First commissioning of the HLS-II storage ring*

Transcription

1 Submitted to Chinese Physics C First commissioning of the HLS-II storage ring* LIU Gang-Wen( 刘刚文 ) XUAN Ke( 宣科 ) 1) XU Wei( 徐卫 ) WANG Lin( 王琳 ) LI Wei-Min( 李为民 ) LI Jing-yi( 李京祎 ) National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 2329, China Abstract: To meet the increasing requirements of synchrotron radiation users, the upgrade project to enhance the performance of Hefei Light Source (HLS), named HLS-II, was launched in 21, and in 214 the first commissioning of HLS-II was successfully completed. After the commissioning, the main design goals for the HLS-II storage ring have been achieved, with natural emittance of electron beam lower than 4 nm rad at 8 MeV, five insertion devices installed in straight sections and root mean square (rms) jitter of closed orbit smaller than 4 μm, making HLS- II at a higher level among the same class of machines in the world. This paper reports on the results of the commissioning of the HLS-II storage ring, which includes linear optics correction, compensation of insertion devices effect and closed orbit feedback. Key words: HLS-II, storage ring, calibration, compensation, feedback PACS: 29.2.db, Fh 1 Introduction The Hefei Light Source (HLS) was a second generation synchrotron radiation light source, which had been operated for over two decades in the VUV to soft X-ray range with natural emittance of 166 nm rad at electron beam energy of 8 MeV. To better satisfy the increasing requirements of synchrotron radiation experiments, an upgrade project of HLS to enhance its performance, named HLS-II, was proposed and then launched in July, 21. The philosophy for the design of the HLS-II storage ring is to reduce the natural emittance of electron beam and increase the number of straight sections for insertion devices while maintaining the former foundation. Following this philosophy, the new storage ring lattice was designed [1] and studied [2, 3]. The circumference of the new storage ring remains m. Its lattice structure is changed to 4 DBA from the former 4 TBA to provide more straight sections for insertion devices. In the new lattice, there are eight straight sections in total applied for beam injection, RF cavity and insertion devices. Strong focusing quadrupoles are employed to obtain lower natural emittance of less than 4 nm rad at the nominal energy of 8 MeV in the achromatic mode. Besides, the full energy injection scheme is to be adopted for the new storage ring, which requires raising the linac energy. So a new linac is designed with the capability of raising beam energy up to a maximum of 96 MeV instead of the former 2 MeV linac. Table 1 shows the main design parameters of the HLS-II ring. * Supported by NSFC( , ) 1) xuanke@ustc.edu.cn

2 Table 1: Main parameters of the HLS-II storage ring Beam energy 8 MeV Beam current 3 ma Natural emittance < 4 nm rad Beam lifetime > 5 hours RF Frequency 24 MHz Harmonic number 45 Natural energy spread (rms).47 Slow orbit drifts <.1 σ To achieve the design goals within the planned schedule, the whole machine including the injector and the storage ring has been commissioned with great efforts. Because the structure parameters of the undulators deviate slightly from the design value, the lattice structure of the HLS- II storage ring is redesigned from four superperiods to two superperiods. The redesigned transverse beta functions are shown in Fig.1. Fig. 1. Transverse beta functions of the HLS-II storage ring. In this paper, the commissioning results of HLS-II storage ring are presented in detail. The new lattice is calibrated and corrected with LOCO and the result of which is presented in section 2. The section 3 mainly indicates the compensation results of wiggler and undulator. The new orbit feedback system in HLS-II storage ring is introduced in section 4 and the stability of the orbit is shown. Finally, the results of HLS-II storage ring commissioning are summarized in conclusion section. 2 Lattice calibration and optics correction Before the optics correction, the beta functions and tunes of the HLS-II storage ring lattice are different from the design values. Especially for the vertical beta function the maximum beating is up to 97.6%. Besides, the beam lifetime is very poor, only 1.5 hours. To make the beam optics meet the design values, the program LOCO (Linear Optics from Closed Orbits) [4] widely used for linear optics calibration around the world [5-7] is adopted to calibrate the upgrade storage ring lattice and correct its optics. The measured response matrix is obtained with all of insertion devices out of work and a beam

3 current of about 6 ma. After fitting the response matrix with LOCO, the beta functions are corrected, whose values at all of quadrupoles in the vertical plane, for example, are shown in Fig. 2 compared with the uncorrected and the design values. The blue bars indicate the design values of vertical beta function, and the red and the yellow are the values before and after the optics correction respectively. It is clear to see that after the correction the vertical beta function is very close to the design value. The maximum relative difference from the design value is reduced from 97.6% to 6.8%, which is at a higher level compared with other light sources of the same class in the world [8, 9]. Fig. 2. Comparison of the vertical beta function values after and before the optics correction, where the green line expresses the design value, the blue stars and red circles show the beta functions at the position of each BPM before and after compensation respectively. To make sure the results are reliable, the residual differences between the measured response matrix and that calculated from the model are normalized with the resolution of the BPMs and the distribution of the normalized results is statistically analyzed, which is shown in Fig. 3. From the figure, we can see that the distribution is normal and the FWHM (full width at half maximum) is less than 2, which means that the difference between the measured and calculated response matrices can converge into the area that is limited by the measurement accuracy of the BPMs. Therefore after fitting with LOCO the theoretical model can accurately reflect the real storage ring lattice. Fig. 3. Distribution of the normalized differences between the measured and calculated response matrices.

