Zeran Zhou

Development of transverse feedback system and instabilities suppress at HLS

In order to cure and damp coupled bunch instabilities (CBI), a bunch-by-bunch (BxB) measurement and transverse feedback system is under commission at Hefei light source (HLS). In this paper we introduce the overview of the measurement and analogy transverse BxB feedback system in emphases. The measurement system is dedicated to observe the beam instability and works as a part of the analogy and digital transverse BxB feedback system. The development of several characterized components and stripline feedback kicker as well as the experiment result of the feedback system in HLS ring is also presented in this paper.

Development of digital transverse bunch-by-bunch feedback system of HLS

Hefei light source (HLS) operates at high beam current with many bunches. Multi-bunch instabilities degrade beam quality. An FPGA based digital transverse bunch-by-bunch feedback system was under development in HLS to suppress beam instabilities. The design of the digital feedback system and primary experiment results are presented in this paper. Further improving of the feedback system and investigating of the characteristics of the feedback loop are the future work.

Bunch Current Measurement Using a High-Speed Photodetector at HLS II

This paper presents a novel bunch current measurement system based on an ultrafast photodetector and a high-speed digitizer at Hefei light source II (HLS II). We use a metal–semiconductor–metal photodetector to measure the emitted optical synchrotron radiation intensity directly, representing the bunch current intensity. To achieve bunch-by-bunch resolution, the sampling rate of the system is nearly 225 GS/s, which is achieved via a dedicated equivalent sampling algorithm. The detailed description of the experimental setup and the equivalent sampling algorithm are presented. According to preliminary tests of the daily operation mode and single-bunch mode, the measured root-mean-square of the beam current is ~1%, which shows that the new system satisfies the requirements for high-precision bunch current measurements. In addition, experimental results of the “HLS” Morse-code fill pattern mode demonstrate that this system could also be a convenient and robust tool for beam top-up modes in the future.

Beam size and position measurement based on logarithm processing algorithm in HLS II

A logarithm processing algorithm to measure beam transverse size and position is proposed and preliminary experimental results in Hefei Light Source II(HLS II) are given. The algorithm is based on only 4 successive channels of 16 anode channels of multianode photomultiplier tube(MAPMT) R5900U-00-L16, which has typical rise time of 0.6 ns and effective area of 0.8×16 mm for a single anode channel. In the paper, we first elaborate the simulation results of the algorithm with and without channel inconsistency. Then we calibrate the channel inconsistency and verify the algorithm using a general current signal processor Libera Photon in a low-speed scheme. Finally we get turn-by-turn beam size and position and calculate the vertical tune in a high-speed scheme. The experimental results show that measured values fit well with simulation results after channel differences are calibrated, and the fractional part of the tune in vertical direction is 0.3628, which is very close to the nominal value 0.3621.A logarithm processing algorithm to measure beam transverse size and position is proposed and preliminary experimental results in Hefei Light Source II (HLS II) are given. The algorithm is based on only 4 successive channels of 16 anode channels of multianode photomultiplier tube (MAPMT) R5900U-00-L16 which has typical rise time of 0.6 ns and effective area of 0.8x16 mm for a single anode channel. In the paper, we firstly elaborate the simulation results of the algorithm with and without channel inconsistency. Then we calibrate the channel inconsistency and verify the algorithm using general current signal processor Libera Photon in low-speed scheme. Finally we get turn-by-turn beam size and position and calculate the vertical tune in high-speed scheme. The experimental results show that measured values fit well with simulation results after channel differences are calibrated and the fractional part of the tune in vertical direction is 0.3628 which is very close to the nominal value 0.3621.

Longitudinal electron bunch diagnostics using coherent transition radiation at the IRFEL

A longitudinal electron bunch diagnostics system is developing to measure the longitudinal bunch charge distribution for the new IRFEL at National Synchrotron Radiation Laboratory (NSRL). We use a Martin-Puplett interferometer, which is essentially a Michelson interferometer, to measure the spectrum of the coherent transition radiation produced by electrons through a thin metallic foil. Frequency components of coherent transition radiation have a relationship with the bunch form factor, which is described by the square modulus of the Fourier transform of the bunch distribution. Then several techniques, including a Kramers-Kronig analysis, have been applied to determine the longitudinal bunch charge distribution. The details of the design and theoretical investigation will be described in this paper.

