Beam dynamics in dielectric laser acceleration

Microbial quality and Aflatoxin levels of sorghum grains (Sorghum bicolor) irradiated with gamma rays, low energy electron beam (LEEB) and high energy electron beam (HEEB)

High-heat-flux test of commercial graphite tiles by high current and low energy electron beam

Simulations of a Dielectric Laser Accelerator (DLA) are presented. Where in an electron beam is passed through a double grating structure of a dielectric material, whilst laser light polarized in the direction of travel of the electron beam is applied to both sides. This results in acceleration of the electron beam. The LSP (Large Scale Plasma) code is a particle in cell code which is applied in a novel way to model the DLA. Ultimately answers are needed to the questions: what is the acceleration gradient, how does this depend on the starting energy of the electrons and what is the current that can be accelerated through the DLA? DLA's have great potential in many fields, from providing compact linear accelerators, x-ray light sources and medical devices to high energy particle research.

Electron accelerators have a wide range of applications in basic research, medicine and industry. In order to reduce costs and size of future devices and facilities, advanced compact accelerator concepts are subject to extensive research and development efforts in accelerator physics. A promising approach is the dielectric laser accelerator (DLA). The electromagnetic near-fields of an ultra-short laser pulse scattered by a dielectric structure are used to accelerate electrons. Dielectrics increases the achievable accelerating gradient by a factor of at least 10 in comparison to metallic structures. Recent progress in the development of femtosecond lasers and lithographic techniques in semiconductor fabrication has paved the way to experimental demonstration of DLAs in 2013. Possible applications of DLAs are in attosecond science, in ultrafast electron-microscopy and -diffraction, and in providing relativistic electrons for lithography and radiotherapy. The use of near-fields in structures with period lengths in the order of micrometers leads to complicated particle dynamics during acceleration. Theoretical descriptions of the electromagnetic fields and the resulting particle dynamics are presented in this thesis. In order to confine the particles in the accelerating structures, a focusing scheme is introduced which is adapted from ion accelerators. This allows to transport particles through a DLA structure of arbitrary length. Due to non-linear dynamics of the accelerated electrons, the particle trajectories are calculated numerically. A simulation tool based on the analytical description of the electromagnetic fields in the structures is implemented. Since DLA structures are quasiperiodic, the description simplifies to one complex number per structure period, i.e., the spatial harmonic of the synchronous accelerating mode. The required computational effort is thus significantly reduced as compared to the full electromagnetic fields. Many applications of DLAs require particle distributions of high charge. The beam dynamics are additionally affected by wake fields, which are generated by the interaction of the charge distribution ensemble with the dielectric structures. Detailed studies of wake fields in DLA structures are discussed in this thesis. Moreover, first measurements of energy spectrum modulations caused by wake fields are presented. The wake field effects can lead to instabilities. For the analysis of possible instabilities, wake fields are integrated in the aforementioned numerical simulation tool. This extended tool is applied to compare particle dynamics with simplified analytical models of longitudinal and transverse wake effects and instabilities. These studies provide estimates for the maximum attainable charges of an accelerated particle bunch. For higher bunch charges, the effects from the wake fields exceed the accelerating and focusing laser fields and thus lead to beam loss. Finally, a possible damping mechanism is presented in order to counteract the instabilities and increase the bunch charge limits.

In Dielectric Laser Accelerators (DLAs) apertures are in submicrometer range. Intensity effects caused by wake-fields are the critical limitations at relativistic energies. To estimate the intensity limits, we present simulations of longitudinal and transverse wakefields for different DLA grating geometries. These simulations enable to calculate the longitudinal beam loading limit. Additionally, coherent transverse beam stabilities are analyzed. Based on these studies, we estimate the influence of wakes on DLA experiments at 3 GeV planned at SwissFEL at the Paul Scherrer Institute (PSI). However, most models of transverse instabilities are valid only for linearized wakes, which holds at most in a fraction of the aperture. To take this into account, we outline the integration of a nonlinear wake kick in our simplified 6D particle tracking code DLAtrack6D. For verification, we compare both the linear and nonlinear tracking results to full Particle-In-Cell simulations.

