New chromaticity compensation approach and dynamic aperture increase in the SSRF storage ring

Strong chromatic sextupoles used to compensate natural chromaticities in the third generation light source storage ring usually reduce dynamic aperture drastically. Many optimization methods can be used to find solutions that provide large dynamic apertures. This paper discusses a new optimization approach of sextupole strengths with step-by-step procedure, which is applied in the SSRF storage ring, and a better solution is obtained. Investigating driving terms generated by the sextupoles in every step can analyze their convergences and guide the weight setting among different terms in object function of the single resonance approach based on the perturbation theory.

Beam-beam effects and mitigation in a future proton-proton collider

The beam-beam effects in a hadron collider with an unprecedented energy scale were studied.These effects are strongly related to the attainable luminosity of the collider.Long-range interactions were identified as the major factor limiting the dynamic aperture,which is strongly dependent on the crossing angle,β*,and bunch population.Different mitigation methods of the beam-beam effects were addressed,with a focus on the compensation of long-range interactions by electric curren wires.The CEPC-SPPC project is a two-stage large circular collider,with a first-stage circular electron-positron collider(CEPC)and a second-stage super proton-proton collider(SPPC).The design of the SPPC aims to achieve a center-of-mass energy of 75 TeV and peak luminosity of approximately 1×10^(35) cm^(-2)s^(-1).We studied the beam-beam effects in the SPPC and tested the effectiveness of the mitigation methods.We found that with compensation using electric current wires,the dynamic aperture is at an acceptable level.Moreover,considering the significant emittance damping in this future proton-proton collider the beam-beam effects and compensation are more complicated and are studied using long-term tracking.It was found that with a smaller emittance,the head-on interactions with a crossing angle become more prominent in reducing the beam stability,and combined head-on and long-range compensation is needed to improve the beam quality.When the reduction in population owing to burnoff was included,it was found that the coupling between the transverse and longitudinal planes at smaller emittance is the main driving source of the instabilities.Thus,crab cavities and emittance control are also necessary than just the compensation of the long-range interactions to improve the beam stability.This study serves as an example for studying the beam-beam effects in future proton-proton colliders.

International Linear Collider damping ring lattice design with two wiggler sections

The International Linear Collider （ILC）, which is based on super-conducting RF acceleration technology, requires the damping rings to provide beams with extremely small equilibrium emittance, and large acceptance to ensure good injection efficiency for high-emittance, and high-energy spread beam from the positron source. In order to reduce the cost for ILC damping rings, an alternative lattice which is different from the baseline configuration design has been designed with modified FODO arc cells, and the total quadrupole number has been reduced by half. At the same time, to decrease the total cost involved in constructing access shafts needed to supply power, cryogenics etc. for the wigglers and other systems, the number of wiggler sections is decreased from 8 to 4, and further to 2. This new lattice has been optimized to have a good dynamic aperture. This alternative ILC damping ring lattice design will reduce the cost largely as compared with the baseline design.

Lattice optimization for the HLS-Ⅱ storage ring

The upgrade project of the Hefei Light Source （HLS）, named HLS-Ⅱ, is under way, whose storage ring will be reconstructed. The HLS-Ⅱ storage ring has lower emittance and more straight sections available for insertion devices compared with the present HLS storage ring. The scan method is applied to the linear lattice optimization for the HLS-Ⅱ storage ring to get thorough information about the lattice. To reduce the computation amount, several scans with different grid spacing values are conducted. And, the calculation of the chromatic sextupole strength for the achromatic mode is included in the scan, which is useful for nonlinear lattice optimization. To better analyze the obtained solutions in the scan, the lattice properties and the variables of quadrupole strengths are statistically analyzed. And, the process of selecting solutions is described in detail, including the choice of the working point, the settings for the emittance and optical functions, and the restriction of maximum magnet strength. Two obtained lattices, one for the achromatic mode and the other for the non-achromatic mode, are presented, including their optical functions and optimized dynamic apertures.

