Compact Particle Accelerators Research Articles

Study of the transfer and matching line for a PWFA-driven FEL

The development of compact accelerator facilities providing high-brightness beams is one of the most challenging tasks in the field of next-generation compact and cost affordable particle accelerators. Recent results obtained at SPARC_LAB show evidence of the FEL laser by a compact (3 cm) particle driven plasma-based accelerator. This work is carried out in the framework of the SPARC_LAB activities concerning the R&D on plasma wakefield accelerators for the realization of new compact plasma based facilities, i.e EuPRAXIA@SPARC_LAB. The work here presented is a theoretical study demonstrating a possible scheme concerning the implementation of an innovative array of discharge capillaries, operating as active-plasma lenses, and one collimator to build an unconventional transport line for bunches outgoing from plasma accelerating module. Taking advantage of the symmetric and linear focusing provided by an active-plasma lens, the witness is captured and transported along the array without affecting its quality at the exit of the plasma module. At the same time the driver, being over-focused in the same array, can be removed by means of a collimator.

Non-resonant ultra-fast multipactor regime in dielectric-assist accelerating structures

The objective of this work is the evaluation of the risk of suffering a multipactor discharge in an S-band dielectric-assist accelerating (DAA) structure for a compact low-energy linear particle accelerator dedicated to hadrontherapy treatments. A DAA structure consists of ultra-low loss dielectric cylinders and disks with irises which are periodically arranged in a metallic enclosure, with the advantage of having an extremely high quality factor and very high shunt impedance at room temperature, and it is therefore proposed as a potential alternative to conventional disk-loaded copper structures. However, it has been observed that these structures suffer from multipactor discharges. In fact, multipactor is one of the main problems of these devices, as it limits the maximum accelerating gradient. Because of this, the analysis of multipactor risk in the early design steps of DAA cavities is crucial to ensure the correct performance of the device after fabrication. In this paper, we present a comprehensive and detailed study of multipactor in our DAA design through numerical simulations performed with an in-house developed code based on the Monte–Carlo method. The phenomenology of the multipactor (resonant electron trajectories, electron flight time between impacts, etc.) is described in detail for different values of the accelerating gradient. It has been found that in these structures an ultra-fast non-resonant multipactor appears, which is different from the types of multipactor theoretically studied in the scientific literature. In addition, the effect of several low electron emission coatings on the multipactor threshold is investigated. Furthermore, a novel design based on the modification of the DAA cell geometry for multipactor mitigation is introduced, which shows a significant increase in the accelerating gradient handling capabilities of our prototype.

Optimized plasma waveguides for single-stage bubble acceleration of monoenergetic electron beams with enhanced quality

The laser wakefield acceleration in bubble regime is now a promising method for producing high-quality electron beams, which is crucial to the development of the next-generation compact and low-cost particle accelerators. In this paper, we demonstrate the possibility of controlling particle injection and electron beam quality in sub-petawatt laser pulse interaction with simple plasma waveguides with a radially step-like density profile. Fully relativistic particle-in-cell calculations for different laser intensities indicate that by considering a sufficiently low electron density for a central plasma channel to ensure both increased acceleration length and formation of a well-structured bubble, one can optimize the surrounding plasma density and central channel diameter to reach the best injection conditions and produce a high-quality monoenergetic electron beam with improved peak energy and reduced energy spread. Calculations are performed for 33 fs, 140–315 TW laser pulses interacting with a ∼2 cm long plasma waveguide. It is demonstrated that by using the proposed scheme in optimized conditions, monoenergetic electron beams with peak energies 3–4 GeV and energy spreads less than 1.5% can be obtained in a single acceleration length of 1.85 cm. The beam charge and conversion efficiency of laser energy into the beam energy were also enhanced to values around 195 pC and 14.7%, respectively.

Adaptive autoencoder latent space tuning for more robust machine learning beyond the training set for six-dimensional phase space diagnostics of a time-varying ultrafast electron-diffraction compact accelerator.

