Beam Current Profile Research Articles

Undulator linear taper control at the European X-Ray Free-Electron Laser facility

Undulator tapering controls the resonance properties of the free-electron laser (FEL) amplification process. Wakefield energy losses in an undulator’s vacuum chamber are one of the factors that determine the undulator’s linear taper. While another contribution to energy losses, namely the losses due to spontaneous radiation, can be calculated analytically, estimating wakefield energy losses requires detailed knowledge of the chamber geometry and the electron beam current profile. We introduce a method for the automatic estimation of wakefield energy losses, which leverages noninvasive THz diagnostics, a current profile reconstruction algorithm enhanced with machine learning, and a recently developed analytical wakefield function for the European XFEL’s undulator beamline. The correctness of this method was validated by directly measuring wakefield-induced electron beam energy losses in the undulator section. This, in turn, enables the prediction of the optimal linear taper in the undulator. Published by the American Physical Society 2024

Study of nanosecond pulsed hollow-cathode electron beam generation and transportation based on PIC-MCC simulation

This work employs the PIC-MCC (Particle-in-Cell Monte-Carlo collisions) method to study the nanosecond pulsed electron beam time-varied beam current profiles through the hollow-cathode discharge plasma evolution and electron beam transportation process. The electron beam’s energy and current distribution characteristics at the anode hole are investigated, and the simulation’s grid and step sizes are set as 0.04 µm and 5×10-13s, respectively. Experimental parameters are used in the simulation, and experimental results are also used to verify the simulation accuracy. The simulations investigate the transportation characteristics of the electron beam under different conditions and obtain the effects of voltage, current, and ion density on the distribution characteristics of the electron beam. The electron beam current generated by the hollow-cathode discharge is gradually transformed from a random distribution in the central region to a Gaussian distribution. Due to the ion-forced channel formed by the runaway electron pre-ionization, the high-energy electron beam achieves focused transport over a certain distance, and the high-energy electron beam has a smaller Gaussian radius than the source input. Besides, the ion-focusing channel formed by the high-energy electron transportation process can realize the transportation of the high-current electron beam in the conduction stage.

Optimization of transformer ratio and beam loading in a plasma wakefield accelerator with a structure-exploiting algorithm

Plasma-based acceleration has emerged as a promising candidate as an accelerator technology for a future linear collider or a next-generation light source. We consider the plasma wakefield accelerator (PWFA) concept where a plasma wave wake is excited by a particle beam and a trailing beam surfs on the wake. For a linear collider, the energy transfer efficiency from the drive beam to the wake and from the wake to the trailing beam must be large, while the emittance and energy spread of the trailing bunch must be preserved. One way to simultaneously achieve this when accelerating electrons is to use longitudinally shaped bunches and nonlinear wakes. In the linear regime, there is an analytical formalism to obtain the optimal shapes. In the nonlinear regime, however, the optimal shape of the driver to maximize the energy transfer efficiency cannot be precisely obtained because currently no theory describes the wake structure and excitation process for all degrees of nonlinearity. In addition, the ion channel radius is not well defined at the front of the wake where the plasma electrons are not fully blown out by the drive beam. We present results using a novel optimization method to effectively determine a current profile for the drive and trailing beam in PWFA that provides low energy spread, low emittance, and high acceleration efficiency. We parameterize the longitudinal beam current profile as a piecewise-linear function and define optimization objectives. For the trailing beam, the algorithm converges quickly to a nearly inverse trapezoidal trailing beam current profile similar to that predicted by the ultrarelativistic limit of the nonlinear wakefield theory. For the drive beam, the beam profile found by the optimization in the nonlinear regime that maximizes the transformer ratio also resembles that predicted by linear theory. The current profiles found from the optimization method provide higher transformer ratios compared with the linear ramp predicted by the relativistic limit of the nonlinear theory.

