分类： 物理学 >> 核物理学 提交时间： 2023-06-16
摘要： Abstract Main quadrupole magnets are critical for the Circular Electron and Positron Collider (CEPC), and are specifically designed as dual aperture quadrupole (DAQ) magnets. However, the field crosstalk between the two apertures presents challenges. As the CEPC will work at four beam energies of Z, W, Higgs and ttbar mode, the DAQ magnets will operate at four field gradients spanning from 3.18 to 12.63 T/m. The first short quadrupole magnet prototype with the bore diameter of 76 mm and magnetic length of 1.0 m revealed the problems of large magnetic field harmonics and a magnetic center shift within the beam energy range. Accordingly, a compensation method was proposed in this work to solve the field crosstalk effect. By adjusting the gap height at the middle of the two apertures, the field harmonics and magnetic center shift are significantly reduced. After optimization, the short prototype was modified using a new scheme. The field simulations are validated from the magnetic measurement results. Further, the multipole field meets the requirements at the four beam energies. The detailed magnetic field optimization, field harmonics adjustment, and measurement results are presented herein.
分类： 地球科学 >> 地球科学史 提交时间： 2018-09-18 合作期刊: 《干旱区科学》
摘要： Reduced tillage provides ecological and economic benefits to arable land on the Loess Plateau of China, where soil erosion has long been a serious problem and soil water availability is largely restricted. However, high abundances of weeds in reduced tillage systems cause significant yield losses. In this study, we explored the effects of no-tillage and stubble retention on the number and density of weeds and weed seeds in a 12-year maize-winter wheat-common vetch rotation on the Loess Plateau. Four treatments including conventional tillage, no-tillage, conventional tillage+stubble retention and no-tillage+stubble retention were designed and applied. We found that no-tillage increased the number of weed species and weed density in most of the crops, while stubble retention decreased weed density in maize and tended to suppress weeds in both no-tillage treatments (no-tillage and no-tillage+stubble retention). No-tillage led to an increase in the number of weed species in the weed seedbank and tended to increase seed density during the spring growth of winter wheat, but it decreased seed density during post-vetch fallow. Stubble retention tended to reduce seed density during the spring growth of winter wheat and post-vetch fallow. We concluded that no-tillage can promote weeds in the experimental crop rotation, while stubble retention suppresses weeds in untilled fields. The combined effects of stubble retention and no-tillage on weed suppression varied among the three crops. Based on these results, we recommend stubble retention in untilled legume-crop rotations on the Loess Plateau to improve the control of weeds.
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