Non-interceptive beam energy spread measurement with a 4-stripline BPM
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摘要: 能散度是反映束流品质的重要参数，采用传统测量方法测量时打到靶上的束流不能得到利用，测得的数据也是多个宏脉冲的平均值，利用该方法的测量结果进行束流调节时，要等待荧光靶从束流轨道中反复插入和提出，调节时间很长。基于单个四条带束流位置检测器，结合发射度测量的方法，实现了非拦截式能散度测量方法。该方法能在不加入额外设备的情况下对每个束流宏脉冲的能散度进行测量，测量结果与传统的拦截式测量方法结果相吻合。进行了误差分析，指出为了减小测量系统误差，需要尽可能使束流通过束流位置检测器中心。Abstract: In linear accelerator free-electron laser facilities, energy spread is an important parameter reflecting the quality of the beam. The traditional measurement method, which uses a bending magnet, fluorescent screens and cameras, is an interceptive measurement method. The beam hitting the screen cannot be utilized during the measurement, and the measured data is the average of multiple macro pulses. Moreover, when using the measurement results of this method for beam tuning, the commissioning may take a long time since the fluorescent screens need to be repeatedly inserted and raised from the beam trajectory, thus new non-interceptive measurement methods need to be developed. This paper implements such a method using a single 4-stripline beam position monitor in conjunction with the emittance measurement method. This method is able to measure the energy spread of each macro beam pulse without inserting extra equipment. The measurement results are in agreement with those of the conventional interceptive measurement method. It is indicated by the error analysis that in order to reduce the measurement systematic error, it is necessary to make the beam pass through the center of the beam position monitor.
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图 3 x方向（左）和y方向（右）使用四极铁扫描法发射度测量的结果
Figure 3. Emittance measurement with quadrupole variation in horizontal and vertical direction
表 1 不同束流参数的发射度测量结果
Table 1. Results of emittance measurements
No. beam energy/MeV βx/(mm·mrad−1) βy/(mm·mrad−1) $ { \epsilon}_{x} $/(mm·mrad) $ { \epsilon}_{y} $/(mm·mrad) 1 9.9 2.95 0.05 1.80 0.87 2 10.1 2.50 0.04 1.67 1.10 
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表 2 不同束流参数下的能散测量结果
Table 2. Results of energy spread measurements with different beam parameters
No. beam energy/MeV Qr/mm2 interceptive measurement
δ/10−3BPM measurement
δ/10−31 9.9 5.27 4.85±0.20 4.87±1.27 2 10.1 4.09 6.61±0.25 6.70±0.81
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