrons. The mechanisms leading to this distortion are discussed in detail. The distortion itself is described by means of analytic calculations for simplified beam distributions and a full simulation model for realistic distributions. Simple relation for minimum magnetic field scaling with beam parameters for avoiding profile distortions is presented. Further, application of machine learning algorithms to the problem of reconstructing the actual beam profile from distorted measured profile is presented. The obtained results show good agreement for tests on simulation data. The performance of these algorithms indicate that they could be very useful for operations of IPMs on high brightness beams or IPMs with weak magnetic field.

A Study on the negative ion beam production in the ININ sputtering ion source

To improve the beam brightness produced by a Source of Negative Ions by Cesium Sputtering we studied the beam generation in the 12~MeV Vandergraff linear accelerator at Instituto Nacional de Investigaciones Nucleares. Results of 3D particle tracking simulations of the ion source and beamline have been compared with measurements, with better agreement than traditional codes that only take into account the negative beam, and they determine a suppression in the Cs$^{+}$ production due to space charge, which in turn explains the intensity limits for negative beam production in both ionizers, and the best way to overpass them. Also, the beam dynamics variation due to the erosion of the target inside the cathode has been determined, helping to prevent beam losses and enhance the beam brightness.

Beam Control in an Intracavity Frequency-Doubling Semiconductor Disk Laser

In this paper, we have demonstrated a 1.3 W green laser using a V-shaped intracavity frequency doubling 1036 nm semiconductor disk laser. The beam quality of the fundamental and second harmonic generation (SHG) laser is investigated. It has been found that the output lasers at the fundamental and SHG wavelength both suffer from reduced beam quality along with power scaling. The measured beam profile is elliptical under high power operation. In order to improve the SHG laser beam, an intracavity aperture is employed to control the mode characteristics of fundamental frequency light. By fine-tuning the aperture, a nearly circular beam profile with improving beam quality and brightness is realized.

Beam brightness and its reduction in a 1.2-MV cold field-emission transmission electron microscope

In this paper we discuss probe properties in terms of probe currents, probe sizes, energy spread, virtual source sizes, and brightness in a 1.2-MV cold field-emission (cold FE) transmission electron microscope (TEM) equipped with a magnetic gun lens. The probe size increased gradually in proportion to the (3/8)th power of the probe current, very unusual behavior in cold FE guns but typical behavior in thermionic guns. This is due to the magnetic gun lens, which caused large emission angles for electron beams in the probe before being limited by aberrations at the gun and acceleration tube. The brightness reached the maximum at 1.62 × 1014 A/(m2sr) and then decreased with increasing the emission current. The energy spread of the beam, including the Boersch effect, was 0.32–0.50 eV, comparable to that of conventional cold FE guns without magnetic gun lens. Experimental analysis indicated that neither noises nor aberrations caused the probe size increase under the optimized illumination condition. The virtual source size increased from 11.1 to 24.7 nm with the increase in the emission current. To describe this behavior, we conjecture that this brightness reduction is due to increase in the virtual source size caused by trajectory displacement created by stochastic Coulomb interactions near the emitter.

Quadrupole pre-filter/mass filter transmission profiles: Experiment and simulation.

Quadrupole pre-filter transmission profiles are shown to be affected by the presence of trapped ions within the pre-filter of a quadrupole mass spectrometer. Trapped ions are the result of ions reflected at the pre-filter/mass filter boundary and lead to space charge effects that can affect the shape of the pre-filter transmission profile, ion beam stability, peak width and peak intensity. The development of high sensitivity mass spectrometers with bright ion beams leads to greater numbers of trapped ions. Experimental pre-filter transmission profiles have been measured with and without an empty step to clear the pre-filter region of trapped ions showing the extent that space charge has on ion transmission through the pre-filter. Ion trajectory simulations, using a restricted set of initial conditions, have been used to help understand the structure of the pre-filter transmission profile. Experiments show that the effects of trapped ions within the pre-filter can be easily seen in the shape of the pre-filter transmission profile, ion beam stability and peak shape. The pre-filter transmission profile shows structure which is related to the secular frequency of the ion of interest which is related to the Mathieu parameter q of the pre-filter. Simulations show that the nodes observed in the experimental transmission profile are spaced by half the secular period of the ion. The structure of the pre-filter transmission profile can be observed experimentally when steps are taken to remove trapped ions from within the pre-filter. A better understanding of the factors contributing to the shape of the pre-filter transmission profile from simulations has been demonstrated.

