I. W. Choi

Gwangju Institute of Science and Technology, Kwangju, Gwangju, South Korea

Are you I. W. Choi?

Claim your profile

Publications (49)67.86 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We have developed a 0.1-Hz-repetition-rate, 30-fs, 1.5-PW Ti:sapphire laser system for the research on high field physics. In this paper, we describe the design and output performance of the PW Ti:sapphire laser and its applications in the generation of relativistic high order harmonic generation and the acceleration of charged particles (protons and electrons). In the experiment on relativistic harmonic generation, the harmonic order dramatically extended up to 164th that corresponds to 4.9 nm in wavelength, and the dramatic extension was explained by the oscillatory flying mirror model. Recently, we could accelerate protons up to 45 MeV from a 10-nm polymer target and show the change in the acceleration mechanism from target normal sheath acceleration to radiation pressure acceleration. The femtosecond high power laser system is a good candidate for developing a compact electron accelerator as well. The generation of multi-GeV electron beam was observed from an injection scheme when a PW laser pulse was focused by a long focal length spherical mirror.
    Proc SPIE 05/2013;
  • [Show abstract] [Hide abstract]
    ABSTRACT: The enhancement of laser-driven proton acceleration mechanism in TNSA regime has been demonstrated through the use of advanced nanostructured thin foils. The presence of a monolayer of polystyrene nanospheres on the target front-side has drastically enhanced the absorption of the incident laser beam, leading to a consequent increase in the maximum proton beam energy and total laser conversion efficiency. The experimental measurements have been carried out at the 100 TW and 1 PW laser systems available at the APRI-GIST facility. Experimental results and comparison with particle-in-cell numerical simulations are presented and discussed.
    01/2013;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Nanostructured thin plastic foils have been used to enhance the mechanism of laser-driven proton beam acceleration. In particular, the presence of a monolayer of polystyrene nanospheres on the target front side has drastically enhanced the absorption of the incident 100 TW laser beam, leading to a consequent increase in the maximum proton energy and beam charge. The cutoff energy increased by about 60% for the optimal spheres' diameter of 535 nm in comparison to the planar foil. The total number of protons with energies higher than 1 MeV was increased approximately 5 times. To our knowledge this is the first experimental demonstration of such advanced target geometry. Experimental results are interpreted and discussed by means of 21/2-dimensional particle-in-cell simulations.
    Physical Review Letters 12/2012; 109(23):234801. · 7.73 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The pointing instability of energetic electron beams generated from a laser-driven accelerator can cause a serious error in measuring the electron spectrum with a magnetic spectrometer. In order to determine a correct electron spectrum, the pointing angle of an electron beam incident on the spectrometer should be exactly defined. Here, we present a method for absolutely calibrating the electron spectrum by monitoring the pointing angle using a scintillating screen installed in front of a permanent dipole magnet. The ambiguous electron energy due to the pointing instability is corrected by the numerical and analytical calculations based on the relativistic equation of electron motion. It is also possible to estimate the energy spread of the electron beam and determine the energy resolution of the spectrometer using the beam divergence angle that is simultaneously measured on the screen. The calibration method with direct measurement of the spatial profile of an incident electron beam has a simple experimental layout and presents the full range of spatial and spectral information of the electron beams with energies of multi-hundred MeV level, despite the limited energy resolution of the simple electron spectrometer.
    The Review of scientific instruments 06/2012; 83(6):063301. · 1.52 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report the manufacturing of an (ultra-)thin foil target made of conjugated polymer, poly(9,9′-dioctylfluorene-co-benzothiadiazole) (F8BT), and the simultaneous observation of laser-accelerated ions and second harmonic radiation, when irradiated with ultrahigh-contrast laser pulse at a maximum intensity of 4 × 1019 W/cm2. Maximum proton energy of 8 MeV is achieved along the target normal direction. Strong second harmonic with over 6% energy ratio compared to fundamental is emitted along the specular direction. Two-dimensional particle-in-cell simulations confirm the simultaneous generation of protons and high-order harmonics, which demonstrates the feasibility of applications requiring particle and radiation sources at once, effectively using the same laser and target.
