Beam Diagnostics Research Articles

Features of the formation and diagnostics of powerful metal ion beams with submillisecond duration

The article presents the results of experimental studies on the formation of repetitively-pulsed beams of titanium ions with submillisecond pulse duration from the continuous vacuum arc discharge plasma. The influence of ion-electron emission on the overestimation of the measured ion current with a power density of tens and hundreds of kilowatts per square centimeter was studied using various methods. It has been established that at an average energy of titanium ions of about 80 keV, due to ion-electron emission, the measured current is three times higher than the real ion beam current. Complicating the measurement of the current and energy parameters of the ion beam, the ion-electron emission ensures the neutralization of its space charge during the entire duration of the pulse. An increase in the ion current density due to ballistic focusing to more than 1 A/cm2 does not change the ion-electron emission coefficient. It is shown that, as a result of incomplete ion beam space charge neutralization, its crossover is shifted by 20 mm beyond the geometric focus of the system. The possibility of forming submillisecond high-intensity titanium ion beams with a current density exceeding 2.8 A/cm2, a power density exceeding 200 kW/cm2, and a pulse energy exceeding 100 J has been experimentally demonstrated. Data are obtained on the change in the current density over the ion beam cross-section depending on the accelerating voltage amplitude, the vacuum arc discharge current, and the distance from the focusing electrode.

Imaging Extreme Ultraviolet Radiation Using Nanodiamonds with Nitrogen-Vacancy Centers.

Extreme ultraviolet (EUV) radiation with wavelengths of 10-121 nm has drawn considerable attention recently for its use in photolithography to fabricate nanoelectronic chips. This study demonstrates, for the first time, fluorescent nanodiamonds (FNDs) with nitrogen-vacancy (NV) centers as scintillators to image and characterize EUV radiations. The FNDs employed are ∼100 nm in size; they form a uniform and stable thin film on an indium-tin-oxide-coated slide by electrospray deposition. The film is nonhygroscopic and photostable and can emit bright red fluorescence from NV0 centers when excited by EUV light. An FND-based imaging device has been developed and applied for beam diagnostics of 50 nm and 13.5 nm synchrotron radiations, achieving a spatial resolution of 30 μm using a film of ∼1 μm thickness. The noise equivalent power density is 29 μW/(cm2 Hz1/2) for the 13.5 nm radiation. The method is generally applicable to imaging EUV radiation from different sources.

(Invited) Ionic Liquid Electrospray Ion Sources for Space Propulsion

The number of microspacecraft launched has significantly increased since 2013, and more than 300 nano/microsatellites (< 50 kg) were launched in 2017. Although some microspacecraft already have a propulsion system, a commonly used propulsion system is a cold-gas thruster, which has a low delta-v (a few 10s of m/s), and thus, high delta-v (> 1 km/s) thrusters are required for more advanced missions. One of the candidates for such thrusters is ion thrusters, which were already mounted on two 50 kg-class microsatellites and successfully operated in space. However, it is not easy to miniaturize ion thrusters so that they can be mounted on even 10 kg-class nanosatellites unless high-pressure gas-feeding systems are removed. Moreover, ion thrusters require electron sources to neutralize ion beams, which consume additional propellants and power. Since most recently launched microspacecraft are less than 10 kg, more compact thrusters are required. Ionic liquid electrospray ion sources are attractive devices for such electric propulsion, especially for microspacecraft. Electrospray thrusters typically consist of many emitters, an extractor electrode, and an accelerator electrode optionally. When a high voltage between the emitter and the extractor is applied, the ionic liquid is transported to the emitter tips, and a strong electric field deforms the ionic liquid into a conical shape known as a Taylor cone. When the force of the electric field is stronger than the surface tension pressure of the ionic liquid, ions evaporate directly from the liquid surface. The extracted ions are then accelerated by the potential difference between the emitter and extractor to produce the thrust. Since the ionic liquid is composed of only cations and anions without solvent, its vapor pressure is negligible; therefore, it can exist in the liquid phase in vacuum, simplifying the propellant feed system and reducing the size of the entire propulsion system. Moreover, the direct acceleration of ions without discharges can be possible, and no neutralizers are required, which also contributes to the power and size reduction together with high efficiency. The key component of electrospray thrusters is the emitter, where there are typically three types of structures: externally-wetted (or needle-shaped), internally-wetted (or capillary), and porous ones. Each has ion emission characteristics depending on the structure. Here, we have fabricated these three types of emitter structures with a wide range of scale: 1 to 100 micrometers. Using these emitters, we have conducted ion emission experiments and beam diagnostics. We obtained a high current density over a few 10 mA/cm2 with 1-micrometer scale emitters, a high current of approximately 1 mA with porous emitters, and a high efficiency with externally-wetted emitters. The emitter structures as mentioned earlier and experimental results will be presented at the conference site.

