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Updated: January 2012
Below is a list of my contributions to knowledge through journals and conferences.
Under Construction
Journal Articles
- All-Dielectric Invisibility Cloaks Made of BaTiO3-Loaded Polyurethane Foam (paper)
Di Bao, Khalid Rajab, Yang Hao, Efthymios Kallos, Wenxuan Tang, Christos Argyropoulos, Yongzhe Piao, Shoufeng Yang
New Journal of Physics, Vol 13, pp.103023 (2011)
DOI 10.1088/1367-2630/13/10/1030233
Abstract Transformation optics has led the way in the development of electromagnetic invisibility cloaks from science fiction to engineering practice. Invisibility cloaks have been demonstrated over a wide range of the electromagnetic spectrum, and with a variety of different fabrication techniques. However, all previous schemes have relied on the use of metamaterials consisting of arrays of sub-wavelength inclusions. We report on the first cloaking structure made of a high-κ dielectric-loaded foam mixture. A polyurethane foam mixed with different ratios of barium titanate is used to produce the required range of permittivities, and the invisibility cloak is demonstrated to work for all incident angles over a wide range of microwave frequencies. This method will greatly facilitate the development and large-scale manufacture of a wide range of transformation optics-based structures.
- Study of an Optical Nanolens with the Parallel Finite Difference Time Domain Technique (paper)
Christos Argyropoulos, Efthymios Kallos, Yang Hao
Radio Science 46, RS0E06 (2011)
DOI 10.1029/2010RS004613
Abstract In this paper a three‐dimensional dispersive finite difference time domain based on the message passing interface architecture is applied for the full wave modeling of a metallic nanolens operating at optical frequencies. The subwavelength imaging potential of the nanodevice is thoroughly studied. The issue of symmetry in the nanolens is exploited, which is shown to have a direct result at the dynamic behavior of the nanolens.
- Comparison of Frequency Responses of Cloaking Devices Under Nonmonochromatic Illumination (paper)
Efthymios Kallos, Christos Argyropoulos, Yang Hao, Andrea Alù
Phys. Rev. B 84, 045102 (2011)
DOI 10.1103/PhysRevB.84.045102
Abstract Plasmonic coatings have been proposed as a robust method to suppress the scattering signature of conventional dielectric objects, typically by surrounding a given object of moderate size with an isotropic, homogeneous, dispersive material that has a low or negative permittivity. In contrast to transformation-based cloaking devices, where highly anisotropic inhomogeneous materials achieve invisibility by electromagnetically isolating a certain region of space, plasmonic cloaks operate under the principle of scattering cancellation and usually allow field energy to enter the device's core. So far, plasmonic devices have been mostly examined using single-frequency, plane-wave excitations. In this work, the performance of such plasmonic cloaks when illuminated by more realistic, broadband nonmonochromatic pulses is investigated and compared with other cloaking mechanisms. The two-dimensional total-field scattered-field method is used within the finite-difference time-domain dispersive numerical technique in order to simulate time domain effects when temporally Gaussian pulses are launched toward cylindrical moderately-sized dielectric objects which are covered with appropriate plasmonic coatings. The results are compared to the performance of transformation-based cloaks, finding that the plasmonic cloaks may suppress scattering more effectively over a wider frequency range.
- Discrete Coordinate Transformation for Designing All-Dielectric Devices (paper)
Wenxuan Tang, Christos Argyropoulos, Efthymios Kallos, Di Bao, Yang Hao
IEEE Transactions on Antennas and Propagation, vol.58, no12, pp.3795 (2010)
DOI
10.1109/TAP.2010.2078475
Abstract
Transformation electromagnetics provides a practical approach to control electromagnetic fields at will. Based on this principle, novel devices such as the invisible cloak have been proposed. Here we examine the extension of this technique as applied to the design of flat devices in antenna systems. A method using discrete coordinate transformation is proposed, which allows the conversion of conventional devices with curved shapes into flat systems, while preserving their non-dispersive, isotropic, broadband, and lossless properties. Two specific design examples, a flat reflector and a flat lens embedded in free space, are presented. To avoid the loss and narrow bandwidth issues typically present in metamaterials, appropriate approximations and simplifications are introduced to make the all-dielectric devices, which are more practical to build. It is also shown that the discrete coordinate transformation is valid for both the E and H polarizations, as long as the local coordinates of the system remain near-orthogonal. Finite-Difference Time-Domain simulations are used to verify the performances of these designs, and show that the all-dielectric devices have similar broadband performances compared to the conventional ones, while possessing the advantages of flat profiles and small volumes.
- Bandwidth Evaluation of Dispersive Transformation Electromagnetics Based Devices (paper)
Christos Argyropoulos, Efthymios Kallos, Yang Hao
Applied Physics A: Materials Science and Processing, Vol. 103, Number 3, pp.715–719 (2010)
DOI 10.1007/s00339-010-6206-y
Abstract
In this paper, the transient responses of some devices which are based on transformation electromagnetics are studied, such as invisible cloaks and concentrators, by using the Finite-Difference Time-Domain (FDTD) numerical technique. In particular, effects of the inherent losses as well as the coating size of the ideal cylindrical cloak on its bandwidth and cloaking performance are examined. In addition, it is demonstrated that the performance of transformation electromagnetics based devices is affected by the material parameters in the design, although they may behave nicely under monochromatic plane wave illuminations. The obtained results are of interest for the future practical implementation of these structures.
- FDTD Analysis of the Optical Black Hole (paper)
-->Selected for Spotlight on Optics (intro)
Christos Argyropoulos, Efthymios Kallos, Yang Hao
JOSA B, Vol. 27, Issue 10, pp.2020-2025 (2010)
DOI 10.1364/JOSAB.27.002020
Abstract
An optical black hole is studied using a parallel radially dependent finite-difference time-domain (FDTD) simulation technique. The device requires non-dispersive metamaterial structures and is capable of broadband operation, based on transformation optics. Excellent absorption is demonstrated for different angles of wave incidence and illumination excitation types. In addition, a practical device, which is made to be matched to free space, is proposed, and the relevant physics is explored. Finally, peculiar phase distributions of the electromagnetic waves are observed inside the radially dependent permittivity material of the devices.