    Applied Physics Letters 11/2011; 99(18). · 3.79 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Single-shot nanometer-scale imaging techniques have become important because of their potential application in observing the structural dynamics of nanomaterials. We report here the image reconstruction results obtained using single-shot Fourier transform x-ray holography with an x-ray laser driven by a table top laser system. A minimum resolution of 87 nm was obtained from the reconstructed image. We could also discriminate the aggregates of carbon nanotubes, which shows the feasibility of single-exposure nanoimaging for real specimens using a laser-driven x-ray laser.
    Applied Physics Letters 03/2011; 98(12):121105-121105-3. · 3.79 Impact Factor
  • K.H. Pae, I.W. Choi, J. Lee
    [Show abstract] [Hide abstract]
    ABSTRACT: The characteristics of high energy protons generated from thin carbon-proton mixture targets via circularly polarized intense laser pulses are investigated using two-dimensional particle-in-cell simulations. It is found that the density ratio n between protons and carbon ions plays a key role in determining the acceleration dynamics. For low n values, the protons are mainly accelerated by the radiation pressure acceleration mechanism, resulting in a quasi-monoenergetic energy spectrum. The radiation pressure acceleration mechanism is enhanced by the directed-Coulomb-explosion of carbon ions which gives a high proton maximum energy, though a large energy spread, for high n values. From a proton acceleration point of view, the role of heavy ions is very important. The fact that the proton energy spectrum is controllable based on the target composition is especially useful in real experimental environments.
    Laser and Particle Beams 02/2011; 29(01):11 - 16. · 2.02 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: An ultrahigh contrast laser pulse of over 1011 for 6 ps before the main pulse was achieved by employing a double plasma mirror installed at the end of a 100TW Ti:sapphire laser system. Spatial beam qualities such as focusability and stability were found to be extremely sensitive in the range of 14–360J/cm2 on the double plasma mirror, while ultrahigh contrast was maintained. At the fluence of 90J/cm2 the focusability of the ultrahigh contrast laser was not degraded, and the stability was very close to that obtained without the double plasma mirror when the 2-dimensional normalized standard deviation and the correlation function for several laser beam profiles were analyzed. These results are requisites for carrying out relativistic laser-plasma interactions with ultrahigh contrast laser pulses, enabling the use of ultrathin solid targets.
    Applied Physics B 01/2011; 104(1):81-86. · 1.78 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Nanometer-scale imaging techniques using only a single shot of ultrafast x- ray pulse are becoming increasingly important because of their potential application in the real-time investigations of nanomaterials. X-ray lasers have a great potential to achieve single-shot nano-imaging due to their coherence and high flux. Here, we will report on single-shot Fourier Transform hologram using Ni-like Ag x-ray laser driven by a single-profiled pumping pulse. We achieved lensless Fourier transform holograms with a single pulse of x-ray laser and the reconstructed image has a minimum resolution of 87 nm.
    X-Ray Lasers 2010. 01/2011;
  • K. H. Pae, I. W. Choi, J. Lee
    [Show abstract] [Hide abstract]
    ABSTRACT: Via three-dimensional particle-in-cell simulations, the self-mode-transition of a laser-driven electron acceleration from laser wakefield to plasma-wakefield acceleration is studied. In laser wakefield accelerator (LWFA) mode, an intense laser pulse creates a large amplitude wakefield resulting in high-energy electrons. Along with the laser pulse depletion, the electron bunch accelerated in the LWFA mode drives a plasma wakefield. Then, after the plasma wakefield accelerator mode is established, electrons are trapped and accelerated in the plasma wakefield. The mode transition process and the characteristics of the accelerated electron beam are presented.
    Physics of Plasmas 12/2010; 17(12):123104-123104-5. · 2.38 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: A proton energy spectrometer system is composed of a time-of-flight spectrometer (TOFS) and a Thomson parabola spectrometer (TPS), and is used to characterize laser-accelerated protons. The TOFS detects protons with a plastic scintillator, and the TPS with a CR-39 or imaging plate (IP). The two spectrometers can operate simultaneously and give separate time-of-flight (TOF) and Thomson parabola (TP) data. We propose a method to calibrate the TOFS and IP by comparing the TOF data and the TP data taken with CR-39 and IP. The absolute response of the TOFS as a function of proton energy is calculated from the proton number distribution measured with CR-39. The sensitivity of IP to protons is obtained from the proton number distribution estimated with the calibrated TOFS. This method, based on the comparison of the simultaneously measured data, gives more reliable results when using laser-accelerated protons as a calibration source. The calibrated spectrometer system can be used to measure absolutely calibrated energy spectra for the optimization of laser-accelerated protons.
