Free Electron Beam Research Articles

Circular dichroism in XUV + IR multiphoton ionization of atoms

Circular dichroism (CD) is theoretically considered for two-colour multiphoton ionization of arbitrary atomic subshells. In particular, p-subshell ionization is analysed and compared with s-subshell ionization. Simple analytical expressions for the CD are obtained for both s- and p-subshell ionization. The calculations performed for Ne(2p) ionization by an extreme ultraviolet pulse in the presence of an infrared laser pulse show that the CD in this case is appreciably larger than in previously discussed s-shell ionization. It makes this case favourable for applications as a sensitive tool for measuring the helicity of short-wavelength free-electron laser beams.

Investigation of the clinical potential of scattering foil free electron beams

Electron beam therapy has been an important radiation therapy modality for many decades. Studies have been conducted recently for more efficient and advanced delivery of electron beam radiation therapy. X-ray contamination is a common problem that exists with all of the advanced electron beam therapy techniques such as Bolus Electron conformal therapy, segmented electron conformal therapy, and modulated electron arc therapy. X-ray contamination could add some limitations to the advancement and clinical utility of those electron modalities. It was previously shown in the literature that the scattering foil is one of the major accelerator parts contributing to the generation of bremsstrahlung photons. Thus, in this work we investigate the dosimetric characteristics of scattering foil free (SFF) electron beams and the feasibility of using those beams for breast cancer boosts. The SFF electron beams were modeled and simulated using the Monte Carlo method. CT scans of six previously treated breast patients were used for the treatment plan generation utilizing our in-house Monte Carlo-based treatment planning system. Electron boost plans with conventional beams and the SFF beams were generated, respectively, for all patients. A significant reduction of the photon component was observed with the removal of the primary scattering foil for beam energies higher than 12 MeV. Flatness was greatly affected but the difference in flatness between conventional and SFF beams was much reduced for small cone sizes, which were often used clinically for breast boosts. It was found that the SFF electron beams could deliver high-quality dose distributions as conventional electron beams for boost treatments of the breast with an added advantage of a further reduced dose to the lung and the heart.

Quantum simulation of a spin polarization device in an electron microscope

A proposal for an electron-beam device that can act as an efficient spin-polarization filter has been recently put forward (Karimi et al 2012 Phys. Rev. Lett. 108 044801). It is based on combining the recently developed diffraction technology for imposing orbital angular momentum to the beam with a multipolar Wien filter inducing a sort of artificial non-relativistic spin–orbit coupling. Here we reconsider the proposed device with a fully quantum-mechanical simulation of the electron-beam propagation, based on the well-established multi-slice method, supplemented with a Pauli term for taking into account the spin degree of freedom. Using this upgraded numerical tool, we study the feasibility and practical limitations of the proposed method for spin polarizing a free electron beam.

TU‐E‐108‐10: Investigation of the Clinical Potential of Scattering Foil Free Electron Beams

Purpose: The photon contamination is a common problem for all advanced modulated electron beam therapy techniques. The scattering foil is one of the major accelerator parts contributing to the generation of bremsstrahlung photons. This work aims to characterize the scattering foil free (SFF) beams from the Siemens Artiste accelerator and to study the feasibility of using those beams for breast cancer boost treatment utilizing our in‐house Monte Carlo based treatment planning system. Methods: Percentage depth doses and lateral profiles were measured for all available energies with the primary scattering foil removed. All beam characteristics were compared to those of the conventional electron beams. The SFF electron beams were modeled and simulated using the Monte Carlo method. CT scans of six previously treated breast patients were used for the treatment plan generation. Electron boost plans with conventional beams and the SFF beams were generated, respectively, for all the patients. Results: A significant reduction of the photon component was observed with the removal of the primary scattering foil for beam energies higher than 12MeV. Slight increase in beam energy was only noticed with the 18MeV and 21MeV beams. Ion chamber measurements showed higher dose per monitor unit especially at the higher energies. Flatness was greatly affected, however the difference in flatness between conventional and SFF beams was much reduced for small cone sizes and can be clinically insignificant for breast boosts. Removal of the secondary scattering foil could further reduce photon contamination but the beam flatness was also drastically deteriorated. Results from the electron boost plans showed similar target coverage with much reduced dose to the critical structures. Conclusion: The SFF electron beams can deliver high‐quality dose distributions as conventional electron beams for boost treatments with an added advantage of further reduced dose to the lung and heart. This work has been supported by a UICC American Cancer Society Beginning Investigators Fellowship funded by the American Cancer Society.

Plasma Damage Mechanism of Electron Beam Curing Process for Spin on Dielectrics

This letter presents the plasma damage mechanism of electron beam curing process for spin on dielectrics. Device damage is studied using the antenna gate metal-oxide-semiconductor field-effect transistor (MOSFET) in terms of the threshold voltage variation as a function of electron beam conditions such as ambient and cathode voltage. Threshold voltages of nMOSFET are decreased by an electron beam curing process without antenna ratio dependency. The electron energy and interlayer dielectric thickness between active devices and metal layers largely affect the variation of threshold voltage. From the experimental results, it is concluded that device damage induced by an electron beam curing process is characterized as radiation damage rather than electron charging damage. For the damage free electron beam curing process, it is essential to control the penetration depth of high-energy electrons by adjusting the cathode voltage while considering the dielectric thickness over active devices.

