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In a previous paper (FEL05), we presented results showing that the Earth's field might give a significant effect on the LCLS undulator performance due to a large concentration of the field by the undulator poles. Based on the result of model calculation, we decided to shield the Earth's field by surrounding the undulator backing structure with a 1-mm-thick mu-metal sheet. First, the effect of the shield was modeled using the code RADIA. According to the calculation, the shielding factor of a C-shape mu-metal shield was better than a factor of 8. Second, we measured the Earth's field shielding effect without an undulator. In our measurement laboratory, the vertical component of the Earth's field was 0.5 gauss. It was suppressed to smaller than 0.1 gauss with the shield. After these background measurements, we examined the effect of the shield with an undulator in place. The measurement results show very good agreement with the model calculation.
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