Bunch Emittance Research Articles

SIMULATION OF THE IDENTICAL PLATEAUS FORMATION ON PLASMA WAKEFIELD FOR LONG DRIVER-BUNCH AND WITNESS-BUNCHES

We formulate scenario for the simultaneous enhancement of the charge of accelerated electrons, improving such characteristics as the transformer ratio, efficiency, low energy spread, and emittance of bunches of accelerated electrons in the blowout regime by the simultaneous achievement of the identical plateaus on the accelerating wakefields and other identical plateaus on the decelerating wakefields.

Overview of CW electron guns and LCLS-II RF gun performance

Various continuous-wave (CW) electron gun technologies are reviewed, including DC, superconducting radio frequency RF (SRF), hybrid DC-SRF and normal-conducting RF. Also, the SLAC Linac Coherent Light Source II (LCLS-II) normal-conducting RF gun and injector are described, and the performance to date, including the bunch emittance achieved and the dark current observed, is presented.

Field enhanced compact S-band gun employing a pin cathode

S-band RF-guns are highly developed for production of low emittance relativistic electron bunches, but need powerful klystrons for driving. Here, we present the design and first experimental tests of a compact S-band gun, which can accelerate electrons up to 180 keV powered by only 10 kW from a compact rack-mountable solid-state amplifier. A pin-cathode is used to enhance the RF electric field on the cathode up to 100 MV/m as in large-scale S-band guns. An electron bunch is generated through photoemission from a flat copper surface on the pin ex-cited by a UV laser pulse followed by a focussing solenoid producing a low emittance bunch with 0.1 mm mrad transverse emittance for up to 100 fC bunch charge. We are currently in the conditioning phase of the gun and first experiments show good agreement with simulations. The compact gun will serve three purposes: (i) it can be used directly for ultrafast electron diffraction; (ii) as an injector into a THz booster producing 0.3 MeV to 2 MeV electron bunches for ultrafast electron diffraction; (iii) The system in (ii) serves as an injector into a THz linear accelerator producing a 20 MeV beam for the AXSIS X-ray source project.

Ultimate bunch length and emittance performance of an MeV ultrafast electron diffraction apparatus with a dc gun and a multicavity superconducting rf linac

We present the design of a high repetition rate MeV energy ultrafast electron diffraction instrument based on a DC photoelectron gun and an 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 $10^5$ 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 $10^5$ 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 UED machines, we simulate the trade-off between the number of cavities used and achievable bunch length and emittance.

Pulse-length Effect on Laser Wakefield Acceleration of Electrons by Skewed Laser Pulses

Laser pulse-length-related control and optimization of electron acceleration from laser wakefield acceleration (LWFA) by a shaped laser pulse is proposed to investigate in this study. The laser pulse shape changes and becomes skewed due to the variation in refractive index during propagation in plasmas. The optimum laser pulse-length for excitation of plasma wave in linear regime $(a_{0} is $\tau _{L}=\lambda _{p} / \sqrt {2\pi c} $ , but, in nonlinear regime $(a_{0}>1)$ , is not quite certain. Here, laser strength parameter $(a_{0})$ is defined as the ratio $eE_{L}/\text {m}\omega \text {c}$ . In nonlinear case, the variation in pulse skewedness at an optimized laser pulse-length produces different evolution of the wakefield that might affect the electron beam parameters. The present investigation predicts an optimized laser pulse-length (with an optimize ratio of the leading to the trailing pulse edge duration) 35 fs and laser strength parameter $a_{0}=2$ to control the electron bunch charge, emittance, and energy wakefield. Injected bunch charge is enhanced about 20%, and beam emittance is reported to reduce about 40% by employing the skewed Gaussian laser pulse in place of Gaussian pulse for LWFA. Energy-spread of accelerated electron bunch is also reduced from 18% to 6.6% for $s_{\alpha }=0.45$ for 35-fs laser pulse. The electron bunch parameters can also be controlled and optimized using skewed laser pulse and optimized pulse-length.

Experimental demonstration of novel beam characterization using a polarizable X-band transverse deflection structure

The PolariX TDS (Polarizable X-Band Transverse Deflection Structure) is an innovative TDS-design operating in the X-band frequency-range. The design gives full control of the streaking plane, which can be tuned in order to characterize the projections of the beam distribution onto arbitrary transverse axes. This novel feature opens up new opportunities for detailed characterization of the electron beam. In this paper we present first measurements of the Polarix TDS at the FLASHForward beamline at DESY, including three-dimensional reconstruction of the charge-density distribution of the bunch and slice emittance measurements in both transverse directions. The experimental results open the path toward novel and more extensive beam characterization in the direction of multi-dimensional-beam-phase-space reconstruction.

