After three years of upgrading work, the Pohang Light Source-II (PLS-II) is now successfully operating. The final quantitative goal of PLS-II is a top-up user-service operation with beam current of 400 mA to be completed by the end of 2014. During the beam store test up to 400 mA in the storage ring (SR), it was observed that the vacuum pressure around the radio frequency (RF) window of the superconducting cavity rapidly increases over the interlock level limiting the availability of the maximum beam current storing. Although available beam current is enhanced by setting a higher RF accelerating voltage, it is better to keep the RF accelerating voltage as low as possible in the long time top-up operation. We investigated the cause of the window vacuum pressure increment by studying the changes in the electric field distribution at the superconducting cavity and waveguide according to the beam current. In our simulation, an equivalent physical modeling was developed using a finite-difference time-domain code. The simulation revealed that the electric field amplitude at the RF window is exponentially increased as the beam current increases, thus this high electric field amplitude causes a RF breakdown at the RF window, which comes with the rapid increase of window vacuum pressure. The RF accelerating voltage of PLS-II RF system was set to 4.95 MV, which was estimated using the maximum available beam current that works as a function of RF voltage, and the top-up operation test with the beam current of 400 mA was successfully carried out.

A: The Pohang Light Source-II (PLS-II) is currently providing a top-up mode user-service operation with maximum available beam current of 400 mA and a beam emittance of below 10 nmrad. The dimension of the beam bunch shortened to accomplish a low beam emittance of below 10 nm-rad from a high beam current of 400 mA increases the bunch charge density. As a result, the electron beam lifetime is significantly degraded and a high gradient of power is lost in the vacuum components of the storage ring. A study on how to reduce the bunch charge density without degrading beam emittance found that reducing the RF accelerating voltage (V acc ) can lower the bunch charge density by lengthening the bunch in the longitudinal direction. In addition, the V acc required for stable operation with beam current of 400 mA can be reduced by lowering the external cavity quality factors (Q ext values) of the superconducting cavities (SCs). To control the Q ext values of SCs gradually without accessing the accelerator tunnel, a remote control motorized three-probetuner was installed in the transmission line of each SC. The optimum installation position of the three-probe-tuner was determined by using a finite-difference time-domain (FDTD) simulation and by experimenting on various installation positions of the three-probe-tuner. The Q ext values of all the SCs were lowered to 1.40 × 10 5 , and then, the V acc required to store the beam current of 400 mA was decreased from 4.8 MV to 4.2 MV, which corresponds to 10% lengthening of the beam bunches. The stable operation with the reduced V acc was confirmed during a 400 mA ten-day top-up mode user-service. Currently, the RF system of the PLS-II storage ring delivers the user-service operation with lowered Q ext values to reduce the power loss at the vacuum components as well as the cryogenic heat load of SCs, and no significant problems have been found. This method of reducing the V acc may also be applied in other synchrotron facilities. K: Instrumentation for synchrotron radiation accelerators; Instrumentation for particle accelerators and storage rings -high energy (linear accelerators, synchrotrons) 1Corresponding author.

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The Pohang Light Source-II (PLS-II) is a synchrotron user-service facility that should provide a user-service period of 190 days per year with stable operation. A series of beam dump problems related to the superconducting radio-frequency (SRF) system but of unknown cause occurred twenty-five times from 2016 to 2018, which severely degraded the stable operation of the PLS-II and the SRF system. Intensive investigation has shown that the relevant beam dumps occurred when the helium (He) flow control valves operated momentarily during the adsorber regeneration process of the cryogenic system. As the control valves operate momentarily, the He vessel pressure of the cryomodule instantaneously fluctuates, and the resonance frequency of the SRF cavity accordingly changes. If the instantaneous change in the resonance frequency is larger than the low-level RF (LLRF) control ability, then incorrect handling of LLRF control occurs, resulting in a beam dump. The instantaneous changes in the He vessel pressure occur via three different mechanisms. Solutions for each beam dump mechanism were proposed and implemented. After each solution was carried out, no beam dump occurred during the adsorber regeneration process. The relation between the He vessel pressure and the resonance frequency of the SRF cavity was also studied to see how the solution works.

We have estimated the problems of high energy application to memory device fabrication in the point of wafer crystalline structure. Following some series of experiments, we have found the different implantation damage induced by the variations of off-cut and azimuth angle of wafer and the difference can be reduced if the tilted wafer is adopted. The using of off-angle wafer also minimize the shadowing effect depended on the tilt angle of implantation because there is no need to use the tilting method in ion implantation process.