Revealing phase transitions and instabilities in pseudospark discharges

AbstractPseudospark discharge is a low‐pressure high‐voltage pulsed discharge with hollow electrodes, and the overall discharge experiences several distinctive sub‐phases. The instability phenomena, characterised by the sudden oscillations or distortions of discharge voltage and current, restrict the applications of pseudospark discharge. This study presents an investigation into phase transitions and instabilities in pseudospark discharge by combining the evidences from multiple discharge waveforms, time‐resolved images, electron beam current profiles, 2D kinetic simulation results, and the spectral line emission of cathode material. The process of a normal pseudospark discharge is firstly discussed, and the threshold conditions of the transition from the superdense glow discharge (SGD) phase to the high current phase with arc cathode spots are addressed. According to temporal profiles and voltage‐current characteristics, two types and four occurrences of instabilities are distinguished: type I—in the pre‐breakdown phase; type II‐1—in the SGD phase; type II‐2—in the transition of cathode spots propagating from cathode hole edge to cathode plane; type II‐3—in the re‐initiation of spots near current reversal. It is found that the instability of type I is caused by the stepwise penetration of virtual anode plasma, and type II is closely related to the cyclic nature and finite lifetime of cathode spots. It is induced by the fact that the decay of cathode spots is faster than the formation under certain conditions, and the existing spots are not able to provide the current required by the external circuit. Important features and the suppression methods of the instabilities are discussed based on the proposed mechanisms.

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.

Medipix3 for dosimetry and real-time beam monitoring: first tests at a 60 MeV proton therapy facility

Charged particle therapy (CPT) is an advanced modality of radiation therapy which has grown rapidly worldwide, driven by recent developments in technology and methods of delivery. To ensure safe and high quality treatments, various instruments are used for a range of different measurements such as for quality assurance, monitoring and dosimetry purposes. With the emergence of new and enhanced delivery techniques, systems with improved capabilities are needed to exceed existing performance limitations of conventional tools. The Medipix3 is a hybrid pixel detector able to count individual protons with millisecond time resolution at clinical flux with near instant readout and count rate linearity. The system has previously demonstrated use in medical and other applications, showing wide versatility and potential for particle therapy. In this work we present measurements of the Medipix3 detector in the 60 MeV ocular proton therapy beamline at the Clatterbridge Cancer Centre, U.K. The beam current and lateral beam profiles were evaluated at multiple positions in the treatment line and compared with EBT3 Gafchromic film. The recorded count rate linearity and temporal analysis of the beam structure was measured with Medipix3 across the full range of available beam intensities, up to 3.12 × 1010 protons/s. We explore the capacity of Medipix3 to provide non-reference measurements and its applicability as a tool for dosimetry and beam monitoring for CPT. This is the first known time the performance of the Medipix3 detector technology has been tested within a clinical, high proton flux environment.

Electro-optic sampling beam position monitor for relativistic electron beams

An electro-optic sampling beam position monitor for installation at the SLAC National Accelerator Laboratory FACET-II facility is described. Simulations of the detector signal were performed using realistic electron beam current profiles, demonstrating that this single-shot, non-destructive, compact detector is capable of resolving the relative transverse offset and longitudinal separation of an ultra-relativistic, two-bunch electron beam to order 1 μm and 10 fs, respectively. A design study was performed to optimize the detector’s response to the ultra-relativistic two-bunch electron beam utilized in plasma wakefield accelerator experiments. In addition, the expected sensitivity to beam tilt is studied for typical electron beams used in free electron lasers.