Damage investigation on tungsten and diamond diffractive optics at a hard x-ray free-electron laser

Focusing hard x-ray free-electron laser radiation with extremely high fluence sets stringent demands on the x-ray optics. Any material placed in an intense x-ray beam is at risk of being damaged. Therefore, it is crucial to find the damage thresholds for focusing optics. In this paper we report experimental results of exposing tungsten and diamond diffractive optics to a prefocused 8.2 keV free-electron laser beam in order to find damage threshold fluence levels. Tungsten nanostructures were damaged at fluence levels above 500 mJ/cm(2). The damage was of mechanical character, caused by thermal stress variations. Diamond nanostructures were affected at a fluence of 59 000 mJ/cm(2). For fluence levels above this, a significant graphitization process was initiated. Scanning Electron Microscopy (SEM) and µ-Raman analysis were used to analyze exposed nanostructures.

Soft x-ray free-electron laser imaging by LiF crystal and film detectors over a wide range of fluences

LiF crystal and film detectors were used to measure the far-field fluence profile of a self-amplified spontaneous-emission free-electron laser beam and diffraction imaging with high spatial resolution. In these measurements the photoluminescence (PL) response of LiF crystal and film was compared over a wide range of soft x-ray fluences. It was found that the soft x-ray fluence dependences of LiF crystal and film differ. At low fluence, the LiF crystal shows higher PL response compared to LiF film, while this comparison is the opposite at higher fluence. Accurate measurement of LiF crystal and film PL response is important for precise characterization of the spatial, spectral, and coherence features of x-ray beams across the full profile and in localized areas. For such measurements, crucial LiF detector attributes are high spatial resolution and high dynamic range.

NOx removal with multiple pulsed electron beam free of catalysts or reagents

A catalyst free approach for nitrogen oxides (NOx) removal has been developed at the United States Naval Research Laboratory. Our goals were to assess the ability of pulsed electron beam to enhance NOx removal at potential lower capital cost with greater efficiency than other large scale NOx removal methods. Removal efficiency over 95% has been attained for NOx concentrations of 1000 parts per million (ppm), 500 ppm and 200 ppm in nitrogen atmosphere. The NOx concentration dropped from 204 ppm to below 4.8 ppm after 10 shots supplying a total dose of 65 kGy. The resultant chemicals after catalyst free pulsed electron beam processing of NOx are nitrogen and oxygen, same as components of air. Pulsed electron beams in a catalyst free approach remove a larger percentage of NOx than continuous wave electron beam with a catalyst. Catalyst free approach removes issues of handling, collecting, transporting and efficiently distributing chemical byproducts. Pulsed electron beams are as efficient as continuous wave electron beams for small removal percentages and have a significant advantage at higher fractional removal percentages of NOx. Preferential destruction of NO species relative to the removal of NO2 species is observed in the pulsed electron beam reaction chamber. The energy required to remove a kilogram of NOx is nearly the same at pressures of 1.16 atmospheres and 1.02 atmospheres.

Natively Inhibited Trypanosoma brucei Cathepsin B Structure Determined by Using an X-ray Laser

The Trypanosoma brucei cysteine protease cathepsin B (TbCatB), which is involved in host protein degradation, is a promising target to develop new treatments against sleeping sickness, a fatal disease caused by this protozoan parasite. The structure of the mature, active form of TbCatB has so far not provided sufficient information for the design of a safe and specific drug against T. brucei. By combining two recent innovations, in vivo crystallization and serial femtosecond crystallography, we obtained the room-temperature 2.1 angstrom resolution structure of the fully glycosylated precursor complex of TbCatB. The structure reveals the mechanism of native TbCatB inhibition and demonstrates that new biomolecular information can be obtained by the "diffraction-before-destruction" approach of x-ray free-electron lasers from hundreds of thousands of individual microcrystals.

Coherent terahertz radiation from high-harmonic component of modulated free-electron beam in a tapered two-asymmetric grating structure

Based on the mechanism of incoherent diffraction radiation excited by an electron bunch in a waveguide with periodic structure, this paper presents the concept of coherent terahertz (THz) radiation from the high-harmonic component of a modulated free-electron beam in a tapered two-asymmetric grating structure. The results show that in this mechanism 0.43 THz radiation can be generated with 10 A/cm2 current density, and the efficiency can reach 0.5%. Because of the low required current density and relative high efficiency, this concept shows the application potential for electron-beam-driven terahertz sources.

Experimental study of multichromatic terahertz wave propagation through planar micro-channels

Previous theoretical and numerical studies [Y. M. Shin and L. R. Barnett, Appl. Phys. Lett. 92, 091501 (2008) and Y. M. Shin et al., Appl. Phys. Lett. 93, 221504 (2008)] have reported that a planar micro-channel with an asymmetric corrugation array supports strongly confined propagation of broadband THz plasmonic waves. The highly broad spectral response is experimentally demonstrated in the near-THz regime of 0.19–0.265 THz. Signal reflection and transmission tests on the three designed micro-channels including directional couplers resulted in a full-width-half-maximum bandwidth of ∼50-60 GHz with an insertion loss of approximately −5 dB, which is in good agreement with simulation data. These micro-structures can be utilized for free electron beam and electronic/optic integrated devices.

