Changbum Kim

Two-dimensional simulation of photon acceleration by using laser wake fields

Two-dimensional simulations of photon acceleration by using a laser wake field are presented with a fully electromagnetic and relativistic particle-in-cell code. The frequency increase of about 10% is observed, which is saturated mainly by diffraction and dispersion of the laser pulse. Images of electron density and laser field profiles are presented.

Generation, transport, and detection of linear accelerator based femtosecond-terahertz pulses

The generation and detection of intense terahertz (THz) radiation has drawn a great attention recently. The dramatically enhanced energy and peak electric field of the coherent THz radiation can be generated by coherent superposition of radiated fields emitted by ultrafast electron bunches. The femtosecond (fs)-THz beamline construction at the Pohang Accelerator Laboratory (PAL) was completed in the end of 2009. The fs-THz beamline at PAL can supply ultrafast and intense fs-THz radiation from a 75 MeV linear accelerator. The radiation is expected to have frequency up to 3 THz (∼100 cm(-1)) and the pulse width of <200 fs with pulse energy up to 10 μJ. This intense THz source has great potential for applications in nonlinear optical phenomena and fields such as material science, biomedical science, chemistry, and physics, etc.

Lithium-induced reconstructions of the Si(001) surface

Abstract We report the new LEED observation that lithium atom adsorption on a Si(001) surface at room temperature or below produces a series of ordered phases, (2×2):Li → (2×1):Li → c(3√2 × √2)R45°:Li → streaky c(3√2 × √2)R45°:Li → (4×1):Li → (1×1):Li, depending on adsorbate coverage. This observation partially contradicts previous studies which reported only two phases in the series. We discuss possible origins of the controversy. We then propose a structural model that may account for the sequential appearance of the phases most naturally by invoking a spin-flip mechanism of an impurity stabilized magnetic system. The results suggest that the first phase be a reconstruction of the silicon substrate induced by Li adsorption where asymmetric dimers arrange themselves ferromagnetically and antiferromagnetically along the [110] and [110] directions, respectively.

Ion beam nitridation of a Si(111) surface : effects of ion reactivity and thermal treatment

Abstract Ion beam nitridation of Si has been investigated in a low energy ion-surface collision chamber. Silicon nitride layer was generated on a Si(111) surface by N + ion beam reaction and post-annealing, and the resulting surface was examined in situ by Auger electron spectroscopy (AES), ultraviolet photoemission spectroscopy (UPS), and low energy electron diffraction (LEED). Initial reaction of N + ions with a Si at room temperature produces surface nitrides of various chemical states. Ion beam reaction to a saturation results in a nitride layer which mostly contains the sp 2 nitrides of D 3h symmetry. The sp 2 nitrides are randomly oriented at room temperature. Annealing of the reacted surface above 900°C changes the LEED pattern from a featureless diffuse background directly to the well-defined “quadruplet” pattern, indicating crystallization of the disordered sp 2 nitrides into small domains. The “8 × 8” LEED phase, which is an intermediate phase of thermal nitridation, is not produced in ion beam reaction. The detailed nature of the nitride layer is discussed based on spectroscopic features.

Heavy ion-surface interaction at low energy: scattering of 3–300 eV Cs+, Xe+, and Ar+ from the Si surface

In order to understand the nature of a hyperthermal collision between a heavy projectile and a light atom surface, Cs + , Xe + , and Ar + ion beams are scattered from Si surfaces for collision energies of 3-300 eV. The scattered ions are analyzed for their mass and kinetic energy. The kinetic energy of the scattered ions is much higher than expected from a classical binary collision model, for example, the ratio of scattered to incident ion energy (E s /E i ) is 0.64 for a Cs + -Si(111) collision at an incident energy of 3 eV and a scattering angle of 90°. The E s /E i for Cs + decreases sharply with increasing energy up to 50 eV. For Xe + and Ar + scattering from Si(100), even higher E s /E i values are measured at low collision energy, but this is attributed to preferential neutralization of the slow noble gas ions on the surface and the resulting upshift in their ionic energy distribution. Molecular dynamics classical trajectory simulation of heavy projectile-light atom surface collisions reveals that the lattice atoms collectively respond during a hyperthermal collision, resulting in a drastic E s /E i increase compared to the collision of a light projectile. An important parameter that determines the E s /E i ratio is the time scale of the collision relative to the energy propagation inside a solid.