Environment-aware motion planning for EAST Optical Diagnostic Beam Orientation Robot

EAST Optical Diagnostic Beam Orientation Robot (EODBOR) is designed for the calibration of the visible optical diagnostic system before the plasma experiment, which can orientate light beams with high precision. At present, the motion of EODBOR is purely controlled by the operators during the whole calibration process since planning algorithms based on prior knowledge of the environment cannot be adopted due to the intrinsically unstructured property of EAST’s vacuum vessel. To automate the calibration, we upgraded our previous system and propose a method for environment-aware motion planning based on Robot Operating System (ROS). In this method, the probabilistic octree is used to represent unknown objects in the environment, which is updated with point clouds from an RGB-D camera mounted at the end of the manipulator. We verify the feasibility and stability of the proposed method through simulations and experiments.

Direct current measurements of the SPIDER beam: a comparison to existing beam diagnostics

For negative ion beam sources there are several methods of measuring the accelerated beam current, most commonly electrical measurements at the power supply and calorimetric measurements. On SPIDER, the ITER Heating Neutral Beam full-scale beam source prototype, electrical measurements at the acceleration grid power supply (AGPS) are complemented by polarizing the diagnostic calorimeter STRIKE to provide an additional electrical measurement of the accelerated current. This is in addition to the calorimetric measurements provided by STRIKE. These diagnostics give differing measurements of the beam current. Exploiting the reduced number of open apertures on SPIDER a new beam diagnostic has been installed to measure the individual beamlet currents directly. The so called Beamlet Current Monitor (BCM) has been used to measure the current of five beamlets during the most recent SPIDER campaign. This work compares the BCM current to the electrical measurements at the AGPS and STRIKE. The average BCM current agrees well with the STRIKE electrical measurements, indicating that the AGPS overestimates the beam current. The individual beamlets are compared to the STRIKE calorimetric measurements, showing similar current trends with the source parameters.

Optimization of a fast deuterium diagnostic method based on visible energetic 3He spectroscopy for high electron density plasmas.

Fast ions play a crucial role in plasma heating, and their behavior in the plasma must be accurately understood. A diagnostics method based on charge exchange emission from the n = 4 - 3 transition (λ0 = 468.6nm) of energetic 3He produced by the deuteron-deuteron reaction has been proposed as a for fast deuterons with energies in the order of MeV. The proposed method has the following advantages: No beam emission interferes with the spectra, the direction of the measuring line of sight, and the injection angle of the diagnostic beam can be freely determined. In previous studies, due to competing bremsstrahlung, it was expected that the proposed method will not be practical in the case of high electron density operation. This paper makes the proposed method available for measurement even at high electron densities by optimizing the measurement line of sight direction and the diagnostic beam incidence angle. This allows an electron density five times larger than the range of applications shown in previous studies. This result will contribute to measure of DT alpha in ITER.

Bragg Curve Detection of Low-Energy Protons by Radiophotoluminescence Imaging in Lithium Fluoride Thin Films.

Lithium fluoride (LiF) crystals and thin films are utilized as radiation detectors for energy diagnostics of proton beams. This is achieved by analyzing the Bragg curves in LiF obtained by imaging the radiophotoluminescence of color centers created by protons. In LiF crystals, the Bragg peak depth increases superlinearly with the particle energy. A previous study has shown that, when 35 MeV protons impinge at grazing incidence onto LiF films deposited on Si(100) substrates, the Bragg peak in the films is located at the depth where it would be found in Si rather than in LiF due to multiple Coulomb scattering. In this paper, Monte Carlo simulations of proton irradiations in the 1-8 MeV energy range are performed and compared to experimental Bragg curves in optically transparent LiF films on Si(100) substrates. Our study focuses on this energy range because, as energy increases, the Bragg peak gradually shifts from the depth in LiF to that in Si. The impact of grazing incidence angle, LiF packing density, and film thickness on shaping the Bragg curve in the film is examined. At energies higher than 8 MeV, all these quantities must be considered, although the effect of packing density plays a minor role.

