Compressed Electron Beam Research Articles

Noninvasive time-sorting in radio frequency-compressed ultrafast electron diffraction.

We demonstrate a noninvasive time-sorting method for ultrafast electron diffraction (UED) experiments with radio frequency (rf)-compressed electron beams. We show that electron beam energy and arrival time at the sample after the rf compression are strongly correlated, such that the arrival time jitter may be corrected through the measurement of the beam energy. The method requires minimal change to the infrastructure of most of the UED machines and is applicable to both keV and MeV UED. In our experiment with ∼3 MeV beam, the timing jitter after the rf compression is corrected with a 35-fs root mean square (rms) accuracy, limited by the energy stability. For keV UED with a high energy stability, sub-10 fs accuracy in time-sorting should be readily achievable. This time-sorting technique allows us to retrieve the 2.5 THz oscillation related to coherent A1g phonon in the laser-excited Bismuth film and extends the temporal resolution of UED to a regime far beyond the 100–200 fs rms jitter limitation.

Adaptive model tuning studies for non-invasive diagnostics and feedback control of plasma wakefield acceleration at FACET-II

Beam-driven plasma wakefield acceleration (PWFA) achieves the same energy gain in a single meter, for which conventional accelerators require several kilometers, however much work is still required to match the beam quality of conventional accelerators. The PWFA processes to be studied at the FACET-II facility will utilize extremely short (down to a few fs) high energy (10 GeV), high charge (few nC) electron and eventually positron bunches. The PWFA process is extremely sensitive to the detailed longitudinal current profiles of these bunches and it would be of great benefit to have precise measurement and control of these profiles. We present an adaptive model tuning technique for FACET-II to adaptively tune online models based on real time accelerator and beam data to continuously provide a non-invasive diagnostic which predicts the longitudinal phase space (LPS) of extremely short and highly compressed electron beams, which otherwise require destructive X-band transverse deflecting cavity (XTCAV)-based measurements. Based on simulation studies, our method has the potential for: (1). The development of a non-invasive longitudinal phase space diagnostic by adaptively tuning models based on non-invasive measurements such as energy spread spectra which could be recorded at all of the bunch compressors in the facility. (2). Utilize these diagnostic to perform model-independent feedback-based to achieve desired longitudinal phase space distributions. (3). Utilize the model-independent feedback approach to maximize energy gain while minimizing emittance growth and energy gain variance of the PWFA process by tuning accelerator parameters, while monitoring what longitudinal phase space the algorithm has found, information that will be useful for further analytical and simulation studies.

Hollow high-quality electron beam generation by cyclotron autoresonance in laser-induced acceleration

In this study, the dynamics of electrons by a focused Gaussian laser pulse in the presence of an ultrashort axial magnetic field have been investigated numerically. The cyclotron resonance of the electrons due to the external magnetic field were theoretically investigated. An optimized magnetic field can enhance the electron energy gradient with ultra-low scattering. The simulation provides a circulating hollow electron beam suitable for high-precision and high-energy particle physics experiments. The features of output electron beam such as mean energy gain, energy spectra, spatial emittance and angular scattering distribution were examined and their concerning physical explanations have been presented. It has been found that an electron bunch with about MeV initial energy becomes a compressed electron beam with about 3 GeV energy, after interacting with the laser pulse applied by a nanosecond pulsed magnetic field.

Investigation of Bunch Compressor and Compressed Electron Beam Characteristics by Coherent Transition Radiation

A magnetic chicane bunch compressor for a new compact accelerator-based terahertz (THz) radiation source at the Institute of Advanced Energy, Kyoto University, was completely installed in March 2016. The chicane is employed to compress an electron bunch with an energy of 4.6 MeV generated by a 1.6-cell photocathode radio frequency (RF)-gun. The compressed bunch is injected into a short planar undulator for THz generation by coherent undulator radiation (CUR). The characteristics of the bunch compressor and the compressed bunch were investigated by observing the coherent transition radiation (CTR). The CTR spectra, which were analyzed by using a Michelson interferometer, and the compressed bunch length were also estimated. The results were that the chicane could compress the electron bunch at a laser injection phase less than 45 degrees, and the maximum CTR intensity was observed at a laser injection phase around 24 degrees. The optimum value of the first momentum compaction factor was around −45 mm, which provided an estimated rms bunch length less than 1 ps.

