Sub-picosecond Electron Pulses Research Articles

Simulation of the response of a diamond-based radiation detector to ultra-short and intense high-energy electron pulses

Single-crystal synthetic diamond sensors have been widely used in radiation dosimetry and beam diagnostics. The foreseen harsh radiation environment in electron-positron colliders at the luminosity frontier requires a thorough investigation of diamond’s response to large radiation bursts, in particular, to intense high-energy electron pulses. In this article, a two-step numerical simulation approach (Sentaurus + LTspice) is proposed to explore this topic. The time response of the diamond detector is simulated using TCAD-Sentaurus while the transmission effect of the electronic circuit is taken into account using LTspice. Good agreement is observed between results of the numerical simulation and preliminary experimental data from detector’s exposure to high-energy sub-picosecond electron pulses, on both the amplitude and the shape of the induced signals. This simulation combination is a novel approach to designing and optimizing diamond detectors for radiation and beam loss monitoring in particle physics experiments.

Evolution of picosecond surface electric fields generated by photon-induced charge emission from La0.67 Sr0.33MnO3 films

In recent years, transient electric field is becoming a means for the generation of hidden states or phase transitions. In this study, the spatial-temporal distribution of picosecond surface electric fields, associated with the femtosecond-laser-induced charge emission from ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_{3}$ (LSMO) single-crystal films, were interrogated by subpicosecond electron pulses and reconstructed by a three-layer theoretical model. A strong picosecond surface electric field, on the order of 100 kV/m, was generated on LSMO films. Such self-induced transient electric fields may be a promising means for controlling ultrafast processes in materials.

Design and simulation of a LINAC for high energy electron radiography research

We designed an electron linear accelerator (LINAC) for High Energy Electron Radiography (HEER) studies, which consists of two different guns, either a thermionic cathode or a photocathode RF gun, for diagnosing different targets. In this paper, we present the simulation studies and optimization of the LINAC based on an S-band thermionic RF gun. The LINAC is capable of generating sub-picosecond electron pulses with low energy spread (0.13%) and large micro-bunch charge (143 pC). The beam dynamics of the LINAC is studied and optimized by different beam dynamics simulation codes. The entrance hole induced field distortion in the alpha magnet is also reported. The space charge effects are studied by the General Particle Tracer (GPT) code. The results showed that the space charge forces had a significant influence on beam dynamics in such a low energy LINAC.

Proof-of-Principle Experiment of Velocity Bunching for Ultra-Short Electron Pulse Production

An accelerator test facility, t-ACTS, established at Research Center for Electron Photon Science, Tohoku University, equips an injector consisted with a thermionic RF gun together with an energy filter and a 3 m traveling wave accelerating structure. A long-period undulator has been also installed for provide THz superradiance. Velocity bunching scheme proposed by Serafini and Ferrario is employed for ultra-short electron pulses production. A non-relativistic electron bunch, which is slightly slower than the velocity of light, is injected into the accelerating structure, and then the longitudinal phase space of the bunch is being rotated during acceleration. According to a numerical simulation, ∼ 50 fs bunch can be produced by using the t-ACTS accelerator configuration. Proof-of-principle experiment of velocity bunching has been carried out by observing sub-picosecond electron pulse using a streak camera. We have succeeded in producing a sub-picosecond electron pulse in the t-ACTS. The details of the experiment for ultra-short electron pulse are described in this paper.

Simultaneous investigation of ultrafast structural dynamics and transient electric field by sub-picosecond electron pulses

The ultrafast structure dynamics and surface transient electric field, which are concurrently induced by laser excited electrons of an aluminum nanofilm, have been investigated simultaneously by the same transmission electron diffraction patterns. These two processes are found to be significantly different and distinguishable by tracing the time dependent changes of electron diffraction and deflection angles, respectively. This study also provides a practical means to evaluate simultaneously the effect of transient electric field during the study of structural dynamics under low pump fluence by transmission ultrafast electron diffraction.

Generation of a comb electron beam to drive SASE FEL radiation spikes

A radiofrequency electron gun followed by a compressor can generate trains of subpicosecond electron pulses by illuminating the photocathode with a comb laser pulse. This kind of electron beams can generate trains of single radiation spikes in a SASE-FEL. The dynamics of different electron beam trains traveling in an accelerator is investigated by PARMELA simulations. A set of parameters relative to the SPARC machine are studied with the intent of generating a train of single radiation spikes in a 500 nm SASE-FEL.

