themos.kallos at elec.qmul.ac.uk

Resume (one page)          Curriculum Vitae

What's new:

 - Two papers from the AAC conference in the summer are uploaded; the upcoming paper on IEEE transactions on capillary discharges is listed (Feb 2009)
 - Two new posters for the APS 2008 Conference have been uploaded (Nov 2008)
 -  I am currently a post-doctoral researcher at the Antenna Group of Queen Mary University in London (Sep 2008)
 -  The paper on electron bunch train generation was published online (Aug 2008)
 -  I successfully defended my thesis and got it approved the thesis editors (get a copy here).
    Currently looking for postdocs! (Jul 2008)
 -  The talk at the EPAC 2008 conference was a smashing success - get the slides here
    (email me for a version with the animations and videos) (Jun 2008)
 -  The double-bunch paper was published online! (Feb 2008)
 -  A single-page resume is up (Feb 2008)
 -  An updated CV was uploaded (Jan 2008)
 -  The two PRL paper abstracts and two new posters are up
 -  The presentation on the evidence for dark matter is added.
 -  Presentation on Biofuels added (Dec 2007)
 -  Include all the abstracts to the papers (Sep 2007)

Some essential information about myself

I was born in Lamia, Greece on January 2nd, 1981. I have been an undergraduate student in the National Technical University of Athens from 1998 to 2003, in the School of Electrical and Computer Engineering. I did my Ph.D. in Electrical Engineering at the University of Southern California in Los Angeles, under the supervision of Professor Thomas Katsouleas working on plasma accelerators. I am currently a post-doctoral researcher at the School of Electronic Engineering and Computer Science at Queen Mary University of London working on metamaterials and cloaking applications.

I am a student member of the IEEE, with an undeniable, amateurish love for physics. I promised myself to work on a subject of engineering but as close to physics as possible. Engineering comes and goes with time but physics timeless; a discovery is always a discovery and a physical law is always a physical law.

On Mematerials

Metamaterials are a special class of man-made artificial materials that can be designed such that they control light in unusual ways. For example, one can make materials that exhibit a negative refractive index, thus bending light in the opposite direction compared to normal materials, or making a perfect lens that can capture every single detail of an image.

We are currently working on using metamaterials to create cloaking devices, i.e. to put a cover around an object that would render it invisible. It is an exciting application of metamaterials that have drawn a lot of interest in the past 2-3 years.

On Plasma Accelerators

The pioneer of my research field was John Dawson from UCLA. He was my advisor's advisor.

The term "Accelerators" applies because we accelerate particles (mostly electrons) to very high speeds. Most of the interesting physics done in the world today uses huge accelerator machines that smash particles together in order to discover new laws. However these machines (e.g. LHC, SLAC, RHIC) are many miles long and cost billions of dollars to build and operate. They use microwave electric fields to accelerate particles.

Here is were the "Plasma" comes into place. We use the much stronger electric fields that are generated inside plasmas (a charged gas) to accelerate the particles. In April of 2006 our group was involved in an experiment that doubled the energy of the 3km long Stanford Accelerator in just 1 meter length of plasma.

My research involves theoretical calculations, simulations, and experimental work on a different experiment done at the Accelerator Test Facility of the Brookhaven National Lab. Our experiment is different in the way we try to generate the electric fields inside the plasma: instead of a single bunch of electrons we use 100 bunches of electrons and try to generate the field like a harmonic oscillator driven with an external periodic force (like a mother pushing her child on the swing again and again). It is called the multibunch experiment, and it is the first of its kind ever done in the world.

Research group website           Research group blog (quite technical)

Publications
as of February 2009

• "A High Density Hydrogen-Based Capillary Plasma Source for Particle-Beam-Driven Wakefield Accelerator Applications"
  Hao Chen, Efthymios Kallos, Patric Muggli, Thomas C. Katsouleas and Martin A. Gundersen
  IEEE Transactions on Plasma Science (2009), to appear
  
  Abstract: We report the generation of variable plasma densities up to 10¹⁹ 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 H_α 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.
  
  

• "Simulations of a high-transformer-ratio plasma wakefield accelerator using multiple electron bunches"
  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) (paper)
  
  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.
  
  

• "A mask technique for the generation of trains or microbunches with subpicosecond spacing and length "
  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) (paper)
  
  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"
  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) (paper, presentation)
  
  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"
  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) (paper)
  
  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).
  