Quantitative evaluation of dosimetric uncertainties associated with small electron fields

This paper describes the implementation of physical safety system for the new low energy electron beam (EB) accelerator installed at Block 43T Nuclear Malaysia. The low energy EB is a locally designed and developed with a target energy of 300 keV. The issues on radiation protection have been addressed by the installation of radiation shielding in the form of a bunker and installation radiation monitors. Additional precaution is needed to ensure that personnel are not exposed to radiation and other physical hazards. Unintentional access to the radiation room can cause serious hazard and hence safety features must be installed to prevent such events. In this work we design and built a control and monitoring system for the shielding door. The system provides signals to the EB control panel to allow or prevent operation. The design includes limit switches, key-activated switches and emergency stop button and surveillance camera. Entry procedure is also developed as written record and for information purposes. As a result, through this safety implementation human error will be prevented, increase alertness during operation and minimizing unnecessary radiation exposure.

BackgroundDry food products are often highly contaminated, and dry stress-resistant microorganisms, such as certain types of Salmonella and bacterial spores, can be still viable and multiply if the product is incorporated into high moisture food products or rehydrated. Traditional technologies for the decontamination of these products have certain limitations and drawbacks, such as alterations of product quality, environmental impacts, carcinogenic potential and/or lower consumer acceptance. Cold atmospheric pressure plasma (CAPP) and low energy electron beam (LEEB) are two promising innovative technologies for microbial inactivation on dry food surfaces, which have shown potential to solve these certain limitations. Scope and approachThis review critically summarizes recent studies on the decontamination of dry food surfaces by CAPP and LEEB. Furthermore, proposed inactivation mechanisms, product-process interactions, current limitations and upscaling potential, as well as future trends and research needs for both emerging technologies, are discussed. Key findings and conclusionsCAPP and LEEB are nonthermal technologies with a high potential for the gentle decontamination of dry food surfaces. Both technologies have similarities in their inactivation mechanisms. Due to the limited penetration depth of both technologies, product-process interactions can be minimized by maintaining product quality. A first demonstrator with Technology Readiness Level (TRL) 7 for LEEB has already been introduced into the food industry for the decontamination of herbs and spices. Compared with LEEB, CAPP is at the advanced development stage with TRL 5, for which further work is essential to design systems that are scalable to industrial requirements.

Computer aided simulations of high current small energy electron beam channels

Conventionally, x-ray microanalysis on scanning electron microscopes (SEM) with energy dispersive spectrometers (EDS) has been performed with relatively high primary energies (>10 kv). for most samples this results in reasonably good separation of the generated x-ray line series from different elements enabling unambiguous identification and therefore accurate qualitative analysis. Under these circumstances it is widely accepted that quantitative analysis of polished bulk samples is possible on a routine basis with relative errors around 1-5% and detection limits of the order of 0.1%.However, in order to address the analysis requirements of new advanced materials with sub-micron features, there is growing interest in performing x-ray microanalysis at low beam energies(<5kv). this is now a more realistic goal due to the routine availability of field emission sem's which can operate with much improved beam sizes at low beam energies with sufficient beam current to perform practical microanalysis, in conjunction with the improved low energy performance of current, commercially available EDS systems.

This study investigated the microbial quality of African nutmeg powder irradiated with a high energy electron beam and Gamma rays. African nutmeg samples were purchased from a local market in Accra, Ghana, cleaned, milled, packaged, and irradiated separately using a high energy electron beam (with energy electrons of 9 MeV) and a 60Co gamma source (dose rate of 2 kGy/h) at doses of 2, 4, 6 and 8 kGy. Un-irradiated African nutmeg powder served as a control. The microbial quality of the treated samples was determined using the ISO, 4832:2006 colony count technique. The presumptive microorganisms were identified using Matrix Assisted Laser Desorption Ionisation-Time of Flight Mass Spectrometry (MALDI-TOF MS). High energy electron beam and gamma rays significantly reduced (p < 0.05) the microbial loads of the African nutmeg powder with increasing irradiation dose. Microorganisms isolated and identified were Cytobacillus kochii, Cronobacter spp, Pseudomonas stutzeri, Acinetobacter pittii, and Aspergillus flavus. The results from this study suggest that gamma and high energy electron beam irradiators are equally effective and hence a high energy electron beam can be a good alternative to gamma irradiation for decontaminating African nutmeg powder with an effective dose of 6 kGy at which microorganisms were not detected.