Generation of short THz bunch trains in a RF photoinjector

A radiofrequency electron gun joined to a compressor can generate trains of THz sub-picosecond electron pulses by illuminating the photocathode with trains of laser pulses. Assuming a prompt electron emission, each laser pulse generates an electron disk at the cathode. The disk train evolves toward a slightly modulated slug having a peculiar sawtooth energy modulation. This kind of energy modulation is transformed into a density modulation within a velocity-bunching compressor recovering at a good extent the initial multi-disk profile. The electron beam evolution is analyzed through simulations looking at its features as function of pulse and frequency characteristics of the laser and of the accelerator parameters.

Review and analysis of autocorrelation electron bunch length measurements

Sub-picosecond electron pulses become increasingly important for the development of next generation light sources to study chemical and biological processes. Such processes occur on picosecond or sub-picosecond time scales and the ability to observe such reactions in real time is of great scientific interest. These and other research opportunities fuel large scale efforts toward the generation of femtosecond electron pulses. A critical part in this endeavor is the ability to measure and correctly analyze femtosecond pulses which is beyond the capabilities of even the most advanced streak cameras leaving a spectroscopic method as the presently most efficient way to measure such short pulses. This method is based on an autocorrelation technique performed in a Michelson interferometer. The technique, however, suffers from limited suitable optical materials available for the far infrared spectral regime. In this work the effects of such limitations and other problem areas have been investigated and their impact on femtosecond pulse measurements is discussed in detail both qualitatively and quantitatively. Furthermore, this technique can also be applied to nondestructive coherent diffraction radiation which displays its own limitations and problems as will be discussed.

Motivation and development of ultrafast laser-based accelerator techniques for chemical physics research

The products of radiation induced chemical reactions are determined by rapid primary processes such as energy transfer, thermalization and solvation. Ultrafast photoionization experiments on liquid water demonstrate that these initial events occur on time scales <5ps and involve a complicated interplay between electronic relaxation and vibrational energy redistribution. These experiments also show that the chemical processes originating from ionizing radiation are unique and cannot be reproduced by laser photons alone. Due to the lack of a suitable femtosecond source of ionizing radiation, knowledge of the primary processes in radiation chemistry remains poor. To address this issue a 20TW laser system has been constructed to obtain subpicosecond electron pulses with energies in the 1–10MeV range. In addition to the production of femtosecond electron pulses, future efforts will be directed towards using this laser for accelerating heavier particles such as protons and generating hard X-rays.

Femtosecond electron bunches from an RF-gun

Sub-picosecond electron pulses become a tool of increasing importance to study dynamics at an atomic level. Such electron pulses can be used directly or be converted into intense coherent far infrared radiation or equally short X-ray pulses. In principle, sub-picosecond electron pulses can be obtained in large, high-energy electron linear accelerator systems by repeatedly applying an energy slew and magnetic compression . Another process is the production of short electron pulses at low energies from an RF-gun with a thermionic cathode together with a bunch compressing α -magnet. In this paper, we present a systematic analysis of capabilities and limits of sub-picosecond electron pulses from such a source. We discuss particular parameter choices as well as the impact of geometric and electric specifications on the 6-dimensional phase space electron distribution . Numerical beam simulations with the computer code PARMELA are performed including effects and limitations due to space charge forces. While the production of femtosecond electron bunches is of primary concern, we also consider the preservation of such short bunches along a beam transport line.

Femtosecond electron bunches from an RF-gun

Sub-picosecond electron pulses become a tool of increasing importance to study dynamics at an atomic level. Such electron pulses can be used directly or be converted into intense coherent far infrared radiation or equally short X-ray pulses. In principle, sub-picosecond electron pulses can be obtained in large, high-energy electron linear accelerator systems by repeatedly applying an energy slew and magnetic compression. Another process is the production of short electron pulses at low energies from an RF-gun with a thermionic cathode together with a bunch compressing α-magnet. In this paper, we present a systematic analysis of capabilities and limits of sub-picosecond electron pulses from such a source. We discuss particular parameter choices as well as the impact of geometric and electric specifications on the 6-dimensional phase space electron distribution. Numerical beam simulations with the computer code PARMELA are performed including effects and limitations due to space charge forces. While the production of femtosecond electron bunches is of primary concern, we also consider the preservation of such short bunches along a beam transport line.

