Sun Kook Kim

First lasing of the KAERI millimeter-wave free electron laser

Abstract A millimeter-wave free electron laser driven by a recirculating electrostatic accelerator has been developed. The energy and the current of the electron beam are 430 keV and 2 A, respectively. In the first oscillation experiment, the FEL has lased at the wavelength of 10 mm with the pulsewidth of 10–30 μs. The peak power is about 1 kW.

Design and fabrication of a diode-side-pumped Nd:YAG laser with a diffusive optical cavity for 500-W output power

A ray-tracing code has been developed, and the design parameters of the laser pump head were analyzed in terms of crystal diameter, doping concentration, and optical cavity diameter. According to the numerical analysis, we fabricated an efficient diode-pumped Nd:YAG laser and experimentally obtained 500-W output power. The output power is close to the numerically calculated output power of approximately 450 W and corresponds to an optical-to-optical conversion efficiency of 46.7% and an optical slope efficiency of 49%.

Transient behavior of the terminal voltage in a recirculating electrostatic accelerator for a long‐pulse free‐electron laser

For long‐pulse, single‐mode operation of free‐electron lasers, transient behavior of the terminal voltage in a recirculating electrostatic accelerator is analyzed. It is found that the rate of terminal voltage drop depends on the position where the current loss of electron beam occurs. The rate is measured to be 507 V/A μs for the current loss in the terminal and 100 V/A μs for the current loss in the deceleration tube. By increasing the capacitances between the adjacent electrodes of a deceleration tube, the accelerator becomes more stable with low rate of terminal voltage drop and increased recovery efficiency.

Design of a Diode-Pumped Yb:YAG Rod Laser with High Efficiency by Using a Diffusive Optical Cavity.

A diode-pumped Yb:YAG rod laser is designed using a fivefold symmetric diffusive optical cavity to improve the pump absorption efficiency of the pump power. By using a ray-tracing method, the optimum design parameters for the doping density, the radius of the diffusive optical cavity, and the radius of the crystal rod are determined by analyzing the absorbed pump power, the distribution profile of the absorbed power, and the threshold absorbed pump power. Based upon these analyses, we show that a highly efficient Yb:YAG rod laser is feasible with the fivefold symmetric diffusive optical cavity. The pump absorption efficiency of 63.6%, which is calculated for the optimal design parameters, corresponds to a 2.4 times improvement in comparison to that obtained by Sumida et al. [Laser Focus World (June, 1999) 63]. By using the absorbed power distribution, we achieved a more realistic quasi-three-level laser output power calculation. In addition, we developed a realistic code, utilizing the absorbed power distribution, for calculating the output power of a quasi-three-level laser. For an incident pump power of 600 W, the laser output power is calculated to be 126 W with optical-to-optical efficiency of 21% and slope efficiency of 42.9%.

2-D SIMULATION OF THE WAVEGUIDE FREE-ELECTRON LASER WITH A HELICAL UNDULATOR

Abstract For the free-electron lasers with helical undulators, we have developed a 2-D simulation code, Using this code we have simulated the KAERI FEL oscillator which is using the helical undulator. From this simulation, we see that the output power of the FEL is about 10 kW.

Measurement of electron bunch timing jitter using wakefield analysis

A wakefield is generated when electron bunches pass through discontinuities of beam pipes, such as rf cavities. We analyze the relation between the timing jitter of the bunches and the wakefield. From the analysis we derive a mathematical expression through which the jitter as well as the amplitude fluctuation of the bunches can be obtained by measuring the power spectrum of the wake. A measurement of the bunch jitter using this frequency-domain based wakefield analysis is compared with that measured by a time-domain based sampling oscilloscope. The two measurement results correspond well each other.

Start-up of lasing in compact far-infrared free-electron laser driven by 8-MeV microtron

We have developed a compact Far Infrared Free Electron Laser (FIR FEL) on the base of 8 MEV microtron, which can provide 6 microsecond(s) up to 70 mA macropulse beam current. Beam line consists of bending and steering magnets. Optical Transition Radiation screen and 3 doublets of quadruples to provide matching of microtron output electron beam parameters with optimized parameters for FEL operation. A 2 m length, 25 mm period and 5.7 mm gap undulator has extremely low field error of 0.05% for the magnet field peak amplitude in the range 5.5 - 6.5 kG. Inside the undulator the electron beam passes through 2 mm X 20 mm aperture 2779 mm length planar waveguide. On both sides of the waveguide are installed two cylindrical 3 m curvature mirrors. They form confocal type free space mode in horizontal plane and waveguide mode in vertical plane of the optical resonator. A hole in output mirror provides coupling ratio about 1 - 2%. During experiments we have observed the coherent effect in spontaneous emission with power enhancement approximately 1000 times. It was observed too the long wave radiation, which can be explained by vertical betatron oscillation of electrons in undulator. The experiments with FIR resonator and Ge-Ga liquid He cooled detector shown coherent generation and tunability of it power with tuning of the length of the resonator. The enhancement of outcoupled signal in 50 times was detected with changing of resonator length. This phenomenon can be explained as startup of the lasing process in FIR resonator.

Design of the optimized V-shape duct for the diode array pumped Nd:YAG laser system

We have determined optimum slope-angle and width of the duct, and found the superior characteristic of the V-shape duct compared to that of the compound parabolic concentrator (CPC) from the analysis of the ray and ray-angle distributions and the absorbed power calculation within the laser crystal.