Low-emittance Beam Research Articles

Transverse recoil imprinted on free-electron radiation

Phenomena of free-electron X-ray radiation are treated almost exclusively with classical electrodynamics, despite the intrinsic interaction being that of quantum electrodynamics. The lack of quantumness arises from the vast disparity between the electron energy and the much smaller photon energy, resulting in a small cross-section that makes quantum effects negligible. Here we identify a fundamentally distinct phenomenon of electron radiation that bypasses this energy disparity, and thus displays extremely strong quantum features. This phenomenon arises when free-electron transverse scattering occurs during the radiation process, creating entanglement between each transversely recoiled electron and the photons it emitted. This phenomenon profoundly modifies the characteristics of free-electron radiation mediated by crystals, compared to conventional classical analysis and even previous quantum analysis. We also analyze conditions to detect this phenomenon using low-emittance electron beams and high-resolution X-ray spectrometers. These quantum radiation features could guide the development of compact coherent X-ray sources facilitated by nanophotonics and quantum optics.

HVE ion sources for medium and high-energy accelerator systems

Since decades, High Voltage Engineering (HVE) manufactures particle accelerator systems for research and industry. HVE’s product line includes Singletron and Tandetron accelerator systems with terminal voltages of up to 6 MV. They are dedicated to a wide range of applications including ion implantation and irradiation, Ion Beam Analysis (IBA), Accelerator Mass Spectrometry (AMS), and neutron reference fields. In this paper, we give an overview of the different positive and negative ion sources that are applied in these systems. We focus especially on the recent development and the performance of a compact 2.45 GHz permanent magnet ECR bipolar ion source (HVE Model SO-160), used for negative light-ion injection into tandem accelerators. It generates high-current, low-emittance light-ion beams at 30 keV energy. The novelty in the design of this ion source is that it combines direct negative extraction for H − with positive extraction for He + that is followed by charge exchange to He − in a Na-based electron donor canal. The source produces in excess of 250 eµA of H − and more than 20 eµA of He −. It is expected the design allows for substantially higher currents, especially for H −.

Design of a kilohertz repetition rate, low-emittance S-band photoinjector

Low-emittance photoinjector-enabled cutting-edge scientific instruments, such as free-electron lasers, inverse Compton scattering light sources, and ultrafast electron diffraction, will greatly benefit from the improved repetition rate. In this paper, we proposed a specifically designed S-band radio frequency (RF) photoinjector to obtain low emittance and kilohertz (kHz) high-repetition rates simultaneously. By lowering the gradient, much lower RF power is needed to feed the electron gun, and then the heat problem is much easier to handle. Meanwhile, by optimizing the length of the gun’s first cell from the normal case of 0.6-cell to 0.4-cell, the launch phase and the extraction field are significantly improved, thus ensuring the generation of low-emittance electron beams. In our design, the proposed 1.4-cell RF gun can work effectively under different field gradients ranging from 30 MV/m to 100 MV/m. For a standard case of 60 MV/m, 2.5 MW peak RF power with μs level pulse width is sufficient, thus offering the feasibility of improving the repetition rate to kHz level with a standard 5 MW irradiation klystron. In addition, simulated electron beams with a low emittance of 0.29 mm.mrad@200 pC can be generated by this proposed photoinjector, showing that this high-repetition rate injector holds the potential to deliver high-quality beams comparable to those of state-of-the-art S-band photoinjectors. Combining the merits of low emittance and high-repetition rate, this proposed photoinjector will provide a new possibility for future free-electron laser facilities operating at repetition rates ranging from kHz to tens of kHz.

Slice emittance measurements using a slit-grid system and a fast wall-current monitor.

The time evolution of beam properties in an electron bunch with the duration of a nanosecond was measured with a time resolution of several tens of picoseconds. A combination of horizontal and vertical slits cuts the beamlet from the original beam, with the current waveform of the beamlet measured using a fast wall-current monitor. The reconstruction of the waveform data obtained by scanning these two slits over the entire beam area provided the time evolution of the spatial profile. A similar measurement using two horizontal (vertical) slits separated by a certain distance also provides the time evolution of the phase-space profile. Using this method, the initial beam extracted from the CeB6 thermionic electron gun of the x-ray free-electron laser (XFEL) SACLA was evaluated. Although the slice emittance in the bunch was measured to be constant, the centroid of the spatial profile moved in the transverse direction by a few hundred micrometers in the 0.6ns flat-top region. This movement arises from the temporal variation in the rectangular high-voltage pulse of the beam chopper and can cause an increase in the projected emittance. These measurements are important for evaluating the conditions of the initial beam emitted from the cathode and processed downstream of the gun. Hence, the proposed diagnostic system will play an important role in developing an extremely low-emittance electron beam or an artificial electron beam with a multi-bunch or micro-bunch structure that enhances the brightness of the XFEL light.

