Pulsed Electron Source Research Articles

Time resolved electron microscopy for in situ experiments

Transmission electron microscopy has functioned for decades as a platform for in situ observation of materials and processes with high spatial resolution. Yet, the dynamics often remain elusive, as they unfold too fast to discern at these small spatial scales under traditional imaging conditions. Simply shortening the exposure time in hopes of capturing the action has limitations, as the number of electrons will eventually be reduced to the point where noise overtakes the signal in the image. Pulsed electron sources with high instantaneous current have successfully shortened exposure times (thus increasing the temporal resolution) by about six orders of magnitude over conventional sources while providing the necessary signal-to-noise ratio for dynamic imaging. We describe here the development of this new class of microscope and the principles of its operation, with examples of its application to problems in materials science.

An ultracold electron source as an injector for a compact SASE-FEL

Ultracold electron sources based on near-threshold photoionization of laser-cooled atomic gases can produce ultrashort electron pulses with a brightness potentially exceeding conventional pulsed electron sources. They are presently being developed for single shot ultrafast electron diffraction, where a bunch charge of 100 fC is sufficient. For application as an injector for x-ray free electron lasers (FEL) a larger bunch charge is generally required. Here we present preliminary calculations of an ultracold electron source operating at bunch charges up to 1 pC. We discuss the relevant bunch degradation processes that occur when the charge is increased. Using general particle tracer tracking simulations we show that bunches can be produced of sufficient quality for driving a 1 Å self amplified spontaneous emission free electron laser (SASE-FEL) at 1.3 GeV electron energy. In addition we speculate on the possibility of using the ultracold source for driving a 15 MeV SASE-FEL in Compton backscatter configuration into the quantum FEL regime.

Circumventing Scherzer's Theorem: Large Numerical Aperture Objective Lenses for Pulsed Electron Microscopy

Scherzer’s theorem states that all static round magnetic and electro-static electron lenses possess positive spherical aberration [1]. As a result, in recent years, much effort has been invested in techniques to correct for the spatial aberrations (Cs) of objective lenses in high resolution electron microscopy. For pulsed electron sources, such as those required for ultrafast transmission electron microscopy (UTEM), an alternative strategy for Cs aberration correction is possible through the use of the dynamic lensing properties of RF cavities [2] that, when operated in the appropriate oscillation mode, can possess negative spherical aberration [3]. For single-shot imaging UTEM, which requires ~ 10 8 electrons/pulse, simulations indicate that a large numerical aperture (NA > 0.1) objective lens will also be required to mitigate against deleterious space-charge effects [4] – effectively by minimizing the time electrons experience a high charge density. Here, we present a theoretical analysis of the possibility of using the focusing properties of RF cavities to compensate for the spherical aberration of large NA objective lenses for pulsed electron microscopy.

Factors Influencing the Electron Yield of Needle-Ring Pulsed Corona Discharge Electron Source for Negative Ion Mobility Spectrometer

A simple negative ion mobility spectrometer (IMS) is designed and used to investigate the factors that influence the number and efficiency of electrons generated by the needle-ring pulsed corona discharge electron source. Simulation with Ansoft Maxwell 12 is carried out to analyze the electric field distribution within the IMS, and to offer the basis and foundation for analyzing the measurement results. The measurement results of the quantities of electrons show that when the drift electric field strength and the ring inner diameter rise, both the number of effective electrons and the effective electron rate are increased. When the discharge voltage becomes stronger, the number of effective electrons goes up while the effective electron rate goes down. In light of the simulation results, mechanisms underlying the effects of drift electric field strength, ring inner diameter, and discharge voltage on the effective electron number and effective electron rate are discussed. These will make great sense for designing negative ion mode IMS using the needle-ring pulsed corona discharge as the electron source.

Effective temperature of an ultracold electron source based on near-threshold photoionization

We present a detailed description of measurements of the effective temperature of a pulsed electron source, based on near-threshold photoionization of laser-cooled atoms. The temperature is determined by electron beam waist scans, source size measurements with ion beams, and analysis with an accurate beam line model. Experimental data is presented for the source temperature as a function of the wavelength of the photoionization laser, for both nanosecond and femtosecond ionization pulses. For the nanosecond laser, temperatures as low as 14 ± 3 K were found; for femtosecond photoionization, 30 ± 5 K is possible. With a typical source size of 25 μm, this results in electron bunches with a relative transverse coherence length in the 10⁻⁴ range and an emittance of a few nm rad.