To evaluate the main characteristics of the Exradin W1 scintillator as a dosimeter and to estimate measurement uncertainties when used in radiotherapy. We studied the calibration procedure, energy and modality dependence, short-term repeatability, dose-response linearity, angular dependence, temperature dependence, time to reach thermal equilibrium, dose-rate dependence, water-equivalent depth of the effective measurement point, and long-term stability. An uncertainty budget was derived for relative and absolute dose measurements in photon and electron beams. Exradin W1 showed a temperature dependence of -0.225% °C(-1). The loss of sensitivity with accumulated dose decreased with use. The sensitivity of Exradin W1 was energy independent for high-energy photon and electron beams. All remaining dependencies of Exradin W1 were around or below 0.5%, leading to an uncertainty budget of about 1%. When a dual channel electrometer with automatic trigger was not used, timing effects became significant, increasing uncertainties by one order of magnitude. The Exradin W1 response is energy independent for high energy x-rays and electron beams, and only one calibration coefficient is needed. A temperature correction factor should be applied to keep uncertainties around 2% for absolute dose measurements and around 1% for relative measurements in high-energy photon and electron beams. The Exradin W1 scintillator is an excellent alternative to detectors such as diodes for relative dose measurements.

The Dielectric Laser Acceleration group at SLAC uses micro-fabricated dielectric grating structures and conventional infrared lasers to accelerator electrons. These structures have been estimated to produce an accelerating gradient up to 2 orders of magnitude greater than that produced by conventional RF accelerators. The success of the experiment depends on both the laser damage threshold of the structure and the timing overlap of femtosecond duration laser pulses with the electron bunch. In recent dielectric laser acceleration experiments, the laser pulse was shorter both temporally and spatially than the electron bunch. As a result, the laser is theorized to have interacted with only a small portion of the electron bunch. The detection of this phenomenon, referred to as partial population modulation, required a new approach to the data analysis of the electron energy spectra. A fitting function was designed to separate the accelerated electron population from the un-accelerated electron population. The approach was unsuccessful in detecting acceleration in the partial population modulation data. However, the fitting functions provide an excellent figure of merit for previous data known to contain signatures of acceleration.

The X-ray free-electron laser (XFEL) based on self amplified spontaneous emission (SASE) is an unprecedented coherent hard X-ray source [1,2]. However, due to its huge size and construction cost, it would not be easy to have this magnificent scientific facility in your neighborhood. Therefore, there have been studies on why it must be so big (why its electron beam energy must be so high and why its undulator is so long) and how its size can be reduced (how its electron energy can be lowered) without degrading radiation quality [3, 4]. Size reduction of a XFEL machine is basically reduced to how a lower energy electron beam can generate a XFEL without degrading the radiation quality. The reason a high energy electron beam is needed is that a very small electron beam emittance is needed for lasing as the relation = n/γ indicates. Here, is the electron beam emittance, n is the gun emittance, and γ is the relativistic factor. Hence, if n is low enough, a lower electron energy can make the same . A compact XFEL without its quality being degraded obviously means a XFEL with high FEL efficiency. A FEL’s efficiency can be roughly estimated by using the FEL parameter ρ. We can make ρ high enough, even at a low electron beam energy, by using a low emittance gun as explained above. Therefore, the issue is reduced to developing an electron gun with improved gun emittance. The emittance performance of the photo-cathode gun was widely known to be around 1−1.2 μrad, and the design of Linac Coherent Light Source (LCLS) was based on this number [5]. However, in the recent LCLS com-

In many countries, lemons are waxed for post-harvest protection. However, in Japan, lemons are typically sold unwaxed because they are a special domestic product whose peels are often consumed directly or used in cooking. This practice poses issues for Japanese-island-grown lemons, which may require a minimum of one week for transport. Unwaxed lemons are generally susceptible to mold contamination and have a short shelf life of only two weeks, making their transportation from the island to the mainland impractical. To address these issues, we developed an irradiation-based method that uses soft (low-energy) electron beams (EBs) to exclusively sterilize mold on lemon peels. Thereafter, we investigated the effect of the treatment on the shelf life of the lemons by uniformly irradiating them with EBs. The results revealed that while the proposed electron irradiation sterilized the mold, it triggered the browning of the lemon peel, rendering the method unsuitable for long-term storage.