A compact tunable quadrupole lens for brighter and sharper ultra-fast electron diffraction imaging

In this article, we report our proof-of-principle design and experimental commissioning of a broadly tunable and low-cost transverse focusing lens system for MeV-energy electron beams. The lens system based on electromagnetic (EM) quadrupoles has been built as a part of the existing instrument for ultra-fast electron diffraction (UED) experiments at the Accelerator Test Facility II (ATF-II) at Brookhaven National Laboratory (BNL). We experimentally demonstrated the independent control of the size and divergence of the beam with the charge ranging from 1 to 13 pC. The charge density and divergence of the beam at the sample are the most important factors determining the quality of the Bragg-diffraction image (BDI). By applying the Robust Conjugate Directional Search (RCDS) algorithm for online optimization of the quadrupoles, the transverse beam size can be kept constant down to 75 µm from 1 to 13 pC. The charge density is nearly two orders of magnitude higher than the previously achieved value using a conventional solenoid. Using the BDI method we were able to extract the divergence of the beam in real-time and apply it to the emittance measurement for the first time. Our results agree well with simulations and with the traditional quadrupole scan method. The real-time divergence measurement opens the possibility of online optimization of the beam divergence (<0.2 mrad) at the sample with the increased beam charge. This optimization is crucial for the future development of single-shot ultra-fast electron microscope (UEM). Finally, we demonstrated BDI with significant improvement, up to 3 times higher peak intensity and 2 times sharper Bragg-diffraction peaks at 13 pC. The charge is now limited by the laser power and increasing charge may improve the quality of BDI further. The capability we demonstrated here provides us with opportunities for new sciences using near-parallel, bright and ultrafast electron beams for single-shot imaging, to directly visualize the dynamics of defects and nanostructured materials, or even record molecular movie, which are impossible using present electron-beam technologies.

Design and fabrication of a fused 7 × 1 35/50 µm into 125/250 µm fiber combiner

We present the design and construction of a fiber combiner comprising 7 × 1 35/50 µm input fibers with thin cladding and an output fiber with 125 µm diameter to maintain the high beam brightness. A transmission higher than 98% between the input fibers and the tapered fiber bundle is achieved. The coupling efficiency between the bundle and the output fiber exceeds 95%. The-near field beam profiles of the fabrication steps are measured and analyzed. Due to the thin cladding of the input fibers, we analyze optical tunneling effects with respect to the numerical aperture of the light.

Controllable beam reshaping by mixing square-shaped and hexagonal optical vortex lattices

In the present work we show experimentally and by numerical calculations a substantial far-field beam reshaping by mixing square-shaped and hexagonal optical vortex (OV) lattices composed of vortices with alternatively changing topological charges. We show that the small-scale structure of the observed pattern results from the OV lattice with the larger array node spacing, whereas the large-scale structure stems from the OV lattice with the smaller array node spacing. In addition, we demonstrate that it is possible to host an OV, a one-dimensional, or a quasi-two-dimensional singular beam in each of the bright beams of the generated focal patterns. The detailed experimental data at different square-to-hexagonal vortex array node spacings shows that this quantity could be used as a control parameter for generating the desired focused structure. The experimental data are in excellent agreement with the numerical simulations.