Alignment of CS2 in intense nanosecond laser fields probed by pulsed gas electron diffraction

A pulsed gas electron diffraction apparatus was developed and applied to investigate an alignment process of molecules in intense laser fields. A two-dimensional (2D) electron diffraction pattern of jet-cooled CS2 in intense nanosecond laser fields (1064 nm, ∼0.64 TW/cm2, 10 ns) was measured using short-pulsed 25 keV electron beam packets (∼7 ns) generated by irradiating a tantalum photocathode with the 4th harmonics of pulsed YAG laser light. The observed anisotropic 2D diffraction pattern was analyzed quantitatively by taking into account the spatio-temporal distributions of the laser pulses, the electron beam packets, and the molecular beam through a numerical simulation of the observed diffraction pattern. The anisotropy of the spatial distribution of molecular axes of CS2 in the laser polarization direction is accounted for by the effect of the intense laser fields. Considering the spatio-temporal averaging effect, the temporal pulse width of an electron packet required for real-time probing of the alignment process of molecules in intense nanosecond laser fields is discussed. A numerical simulation of temporal and spatial profiles of an electron packet is also performed to examine conditions for generating sub-picosecond ultrashort electron pulse for real-time probing of ultrafast molecular dynamics by the pulsed gas electron diffraction method.

Diagnostics of sub-picosecond electron pulse using the fluctuation method

Numerical analysis and measurement of longitudinal bunch length of subpico- and picosecond electron beams by the fluctuation method has been carried out at the S-band twin linear accelerators at Nuclear Engineering Research Laboratory (NERL), University of Tokyo. The diagnostics method utilizes a statistical analysis of shot-noise-driven fluctuations in incoherent radiation. Measurements were performed on Cherenkov radiation emitted by ∼ 1.0 ps (FWHM), 20 MeV electron bunch. As a result, the fluctuation method indicated 5 times longer than that by the streak camera. The discrepancy was caused by neglecting the influence of the transverse emittance. Then, the influence of the transverse profile upon the measurement was analyzed and discussed.

Electron pulse broadening due to space charge effects in a photoelectron gun for electron diffraction and streak camera systems

The electron pulse broadening and energy spread, caused by space charge effects, in a photoelectron gun are studied analytically using a fluid model. The model is applicable in both the photocathode-to-mesh region and the postanode electron drift region. It is found that space charge effects in the photocathode-to-mesh region are generally unimportant even for subpicosecond pulses. However, because of the long drift distance, electron pulse broadening due to space charge effects in the drift region is usually significant and could be much larger than the initial electron pulse duration for a subpicosecond electron pulse. Space charge effects can also lead to a considerable electron energy spread in the drift region. Temporal broadening is calculated for an initial electron pulse as short as 50 fs with different electron densities, final electron energies, and drift distances. The results can be used to design electron guns producing subpicosecond pulses for streak cameras as well as for time resolved electron diffraction.

Development of a pump-probe facility combining a far-infrared source with laser-like characteristics and a VUV free electron laser

The TESLA Test Facility (TTF) at DESY is a facility producing sub-picosecond electron pulses for the generation of VUV or soft X-ray radiation in a free electron laser (FEL). The same electron pulses would also allow the direct production of high-power coherent radiation by passing the electron beam through an undulator. Intense, coherent far-infrared (FIR) undulator radiation can be produced from electron bunches at wavelengths longer than or equal to the bunch length. The source described in this paper provides, in the wavelength range 50– 300 μm , a train of about 1– 10 ps long radiation pulses, with about 1 mJ of optical energy per pulse radiated into the central cone. The average output power can exceed 50 W . In this conceptual design, we intend to use a conventional electromagnetic undulator with a 60 cm period length and a maximum field of 1.5 T . The FIR source will use the spent electron beam coming from the VUV FEL which allows one to significantly extend the scientific potential of the TTF without interfering with the main option of the TTF FEL operation. The pulses of the coherent FIR radiation are naturally synchronized with the VUV pulses from the main TTF FEL, enabling pump-probe techniques using either the FEL pulse as a pump or the FIR pulse as a probe, or vice versa.