Visualizing lattice dynamic behavior by acquiring a single time-resolved MeV diffraction image

We explore the possibility of visualizing the lattice dynamics behavior by acquiring a single time-resolved mega-electron-volt ultrafast electron diffraction (UED) image. Conventionally, multiple UED shots with varying time delays are needed to map out the entire dynamic process. The measurement precision is limited by the timing jitter between the pulses of the pump laser and the electron probe, the intensity fluctuation of probe pulses, and the premature sample damage. Inspired by the early transient spectroscopy studies via an ultrashort-pulse pump/long-pulse probe scheme, we show that, by converting the longitudinal time of an electron pulse to the transverse position of a Bragg peak on the detector, one can obtain the full lattice dynamic process in a single electron pulse. This time-to-position mapping can be achieved by the combination of longitudinally shaping the electron beam and introducing a time-dependent transverse kick after electrons are diffracted from the sample. We propose a novel design of time-resolved UED facility with the capability of capturing a wide range of dynamic features in a single diffraction image. To achieve the best possible temporal resolution, we implement a real-time tuning scheme for optimizing the match between the electron bunch length and the lattice dynamic timescale, varying in the sub-picosecond to tens of picosecond (ps) range depending on the specific process. This timescale match is in favor of the ultrafast phenomenon, which requires a 10 fs temporal resolution for resolving the sub-ps oscillation. A state-of-the-art photocathode gun being developed by Euclid could extend the timescale to hundreds of ps. To study the radiation damage and to mitigate such effect, longitudinally shaping the photocathode drive laser pulse (demonstrated in a previous study) can control and manipulate the electron beam current profile with a tunable periodical structure. Furthermore, we present numerical evidence illustrating the capability of acquiring a single time-resolved diffraction image based on the case-by-case studies of different lattice dynamics behaviors.

Variable beam entrance Faraday cup system for pulsed electron beam current profile characterization.

A high accuracy variable beam entrance Faraday cup (VBEFC) system is designed in this work. The presented VBEFC system is designed for the beam current profile measurement of the transient hollow cathode discharge (THCD) generated pulsed electron beam, which is a new source of high energy flux for metallic material processing. By proper designs of the beam entrance array, fast response electron collector, grounding, and shielding, this VBEFC system is capable of determining the radial profile and its temporal evolution of the THCD generated electron beam. The results of the electron beam current and current density distributions collected at varying radial locations and times under multiple voltages are presented in this paper. The experimental results show that both the amplitude and the current density of the THCD electron beam at a given radius always increased with the increase in the accelerating voltage, which is coincident with the self-focused propagation of the electron beam promoted by the voltage.

Response of beam focusing to plasma fluctuation in a filament-arc-type negative ion source

Beam focusing is one of the most important elements for the stable and safe operation of high power negative ion beams, such as neutral beam injection into magnetically confined fusion plasmas. In order to investigate impacts of the source plasma fluctuation on beam focusing, a simultaneous measurement of the source plasma fluctuation and the beam current profile has been carried out in the research-and-development negative ion source at the National Institute for Fusion Science. The responses of beam width and of the beam centre deviation are observed for the first time, indicating the importance of the source plasma stability for the negative ion beam focusing.

Single Shot Characterization of High Transformer Ratio Wakefields in Nonlinear Plasma Acceleration.

Plasma wakefields can enable very high accelerating gradients for frontier high energy particle accelerators, in excess of 10 GeV/m. To overcome limits on single stage acceleration, specially shaped drive beams can be used in both linear and nonlinear plasma wakefield accelerators (PWFA), to increase the transformer ratio, implying that the drive beam deceleration is minimized relative to acceleration obtained in the wake. In this Letter, we report the results of a nonlinear PWFA, high transformer ratio experiment using high-charge, longitudinally asymmetric drive beams in a plasma cell. An emittance exchange process is used to generate variable drive current profiles, in conjunction with a long (multiple plasma wavelength) witness beam. The witness beam is energy modulated by the wakefield, yielding a response that contains detailed spectral information in a single-shot measurement. Using these methods, we generate a variety of beam profiles and characterize the wakefields, directly observing transformer ratios up to R=7.8. Furthermore, a spectrally based reconstruction technique, validated by 3D particle-in-cell simulations, is introduced to obtain the drive beam current profile from the decelerating wake data.