Next generation high brightness electron beams from ultrahigh field cryogenic rf photocathode sources

Recent studies of the performance of radio-frequency (rf) copper cavities operated at cryogenic temperatures have shown a dramatic increase in the maximum achievable surface electric field. We propose to exploit this development to enable a new generation of photoinjectors operated at cryogenic temperatures that may attain, through enhancement of the launch field at the photocathode, a significant increase in five-dimensional electron beam brightness. We present detailed studies of the beam dynamics associated with such a system, by examining an S-band photoinjector operated at $250\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ peak electric field that reaches normalized emittances in the 40 nm-rad range at charges (100--200 pC) suitable for use in a hard x-ray free-electron laser (XFEL) scenario based on the LCLS. In this case, we show by start-to-end simulations that the properties of this source may give rise to high efficiency operation of an XFEL, and permit extension of the photon energy reach by an order of magnitude, to over 80 keV. The brightness needed for such XFELs is achieved through low source emittances in tandem with high current after compression. In the XFEL examples analyzed, the emittances during final compression are preserved using microbunching techniques. Extreme low emittance scenarios obtained at pC charge, appropriate for significantly extending temporal resolution limits of ultrafast electron diffraction and microscopy experiments, are also reviewed. While the increase in brightness in a cryogenic photoinjector is mainly due to the augmentation of the emission current density via field enhancement, further possible increases in performance arising from lowering the intrinsic cathode emittance in cryogenic operation are also analyzed. Issues in experimental implementation, including cavity optimization for lowering cryogenic thermal dissipation, external coupling, and cryocooler system, are discussed. We identify future directions in ultrahigh field cryogenic photoinjectors, including scaling to higher frequency, use of novel rf structures, and enabling of an extremely compact hard x-ray FEL.

Twin-beam intensity-difference squeezing below 10 Hz.

We report the generation of strong, bright-beam intensity-difference squeezing down to measurement frequencies below 10 Hz. We generate two-mode squeezing in a four-wave mixing (4WM) process in Rb vapor, where the single-pass-gain nonlinear process does not require cavity locking and only relies on passive stability. We use diode laser technology and several techniques, including dual seeding, to remove the noise introduced by seeding the 4WM process as well as the background noise. Twin-beam intensity-difference squeezing down to frequencies limited only by the mechanical and atmospheric stability of the lab is achieved. These results should enable important low-frequency applications such as direct intensity-difference imaging with bright beams on integrating detectors.

Terahertz-based subfemtosecond metrology of relativistic electron beams

Photons, electrons, and their interplay are at the heart of photonic devices and modern instruments for ultrafast science [1-10]. Nowadays, electron beams of the highest intensity and brightness are created by photoemission with short laser pulses, and then accelerated and manipulated using GHz radiofrequency electromagnetic fields. The electron beams are utilized to directly map photoinduced dynamics with ultrafast electron scattering techniques, or further engaged for coherent radiation production at up to hard X-ray wavelengths [11-13]. The push towards improved timing precision between the electron beams and pump optical pulses though, has been stalled at the few tens of femtosecond level, due to technical challenges with synchronizing the high power rf fields with optical sources. Here, we demonstrate attosecond electron metrology using laser-generated single-cycle THz radiation, which is intrinsically phase locked to the optical drive pulses, to manipulate multi-MeV relativistic electron beams. Control and single-shot characterization of bright electron beams at this unprecedented level open up many new opportunities for atomic visualization.

Optomechanically induced entanglement

We present a special quantum phenomenon named optomechanically induced entanglement in the conventional single-cavity optomechanical system driven by a strong pump input field and a relatively weak probe input field. Bipartite entanglement between the pump and probe output fields can be achieved under realistic experimental conditions when the input pump (probe) field is blue detuned by the mechanical frequency (resonant) with respect to the cavity field. The physical origin is the mechanical oscillator displacement, which plays a role similar to the atomic coherence for the well-known electromagnetically induced transparency in the traditional \ensuremath{\Lambda}-type atomic system. This scheme provides an alternative, convenient way to generate nondegenerate entangled bright light beams by using only coherent laser fields, and may bring great facility in realistic quantum information processing protocols.

