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Operational experience with nanocoulomb bunch charges in the Cornell photoinjector
Abstract and Figures
Characterization of 9–9.5 MeV electron beams produced in the dc-gun based Cornell photoinjector is given for bunch charges ranging from 20 pC to 2 nC. Comparison of the measured emittances and longitudinal current profiles to optimized 3D space charge simulations yields excellent agreement for bunch charges up to 1 nC when the measured laser distribution is used to generate initial particle distributions in simulation. Analysis of the scaling of the measured emittance with bunch charge shows that the emittance scales roughly as the square root of the bunch charge up to 300 pC, above which the trend becomes linear. These measurements demonstrate that the Cornell photoinjector can produce cathode emittance dominated beams meeting the emittance and peak current specifications for next generation free electron lasers operating at high repetition rate. In addition, the 1 and 2 nC results are relevant to the electron ion collider community.
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... Quadrupole stray fields in or prior to solenoids couple the transverse phase spaces and dilute the two-dimensional emittances. The successful correction of these stray quadrupole fields in solenoid magnets and from rf couplers in photoelectron guns has been well studied and demonstrated with downstream correction quadrupoles [46][47][48][49][50]. Previously, sextupole aberrations in solenoid magnets and their correction have been considered theoretically and in simulation, and it was found that a similar correction procedure with a downstream sextupole corrector magnet would be successful [51]. ...
... We suspect this to be caused by the design of the iron yoke of the solenoids, which attach as two separate halves. We believe asymmetry in these two halves to be the primary source of quadrupole moments [48]. For the purposes of characterizing the stray quadrupole moments from the solenoids, the first solenoid was energized for the measurements in this section. ...
Ultrafast electron diffraction (UED) is a technique in which short-pulse electron beams can probe the femtosecond-scale evolution of atomic structure in matter driven far from equilibrium. As an accelerator physics challenge, UED imposes stringent constraints on the brightness of the probe electron beam. The low normalized emittance employed in UED, often at the 10-nm scale and below, is particularly sensitive to both applied field aberrations and space-charge effects. The role of aberrations is increasingly important in small probe systems that often feature multiple orders of magnitude variations in beam size during transport. In this work, we report the correction of normal quadrupole, skew quadrupole, and sextupole aberrations via dedicated corrector elements in an ultrafast electron microdiffraction beamline. To do this, we generate precise four-dimensional phase space maps of rf-compressed electron beams and find excellent agreement with aberration-free space-charge simulations. Finally, we discuss the role a probe-forming aperture can play in improving the brightness of bunches with appreciable space-charge effects.
... Quadrupole stray fields in or prior to solenoids couple the transverse phase spaces and dilute the two dimensional emittances. The successful correction of these stray quadrupole fields in solenoid magnets and from rf couplers in photoelectron guns has been well studied and demonstrated with downstream correction quadrupoles [46][47][48][49][50]. Previously, sextupole aberrations in solenoid magnets and their correction have been considered theoretically and in simulation, and it was found that a similar correction procedure with a downstream sextupole corrector magnet would be successful [51]. ...
... We suspect this to be caused by the design of the iron yoke of the solenoids, which attach as two separate halves. We believe asymmetry in these two halves to be the primary source of quadrupole moments [48]. For the purposes of characterizing the stray quadrupole moments from the solenoids, the first solenoid was energized for the measurements in this section. ...
Ultrafast electron diffraction (UED) is a technique in which short-pulse electron beams can probe the femtosecond-scale evolution of atomic structure in matter driven far from equilibrium. As an accelerator physics challenge, UED imposes stringent constraints on the brightness of the probe electron beam. The low normalized emittance employed in UED, often at the 10 nm scale and below, is particularly sensitive to both applied field aberrations and space charge effects. The role of aberrations is increasingly important in small probe systems that often feature multiple orders of magnitude variations in beam size during transport. In this work, we report the correction of normal quadrupole, skew quadrupole, and sextupole aberrations via dedicated corrector elements in an ultrafast electron micro-diffraction beamline. To do this, we generate precise 4-dimensional phase space maps of rf-compressed electron beams, and find excellent agreement with aberration-free space charge simulations. Finally, we discuss the role a probe-forming aperture can play in improving the brightness of bunches with appreciable space charge effects.