- A Broadband Simplified Free Space Cloak Realized by Non-Magnetic Dielectric Cylinders (paper)
Di Bao, Efthymios Kallos, Wenxuan Tang, Christos Argyropoulos, Yang Hao, Tie Jun Cui
Frontiers of Physics in China, Vol.5, No.3, pp.319-323 (2010)
DOI 10.1007/s11467-010-0010-1
Abstract
In this paper, the properties of cylindrical high permittivity dielectric particles are studied. A design for broadband reduction of the scattering signature of metallic objects is proposed by implementing simplified ground-plane cloaking schemes. The devices are functional in the presence of a ground plane as well as in free space ranging from 4 GHz to 10 GHz. The required dielectric map for the cloak is achieved by means of manipulating the dimensions of the periodically distributed dielectric cylinders embedded in a host medium with a permittivity close to one. The scattering reduction effects are verified through simulation results. The proposed all dielectric cloaks are advantageous over other schemes due to their non-dispersive nature, the broad bandwidth, the low loss, and the ease of fabrication.
- Dispersive Cylindrical Cloaks under Non-Monochromatic Illumination (paper, arxiv)
Christos Argyropoulos, Efthymios Kallos, Yang Hao
Phys. Rev. E 81, 016611 (2010)
DOI 10.1103/PhysRevE.81.016611
Abstract
Transformation-based cylindrical cloaks and concentrators are illuminated with nonmonochromatic waves and unusual effects are observed with interesting potential applications. The transient responses of the devices are studied numerically with the finite-difference time-domain method and the results are verified with analytical formulas. We compute the effective bandwidth of several cloaking schemes as well as the effect of losses on the performance of the structures. We also find that narrowband behavior, frequency shift effects, time delays, and spatial disturbances of the incoming waves are dominant due to the inherently dispersive nature of the devices. These effects are important and should be taken into account when designing metamaterial-based devices.
- FDTD Modelling of Electromagnetic Cloaks (amazon.com link)
Christos Argyropoulos, Efthymios Kallos, Yan Zhao, and Yang Hao
Metamaterials: Theory, Design and Applications, Chapter 7, Springer (2009)
- Ground-Plane Quasi-Cloaking for Free Space (paper, arxiv)
Efthymios Kallos, Christos Argyropoulos, Yang Hao
Phys. Rev. A 79, 063825 (2009)
DOI 10.1103/PhysRevA.79.063825
Abstract
Ground-plane cloak designs are presented, which minimize scattering of electromagnetic radiation from metallic objects in the visible spectrum. It is showed that simplified ground-plane cloaks made from only a few blocks of all-dielectric isotropic materials, either embedded in a background medium or in free space, can provide considerable cloaking performance while maintaining their broadband nature. A design which operates isolated in free space that cloaks radiation originating from a specified direction is also analyzed. These schemes should be much easier to be demonstrated experimentally compared to full designs.
- Manipulating Loss in Electromagnetic Cloaks for Perfect Wave Absorption (paper, arxiv)
Christos Argyropoulos, Efthymios Kallos, Yang Hao
Optics Express, Vol.17, No.10, pp.8467-8475 (2009)
DOI 10.1364/OE.17.008467
Abstract
We examine several ways to manipulate the loss in electro-magnetic cloaks, based on transformation electromagnetics. It is found that, by utilizing inherent electric and magnetic losses of metamaterials, perfect wave absorption can be achieved based on several popular designs of electromagnetic cloaks. A practical implementation of the absorber, consisting of ten discrete layers of metamaterials, is proposed. The new devices demonstrate super-absorptivity over a moderate wideband range, suitable for both microwave and optical applications. It is corroborated that the device is functional with a subwavelength thickness and, hence, advantageous compared to the conventional absorbers.
- A High Density Hydrogen-Based Capillary Plasma Source for Particle-Beam-Driven Wakefield Accelerator Applications (paper)
Hao Chen, Efthymios Kallos, Patric Muggli, Thomas C. Katsouleas and Martin A. Gundersen
IEEE Transactions on Plasma Science, Vol.37, No.3, pp.456-562 (2009)
DOI 10.1109/TPS.2008.2011799
Abstract
We report the generation of variable plasma densities up to 10^19 cm^-3 in hydrogen-filled, hollow cathode capillary discharges, and consider their applications as a practical plasma source for particle-beam-driven plasma wakefield accelerators. The capillary consists of a transparent, cylindrical borosilicate glass tube. The plasma density is determined as a function of time, using Stark broadening of the Hα line, with a resolution of 50 ns, and is found to decay exponentially with a typical time constant of several hundreds of nanoseconds. The time delay between the discharge and the drive electron beam can therefore be tuned to reach the density appropriate for the maximum acceleration gradient. The dependence of the plasma density on the capillary geometry and gas pressure is discussed, and the results of optical studies of the discharge channel formation process are presented. Implications of the results for beam-driven plasma accelerators are discussed.
- Generation of Trains of Electron Microbunches with Sub-picosecond Spacing (paper)
Patric Muggli, Vitaly Yakimenko, Wayne D. Kimura, Marcus Babzien, Efthymios Kallos and Karl Kusche
Phys. Rev. Lett. 101, 054801 (2008)
DOI 10.1103/PhysRevLett.101.054801
Abstract
We demonstrate that trains of subpicosecond electron microbunches, with subpicosecond spacing, can be produced by placing a mask in a region of the beam line where the beam transverse size is dominated by the correlated energy spread. We show that the number, length, and spacing of the microbunches can be controlled through the parameters of the beam and the mask. Such microbunch trains can be further compressed and accelerated, and have applications to free electron lasers (FELs) and plasma wakefield accelerators (PWFAs).
