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석희용 한국물리학회 2022 물리학과 첨단기술 Vol.31 No.9

Envelope-Kinetic Method for the Simulation of Raman Backward Laser Amplification in a Plasma

허민섭,석희용 한국물리학회 2007 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.50 No.I

A new envelope-kinetic method for the simulation of Raman backscattering and laser amplification is presented. In the new scheme, the plasma wave envelope is obtained from the envelope-kinetic equation. For the self-consistent calculation of the kinetic term, a set of test particles is employed, and their motion is traced. The benchmark results of the new scheme against the averaged particle-in-cell (aPIC) show quite reasonable agreement while the computation speed increases by a factor of more than 10, depending on the parameters.

Energy Enhancement of the Self-Modulated Laser Wakefield Acceleration by Using the Plasma Density Gradient

유승훈,석희용,Hyojae Jang,Ilmoon Hwang,Jae Hoon Kim,허민섭,한상준 한국물리학회 2007 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.51 No.I

If GeV electron energies are to be achieved in the laser wakefield accelerator (LWFA), it is necessary to propagate an intense laser pulse over a long distance in a plasma without disruption. Many LWFA experiments have been done in the high-power, long-pulse, self-modulated regions, where self-guiding due to relativistic focusing appears to play a role in extending the acceleration length. However, there are strong instabilities and an electron dephasing problem due to the velocity difference between the electron beam and the wake wave. Our study shows that the upward density gradient scheme can be used to avoid the dephasing problem. In the study, particle-in-cell simulations were performed to search for the optimal conditions of the density gradient for the ongoing experiment. The simulation results show that the maximum energy of the accelerated electrons can be increased by about 50 \% in the upward density gradient case. Moreover, we present a brief plan of ongoing experimental research using the laser system at the Korea Electrotechnology Research Institute (KERI).

Plasma Channel Generation for Electron Acceleration with a Laser-Induced Density Gradient

Hyojae Jang,석희용,Jaehoon Kim,허민섭,조무현,유승훈,남궁원 한국물리학회 2007 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.50 No.I

Injection is a critical issue for the generation of a small-energy-spread, high-energy electron beam by using a laser wakefield accelerator (LWFA). By using a steep downward density gradient of plasma electrons at a gas target, a portion of the background electrons can be trapped locally in the laser wakefield, and they are accelerated by the field. To make a steep downward electron density gradient, we generated a pre-plasma using a 200-ps 800-mJ Nd:Glass laser, and we measured its electron density by using a Mach-Zehnder interferometer. Due to the shock structure of the plasma with a background neutral gas, a steep density gradient ($\sim 8\times10^{18}$ cm$^{-3}$/20 $\mu$m) could be generated. In the near future, a 20-TW, 30-fs Ti:Sapphire laser will be sent to this pre-plasma to generate an electron beam. In this paper, some preliminary experimental results and an ongoing work are reported.

고출력 레이저를 이용한 입자 가속

김광훈,이해준,석희용 한국광학회 2003 광학과 기술 Vol.7 No.1

Attosecond Relativistic Electron Beam by Using an Ultrashort Laser Pulse and a Thin Plasma Layer

Victor V. Kulagin,석희용,허민섭 한국물리학회 2007 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.50 No.I

Attosecond ($10^{-15}$ s) electron beams will have some important applications in physics, chemistry, material science, {\it etc.}, where ultrafast phenomena play an important role. Hence, how to generate such ultrashort electron beams is an important issue. Here, we propose to use a thin plasma layer illuminated normally by an ultra-intense femtosecond laser pulse having a sharp rising edge (rising time $\sim$ laser oscillation period). In this process, the plasma layer is compressed nonadabatically by the laser pulse, and all electrons are synchronously accelerated to ultra-relativistic velocities by several half-cycles of the laser field. In an experiment, a solid nanofilm, a taped electron beam, or a thin gas jet can be used as possible targets. For these types of targets, we show the generation of an attosecond high-energy electron beam by using particle-in-cell (PIC) simulations.