... The technical layout of the CBETA accelerator is shown in Fig. 2.13. The acceleration chain [66][67][68] begins with a DC photoelectron gun operated at 300 kV, a pair of emittance-compensating solenoids, and a normal-conducting buncher cavity. This is immediately followed by the superconducting injector cryomodule (ICM), accelerating the beam to the target injection energy of 6 MeV. ...
... To determine suitable machine settings for the low-energy injector, a Multi-Objective Genetic Algorithm optimization (MOGA) [66][67][68], is applied to 3D space-charge simulations of the beam passing through injector, merger, and MLC. Orbit correction for 7 simultaneous beams at 4 different energies in the same beam transport is not trivial. ...
Energy-recovery linacs (ERLs) have been emphasised by the recent (2020) update of the European Strategy for Particle Physics as one of the most promising technologies for the accelerator base of future high-energy physics. The current paper has been written as a base document to support and specify details of the recently published European roadmap for the development of energy-recovery linacs. The paper summarises the previous achievements on ERLs and the status of the field and its basic technology items. The main possible future contributions and applications of ERLs to particle and nuclear physics as well as industrial developments are presented. The paper includes a vision for the further future, beyond 2030, as well as a comparative data base for the main existing and forthcoming ERL facilities. A series of continuous innovations, such as on intense electron sources or high-quality superconducting cavity technology, will massively contribute to the development of accelerator physics at large. Industrial applications are potentially revolutionary and may carry the development of ERLs much further, establishing another shining example of the impact of particle physics on society and its technical foundation with a special view on sustaining nature.
... Typically, simulations are conducted with ideal initial beam distributions, or only a few example distributions from measurements. This can be a significant source of discrepancy between model predictions and measured data [21]. Overall, simulations tend to represent ideal conditions, and therefore ideal beam dynamics. ...
... Using the measured laser distribution to sample particles for space charge simulation can minimize discrepancies between measurement and simulated output values [21]. Therefore, in order to attempt to create more realistic simulated data, real laser profiles were used to generate particle distributions to be tracked in LUME-Astra. ...
Machine learning models of accelerator systems ("surrogate models") are able to provide fast, accurate predictions of accelerator physics phenomena. However, approaches to date typically do not include measured input diagnostics, such as the initial beam distributions, which are critical for accurately representing the beam evolution through the system. In addition, these inputs often vary over time, and models that can account for these changing conditions are needed. As beam time for measurements is often limited, simulations are in some cases needed to provide sufficient training data. These typically represent the designed machine before construction; however, the behavior of the installed components may be quite different due to changes over time or static differences that were not modeled. Therefore, surrogate models that can leverage both simulation and measured data successfully are needed. We introduce an approach based on convolutional neural networks that uses the drive laser distribution and scalar settings as inputs for a photoinjector system model (here, the Linac Coherent Light Source II, LCLS-II, injector frontend). The model is able to predict scalar beam parameters and the transverse beam distribution downstream, taking into account the impact of time-varying non-uniformities in the initial transverse laser distribution. We also introduce and evaluate a transfer learning procedure for adapting the surrogate model from the simulation domain to the measurement domain, to account for differences between the two. Applying this approach to our test case results in a model that can predict test sample outputs within a mean absolute percent error of 7.6%. This is a substantial improvement over the model trained only on simulations, which has an error of 112.7% when applied to measured data. While we focus on the LCLS-II Injector frontend, these approaches for improving ML-based online modeling of injector systems could be easily adapted to other accelerator facilities.
... This in fact means that electron bunches obtained using plasma based acceleration schemes are not a viable option as the energy gradient is too large. However, for the traditional linear accelerators the requirement of a long bunch is beneficial for the Thomson scheme: as was shown in [13] 1) the long bunch reduces space-charge forces 2) allows for much larger bunch charge, of several nano Coulombs [14][15][16][17] . 3) the energy spread of the electrons and its correlation is controllable. ...
