Picosecond Electron Bunches Research Articles

Photoemission of Picosecond Electron Bunches with Large Charge in RF Guns

Photoemission of Picosecond Electron Bunches with Large Charge in RF Guns

An Improved Model for Photoemission of Space Charge Dominated Picosecond Electron Bunches: Theory and Experiment

An Improved Model for Photoemission of Space Charge Dominated Picosecond Electron Bunches: Theory and Experiment

Measurement of bunch length and temporal distribution using accelerating radio frequency cavity in low-emittance injector

We demonstrate an experimental methodology for measuring the temporal distribution of pico-second level electron bunch with low energy using radial electric and azimuthal magnetic fields of an accelerating (hbox {TM}_{01} mode) radio frequency (RF) cavity that is used for accelerating electron beams in a linear accelerator. In this new technique, an accelerating RF cavity provides a phase-dependent transverse kick to the electrons, resulting in the linear coupling of the trajectory angle with the longitudinal position inside the bunch. This method does not require additional devices on the beamline since it uses an existing accelerating cavity for the projection of the temporal distribution to the transverse direction. We present the theoretical basis of the proposed method and validate it experimentally in the compact-energy recovery linac accelerator at KEK. Measurements were demonstrated using a 2-cell superconducting booster cavity with a peak on-axis accelerating field (E_0) of 7.21 MV/m.

Time-resolved cathodoluminescence of DNA triggered by picosecond electron bunches

Despite the tremendous importance of so-called ionizing radiations (X-rays, accelerated electrons and ions) in cancer treatment, most studies on their effects have focused on the ionization process itself, and neglect the excitation events the radiations can induce. Here, we show that the excited states of DNA exposed to accelerated electrons can be studied in the picosecond time domain using a recently developed cathodoluminescence system with high temporal resolution. Our study uses a table-top ultrafast, UV laser-triggered electron gun delivering picosecond electron bunches of keV energy. This scheme makes it possible to directly compare time-resolved cathodoluminescence with photoluminescence measurements. This comparison revealed qualitative differences, as well as quantitative similarities between excited states of DNA upon exposure to electrons or photons.

Direct Observation of Spatiotemporal Dynamics of Short Electron Bunches in Storage Rings.

In recent synchrotron radiation facilities, the use of short (picosecond) electron bunches is a powerful method for producing giant pulses of terahertz coherent synchrotron radiation. Here we report on the first direct observation of these pulse shapes with a few picoseconds resolution, and of their dynamics over a long time. We thus confirm in a very direct way the theories predicting an interplay between two physical processes. Below a critical bunch charge, we observe a train of identical THz pulses (a broadband Terahertz comb) stemming from the shortness of the electron bunches. Above this threshold, a large part of the emission is dominated by drifting structures, which appear through spontaneous self-organization. These challenging single-shot THz recordings are made possible by using a recently developed photonic time stretch detector with a high sensitivity. The experiment has been realized at the SOLEIL storage ring.

Simulation of Coherent Diffraction Radiation Generation by Pico-Second Electron Bunches in an Open Resonator

In this report we present new approach for calculation of processes of diffraction radiation generation, storage and decay in an open resonator based on generalized surface current method. The radiation characteristics calculated using the developed approach were compared with those calculated using Gaussian-Laguerre modes method. The comparison shows reasonable coincidence of the results that allows to use developed method for investigation of more complicated resonators.

Theoretical analysis and simulation of the influence of self-bunching effects and longitudinal space charge effects on the propagation of keV electron bunch produced by a novel S-band Micro-Pulse electron Gun

As an important electron source, Micro-Pulse electron Gun (MPG) which is qualified for producing high average current, short pulse, low emittance electron bunches steadily holds promise to use as an electron source of Coherent Smith-Purcell Radiation (CSPR), Free Electron Laser (FEL). The stable output of S-band MPG has been achieved in many labs. To establish reliable foundation for the future application of it, the propagation of picosecond electron bunch produced by MPG should be studied in detail. In this article, the MPG which was working on the rising stage of total effective Secondary Electron Yield (SEY) curve was introduced. The self-bunching mechanism was discussed in depth both in the multipacting amplifying state and the steady working state. The bunch length broadening induced by the longitudinal space-charge (SC) effects was investigated by different theoretical models in different regions. The 2D PIC codes MAGIC and beam dynamic codes TraceWin simulations were also performed in the propagation. The result shows an excellent agreement between the simulation and the theoretical analysis for bunch length evolution.

Mapping transient electric fields with picosecond electron bunches

Transient electric fields, which are an important but hardly explored parameter of laser plasmas, can now be diagnosed experimentally with combined ultrafast temporal resolution and field sensitivity, using femtosecond to picosecond electron or proton pulses as probes. However, poor spatial resolution poses great challenges to simultaneously recording both the global and local field features. Here, we present a direct 3D measurement of a transient electric field by time-resolved electron schlieren radiography with simultaneous 80-μm spatial and 3.7-ps temporal resolutions, analyzed using an Abel inversion algorithm. The electric field here is built up at the front of an aluminum foil irradiated with a femtosecond laser pulse at 1.9 × 10(12) W/cm(2), where electrons are emitted at a speed of 4 × 10(6) m/s, resulting in a unique "peak-valley" transient electric field map with the field strength up to 10(5) V/m. Furthermore, time-resolved schlieren radiography with charged particle pulses should enable the mapping of various fast-evolving field structures including those found in plasma-based particle accelerators.