Collimated GeV attosecond electron–positron bunches from a plasma channel driven by 10 PW lasers

High-energy positrons and bright γ-ray sources are of great importance both in fundamental research and for practical applications. However, collimated GeV electron–positron pair jets and γ-ray flashes are still rarely produced in the laboratory. Here, we demonstrate that by irradiating a near-critical-density plasma channel with two 10 PW-scale laser pulses, highly directional GeV electron–positron pairs and bright γ-ray beams can be efficiently generated. Three-dimensional particle-in-cell simulations show the formation of GeV positron jets with high density (8×1021/ cm3), attosecond duration (400 as), and a divergence angle of 14°. Additionally, ultrabright [2×1025 photons s−1 mm−2 mrad−2 (0.1% bandwidth)−1] collimated attosecond (370 as) γ-ray flashes with a laser energy conversion efficiency of 5.6% are emitted. These features show the significant advantage of using a plasma channel as compared with a uniform plasma and thus open up new possibilities for a wide variety of applications.

Deterministic quantum entanglement among multiple quantum nodes

Quantum entanglement is a significant quantum resource, which plays a central role in quantum communication. For realizing quantum information network, it is important to establish deterministic quantum entanglement among multiple spatial-separated quantum memories, and then the stored entanglement is transferred into the quantum channels for distributing and transmitting the quantum information at the user-control time. Firstly, we introduce the scheme of deterministic generation polarization squeezed state at 795 nm. A pair of quadrature amplitude squeezed optical fields are prepared by two degenerate optical parameter amplifiers pumped by a laser at 398 nm, and then the polarization squeezed state of light appears by combining the generated two quadrature amplitude squeezed optical beams on a polarizing beam splitter. Secondly, we present the experimental demonstration of tripartite polarization entanglement described by Stokes operators of optical field. The quadrature tripartite entangled states of light corresponding to the resonance with D1 line of rubidium atoms are transformed into the continuous-variable polarization entanglement via polarization beam splitter with three bright local optical beams. Finally, we propose the generation, storage and transfer of deterministic quantum entanglement among three spatially separated atomic ensembles. By the method of electromagnetically induced transparency light-matter interaction, the optical multiple entangled state is mapped into three distant atomic ensembles to build the entanglement among three atomic spin waves. Then, the quantum noise of entanglement stored in the atomic ensembles is transferred to the three space-seperated quadrature entangled light fields through three quantum channels. The existence of entanglement among the three released beams verifies that the system has the ability to maintain the multipartite entanglement. This protocol realizes the entanglement among three distant quantum nodes, and it can be extended to quantum network with more quantum nodes. All of these lay the foundation for realizing the large-scale quantum network communication in the future.

Comparison of coherence-area measurement techniques for bright entangled twin beams

Quantum states of light with multiple spatial modes are fundamental for quantum imaging and parallel quantum information processing. Thus, their characterization, which can be achieved through measurements of the coherence area, is an important area of research. We present a comparative study between two different measurement techniques for the coherence area of bright entangled twin beams of light generated with a four-wave mixing process in a hot rubidium vapor cell. The first one provides a direct characterization of the size of the coherence area and is based on correlation measurements between spatial intensity fluctuations of the twin beams with an electron-multiplying charge-coupled-device camera. The second one provides an indirect measure and is based on a noise analysis of different spatial regions of the twin beams in the time domain with a single photodiode. We show that the indirect technique, which can be implemented with a significantly less complicated measurement scheme, gives an estimate of the size of the coherence area consistent with the direct measurement technique performed in the spatial domain.

Japan’s next neutrino detector to have huge tanks, bright beam

Japan’s next neutrino detector to have huge tanks, bright beam