Simulation of grazing-incidence coherent imaging

A method is proposed for simulating optical object images formed by oblique or grazing-incidence coherent beams. The theoretical approach relies on the solution of a parabolic equation, which generalises the Fresnel integral. Our numerical results are given for experimental conditions close to those realised when use is made of modern soft X-ray lasers. The newly developed method may also be employed to simulate X-ray imaging systems developed around synchrotron and free-electron laser beams.

Transmitting Hertzian Optical Nanoantenna with Free-Electron Feed

A pair of coupled gold nanorods excited by a beam of free electrons acts as a transmitting Hertzian antenna in the optical part of the spectrum. Significantly enhanced resonant emission is observed from the antenna when the electron beam is injected around the junction between the rods, where the local density of electromagnetic states is elevated.

Diamond X-ray beam-position monitoring using signal readout at the synchrotron radiofrequency

Single-crystal diamond is a material with great potential for the fabrication of X-ray photon beam-position monitors with submicrometre spatial resolution. Low X-ray absorption combined with radiation hardness and excellent thermal-mechanical properties make possible beam-transmissive diamond devices for monitoring synchrotron and free-electron laser X-ray beams. Tests were made using a white bending-magnet synchrotron X-ray beam at DESY to investigate the performance of a position-sensitive diamond device using radiofrequency readout electronics. The device uniformity and position response were measured in a 25 microm collimated X-ray beam with an I-Tech Libera ;Brilliance' system. This readout system was designed for position measurement and feedback control of the electron beam in the synchrotron storage ring, but, as shown here, it can also be used for accurate position readout of a quadrant-electrode single-crystal diamond sensor. The centre-of-gravity position of the F4 X-ray beam at the DORIS III synchrotron was measured with the diamond signal output digitally sampled at a rate of 130 Msample s(-1) by the Brilliance system. Narrow-band filtering and digital averaging of the position signals resulted in a measured position noise below 50 nm (r.m.s.) for a 10 Hz bandwidth.

Light Well: A Tunable Free-Electron Light Source on a Chip

The passage of a free-electron beam through a nanohole in a periodically layered metal-dielectric structure creates a new type of tunable, nanoscale radiation source--a "light well". In the reported demonstration, tunable light is generated at an intensity of approximately 200 W/cm(2) as electrons with energies in the 20-40 keV range are injected into gold-silica well structures with a lateral size of just a few hundred nanometers.

Non-Localisation des Électrons en Mouvement

We consider a beam of monocinetic free electrons flowing parallel to the Ox3 axis ; the beam will be uniformly distributed corresponding to a mathematical constant density ρ, and we suppose that this beam meets a uniform magnetic field H, parallel to Ox3, which is perpendicular to a plane (P) bounding the field H. We prove that the localization of these electrons is inconsistent with the Dirac equation and the electrons are extended in the space.

Spin-polarized free electron beam interaction with radiation and superradiant spin-flip radiative emission

The problems of spin-polarized free-electron beam interaction with electromagnetic wave at electron-spin resonance conditions in a magnetic field and of superradiant spin-flip radiative emission are analyzed in the framework of a comprehensive classical model. The spontaneous emission of spin-flip radiation from electron beams is very weak. We show that the detectivity of electron spin resonant spin-flip and combined spin-flip/cyclotron-resonance-emission radiation can be substantially enhanced by operating with ultrashort spin-polarized electron beam bunches under conditions of superradiant (coherent) emission. The proposed radiative spin-state modulation and the spin-flip radiative emission schemes can be used for control and noninvasive diagnostics of polarized electron/positron beams. Such schemes are of relevance in important scattering experiments off nucleons in nuclear physics and off magnetic targets in condensed matter physics.

On Possible Mechanism of Spontaneous Radiation from Non-Relativistic Free Electron Beam in the Motz Undulator

On Possible Mechanism of Spontaneous Radiation from Non-Relativistic Free Electron Beam in the Motz Undulator

A radiation generator based on the induced transition effect

The possibility of generating electromagnetic radiation using a beam of free electrons in an empty resonator with ideally conducting walls is examined. It is shown that a regime can exist in which the number of photons in the resonator exhibits exponential growth, provided that the electron bunch length is sufficiently large.

Distributed real-time control and data acquisition of free-electron laser beams

Implementation of a feedback control scheme and its associated user interface and data acquisition modules in a distributed real time computing environment are described. The distributed system consists of an ethernet network interconnecting real time processor modules to host workstations and other embedded controllers. The goal of the feedback control scheme is to maintain positions of the electron beam in the middle of the vacuum chamber in the presence of electromagnetic interference and noise. Technical details of implementing the data acquisition and control scheme and their integration with the existing control system modules are presented, along with experimental results.