Quasi-Monoenergetic Electron-Beam Generation Using a Laser Accelerator for Ultra-Short X-ray Sources

J Kim,고도경,석희용,H Jang,김형택,I Hwang,최일우,J Lim,J. Lee,J. H. Sung,K.-H. Hong,허민섭,N Hafz,유승훈,유태준,T. M. Jeong,V Kulagin,Y.-C. Noh 한국물리학회 2007 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.51 No.I

Two types of electron acceleration methods have been conducted to generate quasi-monoenergetic electron beams. Multi-MeV quasi-monoenergetic high-charge electron beams were generated at Korea Electrotechnology Research Institute (KERI) from self-modulated laser wakefield acceleration by using a collimator with a 2 TW (1.4 J/700 fs) Nd:glass/Ti:sapphire hybrid laser system and a supersonic nitrogen gas jet. The peak electron energy was 3.6 MeV, and the energy spread was 4 MeV. These electron beams are useful for the generation of short-pulse X-rays in the water window region, which is 250 eV -- 500 eV (2.5 -- 5 nm), by using Thomson scattering. The calcualted photon spectrum indicates the scattered photon covers the water window region. This can be used for a high spatial and temperal resolution microscope for medical imaing. To generate higher-energy electron beams with small energy spread, a laser wakefield acceleration experiment with a sharp downward electron density gradient was conducted with a 100 TW laser system at Advanced Photon Research Insistitute (APRI). With the electron density gradient, some background plasma electrons could be locally injected in the laser wake wave and a small energy spread was expected. Using the pre-pulse, we could generate sharp downward electron density gradients. The gradient scale length was 20 $\mu$m for a 25 \% density change. With this electron density gradient, we could get more reproducible electron beams than we could without the density gradient.

Highly-efficient 20 TW Ti:sapphire laser system using optimized diverging beams for laser wakefield acceleration experiments

남인혁,김민석,이태희,이승우,석희용 한국물리학회 2015 Current Applied Physics Vol.15 No.4

We developed a highly efficient and compact Ti:sapphire multipass amplifier that uses divergent seed beams to generate 20 TW/40 fs laser pulses with a repetition rate of 10 Hz. The laser system consists of a mode-locked oscillator, a regenerative amplifier, and a multipass power amplifier. The thermal lensing effect is very important in this system, especially in the multipass amplifier, as it limits the conversion efficiency. In order to compensate the thermal lensing effect, we calculated the optimum divergence of the seed beam and used the result for the multipass amplifier, where the thermal focusing is taken into account. In this way, we achieved a very high conversion efficiency of 41%, which is close to the theoretical limit. The laser system was then used with a capillary gas cell to generate stable high-energy electron beams with electron energies of about 150 MeV and a beam divergence of 4 ± 1 mrad. In this paper, details of the laser system development and experimental results for electron generation are presented.

Bunching of Electron Beams by Ultra-Relativistic Laser Pulses

Victor V. Kulagin,석희용,허민섭,Vladimir A. Cherepenin 한국물리학회 2006 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.48 No.4

The bunching of an electron beam by an ultra-relativistic laser pulse in vacuum is considered. The one-dimensional theory describing this process is elaborated. The laser pulse is shown to compress the electron beam and to generate fast density modulations (microbunching) in it. Two spatial harmonics can be present simultaneously in longitudinal density modulations of the electron beam - one with the laser wavelength and the other with half of the laser wavelength, and the ratio of the amplitudes of the harmonics depends on the duration of the laser pulse front. The average density of the electron beam (slow density modulation) can be controlled by changing the form of the laser pulse envelope. The number of microbunches in the compressed electron beam can be changed by varying the amplitude of the laser pulse and the initial length of the electron beam, and for certain conditions, only one electron bunch with an attosecond length can be produced. The results of the theory are compared with 1D PIC (Particle-In-Cell) simulations, and a good agreement is found.O?

Enhanced betatron radiation by a modulating laser pulse in laser wakefield acceleration

이승우,엄한섭,강태연,허민섭,석희용 한국물리학회 2019 Current Applied Physics Vol.19 No.4

We propose a new idea to enhance and control the betatron radiation by using a modulating laser pulse in laser wakefield acceleration. In this scheme, a high-power laser pulse is used for self-trapping and acceleration of the plasma electrons and the accelerated electron beam is modulated by a separately-propagating laser pulse for large amplitude betatron oscillations and microbunching. In this way, the relatively low power modulating laser pulse can enhance the X-ray photon flux and energy significantly. We performed two-dimensional particle-in-cell simulations to demonstrate the idea and the results show that a sub-TW laser pulse is enough for electron beam modulation and it can generate easily-controllable fs X-ray pulses with a wide range of photon energies from soft X-rays to hard X-rays.