Design of Bunch Length Measurement System at the IRFEL Using a Martin-Puplett Interferometer

Electron bunch length measurement is of great significance for optimizing IRFEL performance. An optical autocorrelation system using coherent transition radiation (CTR) would be set up to measure the electron bunch length at the IRFEL. CTR can be occurred when short electron bunches traverse a vacuum-metal interface. A Martin-Puplett interferometer allowed measurement of the autocorrelation of the CTR signal. The basic principle and the main components of Martin-Puplett interferometer are elaborated in this paper. INTRODUCTION With the development of the high pulse intensity and ultrashort pulse duration of the Infrared Free Electron Laser (IRFEL), which overcomes the shortages of traditional infrared spectroscopy, such as low sensitivity, low spacial and low time resolution. This project will significantly promote the progress in energy chemistry, and establish a fundamental scientific research to reach the international advanced level. A schematic view is shown in Fig. 1, with the design parameters listed in Table 1. The measurements of bunch length can fall to two categories: time domain measurements and frequency domain measurements [1]. In the time domain techniques, streak camera is usually used. But the resolution limit of streak camera is 200 fs and streak camera is too expensive. Instead of time domain techniques, frequency domain measurements using Coherent Transition Radiation (CTR) is employed. Figure 1: Schematic view of NSRL IRFEL. Table 1: Parameters of the IRFEL

Modified Trigger Mode of Streak Camera to Measure Bunch Longitudinal Distribution in HLS II

In Hefei Light Source, the streak camera was used to measure the bunch length and longitudinal distribution using synchronous light. As the RF frequency of HLS II was 204 MHz, the streak camera worked at the frequency of 102 MHz (half of 204 MHz). Because of the bunch lengthening, the streak camera faced the problem, the streak image on the phosphor screen will overlap when the bunch length was above 200.5 ps@5% linear error and 10% overlap. In order to solve this problem, an effective solution was to change the working frequency of the streak camera to 136 MHz (two thirds of 204 MHz), and then the streak image on the phosphor screen will overlap when the bunch length was above 285.6 ps@5% linear error and 10% overlap. So a front-end electronic was needed before the synchronizing signals feed into the streak camera. The frontend electronic was designed to convert the 204 MHz synchronizing signal to 136 MHz. INTRODUCTION Hefei Light Source is an electron storage ring with 800 MeV energy, and its RF frequency is 204 MHz. When it is working under single-bunch patterns, the bunch space is 220 ns and the natural bunch length is 110 ps [1]. Besides when it is working under multi-bunch patterns, the bunch length is about 240~305 ps while the beam current is from 100 to 200 mA [2]. Because of the bunch lengthening, the bunch length is increasing rapidly along with the increase of the beam current [3]. The streak camera used to work at a frequency of 102 MHz, but the measurement got error when the bunch length was above 200.5 ps @5% linear error and 10% overlap. It has been confirmed that the streak camera could avoid the error if changing its working frequency to 136 MHz when the bunch length is under 285.6 ps in theory, so we designed and made a front-end electronic to realize the frequency conversion from 204 MHz to 136 MHz. In this paper we will introduce operating principles of the streak camera, synchronous sweep under different trigger modes and the front-end electronic. INTRODUCTION TO STREAK CAMERA A streak camera developed at Optronis Company in Germany was adopted in HLS II. Streak camera is an instrument to measure ultra-high-fast optical phenomenon and it can provide intensity, time and location information of the light. Figure 1: Operating principle of the streak camera. Figure 1 shows operating principles of the streak camera. The light being measured first passes a slit, and is formed into a slit image on the photocathode of streak tube by an optics system. There, four optics pulses which have slightly difference in space and time and different intensities reach the photocathode through the slit. The light reaching the photocathode is converted into series of electrons proportional to the light intensity. Then, these electrons pass a pair of accelerating electrodes where they are accelerated and struck against the phosphor screen. When electrons generated by four optics pluses pass a pair of sweep electrodes, the relationship between the incident light and the high voltage applied to the sweep electrode is shown in Fig. 2. Figure 2: Operating timing (at time of sweep). During the high-speed sweep, the electrons having a slight time lag will have a different angle deflection in vertical direction and then enter the MCP. When passing the MCP, the electrons are amplified thousands of times, and then strike against the phosphor screen converting into light again. On the phosphor screen, the phosphor image on the uppermost position corresponds to the earliest optics pulse and other images are in order from the top to the bottom. Meanwhile, the brightness of the phosphor images is in direct proportion to the intensity of incident optics pulses [4]. In this way, we can obtain the incident optics intensity from the brightness of the phosphor image, and the time and position from the location of the phosphor image [4]. The streak camera measurement system in HLS II is composed of trigger unit, synchrotron light extracting unit, OPTOSCOPE streak camera, readout unit, frame-grabber board and computer. The system configuration is shown in Fig. 3 [5]. _________________ * Supported by the National Science Foundation of China (11575181, 11175173) # Corresponding author (email: bgsun@ustc.edu.cn) MOPMB041 Proceedings of IPAC2016, Busan, Korea ISBN 978-3-95450-147-2 184 C op yr ig ht