- High-gradient Plasma Wakefield Acceleration with two subpicosecond electron bunches (paper)
Efthymios Kallos, Tom Katsouleas, Wayne D. Kimura, Patric Muggli, Igor Pavlishin, Igor Pogorelsky, Daniil Stolyarov, Vitaly Yakimenko
Phys. Rev. Lett. 100, 074802 (2008)
DOI 10.1103/PhysRevLett.100.074802
Abstract
A plasma wakefield experiment is presented where two 60-MeV subpicosecond electron bunches are sent into a plasma produced by a capillary discharge. Both bunches are shorter than the plasma wavelength, and the phase of the second bunch relative to the plasma wave is adjusted by tuning the plasma density. It is shown that the second bunch experiences a 150 MeV/m loaded accelerating gradient in the wakefield driven by the first bunch. This is the first experiment to directly demonstrate high-gradient, controlled acceleration of a short-pulse trailing electron bunch in a high-density plasma.
- Plasma-Based Advanced Accelerators at the Brookhaven Accelerator Test Facility (paper)
I. V. Pogorelsky, M. Babzien, K. P. Kusche, I. V. Pavlishin, V. Yakimenko, C. E. Dilley, S. C. Gottschalk, W. D. Kimura, T. Katsouleas, P. Muggli, E. Kallos, L. C. Steinhauer, A. Zigler, N. Andreev, D. B. Cline and F. Zhou
Laser Physics, Vol.16, No.2, pp. 259-266 (2006)
DOI 10.1134/S1054660X06020095
Abstract
The Accelerator Test Facility at Brookhaven National Laboratory (BNL ATF) offers to its users a unique combination of research tools that include a high-brightness 70-MeV electron beam, a mid-infrared (λ= 10μm) CO2 laser of terawatt power, and a capillary discharge as a plasma source. These cutting-edge technologies have enabled us to launch a new R&D program at the forefronts of advanced accelerators and radiation sources. The main subjects that we are researching are innovative methods of producing wakes in a linear regime using plasma resonance with the electron microbunch train periodic to the laser’s wavelength and so called “seeded” laser wakefield acceleration (LWFA) that is driven and probed by a combination of electron and laser beams. We describe the present status of the ATF experimental program, including simulations and preliminary experiments; in addition, we review previous ATF experiments that were the precursors to the present program. They encompass our demonstration of longitudinal- and transverse-field phasing inside the plasma wave, plasma channeling of intense CO2 laser beams, and the generation of e -beam microbunch trains by the inverse FEL technique.
Conference Proceedings
- Experimental Verification of Carpet Cloak Realized with Dielectric Cylinders (paper)
Di Bao, Wenxuan Tang, Christos Argyropoulos, Efthymios Kallos and Yang Hao
Antenna and Propagation Society International Symposium (APSURSI), pp.2861, Spokane, WA (2011)
DOI 10.1109/APS.2011.5997123
Abstract
Based on coordinate transformation, a carpet cloak made from nonmagnectic dielectric cylinders is designed, simulated and tested experimentally in this paper. By manipulating the periodicity of the dielectric cylinders, it is possible to achieve the required dielectric map and therefor the propagate direction of the electromagnetic waves. The scattering reduction effects of the cloak are verified through both simulation and experiment from 7 GHz to 9 GHz with the incidence of an open-end waveguide. The proposed all-dielectric cloak is broad band, low loss and easy to fabricate.
- Flat Devices Design for Antenna Systems Using Coordinate Transformation (paper)
Wenxuan Tang, Christos Argyropoulos, Efthymios Kallos, Di Bao, Wei Song, Yang Hao
Antenna and Propagation Society International Symposium (APSURSI), Toronto, Canada (2010)
DOI 10.1109/APS.2010.5561005
Abstract
The discrete coordinated transformation provides us a scheme to design flat all-dielectric devices. Such novel devices have flat profiles and are easy to be controlled and set up in the antenna systems. A flat reflector and a flat lens are designed as examples. The FDTD based simulation results have proved good agreement between the transformed devices and the conventional ones.
- Study of an Optical Metallic Nanolens with a Parallel FDTD Technique (paper)
Christos Argyropoulos, Efthymios Kallos, Atiqur Rahman, Yang Hao
International Symposium on Electromagnetic Theory (EMTS), Berlin, Germany, pp.261-264 (2010)
DOI 10.1109/URSI-EMTS.2010.5636979
Abstract
In this paper, a parallel three-dimensional dispersive FDTD method is introduced to model a nanolens. The device has its potential in subwavelength imaging at optical frequencies. The finiteness of such a nano-device and its impact on the system dynamic behavior is numerically exploited.
- Frequency Response of Plasmonic Cloaking Devices under Non-monochromatic Illumination (presentation)
Efthymios Kallos, Christos Argyropoulos, Yang Hao, Andrea Alu
IEEE International Symposium on Antennas & Propagation (AP-S), Toronto, Canada (2010)
Abstract
Plasmonic coatings have been proposed as a robust method to suppress the scattering signatures of conventional dielectric objects, typically by surrounding an object with an isotropic, homogenous, dispersive material that has a low or negative permittivity. In contrast to transformation-based cloaking devices, where highly anisotropic
inhomogeneous materials achieve invisibility by electromagnetically isolating a certain
region of space, plasmonic devices operate under the principle of scattering cancellation
and usually allow field energy to enter the device's core. So far, plasmonic devices have been mostly examined using single frequency, plane wave excitations. In this work, the performance of such plasmonic cloaks when illuminated by more realistic, broadband non-monochromatic pulses is investigated. Specifically, the 2D Total-Field Scattered-Field (TFSF) method is used within the Finite Difference Time Domain (FDTD) dispersive numerical technique in order to simulate time domain effects when temporally Gaussian pulses with bandwidths between 100 THz – 400 THz, centered at tunable frequencies around 600 THz, are launched towards cylindrical dielectric objects of diameter ~λ/4, which are covered with appropriate plasmonic coatings. It is found that the plasmonic coating can suppress the scattering from a moderately sized object almost perfectly at its design frequency, which may be tuned by varying the Drude parameters of the plasmonic materials. In addition, the limited value of material parameters of the plasmonic devices avoids extreme wavefront bending, allowing for good performance over a relatively wide frequency range (at least ~30%).