Thomson/Compton scattering is a method to produce high energy photons through the collision of low energy photons in a laser pulse onto relativistic electrons. In the linear (inco-herent) Thomson/Compton regime, the flux scales linearly with the number of primary particles and the bandwidth of the produced photons depend, amongst other factors, on the energy spread of them. In general, an increase of the primary particles is connected to a larger energy spread (e.g. non-constant acceleration gradients, collective effects, etc). Therefore their number is restricted by the desired band-width, and thus limits the flux. In our previous (theoretical) studies we showed that the ideal Thomson spectrum can be retrieved when an electron bunch with a linear energy correlation of several percent collides with a matched linearly chirped laser pulse. Here we extend the scheme to allow for higher order energy correlations and quantify how the electron distribution influences the bandwidth. Furthermore we discuss the practical viability to maximize the primary particles, with the focus on linear accelerators (LINACS) for the electrons and laser pulses based on the chirped pulse amplification (CPA) scheme. These could potentially provide up to tens of nano-Coulomb electron bunches and tens, or even over a hundred, Joule lasers pulses respectively.
... Assuming an electron beam waist of σ r ¼ 5 μm, the normalized transverse emittance ϵ n ¼ γβσ θ σ r of the electron beam corresponding to the angular spread at this point is ϵ n ¼ 500 nm rad. This is much larger than the emittance of electron bunches from photoinjectors with a charge of tens of pC [24,25]. ...
Imposing a density modulation on an electron beam may improve the brightness of a Thomson source by orders of magnitude via superradiant emission. In this paper, we analytically and numerically analyze electron beam modulation via the ponderomotive force due to the copropagating beat wave formed by two laser pulses at different frequencies. We show that energy modulation favorably scales with electron beam energy but is limited by the interaction length imposed by the finite waist of the laser pulses. Additionally, the effect of initial emittance and energy spread on the quality of microbunching is studied. Finally, we propose a superradiant extreme ultraviolet Thomson source based on ponderomotive bunching.
... Recently, the developments of the above machines have dramatically required the electron beam with not only high brightness but also high repetition rate. Therefore, electron guns capable of operating at MHz-class repetition rates are intensely studied in the past few years, and several groups around the world have proposed different schemes including the direct current (DC) gun [1], Superconducting RF gun [2] and very high frequency (VHF) gun [3]. ...
A 217 MHz VHF gun operating in CW mode is designing at Tsinghua University. The cathode gradient is designed to be 30 MV/m to accelerate the electron bunches up to 878 keV. The cavity profile is optimized in CST to minimize the input power, peak surface electric field, and peak wall power density. The multipacting analysis and the thermal analysis are also presented in this paper. Further gun shape optimization and mechanical design are ongoing.
... In the following section, we will limit ourselves to optimization of the beamline's single-bunch performance, deferring a discussion of the effects of multibunch stability until the following section. All of our simulation work is performed with General Particle Tracer (GPT) [33,34], using a Multiobjective Genetic Algorithm (MOGA) optimization [22,35,36]. Although the gun, buncher, and solenoid field maps are cylindrically symmetric, the SRF cavity field maps included the effects of the input couplers and are fully 3D. ...
We present the design of a high repetition rate MeV energy ultrafast electron diffraction instrument based on a dc photoelectron gun and an superconducting rf (SRF) linac with multiple independently controlled accelerating and bunching cavities. The design is based on the existing Cornell photoinjector, which can readily be applied to the presented findings. Using particle tracking simulations in conjunction with multiobjective genetic algorithm optimization, we explore the smallest bunch lengths, emittance, and probe spot sizes achievable. We present results for both stroboscopic conditions (with single electrons per pulse) and with 105 electrons/bunch which may be suitable for single-shot diffraction images. In the stroboscopic case, the flexibility provided by the many-cavity bunching and acceleration allows for longitudinal phase space linearization without a higher harmonic field, providing sub-fs bunch lengths at the sample. Given low emittance photoemission conditions, these small bunch lengths can be maintained with probe transverse sizes at the single micron scale and below. In the case of 105 electrons per pulse, we simulate state-of-the-art 5D brightness conditions: rms bunch lengths of 10 fs with 3-nm normalized emittances, while now permitting repetition rates as high as 1.3 GHz. Finally, to aid in the design of new SRF-based ultrafast electron diffraction machines, we simulate the trade-off between the number of cavities used and achievable bunch length and emittance.