    Measurement Science and Technology 10/2009; 20(11):115112. · 1.44 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Laser‐driven plasma accelerators are gaining much attention by the advanced accelerator community due to the potential these accelerators hold in miniaturizing future high‐energy and medium‐energy machines. In the laser wakefield accelerator (LWFA), the ponderomotive force of an ultrashort high intensity laser pulse excites a longitudinal plasma wave or bubble. Due to huge charge separation, electric fields created in the plasma bubble can be several orders of magnitude higher than those available in conventional microwave and RF‐based accelerator facilities which are limited (up to ∼100 MV∕m) by material breakdown. Therefore, if an electron bunch is injected into the bubble in phase with its field, it will gain relativistic energies within an extremely short distance. Here, in the LWFA we show the generation of high‐quality and high‐energy electron beams up to the GeV‐class within a few millimeters of gas‐jet plasmas irradiated by tens of terawatt ultrashort laser pulses. Thus we realize approximately four orders of magnitude acceleration gradients higher than available by conventional technology. As a practical application of the stable high‐energy electron beam generation, we are planning on injecting the electron beams into a few‐meters long conventional undulator in order to realize compact X‐ray synchrotron (immediate) and FEL (future) light sources. Stable laser‐driven electron beam and radiation devices will surely open a new era in science, medicine and technology and will benefit a larger number of users in those fields.
    AIP Conference Proceedings. 07/2009; 1153(1):167-175.
  • [Show abstract] [Hide abstract]
    ABSTRACT: As a benchmark experiment to realize a novel and compact laser driven proton accelerator whose significant features are high number density in a short pulse width (∼ns), 2.4 MeV laser driven proton beam is stably focused at 1 Hz repetition rate by using a pair of permanent quadrupole magnets (PMQs) with large apertures whose diameters and field strengths are 3.5 cm and 55 T∕m for the first and 2.3 cm and 60 T∕m for the second magnets, respectively. The proton beam has been focused to a focal spot of 3×8 mm2 in horizontal and vertical direction (full width at half maximum) at 650∼mm from the source, which is well reproduced by the simulation. The further optimization of the focusing system will sure to pave a way to the novel proton accelerator with which we can investigate the physics appeared in the short time scale as well as that in a high energy density matter, oncology, astrophysics, and so on.
    AIP Conference Proceedings. 07/2009; 1153(1):151-158.
  • [Show abstract] [Hide abstract]
    ABSTRACT: By using particle-in-cell simulations, a new method for energetic collimated proton generation via intense short pulse laser-thin foil interactions is presented. To enhance the electron heating efficiency, a small hole is bored at the center of a thin foil target. The small hole combines target heating mechanisms effectively, which results in a high proton maximum energy. While an ultraintense, ultrashort laser pulse propagates through a small hole (diameter<laser spot size), the laser pulse drives electrons pulled out from the hole inner wall effectively inside the hole. When these electrons leave the target, a strong sheath field is formed between the electrons and the target rear surface and this accelerates protons from the rear surface of the target. The effective combination of the laser longitudinal ponderomotive force with the transverse heating (by E field) mechanism results in highly efficient electron heating of the hole target. When the rear part of the hole is filled with a proton-electron contamination layer, energetic collimated protons are produced. The scaling of the maximum proton energy of a hole target over a wide range of laser pulse intensities is presented and compared with that of a simple planar target.
    Physics of Plasmas 07/2009; 16(7):073106-073106-5. · 2.38 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: An ion spectrometer, composed of a time-of-flight spectrometer (TOFS) and a Thomson parabola spectrometer (TPS), has been developed to measure energy spectra and to analyze species of laser-driven ions. Two spectrometers can be operated simultaneously, thereby facilitate to compare the independently measured data and to combine advantages of each spectrometer. Real-time and shot-to-shot characterizations have been possible with the TOFS, and species of ions can be analyzed with the TPS. The two spectrometers show very good agreement of maximum proton energy even for a single laser shot. The composite ion spectrometer can provide two complementary spectra measured by TOFS with a large solid angle and TPS with a small one for the same ion source, which are useful to estimate precise total ion number and to investigate fine structure of energy spectrum at high energy depending on the detection position and solid angle. Advantage and comparison to other online measurement system, such as the TPS equipped with microchannel plate, are discussed in terms of overlay of ion species, high-repetition rate operation, detection solid angle, and detector characteristics of imaging plate.