High-quality beam generation using an RF gun and a 150 MeV microtron

Low-emittance sub-picosecond electron pulses are expected to be used in a wide field, such as free electron laser, laser acceleration, femtosecond X-ray generation by Inverse Compton scattering, pulse radiolysis, etc. In order to produce the low-emittance sub-picosecond electron pulse, we are developing a compact Racetrack Microtron (RTM) with a new 5 MeV injection system adopting a laser photo cathode RF gun (Washio et al., Seventh China–Japan Bilateral Symposium on Radiation Chemistry, October 28, Cengdu, China, 1996). The operation of RTM has been kept under a steady state of beam loading for long pulse mode so far (Washio et al., J. Surf. Sci. Soc. Jpn. 19 (2) (1998) 23). In earlier work (Washio et al., PAC99, March 31, New York, USA, 1999), we have succeeded in the numerical simulation for the case of single short pulse acceleration. Finally, the modified RTM was demonstrated as a useful accelerator for a picosecond electron pulse generation under a transient state of beam loading. In the simulation, a picosecond electron pulse was accelerated to 149.6 MeV in RTM for the injection of 5 MeV electron bunch with a pulse length of 10 ps (FWHM), a charge of 1 nC per pulse, and an emittance of 3 πmm mrad.

Subpicosecond electron single-beam diagnostics by a coherent transition radiation interferometer and a streak camera

Abstract Longitudinal bunch distributions of subpicosecond and picosecond electron beams have been evaluated by the coherent transition radiation (CTR) Michelson interferometer with the reconstruction procedure obtained from the interferograms. The results were compared with those obtained from the femtosecond streak camera whose time resolution is 200 fs at the full-width at half-maximum (FWHM). From the comparison, the reliability of the interferometry to evaluate the subpicosecond electron pulse, which is close to the time resolution of the femtosecond streak camera, has been discussed in detail. Further, the validity of the application of the Kramers–Kronig relations and the extrapolation of missing data in the measurements is also discussed.

Production and utilization of synchronized femtosecond electron and laser single pulses

A subpicosecond (700 fs at FWHM) electron pulse from the S-band (2.856 GHz) linear accelerator (linac) of the NERL (Nuclear Engineering Research Laboratory) was synchronized with a femtosecond (100 fs at FWHM) laser pulse from a T 3 (table-top terawatts) laser with a picosecond time whose standard deviation is 3.7 ps. Then we generated a picosecond characteristic X-ray pulse by irradiating through the electron pulse a Cu target (Kα, 8.1 keV, 1.54Å) and obstained the Bragg diffraction from a NaCl ionic monocrystal using a high sensitivity X-ray imaging plate. Further, we discuss its applications to observe lattice vibration of the monocrystal by using the synchronized laser (pump) and X-ray (probe).

Observation of SASE at 47 μm

Abstract Coherent, far-infrared undulator radiation from sub-picosecond electron pulses has been observed at the Stanford SUNSHINE facility. Measured intensities exceed theoretical prediction for spontaneous radiation by more than an order of magnitude. The forward-radiated energy from a 16 MeV electron beam travelling in a single pass through an undulator with a strength parameter K = 0.6 grows exponentially and is consistent with predictions of Self-Amplified Spontaneous Emission (SASE) with a gain length of 45.4 cm or 5.9 undulator periods.

Measurement of subpicosecond electron pulses.

A bunch-length measuring method has been developed to measure the subpicosecond electron pulses generated at the Stanford University Short Intense Electron Source (SUNSHINE) facility. This method utilizes a far-infrared Michelson interferometer to measure coherent transition radiation emitted at wavelengths longer than or equal to the bunch length via optical autocorrelation. To analyze the measurement, a simple and systematic way has also been developed, which considers interference effects on the interferogram caused by the beam splitter; hence the electron bunch length can be easily obtained from the measurement. This simple, low-cost, frequency-resolved autocorrelation method demonstrates subpicosecond resolving power that cannot be achieved by existing time-resolved methods.