Development of the sheet electron beam focusing system based on thermionic and field emission cathodes

Synthesis method has been developed for modeling the electron-focusing systems formed compressed sheet electron beam with a field emission cathode. The results of modeling by synthesis of a convergent sheet electron beam with a cross section of 0.15·0.8 mm2 and a current of 100 mA with magnetic shielding of the thermionic and field emission cathodes are presented. The estimated electric field on the cathode is 0.2·105 V/cm at the anode voltage of 20 kV. Experimental investigations of the current-voltage characteristic of an electron gun with impregnated flat cathode in a pulsed mode are conducted, where the collector current is 100 mA.

Determination of the ReA Electron Beam Ion Trap electron beam radius and current density with an X-ray pinhole camera.

The Electron Beam Ion Trap (EBIT) of the National Superconducting Cyclotron Laboratory at Michigan State University is used as a charge booster and injector for the currently commissioned rare isotope re-accelerator facility ReA. This EBIT charge breeder is equipped with a unique superconducting magnet configuration, a combination of a solenoid and a pair of Helmholtz coils, allowing for a direct observation of the ion cloud while maintaining the advantages of a long ion trapping region. The current density of its electron beam is a key factor for efficient capture and fast charge breeding of continuously injected, short-lived isotope beams. It depends on the radius of the magnetically compressed electron beam. This radius is measured by imaging the highly charged ion cloud trapped within the electron beam with a pinhole camera, which is sensitive to X-rays emitted by the ions with photon energies between 2 keV and 10 keV. The 80%-radius of a cylindrical 800 mA electron beam with an energy of 15 keV is determined to be r(80%) = (212 ± 19)μm in a 4 T magnetic field. From this, a current density of j = (454 ± 83)A/cm(2) is derived. These results are in good agreement with electron beam trajectory simulations performed with TriComp and serve as a test for future electron gun design developments.

Velocity bunching in travelling wave accelerator with low acceleration gradient

We present the analytical and simulated results of our study of the influence of the acceleration gradient in the velocity bunching process, which is a bunch compression scheme that uses a travelling wave accelerating structure as a compressor. Our study shows that the bunch compression application with low acceleration gradient is more tolerant to phase jitter and more successful in obtaining a compressed electron beam with symmetrical longitudinal distribution and low energy spread. We also present a transverse emittance compensation scheme to compensate the emittance growth caused by the increase of the space charge force in the compressing process, which is easy to adjust for different compression factors.

Specific features of accumulating electrons in high-current trap

A symmetric dynamics of the virtual cathode (VC) formation in an electron beam and a scenario for attaining a compressed electron beam state have been numerically simulated. The parameters of this process are determined, including the moment of VC appearance and the total amount of charge accumulated by this moment. The established symmetric scenario qualitatively differs from those previously described in the literature.

Characterization and mitigation of coherent-optical-transition-radiation signals from a compressed electron beam

The Advanced Photon Source (APS) injector complex includes an option for rf photocathode (PC) gun beam injection into the 450-MeV S-band linac. At the 150-MeV point, a four-dipole chicane was used to compress the micropulse bunch length from a few ps to sub-0.5 ps (FWHM). Noticeable enhancements of the optical transition radiation (OTR) signal sampled after the APS chicane were then observed as has been reported in the Linac Coherent Light Source (LCLS) injector commissioning. A far-infrared (FIR) coherent transition radiation detector and interferometer were used to monitor the bunch compression process and correlate the appearance of localized spikes of OTR signal (5 to 10 times brighter than adjacent areas) within the beam-image footprint. We have performed spectral-dependency measurements at 375 MeV with a series of bandpass filters centered in 50-nm increments from 400 to 700 nm and with an imaging spectrometer and observed a broadband enhancement in these spikes. Mitigation concepts of the observed coherent OTR, which exhibits an intensity enhancement in the red part of the visible spectrum as compared to incoherent OTR, are described.

