Sung-Ju Park

Synchrotron Radiation Facilities in Korea: Pohang Light Source and Future XFEL Project

The Pohang Light Source (PLS) at the Pohang Accelerator Laboratory (PAL) is a third-generation light source, the only synchrotron radiation facility in Korea, and the fifth machine of its kind in the world (see Figure 1). In 1988, PAL was organized for the construction of the PLS. Ground-breaking was celebrated in 1991, and PLS construction was completed in 1994. In 1995, the PLS opened two beamlines to public users. The PLS was initially operated at 2.0 GeV in 1995. Since 2002, the energy of the electron beam has been upgraded to 2.5 GeV (see Table 1 for the principal parameters of PLS). Remarkable increases in the number of beamlines, users, and scientific results have been achieved since the opening of the PLS in 1995. Two or three beamlines have been added each year for the past 15 years, and as of February 2009 we have in total 27 beamlines in operation and 3 beamlines under construction, which will be completed by the end of 2009 (Figures 2 and 3).

Experiments with a radial multichannel pseudospark switch for extremely high Coulomb transfer

Radial multichannel pseudospark switches for extremely high-charge transfer were designed, constructed, and tested. The characteristics of the switches were measured for different electrode geometries and materials in hydrogen and helium as function of gas pressure. The switches were operated in a 100 kJ capacitor bank and conducted an effective charge up to 31 C. A sealed-off radial multichannel pseudospark switch was also manufactured and tested with a 300 kJ capacitor bank. The switch was successfully tested up to 105 kA peak current and 78 C charge transfer. The experimental results demonstrated that a radial multichannel pseudospark switch with SiC electrodes and triggered by a high dielectric trigger has the capability of extremely high charge transfer under pulsed operation.

Development of Air Core Type 500 kV HV Pulse Transformer

Cylindrical type air core pulse transformers capable of passing high voltage and energy pulse waveforms with high efficiency and low distortion require a much more delicate design balance of physical dimensions and electrical parameters than iron or ferrite core units. The structure of an air core high voltage pulse transformer is relatively simple, but considerable attention is needed to prevent breakdown between transformer windings. Since the thickness of the windings in spiral type is on the order of sub-millimeter, field enhancement at the edge of the windings is very high. It is, therefore, important to find proper electrical insulation parameter to make the system compact. Several shapes of the winding are considered for air core pulse transformer development. Requirements of the transformer are 170 nH inductance at primary side, 500 kV. In this paper, we will describe a design procedure, parameters, measurement and experiment results of air core type 500 kV HV pulse transformer.

Progress of KSTAR 5-GHz Lower Hybrid Current Drive System

Abstract A 5-GHz steady-state lower hybrid (LH) current drive (LHCD) system is planned to support steady-state and advanced tokamak operation on the Korea Superconducting Tokamak Advanced Research (KSTAR) experiment. As an initial phase, a pulsed 5-GHz, 500-kW LHCD system has been installed in KSTAR for basic experimental studies of the LH coupling and flux saving in the plasma current ramp-up, prior to long-pulse noninductive operation in KSTAR. A Toshiba-made klystron developed in collaboration with Pohang University of Science and Technology in 2006 is utilized for the initial KSTAR LHCD system. The LH launcher is designed as a fully active waveguide grill type with a parallel refractive index n[parallel] value ranging from 1.8 to 4.3 and with high directivity. In the initial stage, the LH launcher consists of eight columns of four-way power splitters and two columns of dummy waveguides, one on each side. The operational n[parallel] value is fixed at 2.1 but can be adjusted by replacing waveguide components external to the vacuum vessel. Since the target operation pulse duration of the initial LHCD system is 2 s with an output power of 500 kW at the klystron window, the prototype klystron was recently successfully conditioned to a radio frequency power of 514 kW for a maximum pulse duration of 3 s using a matched dummy load (voltage standing wave ratio of 1.16:1). This paper presents the progress of the initial KSTAR LHCD system and the performance test results of the prototype klystron. The research plan aiming at steady-state LHCD operation in KSTAR is also described in this paper.

