Hefei Light Source Research Articles

Ro-vibrational Distribution of NO+ Dissociated from NO2+ Ions in the a3B2 and b3A2 States: A Slow "Impulsive" Dissociation Example Revealed from Threshold Photoelectron-Photoion Coincidence Imaging.

To clarify the contentions about dissociative photoionization mechanism of nitrogen dioxide via the a3B2 and b3A2 ionic states, a new threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging has been conducted in the 12.8-14.0 eV energy range at the Hefei Light Source. The fine vibrational-resolved threshold photoelectron spectrum agrees well with the previous measurements. The ro-vibrational distributions of NO+, as the unique fragment ion in the dissociation of NO2+ in specific vibronic levels of a3B2 and b3A2 states, are derived from the recorded TPEPICO velocity images. A "cold" vibrational (v+ = 0) and "hot" rotational population is observed at the a3B2(0,3,0) and (0,4,0) vibronic levels, while the dissociation of NO2+ in b3A2(0,0,0) leads to the NO+ fragment with both hot vibrational and rotational populations. With the aid of the quantum chemical calculations at the time-dependent B3LYP level, minimum energy paths on the potential energy surfaces of the a3B2 and b3A2 states clarify their adiabatic dissociation mechanisms near the thresholds, and this study proposes reliable explanations for the observed internal energy distributions of fragment ions. Additionally, this study provides valuable insights into the application of the classical "impulsive" model on an overall slow dissociation process.

A wavelength calibration method for variable-included-angle plane grating monochromators with sine drives

The wavelength calibration is a key step for a variable-included-angle plane grating monochromator (VIA-PGM). At present, the wavelength calibration of the VIA-PGM is done separately for sine drives of the optical components. But the positions of the sine drives are linked by the grating equation and the focusing condition. A new wavelength calibration method for VIA-PGM is presented in which the relationship between the wavelength and both sine drive positions are included in a mathematical model. By this method the structural parameters for both sine drives are calibrated at the same time. It has been successfully applied to the wavelength calibration of a VIA-PGM in the Hefei Light Source, China. The results show that this method is effective and can perform accurate wavelength calibration.

Effective improvement of beam lifetime based on radiofrequency phase modulation at the HLS-II storage ring

A radiofrequency (RF) phase modulation method is applied to the Hefei Light Source II storage ring to deeply investigate its longitudinal beam characteristics and improve the beam lifetime. A theoretical analytical model and corresponding experimental measurements of single bunch length and island phenomena are examined. From a series of online machine experiments, we demonstrate that the suitable phase modulation amplitude is 0.02 rad, corresponding to an optimum modulation frequency ranging from 19.6 to 20.7 kHz of the RF system. Furthermore, the overall beam lifetime can be increased by a factor of 2.38 as a result of the beam dilution effect.

Upgrade of low level RF system based on Micro Telecom Computing Architecture (MTCA) for HLS-II LINAC

The performance of Hefei Light Source II (HLS-II) has improved a lot after major maintenance and reconstruction. To further provide continuous and stable light, the RF system of the HLS-II LINAC needs to be upgraded for top-off mode. It is required that the RF power source have long-time stability and reliability, but the old analog low level RF system(LLRF) can’t meet the requirement. Hence a digital low level RF control system based on Micro Telecom Computing Architecture(MTCA) is designed and implemented to control the amplitude and phase of the RF power source. This system is composed of digital board cards based on FPGA, RF board cards, MTCA chassis and a frequency synthesis system. It works at 2856 MHz of S band, with phase and amplitude stability up to 0.2° and 0.04% respectively, which meets the top-off mode requirement of 0.25° RMS phase jitter of the digital low-level RF system in the HLS-II LINAC.

The new infrared beamline at NSRL

National Synchrotron Radiation Laboratory (NSRL) owns the first dedicated synchrotron radiation facility in China, Hefei Light Source (HLS). The HLS was fully upgraded to HLS II in 2016. Following the machine upgrade, the infrared beamline was reconstructed to match the new storage ring and improved its performance. The infrared radiation is extracted from a bending magnet (1 degree port) with an acceptance angle of 65(H) × 55(V) mrad2, which including edge radiation and constant field radiaton. Two endstations were reconstructed. One is for spectroscopy experiment covering from far to near infrared, the other one is for mid-infrared microspectroscopy and imaging experiment. In this paper, we introduce the new infrared beamline and endstations in detail, including optics, experimental techniques developed at two endstations.

Study on beam-induced heating in injection section of Hefei Light Source

Ceramic chambers distributed with metal striplines on the inner surface are installed in the injection section at Hefei Light Source (HLS). Heating on the ceramics chambers has been observed during machine operation. An air compressor is used to cool these chambers due to concerns of overheating during top-up operation mode. To understand the sources of the heating, a series of experiments are performed with various beam currents and bunch filling patterns. The study shows that the heating is mainly caused by the narrow-band impedances of the ceramic chambers and their adjacent vacuum components.