We present a general adaptive latent space tuning approach for improving the robustness of machine learning tools with respect to time variation and distribution shift. We demonstrate our approach by developing an encoder-decoder convolutional neural network-based virtual 6D phase space diagnostic of charged particle beams in the HiRES ultrafast electron diffraction (UED) compact particle accelerator with uncertainty quantification. Our method utilizes model-independent adaptive feedback to tune a low-dimensional 2D latent space representation of ∼1 million dimensional objects which are the 15 unique 2D projections (x,y),...,(z,p_{z}) of the 6D phase space (x,y,z,p_{x},p_{y},p_{z}) of the charged particle beams. We demonstrate our method with numerical studies of short electron bunches utilizing experimentally measured UED input beam distributions.

Comparative study of ultrashort single-pulse and multi-pulse driven laser wakefield acceleration

Laser wakefield acceleration (LWFA) is a promising technique to build compact and powerful particle accelerators. In such accelerators, the electric fields required to accelerate charged particles are sustained by electron density modulations in the plasma. The plasma wave modulating the electron density may be excited by an intense laser pulse. However, propagation of intense laser pulse in plasma is subject to various instabilities which result in significant losses of laser energy, reducing the efficiency of wakefield generation. Using a train of lower intensity pulses instead of a single higher intensity pulse appears to be a more efficient scheme for LWFA. Here we have studied this alternative scheme by applying an ultra-short femtosecond Gaussian laser beam consisting pulse train of a various number of pulses in different cases to underdense plasma. The plasma density modulation and strength of the resulting wakefield have been compared in various cases of multi-pulse and single-pulse lasers, for the same amount of input energies. Here we demonstrate that applying multi-laser pulses of optimally selected lower intensities and proper spacing leads to stronger wakefield generation and more efficient electron acceleration compared to the case of a single pulse of higher energy.

Study of the transmission line for Israeli THz free-electron radiation source

This article establishes a very important part of the transmission line (TL) design of the novel 6 MeV compact particle accelerator. Practically, the entire new array of particle accelerators has been designed and is in the final stages of construction at the Ariel University. Except for a novel hybrid photo-cathode electron gun, which has been developed and built in University of California, Los Angeles's Laboratory, it was commissioned at The Schlesinger Family Center for Compact Accelerators, Radiation Sources and Applications (free electron laser) as a part of an ongoing collaboration between the two universities. In this work, we have built a complete simulator for the analysis and propagation of terahertz pulses through the TL, which meets our specific requirements with a high level of flexibility. To simulate the propagation of an output wave packet of the source through the TL quasi-optical system, the original wave packet is decomposed in terms of free-space Hermite–Gaussian basis while the following transformations of the wave packet is described by the transfer matrices technique. Total radiation losses and spectral properties of the TL transfer are analyzed.

IsoDAR@Yemilab: A report on the technology, capabilities, and deployment

IsoDAR@Yemilab is a novel isotope-decay-at-rest experiment that has preliminary approval to run at the Yemi underground laboratory (Yemilab) in Jeongseon-gun, South Korea. Here, we describe in detail the considerations for installing this compact particle accelerator and neutrino target system at the Yemilab underground facility. Specifically, we describe the caverns being prepared for IsoDAR, and address installation, shielding, and utilities requirements. To give context and for completeness, we also briefly describe the physics opportunities of the IsoDAR neutrino source when paired with the Liquid Scintillator Counter (LSC) at Yemilab, and review the technical design of the neutrino source.

Gyrotrons as High-Frequency Drivers for Undulators and High-Gradient Accelerators

Gyrotrons are used as high-power sources of coherent radiation operating in pulsed and CW regimes in many scientific and technological fields. In this paper, we discuss two of their numerous applications. The first one is in gyrotron-powered electromagnetic wigglers and undulators. The second one is for driving high-gradient accelerating structures in compact particle accelerators. The comparison, between the requirements imposed by these two concepts on the radiation sources on one hand and the output parameters of the currently available high-performance gyrotrons on the other hand, show that they match each other to a high degree. We consider this as a manifestation of the feasibility and potential of these concepts. It is believed that after the first successful proof-of-principle experiments they will find more wide usage in the advanced FEL and particle accelerators.