System for Measuring Emittance Characteristics of Ion Sources

The article presents the results of the development of a system for measuring emittance characteristics of ion sources studied at the IAP NAS of Ukraine with the aim of obtaining the ion beams with a high brightness. The emittance measurement system is based on the scheme of an electrostatic scanner and consists of two main parts: the scanner, which moves in the direction perpendicular to the beam axis using a stepper motor, and the electronic system of control, processing and data acquisition. The electronic system contains a Raspberry pi 3B microcomputer, precision DAC/ADCs, the high-voltage amplifier of a scanning voltage up to ±500 V on deflection plates of the scanner and a wide range current integrator. The determination of the emittance consists in measuring the ion beam intensity distribution when the scanner moves along the x-coordinate and the electrostatic scanning along the x´ angle. The obtained two-dimensional data array allows determining the main characteristics of ion beam: geometric 90% emittance, the root mean square (rms) emittance, the Twiss parameters and phase ellipse of rms emittance, the beam current profile and the angle current density distribution. To test the performance and functionality of the system, the emittance characteristics of the penning type ion source were measured. The working gas was helium, and the beam energy varied within 7–15 keV. At 13 keV of beam energy the following emittances of the He+ ions beam was obtained: 90% emittance is 30 π∙mm∙mrad, rms emittance is 8.4 mm∙mrad, and the normalized rms emittance is equal to 0.022 mm∙mrad. The developed system for measuring the emittance of the ion beams is characterized by a short measurement time of 10-15 minutes.

Experimental investigations of nanosecond-pulse electron beam profile for metal surface treatments

In this paper, a transient hollow cathode discharge (THCD) generated pulsed electron beam is applied for the metal surface treatment. And the profile of this pulsed beam current density distribution is experimentally investigated via a high accuracy variable beam entrance Faraday Cup (VBEFC) system. The influences of the varying beam pulses on the post-treated surface microstructure and properties of 306 stainless steel and 6061 aluminum alloy are investigated in this paper. The experimental results demonstrate that the THCD generated pulsed electron beam profile has a characteristic pattern of concentrated energy flux within a 2 mm circular region where the high energy electrons are the most concentrated. And such a concentrated energy flux pattern can be further promoted by the increased accelerating voltage and the self-focusing space charge effect. The pulsed e-beam surface treatment results on 306 stainless steel show that the surface morphology variations are identical with the evolutions of the beam current profiles under varying accelerating voltages. Additionally, the surface treatment results on 6061 aluminum alloy show that the micro-hardness of the alloy can be improved from the initial 63.1 HV to 85.1 HV by increasing the beam accelerating voltage up to 25 kV, promoting the concentrated energy flux of the generated beam pulse.

Spatiotemporal oscillation of an ion beam extracted from a potential-oscillating plasma source

A radiofrequency oscillation is successfully superimposed on a plasma potential of a filamented source plasma in an ion beam source while maintaining a constant plasma density, in order to investigate effects of oscillating source plasma potential on an extracted ion beam. The experiment is preliminarily performed with a positive argon ion beam source. A class-D amplifier operational over a wide range of a frequency from a few tens of kHz to several MHz is installed; leading the oscillation of the plasma potential in the plasma source for the frequency range being tested. The beam current profile downstream of the extraction grids shows an oscillation of the beam current at the peripheral region of the ion beam; implying that the oscillation of a beam halo is induced by the potential oscillation of the source plasma.

Mechanisms for the mitigation of the hose instability in plasma-wakefield accelerators

In this work we provide an in-depth analysis of mechanisms which mitigate the hose instability in the blowout regime in plasma-wakefield accelerators. We show by means of theory and three-dimensional particle-in-cell simulations that the mitigation mechanisms related to a beam energy spread are effective for parameters as used in major plasma-wakefield accelerator experiments and for various beam energies, beam current profiles, and types of hosing seed. In addition, we establish the theoretical principles for the reduction of the initial hosing seed in tapered vacuum-to-plasma transitions and derive respective analytic predictions which are successfully benchmarked against particle-in-cell simulations. We also investigate the possibility to facilitate efficient and stable acceleration of witness beams in the blowout regime. This work therefore provides a deepened understanding for the methods that allow for the mitigation of hosing, a crucial prerequisite to facilitate stable acceleration of high quality beams in plasma-wakefield accelerators.