  

• "Generation of Trains of Electron Microbunches with Sub-picosecond Spacing"
  Patric Muggli, Vitaly Yakimenko, Wayne D. Kimura, Marcus Babzien, Efthymios Kallos and Karl Kusche
  Phys. Rev. Lett. 101, 054801 (2008) (paper)
  
  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"
  Efthymios Kallos, Tom Katsouleas, Wayne D. Kimura, Patric Muggli, Igor Pavlishin, Igor Pogorelsky, Daniil Stolyarov, Vitaly Yakimenko
  Phys. Rev. Lett. 100, 074802 (2008) (paper)
  
  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 Wakefield Acceleration Utilizing Multiple Electron Bunches"
  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). (poster, paper)
  
  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"
  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). (paper)
  
  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"
  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). (paper)
  
  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 ~10¹⁴ cm^-3 to ~10¹⁸ 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"
  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). (paper)
  
  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"
  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) (paper)
  
  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.
  
   

• "Plasma Simulations and Multibunched Electron Beam Diagnostics"
  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) (poster, presentation, paper)
  
  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"
  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) (paper)
  
  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"
  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) (paper)
  
  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"
  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) (paper)
  
  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 10¹⁷ – 10¹⁵ 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.
  
   

• "Plasma-Based Advanced Accelerators at the Brookhaven Accelerator Test Facility"
  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) (paper)
  
  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.
  
   

• "A Multibunch Plasma Wakefield Accelerator"
  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). (poster, paper)
  
  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.
   

Posters & Presentations Only

• "Multi Bunch Plasma Wakefield Acceleration Experiments"
  Efthymios Kallos, Patric Muggli, Vitaly Yakimenko, Marcus Babzien, Karl Kusche, Adam Lichtl and Jangho Park
  50th Annual Meeting of the American Physical Society's Division of Plasma Physics, Dallas, TX (2008). (presented by P. Muggli)
  
  We have demonstrated a method to produce a train of equidistant drive electron bunches (P. Muggli et al, Phys. Rev. Lett. 2008) followed by a witness bunch, suitable for multi-bunch plasma wakefield acceleration experiments (MB-PWFA).The drive bunches are separated by an adjustable distance d=100 to 450 microns. The witness bunch follows the last drive bunch by 1.5d. At the Brookhaven National Laboratory Accelerator Test Facility this bunch train is sent into a 2cm-long capillary discharge. The electron plasma density is adjusted by varying the timing between the firing capillary discharge and the arrival time of the electron bunch train. Calculations show that the acceleration of the witness bunch is maximum when the plasma wavelength is equal to d (resonant excitation of the wakefield). In the experiment we measure the energy gained by the witness bunch as a function of the plasma density and of the number of drive bunches. This scheme could be used to multiply the energy of the witness bunch in a single PWFA stage. Preliminary experimental results will be presented.
  
  

• "Plasma Wakefield Acceleration Simulations with Multiple Electron Bunches"
  Efthymios Kallos, Patric Muggli, Vitaly Yakimenko, Marcus Babzien, Karl Kusche, Adam Lichtl and Jangho Park
  50th Annual Meeting of the American Physical Society's Division of Plasma Physics, Dallas, TX (2008) (poster) . (presented by P. Muggli)
  
  In the multibunch plasma wakefield accelerator, a train of electron bunches is utilized to excite a high gradient wakefield in a plasma which can be sampled by a trailing short witness bunch. We show that for five drive bunches with 150 pC total charge which can be generated in the Accelerator Test Facility of the Brookhaven National Lab, a wakefield of 140 MV/m can be generated if the plasma density is matched to the bunch train period. In addition, the possibility of ramping the charge per bunch in order to achieve high transformer ratios (>5) is examined, a scenario that is of great interest for a future afterburner collider. The work was supported by the US Department of Energy.
  
  

• "Multi-bunch plasma wakefield acceleration experiments"
  Patric Muggli, Efthymios Kallos, Tom Katsouleas, Vitaly Yakimenko, Marcus Babzien, Karl Kusche, Igor Pogorelsky and Wayne D. Kimura
  35th IEEE International Conference on Plasma Science, Karlsruhe, Germany (2008). (presented by P. Muggli)
  
  

• "Generation of Electron Microbunches Trains with Adjustable Sub-picosecond Spacing"
  Patric Muggli, Efthymios Kallos, Vitaly Yakimenko, Marcus Babzien, Karl Kusche
  Annual Meeting of the American Physical Society St. Louis, MO (2008). (presented by P. Muggli)
  
  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. The particles are selected based on the scattering of their  emittance at the mask. 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 have been produced. The microbunches spacing was  adjusted in the 100 to 300 $\mu $m or 300 fs to 1 ps range. The train  structure is measured using CTR interferometry. Such microbunch trains can  be further compressed and accelerated, and have applications to free  electron lasers (FELs) and plasma wakefield accelerators (PWFAs).
  