- Bandwidth Performance of a Simplified Cloak Realized by Non-Magnetic Dielectric Cylinders (paper)
Di Bao, Efthymios Kallos, Christos Argyropoulos, Yang Hao
The 4th European Conference on Antennas and Propagation (EUCAP), Barcelona, Spain (2010)
Abstract
A design for broadband scattering reduction of metallic objects based on non magnetic dielectric cylinders is proposed, and their bandwidth performance is examined. The devices are functional in the presence of a ground plane as well as directionally in free space between 4 - 10 GHz. The required all-dielectric permittivity distribution for the cloak is achieved by manipulating the dimensions of periodically distributed dielectric cylinders embedded in the host medium with a permittivity close to one. The validity of the scattering reduction is shown through simulation results. The proposed all dielectric cloaks are advantageous over other schemes due to their nature of broad bandwidth, low loss and ease of fabrication
- Study of the Optical Black Hole with the FDTD Method (paper)
Christos Argyropoulos, Efthymios Kallos, Yang hao
Annual Conference of the Applied Computational Electromagnetics Society (ACES) (2010)
Abstract
An optical black hole is studied with a radially-dependent FDTD technique. The device is non- dispersive and can have a broadband operation. It can be constructed with non-resonant metamaterial structures based on the recently proposed transformation electromagnetic method. The perfect absorption is demonstrated for different angles of incidence and excitation types. A matched to free space device is proposed and the relevant physics are explored.
- Bandwidth evaluation of dispersive transformation electromagnetics based devices (paper)
Christos Argyropoulos, Efthymios Kallos, Yang Hao
2nd International Conference on Metamaterials, Photonic Crystals and Plasmonics (META), Cairo, Egypt (2010)
Abstract
In this paper the transient responses of some devices which are based on trans- formation electromagnetics are studied, such as invisible cloaks and concentrators, by using the Finite-Di®erence Time-Domain (FDTD) numerical technique. In particular, e®ects of the inher- ent losses as well as the coating size of the ideal cylindrical cloak on its bandwidth and cloaking performance are examined. In addition, it is demonstrated that the performance of transforma- tion electromagnetics based devices is a®ected by the material parameters in the design, although they may behave nicely under monochromatic plane wave illuminations. The obtained results are of interest for the future practical implementation of these structures.
- Discrete Transformation Electromagnetics and its Applications in Antenna Design (paper)
Wenxuan Tang, Christos Argyropoulos, Efthymios Kallos, Yang Hao
International Workshop on Antenna Technology (IWAT), Lisbon, Portugal (2010)
DOI 10.1109/IWAT.2010.5464682
Abstract
Current designs of electromagnetic cloaks are largely based on the use of metamaterials and a technique so-called ¿transformation optics/electromagnetics¿. Free space cloaks require extreme materials which are difficult to implement in practice, however, the theory of ¿transformation optics/electromagnetics¿ offers a useful design tool for antenna engineers in developing novel antennas. In this paper, a method of discrete transformation is proposed, which offers us an easy way to transform the conventional devices to the all-dielectric flat ones. A flat reflector and a flat lens antenna in the free space are designed as examples. Finite-Difference Time-Domain (FDTD) method based simulation results have proved that these simply constructed all-dielectric devices have very nice performances compared with the conventional ones, while have the advantages of flat profiles and small thicknesses.
- Properties and Applications of Periodic Dielectric Particles as Tunable-Index Materials (paper)
Di Bao, Christos Argyropoulos, Efthymios Kallos,Yang Hao
Loughborough Antennas & Propagation Conference (LAPC), Loughborough, UK (2009)
Abstract
In this paper, the properties of cylindrical high permittivity dielectric particles are studied. The refractive index is calculated with a parameter retrieval method and a Fourier Transform method. It is shown that the periodic particles have low losses, while being non-resonant and broadband. Furthermore, the refractive index can be adjustable according to the dimensions of the particles. Application examples are also presented.
- Simplified Directional Ground-Plane Cloaks (paper)
Efthymios Kallos, Christos Argyropoulos, Yang Hao
International Conference on Electromagnetics in Advanced Applications, Torino, Italy (2009)
Abstract
A directional cloaking device based on the transformation electromagnetics approach for a ground-plane cloak in the visible spectrum is presented. A simplified cloak consisting of 16 all-dielectric blocks is illuminated under plane wave incidence. The field distributions, the scattered energy and the frequency spectrum of the incident pulse are also analyzed. It is found that the simplified cloak works very similar to the full high-resolution cloak. Finite-Difference Time-Domain simulations are used to quantify the electromagnetic performance of the device.
- Plasma Wakefield Accelerators Using Multiple Electron Bunches
Efthymios Kallos
Proceedings of the 2009 IEEE European Particle Accelerator Conference, Vancouver, Canada (2009)
Abstract
For 70 years particle acceleration schemes have been based on the same technology which places particles onto rf electric fields inside metallic cavities. However, since the accelerating gradients cannot be increased arbitrarily due to limiting effects such as wall breakdown, in order to reach higher energies today’s accelerators require km-long structures that have become very expensive to build, and therefore novel accelerating techniques are needed to push the energy frontier further. Plasmas do not suffer from those limitations since they are gases that are already broken down into electrons and ions. In addition, the collective behavior of the particles in plasmas allows for generated accelerating electric fields that are orders of magnitude larger than those available in conventional accelerators. As plasma acceleration technologies mature, one of the main future challenges is to monoenergetically accelerate a second trailing bunch by multiplying its energy in an efficient manner, so that it can potentially be used in a future particle collider. The work presented here analyzes the use of multiple electron bunches in order to enhance certain plasma acceleration schemes.
- Dispersive and Bandwidth Effects using Non-monochromatic Pulses for Ground-Plane Quasi-Cloaks (paper)
Efthymios Kallos, Christos Argyropoulos, Yang Hao
Proceedings of the 3rd International Congress on Advanced Electromagnetic Materials in Microwaves and Optics, pp.280-282, London, UK (2009)
Abstract
We investigate through FDTD simulations bandwidth issues for ground-plane cloaks by launching non-monochromatic pulses against a metal object placed on a ground plane. We find that the cloak designs can be placed in free space while still maintaining excellent cloaking potential over a broad frequency range. Small regions that require dispersive materials have little effect in the performance. The tradeoffs between the design detail of the device and its bandwidth are also examined.