Zeeman spectroscopy as a method for determining the magnetic field distribution in self-magnetic-pinch diodes (invited).

In the self-magnetic-pinch diode, the electron beam, produced through explosive field emission, focuses on the anode surface due to its own magnetic field. This process results in dense plasma formation on the anode surface, consisting primarily of hydrocarbons. Direct measurements of the beam's current profile are necessary in order to understand the pinch dynamics and to determine x-ray source sizes, which should be minimized in radiographic applications. In this paper, the analysis of the C IV doublet (580.1 and 581.2 nm) line shapes will be discussed. The technique yields estimates of the electron density and electron temperature profiles, and the method can be highly beneficial in providing the current density distribution in such diodes.

Beam shaping to improve the free-electron laser performance at the Linac Coherent Light Source

A new operating mode has been developed for the Linac Coherent Light Source (LCLS) in which we shape the longitudinal phase space of the electron beam. This mode of operation is realized using a horizontal collimator located in the middle of the first bunch compressor to truncate the head and tail of the beam. With this method, the electron beam longitudinal phase space and current profile are reshaped, and improvement in lasing performance can be realized. As a result, we present experimental studies at the LCLS of the beam shaping effects on the free-electron laser performance.

Temporal characterization of ultrashort linearly chirped electron bunches generated from a laser wakefield accelerator

A new method for diagnosing the temporal characteristics of ultrashort electron bunches with linear energy chirp generated from a laser wakefield accelerator is described. When the ionization-injected bunch interacts with the back of the drive laser, it is deflected and stretched along the direction of the electric field of the laser. Upon exiting the plasma, if the bunch goes through a narrow slit in front of the dipole magnet that disperses the electrons in the plane of the laser polarization, it can form a series of bunchlets that have different energies but are separated by half a laser wavelength. Since only the electrons that are undeflected by the laser go through the slit, the energy spectrum of the bunch is modulated. By analyzing the modulated energy spectrum, the shots where the bunch has a linear energy chirp can be recognized. Consequently, the energy chirp and beam current profile of those bunches can be reconstructed. This method is demonstrated through particle-in-cell simulations and experiment.

Suppression of microbunching instability via a transverse gradient undulator

The microbunching instability in the linear accelerator (linac) of a free-electron laser facility has always been a problem that degrades the electron beam quality. In this paper, a quite simple and inexpensive technique is proposed to smooth the electron beam current profile to suppress the instability. By directly adding a short undulator with a transverse gradient field right after the injector to couple the transverse spread into the longitudinal direction, additional density mixing in the electron beam is introduced to smooth the current profile, which results in the reduction of the gain of the microbunching instability. The magnitude of the density mixing can be easily controlled by varying the strength of the undulator magnetic field. Theoretical analysis and numerical simulations demonstrate the capability of the proposed technique in the accelerator of an x-ray free-electron laser.

Analysis of the microbunching instability in a mid-energy electron linac**Supported by National Natural Science Foundation of China, (11275253), and Natural Science Foundation of Shanghai City (12ZR1436600)

Microbunching instability usually exists in the linear accelerator (linac) of a free electron laser (FEL) facility. If it is not controlled effectively, the beam quality will be damaged seriously and the machine will not operate properly. In the electron linac of a soft X-ray FEL device, because the electron energy is not very high, the problem can become even more serious. As a typical example, the microbunching instability in the linac of the proposed Shanghai Soft X-ray Free Electron Laser facility (SXFEL) is investigated in detail by means of both analytical formulae and simulation tools. In the study, a new mechanism introducing random noise into the beam current profile as the beam passes through a chicane-type bunch compressor is proposed. The higher-order modes that appear in the simulations suggest that further improvement of the current theoretical model of the instability is needed.