Evolution of bunch envelope and emittance in photo-injectors with application to the AWAKE experiment

We investigate on time evolution of envelopes and emittance of bunched beams in photo-injectors. We carry out a detailed beam dynamic study with focus on calculations of space charge forces. We take the average of the forces over the whole ensemble of particles. This results in space charge coefficients that are largely independent of the bunch distribution. By implementing these coefficients in the beam envelope equations we find a set of modified equations as a model for describing the evolution of the bunch envelope. Our equations crucially decrease the consumed time in particle-in-cell codes for designing today’s photo-injectors. The feasibility and accuracy of the model have been benchmarked against the simulation results from existing particle-in-cell codes with very good agreement. We apply our model for the design of the ultra-short bunch photo-injector of the AWAKE experiment. Moreover, by using assumption of linear external fields we develop the governing equations of the bunch emittances in photo-injectors. These equations allow us to partly quantitatively and partly qualitatively study the effects of external and space charge fields on emittance growth phenomena.

Utilization of RADFET sensors for x-ray undulators: experiential considerations on magnetic field profile before commissioning

Concerning accelerator-based light sources, electrons are accelerated in conformity with specific time structures (i.e. continuous wave, ‘cw’, or bunched), which arise out of accelerating frequencies of employed radiofrequency cavities. Thus, the concept of transverse emittance of electron bunches comes into prominence, simply defining the area of an ellipse in phase space where the electrons are occupied. In nonideal cases (i.e. in real life), when some fraction of these electrons sneak out of a bunch, the concept of ‘energy spread’ automatically arises. Since the energy spread resulting from off-axis relativistic electrons in an insertion device induces considerable radiation damage on magnetic structures of x-ray undulators, radiation-sensitive field effect transistors (RADFETs) are utilized for quantification of radiation intensity. Hence, it is crucial to bring out their influence on magnetic field profiles before commisioning in tunnels. In this respect, two different types of RADFET sensors were investigated over the magnetic poles of an undulator with 40 mm period length (shortened as an U40 undulator). The effects of the sensors on magnetic field were measured using transverse and longitudinal scans through 6.5 m-long magnetic measurement benches (MMBs). The results of two RADFET standard systems are presented, wherein one contains the magnetic material nickel. Finally, novel magnetic measurement techniques for x-ray undulators via unique MMBs are thoroughly explained as well.

Acceleration of electron bunches in plasma wakefields generated by frequency-chirped laser pulse

A comparative study of different injection configurations in the wakefield generated by a chirped laser pulse is done. The preferable electron injection region is considered for different injection schemes. For injection in the chirped laser pulse region, the energy spread has approximately the same final value independently of its injection position while energy spread changes strongly for injection in the wakefield region. It is demonstrated that an externally injected electron bunch with an initial energy of 100 MeV can achieve an energy gain of 8 GeV by using periodical chirped laser pulses. In particular, strong bunch decoherence is observed for off-axis injection in a plasma-channel-guided-laser wakefield. This strong bunch decoherence leads to a considerable increase in the bunch emittance growth. The emittance growth in off-axis injection is at least ten orders of magnitude higher than in on-axis injection.

Improvement of 6D brightness by a 1.4-cell photocathode RF gun for MeV Ultrafast electron diffraction

Recent research indicates that ultrafast electron diffraction and microscopy (UED/M) have unprecedented potential in probing ultrafast dynamic processes, especially in organic and biological materials. However, reaching the required brightness while maintaining high spatiotemporal resolution requires new design of electron source. In order to produce ultrashort electron beam with extreme high brightness, a 1.4-cell RF gun is being developed to reach higher acceleration gradient near the photocathode and thus suppress the space charge effect in the low energy region. Simulation of the 1.4-cell RF photocathode gun shows considerable improvement in bunch length, emittance and energy spread, which all lead to better temporal and spatial resolution comparing to traditional 1.6-cell RF photocathode gun. The results demonstrate the feasibility of sub-ps temporal resolution with normalized emittance less than 0.1 πmm·mrad while maintaining 1 pC electron pulse.

