Study On The Vertical Emittance For The Ssrf Storage Ring

The spectral brightness is an important parameter for the synchrotron radiation light source. The storage ring light source with high brightness can guarantee a high radiation energy density when it is used to irradiate the sample. High brightness synchrotron light can get ideal results with high signal to noise ratio in a very short period of time. Therefore, users care more about the brightness of the storage ring light source. The brightness is the photon flux in the unit area per unit volume, which is proportional to the beam current, inverse proportional to the horizontal emittance and vertical emittance. The beam current is limited by collective effects and the horizontal emittance is decided by the lattice structure, which has been almost fixed in a machine. Reducing the vertical emittance to improve the brightness is a simple and feasible method. Other benefits such as the increase of transverse coherence and the reduction of vertical spot size can be obtained in reducing the vertical emittance.This paper outlines some basic theories of the vertical emittance at first. The contribution to vertical emittance is from three parts: the quantum limit, vertical dispersion and betatron coupling. The first driving contribution to vertical emittance, which is very small and intrinsic to the lattice design, is due to the opening angle of the synchrotron radiation. It is often less than 1 pm rad. Vertical emittance in a storage ring is primarily determined by two effects: spurious vertical dispersion and betatron coupling. The dominant causes of spurious vertical dispersion and betatron coupling are magnetic alignment errors. Spurious vertical dispersion is caused by vertical bending errors from tilts of dipole, tilts of quadrupoles, vertical alignment errors on the quadrupoles and vertical alignment errors of the sextupoles. Betatron coupling is due to quadrupole tilt errors and vertical misalignments of the sextupoles.In this paper, the vertical emittance in the currently operational Shanghai Synchrotron Radiation Facility(SSRF) storage ring is corrected using LOCO(Linear Optics from Closed Orbits). The vertical emittance can be corrected from 12 pm rad to 5 pm rad for the SSRF storage ring. The effect of correcting the vertical emittance by linear fitting and full fitting is compared when using LOCO to compute the model response matrix.This paper also studies about the vertical emittance in the SSRF Phase II beamline project(SSRF Phase II). The sensitivity of closed orbit distortion(COD) and vertical emittance to magnetic alignment errors in the SSRF Phase II is analyzed. If the alignment tolerances of all magnets in the SSRF Phase II project are the same as those in the currently operational SSRF storage ring, the misalignment on the quadrupole magnets has a profound effect on the vertical COD, and the vertical emittance is primarily determined by vertical misalignments of the sextupole magnets. Singular values decomposition(SVD) is used to correct the vertical COD. Skew quadrupole magnets are utilized to correct the vertical emittance. With 60 skew quadrupole magnets and gradient descent algorithm, we can correct the emittance coupling and keep it below 0.5% for the magnetic alignment errors.