Online Betatron Tune Feedback in the HLS-II Storage Ring

The betatron tune of a storage ring is a critical parameter in the design process and for user operation. A proportional–integral–differential (PID)-based feedback system is developed for the Hefei light source II (HLS-II) storage ring to stabilize the betatron tune. This feedback system adopts a tune knob using global mechanism, which can precisely adjust the tune. A simulation using an intelligent algorithm is carried out to optimize the PID coefficients. Online experiments show that this system can effectively correct the tune shift, which is caused by the gap change of insertion devices and other factors. The tune feedback system can improve the tune stability to the order of $10^{-4}$ , which can be employed as a useful tool for stable operation of storage ring-based light sources. This paper reports the details of the development of this tune feedback system. Some simulation and online experiment results are also presented.

Dissociation dynamics of energy-selected ions using threshold photoelectron-photoion coincidence velocity imaging

Threshold photoelectron-photoion coincidence (TPEPICO) is a powerful method to prepare and analyze internal energy- or state-selected ions. Here, we review the state-of-the-art TPEPICO imaging technique combining with tunable vacuum ultraviolet (VUV) synchrotron radiation and its recent applications at Hefei Light Source (HLS), especially on the fundamental data measurement and the dissociation dynamics of ions. By applying the double velocity map imaging for both electrons and ions in coincidence, the collection efficiency of the charged particles, the electron energy resolution and the resolving power of the released kinetic energy in dissociation have been greatly improved. The kinetic energy and the angular distributions of fragment ions dissociated from parent ions with definitive internal energy or state have been acquired directly from TPEPICO images. Some dissociation mechanisms involving non-adiabatic quantum effects, like conical intersection and internal conversion, have been revealed. Moreover, the mass-selected threshold photoelectron spectroscopy (MS-TPES) shows tremendous advantages in isomer-specific analysis of complex systems.

Development of a control system for the fourth-harmonic cavity of the HLS storage ring

Harmonic RF cavities are commonly used in storage rings to lengthen the bunches and thus suppress the beam’s instabilities and increase its Touschek lifetime. The voltage and phase of the electromagnetic fields in the harmonic cavity are of great importance for stretching the bunch. In the Hefei Light Source storage ring, a passive fourth-harmonic cavity is installed, and the cavity is monitored and controlled by an analog control module provided by its manufacturer. To vary and maintain the voltage of the harmonic cavity in a more effective way, a digital proportional, integral, and derivative feedback system based on the Experimental Physics and Industrial Control System is developed on top of the analog control module. This paper reports the details of the development of this voltage control system. Some test and operational results are also presented.

Automatic data archiving and visualization at HLS-II

The control system of Hefei Light Source II (HLS-II) is a distributed system based on the experimental physics and industrial control system (EPICS). It is necessary to maintain the central configuration files for the existing archiving system. When the process variables in the control system are added, removed, or updated, the configuration files must be manually modified to maintain consistency with the control system. This paper presents a new method for data archiving, which realizes the automatic configuration of the archiving parameters. The system uses microservice architecture to integrate the EPICS Archiver Appliance and RecSync. In this way, the system can collect all the archived meta-configuration from the distributed input/output controllers and enter them into the EPICS Archiver Appliance automatically. Furthermore, we also developed a web-based GUI to provide automatic visualization of real-time and historical data. At present, this system is under commissioning at HLS-II. The results indicate that the new archiving system is reliable and convenient to operate. The operation mode without maintenance is valuable for large-scale scientific facilities.

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

This contribution presents a novel bunch current measurement system based on an ultra-fast photodetector and a high-speed digitizer at Hefei Light Source II (HLS II). In order to achieve bunch-by-bunch resolution, the sampling rate of the system is nearly 225 GS/s via a dedicated equivalent sampling algorithm. According to preliminary tests of daily operation mode and single-bunch mode, the root-mean-square (rms) of current relative error distribution is 1.03%, which illustrates the new system satisfies requirements for high-precision bunch current measurement. In addition, experiment results of "HLS" Morse code fill pattern mode demonstrate this system also could be a convenient and robust tool for beam top-up mode in the future.

Multiple-bunch-length operating mode design for a storage ring using hybrid low alpha and harmonic cavity method

In this paper we design a simultaneous three bunch length operating mode at the HLS-II (Hefei Light Source II) storage ring by installing two harmonic cavities and minimizing the momentum compaction factor. The short bunches (2.6mm) presented in this work will meet the requirement of coherent millimeter-wave and sub-THz radiation experiments, while the long bunches (20mm) will efficiently increase the total beam current. Therefore, this multiple-bunch-length operating mode allows present synchrotron users and coherent millimeter-wave users (or sub THz users) to carry out their experiments simultaneously. Since the relatively low energy characteristic of HLS-II we achieve the multiple-bunch-length operating mode without multicell superconducting RF cavities, which is technically feasible.