  

• "Multi-bunch Plasma Wakefield Accelerator Experiments at the ATF "
  Patric Muggli, Efthymios Kallos, Tom Katsouleas, Igor Pogorelsky, Vitaly Yakimenko, Karl Kusche, Marcus Babzien and Wayne D. Kimura
  Annual Meeting of the American Physical Society St. Louis, MO (2008). (presented by P. Muggli)
  
  Abstract: We present initial results obtained with a plasma wakefield accelerator driven by a train of microbunches. The microbunch train is produced with a masking technique [P. Muggli et al., this conference]. The plasma is produced in a cm-long gas-filled capillary discharge. The plasma density is measured using Stark broadening of the hydrogen Hα line. It is adjusted such that the plasma wavelength is equal to the microbunches spacing. In this case, The train resonantly drives the wake, and the accelerating field behind the train with a variable number of microbunches is maximized. The energy loss of each microbunch increases with the microbunch number and depends on the charge in each microbunch. The accelerating wake field is sampled by a witness bunch following the drive train. This multi-bunch method could be used to multiply the energy of a future linear particle collider with a high efficiency. The experimental set-up, as well as detailed experimental results will be presented.
  
  

• "A high gradient plasma wakefield accelerator using two subpicosecond electron bunches"
  Efthymios Kallos, Tom Katsouleas, Patric Muggli, Igor Pogorelsky, Vitaly Yakimenko, Igor Pavlishin, Karl Kusche, Daniil Stolyarov, and Wayne D. Kimura
  49th Annual Meeting of the American Physical Society's Division of Plasma Physics, Orlando, FL (2007). (poster)
  
  Abstract: A high gradient plasma wakefield accelerator was tested at the Accelerator Test Facility of Brookhaven National Lab. Two ~100 fs electron bunches with total charge of 0.5 nC separated by ~500 fs were fed into a 6 mm long high density (1e14/cc to 1e17/cc) plasma generated by an ablative capillary discharge. The drive bunch created a ~300 MV/m wakefield that was sampled by the short witness bunch. The relative position of the witness bunch with respect to the drive bunch wakefield could be adjusted by varying the plasma density, thus allowing controllable energy loss or energy gain with small energy spread. The experimentally observed energy shifts are in good agreement with 2D model predictions.
  
   

• " High Density Plasma in a High Pressure Hydrogen Capillary Discharge"
  Hao Chen, Efthymios Kallos, Patric Muggli, Andras Kuthi, Tom Katsouleas, Martin Gundersen
  IEEE Pulsed Power Plasma Science Conference, Albuquerque, NM (2007). (poster by Hao Chen)
  
  Abstract: Measurements of electron plasma density generated in a hydrogen-filled hollow cathode capillary discharge are reported. The capillary is made of borosilicate glass. H_α line and H_β line stark broadening are used to diagnose the plasma density. A 60KV peak voltage, 75ns pulse was applied, which was in excess of the breakdown voltage of the high pressure (>1 atm) hydrogen capillary. Preliminary results indicate that ~10¹⁹ cm^-3 electron density can be obtained. The dependence of the plasma density on the capillary length; the capillary diameter; the gas pressure; the electrode geometry; and the discharge V-I characteristics will be presented, and potential applications to plasma wakefield accelerators will also be discussed.
  
   

• "Capillary Plasma Density Diagnostics for Resonant Plasma Wakefield Experiment "
  Efthymios Kallos, Tom Katsouleas, Patric Muggli, Ilan Ben-Zvi, Igor Pogorelsky, Vitaly Yakimenko, Igor Pavlishin, Karl Kusche, Marcus Babzien, Daniil Stolyarov, and Wayne D. Kimura
  48th Annual Meeting of the American Physical Society's Division of Plasma Physics, Philadelphia, PA (2006). (poster)
  
  Abstract: The wakefield generated in a plasma from an electron beam can be enhanced if instead of a single bunch the beam is modulated into multiple bunches. Then the wakefields generated from the microbunches can add up in phase if the plasma density is tuned precisely at the separation between them. In the experimental setup at Brookhaven’s Accelerator Test Facility the 45MeV electron beam is IFEL modulated into 150 microbunches 10.6μm apart. Here we present plasma simulations that confirm the wakefield enhancement and diagnostics we performed to tune the plasma density (Stark broadening, HeNe laser interferometry).
  