Status and future plans for C3 R&D

C3 is an opportunity to realize an e + e - collider for the study of the Higgs boson at √s = 250 GeV, with a well defined upgrade path to 550 GeV while staying on the same short facility footprint [2,3]. C3 is based on a fundamentally new approach to normal conducting linear accelerators that achieves both high gradient and high efficiency at relatively low cost. Given the advanced state of linear collider designs, the key system that requires technical maturation for C3 is the main linac. This paper presents the staged approach towards a facility to demonstrate C3 technology with both Direct (source and main linac) and Parallel (beam delivery, damping ring, ancillary component) R&D.The primary goal of the C3 Demonstration R&D Plan is to reduce technical and cost risk by building and operating the key components of C3 at an adequate scale. This R&D plan starts with the engineering design, and demonstration of one cryomodule and will culminate in the construction of a 3 cryomodule linac with pre-production prototypes. This R&D program would also demonstrate the linac rf fundamentals including achievable gradient and gradient stability over a full electron bunch train and breakdown rates. It will also investigate beam dynamics including energy spread, wakefields, and emittance growth. This work will be critical to confirm the suitability of the C3 beam parameters for the physics reach and detector performance in preparation for a Conceptual Design Report (CDR), as well as for follow-on technology development and industrialization.The C3 Demonstration R&D Plan will open up significant new scientific and technical opportunities based on development of high-gradient and high-efficiency accelerator technology. It will push this technology to operate both at the GeV scale and mature the technology to be reliable and provide high-brightness electron beams.The timeline for progressing with C3 technology development will be governed by practical limitations on both the technical progress and resource availability. It consists of four stages: Stage 0) Ongoing fundamental R&D on structure prototypes, damping and vibrations. Stage 1) Advancing the engineering maturity of the design and developing start-to-end simulations including space-charge and wakefield effects. This stage will include testing of strucutres operating at cryogenic temperatures. Beam tests would be performed with high beam current to test full beam loading. Stage 2) Production and testing of the first cryomodule at cryogenic temperatures. This would provide sufficient experimental data to compile a CDR and it is anticipated for Stage 2 to last 3 years and to culminate with the transport of photo-electrons through the first cryomodule. Stage 3) Updates to the engineering design of the cryomodules, production of the second and third cryomodule and their installation. Lower charge and lower emittance beams will be used to investigate emittance growth. The successful full demonstration of the 3 cryomodules to deliver up to a 3 GeV beam and achieve the C3five gradient will allow a comprehensive and robust evaluation of the technical design of C3 as well as mitigate technical, schedule, and cost risks required to proceed with a Technical Design Report (TDR).

High-Temperature Superconducting Undulator Prototype Coils for Compact Free-Electron Lasers

Short-period and high-field undulators are crucial for the production of coherent light up to X-rays in compact free-electron lasers (FELs). Besides, future colliders like CLIC or FCC-ee demand high-field damping wigglers to reach a low beam emittance. Both applications may benefit from the use of high-temperature superconductors (HTS), e.g. by providing high magnetic field amplitudes or higher operating temperatures. This paper presents and summarizes our HTS undulator prototype coils, designed and manufactured at CERN, made from coated REBCO tape superconductor. The coil design, described and already powered at 77K in previous works, is based on no-insulation (NI) vertical racetracks with a period length of 13 mm, assembled with iron poles. Powering tests of several HTS undulator prototype coils at 4.2 K, performed at KIT, revealed safe operations at high engineering current densities of 2.3 kA/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> with produced magnetic fields of up to 1.5 T at 3.5mm distance from the magnetic iron poles. Reasonable effective time constants of around 100 s were obtained and further powering tests in regions beyond <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> c with 3.6 kA/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> showed the stability of our NI design and created magnetic fields in the region of 2 T.

Design and Beam Dynamic Studies of an Injector for a Compact THz Coherent Radiation Source

An intense narrow-band terahertz (THz) radiation source has been designed to generate a broad tuning range of radiation frequencies between 0.5 THz and 5.0 THz. The THz radiation is produced when a short-bunch electron beam propagates through an undulator. To achieve high-power peak radiation, the source requires high-brightness electron beams with low beam emittance and short bunch length. A proposed design for the photocathode RF gun used as the electron source is presented. The gun with high mode separation and high Q-factor can be achieved for producing a good beam quality. The beam dynamics of the injector have been preliminarily optimized using the software ASTRA and Elegant, investigating the impact of laser pulse shape on electron beam quality. The results of the beam dynamics studies are comprehensively discussed in this paper.