- Characterisation of Electromagnetic Cylindrical Cloaks (paper)
Efthymios Kallos, Christos Argyropoulos, Yang Hao
Proceedings of the 3rd International Congress on Advanced Electromagnetic Materials in Microwaves and Optics, pp.537-539, London, UK (2009)
Abstract
We investigate through FDTD simulations bandwidth issues for ground-plane cloaks by launching non-monochromatic pulses against a metal object placed on a ground plane. We find that the cloak designs can be placed in free space while still maintaining excellent cloaking potential over a broad frequency range. Small regions that require dispersive materials have little effect in the performance. The tradeoffs between the design detail of the device and its bandwidth are also examined.
- Examining the Limitations of Ideal Cylindrical Cloaks through Dispersive Finite-Difference Time-Domain Simulations (paper)
Christos Argyropoulos, Efthymios Kallos, Yang Hao
IEEE International Symposium on Antennas & Propagation, North Charleston, SC (2009)
DOI 10.1109/APS.2009.5172319
Abstract
In this paper the limitations of the cloak's transient response were studied. It can be concluded that the ideal cylindrical cloak, even though it is supposed to be ldquoidealrdquo, has some drawbacks. The highly dispersive nature of the device is the main reason for these limitations, which are directly affecting its performance. It is deduced that the cloak is very narrowband, ideally operates correctly only at one frequency. In order to avoid these restraints, broadband active metamaterials have to be designed or, alternative, novel non-dispersive cloak designs have to be conceived.
- FDTD Simulations of Approximate Ground-Plane Cloaks (paper)
Efthymios Kallos, Wei Song, Christos Argyropoulos, Yang Hao
IEEE International Symposium on Antennas & Propagation, North Charleston, SC (2009)
DOI 10.1109/APS.2009.5172327
Abstract
In this paper we use a coordinate transformation to create a cloak that covers a triangular-shaped conducting object placed on a ground plane. A finite-difference time-domain (FDTD) algorithm that uses non-orthogonal mesh cells (NFDTD) is initially utilized to test the full cloak. Using the coordinate transformation method, we have designed a ground-plane cloak that is an approximate version of the full scale cloak. Despite its simple structure, which consists of only 16 blocks of all-dielectric materials, the approximate cloak is found through FDTD simulations to work very well when an incident pulse is impinging on an object placed on a flat ground plane. The scattering signature is significantly suppressed and we expect such cloaks to be fairly easy to implement in reality.
- Simulations of a High-Transformer-Ratio Plasma Wakefield Accelerator Using Multiple Electron Bunches (paper)
Efthymios Kallos, Patric Muggli, Tom Katsouleas, Vitaly Yakimenko, Jangho Park and Karl Kusche
Proceedings of the 13th Advanced Accelerator Concepts Workshop, Santa Cruz, CA, Vol.1086, pp.580-585 (2008)
DOI 10.1063/1.3080972
Abstract
Particle-in-cell simulations of a plasma wakefield accelerator in the linear regime are presented, consisting of four electron bunches that are fed into a high-density plasma. It is found that a high transformer ratio can be maintained over 40cm of plasma if the charge in each bunch is increased linearly, the bunches are placed 1.5 plasma wavelengths apart and the bunch emmitances are adjusted to compensate for the nonlinear focusing forces. The generated wakefield is sampled by a test witness bunch whose energy gain after the plasma is six times the energy loss of the drive bunches. The possibility of extending these schemes to the blowout regime is also examined and it is found to be complicated without shaping the drive bunches. This work was supported by the US Department of Energy.
- Contrast of Subpicosecond Microelectron Bunch Trains (paper)
Patric Muggli, Vitaly Yakimenko, Jangho Park, Efthymios Kallos, Karl Kusche and Marcus Babzien
Proceedings of the 13th Advanced Accelerator Concepts Workshop, Santa Cruz, CA, Vol.1086, pp.683-688 (2008)
DOI 10.1063/1.3080990
Abstract
We present experimental results obtained with a method for producing trains of microbunches with time separation and length of less than a picosecond. The method uses a solid mask placed in a region of the beam line where the bunch transverse size is dominated by its correlated energy spread. The mask spoils the emittance of selected bunch slices. The particles scattered by the solid parts of the mask are lost along the beam line. The modulation in energy of the beam charge therefore also corresponds to a modulation of the beam current in time. The mask and beam parameters can be chosen to design the bunch current profile for particular applications, such as plasma wakefield accelerators (PWFAs) or free electron lasers (FELs).
- Some Experimental Results of a Plasma Wakefield Accelerator Using Multiple Electron Bunches (paper, presentation)
Efthymios Kallos, Tom Katsouleas, Patric Muggli, Igor Pavlishin, Igor Pogorelsky, Daniil Stolyarov, Vitaly Yakimenko, and Wayne D. Kimura
Proceedings of the 2008 IEEE European Particle Accelerator Conference, Genoa, Italy, pp.1912-1914 (2008)
Abstract
We present some preliminary experimental results of a plasma wakefield accelerator technique which utilizes multiple electron bunches in order to drive a plasma wave. The experiments were performed at the Accelerator Test Facility of Brookhaven National Laboratory where 5-8 equidistant bunches with a spacing which was varied between 100-200μm were fed into a 6mm-long capillary discharge plasma. By varying the time delay of the bunches with respect to the discharge different plasma densities could be tuned, and the effects of the plasma on the bunches were recorded. Such multiple bunch schemes are of great interest because they can provide increased efficiencies and high transformer ratios for advanced accelerators.