4 3 Lattice compensation of insertion devices effect Five insertion devices [1] have been equipped in the HLS-II storage ring. The periodic magnetic field in these insertion devices leads to tune shift and beta function distortion, which is similar to the focusing effect of the fringe field in dipoles. It is very necessary to compensate the HLS-II storage ring lattice disturbed by these insertion devices. The compensation of the wiggler and in vacuum undulator (IVU) is the priority due to their serious effect on the beam in the storage ring. The wiggler is the main insertion device to reduce the beam emittance in the HLS-II storage ring. A hybrid permanent magnet wiggler is installed with the deflection parameter of and the length of 156 mm. We can estimate theoretically that the vertical tune shift can be as much as.1926 and the beta beating will be % at the most with the effect of the wiggler. Considering these serious effects on the beam, the lattice is compensated for with all of quadrupoles in the storage ring. After the lattice compensation under the condition that the gap of wiggler is 32 mm, the beta functions at each quadrupole are measured. Fig. 4 shows the vertical beta function measured after the compensation, which are compared with their values before the compensation and their design values. We can see that the vertical beta function values have been improved greatly after the compensation. The maximum beta beating is only about 1.5%, which indicates that the measured vertical beta function values almost meet the theoretical values. The distribution of the normalized residual errors is also statistically analyzed just as mentioned in the previous section, and the FWHM is still less than 2, which shows that the results of the compensation are reliable. Fig. 4. Comparison of the vertical beta function values before and after the compensation of the wiggler, where the green line expresses the design value, the blue stars and red circles show the beta functions at the position of each BPM before and after compensation respectively. To compensate the effect of the wiggler, the strengths of all quadrupoles are adjusted. The strength change of each quadrupole is presented in Fig. 5. It is clear to see that the strengths of the two quadrupoles Q12 and Q13 around the wiggler have the largest changes, and other quadrupole strengths also have obvious changes. After the global compensation, the measured transverse tunes are [4.4442, ] against the theoretical values [4.4448, ], which are more precise than the measured ones [4.456, ] before the compensation. Besides, the beam life time is also increased from 3.2 hours to 7.1 hours. The wiggler has two working modes. One is that the gap

5 of the wiggler is tuned to the maximum of 15 mm, and the other is that the gap is tuned to the minimum of 32 mm. The lattices for both modes have been well compensated for and recorded in the control system, which can be chosen conveniently according to the user s need. Fig. 5. Changes of all the quadrupole strengths after the compensation The IVU in HLS-II storage ring is a hybrid permanent magnet IVU with deflection parameter in the range from.67 to 3.96 and gap in the range from 1 mm to 45 mm. The effect on the beam of the IVU is also estimated with the vertical tune increase of.1421 and the relative beta beating of as much as 115.1%. During the operation of HLS, the gap of the IVU is often adjusted in order to provide lights of different energies for the users. The effect of the IVU must be compensated for each time after its gap changes to reduce the distortion of the optics. In order to reduce the effect on other beam lines, the local compensation is adopted to minimize the effect caused by this IVU. Four quadrupoles nearby the IVU are chosen to compensate for the lattice, and their layout is shown in Fig. 6. Fig. 6. The magnet layout around the IVU. The designed strengths of the four quadrupoles QF1, QD1, QD2 and QF2 are , , and 2.773, respectively. With the IVU, the lattice is compensated with different gaps of the IVU. When the gap decreases, the vertical focusing strength of the IVU increases, which leads to the increase of the vertical tune. So in order to offset the extra vertical focusing, the strengths of QD1 and QD2 should decrease. To balance the horizontal focusing increase caused by the decrease of QD1 and QD2, the strengths of QF1 and QF2 should decrease at the same time to keep the horizontal tune fixed. The change of the quadrupole strengths after compensation under different gaps of the IVU is shown in Table 2, which agrees with our analysis. Note that the strengths of QD1 and QD2 are negative, so their positive change in Table 2 means decrease of their strengths.