New Commissioning Results of the MIST-1 Multicusp Ion Source

For the sterile neutrino experiment IsoDAR (Isotope Decay-At-Rest), we have developed a compact particle accelerator system delivering a 10 mA, continuous wave (cw) proton beam at 60 MeV to a neutrino production target. The accelerator comprises a compact isochronous cyclotron, an RFQ embedded in the cyclotron yoke, and an ion source. To reduce space charge effects during injection and acceleration, we are accelerating H+ 2 instead of protons. To produce the needed cw H+ 2 beam current of 10 mA (nominal) at the required purity and quality, we have built a new filament driven, multicusp ion source (MIST-1). Here we report commissioning results for long-time running at reduced power, demonstrating the feasibility of the design. Highlights include an H+ 2 beam current density of ≈ 10 mA/cm2, ≈ 80% H+ 2 fraction, and extrapolated emittances of 0.05 π-mm-mrad (RMS, normalized) after extraction. We also present high fidelity simulations that are in good qualitative and quantitative agreement with emittance measurements in our test beam line.

Order-of-magnitude beam current improvement in compact cyclotrons

There is great need for high intensity proton beams from compact particle accelerators in particle physics, medical isotope production, and materials- and energy-research. To address this need, we present, for the first time, a design for a compact isochronous cyclotron that will be able to deliver 10 mA of 60 MeV protons—an order of magnitude higher than on-market compact cyclotrons and a factor four higher than research machines. A key breakthrough is that vortex motion is incorporated in the design of a cyclotron, leading to clean extraction. Beam losses on the septa of the electrostatic extraction channels stay below 120 W (40% below the required safety limit), while maintaining good beam quality. We present a set of highly accurate particle-in-cell simulations, and an uncertainty quantification of select beam input parameters using machine learning, showing the robustness of the design. This design can be utilized for beams for experiments in particle and nuclear physics, materials science and medical physics as well as for industrial applications.

Diagnostic of laser wakefield acceleration with ultra – Short laser pulse by using SMILEI PIC code

• Generation of high energy beam with LWFA is the most efficient and low in cost as compared with conventional accelerator. • In LWFA, we firstly performed this acceleration with the combination of up ramp and down ramp density region. • We firstly performed the work related with production of high energy beam with self-injected electrons. • We firstly performed the work with ultra – short laser pulse and polygonal plasma density profile for LWFA. High-energy particle beam is used to probe the local or long range structure and properties of materials. Particle accelerators are also used in material science to understand radiation damage, particularly in studies of structural material to be applied in fusion power generator and satellites. Laser Wakefield Acceleration (LWFA) is a promising technique to build compact and powerful particle accelerators for generating multi-GeV electron beams. The efficient coupling of laser energy with plasma is very important to generate high energy electrons, which is the subject to optimum laser and plasma profiles. In this article, we have used high intensity ultra-short laser pulse and a polygon plasma density profile to analyse the behaviour of LWFA in the bubble regime, which corresponds to the evacuation of plasma electrons in the wake of the laser pulse. Many electrons get self-injected in this wakefield and get accelerated behind the laser pulse up to very high energies, producing high energy beam useful for material processing along with their other applications. We have diagnosed some parameters such as electron density variation, wakefield magnitude and energy spectrum of the electrons accelerated in this mechanism.