Multiplexed photoinjector optimization for high-repetition-rate free-electron lasers

The multiplexing capabilities of superconducting X-ray free-electron lasers (FELs) have gained much attention in recent years. The demanding requirements for photon properties from multiple undulator lines necessitate more flexible beam manipulation techniques to achieve the goal of “beam on demand”. In this paper, we investigate a multiplexed configuration for the photoinjector of high-repetition-rate FELs that aims to simultaneously provide low-emittance electron beams of different charges. A parallel, multi-objective genetic algorithm is implemented for the photoinjector parameter optimization. The proposed configuration could drastically enhance the flexibility of beam manipulation to improve multiplexing capabilities and realize the full potential of the facility.

Virtual commissioning of the European XFEL for advanced user experiments at photon energies beyond 25 keV using low-emittance electron beams

Growing interests in ultra-hard X-rays are pushing forward the frontier of commissioning the European X-ray Free-Electron Laser (XFEL) for routine operation towards the sub-ångström regime, where a photon energy of 25 keV (0.5 Å) and above is desired. Such X-rays allow for larger penetration depths and enable the investigation of materials in highly absorbing environments. Delivering the requested X-rays to user experiments is of crucial importance for the XFEL development. Unique capabilities of the European XFEL are formed by combining a high energy linac and the long variable-gap undulator systems for generating intense X-rays at 25 keV and pushing the limit even further to 30 keV. However, the FEL performance relies on achievable electron bunch qualities. Low-emittance electron bunch production, and the associated start-to-end modelling of beam physics thus becomes a prerequisite to dig into the XFEL potentials. Here, we present the obtained simulation results from a virtual commissioning of the XFEL for the user experiments at 25 keV and beyond, including the optimized electron bunch qualities and corresponding FEL lasing performance. Experimental results at 30 keV from the first test run are presented.

Progress on HTS Undulator Prototype Coils for Compact FEL Designs

Compact free-electron lasers require short accelerator structures followed by high peak field and short-period undulators to produce coherent radiation up to hard X-rays. Likewise, future lepton colliders, like CLIC or FCC-ee, require high-field damping wigglers for the creation of low emittance beams. The application of high-temperature superconductor (HTS) in the form of coated REBCO tape to undulators and wigglers enables higher magnetic fields and larger operating margins. This paper discusses the potential of HTS for a superconducting vertical racetrack (VR) undulator coil geometry as well as results of the development work done on a prototype coil, with a period length of 13 mm, wound from coated REBCO tape conductor. Design and assembly aspects as well as first results from powering tests at 77 K are presented and discussed.

Two-dimensional electron beam size measurements with x-ray heterodyne near field speckles

In this paper we report on recent two-dimensional (2D) electron beam size measurements with a nonconventional synchrotron radiation interferometric technique based on x-ray heterodyne near field speckles (HNFS). The method relies on Fourier analysis of the random speckle patterns generated by a water suspension of nanospheres to assess the full 2D transverse coherence of the incoming x rays. The horizontal and vertical electron beam sizes are then retrieved by means of statistical optics approaches. The manuscript thoroughly describes the HNFS technique, and shows experimental results obtained at the ALBA Synchrotron Light Source. By changing the machine coupling, beam sizes as small as $5\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ are measured, thus improving on past measurements reported in the literature and proving the HNFS diagnostics suitable for low-emittance particle beams.

Positron sources: from conventional to advanced accelerator concepts-based colliders

Positron sources are the key elements for the future and current lepton collider projects such as ILC, CLIC, SuperKEKB, FCC-ee, Muon Collider/LEMMA, etc., introducing challenging critical requirements for high intensity and low emittance beams in order to achieve high luminosity. In fact, due to their large production emittance and constraints given by the target thermal load, the main collider parameters such as the peak and average current, the emittances, the damping time, the repetition frequency and consequently the luminosity are determined by the positron beam characteristics. In this paper, the conventional positron sources and their main properties are explored for giving an indication to the challenges that apply during the design of the advanced accelerator concepts. The photon-driven positron sources as the novel approach proposed, primarily for the future linear colliders, are described highlighting their variety and problematic.

