Youngdo Joo

Studies and optimization of Pohang Light Source-II superconducting radio frequency system at stable top-up operation with beam current of 400 mA

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.

Emission test of electron emitter using easy-replaceable-emitter test bench

The emission test is conducted using an easy-replaceable-emitter emitter test bench. A simple cylindrical button type emitter with the diameter of 6 mm is installed vertically into an emitter cup holder. The emitter is heated by a tungsten wire heater located around the cup holder. The emitter temperature is measured by an optical pyrometer at a given heater power. A high voltage pulse power supply gives the anode-cathode gap voltage up to 20 kV with the pulse width and repetition rate of 15 us and 50 Hz, respectively. The emitted current from the emitter surface is captured at a faraday cup and is measured using current transformer. The test bench is installed in the vacuum chamber with easy access door thus the emitter can be easily replaceable. The emitters with various combinations of tungsten grain structure and impregnations will be tested under the various emitter activation processes. The detailed test result for the emitter will be presented.

Development of new S-band RF window for S-band RF system of Pohang Light Source-II linear accelerator

The RF windows have been used in the Pohang Accelerator Laboratory, Pohang Light Source-II (PLS-II) must meet the conditions for operating at a peak power of 80 MW and a repetition rate of 10 Hz. In these operational conditions of the PLS-II, the original RF window that was designed for a peak power of 60 MW is more likely to inflict damages that cause vacuum leaks. Therefore, for the new RF window, the circular cavity of TE012 mode with two coupling irises design is employed to reduce the peak electric field on the ceramic surface, while the circular cavity of TE111 mode is used for the original RF window. The finite-difference time-domain (FDTD) simulation was used for the design. For this paper, we present the result of the prototype and its high power test.

Status of the PLS-II Synchrotron Light Source

Pohang light source (PLS) had been operated for 14 years successfully. The number of beamlines of PLS was two at the beginning stage. The number of beam-lines has been increased two or three every year. And the total beam-lines in operation became 28 in 2010 and 3199 users had visited PAL for their experiments [1]. The user community gradually extended and they became to require brighter beam to see small signal from a sample. To meet this requirement of the user community, the PLS machine was upgraded as PLS-II. The main goals of the PLS-II machine are to increase the beam energy from 2.5 to 3 GeV, and to increase the number of insertion device (ID) by the factor of two (20 IDs). This PLS-II upgrade project had been completed in 2011 and user service has been started in 2012. We will report the current status of PLS-II machine in this paper.

Study on Vertical Position Reading Noise from Beam Position Monitor in Pohang Light Source Storage Ring Vacuum Chamber

A sudden step change is observed in the vertical position readings from beam position monitors (BPMs) mounted at the several sector vacuum chambers of Pohang Light Source. To study the source of this sudden step change, we measured the RF transmission scattering matrix (S₂₁) through the pickup electrodes of BPMs mounted at the both ends of the sector vacuum chamber. The measured S₂₁ graph of the sector vacuum chambers suffering sudden step change has a peak in the BPM operation frequency bandwidth. Otherwise that of the other sector vacuum chambers doesn’t have a peak. It is shown by the numerical simulation that the peak found in the BPM operation frequency bandwidth corresponds to the longitudinal harmonic of transverse electric resonance mode.