分流环型绝热过渡区双绕螺旋线Wiggler束流传输实验

THE DESIGN WITH THE STAGGERED LOOPS AT THE ENTRANCE REGION OF BIFILAR HELICAL WIGGLER AND ITS e BEAM TRANSPORT EXPERIMENT

束流传输实验结果表明，分流环型过渡区双绕螺旋线圆极化Ｗｉｇｇｌｅｒ的端口区磁场，具有良好对称性，能基本保证磁场强度由零绝热过渡到均匀区，没有磁场畸变，因而对电子束流的能散度影响小，端口束流损失少。当入口端γ＝２的５００Ａ束流时，有约３５０Ａ束流传输到均匀区，而在相同条件下，喇叭口型绝热过渡区Ｗｉｇｇｌｅｒ只有１２０Ａ的束流传输到３４ｃｍ处的均匀区。分流环型变磁场技术可用于双绕螺旋线Ｗｉｇｇｌｅｒ变参自由电子激光。

AbstractThe problem in free electron laser experiment with a bifilar helical wiggler is that sharp char-ges of magnetic field amplitude at the wiggler entrance can easily increase the beam perpendiculartemperature, and a poor quality, high temperature beam substantially reduce the gain of FEL.However,e beam can adiabatically transport from the entrance to the interaction region of thewiggler,if the wiggler entrance region is designed to adiabatic profile with staggered loops,illus-trated in Fig.2.We know,the strength of an infinite wiggler can be describe by formula(1).The staggered termination wiggler equvialent circuit shows as Fig.3.Carefully adjusting theimpedance of staggered loops,the current conducting through the wiggler winging wire at the en-trance region can contribute to form the pattern of the field profile which FEL experiment need. Take out any periodic unit from the equivalent circuit, shown in Fig.4,assuming the totalimpedance in front of the unit is R , thus obtain fromulae(2 )and(3). If we have known the current profile conducting the winging wire at the wiggler entrance re-gion, the impedanee ofdifference loop can be obtained by solving eqs(2) and eqs(3). Supposingthe current profile at the entrance region is I_1,I_2,…,I_i,I_(i+1),…,I_N, the magnetic field of helicalwiggler in entrance region can be described by equations(4 ) and(5). The adiabatic variation of the magnetic field at the wiggler entrance region can be describedby formula(6).From equ.(1) and equ.(5),we can obtain expression of the current profile atthe entrance region with equ.(7).After average over halfwiggler period, the step current in eachλ_w/2 unite of the winging wire at the entrance region is equation(8). So we can evaluate out the impedance value of each loop corresponding to the current profileby use of above mentioned formula. For example,a bifilar helical wiggler With parameters=3.45cm and N=7(N is periodic number of the entrance region),the necessary impedance foreach loop lists in table 1.We have performed numerical simulation and experiment measurement ofthe field at the entrance region designed according to parameters shown in table 1,the field pro-file of numerical simulation and experiment measurement at the entranee region show in Fig.5 andFig.6.From Fig.6 and Fig.7,we can obtain that the experiment measurement results show agood agreement with numerical simulation,and provide a verification of rationality for thedesign technique. The layout of experiment apparatus for e beam transport is delayed in Fig.7,and its con-trol diagram of the experiment is shown in Fig.8,the results of e-beam transporting experi-ment,shown in Fig.9,reveal that 350A e-beam current from 500A at entrance is transported tointeraction action region,however in same experiment conditions,only about 120A e-beamcurrent that is clearly shown in Fig.1 has been observated at the interaction region of helicalwiggler with flareing tapper entrance.