Observation of coherent terahertz edge radiation from compressed electron beams

Coherent radiation emitted from a compressed electron bunch as it traverses the sharp edge regions of a magnetic chicane has been investigated at the Brookhaven National Laboratory Accelerator Test Facility. Electron beam measurements using coherent transition radiation interferometry indicate a 100 fs rms bunch accompanied by distinct distortions in energy spectrum due to strong self-fields. These self-fields are manifested in emitted high power THz radiation, which displays signatures of the phenomenon known as coherent edge radiation. Radiation characterization studies undertaken include spectral analysis, far-field intensity distribution, polarization, and dependence on the electron bunch length. The observed aspects of the beam and radiation allow detailed comparisons with start-to-end simulations.

Measuring the virtual cathode velocity

An electron beam with a virtual cathode (VC) transported in a two-section drift tube of a SINUS-7 high-current electron accelerator has been studied. The dependence of the VC velocity on the injected current has been experimentally determined for the first time. It is established that the VC motion in the drift tube is accompanied by microwave oscillations, which are caused by transient processes involved in the formation of a compressed electron beam.

Simulation of electron-optical forming systems with highly compressed electron beams

A method of synthesis is developed, equations are derived, and the results are obtained via this method for the simulation of systems that form high-intensity electron beams with a high compression. The relatively high compression of a beam is realized owing to both a faster (in comparison with a variation in the accompanying magnetic field) increase in the axial component of the magnetic field at a certain plane in the gun region and to selection of the length of the region in the gun where the potential increases. Electron-optical systems with beams whose structure is substantially affected by electron thermal velocities are considered. Relationships that allow determination of the initial parameters of the beam and the focusing magnetic field during the synthesis of such systems are presented.

Validity of fixing a zero forming electrode at a non-Pierce angle for optimization of a highly compressed electron beam

Substantiation of applying a zero forming electrode with a non-Pierce angle for optimal beam compression is shown to rely on an invalid theoretical model, the use of which may lead to a confusing situation in which the intrinsic properties of the calculation algorithm are analyzed rather than the real physics of the phenomenon under investigation.

Compression and microbunching of electron beams by ultra-intense laser pulses

The formation of coherent structures, induced by a super-intense plane electromagnetic wave with a sharp rising edge in an ensemble of electrons (electron beam) in vacuum, is considered. The theory describing this process is elaborated. It is shown that the laser pulse can strongly compress the electron beam and also generate fast density modulations (microbunching) in it. Depending on the duration of a laser pulse front, two harmonics can be present simultaneously in longitudinal density modulations of the electron beam—one with laser wavelength and the other with half of the laser wavelength. By changing the form of the laser pulse envelope, one can control the average density of the electron beam (slow density modulation). By varying the laser pulse amplitude and initial length of the electron beam, it is possible to change the number of microbunches in the compressed electron beam, and for certain conditions only one electron bunch can be produced with ultrashort length smaller than the laser wavelength (attosecond length electron beam). The results of the theory are compared with 1D PIC (particle-in-cell) simulations and a good agreement is found.