Smoothing analysis of HLSII storage ring magnets**Supported by National Natural Science Foundation of China (11275192) and the Upgrade Project of Hefei Light Source

Hefei Light Source (HLS) has been upgraded to improve the quality and stability of the synchrotron light, and the new facility is named HLSII. However, a final accurate adjustment is required to smooth the beam orbit after the initial instalment and alignment of the magnets. We implement a reliable smoothing method for the beam orbit of the HLSII storage ring. In addition to greatly smoothing and stabilizing the beam orbit, this method also doubles the work efficiency and significantly reduces the number of magnets adjusted and the range of the adjustments.

Transverse beam size measurement system using visible synchrotron radiation at HLS II**Supported by National Natural Science Foundation of China (11105141, 11175173) and Upgrade Project of Hefei Light Source

An interferometer system and an imaging system using visible synchrotron radiation (SR) have been installed in the Hefei Light Source (HLS) II storage ring. Simulations of these two systems are given using Synchrotron Radiation Workshop (SRW) code. With these two systems, the beam energy spread and the beam emittance can be measured. A detailed description of these two systems and the measurement method is given in this paper. The measurement results of beam size, emittance and energy spread are given at the end.

The vacuum ultraviolet beamline/endstations at NSRL dedicated to combustion research

An undulator-based vacuum ultraviolet (VUV) beamline (BL03U), intended for combustion chemistry studies, has been constructed at the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. The beamline is connected to the newly upgraded Hefei Light Source (HLS II), and could deliver photons in the 5-21 eV range, with a photon flux of 10(13) photons s(-1) at 10 eV when the beam current is 300 mA. The monochromator of the beamline is equipped with two gratings (200 lines mm(-1) and 400 lines mm(-1)) and its resolving power is 3900 at 7.3 eV for the 200 lines mm(-1) grating and 4200 at 14.6 eV for the 400 lines mm(-1) grating. The beamline serves three endstations which are designed for respective studies of premixed flame, fuel pyrolysis in flow reactor, and oxidation in jet-stirred reactor. Each endstation contains a reactor chamber, an ionization chamber where the molecular beam intersects with the VUV light, and a home-made reflectron time-of-flight mass spectrometer. The performance of the beamline and endstations with some preliminary results is presented here. The ability to detect reactive intermediates (e.g. H, O, OH and hydroperoxides) is advantageous in combustion chemistry research.

Beam size and position measurement based on logarithm processing algorithm in HLS II**Supported by National Natural Science Foundation of China (11005105, 11175173)

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.

Central RF frequency measurement of the HLS-II storage ring**Supported by National Natural Science Foundation of China (11105141, 11175173) and the upgrade project of Hefei Light Source

Central RF frequency is a key parameter of storage rings. This paper presents the measurement of central RF frequency of the HLS-II storage ring with the sextupole modulation method. Firstly, the basis of central RF frequency measurement of the electron storage ring is briefly introduced. Then, the error sources and the optimized measurement method for the HLS-II storage ring are discussed. The workflow of a self-compiled Matlab script used in central RF frequency measurement is also described. Finally, the results achieved by using two data processing methods to cross-check each other are shown. The measured value of the central RF frequency demonstrates that the circumference deviation of the HLS-II storage ring is less than 1 mm.

Simulation of storage ring injection of HLS II

The Hefei light source (HLS) has been upgraded into HLS II, with increased brightness and stability. In this paper, the new injection system is described. Elegant program is used to simulate the particles motion in 6D phase space. In the process of tracking simulation of electrons, the system tolerances and magnet errors are introduced. Influences to the injection efficiency from beam injection deviation are studied. These provide important reference for realizing the Top-up injection.

Broadband impedance calculations and single bunch instabilities estimations of of the HLS-II storage ring**Supported by Natural Science Foundation of China (11175182, 11175180)

The upgrade project of the Hefei Light Source storage ring is under way. In this paper, the broadband impedances of resistive wall and coated ceramic vacuum chamber are calculated using the analytic formula, and the wake fields and impedances of other designed vacuum chambers are simulated by CST code, and then a broadband impedance model is obtained. Using the theoretical formula, longitudinal and transverse single bunch instabilities are discussed. With the carefully-designed vacuum chamber, we find that the thresholds of the beam instabilities are higher than the beam current goal.

Producing terahertz coherent synchrotron radiation at the Hefei Light Source**Supported by National Natural Science Foundation of China (11375176)

This paper theoretically proves that an electron storage ring can generate coherent radiation in the THz region using a quick kicker magnet and an AC sextupole magnet. When the vertical chromaticity is modulated by the AC sextupole magnet, the vertical beam collective motion excited by the kicker produces a wavy spatial structure after a number of longitudinal oscillation periods. The radiation spectral distribution was calculated from the wavy bunch parameters at the Hefei Light Source (HLS). When the electron energy is reduced to 400 MeV, extremely strong coherent synchrotron radiation (CSR) at 0.115 THz should be produced.