Probing ultrafast laser plasma processes inside solids with resonant small-angle x-ray scattering

Extreme states of matter exist throughout the universe e.g. inside planetary cores, stars or astrophysical jets. Such conditions are generated in the laboratory in the interaction of powerful lasers with solids, and their evolution can be probed with femtosecond precision using ultra-short X-ray pulses to study laboratory astrophysics, laser-fusion research or compact particle acceleration. X-ray scattering (SAXS) patterns and their asymmetries occurring at X-ray energies of atomic bound-bound transitions contain information on the volumetric nanoscopic distribution of density, ionization and temperature. Buried heavy ion structures in high intensity laser irradiated solids expand on the nanometer scale following heat diffusion, and are heated to more than 2 million Kelvin. These experiments demonstrate resonant SAXS with the aim to better characterize dynamic processes in extreme laboratory plasmas.

An adaptive approach to machine learning for compact particle accelerators

Machine learning (ML) tools are able to learn relationships between the inputs and outputs of large complex systems directly from data. However, for time-varying systems, the predictive capabilities of ML tools degrade if the systems are no longer accurately represented by the data with which the ML models were trained. For complex systems, re-training is only possible if the changes are slow relative to the rate at which large numbers of new input-output training data can be non-invasively recorded. In this work, we present an approach to deep learning for time-varying systems that does not require re-training, but uses instead an adaptive feedback in the architecture of deep convolutional neural networks (CNN). The feedback is based only on available system output measurements and is applied in the encoded low-dimensional dense layers of the encoder-decoder CNNs. First, we develop an inverse model of a complex accelerator system to map output beam measurements to input beam distributions, while both the accelerator components and the unknown input beam distribution vary rapidly with time. We then demonstrate our method on experimental measurements of the input and output beam distributions of the HiRES ultra-fast electron diffraction (UED) beam line at Lawrence Berkeley National Laboratory, and showcase its ability for automatic tracking of the time varying photocathode quantum efficiency map. Our method can be successfully used to aid both physics and ML-based surrogate online models to provide non-invasive beam diagnostics.

Design of a multichannel photonic crystal dielectric laser accelerator

To be useful for most scientific and medical applications, compact particle accelerators will require much higher average current than enabled by current architectures. For this purpose, we propose a photonic crystal architecture for a dielectric laser accelerator, referred to as a multi-input multi-output silicon accelerator (MIMOSA), that enables simultaneous acceleration of multiple electron beams, increasing the total electron throughput by at least 1 order of magnitude. To achieve this, we show that the photonic crystal must support a mode at the Γ point in reciprocal space, with a normalized frequency equal to the normalized speed of the phase-matched electron. We show that the figure of merit of the MIMOSA can be inferred from the eigenmodes of the corresponding infinitely periodic structure, which provides a powerful approach to design such devices. Additionally, we extend the MIMOSA architecture to electron deflectors and other electron manipulation functionalities. These additional functionalities, combined with the increased electron throughput of these devices, permit all-optical on-chip manipulation of electron beams in a fully integrated architecture compatible with current fabrication technologies, which opens the way to unconventional electron beam shaping, imaging, and radiation generation.

Raman Shifting Induced by Cascaded Quadratic Nonlinearities for Terahertz Generation

AbstractCascaded quadratic optical nonlinearities are well known for producing effective third‐order nonlinear optical effects such as self‐phase modulation or self‐steepening. As a result, they have been extensively applied in areas such as mode‐locking and pulse compression. In this article, a regime of cascaded quadratic nonlinearities involving highly phase‐matched second‐order interactions is introduced, which produce an effective third‐order nonlinearity analogous to Raman shifting rather than the typical case of self‐phase modulation. This results in a continuous red‐shift of the optical pump frequency rather than spectral broadening. This phenomenon is particularly relevant to terahertz generation, where a continuous red‐shift of the pump frequency resolves current issues of dispersion and laser‐induced damage. In the absence of absorption or dispersion, the presented method results in optical‐to‐terahertz energy conversion efficiencies that approach 100%, which is not possible with conventional cascaded difference‐frequency generation. Designs of aperiodically poled lithium niobate structures are presented, which result in energy conversion efficiencies of 35% even in the presence of dispersion and absorption. The presented work thus addresses an important bottleneck in terahertz generation, which paves the way for the development of compact particle accelerators, X‐ray free‐electron lasers, advanced electron‐beam diagnostics, and various experiments in condensed‐matter physics and chemistry.