- Generation of Electron Microbunches Trains with Adjustable Sub-picosecond Spacing for PWFA and FEL applications (paper)
Patric Muggli, Efthymios Kallos, Vitaly Yakimenko, Marcus Babzien and Karl Kusche
Proceedings of the 2008 IEEE European Particle Accelerator Conference, Genoa, Italy, pp.2830-2832 (2008)
Abstract
We demonstrate that trains of subpicosecond electron microbunches, with subpicosecond spacing, can be produced by placing a mask in a large dispersion region of the beam line where the beam transverse size is dominated by the correlated energy spread. The particles are selected based on the scattering of their emittance at the mask. The electrons that hit the solid arts of the mask are subsequently lost. The mask spatial pattern is converted into a time pattern in the dispersion-free region of the beam line. The experiment was performed with the Brookhaven National Laboratory Accelerator Test Facility 60 MeV beam. We show that the number, length, and spacing of the microbunches can be controlled through the parameters of the beam and the mask. Trains with one to eight equidistant microbunches are produced. The microbunches spacing is adjusted in the 100 to 300 microns or 300 fs to 1 ps range and comparable microbunch length. The train structure is measured using CTR interferometry, and is stable in time and energy. Such microbunch trains can be further compressed and accelerated, and have applications to free electron lasers (FELs) and plasma wakefield accelerators (PWFAs).
- Plasma Wakefield Acceleration Utilizing Multiple Electron Bunches (paper)
Efthymios Kallos, Tom Katsouleas, Patric Muggli, Igor Pavlishin, Igor Pogorelsky, Daniil Stolyarov, Vitaly Yakimenko, Wayne D. Kimura
Proceedings of the 2007 IEEE Particle Accelerator Conference, Albuquerque, NM, pp.3070-3072 (2007)
DOI 10.1109/PAC.2007.4440671
Abstract
We investigate various plasma wakefield accelerator schemes that rely on multiple electron bunches to drive a large amplitude plasma wave, which are followed by a witness bunch at a phase where it will sample the high acceleration gradient and gain energy. Experimental verifications of various two bunch schemes are available in the literature; here we provide analytical calculations and numerical simulations of the wakefield dependency and the transformer ratio when M drive bunches and one witness bunch are fed into a high density plasma, where M is between 2 and 10. This is a favorable setup since the bunches can be adjusted such that the transformer ratio and the efficiency of the accelerator are enhanced compared to single bunch schemes. The possibility of a five bunch ILC afterburner to accelerate a witness bunch from 100 GeV to 500 GeV is also examined.
- Generation and Characterization of the Microbunched Beams with a Mesh Target (paper)
Patric Muggli, Efthymios Kallos, Vitaly Yakimenko, Marcus Babzien, Karl Kusche, Wayne D. Kimura
Proceedings of the 2007 IEEE Particle Accelerator Conference, Albuquerque, NM, pp.3073-3075 (2007)
DOI 10.1109/PAC.2007.4440674
Abstract
We use a wire mesh mask placed in a dispersive region of the Accelerator Test Facility (ATF) at Brookhaven National Laboratory to produce a train of picosecond microbunches. The bunch spacing and charge can be tailored for specific applications. We plan on using this method to generate a train of drive bunches and a witness bunch for plasma wakefield accelerator experiments.
- Plasma Wakefield Acceleration Experiments Using Two Subpicosecond Electron Bunches (paper)
Patric Muggli, Wayne D. Kimura, Efthymios Kallos, Tom Katsouleas, Karl Kusche, Igor Pavlishin, Igor Pogorelsky, Vitaly Yakimenko
Proceedings of the 2007 IEEE Particle Accelerator Conference, Albuquerque, NM, pp.3079-3081 (2007)
DOI 10.1109/PAC.2007.4440672
Abstract
Two subpicosecond electron bunches, separated in energy by approximately 2 MeV and in time by 0.5-1 ps, are sent through a capillary discharge plasma. The plasma density is varied from ~10614 cm-3 to ~10618 cm^-3. A 1-D plasma wakefield acceleration (PWFA) model indicates the net wakefield produced by the bunches will depend on their relative charge, temporal separation, and the plasma density. The wakefield of the first bunch will also affect the amount of energy gain or loss of the second bunch. During measurements of the energy spectrum of the bunches, we observed a difference in the amount of loss depending on the plasma density. Indication of gain was also observed.
- Femtosecond Microbunched Electron Beam - A New Tool for Advanced Accelerator Research (paper)
Igor Pogorelsky, Marcus Babzien, Ilan Ben Zvi, Karl Kusche, Igor Pavlishin, Vitaly Yakimenko, C. Dilley, S. Gottschalk, Wayne D. Kimura, S. Steinhauer, Efthymios Kallos, Tom Katsouleas, Patric Muggli, A. Zigler, Sammer Banna, Levi Schächter, David Cline, Feng Zhou, Y. Kamiya, T. Kumita
Proceedings of the Third International Conference on Superstrong Fields in Plasmas, Vol. 827, No.1, pp. 297-307 (2006)
DOI 10.1063/1.2195220
Abstract
We employed periodic trains of femtosecond electron bunches for testing several novel concepts of acceleration. A microwave-driven linac sends a 45-MeV electron beam (e-beam) through a magnetic wiggler wherein the e-beam energy is modulated via the inverse free electron laser (IFEL) technique by interacting with a 30-GW CO2 laser beam, so creating 3 fs long microbunches separated by a 30 fs laser period. We show several examples of utilizing such a femtosecond bunch train in advanced accelerator and radiation source research. We demonstrated that microbunching improves the performance of the laser acceleration process compared to the previously investigated single-bunch technique. Specifically, microbunches were phased to the electromagnetic wave of the CO2 laser beam inside a matched tapered wiggler where ~80% of electrons gained energy as an ensemble while maintaining a narrow energy spread (i.e., monoenergetic). Another plasma wakefield acceleration (PWFA) experiment explored resonant wakefield excitation in an electric discharge plasma with the plasma frequency matched to that of the CO2 laser. Simulations predict orders-of-magnitude enhancement in the wakefield's amplitude compared with that attained with single bunches. In the Particle Acceleration by Stimulated Emission of Radiation (PASER) experiment, we tested a prediction that an active laser medium can produce particle acceleration by stimulating the emission of radiation. The process benefits from the action of a periodic train of microbunches resonating with the laser transition. Finally, we analyze prospects for using partially coherent x-ray sources based on Thomson backscattering from the electron microbunch train.
