Ultrashort Electron Beam Research Articles

Advanced diagnosis of the temporal characteristics of ultra-short electron beams

Monitoring the temporal structure of an ultra-short electron beam is an indispensable function in order to tune a machine to obtain a highly qualified beam for a recent sophisticated accelerator, such as an X-ray free electron laser (XFEL), and to maintain stable X-ray laser operation. For this purpose, various instruments, such as an HEM11-mode RF beam deflector (RFDEF), a screen monitor (SCM), an electro-optic (EO) sampling method that uses a ZnTe crystal, and a beam position monitor (BPM) have been developed. The SCM that is used to observe the deflected beam image has a position resolution of 2.5 μm, which corresponds to a temporal resolution of 0.5 fs and it is installed at a position 5 m downstream from the RFDEF. The EO sampling method showed the ability to observe an electron bunch length for up to 300 fs (FWHM) at the SCSS test accelerator. The phase reference cavity of the BPM has an additional function of providing beam arrival timing information. A test for the BPM showed temporal fluctuation of 46 fs on the beam arrival timing at the test accelerator. These monitors with high temporal resolutions allow us to achieve the fine beam tuning demanded for the XFEL. The above-mentioned activities are described in this paper as a review article.

Development of a coherent THz radiation source based on the ultra-short electron beam and its applications

At the National Institute of Advanced Industrial Science and Technology (AIST), a coherent terahertz (THz) radiation source has been developed based on an ultra-short electron beam using an S-band compact electron linac. The designed THz pulse has a high peak power of more than 1 kW in the frequency range 0.1–2 THz. The entire system is located in one research room of about 10 m square. The linac consists of a laser photocathode rf gun (BNL type) with a Cs 2Te photocathode load-lock system and two 1.5-m-long S-band accelerator tubes. The electron beam can be accelerated up to approximately 42 MeV. The electron bunch was compressed to less than 1 ps (rms) with a magnetic bunch compressor. The coherent synchrotron radiation (CSR) of the THz region was generated from the ultra-short electron bunch at the 90° bending magnet, and it was extracted from a z-cut quartz window for THz applications. In this work, the THz scanning transmission imaging was successfully demonstrated for measuring the freshness of a vegetable leaf over a period of time.

Generation and applications of ultra-short electron beams in energy-recovery linacs

Energy-recovery linacs (ERLs) are in operation and under development in the world for future light sources, which cover a wide range of photon energy, from THz to γ-rays, by using various kinds of photo-emission processes, undulator radiation, free-electron lasers, laser Compton scattering (LCS), and coherent radiation. Availability of ultra-short electron beams is an essential feature of ERLs for future light sources. In this paper, we provide an overview of the generation and applications of ultra-short electron beams in ERLs. Magnetic bunch compression and velocity bunching are the two schemes of ultra-short electron beam generation that are presented with the Compact ERL test facility as an example.

Ultra-short electron beams based spatio-temporal radiation biology and radiotherapy

Deeply understanding the basic mechanisms of radiation damage in vitro and on living cells, starting from the early radical and molecular processes to mutagenic DNA lesions, cell signalling, genomic instability, apoptosis, microenvironment and Bystander effects, radio sensitivity should have many practical consequences such as the customization of cancer radiotherapy or radioprotection protocols. In this context, innovative laser-plasma accelerators provide ultra-short particle beams (electrons, protons) with parameters of interest for radiation biology and medical physics. This review article approaches some complex links that exist between radiation physics of new pulsed irradiation sources and potential biomedical applications. These links concern mainly the understanding of spatio-temporal events triggered by a radiation, within a fluctuating lapse of time from the initial energy deposition to primary damages of biological interest.

High quality single shot diffraction patterns using ultrashort megaelectron volt electron beams from a radio frequency photoinjector

Single shot diffraction patterns using a 250-fs-long electron beam have been obtained at the UCLA Pegasus laboratory. High quality images with spatial resolution sufficient to distinguish closely spaced peaks in the Debye-Scherrer ring pattern have been recorded by scattering the 1.6 pC 3.5 MeV electron beam generated in the rf photoinjector off a 100-nm-thick Au foil. Dark current and high emittance particles are removed from the beam before sending it onto the diffraction target using a 1 mm diameter collimating hole. These results open the door to the study of irreversible phase transformations by single shot MeV electron diffraction.

10.1007/s11454-008-1017-2

The formation of a runaway electron beam in helium and nitrogen at a generator voltage of 25 kV is studied experimentally. At low generator voltages, an ultrashort avalanche electron beam (UAEB) is shown to form at the flat top of the voltage pulse and its delay time relative to the leading edge of the pulse may attain several tens of nanoseconds. The conditions of runaway electron beam generation depend on the pressure in the gas-filled diode. The FWHM of the beam current varies from 200 ps to several nanoseconds. Beam electron energy distributions at different pressures are obtained. It is found that, if the gap is preionized by an additional source, the UAEB generation conditions break.