Continuous-wave (CW) electron sources with sub-micron normalized emittance are of critical importance for advanced linear accelerator based light sources. The DC-SRF-II gun, designed as such a source, has been newly constructed and brought into operation. To characterize the performance of the gun, we have developed an emittance measurement system based upon single slit scanning method under a dedicated beam diagnostics mode. With this system, the measurement of sub-micron normalized emittance, together with a reconstruction of the phase space distribution, has been achieved with high reproducibility. This paper presents a detailed description of the system.
High-energy, high-dose, microfocus X-ray computed tomography (HHM CT) is one of the most effective methods for high-resolution X-ray radiography inspection of high-density samples with fine structures. Minimizing the effective focal spot size of the X-ray source can significantly improve the spatial resolution and the quality of the sample images, which is critical and important for the performance of HHM CT. The objective of this study is to present a 9 MeV HHM CT prototype based on a high-average-current photo-injector in which X-rays with about 70μm focal spot size are produced via using tightly focused electron beams with 65/66μm beam size to hit an optimized tungsten target. In digital radiography (DR) experiment using this HHM CT, clear imaging of a standard 0.1 mm lead DR resolution phantom reveals a resolution of 6 lp/mm (line pairs per mm), while a 5 lp/mm resolution is obtained in CT mode using another resolution phantom made of 10 mm ferrum. Moreover, comparing with the common CT systems, a better turbine blade prototype image was obtained with this HHM CT system, which also indicates the promising application potentials of HHM CT in non-destructive inspection or testing for high-density fine-structure samples.
Many new applications for electron accelerators require high-brightness, high-average power beams, and most rely on photocathode-based electron injectors as a source of electrons. To achieve such a photoinjector, one requires both a high-power laser system to produce the high average current beam, and also a system at reduced repetition rate for electron beam diagnostics to verify high beam brightness. Here we report on two fiber laser systems designed to meet these specific needs, at 50 MHz and 1.3 GHz repetition rate, together with pulse pickers, second harmonic generation, spatiotemporal beam shaping, intensity feedback, and laser beam transport. The performance and flexibility of these laser systems have allowed us to demonstrate electron beam with both low emittance and high average current for the Cornell energy recovery linac.
We present the results of transverse emittance and longitudinal current
profile measurements of high bunch charge (greater than or equal to 100 pC)
beams produced in the DC gun-based Cornell Energy Recovery Linac Photoinjector.
In particular, we show that the cathode thermal and core beam emittances
dominate the final 95% and core emittance measured at 9-9.5 MeV. Additionally,
we demonstrate excellent agreement between optimized 3D space charge
simulations and measurement, and show that the quality of the transverse laser
distribution limits the optimal simulated and measured emittances. These
results, previously thought achievable only with RF guns, demonstrate that DC
gun based photoinjectors are capable of delivering beams with sufficient single
bunch charge and beam quality suitable for many current and next generation
accelerator projects such as Energy Recovery Linacs (ERLs) and Free Electron
Lasers (FELs).
We report on the direct amplification of picosecond pulses to megawatt peak power and 150 W average power using a Yb-doped rod-type fiber master oscillator power amplifier. 3 ps pulses at 50 MHz repetition rate are amplified in a 55 cm rod-type fiber amplifier to produce linearly polarized pulses with 3 μJ pulse energy and 1 MW peak power. These results extend megawatt picosecond fiber amplifiers to the 100 W average power regime for the first time. Frequency doubling of the pulses yields 70 W green average power, 1.4 μJ pulse energy, and 500 kW peak power.
