J.Y. Huang

Electron and photon beam diagnosis in PLS storage ring

The beam diagnostic system in PLS storage ring, which is in commissioning now, is described. Components of the electron beam diagnostic systems are: 4 fluorescent screens for the first‐turn beam detection, injection screen monitor, dc current transformer, beam scraper, betatron and synchrotron tune measurement system, and 108 beam position monitors composed of button style pickup electrodes. Among 108 BPMs, 96 have narrow band receivers and 12 have wideband receivers. With narrow BPMs, we achieved more than 65dB input dynamic range and better than 5 μmrms resolution. At every beamline front end, there is an x‐ray beam position monitor, composed of a pair of gold‐plated diagonal electrodes, for the beamline beam steering and fast local orbit steering. Transverse beam sizes will be measured with two linear image sensor arrays and a 2d CCD array. A sampling oscilloscope and an optical‐to‐electrical converter module with 17ps risetime will be used for the measurement of short bunch length.

A new PLS-II in-vacuum undulator and characterization of undulator radiation

This paper describes the result of overall studies from development to characterization of undulator radiation. After three years of upgrading, PLS-II has been operating successfully since 21st March 2012. During the upgrade, we developed and installed an in-vacuum undulator (IVU) that generates brilliant X-ray beam. The IVU with a 3 GeV electron beam generates undulator radiation up to ~ 21 keV by using 11th higher harmonic. The characterizations of the undulator radiation at an X-ray beam line in PLS-II agreed well with the simulation. Based on this performance demonstration, the in-vacuum undulator is successfully operating at PLS-II.

Beam Energy Drift and Energy Feedback System in the PLS 2.5GeV Linac

In PAL_XFEL, the specifications of the beam energy spread and rf phase are 0.03 %(rms) and 0.01 degrees(rms) respectively. We examined the rf performance in the present PLS 2.5 GeV Linac. The beam energy is changed as much as 0.8% by cooling temperature, air condition, and modulator high voltage jitter. The main factor to change the beam energy is the rf phase drift according to the variation of environmental condition. Dependence of the beam position drift on the rf phase was also examined. For the improvement of the injection beam stability, rf phase feedback system is designed and tested. This paper describes the rf phase feedback and the beam energy feedback system.

Exact determination of HOM frequency in storage ring RF cavity

A formula for the determination of higher-order mode (HOM) frequency in an active storage ring RF cavity is derived, which utilizes the frequency tuning mechanism of the low-level RF feedback system. It helps us to understand the behavior of coupled bunch instabilities in a storage ring whenever we change the operating beam current, the cavity gap voltage, and the ring RF frequency as well. This analysis includes the fundamental mode frequency detuning for beam loading and Robinson damping. At any operating conditions of the RF cavity, the HOM frequencies can be precisely calculated with the low-power measurement data of the frequency shift as a function of the tuner position. The formula shows a good agreement with the measurement results.

Effects of an elliptically polarizing undulator on the beam dynamics in PLS-II

The non-linear effects caused by the intrinsic field’s transverse roll-off in an elliptically polarizing undulator with a 72-mm period and a maximum peak field of 0.72 T (EPU72) on the dynamics aperture of the Pohang Light Source II (PLS-II) were investigated. A kick-map and a frequency map analysis both demonstrated that EPU72 will not reduce the lifetime or cause injection problems in PLS-II.

Analysis and control of the photon beam position at PLS-II

The variation of the photon beam position in a beamline, which is a critical issue for user experiments, is analyzed and corrected through the correlation link with the electron beam position in the storage ring.

High accuracy and high stability magnet power supply by four-phase buck type DC/DC converter

Linear accelerator and Beam Transfer Line magnet power supply (MPS) in Pohang Accelerator Laboratory have been upgraded from convention scheme to switching scheme for the stable beam injection and the 4th generation light source research. New MPS consists of two-main power board (MPB), which are regulator, CPU and control power boards. The size of each board is only 100 mm times 240 mm. The MPB consists of full-bridge scheme switch. The functions of the one MPB are that they operate as two parallel buck (PB) DC/DC converter. Switching frequency is 42 kHz at MPB. Two-MPB is operated as synchronous four-PB DC/DC converter. Input and output of the MPS are 3p 35Vac and 80Adc/50Vdc, respectively. Its resolution and stability are 16-bit and <20 ppm, respectively. To eliminate generated switching noise from main switch, specially designed 2-stage LC output filter is adopted in output station. The 1st-stage filter is included in the MPB. The 2nd-stage filter is located in output stage of MPB as separated PCB. High accuracy CT for sensing of output current is located in the summing stage of synchronous four-PB type DC/DC converter. This paper describes the design concept and performance of the MPS with four-phase buck type DC/DC converter.

HOM analysis and design of its removal system for 3rd harmonic RF cavity in PLS

Pohang Accelerator Laboratory has prepared to SRF 3rd harmonic cavity to increase beam lifetime and to damp orbit instability by lengthening electron bunch in PLS. The SRF cavity was developed and its vertical test was done already with success. Higher order modes were analyzed to optimize its performance in beam orbit. Most of them are not effective to electron beam, while the others have possibility to impact orbit stability. These harmful HOMs can be removed by HOM absorber installed in beam pipe.

Compact output filter for switching frequency elimination at the PLC Linac new magnet power supply

Pohang Accelerator Laboratory Linac magnet power supply (MPS) has been upgraded for the stable beam injection and 4 generation light source research, in 2006. MPSs are developed as compact configuration by using four quadrant switching scheme with power MOSFET. In case of 10 V, +/-10A bipolar output operation, main circuit is operated as one or three quadrant, and in case of 50 V, 50 A unipolar output operation, main circuit is operated as parallel one quadrant. Switching frequency of MOSFET switch is above than 40 kHz. MPS consists of main power board, control power board, regulator board and CPU board. The size of each board has 100 mm width and 250 mm depth. Main power board made of four- quadrant MOSFET switch, driver and output filter. Output filter must be perfectly eliminating switch frequency and have compact size. In this paper, we report on development and characteristics of compact output filter of the new MPS for PAL linac.

Beam diagnostic system for high intensity proton linac at KAERI

A 100-MeV high-intensity (20 mA CW) proton linac is being built in the KAERI (Korea Atomic Energy Research Institute), Korea. Beam diagnostic system for the machine require to include BCMs (Beam Current Monitors), BPPMs (Beam Position and Phase Monitors), and other devices. BCMs are of special importance for high-intensity proton linacs, since the control of beam loss is critical. They should have high stability and resolution better than 5E-4. BPPMs are for measuring beam position and phase (w.r.t RF) simultaneously, that are crucial for the successful commissioning and operation of the accelerator. All diagnostic devices are required to operate both in CW and pulsed (several ms) modes. Pulsed mode operation is useful when machine tuning and commissioning. In this article, we report the developmental status of beam diagnostic devices for the KOMAC accelerator, with detailed description on the BPPM PU design.