   

• "Plasma Based Wakefield Acceleration using a 65MeV Multibunched Electron Beam"
  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
  47th Annual Meeting of the American Physical Society's Division of Plasma Physics, Denver, CO (2005) (poster)
   

• "The IFEL Wiggler Microbuncher at Brookhaven's Accelerator Test Facility"
  Efthymios Kallos, Tom Katsouleas, Vitaly Yakimenko, and Igor Pogorelsky
  11th Advanced Accelerator Concepts Workshop, Stony Brook, NY (2004) (poster)
   

Ph.D. Dissertation [August 2008]

Plasma Wakefield Accelerators Using Multiple Electron Bunches

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.
 

Dissertation (4.2MB PDF, English)

This is a word cloud of the abstract of my dissertation as plotted by wordle.

Dark Matter Presentation [December 2007]

Dark Matter Evidence

Abstract:
This presentation is a literature survey that summarizes the major evidence that support the existence of Dark Matter. I analyze the galaxy rotation curves, the motions of galaxies inside clusters, the baryonic mass density estimated from Big Bang Nucleosynthesis, the data accumulated from the cosmic microwave background radiation (WMAP, COBE) and finally the dark matter estimates from gravitationally lensed data such as the bullet cluster. Alongside I present some proof for the existence of Dark Energy, as well as evidence that the universe is flat.
 

Presentation       (PDF, English)
(email me for the videos)

Research Proposal [May 2007]

Capillary Discharges as Plasma Sources for Wakefield Acceleration Experiments

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 (~10¹⁹ 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.
 

Introduction        (PDF, English)

Thesis                (PDF, English)

Biofuels Presentation [November 2007]

Cellulosic Ethanol and the future of biofuels: From carbohydrates to hydrocarbons

Abstract:
This report is aimed at providing a summary of the field of biofuels: the production of liquid fuels from plants. Biofuels are not aiming at solving the world energy problem, but rather at providing a viable alternative to the transportation fuels which are presently derived almost in their entirety from imported oil. Rising oil prices, instabilities in the oil-producing regions of the world and greenhouse gas emissions from fossil fuels provide the motivation behind a field in ferment.

As opposed to other renewable intermittent energy technologies such as photovoltaic cells and wind farms, which require (currently inefficient) electrical storage mechanisms in order to function reliably over long periods of time, plants absorb solar energy and store it chemically inside their biomass. It is estimated by our report that 1TW of average power is stored into available for biofuel production biomass in the United States only (the global power consumption is during the year 2007 at 15TW). Even if a small fraction of that stored energy can be retrieved from the biomass, a significant portion of motor fuel could be replaced.

The first chapter summarizes the present global situation in terms of energy demand, CO₂ emissions and oil consumption. Chapter 2 provides a basic background on biofuels and examines their potential from an energy perspective. Chapter 3 provides an overview of the biofuel landscape in the United States, which is currently relying on ethanol fuel derived from corn kernels to provide 3% of its transportation fuels, although this type of ethanol could not be expanded into large scale. Chapter 4 examines the details of producing ethanol from the cellulose molecules that comprise the plant walls, which, if harnessed properly, can have much higher efficiencies and energy outputs than crop-derived ethanol because it can consume non-traditional biomass which is not used directly for other purposes. Chapter 5 describes briefly other biofuel production techniques, such as Biodiesel (popular in Germany), sugarcane-derived ethanol (successful in Brazil), Biobutanol and algae cultivation. Finally, we summarize the report in chapter 6.

 The document was prepared as a requirement of the ENE505 class at USC (Energy and the Environment) under prof. Ravindra.

Presentation

Technical Report

Diploma Thesis [May 2003]

Study of the Smith-Purcell Effect

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.
 

Thesis        (PDF, in Greek)

Some of my undergraduate writings (Greek):

Neutrino Oscillations
A synopsis of the neutrino history, the recent experimental results and their theoretical ramifications.

Shape Resonance in Helium
A special kind of quantum mechanical resonance that is caused due to the potential on the Helium ion.

The Hydrogen Atom
A complete treatment of the Hydrogen Atom, the quantum numbers and how they arise as a result of Schrodinger's equation.

Theory of Relativity
A simple mathematical introduction to the special theory of relativity, including some interesting problems.

Black Holes
Why are the black holes created, and some brief applications and consequences.

Propagation in Plasma
A technical project about dispersion relations in plasma when a magnetic field is applied.

The Obsolete Magnetic Field
How the Magnetic Field is just a relativistic electric field.

Active Contours
How the active contours have evolved as a highly important method in solving computer vision problems.

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