We present a detailed study of the six-dimensional phase space of the
electron beam produced by the Cornell Energy Recovery Linac Photoinjector, a
high-brightness, high repetition rate (1.3 GHz) DC photoemission source
designed to drive a hard x-ray energy recovery linac (ERL). A complete
simulation model of the injector has been constructed, verified by measurement,
and optimized. Both the horizontal and vertical 2D transverse phase spaces, as
well as the time-resolved (sliced) horizontal phase space, were simulated and
directly measured at the end of the injector for 19 pC and 77 pC bunches at
roughly 8 MeV. These bunch charges were chosen because they correspond to 25 mA
and 100 mA average current if operating at the full 1.3 GHz repetition rate.
The resulting 90% normalized transverse emittances for 19 (77) pC/bunch were
0.23 +/- 0.02 (0.51 +/- 0.04) microns in the horizontal plane, and 0.14 +/-
0.01 (0.29 +/- 0.02) microns in the vertical plane, respectively. These
emittances were measured with a corresponding bunch length of 2.1 +/- 0.1 (3.0
+/- 0.2) ps, respectively. In each case the rms momentum spread was determined
to be on the order of 1e-3. Excellent overall agreement between measurement and
simulation has been demonstrated. Using the emittances and bunch length
measured at 19 pC/bunch, we estimate the electron beam quality in a 1.3 GHz, 5
GeV hard x-ray ERL to be at least a factor of 20 times better than that of
existing storage rings when the rms energy spread of each device is considered.
These results represent a milestone for the field of high-brightness,
high-current photoinjectors.
We have constructed a very high voltage DC photoemission electron gun as the electron source of an injector for an Energy Recovery Linac (ERL) based synchrotron radiation light source. The gun is designed to deliver 100 mA average beam current in a 1300 MHz CW bunch train (77 pC/bunch), and to operate up to 750 kV cathode potential. Negative electron affinity (NEA) photocathodes are used for their small thermal emittance and high quantum efficiency. A load-lock system allows introduction, cleaning, and activation of cathodes outside of the electron gun. Cathodes are cleaned by heating and exposure to atomic hydrogen, and activated with cesium and nitrogen trifluoride. Cathode electrodes of 316 LN stainless and Ti4V6Al have been used with a beryllium anode. The internal surface of the ceramic insulator has a high resistivity fired coating, providing a charge drainage path. Non-evaporable getter (NEG) pumps provide a very high pumping speed for hydrogen. Operating experience with this gun will be presented.
We present a comparison between space charge calculations and direct measurements of the transverse phase space for space charge dominated electron bunches after a high voltage photoemission DC gun followed by an emittance compensation solenoid magnet. The measurements were performed using a double-slit setup for a set of parameters such as charge per bunch and the solenoid current. The data is compared with detailed simulations using 3D space charge codes GPT and Parmela3D with initial particle distributions created from the measured transverse and temporal laser profiles. Beam brightness as a function of beam fraction is calculated for the measured phase space maps and found to approach the theoretical maximum set by the thermal energy and accelerating field at the photocathode.
Sodium potassium antimonide photocathodes with Quantum Efficiency (QE) in the range of few percent have been grown, and their photoemission properties are measured. We report the intrinsic emittance and response time of electron bunches extracted from this material. It is possible to recover the QE of an overheated cathode by simple potassium addition, and the cathode is rugged enough to deliver tens of mA of average current with no or minimal degradation.
High-power, high-brightness electron beams are of interest for many applications, especially as drivers for free electron lasers and energy recovery linac light sources. For these particular applications, photoemission injectors are used in most cases, and the initial beam brightness from the injector sets a limit on the quality of the light generated at the end of the accelerator. At Cornell University, we have built such a high-power injector using a DC photoemission gun followed by a superconducting accelerating module. Recent results will be presented demonstrating record setting performance up to 65 mA average current with beam energies of 4–5 MeV.
A future electron-ion collider at BNL
- Erhic
eRHIC, A future electron-ion collider at BNL, in Proceedings of the 9th European Particle Accelerator Conference,
Lucerne, 2004 (EPS-AG, Lucerne, 2004).