Interaction of an ultrashort laser pulse and relativistic electron beam in a corrugated plasma channel

Copropagation of a laser pulse and a relativistic electron beam in a corrugated plasma channel has been proposed for the direct laser acceleration of electrons [Palastro, Phys. Rev. E 77, 036405 (2008)]. The corrugated plasma channel allows for the guiding of laser pulses composed of subluminal spatial harmonics. Phase matching between the electron beam and the spatial harmonics results in acceleration, but for high beam densities, the pulse energy can be rapidly depleted. This depletion may result in interaction times shorter than the waveguide length limited time or pulse length dephasing time. We present an analytic model and self-consistent simulations of the electron beam-laser pulse interaction. A linear dispersion relation is derived. The effect of the electron beam on the pulse after the occurrence of axial bunching is examined. Injection of axially modulated electron beams is also explored. In particular, we find that a properly phased electron beam can transfer energy to the laser pulse as an inverse process to acceleration.

Measurements and Simulations of Ultralow Emittance and Ultrashort Electron Beams in the Linac Coherent Light Source

The Linac Coherent Light Source (LCLS) is an x-ray free-electron laser project presently in a commissioning phase at the SLAC National Accelerator Laboratory. We report here on very low-emittance measurements made at low bunch charge, and a few femtosecond bunch length produced by the LCLS bunch compressors. Start-to-end simulations associated with these beam parameters show the possibilities of generating hundreds of GW at 1.5 A x-ray wavelength and nearly a single longitudinally coherent spike at 1.5 nm with 2-fs duration.

Energy distribution of runaway electrons generated by a nanosecond discharge in atmospheric-pressure air

The spectra of an ultrashort avalanche electron beam generated by a nanosecond discharge in atmospheric-pressure air were investigated. The temporal characteristics of the beam current pulses, gap voltage, and discharge current in a gas diode were measured with a time resolution of ∼0.1 ns. A simple technique was developed for recovering electron spectra from the curves of beam attenuation by aluminum foils. The effect of the cathode design, electrode gap length, and generator parameters on the electron spectra were studied using seven setups. It is shown that generation of electrons with anomalously high energies requires the use of cathodes with increased curvature radius.

Electron flux spatial distribution in an ultrashort avalanche electron beam generated at atmospheric air pressure

The results of experimental study on generation of ultrashort avalanche electron beams (UAEB) in gas-filled diodes are considered. The spatial distribution of the flux of runaway electrons and X-rays generated in the gas diode fed by nanosecond high-voltage pulses was studied. It was shown that the UAEB in the gas-filled diode (at an air pressure of 1 atm) with sharply nonuniform electric field is generated from the interelectrode region into a solid angle exceeding 2π sr. Narrowing of the cathode-anode gap results in a decrease in the current amplitude of the beam generated to side walls of the gas diode and an increase in the beam current pulse duration in both axial and radial directions. Current pulses of the beam initiated from the side surface of the tubular cathode were detected.

Nanosecond discharge in sulfur hexafluoride and the generation of an ultrashort avalanche electron beam

A discharge in the presence of a nonuniform electric field and the generation of an ultrashort avalanche electron beam (UAEB) are studied in the insulating gas SF6 at the pressures 0.01–2.50 atm. High-voltage nanosecond pulses (about 150 and 250 kV) and the voltage pulses with an amplitude of 25 kV and a duration of tens of nanoseconds are applied across the gap. An electron beam is obtained behind the AlBe foil with a thickness of 45 μm at a sulfur hexafluoride pressure in a gas-filled diode of up to 2 atm. It is demonstrated that, at relatively high pressures (greater than 1 atm) and in the presence of high-voltage nanosecond pulses across the gap, the UAEB pulse FWHM increases. The spectra of the diffuse and contracted discharges in sulfur hexafluoride are measured.

Effect of a transverse magnetic field on the generation of electron beams in the gas-filled diode

The effect of a transverse magnetic field (0.080 and 0.016 T) on generation of an electron beam in the gas-filled diode is experimentally investigated. It is shown that, at voltage U = 25 kV across the diode and a low helium pressure (45 Torr), the transverse magnetic field influences the beam current amplitude behind a foil and its distribution over the foil cross section. At elevated pressures and under the conditions of ultrashort avalanche electron beam formation in helium, nitrogen, and air, the transverse magnetic field (0.080 and 0.016 T) has a minor effect on the amplitude and duration of the beam behind the foil. It is established that, when the voltage of the pulse generator reaches several hundreds of kilovolts, some runaway electrons (including the electrons from the discharge plasma near the cathode) are incident on the side walls of the diode.