Vibration analysis of a cracked beam using the finite element method

This study is motivated by the condition monitoring and fault diagnostics of structural beams used in large-scale vibrating screens for the mining industry. For the purpose of developing a reliable model-based approach, a new stiffness matrix of a three-dimensional finite element is proposed for modelling a beam with a breathing crack. Using the obtained stiffness matrix, a finite element model was derived for a cracked beam subjected to a bidirectional base excitation. With the model, a computer simulation was conducted to examine the influence of the crack depth on the natural frequencies of the beam. The numerical results show that the crack influences mainly the natural frequencies of the bending modes in the direction of the crack's growth. The simulation also investigates the responses of the beam subjected to a harmonic base excitation in two directions. The numerical results show that the responses of the cracked beam contain several superharmonic components, and the amplitude of superharmonic components increases with the increase in crack depth. Experiments are conducted to validate the proposed dynamic model using both impact testing and forced testing.

Cherenkov diffraction radiation emissions from single electrons and positrons on a fused silica radiator

Beam diagnostics are crucial for smooth accelerator operations. Many techniques rely on instrumentation in which the beam properties are significantly affected by the measurement. Novel approaches aim to use Cherenkov Diffraction Radiation (ChDR) for non-invasive diagnostics. Unlike regular Cherenkov Radiation, the charged particles do not have to move inside of the medium, but it is sufficient for them to move in its vicinity as long as they are faster than the speed of light in the medium. Changes to the beam properties due to ChDR measurements are consequently negligible. To examine ChDR emission under different conditions, we placed a fused silica radiator in the DESY II Test Beam. We observed increases in ChDR intensity for electron and positron momenta between 1GeVc−1 and 5GeVc−1. Additionally, we found a larger photon yield for electrons than positrons for increasing particle momenta. However, the significance of these measurements is strongly limited by the accuracy of the conversion from the measured signal to absolute photon numbers. The results suggest a need for further research into the ChDR generation by electrons and positrons and may find application in the design of future beam diagnostic devices.

Astigmatic electron beam propagation

An astigmatic electron beam with an ellipticity ratio of approximately 2:1 was examined using the Enhanced Modified Faraday Cup (EMFC) diagnostic to measure its properties and show how the power density distribution varies near the beam waist. Results show that the beam has two power density peaks, one on either side of the beam crossover point, resulting from different focal distances of the major and minor axes of the elliptical beam shape. Quantification of the beam properties was used to establish a depth-of-field where the beam diameter and peak power densities are relatively constant. For the 1 kW beam studied here, the depth-of-field measured approximately ±5 mm from the beam crossover point, illustrating how electron beam diagnostics can be used to identify process control limits for repeatable and reliable welds under non-ideal electron beam power density distribution conditions.

Phase Space Reconstruction from Accelerator Beam Measurements Using Neural Networks and Differentiable Simulations.

Characterizing the phase space distribution of particle beams in accelerators is a central part of understanding beam dynamics and improving accelerator performance. However, conventional analysis methods either use simplifying assumptions or require specialized diagnostics to infer high-dimensional (>2D) beam properties. In this Letter, we introduce a general-purpose algorithm that combines neural networks with differentiable particle tracking to efficiently reconstruct high-dimensional phase space distributions without using specialized beam diagnostics or beam manipulations. We demonstrate that our algorithm accurately reconstructs detailed 4D phase space distributions with corresponding confidence intervals in both simulation and experiment using a limited number of measurements from a single focusing quadrupole and diagnostic screen. This technique allows for the measurement of multiple correlated phase spaces simultaneously, which will enable simplified 6D phase space distribution reconstructions in the future.

Simulation of the response of a diamond-based radiation detector to ultra-short and intense high-energy electron pulses

Single-crystal synthetic diamond sensors have been widely used in radiation dosimetry and beam diagnostics. The foreseen harsh radiation environment in electron-positron colliders at the luminosity frontier requires a thorough investigation of diamond’s response to large radiation bursts, in particular, to intense high-energy electron pulses. In this article, a two-step numerical simulation approach (Sentaurus + LTspice) is proposed to explore this topic. The time response of the diamond detector is simulated using TCAD-Sentaurus while the transmission effect of the electronic circuit is taken into account using LTspice. Good agreement is observed between results of the numerical simulation and preliminary experimental data from detector’s exposure to high-energy sub-picosecond electron pulses, on both the amplitude and the shape of the induced signals. This simulation combination is a novel approach to designing and optimizing diamond detectors for radiation and beam loss monitoring in particle physics experiments.