Top-up operation at Pohang Light Source-II

After three years of upgrading work, PLS-II (S. Shin, Commissioning of the PLS-II, JINST, January 2013) is now successfully operating. The top-up operation of the 3 GeV linear accelerator had to be delayed because of some challenges encountered, and PLS-II was run in decay mode at the beginning in March 2012. The main difficulties encountered in the top-up operation of PLS-II are different levels between the linear accelerator and the storage ring, the 14 narrow gap in-vacuum undulators in operation, and the full energy injection by 3 GeV linear accelerator. Large vertical emittance and energy jitter of the linac were the major obstacles that called for careful control of injected beam to reduce beam loss in the storage ring during injection. The following measures were taken to resolve these problems: (1) The high resolution Libera BPM (see http://www.i-tech.si) was implemented to measure the beam trajectory and energy. (2) Three slit systems were installed to filter the beam edge. (3) De-Qing circuit was applied to the modulator system to improve the energy stability of injected beam. As a result, the radiation by beam loss during injection is reduced drastically, and the top-up mode has been successfully operating since 19th March 2013. In this paper, we describe the experimental results of the PLS-II top-up operation and the improvement plan.

Evidence of substrate metallization by Li adsorption on the Si(001) surface

We report an evidence of substrate metallization induced by Li adsorption on the Si(001) surface, based on the combined results of electron energy-loss (EEL) and angle-resolved photoemission (ARP) measurements. The metallic surface at a low dose of Li manifests itself as a loss peak due to an intraband surface plasmon in EEL spectra and a metallic peak in ARP spectra. These peaks are coherently understood in terms of substrate metallization, where electrons from Li adatoms partially occupy the empty substrate surface bands. Furthermore, the unique negative dispersion of the plasmon reveals that local field effects may cause such an anomalous dispersion.

Secondary ion emission from clean and K-covered Ni surfaces near threshold impact energies

Abstract Clean and K-covered Ni surfaces are bombarded with low energy (10–500 eV) beams of He + , Ne + , Ar 2+ , and Kr + ions, and the emitted ion yield is measured as a function of beam energy. The apparent threshold energies for K + and Ni + emission are proportional to the ionic binding energies of K + and Ni + to the Ni surface. From comparison of the ion and neutral yield curves, it is suggested that these ions are emitted via momentum transfer collisions similar to neutral sputtering.

High power beam test and measurement of emittance evolution of a 1.6-cell photocathode RF gun at Pohang accelerator laboratory

A Brookhaven National Laboratory (BNL) GUN-IV type photocathode rf gun has been fabricated to use in femtosecond electron diffraction (FED), femtosecond far infrared radiation (fs-FIR) facility, and X-ray free electron laser (XFEL) facilities at the Pohang Accelerator Laboratory (PAL). The gun consists of a 1.6-cell cavity with a copper cathode, a solenoid magnet, beam diagnostic components and auxiliary systems. We report here the measurement of the basic beam parameters which confirm a successful fabrication of the photocathode RF gun system. The emittance evolution is measured by an emittance meter and compared with the PARMELA simulation, which shows a good agreement.

Searching for new THz science

The generation and detection of intense THz radiation has drawn a great attention recently. The dramatically enhanced energy, peak power, and peak electric field of the coherent THz radiation can be generated from the coherent superposition of the radiated fields emitted by ultrafast electron bunches. The fs-THz beamline construction program at PAL (Pohang Accelerator Laboratory), which is scheduled to be completed by the end of 2009, is underway to generate ultrafast and intense fs-THz radiation from 60 MeV linac (linear accelerator). The radiation is expected to cover up to 3 THz (≈100 cm−1) and the pulse width to be less than 200 fs. The mechanical and optical designs of the beamline are discussed. This intense THz source has a great potential for applications in nonlinear optical phenomena as well as material science, biomedical science, chemistry, physics, etc.