- Generation and Characterization of the Microbunched Beams in the Range from 0.3 to 500 Femtoseconds (paper)
Vitaly Yakimenko, Marcus Babzien, Karl Kusche, Efthymios Kallos, Patric Muggli, and Wayne D. Kimura
Proceedings of the 28th International Free Electron Laser Conference, Berlin, Germany, pp.481-484 (2006)
Abstract
The recent results indicate formation and measurement of the micro bunch structures of the different time scales. Double beam structure produced and characterized at 100 fs - 05. ps range using beam splitting during compression in the magnetic chicane - "dog leg" arrangement. Arbitrary number of 10-50 fs microbunches are sliced out of 5 ps long beam using wire mesh. CSR interferometer is used for detailed characterization of the beams in the two techniques above. 0.3 fs bunches are produced by IFEL and characterized by spectral measurements of the multiple harmonics. Presentation covers experimental results at Brookhaven Accelerator Test Facility.
- Resonant Plasma Wakefield Experiment: Plasma Simulations and Multibunched Electron Beam Diagnostics (paper)
Efthymios Kallos, Patric Muggli, Tom Katsouleas, Vitaly Yakimenko, Daniil Stolyarov, Igor Pogorelsky, Igor Pavlishin, Karl Kusche, Marcus Babzien, Ilan Ben-Zvi and Wayne D. Kimura
Proceedings of the 12th Advanced Accelerator Concepts Workshop, Lake Geneva, WI, AIP Conference Proceedings No.877, pp.520-526 (2006)
DOI 10.1063/1.2409178
Abstract
In the multibunch plasma wakefield acceleration experiment at the Brookhaven National Lab's Accelerator Test Facility a 45 MeV electron beam is initially modulated through the IFEL interaction with a CO2 laser beam at 10.6 µm into a train of short microbunches, which are spaced at the laser wavelength. It is then fed into a high-density capillary plasma with a density resonant at this spacing (1.0 × 1019 cm-3). The microbunched beam can resonantly excite a plasma wakefield much larger than the wakefield excited from the non-bunched beam. Here we present plasma simulations that confirm the wakefield enhancement and the results of a series of CTR measurements performed of the multibunched electron beam.
- Update on Seeded SM-LWFA and Pseudo-Resonant LWFA Experiments (paper)
W D. Kimura, N. E. Andreev, X. Ding, M.Babzien, I. Ben-Zvi, D. B. Cline, S. M. Hooker, E. Kallos, T. C. Katsouleas, K. P. Kusche, S. V. Kuznetsov, P. Muggli, I. V. Pavlishin, I. V. Pogorelsky, A. A. Pogosova, L. C. Steinhauer, D. Stolyarov, A. Ting, V. Yakimenko, A. Zigler, and F. Zhou
Proceedings of the 12th Advanced Accelerator Concepts Workshop, Lake Geneva, WI, AIP Conference Proceedings No.877, pp.534-540 (2006)
DOI 10.1063/1.2409180
Abstract
The Staged Electron Laser Acceleration — Laser Wakefield (STELLA-LW) experiment is investigating two new methods for laser wakefield acceleration (LWFA) using the TW CO2 laser available at the Brookhaven National Laboratory Accelerator Test Facility. The first is seeded self-modulated LWFA where an ultrashort electron bunch (seed) precedes the laser pulse to generate a wakefield that the laser pulse subsequently amplifies. The second is pseudo-resonant LWFA where nonlinear pulse steepening of the laser pulse occurs in the plasma allowing the laser pulse to generate significant wakefields. The status of these experiments is reviewed. Evidence of wakefield generation caused by the seed bunches has been obtained as well as preliminary energy gain measurements of a witness bunch following the seeds. Comparison with a 1-D linear model for the wakefield generation appears to agree with the data.
- Subpicosecond Double Electron Bunch Generation (paper)
Wayne D. Kimura, Vitaly Yakimenko, Marcus Babzien, Xiaoping Ding, Eftymios Kallos, Tom Katsouleas, Karl Kusche, Patric Muggli, Igor Pavlishin, Igor Pogorelsky, Daniil Stolyarov, and Feng Zhou
Proceedings of the 12th Advanced Accelerator Concepts Workshop, Lake Geneva, WI, AIP Conference Proceedings No.877, pp.527-533 (2006)
DOI 10.1063/1.2409179
Abstract
We have demonstrated creating two compressed electron beam bunches from a single 60-MeV bunch. Measurements indicate they have comparable bunch lengths (~100–200 fs) and are separated in energy by ~1.8 MeV with the higher-energy bunch preceding the lower-energy bunch by 0.5–1 ps. A possible explanation for the double-bunch formation process is also presented.
- Plasma Density Measurements in Hydrogen-Filled and Plastic Ablation Discharge Capillaries based on Stark Broadening of Atomic Hydrogen Spectral Lines (paper)
Daniil Stolyarov, Igor Pavlishin, Marcus Babzien, Wayne Kimura, Patric Muggli, Efthymios Kallos and Vitaly Yakimenko
Proceedings of the 12th Advanced Accelerator Concepts Workshop, Lake Geneva, WI, AIP Conference Proceedings No.877, pp.784-791 (2006)
DOI 10.1063/1.2409216
Abstract
Results of plasma density measurements in ablative and hydrogen-filled discharge capillaries are presented. The method of plasma density measurement is based on Stark broadening of atomic hydrogen spectral lines in the plasma due to interaction of the hydrogen atoms with free charges. To ensure the measured plasma density corresponds to the internal portion of the discharge volume, we also examine a possibility to collect the plasma light emission with an optical fiber inserted inside the capillary channel. We studied the time dependence of the plasma density relative to the beginning of the discharge with a temporal resolution of 150 ns. The plasma density was found to vary over a range of 10^15 – 10^17 cm^-3. The dependence of the plasma density upon discharge voltage and hydrogen pressure in the hydrogen-filled capillary was also studied. The possibility of designing a hybrid ablative hydrogen-filled capillary that allows us to simplify the high voltage generator scheme and reach high plasma densities is discussed.
