Jung Yun Huang

Performance of the Pohang Light Source

Abstract The Pohang Light Source has actively supported users since the commissioning in 1994. In 1999, we had two decisive breakthroughs for the machine performance; installation of feedback systems to actively damp the coupled-bunch instabilities, and 2.5 GeV operation through energy ramping of the 2.0 GeV electron beam from the linac. Now we can provide two stable operation modes, 2.0 and 2.5 GeV modes. This paper describes the upgraded machine performance and the beamline status of the Pohang light source. Having started with two beamlines in 1995, the facility now has 12 beamlines operational, 1 beamline under commissioning, and 7 beamlines under construction.

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.

The Beam Based Alignment Technique for the Measurements of Beam Position Monitors Offsets and Beam Offsets from Quadrupoles in the Pohang Light Source

The beam based alignment (BBA) technique is applied to the 2-GeV storage ring of the Pohang Light Source to measure the offsets of beam position monitors. This measurement is particularly necessary for beam position monitors (BPMs) plugged into a long (~10 m) aluminum chamber, since the mechanical deformation of the vacuum chamber is experienced after repeated heating for the outgassing process, and the BPM positions are changed accordingly. A part of the excitation current of each quadrupole magnet is shunted through an electronic shunt circuit. Then, the closed orbit receives a perturbation due to the current reduction. Using two quadrupole magnets, we can measure the offset of each BPM. Also, the BBA technique is applied to measure the beam offsets from the center of quadrupole magnets, and gives information to the survey team about which quadrupole magnets should be aligned mostly. In this process, we introduce the merit function to reduce various errors such as BPM characteristic changes and the lattice imperfection. By minimizing the merit function, we can get the beam offset as the maximized expectation value. This paper presents the BBA technique used and experimental results taken from the 2-GeV Pohang Light Source (PLS) storage ring. When the BPM offset is measured, it is observed that a 3% of the shunt current is suitable.

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.

Commissioning of the PLS 2 GeV storage ring

In Pohang, Korea, construction of a third-generation 2 GeV light source has been in progress since 1988. Installation of the storage ring components started in March 1993 and was completed in August 1994. Initially, only two super-periods out of 12 were baked out in-situ. From September to the end of 1994, the machine commissioning had been carried out. The initial injection energy for the commissioning was chosen to be 1.4 GeV because at this energy the injection kicker magnet can make a design bump, -21 mm. On September 8, the injected beam from the injector linear accelerator was circulated in storage ring more than 500 turns without RF. After accumulating the beam up to 76 mA on October 20, the injection energy was changed to the design value which is 2.0 GeV. At 2.0 GeV, amount of the bumped orbit which the kicker magnet can generate is -15 mm. Additional -6 mm bump is produced by four horizontal corrector magnets located on each side of the injection straight section. On December 24, 1994, the beam was stacked up to 300 mA, exceeding the commissioning goal of 100 mA. From January to March 1995, the ring had been shut down for in-situ bakeout. Since April, the second commissioning has been in progress. The PLS facility will be open to users in July 1995. In this paper, we present the current status of the PLS storage ring commissioning and the future plan for normal operation.

Reconstruction of electron beam distribution using phase-retrieval algorithm

The shapes of light sources such as electron beams can be reconstructed by inverse Fourier transformation of the complex degree of spatial coherency, which can be measured using Youngs interferometer. The application of the phase-retrieval algorithm to reduce phase measurement errors in the complex degree of spatial coherency is numerically studied using an electron beam with an asymmetric distribution. This application is demonstrated with experimental data measured at the diagnostic beamline at the Pohang Accelerator Laboratory.

Experimental approaches for the beam dynamics study in the PC RF gun at the PAL

A high-brightness electron beam is emitted from a photo-cathode (PC) RF gun for use in the FIR (Far Infrared) facility being built at the Pohang Accelerator Laboratory (PAL). The beam dynamics study for the PAL XFEL injector is essential to generate low emittance electron beam from the PC RF gun. The XFEL injector requires 1 nC beam with short bunch length and low emittance. These conditions are simulated with PARMELA code and then are realized on experimental conditions. The experimental conditions for the XFEL injector are measured with beam diagnostic devices such as ICT and Faraday cup for charge measurement, a spectrometer for beam energy measurement. In this article, we present the experimental approaches of the beam dynamics study for the XFEL injector.

On Landau Damping by Bunch Filling Pattern in the Pohang Light Source Storage Ring

The stable beam current of the Pohang Light Source (PLS) is normally limited to about 150 mA despite the design value of 400 mA at 2.0 GeV due to the coupled bunch mode instabilities (CBMIs) which are generated by the interaction between higher order modes of RF cavities and circulating beams. In order to improve this situation, a longitudinal feedback system (LFS) using parallel digital signal processors was installed during the summer maintenance period in 1999. Besides the cure of the CBMIs, this programmable LFS is useful for various beam diagnostics such as the synchrotron oscillation phases and their spreads, and the beam pseudo-spectra that are the beam spectra without beam revolution harmonics. With this bunch-to-bunch diagnostic information, it is found that properly arranged bunch filling patterns can increase the stored beam current significantly without an active longitudinal feedback due to Landau damping by bunch-to-bunch synchrotron frequency spreads.