High-quality 5 GeV electron bunches with resonant multi-pulse ionization injection

The production of high-quality electron bunches in laser wakefield acceleration relies on the possibility of injecting ultra-low emittance bunches in the plasma wave. A new bunch injection scheme (resonant multi-pulse ionization, ReMPI) has been conceived and studied, in which electrons extracted by ionization are trapped by a large-amplitude plasma wave driven by a train of resonant ultrashort pulses. Such a train of pulses can be obtained in a very efficient, compact and stable way, by phase manipulation in the laser front-end. The ReMPI injection scheme relies on currently available laser technology and is being considered for the implementation of future compact x-ray free electron laser schemes. Simulations show that high-quality electron bunches with an energy of up to 5 GeV and a peak current exceeding 2 kA, with normalized emittance of below 0.1 mm × mrad and a slice energy spread of below 0.1%, can be obtained with a single stage.

Properties of THz coherent undulator radiation generated from a compact accelerator source at Kyoto University

A compact accelerator-base source of THz Coherent Undulator Radiation (THz-CUR) at the Kyoto University has been developed with the purpose of providing intense quasimonochromatic and tunable THz-CUR at frequencies below 1 THz. The source is driven by a compact accelerator using a photocathode RF gun, which emits an electron beam with a fixed energy of 4.6 MeV and high bunch charge. The THz-CUR from our source can be generated when a compressed electron bunch passes through a planar undulator. In this study, we investigate the properties of this radiation, including the total radiation energy, spatial distribution, and power spectrum. With an electron beam of 160 pC bunch charge, the total radiation energy of THz-CUR at 0.16 THz was 1.2 µJ in the micropulse. The THz-CUR covering the frequency range from 0.16 THz to 0.65 THz could be produced by changing the magnetic field of the undulator at a 60 pC bunch charge. Due to the influence of the space charge forces causing the degradation of electron beam qualities, saturation of the radiation energy occurred, and the radiation power spectrum at a frequency of 0.65 THz could not be observed in the high charge condition (160 pC). The effects of bunch lengthening, energy spread, and emittance growth and the influence of the phase error on the generation of THz radiation are also discussed in this paper. This opens up the possibility of understanding the generation of THz-CUR and sheds further light on the enhancement of the radiation power.

Analytical and numerical analysis of longitudinally coupled transverse dynamics of Pulse Picking by Resonant Excitation in storage rings serving timing and high-flux users simultaneously

Abstract The pulse picking by resonant excitation (PPRE) method is applied at BESSY II to provide single bunch light to timing users while operating a multi-bunch filling pattern. This method can provide single bunch light to all beamlines simultaneously since the emittance of a selected bunch is increased by a quasi-resonant incoherent excitation close to the first synchrotron sideband of the betatron oscillation frequency. This gives high-flux to users by separating the synchrotron radiation from one horizontally enlarged bunch from the light of the multi-bunch beam. The properties of the excited bunch depend strongly on lattice parameters such as beta-functions and horizontal chromaticity, and the frequency and amplitude of the excitation signal. A theoretical interpretation by applying a linear transfer-matrix analysis based on the Liouville’s theorem is derived to illustrate a quantitative relation between the beam size and lattice parameters. In addition, measurements and numerical simulations show a monotonic increase of the timing bunch emittance as a function of the excitation amplitude at the first synchrotron sideband of the betatron oscillation frequency. The observed excitation-amplitude-dependent features of the transverse bunch size are confirmed by measurements with two independent diagnostics at BESSY II.

Overtaking collision effects in a cw double-pass proton linac

The recirculating superconducting proton linac has the advantage of reducing the number of cavities in the accelerator and the corresponding construction and operational costs. Beam dynamics simulations were done recently in a double-pass recirculating proton linac using a single proton beam bunch. For continuous wave (cw) operation, the high-energy proton bunch during the second pass through the linac will overtake and collide with the low-energy bunch during the first pass at a number of locations of the linac. These collisions might cause proton bunch emittance growth and beam quality degradation. In this paper, we study the collisional effects due to Coulomb space-charge forces between the high-energy bunch and the low-energy bunch. Our results suggest that these effects on the proton beam quality would be small and might not cause significant emittance growth or beam blowup through the linac. A 10 mA, 500 MeV cw double-pass proton linac is feasible without using extra hardware for phase synchronization.