Ponderomotive scattering of electrons and its application to measure the pulse duration of ultrafast electron beams

We numerically study the relativistic ponderomotive scattering of an ultrafast electron beam with a focused high-intensity laser pulse using a three-dimensional test-particle code. The temporal evolution of the spatial distribution and the energy spectrum of the scattered electron beam are investigated. The method using the ponderomotive scattering effect to measure the pulse duration of an ultrashort electron beam is discussed in detail. Taking into account the group velocity mismatch, the interaction geometry, and electron energy spectrum modified by the ponderomotive forces, the pulse duration of ultrashort electron beams can be retrieved by the ponderomotive scattering cross-correlation method.

Generation of runaway electron subnanosecond pulses in nitrogen and helium at a voltage of 25 kV across the gap

The formation of a runaway electron beam in helium and nitrogen at a generator voltage of 25 kV is studied experimentally. At low generator voltages, an ultrashort avalanche electron beam (UAEB) is shown to form at the flat top of the voltage pulse and its delay time relative to the leading edge of the pulse may attain several tens of nanoseconds. The conditions of runaway electron beam generation depend on the pressure in the gas-filled diode. The FWHM of the beam current varies from 200 ps to several nanoseconds. Beam electron energy distributions at different pressures are obtained. It is found that, if the gap is preionized by an additional source, the UAEB generation conditions break.

Generation regimes for the runaway-electron beam in gas

The generation of the runaway-electron beam in nitrogen and helium is studied at an oscillator voltage of about 25 kV. Various regimes of the electron beam generation with a pulse duration ranging from 200 ps to several nanoseconds are realized depending on the pressure in the gas diode. An ultrashort avalanche electron beam (UAEB) with a low (10–20%) variation in the voltage across the gap is obtained at a relatively high pressure in the gas diode. The UAEB generation can be delayed relative to the leading edge of the voltage pulse by tens of nanoseconds at low oscillator voltages.

Attosecond Relativistic Electron Beam by Using an Ultrashort Laser Pulse and a Thin Plasma Layer

Attosecond (10−15 s) electron beams will have some important applications in physics, chemistry, material science, etc., where ultrafast phenomena play an important role. Hence, how to generate such ultrashort electron beams is an important issue. Here, we propose to use a thin plasma layer illuminated normally by an ultra-intense femtosecond laser pulse having a sharp rising edge (rising time ∼ laser oscillation period). In this process, the plasma layer is compressed nonadabatically by the laser pulse, and all electrons are synchronously accelerated to ultra-relativistic velocities by several half-cycles of the laser field. In an experiment, a solid nanofilm, a taped electron beam, or a thin gas jet can be used as possible targets. For these types of targets, we show the generation of an attosecond high-energy electron beam by using particle-in-cell (PIC) simulations.

Ultrashort electron beams generated on the flat part of a voltage pulse in nitrogen and helium

In the case of voltage pulses with a small amplitude, an ultrashort avalanche electron beam (UAEB) in a diode filled with nitrogen or helium is generated on a flat part of the pulse. UAEBs obtained at a voltage of 25 kV have a full width at half maximum (FWHM) of about 200 ps and are delayed relative to the voltage pulse front by a time reaching tens of nanoseconds. Waveforms of the electron beam current pulse with several peaks of subnanosecond duration have been observed. At elevated pressures in a gas-filled diode, the voltage across the gap decreases by 10–20% during the electron beam generation.

Ultra short electron beam bunches from a laser plasma cathode

The fluctuation of the electron bunch duration due to energy spectrum instability in a laser plasma cathode has been examined. Previous experiments clearly proved that a laser plasma cathode can generate ultrashort electron bunches with a bunch duration of 130fs (FWHM) and a geometrical emittance 0.07πmmmrad. The effect of temporal elongation of electron bunches due to their energy spread is estimated and the results are in good agreement with previous experiments. It is also clarified that the instability of the energy spectrum not only leads to a fluctuation of the bunch shape but also to a time-of-flight jitter, affecting possible future applications of a laser plasma cathode.

Effect of the amplitude and rise time of a voltage pulse on the formation of an ultrashort avalanche electron beam in a gas diode

The effect of the amplitude and rise time of a voltage pulse from a RADAN-303 pulser on the formation of an ultrashort avalanche electron beam (UAEB) in a gas diode is experimentally investigated. It is shown that, when the open-circuit voltage of the pulser exceeds an optimum value, the beam current amplitude and the gap voltage under which the UAEB is generated decrease.

Discharge current and ultrashort avalanche electron beam current in a volume nanosecond gas discharge in inhomogeneous electric field

The moment of generation of an ultrashort avalanche electron beam (UAEB) relative to the discharge current pulse front has been determined in a volume discharge formed upon nanosecond breakdown of an air gap at atmospheric pressure in an inhomogeneous electric field. The UAEB current reaches maximum on the front of the discharge current pulse, ∼100 ps before the peak of this current. Bias currents with amplitude above 1 kA have been observed. The amplitude of this current increases due to the charging of a capacitor formed by the flat metal anode and a dense discharge plasma expanding from the cathode.