Juho Hong

Reduction of Multipole Fields in Photocathode RF Gun

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.

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.

Note: Recent achievements at the 60-MeV Linac for sub-picosecond terahertz radiation at the Pohang Accelerator Laboratory

A femtosecond (fs) terahertz (THz) linac has been constructed to generate fs-THz radiation by using ultrashort electron beam at the Pohang Accelerator Laboratory. To generate an ultrashort electron beam with 60-MeV energy, a chicane bunch compressor has been adopted. Simulation studies have been conducted to design the linac. In this note, recent achievements at 60-MeV linac are presented.

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.

Micro-mover Development and Test in the PAL-XFEL

Abstract Two micro-movers, which are able to control the horizontal, vertical and longitudinal positions as well as the yaw and pitch angles remotely, were developed and installed in the PAL-XFEL linac. The solenoid micromover in the gun section allows beam-based alignment of an electron beam to the solenoid field and the gun RF field. The X-band cavity micro-mover minimizes the transverse wake field effect caused by transverse misalignment between the beam and X-band cavity. Two micro-movers has similar specifications and the same mechanism, but the sizes are different from each other. In this paper, we present the design, manufacture and test results of the micro-movers.

Results Produced after Measuring PAL-ITF Beam Diagnostic Instruments

Pohang Accelerator Laboratory (PAL) built a PAL-ITF at the end of 2012 to successfully complete PAL-XFEL in 2015. The PAL-ITF is equipped with various kinds of diagnostic equipment to produce high-quality electron bunches. An ICT and a Turbo-ICT were installed in the PAL-ITF. A Faraday Cup (FC) is installed at the end of the linear accelerator. These days, the quantity of electric charge occasionally is measured using a BPM Sum value. This paper focuses on the processes and results of electric charge quantity measurements using ICT, TurboICT, FC and BPM. The PAL-ITF is equipped with Stripline-BPM. It is important to find a way to minimize measurement errors that can appear in the process of installing or measuring the BPM. For this, PAL-ITF separately measured the BPM electrode sensitivity and minimized BPM measurement errors through generally calibrating BPM devices by applying Lambertsons Method. A plan was made to minimize BPM measurement errors through applying the BPM electrical calibration method for BPM devices to be used by the PAL-XFEL. This paper examines the processes for checking the performance of the S-BPM installed in the PAL-ITF and the results of its measurements. INTRODUCTION The three main parameters that an injection testing facility should measure are charge, energy and emittance. Although ICT and FC were installed to measure charge, the noise generated in a klystron modulator not only interrupted accurate measurement but prevented low bunch charges under tens of pC from being measured. Due to the changes in the overall voltage level of PALITF, integration of ICT measured value failed to maintain perfect accuracy in terms of methodology (measured value continuously changed by +/5pC). Accordingly, to solve the noise problems and accurately measure the quantity of electric charge, Turbo-ICT was installed. Accurate measurement of beam positions requires not only BPM pickup characteristics but comprehensive methods of checking the BPM system, which can include all factors with potential BPM offset such as alignment in the process of installing cables, electronics and BPM equipment. To check sensitivity and offset of BPM, Lambertsons method was applied. The factors that should be calibrated to improve accuracy and precision are provided in Fig.1 [1][2]. See Fig.2 for types and locations of diagnostic units installed in PAL-ITF [3]. Figure 1: Factors crucial to improving the performance of diagnostic units. Figure 2: Types and locations of diagnostic units installed in PAL-ITF. BEAM CHARGE MONITOR (BCM) In PAL-ITF, instead of installing BCM-IHR-E electronics recommended by Bergoz Instrumentation, ICT output was going to be directly connected to an oscilloscope to measure quantity of electric beam charge, but noise prevented an accurate measurement. As shown in Fig.3, use of 50 ohm impedance matching and low-pass filter resulted in some improvement, but the klystron modulator led to constant slopping of ground-level voltage and measurement failed in low beam charge due to weak ICT output current. Figure 3: Improvement of ICT measurement methods. _____________________ * Work supported by the Ministry of Science, ICT and Future Planning (MSIP) in Korea. † choihyo@postech.ac.kr 5th International Particle Accelerator Conference IPAC2014, Dresden, Germany JACoW Publishing ISBN: 978-3-95450-132-8 doi:10.18429/JACoW-IPAC2014-THPRO021 06 Instrumentation, Controls, Feedback & Operational Aspects T03 Beam Diagnostics and Instrumentation THPRO021 2903 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 14 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I.