Low emittance lattice design for Korea-4GSR

The lattice type of Korea 4th Generation Storage Ring (Korea-4GSR) is determined as hybrid multi-bend achromat type (HMBA) after a comparison of three types of multi-bend achromat (MBA) lattices because HMBA provides sufficiently low beam emittance and good nonlinear beam dynamics with the smallest number of sextupole magnets among the three types of lattices. We also propose the use of bending beamlines with 2 T centre bending magnets (CBMs). Considering the determined lattice type, longitudinal gradient bending magnets and reverse bending magnet were used to reduce the emittance and use of damping wigglers is being considered to reduce emittance further. The ring design result shows that the natural emittance of Korea-4GSR is 58 pm and can be reduced 28% by adopting damping wigglers. Linear and nonlinear dynamics aspects of Korea-4GSR including intrabeam scattering simulation is demonstrated and error effects on Korea-4GSR were also investigated.

Direct measurement of photocathode time response in a high-brightness photoinjector

Electron photoinjectors provide high-brightness electron beams to numerous research applications in physics, chemistry, material, and life sciences. Semiconductor photocathodes are widely used here, as they enable the production of low-emittance beams with variable charge at high repetition rates. One of the key figures of merit of photocathodes is the minimum achievable bunch length. In semiconductor cathodes, this is dominated by scattering effects and varying penetration depths of the extracting photons, which leads to a characteristic electron emission function. We present a method to determine this cathode time response with resolution on the tens of femtoseconds level, breaking the resolution barrier encountered in previous studies. The method is demonstrated with cesium-telluride (Cs2Te) and gold cathodes, revealing response times of (184 ± 41) fs up to (253 ± 58) fs for the semiconductor and an upper limit of (93 ± 17) fs for the metal. Monte Carlo simulations of Cs2Te emission benchmarked to these results give detailed information about the cathode material.

Low-emittance beam injection for a synchrotron radiation source using an X-ray free-electron laser linear accelerator

The full-energy beam injection system from SACLA to the SPring-8 storage ring has been developed. Low-emittance beam injection is required for the future upgrade of SPring-8 due to its small injection beam aperture. The electron beam injection from an X-ray free-electron laser linear accelerator not only satisfies the emittance requirement but also it confers a cost benefit compared to a conventional dedicated injector system. In SPring-8, the SACLA linear accelerator is now delivering X-ray free-electron laser photon beams and serving as a full-time injector of SPring-8 in parallel.2 MoreReceived 13 August 2021Accepted 11 October 2021DOI:https://doi.org/10.1103/PhysRevAccelBeams.24.110702Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBeam injection, extraction & transportBeam opticsBeam optics transportPhysical SystemsFree-electron lasersSynchrotron radiation & free-electron lasersSynchrotron radiation facilitiesAccelerators & Beams

Perspectives towards Sub-Ångström Working Regime of the European X-ray Free-Electron Laser with Low-Emittance Electron Beams

Sub-ångström working regime refers to a working state of free-electron lasers which allows the generation of hard X-rays at a photon wavelength of 1 ångström and below, that is, a photon energy of 12.5 keV and above. It is demonstrated that the accelerators of the European X-ray Free-Electron Laser can provide highly energetic electron beams of up to 17.5 GeV. Along with long variable-gap undulators, the facility offers superior conditions for exploring self-amplified spontaneous emission (SASE) in the sub-ångström regime. However, the overall FEL performance relies quantitatively on achievable electron beam qualities through a kilometers-long accelerator beamline. Low-emittance electron beam production and the associated start-to-end beam physics thus becomes a prerequisite to dig in the potentials of SASE performance towards higher photon energies. In this article, we present the obtained results on electron beam qualities produced with different accelerating gradients of 40 MV/m–56 MV/m at the cathode, as well as the final beam qualities in front of the undulators via start-to-end simulations considering realistic conditions. SASE studies in the sub-ångström regime, using optimized electron beams, are carried out at varied energy levels according to the present state of the facility, that is, a pulsed mode operating with a 10 Hz-repetition 0.65 ms-long bunch train energized to 14 GeV and 17.5 GeV. Millijoule-level SASE intensity is obtained at a photon energy of 25 keV at 14 GeV electron beam energy using a gain length of about 7 m. At 17.5 GeV, half-millijoule lasing is achieved at 40 keV. Lasing at up to 50 keV is demonstrated with pulse energies in the range of a few hundreds and tens of microjoules with existing undulators and currently achievable electron beam qualities.