A study of wake potentials for the pick-ups in storage ring

Calculating wake potentials in accelerators are necessary in order to optimize the accelerator performance in terms of stored current and emittance. Wake potentials due to pick-ups can be an important source for current and emittance degradation. In this paper, we present theoretical analysis and computer simulations of the wake potentials for the beam pipe with and without mounted pick-ups. We introduce generalized wake potential to calculate the transverse wake-field and analyze the effect of pick-ups on these potentials. In addition, the effects of wake potentials due to pick-ups on the bunch length and emittance are studied using ELEGANT particle tracking code.

Emittance preservation in plasma-based accelerators with ion motion

In a plasma-accelerator-based linear collider, the density of matched, low-emittance, high-energy particle bunches required for collider applications can be orders of magnitude above the background ion density, leading to ion motion, perturbation of the focusing fields, and, hence, to beam emittance growth. By analyzing the response of the background ions to an ultrahigh density beam, analytical expressions, valid for nonrelativistic ion motion, are derived for the transverse wakefield and for the final (i.e., after saturation) bunch emittance. Analytical results are validated against numerical modeling. Initial beam distributions are derived that are equilibrium solutions, which require head-to-tail bunch shaping, enabling emittance preservation with ion motion.

The resonant multi-pulse ionization injection

The production of high-quality electron bunches in Laser Wake Field Acceleration relies on the possibility to inject ultra-low emittance bunches in the plasma wave. In this paper, we present a new bunch injection scheme in which electrons extracted by ionization are trapped by a large-amplitude plasma wave driven by a train of resonant ultrashort pulses. In the Resonant Multi-Pulse Ionization injection scheme, the main portion of a single ultrashort (e.g., Ti:Sa) laser system pulse is temporally shaped as a sequence of resonant sub-pulses, while a minor portion acts as an ionizing pulse. Simulations show that high-quality electron bunches with normalized emittance as low as 0.08 mm × mrad and 0.65% energy spread can be obtained with a single present-day 100TW-class Ti:Sa laser system.

Design study of a low-emittance high-repetition rate thermionic rf gun

We propose a novel gridless continuous-wave radiofrequency (rf) thermionic gun capable of generating nC ns electron bunches with a rms normalized slice emittance close to the thermal level of 0.3 mm mrad. In order to gate the electron emission, an externally heated thermionic cathode is installed into a stripline-loop conductor. Two high-voltage pulses propagating towards each other in the stripline-loop overlap in the cathode region and create a quasielectrostatic field gating the electron emission. The repetition rate of pulses is variable and can reach up to one MHz with modern solid-state pulsers. The stripline attached to a rf gun cavity wall has with the wall a common aperture that allows the electrons to be injected into the rf cavity for further acceleration. Thanks to this innovative gridless design, simulations suggest that the bunch emittance is approximately at the thermal level after the bunch injection into the cavity provided that the geometry of the cathode and aperture are properly designed. Specifically, a concave cathode is adopted to imprint an Ƨ-shaped distribution onto the beam transverse phase-space to compensate for an S-shaped beam distribution created by the spherical aberration of the aperture-cavity region. In order to compensate for the energy spread caused by rf fields of the rf gun cavity, a 3rd harmonic cavity is used. A detailed study of the electrodynamics of the stripline and rf gun cavity as well as the beam optics and bunch dynamics are presented.

Beam load structures in a basic relativistic interaction model

Some recent experiments have shown that the beam load in bubble and blow-out experiments is located in a volume as small as a few μm3. Now, we show what kinds of inner structures are possible in such a high dense electron ensemble. Our analysis starts from a first principles model for relativistically corrected mutual electron interaction in a phenomenological bubble model. Discussing 2D and 3D beam load configurations, we show that, depending on the bunch emittance, the beam load might be in a highly ordered and dense configuration, a less ordered but still dense state, or a configuration where each electron performs an individual random motion.

Brilliant GeV electron beam with narrow energy spread generated by a laser plasma accelerator

The production of GeV electron beam with narrow energy spread and high brightness is investigated using particle-in-cell simulations. A controlled electron injection scheme and a method for phase-space manipulation in a laser plasma accelerator are found to be essential. The injection is triggered by the evolution of two copropagating laser pulses near a sharp vacuum-plasma transition. The collection volume is well confined and the injected bunch is isolated in phase space. By tuning the parameters of the laser pulses, the parameters of the injected electron bunch, such as the bunch length, energy spread, emittance and charge, can be adjusted. Manipulating the phase-space rotation with the rephasing technique, the injected electron bunch can be accelerated to GeV level while keeping relative energy spread below 0.5% and transverse emittance below $1.0\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$. The results present a very promising way to drive coherent x-ray sources.