Design of a

It is our endeavor to develop switching technology for the breakdown reliability and reproducibility at extremely high voltages above 1 MV. One of the key objectives of the endeavor is to derive precise breakdown criteria for use in high-voltage applications. We have developed empirical criteria by which the breakdown field strength of a high-pressure SF_6-filled spark gap can be computed. The validity of the computed result has been verified with several experimental data from different investigators. The computed result has shown good agreement in an extended range of pd (pressure times gap distance) and field enhancement factor of the spark gap geometry. Furthermore, a spark gap has been designed and constructed with particular intention of generating a high-pulsed-power source in Pohang Accelerator Laboratory. In parallel, field modeling has been performed to verify the appropriate shape of the highly stressed insulator and electrode design in the spark gap and to evaluate their field enhancements. A series of breakdown voltage tests has demonstrated the function of the spark gap switch as intended up to 9 bars of SF_6. Aside from this, the limitation of the switching behavior has been examined with an aim of operating the designed spark gap to its maximum capability.

\hbox{SF}_{6}

We had designed, fabricated, and tested sealed-off triggered vacuum switches (TVSs) to use as an ETC-gun switch. To fabricate the sealed-off TVS, we developed fabrication processes such as part machining, chemical cleaning, ceramic to metal and metal to metal brazing, metal welding, baking, getter processing and test procedures. The switches fabricated were tested with capacitor banks of 100 kJ and 300 kJ. The prototype switches were successfully tested up to 30 kV peak charging voltage, 100 kA peak current, 1.2 ms pulsewidth, and 74-coulomb integrated charge transfer. It also showed good trigger capability. In addition, we tested rectifier characteristic of the switch. This paper describes test results of the switch characteristics.

-Gas-Filled Spark Gap Switch for High-Voltage Application

The 2-GeV electron linac at Pohang accelerator laboratory (PAL) has been operated continuously as a full energy injector for the Pohang Light Source (PLS) since Dec. 1994. There have been continuous efforts to improve the klystron-modulator (K&M) system to make it more stable and reliable. To improve self-diagnostic, operation, monitoring, and remote communication, we developed a new modulator controller based on an industrial PC platform. The modulator controller mainly consisted of an interlock signal conditioning module, a fast pulse signal-conditioning module, and the main PC platform. Operating system of the PC is Windows NT 4.0, and application software is developed with C++. The interlock signal-conditioning module generally carries digital and slowly varying clean analog signals. The fast pulse signal-conditioning module is used for preconditioning fast pulse and DC signals that inherently have high noise levels, such signals as a beam voltage, a beam current, an EOLC current, a HVDC voltage, and a HVDC current. All operation data of the K&M system can be acquired and saved in real time. We are upgrading the main PLS, control system with EPICS. The industrial PC platform will function as an input/output controller (IOC) in the future EPICS PLS control system.

Development of a triggered vacuum switch for a ETC gun system

There are growing interests in generation, preservation and applications of high brightness electron beam. With the rapid development in the techniques for emittance compensation, laser shaping and generation of ellipsoidal beam, we are approaching the limit--the uncorrelated thermal emittance. In this paper, we describe the design of the angle-resolved photoemission spectroscopy for measurements of thermal emittance for Cu and Mg. The measurement is conducted in a field-free region. The energy spectrum and angular distribution of the electrons will be measured immediately after its emission and further used to reconstruct the initial phase space and the corresponding thermal emittance. We also show how cathode surface roughness and laser incidence angle as well as its polarization state affect the quantum efficiency and thermal emittance.

A new modulator controller of the PLS linac

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.

First principle measurements of thermal emittance for copper and magnesium

The purpose of this experiment is to improve understanding of the high current vacuum arc phenomena in a triggered vacuum switch (TVS). The TVS used in the experiment has a rod array electrode. The cross section of each rod has trapezoidal shape. A spectroscopic measurement was performed up to 150 kA peak current at the center of electrodes. The tested electrode material was Fe. Measured Fe spectrum range was from 200 nm to 900 nm. Measurement result showed that over 90 percent of the charge states were Fe II, and the others were Fe I and Fe III. By assuming a local thermal equilibrium (LTE), electron temperatures of the TVS arc were determined from the relative line intensity ratio of Fe II. The electron temperatures at the center of electrodes were measured as 1.5 eV and 2.5 eV with 26 kA and 64 kA peak currents, respectively. The electron temperature increased with its peak current.