Simultaneous detection of benzene and toluene using a pulsed ion mobility spectrometer

Ion mobility spectrometry (IMS) is a well known method for fast trace gas detection in air. Having simple instrumentation, IMS is especially suited when portability and highest sensitivity are required. However, due to competing chemical ionization processes in the reaction region of IMS, it can be difficult or even impossible to detect small amounts of certain substances in a complex mixture. A prominent example is the detection of low benzene concentrations in the presence of higher toluene concentrations. In this paper we present experimental results of simultaneous toluene and benzene detection by using a pulsed electron source for ionization and demonstrate how this can help to overcome the major issue of atmospheric pressure chemical ionization (APCI).

Semiconducted symphony: ultrafast field emitter array

Semiconducted symphony: ultrafast field emitter array

Dispersion compensation for attosecond electron pulses

We propose a device to compensate for the dispersion of attosecond electron pulses. The device uses only static electric and magnetic fields and therefore does not require synchronization to the pulsed electron source. Analogous to the well-known optical dispersion compensator, an electron dispersion compensator separates paths by energy in space. Magnetic fields are used as the dispersing element, while a Wien filter is used for compensation of the electron arrival times. We analyze a device with a size of centimeters, which can be applied to ultrafast electron diffraction and microscopy, and fundamental studies.

Pulsed electron beams in ion mobility spectrometry

Ion mobility spectrometry is a well-known technique used to analyze trace gases in ambient air. Typically, it works by employing a radioactive source to provide electrons with high energy to ionize the analytes in a series of chemical reactions. During the past ten years non-radioactive sources have been one of the subjects of interest in ion mobility spectrometry, initially in order to replace radioactive sources as a result of general security and regulatory concerns. Among these non-radioactive sources especially pulsed sources have recently been shown to additionally improve the analytic information provided by ion mobility spectrometers. In this review we will describe the progress regarding the application of pulsed non-radioactive electron sources in ion mobility spectrometry and show the recent analytical advances that have been achieved by using pulsed electron beams.

Quantum description and properties of electrons emitted from pulsed nanotip electron sources

We present a quantum calculation of the electron degeneracy for electron sources. We explore quantum interference of electrons in the temporal and spatial domain and demonstrate how it can be utilized to characterize a pulsed electron source. We estimate effects of Coulomb repulsion on two-electron interference and show that currently available pulsed nanotip electron sources operate in the regime where the quantum nature of electrons can be made dominant.

Investigation of dimethyl methylphosphonate (DMMP) with an Ion mobility spectrometer using a pulsed electron source

Ion mobility spectrometry (IMS) has been an important tool for decades in the field of trace gas analysis of substances such as explosives, drugs of abuse or chemical warfare agents. In recent years, its application has been extended to more complex set ups. In this paper we present the application of a standard IMS device equipped with a novel pulsed electron gun for ionization in the investigation of the chemical warfare agent simulant dimethyl methylphosphonate (DMMP). The signal decay times of the reactant ion peak (RIP), the DMMP monomer and dimer have been investigated. Thus, further information could be obtained of the innovative application of different signal decay times in order to filter out signals of contaminants with focus on the decay dependence on the concentration. Additionally, further details regarding the still not fully understood underlying decay mechanisms have been found.

Application of a Nonradioactive Pulsed Electron Source for Ion Mobility Spectrometry

Ion mobility spectrometry (IMS) is a well-known method for detecting hazardous compounds in air. Typical applications are the detection of chemical warfare agents, highly toxic industrial compounds, explosives, and drugs of abuse. Detection limits in the low part per billion range, fast response times, and simple instrumentation make this technique more and more popular. Common ion mobility spectrometers work by employing a radioactive source to provide electrons with high energy to ionize analytes in a series of chemical reactions. General security as well as regulatory concerns related to radioactivity result in the need for a different ionization source which on the other hand produces ions in a similar manner as a radioactive source since the ion chemistry is well-known. Here we show the application of a novel nonradioactive source that produces spectra similar to those obtained with radioactive tritium sources. Using this source in a pulsed mode offers the additional advantage of selecting certain analytes by their recombination time and thus significantly increasing the selectivity. The successful isolation of a target signal in the presence of contaminants using a pulsed electron beam or more precisely the difference in recombination times will be demonstrated for the case of dimethyl-methylphosphonate (DMMP) showing the potential of this source to reduce the possibility for false-positive detection of corresponding chemical warfare agents (CWA) by IMS.