Physics issues in diffraction limited storage ring design

Diffraction limited electron storage ring is considered a promising candidate for future light sources,whose main characteristics are higher brilliance,better transverse coherence and better stability.The challenge of diffraction limited storage ring design is how to achieve the ultra low beam emittance with acceptable nonlinear performance.Effective linear and nonlinear parameter optimization methods based on Artificial Intelligence were developed for the storage ring physical design.As an example of application,partial physical design of HALS(Hefei Advanced Light Source),which is a diffraction limited VUV and soft X-ray light source,was introduced.Severe emittance growth due to the Intra Beam Scattering effect,which is the main obstacle to achieve ultra low emittance,was estimated quantitatively and possible cures were discussed.It is inspiring that better performance of diffraction limited storage ring can be achieved in principle with careful parameter optimization.

Nonlinear optimization of the modern synchrotron radiation storage ring based on frequency map analysis

In this paper, we present a rule to improve the nonlinear solution with frequency map analysis （FMA）, and without frequently revisiting the optimization algorithm. Two aspects of FMA are emphasized. The first one is the tune shift with amplitude, which can be used to improve the solution of harmonic sextupoles, and thus obtain a large dynamic aperture. The second one is the tune diffusion rate, which can be used to select a quiet tune. Application of these ideas is carried out in the storage ring of the Shanghai Synchrotron Radiation Facility （SSRF）, and the detailed processes, as well as better solutions, are presented in this paper. Discussions about the nonlinear behaviors of off-momentum particles are also presented.

Optimal placement of magnets in Indus-2 storage ring

In Indus-2, by optimizing the position of the magnetic elements, using the simulated annealing algorithm, at different locations in the ring with their field errors, the effects on beam parameters have been minimized. Closed orbit distortion and beta beat are considerably reduced by optimizing the dipole and quadrupole magnets positions in the ring. For the Indus-2 storage ring, sextupole optimization gives insignificant improvement in dynamic aperture with chromaticity-correcting sextupoles. The magnets have been placed in the ring with the optimized sequence and storage of the beam has been achieved at injection energy without energizing any corrector magnets. Magnet sorting has led to the easy beam current accumulation and the measurement of parameters such as closed orbit distortion, beta function, dispersion, dynamic aperture etc.

Conceptual design of Hefei advanced light source

The conceptual of Hefei Advanced Light Source, which is an advanced VUV and Soft X-ray source, was developed at NSRL of USTC. According to the synchrotron radiation user requirements and the trends of SR source development, some accelerator-based schemes were considered and compared; furthermore storage ring with ultra low emittance was adopted as the baseline scheme of HALS. To achieve ultra low emittance, some focusing structures were studied and optimized in the lattice design. Compromising of emittance, onmomentum and off-momentum dynamic aperture and ring scale, five bend acromat (FBA) was employed. In the preliminary design of HALS, the emittance was reduced to sub nm·rad, thus the radiation up to water window has full lateral coherence. The brilliance of undulator radiation covering several eVs to keVs range is higher than that of HLS by several orders. The HALS should be one of the most advanced synchrotron radiation light sources in the world.

Optimization of magnet sorting in a storage ring using genetic algorithms

In this paper, the genetic algorithms are applied to the optimization problem of magnet sorting in an electron storage ring, according to which the objectives are set so that the closed orbit distortion and beta beating can be minimized and the dynamic aperture maximized. The sorting of dipole, quadrupole and sextupole magnets is optimized while the optimization results show the power of the application of genetic algorithms in magnet sorting.

Improved step-by-step chromaticity compensation method for chromatic sextupole optimization

The step-by-step chromaticity compensation method for chromatic sextupole optimization and dynamic aperture increase was proposed by E. Levichev and P. Piminov （E. Levichev and P. Piminov, 2006）. Although this method can be used to enlarge the dynamic aperture of a storage ring, it has some drawbacks. In this paper, we combined this method with evolutionary computation algorithms, and proposed an improved version of this method. In the improved method, the drawbacks are avoided, and thus better optimization results can be obtained.

SHER-HIAF ring lattice design

The Super Heavy Experimental Ring （SHER）, which is one of the rings of the next accelerator complex High Intensity Heavy Ion Accelerator Facility （HIAF） at IMP, has to be optimized for e-cooling. Its lattice is designed for two modes： the first is the isochronous mode, which is a time-of-flight mass spectrometer for short-lived secondary nuclei, the second is the storage ring mode, which is used for collecting and cooling the secondary rare isotope beams from the transport line. In order to fulfil its purpose, the ion optics can be set to different ion optical modes.