Preliminary Research of HLS II BLM System

Beam loss monitor(blm) system has been designed in many electron storages in order to indirectly measure lost electrons, which can be used to analysis beam loss mechanism and beam life. It can contribute to beam commissioning and improving stable operation of storage ring. According to lattice structure of the HLS II storage ring, 64 beam loss detectors have been located in the upper, lower, inner, outer side surfaces of vacuum chamber in the HLS II storage ring. Some preliminary researches based on the HLS II blm system have been done. The results in successfully stable operation and unsuccessfully stable operation in beam commissioning stage were compared. Analysis of a sudden lost beam phenomenon were carried out. INTRODUCTION Hefei Light Source II(HLS II) has been upgraded from August 2010 and now enter formal operational phase. It consists of a linac, a transport line and a storage ring. The energy of beam is accelerated to 800MeV when beam reaches the end of the linac and beam is injected into storage ring through the transport line. In order to protect machine, speed up commissioning, conventionally diagnose beam and study beam life, beam loss monitor system is designed for the HLS II storage ring. According to lattice of the HLS II storage ring, beam loss monitors are installed nearby quadrupole magnets[1, 2]. In order to monitor beam loss in different directions of the vacuum chamber, every 16 beam loss monitors are respectively mounted on upper and lower side surfaces of the vacuum chamber in vertical direction and on inner and outer side surfaces of the vacuum chamber in horizontal direction. The layout of beam loss monitors in the HLS II storage ring is shown in Fig. 1. Site installation diagram is shown in Fig. 2. Bergoz’s PIN-diode detectors are used as beam loss monitors in the HLS II storage ring. The detector is composed of two PIN-diodes mounted face to face to form a 2channel coincidence detector in which MIPs (minimum ionizing particles) cause ionizations in both PIN-diodes, a coincidence occurs and an output pulse is generated[3]. BLM SYSTEM STRUCTURE Beam loss monitor system structure is as shown in Fig. 3. The generated pulses in four blm detectors located in the upper, lower, inner, outer side surfaces of vacuum chamber are transmitted to Data Acquisition(DAQ) device to shape pulses, count pulses, and transmit results to PC via ethernet. Labview program on PC publishes results in epics environment and any PC via intranet can acquire blm results. Fig. 4 is Labview program main interface for blm. Blm results of four different orientations in each monitoring position display with histogram. Figure 1: The layout of beam loss monitors in the HLS II storage ring. Figure 2: Site installation diagram for beam loss monitors in the HLS II storage ring. Figure 3: Beam loss monitor system structure diagram. ___________________________________________ * Supported by the National Science Foundation of China (11175173, 11375178) # zhouzr@ustc.edu.cn MOPMB043 Proceedings of IPAC2016, Busan, Korea ISBN 978-3-95450-147-2 190 C op yr ig ht

Design of the Beam Profile Monitors for THz Source Based FEL

To meet requirements of high performance THz-FEL, a compact FEL facility was proposed. In order to characterize the beam, some beam profile monitors were designed. There are four Flags for beam profiles in LINAC,one pop-in monitor for high precision beam profile inside a small-gap undulator, and two screens to measure the beam energy spread and emittance of Linac. On one hand, we need to use software to control the position of these profile monitors, on the other hand, we need screens to display the results. This paper describes how to design and control these monitors, as well as the beam profile image acquisition system. INTRODUCTION The operation of short wavelength Free Electron Lasers(FELs) requires the usage of electron beams with extraordinary beam quality. HUST(Huazhong University of Science and Technology) and NSRL(National Synchrotron radiation Laboratory)/USTC(University of Science and Technology of China) are cooperating to set up the facility. The purpose is to meet strict requirements of high performance of electron beam for THz source based FEL and the facility get compact[1]. The facility is composed of a ITC-Gun, constant gradient travelling wave LINAC, microwave power system, vacuum system, control system, beam diagnostics system and so on. The beam diagnostics system consists of three beam current transformers, one stripline beam position monitor, and a set of beam profile monitors.