Focus on photonics and biomedical optics

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CONCEPT OF ELECTRON SOURCE BASED ON PULSED PLASMA DISCHARGE

CONCEPT OF ELECTRON SOURCE BASED ON PULSED PLASMA DISCHARGE. The lack of continuous electronbeam machine is electron current limitation and heating impact on target. So it is needed a new machine which will givehigher current in the pulsed mode operation. This paper offers on the possibility of pulsed plasma discharge as a pulseelectron source. By using simple equation, it can be achieved some interesting parameters such as plasma current,rise time etc. The result is useful as basic understanding in designing this machine.

Low-Fluence Electron Yields of Highly Insulating Materials

Electron-induced electron yields of high-resistivity high-yield materials - ceramic polycrystalline aluminum oxide and polymer polyimide (Kapton HN) - were made by using a low-fluence pulsed incident electron beam and charge neutralization electron source to minimize charge accumulation. Large changes in the energy-dependent total yield curves and yield decay curves were observed, even for incident electron fluences of < 3 fC/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The evolution of the electron yield as charge accumulates in the material is modeled in terms of electron recapture based on an extended Chung-Everhart model of the electron emission spectrum. This model is used to explain the anomalies measured in highly insulating high-yield materials and to provide a method for determining the limiting yield spectra of uncharged dielectrics. The relevance of these results to spacecraft charging is also discussed.

ULTRACOLD ELECTRON SOURCES

Ultra-cold plasmas with electron temperatures of ~10 K can be created by photo-ionization just above threshold of a cloud of laser-cooled atoms. Recently it was shown 7 by GPT particle tracking simulations that an ultra-cold plasma has an enormous potential as a pulsed bright electron source. Here we discuss these results in the framework of normalized 6D brightness, which allows us to make a proper comparison both with the performance of pulsed, radio-frequency photo-emission sources and with the performance of continuous, needle-like field-emission sources. In addition we speculate on the possibility of using ultra-cold plasmas to realize quantum degenerate electron beams, constituting the ultimate limit in electron beam brightness.

PROPOSAL FOR A HIGH-BRIGHTNESS PULSED ELECTRON SOURCE

We propose a novel scheme for a high-brightness pulsed electron source, which has the potential for many useful applications in electron microscopy, inverse photo-emission, low energy electron scattering experiments, and electron holography. A description of the proposed scheme is presented.

Proposal for a Quantum-Degenerate Electron Source

We propose a pulsed electron source capable of 6D brightness orders-of-magnitude greater than that of existing sources. It could deliver average current up to 0.5 pA and achieve an emittance approaching the quantum limit in each spatial dimension. It could be employed to advantage in electron microscopy, inverse photo-emission, precision low-energy scattering experiments, and electron holography. This source could make possible pump-probe experiments with Angstrom spatial and subpicosecond time resolution. Here, we present the basic concepts of the source, including a generalized expression for the brightness that can be used in the quantum limited case and the analysis of the main issues that must be addressed for successful construction and operation. We have begun an experiment to demonstrate its essential features.

Ultracold Electron Source

We propose a technique for producing electron bunches that has the potential for advancing the state-of-the-art in brightness of pulsed electron sources by orders of magnitude. In addition, this method leads to femtosecond bunch lengths without the use of ultrafast lasers or magnetic compression. The electron source we propose is an ultracold plasma with electron temperatures down to 10 K, which can be fashioned from a cloud of laser-cooled atoms by photoionization just above threshold. Here we present results of simulations in a realistic setting, showing that an ultracold plasma has an enormous potential as a bright electron source.

Growth of ZnO films on C-plane (0 0 0 1) sapphire by pulsed electron deposition (PED)

Thin films of ZnO on C-plane (0 0 0 1) sapphire (α-Al 2O 3) substrates were grown by pulsed electron beam deposition (PED). Intense electron pulses, approximately 0.8 J/70 ns pulse produced by a channelspark pulsed electron source at a rate of 10 Hz were used to ablate sintered polycrystalline ZnO targets at an oxygen pressure of 15 mTorr. During growth, the sapphire substrate temperature was maintained at 700 °C. A 15 min growth produced a 250 nm film, as measured by a Dektak profilometer. Measurements by X-ray diffraction indicate c-axis oriented films. Cathodoluminescence (CL) data show strong band edge emission. Optical absorption data indicate a sharp band edge with clearly visible exciton absorption at room temperature, and resolved A and B excitons at 77 K. Thus, pulsed electron beam deposition of ZnO films is shown to be a viable technique for producing high quality ZnO films.