Generation of picosecond electron bunches in storage rings.

Approaches to generating short X-ray pulses in synchrotron light sources are discussed. In particular, the method of using a superconducting harmonic cavity to generate simultaneously long and short bunches in storage rings and the approach of injecting short bunches from a linac injector into a storage ring for multi-turn circulation are emphasized. If multi-cell superconducting RF (SRF) cavities with frequencies of ∼1.5 GHz can be employed in storage rings, it would be possible to generate stable, high-flux, short-pulse X-ray beams with pulse lengths of 1-10 ps (FWHM) in present or future storage rings. However, substantial challenges exist in adapting today's high-gradient SRF cavities for high-current storage ring operation. Another approach to generating short X-ray pulses in a storage ring is injecting short-pulse electron bunches from a high-repetition-rate linac injector for circulation. Its performance is limited by the microbunching instability due to coherent synchrotron radiation. Tracking studies are carried out to evaluate its performance. Challenges and operational considerations for this mode are considered.

Electron diffraction from an ultracold source

Picosecond electron bunches extracted from a laser-cooled gas cloud could be a new tool for macromolecular studies.

Picosecond electron bunches from GaAs/GaAsP strained superlattice photocathode

GaAs/GaAsP strained superlattices are excellent candidates for use as spin-polarized electron sources. In the present study, picosecond electron bunches were successfully generated from such a superlattice photocathode. However, electron transport in the superlattice was much slower than in bulk GaAs. Transmission electron microscopy observations revealed that a small amount of variations in the uniformity of the layers was present in the superlattice. These variations lead to fluctuations in the superlattice mini-band structure and can affect electron transport. Thus, it is expected that if the periodicity of the superlattice can be improved, much faster electron bunches can be produced.

High-coherence picosecond electron bunches from cold atoms

Ultrafast electron diffraction enables the study of molecular structural dynamics with atomic resolution at subpicosecond timescales, with applications in solid-state physics and rational drug design. Progress with ultrafast electron diffraction has been constrained by the limited transverse coherence of high-current electron sources. Photoionization of laser-cooled atoms can produce electrons of intrinsically high coherence, but has been too slow for ultrafast electron diffraction. Ionization with femtosecond lasers should in principle reduce the electron pulse duration, but the high bandwidth inherent to short laser pulses is expected to destroy the transverse coherence. Here we demonstrate that a two-colour process with femtosecond excitation followed by nanosecond photoionization can produce picosecond electron bunches with high transverse coherence. Ultimately, the unique combination of ultrafast ionization, high coherence and three-dimensional bunch shaping capabilities of cold atom electron sources have the potential for realising the brightness and coherence requirements for single-shot electron diffraction from crystalline biological samples.

Operation of a picosecond narrow-bandwidth Laser–Thomson-backscattering X-ray source

A tunable source of intense ultra-short hard X-ray pulses represents a novel tool for the structural analysis of complex systems with unprecedented temporal and spatial resolution. With the simultaneous availability of a high power short-pulse laser system this provides unique opportunities at the forefront of relativistic light–matter interactions. At Helmholtz-Zentrum Dresden-Rossendorf (HZDR) we demonstrated the principle of such a light source (PHOENIX – Photon Electron collider for Narrow bandwidth Intense X-Rays) by colliding picosecond electron bunches from the ELBE linear accelerator with counter-propagating femtosecond laser pulses from the 150TW Draco Ti:Sapphire laser system. The generated narrowband X-rays are highly collimated and can be reliably adjusted from 12keV to 20keV by tuning the electron energy (24–30MeV). Ensuring the spatial–temporal overlap at the interaction point and suppressing the Bremsstrahlung background a signal to noise ratio of greater than 300 was reached.

Multimode terahertz-wave generation using coherent Cherenkov radiation

Multimode terahertz(THz)-wave generation using coherent Cherenkov radiation (CCR) was investigated. The frequency spectra of CCR, which utilized a metal-wrapped hollow dielectric tube of 7 mm outer radius and a picosecond electron bunch of 27 MeV beam energy, were measured by a Michelson interferometer with a 4.2 K silicon bolometer. In this study, discrete spectral components at frequencies of 0.09, 0.14, and 0.36 THz were observed experimentally and explained as transverse magnetic (TM) modes of TM03, TM04, and TM09, respectively, according to a theoretical calculation for the tube.

Simple method for generating adjustable trains of picosecond electron bunches

A simple, passive method for producing an adjustable train of picosecond electron bunches is demonstrated. The key component of this method is an electron beam mask consisting of an array of parallel wires that selectively spoils the beam emittance. This mask is positioned in a high magnetic dispersion, low beta-function region of the beam line. The incoming electron beam striking the mask has a time/energy correlation that corresponds to a time/position correlation at the mask location. The mask pattern is transformed into a time pattern or train of bunches when the dispersion is brought back to zero downstream of the mask. Results are presented of a proof-of-principle experiment demonstrating this novel technique that was performed at the Brookhaven National Laboratory Accelerator Test Facility. This technique allows for easy tailoring of the bunch train for a particular application, including varying the bunch width and spacing, and enabling the generation of a trailing witness bunch.