- A Multibunch Plasma Wakefield Accelerator (paper)
Efthymios Kallos, Tom Katsouleas, Patric Muggli, Ilan-Ben Zvi, Igor Pogorelsky, Vitaly Yakimenko, Igor Pavlishin, Karl Kusche, Marcus Babzien, Feng Zhou and Wayne D. Kimura
Proceedings of the 2005 IEEE Particle Accelerator Conference, Knoxville, TN, pp.3384-3386 (2005)
Abstract
We investigate a plasma wakefield acceleration scheme where a train of electron microbunches feeds into a high density plasma. When the microbunch train enters such a plasma that has a corresponding plasma wavelength equal to the microbunch separation distance, a strong wakefield is expected to be resonantly driven to an amplitude that is at least one order of magnitude higher than that using an unbunched beam. PIC simulations have been performed using the beamline parameters of the Brookhaven National Laboratory Accelerator Test Facility operating in the configuration of the STELLA inverse free electron laser (IFEL) experiment. A 65 MeV electron beam is modulated by a 10.6 μm CO2 laser beam via an IFEL interaction. This produces a train of ~90 microbunches separated by the laser wavelength. In this paper, we present both a simple theoretical treatment and simulation results that demonstrate promising results for the multibunch technique as a plasma-based accelerator.
Theses
Ph.D. Dissertation
Plasma Wakefield Accelerators Using Multiple Electron Bunches
University of Southern California
Los Angeles, CA (2008)
Abstract:
Particle accelerators are the tools that physicists use today in order to probe the fundamental forces of Nature, by accelerating charged particles such as electrons and protons to high energies and then smashing them together. For the past 70 years the acceleration schemes have been based on the same technology, which is to place the particles onto radio-frequency electric fields inside metallic cavities. However, since the accelerating gradients cannot be increased arbitrarily due to limiting effects such as wall breakdown, in order to reach higher energies today’s accelerators require km-long structures that have become very expensive to built, and therefore novel accelerating techniques are needed to push the energy frontier further.
Plasmas do not suffer from those limitations since they are gases that are already broken down into electrons and ions. In addition, the collective behavior of the particles in plasmas allows for generated accelerating electric fields that are orders of magnitude larger than those available in conventional accelerators. Such wakefields have been demonstrated experimentally, typically by feeding either single electron bunches or laser beams into high density plasmas. As such plasma acceleration technologies mature, one of the main future challenges is to monoenergetically accelerate a second trailing bunch by multiplying its energy in an efficient manner, so that it can potentially be used in a future particle collider.
The work presented in this dissertation is a fruitful combination of theory, simulations and experiments that analyzes the use of multiple electron bunches in order to enhance certain plasma acceleration schemes. Specifically, the acceleration of a trailing electron bunch in a high-gradient wakefield driven by a preceding bunch is demonstrated experimentally for the first time by using bunches short enough to sample a small phase of the plasma wakes.
Additionally, it is found through theoretical analysis and through simulations that by using multiple bunches to drive the wakefields, the energy of a trailing bunch could be efficiently multiplied in a single stage, thus possibly reducing the total length of the accelerator to a more manageable scale. Relevant proof-of-principle experimental results are also presented, along with suggested designs that could be tested in the near future. Furthermore, electron beam and plasma diagnostics are analyzed and presented, which are necessary for properly completing and understanding any plasma wakefield experiment. Finally, certain types of plasma sources that can be used in related experiments are designed, diagnosed and tested in detail.
We are at the very beginning of time for the human race. It is not unreasonable that we grapple with problems. But there are tens of thousands of years in the future. Our responsibility is to do what we can, learn what we can, improve the solutions, and pass them on.
- Richard Feynman
Dissertation (4.2MB pdf)
The image above is a word cloud of the abstract of my dissertation as plotted by wordle.
Research Proposal
Capillary Discharges as Plasma Sources for Wakefield Acceleration Experiments
University of Southern California
Los Angeles, CA (2007)
Abstract:
In this proposal several ablative and gas-filled capillary plasma sources are reviewed, relevant diagnostics that can be utilized to diagnose the peak plasma density and the density evolution over time are investigated, experimental results are presented, and also methods and techniques that can be implemented in order to generate and diagnose high density (~10^19 cm^-3) plasmas are proposed for this realm of densities which is at the moment not reliably available using capillaries. The work is aimed towards utilizing such plasma sources in high gradient plasma wakefield accelerators.
Proposal (3.9MB pdf)
Diploma Thesis
Study of the Smith-Purcell Effect
National & Techical University of Athens
Athens, Greece (2003)
Abstract:
When a charged particle propagates parallel to a periodic structure, energy is radiated in the form of an electromagnetic wave. This type of radiation is caused due to the interaction of the charged particle’s field (such as an electron) with the periodicity of the structure, and belongs to a wide category of phenomena which arise through the interaction of electrons with a medium. The energy radiates under a specific angle with regard to the line of propagation, an angle which depends on the frequency of the particle. So, different frequencies radiate the energy into different angles. This phenomenon was predicted by Frank in 1942 and was experimentally observed in 1953 by Smith and Purcell.
Here we study theoretically the structure which consists of a dielectric waveguide (slab) of specific width, with a sinusoidal periodicity with regard to one of its surfaces. A line current moves parallel to the direction of periodicity and in short distance from it, which causes waves to arise inside the waveguide. These waves are periodical following the period of the structure.
We solve both the homogenous problem (without the source line) and we find the dispersion relation and the propagation factors of the waveguide, as well as the non homogenous problem where the Green function is derived. We use the Floquet theorem for periodic structures, find solutions to the Helmholtz equation, and then apply boundary conditions of continuity to find the unknown coefficients.
The results are calculated arithmetically using Matlab; we draw the electromagnetic fields and the power Poynting vector everywhere in space. Through Poynting vector we derive conclusions about the angle of radiation with respect to frequency. The computer programs are parameterized with respect to the frequency, the geometrical features of the structure and the speed of the source line. Finally, this technique is not limited to sinusoidal structures but applies in any periodic one.
In the beginning, the Universe was created.
This has made a lot of people very angry,
and has been widely regarded as a bad move.
- The Hitch Hiker's Guide to the Galaxy
Thesis (2.9MB pdf, in Greek)
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