Performance of the MICE diagnostic system

Muon beams of low emittance provide the basis for the intense, well-characterised neutrino beams of a neutrino factory and for multi-TeV lepton-antilepton collisions at a muon collider. The international Muon Ionization Cooling Experiment (MICE) has demonstrated the principle of ionization cooling, the technique by which it is proposed to reduce the phase-space volume occupied by the muon beam at such facilities. This paper documents the performance of the detectors used in MICE to measure the muon-beam parameters, and the physical properties of the liquid hydrogen energy absorber during running.

Resonant excitation of very high gradient plasma wakefield accelerators by optical-period bunch trains

Using a periodic electron beam bunch train to resonantly excite plasma wakefields in the quasi-nonlinear (QNL) regime has distinct advantages over employing a single, higher charge bunch. Resonant excitation in the QNL regime can produce plasma electron blowout using very low emittance beams with a small charge per pulse: the local density perturbation is extremely nonlinear, achieving total rarefaction, yet the resonant response of the plasma electrons at the plasma frequency is preserved. Such a pulse train, with inter-bunch spacing equal to the plasma period, can be produced via inverse free-electron laser bunching. To achieve resonance with a laser wavelength of a few microns, a high plasma density is used, with the attendant possibility of obtaining extremely large wakefield amplitude, near 1 TV/m for FACET-II parameters. In this article, we use particle-in-cell simulations to study the plasma response, the beam modulation evolution, and the instabilities encountered, that arise when using a bunching scheme to resonantly excite waves in a dense plasma.

Suppression of coherent synchrotron radiation induced emittance growth during electron-beam injection into plasma wakefields

Coherent synchrotron radiation (CSR) is a collective effect that mainly occurs when the trajectory of an electron beam is bent in a dipole magnet. It affects the electron beam by distorting the phase space along its slice distribution, which leads to emittance growth. Therefore, CSR should be suppressed to transport electron beams without further degradation of the emittance. In linear optics, CSR-induced emittance can be suppressed by controlling the Twiss parameters along the electron-beam transfer line. However, owing to some physical constraints, transfer-line optics may be governed by higher-order terms in the transfer map, and the use of a sextupole magnet to suppress these terms would be very challenging for low-energy-spread and low-emittance beams. Therefore, without using a sextupole magnet, we estimate the region of the Twiss parameters where the first-order terms are dominant along the transfer line by introducing chromatic amplitude. In this region, we can apply the suppression condition that is valid in a linear matrix system. This minimization of the emittance growth becomes even more important when the electron-beam transfer line is used for external injection into a plasma wakefield because mismatched beam conditions could induce an additional increase in the emittance during the acceleration. In this paper, we discuss a method of emittance-growth minimization driven by the CSR effect along the transfer line, which is particularly used for electron-beam injection into plasma wakefields. In addition, using the particle-in-cell simulation, we investigate the evolution of electron beam parameters during the acceleration through plasma wakefields in the presence of the CSR effect on the electron beam. We confirm that the beam emittance growth is minimized when the CSR effect is properly controlled. Otherwise, it is found that 11%--32% emittance growths by the CSR effect along the transfer line lead to additional 20%--40% increase of the maximum slice emittance.

Physics of High-Charge Electron Beams in Laser-Plasma Wakefields

Laser wakefield acceleration (LWFA) and its particle-driven counterpart, plasma wakefield acceleration (PWFA), are commonly treated as separate, though related branches of high-gradient plasma-based acceleration. However, novel proposed schemes are increasingly residing at the interface of both concepts where the understanding of their interplay becomes crucial. Here, we report on experiments covering a wide range of parameters by using nanocoulomb-class quasi-monoenergetic electron beams from LWFA with a 100-TW-class laser. Based on a controlled electron injection, these beams reach record-level performance in terms of laser-to-beam energy transfer efficiency (up to 10%), spectral charge density (regularly exceeding 10 pC/MeV) and divergence (1 mrad full width at half maximum divergence). The impact of charge fluctuations on the energy spectra of electron bunches is assessed for different laser parameters, including a few-cycle laser, followed by a presentation of results on beam loading in LWFA with two electron bunches. This scenario is particularly promising to provide high-quality electron beams by using one of the bunches to either tailor the laser wakefield via beam loading or to drive its own, beam-dominated wakefield. We present experimental evidence for the latter, showing a varying acceleration of a low-energy witness beam with respect to the charge of a high-energy drive beam in a spatially separate gas target. With the increasing availability of petawatt-class lasers the access to this new regime of laser-plasma wakefield acceleration will be further facilitated, thus providing new paths towards low-emittance beam generation for future plasma-based colliders or light sources.