Faster laser pulses boost plasma accelerators

New methods to control how laser pulses propagate inside a plasma could signify the start of a global race to demonstrate truly high-energy compact particle accelerators.

CBETA: First Multipass Superconducting Linear Accelerator with Energy Recovery.

Energy recovery has been achieved in a multipass linear accelerator, demonstrating a technology for more compact particle accelerators operating at higher currents and reduced energy consumption. Energy delivered to the beam during the first four passes through the accelerating structure was recovered during four subsequent decelerating passes. High-energy efficiency was achieved by the use of superconducting accelerating cavities and permanent magnets. The fixed-field alternating-gradient optical system used for the return loop successfully transported electron bunches of 42, 78, 114, and 150MeV in a common vacuum chamber. This new kind of accelerator, an eight-pass energy recovery linac, has the potential to accelerate much higher current than existing linear accelerators while maintaining small beam dimensions and consuming much less energy per electron.

Introduction

4th European Advanced Accelerators Concepts.More than 250 scientists from all over the world gathered from Sep 15 – 21 at the 4th European Advanced Accelerator Concept Workshop (EAAC) on the Island of Elba in Italy. Specialists from accelerator physics, RF technology, plasma physics, instrumentation and the laser field discussed ideas and directions towards a new generation of ultra compact and cost effective accelerators with ground-breaking applications in science, medicine and industry.The EAAC was founded by the European Network for Novel Accelerators (EuroNNAc) in 2013 and has grown in its fourth edition into a meeting with world-wide visibility, rapidly catching up with the long tradition of the Advanced Accelerator Concepts workshop (AAC) in the US. At the EAAC senior scientists from various specialties mix with junior experts and a large community of young students, attracted by the promise and success of compact particle accelerators. In 2019 about 50 PhD students presented their work at the EAAC. Besides the reports on scientific achievements the large diversity in gender, age distribution and nationalities made the EAAC 2019 a special event and a great success for the accelerator field.Group photo of the EAAC 2019 attendants is present in this pdf. Committees List of Workshop Organizing Committee, Programme Committee, International Advisory Committee, Local Organizing Committee and Proceedings Editors are available in this pdf.

Angular Dependence of Copper Surface Damage Induced by an Intense Coherent THz Radiation Beam

In this work, we show the damage induced by an intense coherent terahertz (THz) beam on copper surfaces. The metallic surface was irradiated by multiple picosecond THz pulses generated by the Free Electron Laser (FEL) at the ISIR facility of the Osaka University, reaching an electric field on the sample surface up to ~4 GV/m. No damage occurs at normal incidence, while images and spectroscopic analysis of the surface point out a clear dependence of the damage on the incidence angle, the electric field intensity, and polarization of the pulsed THz radiation. Ab initio analysis shows that the damage at high incidence angles could be related to the increase of the absorbance, i.e., to the increase of the temperature around or above 1000 °C. The experimental approach we introduced with multiple fast irradiations represents a new powerful technique useful to test, in a reproducible way, the damage induced by an intense electric gradient on copper and other metallic surfaces in view of future THz-based compact particle accelerators.

X-ray laser wakefield acceleration in a nanotube

Plasma-based accelerator technology enables compact particle accelerators. In Laser Wakefield Acceleration, with an ultrafast high-intensity optical laser driver, energy gain of electrons is greater if the electron density is reduced. This is because the energy gain of electrons is proportional to the ratio of laser’s critical density to electron density. However, an alternative path for higher energy electrons is increasing the critical density via going to shorter wavelengths. With the advent of Thin Film Compression, we now see a path to a single cycle coherent X-ray beam. Using this X-ray pulse allows us to increase the plasma density to solid density nanotube (carbon or porous alumina) regime and still be under-dense for a Laser Wakefield Acceleration technique. We will discuss some implications of this below.