Numerical and experimental investigations of a microwave interferometer for the negative ion source SPIDER

The electron density close to the extraction grids and the co-extracted electrons represent a crucial issue when operating negative ion sources for fusion reactors. An excessive electron density in the plasma expansion region can indeed inhibit the negative ion production and introduce potentially harmful electrons in the accelerator. Among the set of plasma and beam diagnostics proposed for SPIDER upgrade, a heterodyne microwave (mw) interferometer at 100 GHz is currently being explored as a possibility to measure electron density in the plasma extraction region. The major issue in applying this technique in SPIDER is the poor accessibility of the probing microwave beam through the source metal walls and the long distance of 4 m at which mw modules should be located outside the vacuum vessel. Numerical investigations in a full-scale geometry showed that the power transmitted through the plasma source apertures was above the signal-to-noise ratio threshold for the microwave module sensitivity. An experimental proof-of-principle of the setup to assess the possibility of signal phase detection was then performed. The microwave system was tested on an experimental full-scale test-bench mimicking SPIDER viewports accessibility constraints, including the presence of a SPIDER-like plasma. The outcome of first tests revealed that, despite the geometrical constraints, in certain conditions, the phase detection, and, therefore, electron density measurements are possible. The main issue arises from decoupling the one-pass signal from spurious multipaths generated by mw beam reflections, requiring signal cross correlation analysis. These preliminary tests demonstrate that despite the 4 m distance between the mw modules and the presence of metal walls, plasma density measurement is possible when the 80 mm diameter ports are available. In this contribution, we discuss the numerical simulations, the preliminary experimental tests and suggest design upgrades of the interferometric setup to enhance signal transmission.

A new high parallel-field spectrometer at TRIUMF's β-NMR facility.

A new high field spectrometer has been built to extend the capabilities of the β-detected nuclear magnetic resonance (β-NMR) facility at TRIUMF. This new beamline extension allows β-NMR spectroscopy to be performed with fields up to 200 mT parallel to a sample's surface (perpendicular to the ion beam), allowing depth-resolved studies of local electromagnetic fields with spin polarized probes at a much higher applied magnetic field than previously available in this configuration. The primary motivation and application is to allow studies of superconducting radio frequency (SRF) materials close to the critical fields of Nb metal, which is extensively used to fabricate SRF cavities. The details of the design considerations and implementation of the ultra-high vacuum (UHV) system, ion optics, and beam diagnostics are presented here. Commissioning of the beamline and spectrometer with radioactive ions are also reported here. Future capabilities and applications in other areas are also described.

Proton Bragg peak imaging by colour centre radiophotoluminescence in lithium fluoride thin film radiation detectors on silicon

Optically transparent lithium fluoride (LiF) thin films, thermally evaporated on Si(100) substrates, are under investigation as novel radiation detectors based on radiophotoluminescence for imaging of the full Bragg curves of proton beams produced by a linear accelerator for proton therapy under development at ENEA C.R. Frascati. Proton irradiation induces the formation of stable colour centres in LiF, amongst which the broadband light-emitting F2 and F3+ aggregate defects, whose concentrations are locally proportional to the energy deposited in the material. Their spatial distributions in the irradiated LiF thin films and crystals are carefully measured by acquiring the latent two-dimensional visible fluorescence images with an optical microscope under blue lamp excitation. Several LiF films grown on silicon substrate were irradiated in air at increasing proton energies up to 35 MeV with their surface parallel to the particle beam and a cleaved edge perpendicularly facing it; for each sample, the fluorescence image acquired from the top surface side of the film allows to obtain the depth profile of the energy released by protons. Differences in colour centre distributions detected in LiF films with respect to LiF crystals are presented and discussed. Accurate Monte Carlo simulations allow to fully explain their experimental behaviours, paving the way towards using LiF film radiation detectors on silicon for the advanced diagnostics of proton beams at typical particle energies used for proton therapy.

Smith–Purcell Radiation Driven by the Field of a Standing Laser Wave

Smith–Purcell radiation is well known as a source of quasi-monochromatic electromagnetic radiation that occurs when fast electrons move above a diffraction grating. In this paper, we calculated the Smith–Purcell radiation generation from a flat surface along which there is a field of a standing laser wave. A periodically changing laser field induces a periodic inhomogeneity in the distribution of electrons in the near-surface layer. This periodicity, being an analogue of a diffraction grating, leads to the possibility of generating Smith–Purcell radiation. It is shown that the properties of Smith–Purcell radiation from such an unusual “light” grating are also unusual: the dispersion relation, unlike the standard for Smith–Purcell radiation, does not contain diffraction orders, so that all radiation is concentrated in one peak. The calculated effect makes it possible to control the radiation frequency or angle by changing the laser frequency and may be of interest for the development of new compact radiation sources with tunable characteristics and for non-invasive diagnostics of relativistic electron beams.