Absolute charge calibration of scintillating screens for relativistic electron detection

We report on new charge calibrations and linearity tests with high-dynamic range for eight different scintillating screens typically used for the detection of relativistic electrons from laser-plasma based acceleration schemes. The absolute charge calibration was done with picosecond electron bunches at the ELBE linear accelerator in Dresden. The lower detection limit in our setup for the most sensitive scintillating screen (KODAK Biomax MS) was 10 fC/mm(2). The screens showed a linear photon-to-charge dependency over several orders of magnitude. An onset of saturation effects starting around 10-100 pC/mm(2) was found for some of the screens. Additionally, a constant light source was employed as a luminosity reference to simplify the transfer of a one-time absolute calibration to different experimental setups.

Absolute response of Fuji imaging plate detectors to picosecond-electron bunches

The characterization of the absolute number of electrons generated by laser wakefield acceleration often relies on absolutely calibrated FUJI imaging plates (IP), although their validity in the regime of extreme peak currents is untested. Here, we present an extensive study on the dependence of the sensitivity of BAS-SR and BAS-MS IP to picosecond electron bunches of varying charge of up to 60 pC, performed at the electron accelerator ELBE, making use of about three orders of magnitude of higher peak intensity than in prior studies. We demonstrate that the response of the IPs shows no saturation effect and that the BAS-SR IP sensitivity of 0.0081 photostimulated luminescence per electron number confirms surprisingly well data from previous works. However, the use of the identical readout system and handling procedures turned out to be crucial and, if unnoticed, may be an important error source.

Image-converter tubes for lasers and lasers for image-converter tubes (operating experience of the Photoelectronics Department, GPI, RAS)

Work in the field of electron-optical diagnostics of laser-induced rapid processes performed for almost half a century at the Photoelectronics Department at the General Physics Institute, RAS, is summed up. New femtosecond image-converter tubes have been developed, which provide the time resolution no worse than 100 fs in the slit scan (streak) regime, and photoelectron guns have been built for time-resolved studies of electron diffraction. The results of recent studies on manifold (up to fifty-fold) compression of picosecond electron bunches in nonstationary focusing fields are demonstrated.

Subpicosecond jitter in picosecond electron bunches

We have simulated and measured 60–120fs time jitter of photoelectron pulses emitted by a nitride photocathode at 100GHz rate as in order to evaluate the resolution performance of a previously proposed photonic analog to digital converter. Recently, there has been an increasing demand for high speed analog-to-digital converters (ADCs) for microwave bandwidth signals. State of the art electronic ADCs have reached 10Gigasamples∕second (GS/s), 6–12bit performance [P. W. Juodawlkis, J. C. Twichell, G. E. Betts, J. J. Hargreaves, R. D. Younger, J. L. Wasserman, F. J. O’Donnell, K. G. Ray, and R. C. Williamson, IEEE Microwave Theory Tech. 49, 1840 (2001)]. We have previously introduced a photoelectronic ADC implementation with measured performance of 3bits at 100GS∕s [K. Ioakeimidi, R. Leheny, S. Gradinaru, K. Ma, R. Aldana, J. Clendenin, J. S Harris, R. F. W. Pease, IEEE Trans. Microwave Theory Tech. (to be published); Conference on Lasers and Electro-Optics, Baltimore MD, 1–6 June 2003]. The basic operating principle of the ADC is based on a miniaturized cathode ray tube where a bunch of photoemitted electrons passing through an electric deflection system is directed to a specific detector whence a digital code word emanates. The electron bunch samples the analog deflecting voltage that is then quantized according to the position of the detector receiving the bunch. The fundamental limit of the number of distinguishable voltage levels is the ratio of the deflecting voltage to the energy spread due to diffraction of the electron beam. This allows for up to 12bits at 100GS∕s [R. F. Pease, K. Ioakeimidi, R. Aldana, and R. Leheny, J. Vac. Sci. Technol. B 21, 2826 (2003)]. At a more practical level, the bit resolution is primarily limited by the uncertainty in the emission of each electron bunch (temporal jitter). For 100fs time uncertainty 5bits of resolution are attainable with the nitride cathode for a 50GHz bandwidth analog signal.

Diagnostics of ultrashort electron bunch by coherent transition/diffraction radiation

Measurements of longitudinal bunch length of picosecond electron bunches have been performed by the femtosecond streak camera, the interferometer and the polychromator and their results were compared with one another. As regards the radiation, transition radiation and diffraction radiation were investigated through the diagnostics. As a result, the fine agreements between them within 200 fs were obtained and the reliability of the interferometer and the polychromator for the diagnostics of less than picosecond electron bunch were verified. Furthermore, nondestructive diagnostics for subpicosecond bunches was performed utilizing coherent diffraction radiation (CDR) interferometry.