Study On Magnet Array Cooling Technology Of The Cryogenic Permanent Magnet Undulator

Cryogenic Permanent Magnet Undulator(hereinafter referred to as CPMU)whose magnets work at 50 K to 150 K has magnetic performance 30% to 50% higher than that of a conventional In-vacuum Undulator(hereinafter referred to as IVU).Due to the magnetic characteristics that the remanence and intrinsic coercivity of Nd Fe B and Pr Fe B will increase substantially with the working temperature decreasing,CPMU can gain higher peak field within shorter magnetic period under a given magnetic gap.This is of great significance in improving the brightness of the SR light.At low temperature,CPMU's magnetic field performance,including magnetic field distribution,integral field and phase error,is under the influence of its cryogenic performance.The cooling technology adopted at LN2 temperature is one of the key technologies.Moreover,the way in which CPMU's cooling performance affects its magnetic field performance is one of the key issues.The paper will research the followings:1)Design on overall cooling scheme of CPMU.The scheme draws a conclusion that CPMU prototype can cool the inside girders with a subcooled LN2 circulation system and thermal spacers and use the magnet clamps to cool the magnets through thermal conduction after the design requirements for CPMU prototype,the magnetic properties and structures of magnets,and the working features and requirements for CPMU at all stages of development(magnetic field measurement and correction should be completed before working on-line)are taken into consideration.2)Research on CPMU heatload.Firstly,analyze the requirements set out in the cooling scheme in case of working on-line and magnetic field measurement off-line,analyze comprehensively the sources of heatload under different working conditions,research the key factors affecting the elements of heatload by building a theoretical model,and determine the appropriate working condition for the design;secondly,use the model to analyze the heatload of CPMU prototype so as to minimize heat leakage,optimize the design for structures,perform experimental testing,modify the theoretical analytical model,search a new way for heatload measurement,and demonstrate through experiment that the heatload analysis conducted with thermal spacer heat conduction method is superior to that with LN2 heat transfer method;last but not least,apply the theoretical analytical method and experimental measurement method concerning CPMU heatload verified and corrected on the basis of CPMU prototype to analyze the static heatload of CPMU prototype and to conduct the experiment in case of magnetic field measurement off-line.For CPMU prototype,its measurement off-line is carried out at 120K-170 K and the consumption of cooling power reaches 200W-418 W,both of which meeting the requirements for temperature adjustment in the magnetic field measurement off-line.Besides,the CPMU prototype completes successful the test on-line and detects a maximum of 220 W dynamic heatload,which basically conforms to the estimated value(i.e.200W).3)Research on CPMU magnet circulation system.Firstly,analyze qualitatively the layout of CPMU magnet circulation system,apply finite element method to simulate and analyze various schemes,consider the temperature evenness of CPMU magnets,the flow resistance of subcooled LN2 and the distribution of LN2,and cool the upper and lower girders respectively with two circulation systems connected in parallel;secondly,build a theoretical analytical model concerning heat convection for the circulation systems of CPMU magnets,and determine that the inside diameter of cooling pipe should be 8mm taking LN2 flow state,flow resistance and degree of under cooling of coolant into account;thirdly,according to the analytical result regarding CPMU heatload,analyze the convection parameters for subcooled LN2 in circulation system,study the change law for such parameters as flow resistance,convection heat transfer factor and convection heat resistance,and determine that the convection heat resistance of CPMU cooling structure represents only 1.3% of total resistance,which,therefore,can be ignored.Last,adopt CPMU prototype to experiment and research the circulation system of CPMU magnets.The experiment shows that when the layout is applied to the circulation system,the temperature difference between inlet and outlet of circulation system will slightly affect the temperature evenness of girders.4)Research on cooling structure and thermoregulation scheme of CPMU magnets.Firstly,install different thermal spacers,the innovative cooling structures,to the ends of girder and near the stems according to the analytical result of theory of CPMU heatload to analyze and the distribution features of heat source;secondly,apply finite element analysis software to optimize,simulate and verify the design scheme of thermal spacer;finally,experiment and research the change on temperature distribution and heatload of CPMU prototype in the off-design conditions.In the process of magnetic field measurement off-line,the maximum temperature difference between girders is always lower than 2K,which serves as a powerful guarantee that phase error of magnetic field can meet the design requirements.5)Research on thermal contraction of CPMU magnets.Firstly,conduct theoretical analysis of thermal contraction for CPMU magnetic gap,and build a model;secondly,use the magnetic gap measuring device integrated into magnetic field measurement platform to measure the gap when CPMU is at low temperature,and demonstrate that the analytical result of magnetic gap thermal contraction theory is accurate.Moreover,in case of the phenomenon that the phase error of magnetic field of CPMU prototype varies with the change of temperature,apply finite element analysis software to do simulation research regarding the distribution of thermal contraction of CPMU prototype at low temperature,and analyze the influence of thermal contraction of CPMU magnetic gap on the distribution of peak field.Take account of magnetic field measurement result of CPMU prototype at low temperature,make an analysis,and determine that the variation of magnetic gap arising from thermal contraction of inside girders is one of the key factors affecting the phase error of magnetic field when CPMU is at low temperature,and make clear the improvement direction for CPMU prototype;finally,correct the local magnetic field of CPMU prototype at room temperature in advance to compensate the varying thermal contraction for magnetic gap at low temperature,so that the phase error of CPMU prototype is lower than 3 degrees,at room temperature or at low temperature,reaching the international standard.