Preparation of different carbon stripper foils and application in beam diagnostics

The production of carbon stripper foils that are mounted at different locations of the heavy-ion accelerator is an important task of the target laboratory, and the process for the preparation of carbon foils is constantly improved. Recently, we tested additional heat treatment of the carbon stripping foils following our standard procedure to reduce stress in the foils, since the performance and the durability of these stripping foils is a crucial factor for effective operation. We investigated the properties of foils produced some time ago and of foils freshly produced with the standard procedure, and foils treated thermally after the standard procedure. For the beam diagnostic department at GSI radiation resistant target materials for transverse ion beam profile measurement is an ongoing research topic. In that framework, the feasibility of Optical Transition Radiation (OTR) generation due to ion beam interaction with metal target for beam profile determination has been investigated. Here, carbon stripper foils are applied to enhance the OTR signal. The differently prepared foils were applied in many different experiments of the beam diagnostic department. We describe the performance of the foils during irradiation and compare the features of foils before and after irradiation.

Average and statistical properties of coherent radiation from steady-state microbunching.

A promising accelerator light source mechanism called steady-state microbunching (SSMB) is being actively studied. With the combination of strong coherent radiation from microbunching and high repetition rate of a storage ring, high-average-power narrow-band radiation can be anticipated from an SSMB storage ring, with wavelengths ranging from THz to soft X-ray. Such a novel light source could provide new opportunities for accelerator photon science like high-resolution angle-resolved photoemission spectroscopy and industrial applications like extreme ultraviolet (EUV) lithography. In this paper, a theoretical and numerical study of the average and statistical properties of coherent radiation from SSMB are presented. The results show that 1 kW average-power quasi-continuous-wave EUV radiation can be obtained from an SSMB ring provided that an average current of 1 A and a microbunch train with bunch length of 3 nm can be formed at the radiator which is assumed to be an undulator. Together with the narrow-band feature, the EUV photon flux can reach 6 × 1015 photons s-1 within a 0.1 meV energy bandwidth, which is three orders of magnitude higher than that in a conventional synchrotron source and is appealing for fundamental condensed matter physics and other research. In this theoretical investigation, we have generalized the definition and derivation of the transverse form factor of an electron beam which can quantify the impact of its transverse size on coherent radiation. In particular, it has been shown that the narrow-band feature of SSMB radiation is strongly correlated with the finite transverse electron beam size. Considering the pointlike nature of electrons and quantum nature of radiation, the coherent radiation fluctuates from microbunch to microbunch, or for a single microbunch from turn to turn. Some important results concerning the statistical properties of SSMB radiation are presented, with a brief discussion on its potential applications, for example the beam diagnostics. The presented work is of value for the development of SSMB to better serve potential synchrotron radiation users. In addition, this also sheds light on understanding the radiation characteristics of free-electron lasers, coherent harmonic generation, etc.

Surrogate-based Bayesian inference of transverse beam distribution for non-stationary accelerator systems

Constraints on the beam diagnostics available in real-time and time-varying beam source conditions make it difficult to provide users with high-quality beams for long periods without interrupting experiments. Although surrogate model-based inference is useful for inferring the unmeasurable, the system states can be incorrectly inferred due to manufacturing errors and neglected higher-order effects when creating the surrogate model. In this paper, we propose to adaptively assimilate the surrogate model for reconstructing the transverse beam distribution with uncertainty and underspecification using a sequential Monte Carlo from the measurements of quadrant beam loss monitors. The proposed method enables sample-efficient and training-free inference and control of the time-varying transverse beam distribution.

A novel smart rad-hard fast detection system for Radioactive Ion Beam Tagging and Diagnostics

Radioactive Ion Beams (RIBs) of large intensity (106pps or higher) are at the frontier in nuclear physics. We designed a novel detection system for RIBs diagnostics and tagging based on Silicon Carbide detectors and on custom frontend electronics ready to be coupled with a Real Data Management Unit. The full detection system is designed to measure the spatial distribution of the beam intensity and trajectory with sufficient spatial resolution (of the order of 1-2 mm). In addition, the detection system has to determine the RIB composition that can be obtained from the joint measurement of the energy loss (&gt;E) of the ions passing through the sensors and the time of flight between two sensors or with respect to a given reference signal as the RadioFrequency signal of a Cyclotron. In this paper we present the full design of the proposed system together with the results of the first experimental qualification of the first mini-prototype. The paper also shows the steps towards the final detection system, housed in a DN160 spherical cross and able to cover an active area of 30 mm × 60 mm.