6 Table 2: Changes of the four quadrupole strengths after compensation at different gap values. IVU gap(mm) QF1 (m -2 ) QD1 (m -2 ) QD2 (m -2 ) QF2 (m -2 ) The method of dynamic feed-forward compensation has been adopted to minimize the optics change caused by the change of the IVU gap. The compensation result mentioned above has been recorded in the EPICS IOC. When the gap changes, the linear interpolation algorithm is used to get the exact compensation values of QF1, QD1, QD2 and QF2. Then the strengths of the quadrupoles are set and the lattice is compensated dynamically. Fig. 7 indicates the shift of the transverse tune during the IVU gap changing with the dynamic feed-forward compensation method. The IVU gap decreases from 45mm to 1mm and then increases back to 45mm. It turns out that the horizontal tune shift is smaller than.3 and the vertical tune shift is smaller than.35. The result is benefic for the stable operation of HLS-II V x, V y IVU Gap(mm) v x v y Gap :44 19:47 19:51 19:54 19:57 2:1 2:4 time Fig. 7. Tune shift with the change of the IVU gap. 4 Closed orbit feedback The feedback and correction of the closed orbit distortion is very important for the stable operation of light sources. A slow feedback control system that has been studied in HLS [11] is employed and improved [12] in HLS-II storage ring for closed orbit feedback and correction. As the actuator of the orbit feedback, the quality of the power of corrector magnets directly decides the stability of the beam orbit. To improve the stability of the beam orbit of the upgrade storage ring, the first step is to make sure that the power of correctors is stable enough. The power of correctors is DVDD power, and the algorithm of the digital regulator is proportion integration differentiation (PID). The parameters K p and K I in the PID algorithm have been optimized to improve the stability of the power. The beam orbit before and after the optimization have been compared below in Fig. 8. The left figure shows the measured orbit before the optimization. The jitter scope of the orbit at the position of one BPM is about -2 μm to 2 μm and the standard deviation is about 6 μm. After the optimization, the jitter scope of the orbit showing in the right figure is only about -3μm to 3 μm and its standard deviation is about 1μm. It is obvious to see that the stability of the beam orbit

7 has been improved after the optimization of the corrector power..4 RNG:EPU:BPM:BPM2 std: RNG:EPU:BPM:BPM2 std: x(mm).1 x(mm) : 1:12 2:24 3:36 4:48 : 1:12 2:24 3:36 4:33 : 1:12 2:24 3:36 time time Fig. 8. Horizontal orbit at the position of a BPM. The ultimate performance of the closed orbit feedback and correction is to make the beam running stably on the target orbit, which is normally set on the centers of quadrupoles. Hence another key step is to measure the centers of all quadrupoles accurately using Beam-Based Alignment (BBA) technique. There are 32 BPMs in the HLS-II storage ring employed for BBA to accurately measure the center positions of these 32 quadrupoles, and the measured center position differences from the original beam orbit are shown in Fig. 9. Most of the measurement errors are smaller than 2 μm Hori. Quad Center [mm] Vert. Quad Center [mm] Quad Index Fig. 9. Differences between the center positions of all quadrupoles and original beam orbit. After optimizing the corrector power and measuring all the quadrupole center positions, the orbit feedback and correction can proceed with the beam orbit feedback system, which consists of beam orbit measurement system, orbit corrector and orbit feedback control system. Without the orbit feedback, the beam orbit shifts more and more seriously along with the time and the beam current. The orbit all around the storage ring is monitored in seven hours and the results are shown in Fig. 1. The figure shows that the horizontal orbit shift up to 25 μm and the vertical orbit shift up to 4 μm in 7 hours with different strength of current Quad Index Fig. 1: Beam orbit shift in 7 hours with the feedback off. The blue line shows the change of the beam current, and the other lines in different colors represent the orbit measured from different BPMs.

8 After the beam orbit is corrected with the feedback system, the orbit only shift about 5μm in the horizontal direction and 4μm in the vertical direction at the most in 7 hours, which is presented in Fig. 11. It turns out that the orbit feedback can greatly improve the stability of the beam orbit. And not only that, but the orbit feedback system can also keep the repeatability of the stable orbit. Fig. 11: Beam orbit shift in 7 hours with the feedback on. The Fig. 12 indicates the beam orbit monitored continuously in 1 days. The blue lines indicate the change of the beam current after each injection. We can see that the beam orbit in HLS-II storage ring can remain stable in any operation period with the orbit feedback system, and the rms jitter of the beam orbit is smaller than 4 μm in a long term. Fig. 12: Beam orbit measured continuously in 1 days with the feedback on. 5 Conclusion After the lattice calibration of the HLS-II storage ring,the vertical beta beating is reduced from 97.6% to 6.8%, the transverse tune differences are decreased to.1% in both directions and the beam life time is increased from 1.46 hours to 7.5 hours. After further compensated for the lattice of the insertion devices effect, the maximum vertical beta beating is reduced to only 1.5%, the horizontal tune shift is smaller than.3 and the vertical tune shift is smaller than.35, while the beam life time is almost unchanged. The linear optics of the HLS-II storage ring has met the design value and the performances have achieved the expectation of the upgrade project. Furthermore the stability of the closed orbit is improved greatly with the closed orbit feedback system. The closed orbits in both directions only distort about 4 μm and the rms jitter of the beam orbit is less than 4 μm in a long term. Further lattice calibration and compensation are in progress with all of the insertion devices to make sure the best operation performance of the HLS-II storage ring. The whole commissioning of the storage ring with various subsystems,including transverse feedback system, closed orbit feedback system, beam diagnostic system and etc., will be completed in next work.

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