    The Review of scientific instruments 06/2009; 80(5):053302. · 1.52 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: A pair of conventional permanent magnet quadrupoles is used to focus a 2.4 MeV laser-driven proton beam at a 1 Hz repetition rate. The magnetic field strengths are 55 and 60 T/m for the first and second quadrupoles, respectively. The proton beam is focused to a spot with a size of less than ∼3×8 mm <sup>2</sup> at a distance of 650 mm from the source. This result is in good agreement with the Monte Carlo particle trajectory simulation.
    Applied Physics Letters 03/2009; · 3.79 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Using three different laser systems, we demonstrate a convenient and simple plasma based diagnostic of the contrast of high-power short-pulse lasers. The technique is based on measuring the specular reflectivity from a solid target. The reflectivity remains high even at relativistic intensities above 10(19) W/cm(2) in the case of a high-contrast prepulse-free laser. On the contrary, the specular reflectivity drops with increasing intensities in the case of systems with insufficient contrast due to beam breakup and increased absorption caused by preplasma.
    Applied Physics Letters 01/2009; 94(24). · 3.79 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Spatial characteristics of Ni-like silver x-ray laser using grazing incidence pumping (GRIP) schemes were investigated. We succeeded in lasing from a nickellike silver plasma in the GRIP geometry using both a double pump laser pulse as well as a single-profiled pulse.The transverse profile of the intensity distribution as well as the spatial coherence of both variants of Ni-like Ag x-ray laser were determined by far-field measurements and the Young's double slit interferometry. The x-ray laser pumped by a single-profiled pulse showed the output beam with a highly coherent part of a reasonable volume containing about 3% of the total photon number
    12/2008: pages 143-152;
  • [Show abstract] [Hide abstract]
    ABSTRACT: The authors describe firstly the lasers for high intensity physics experiments at JAEA including J-KAREN and JLITE-X lasers which can deliver 00TW and 0TW laser power, respectively. Secondly the authors describe demonstration of flying mirror technique which will become a new technique to make a coherent tunable x-ray source. Thirdly a femto-second laser driven incoherent soft x-ray source and its imaging applications for nano-structures are described. The th topic includes femto-second laser driven multiple quantum beam generation and its applications using a laser driven thin foil target.
    12/2008: pages 33-42;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We characterized a Ni-like silver soft x-ray laser realized as, what we believe is, a new variant of the grazing incidence pumping (GRIP) scheme. The x-ray laser was pumped by a single profiled laser pulse from a 10 Hz Ti:sapphire laser system. The Ni-like Ag x-ray laser was saturated with a gain coefficient of 76 cm − 1 and an effective gain-length product of 28.2. The spatial characteristics of the new version of GRIP x-ray laser are presented including the far- and near-field beam profiles. Computational modeling of the lasing conditions was used to provide some qualitative explanations of the physical processes occurring in the x-ray laser. Additionally, we obtained some preliminary results on the injection seeding technique applied to the Ni-like Ag active medium using the 59th harmonic of the Ti:sapphire laser pulse as the seed. The single-pulse variant of the pumping scheme proved to be a stable and simple configuration of the table-top x-ray lasers also suitable for the injector-amplifier arrangement.
    Journal of the Optical Society of America B 06/2008; 25(7):B76-B84. · 2.21 Impact Factor

Publication Stats

178 Citations
67.86 Total Impact Points

Institutions

  • 2006–2012
    • Gwangju Institute of Science and Technology
      • Advanced Photonics Research Institute
      Kwangju, Gwangju, South Korea
    • National Institute of Radiological Sciences
      Tiba, Chiba, Japan
  • 2002
    • Korea Advanced Institute of Science and Technology
      • Department of Physics
      Sŏul, Seoul, South Korea
    • Dongshin University
      Rashū, South Jeolla, South Korea
  • 1999–2000
    • Chonbuk National University Hospital
      Sŏul, Seoul, South Korea