BPM Signal Channel Characterization Test based on TDR for HLS II Storage Ring

A new BPM system on the upgraded Hefei light source (HLS II) storage ring is installed. Before the machine commissioning, the BPM system should be carefully tested, such as the conductivity and integrity of BPM signal channels from button electrodes to digital beam position processors (pickups, cables and connectors). This paper presents an experience of signal channel test based on time domain reflection (TDR) for HLS II storage ring BPM system. Basing on the wave propagation method, an analytic expression for the signal from TDR on BPM signal channel is briefly introduced. The conductivity and integrity of the BPM signal channels can be verified by comparing the TDR waveform with theory signal. All the BPM signal channels are tested by the TDR in order to verify electronic characteristic and the usability. And some breakdowns are analysed and handled. INTRODUCTION An upgrade is employed on Hefei Light Source. The energy of injector changed from 200MeV to 800MeV, and the electron storage ring was reconstructed and most of beam diagnosis system were redeveloped include BPM system, as is shown in Fig. 1. Figure 1: Beam diagnosis component of HLSII storage ring. Much of the beam diagnosis system are working on radio frequency, such as BPM system, FCT&ICT measurement system, Bunch by bunch Feedback system, Tune measurement system. In this systems, detectors and processors were distributed in different place and connected by RF cables called as signal channel. The electronic characteristic and the usability of signal channel will influence the performance of measurement system. BPM signal channels were test and evaluated in this paper. The BPM system of HLS II storage ring has 32 units for beam close orbit distortions measurement. Each unit consists of the digital signal processor, signal channels, and pickups. The pick-up is button-type electrode and the beam position signal processor is Libera Brilliance [1]. Since BPM has four button-type electrodes, each unit has four signal channels severally named as channel A, B, C and D. The signal channels between the BPMs and the position signal processors consists of main signal cables, jumper wires, feedthroughs on the button and the connectors between different sections. The schematic of one BPM signal channels is shown in Fig. 2. The type of main signal cables is LMR-400 of the Times Microwave Company and the jumpers are PRC9104. The delay time of the signal are respectively 3.92 ns m and 5.0 ns m , and the attenuation factor of the LMR-400 is about 9.2 dB/100m at 500 MHz [2]. The buttons has a female SMA connector [3]. The idea impedances of main cable and the jumpers are 50 . The impedance of the feedthrough of the button was also designed to be 50 . As a complex system, the damages of the multitudinous devices are probability during the machine installation. So it is necessary to make a dedicated test for the continuity of signal channels before machine commissioning. Figure 2: The schematic of the BPM signal channels. The technique of Time Domain Reflection (TDR) [4] was developed as early as 1960s. A pulse from a signal generator transmit along the transmission line and the reflection of the signal happens when it meet a mismatch of the impedance. The reflecting signal can be received and displayed on the oscilloscope. By observing the wave, the character of the transmission line can be analysed. TDR method can be used to qualitatively test the character of the long-length cables in many engineer fields, as well as in the accelerator beam diagnostics. SIGNAL CHANNEL EVALUTION BASED ON TDR Figure 3 illustrates the method for the evaluation of the signal channel. In the BPM system, button, at the end of signal channel, can be seen as a capacitive device. The jumpers and the feedthrough are much shorter than the main cable. The measurement instrument has variable ___________________________________________ *Supported by National Natural Science Foundation of China(11105141,11175173) #ylyang@ustc.edu.cn Ω Ω 5th International Particle Accelerator Conference IPAC2014, Dresden, Germany JACoW Publishing ISBN: 978-3-95450-132-8 doi:10.18429/JACoW-IPAC2014-THPME145 06 Instrumentation, Controls, Feedback & Operational Aspects T03 Beam Diagnostics and Instrumentation THPME145 3593 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.