High-energy scaling of Compton scattering light sources

No monochromatic ($\ensuremath{\Delta}{\ensuremath{\omega}}_{x}/{\ensuremath{\omega}}_{x}l1%$), high peak brightness [$g{10}^{20}\text{ }\mathrm{photons}/(\mathrm{m}{\mathrm{m}}^{2}\ifmmode\times\else\texttimes\fi{}{\mathrm{mrad}}^{2}\ifmmode\times\else\texttimes\fi{}\mathrm{s}\ifmmode\times\else\texttimes\fi{}0.1%\text{ }\mathrm{bandwidth})$], tunable light sources currently exist above 100 keV. Important applications that would benefit from such new hard x-ray and $\ensuremath{\gamma}$-ray sources include the following: nuclear resonance fluorescence spectroscopy and isotopic imaging, time-resolved positron annihilation spectroscopy, and MeV flash radiography. In this paper, the peak brightness of Compton scattering light sources is derived for head-on collisions and found to scale quadratically with the normalized energy, $\ensuremath{\gamma}$; inversely with the electron beam duration, $\ensuremath{\Delta}\ensuremath{\tau}$, and the square of its normalized emittance, $\ensuremath{\epsilon}$; and linearly with the bunch charge, $e{N}_{e}$, and the number of photons in the laser pulse, ${N}_{\ensuremath{\gamma}}\mathrm{\text{:}}\text{ }{\stackrel{^}{B}}_{x}\ensuremath{\propto}{\ensuremath{\gamma}}^{2}{N}_{e}{N}_{\ensuremath{\gamma}}/{\ensuremath{\epsilon}}^{2}\ensuremath{\Delta}\ensuremath{\tau}$. This ${\ensuremath{\gamma}}^{2}$ scaling shows that for low normalized emittance electron beams (1 nC, $1\text{ }\text{ }\mathrm{mm}\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{mrad}$, $l1\text{ }\text{ }\mathrm{ps}$, $g100\text{ }\text{ }\mathrm{MeV}$), and tabletop laser systems ($1--10\text{ }\mathrm{J}$, 5 ps) the x-ray peak brightness can exceed ${10}^{23}\text{ }\mathrm{photons}/(\mathrm{m}{\mathrm{m}}^{2}\ifmmode\times\else\texttimes\fi{}{\mathrm{mrad}}^{2}\ifmmode\times\else\texttimes\fi{}\mathrm{s}\ifmmode\times\else\texttimes\fi{}0.1%\text{ }\mathrm{bandwidth})$ near $\ensuremath{\hbar}{\ensuremath{\omega}}_{x}=1\text{ }\mathrm{MeV}$; this is confirmed by three-dimensional codes that have been benchmarked against Compton scattering experiments performed at Lawrence Livermore National Laboratory. The interaction geometry under consideration is head-on collisions, where the x-ray flash duration is shown to be equal to that of the electron bunch, and which produce the highest peak brightness for compressed electron beams. Important nonlinear effects, including spectral broadening, are also taken into account in our analysis; they show that there is an optimum laser pulse duration in this geometry, of the order of a few picoseconds, in sharp contrast with the initial approach to laser-driven Compton scattering sources where femtosecond laser systems were thought to be mandatory. The analytical expression for the peak on-axis brightness derived here is a powerful tool to efficiently explore the 12-dimensional parameter space corresponding to the phase spaces of both the electron and incident laser beams and to determine optimum conditions for producing high-brightness x rays.

Highly charged ions produced in a warm electron beam ion trap

A compact electron beam ion trap (WEBIT) working at room temperature without any cryogenic components is described and experimentally investigated. The trap design is based on permanent magnet technology. For the formation of the electron beam a Pierce electron gun equipped with a cathode of high emissivity is used. The ion trap is created by a compressed electron beam passing through a drift tube system consisting of three sections with corresponding electrical trap potentials. X-ray spectra measured with a Si(Li) semiconductor detector indicate the production of Kr34+, Xe44+, Ce48+, Ir64+, and Hg66+ ions.

Studies of a magnetically compressed electron beam

An experimental investigation of the problem of magnetically compressing a perveance 0.54 \times 10^{-6} A/V3/2beam is reported. The feasibility of magnetic compression is demonstrated by passing this beam, compressed in area by a factor of 1000, through a drift tube 0.014 inch in diameter and 1 inch long with 95 percent transmission. The success of the magnetic-compression technique rests on the hitherto unsuspected ability of the beam to damp out undulations and to lose its thermal structure within a relatively short distance. A semiempirical theory from which the field required to maintain the compressed beam can be calculated is also presented.

Studies of a magnetically compressed electron beam

Studies of a magnetically compressed electron beam

Scalloping Behaviour of Compressed Electron Beams in Varying Magnetic and Electric Fields†

ABSTRACT The scalloping behaviour of electron beams in rapidly increasing magnetic fields is studied. It is shown that in order to reduce the scalloping on these compressed beams, it is necessary that the beam see a relatively slow varying magnetic field upon being launched into it. The magnetic field can rise quite fast to almost 80-90% of the final value in the intermediate region followed by another region of smooth variation on to the constant field in the interaction region. The beam scalloping is critically dependent on the magnetic field format and increases rapidly also if the entrance conditions are ‘imperfect’. For compressed Brillouin beams, the beam ripple rises B√f/B√l times faster than for constant-field beams, for given deviation from ideal launching conditions. For backward-wave oscillator applications where it is necessary to vary the beam voltage, it is advantageous to accelerate the beam at the end of magnetic compression. Magnetically compressed Brillouin beams are shown capable of fur...