Yong Woon Parc

Temporally-coherent terawatt attosecond XFEL synchronized with a few cycle laser

Attosecond metrology using laser-based high-order harmonics has been significantly advanced and applied to various studies of electron dynamics in atoms, molecules and solids. Laser-based high-order harmonics have a limitation of low power and photon energies. There is, however, a great demand for even higher power and photon energy. Here, we propose a scheme for a terawatt attosecond (TW-as) X-ray pulse in X-ray free-electron laser controlled by a few cycle IR pulse, where one dominant current spike in an electron bunch is used repeatedly to amplify a seeded radiation to a terawatt level. This scheme is relatively simple, compact, straightforward, and also produces a temporally and spectrally clean pulse. The viability of this scheme is demonstrated in simulations using Pohang accelerator laboratory (PAL)-XFEL beam parameters.

Reduction of Multipole Fields in Photocathode RF Gun

Photocathode rf gun, the high-brightness electron source with extremely low emittance is highly required for future light sources such as X-ray free electron lasers (XFEL). The coupling hole between the waveguide and the cavity of a photocathode rf gun causes asymmetries in the rf fields at the coupler cell. The dipole and quadruple fields are the dominant sources of the transverse rf emittance growth. In the BNL Gun-III, the dipole field is reduced by adding a symmetric pumping hole at the opposite side of the waveguide coupling hole. The dipole field can be reduced further by adjusting the size of the pumping hole. However, the quadruple field cannot be suppressed by the single pumping hole. We have designed new rf cavity in which the quadruple field as well as the dipole one are suppressed. In this design, two additional pumping ports are placed at the 90° positions with respect to the coupling hole and the pumping holes. Beam dynamics simulation for newly designed rf gun shows that the vertical transverse emittance is reduced by about 60% compared to the old one.

Using irregularly spaced current peaks to generate an isolated attosecond X-ray pulse in free-electron lasers

A method is proposed to generate an isolated attosecond X-ray free-electron laser pulse with the peak power beyond 1 TW.

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.

Status of Beam Diagnostic Systems for the PEFP

A proton linear accelerator is currently the construction at the KAERI (Korea Atomic Research Institute) to the PEFP (Proton Engineering Frontier Project) in Korea. We are accomplished the technique development of beam diagnostic system to be currently the construction. We treat beam diagnostics for the high power proton linear accelerator. Prototype beam position & phase monitor (BPPM) electronics was made and tested successfully in one of the beam diagnostic systems. The beam position monitor pickup electrode is a capacitive type (electrostatic type) which has a button form. Button form electrode, in common use around electron synchrotrons and storage rings, are a variant of the electrode with small button form (e.g., sub mm diameter). However, we are designed button form electrode to measure beam position of proton beam. The BCM (Beam Current Monitor) is developed Tuned CT (Current Transformer) for collaborate with Bergoz Instruments. This paper describes the status of beam diagnostic systems for the PEFP.

Isolated terawatt attosecond hard X-ray pulse generated from single current spike

Isolated terawatt (TW) attosecond (as) hard X-ray pulse is greatly desired for four-dimensional investigations of natural phenomena with picometer spatial and attosecond temporal resolutions. Since the demand for such sources is continuously increasing, the possibility of generating such pulse by a single current spike without the use of optical or electron delay units in an undulator line is addressed. The conditions of a current spike (width and height) and a modulation laser pulse (wavelength and power) is also discussed. We demonstrate that an isolated TW-level as a hard X-ray can be produced by a properly chosen single current spike in an electron bunch with simulation results. By using realistic specifications of an electron bunch of the Pohang Accelerator Laboratory X-ray Free-Electron Laser (PAL-XFEL), we show that an isolated, >1.0 TW and ~36 as X-ray pulse at 12.4 keV can be generated in an optimized-tapered undulator line. This result opens a new vista for current XFEL operation: the attosecond XFEL.

Note: Recent achievements at the 60-MeV Linac for sub-picosecond terahertz radiation at the Pohang Accelerator Laboratory

A femtosecond (fs) terahertz (THz) linac has been constructed to generate fs-THz radiation by using ultrashort electron beam at the Pohang Accelerator Laboratory. To generate an ultrashort electron beam with 60-MeV energy, a chicane bunch compressor has been adopted. Simulation studies have been conducted to design the linac. In this note, recent achievements at 60-MeV linac are presented.

The coherency of synchrotron radiation at Pohang Accelerator Laboratory.

The coherency of the synchrotron radiation at Pohang Accelerator Laboratory has been investigated using Youngs interferometer. The electron beam size can be measured precisely using the interferometer. An interferogram using 650 nm light at the diagnostics beamline at Pohang Light Source (PLS) has been measured to determine the electron beam distribution and the spatial coherence length. Interferograms obtained by numerical study are compared with experimental results in order to understand the measured data. From this comparison, the electron beam at PLS is revealed to be a Gaussian distribution with a standard deviation of 210 microm. The spatial coherency length of 650 nm light at PLS is measured to be 0.57 cm, and that of 0.1 nm light at PLS is predicted to be 0.88 microm by the same numerical study.

Simulation Study for Electro-Optic Sampling Measurement of Low-Energy Electron Beam

For the successful optimization of the injector part of X-ray free-electron laser (XFEL) facilities, the temporal properties of the beam at the exit of the gun must be measured in a nondestructive way when the energy of the electron beam is 4–6 MeV. Electro-optic sampling (EOS) is a promising method of measuring the electron bunch length nondestructively. A simulation study is carried out with the pulse propagation method, which utilizes the Fourier transform to investigate the evolution of the electromagnetic pulse inside the electro-optic (EO) crystal. In our work, the properties of the 4.5 MeV electron beam are studied with the rings of charge for several values for the distances between the electron beam and the probe laser. The simulation result shows that the bunch length of the electron beam at the exit of the gun is measurable nondestructively by EOS.

Measurements of Transverse Emittance for RF Photocathode Gun at the PAL

A BNL GUN-IV type RF photo-cathode gun is under fabrication for use in the FIR (Far Infrared) facility being built at the Pohang Accelerator Laboratory (PAL). Performance test of the gun will include the measurement of transverse emittance profile along the longitudinal direction. Precise measurement of the emittance profile will provide powerful tool for the commissioning of the 4GLS (4th generation light source) injectors based on the emittance compensation principle. We are going to achieve this with the use of Pin-hole plate based Emittance-Meter that can be moved along the longitudinal direction. In this article, we present design considerations for the Emittance-Meter which is capable of measuring emittance as low as 1 mm mrad.