The magnetic field gradient measured by sweep-fluorescence method is about 40% larger than theoretical calculation, which is beyond the scope of experimental error. In order to clarify the reasons for this difference, the following series of experiments were conducted. First, the coils were removed, placed on the ground, surrounded by non-magnetic material, and after fixing them according to the spacing between the two coils in the experiment, The pull meter measures its magnetic field distribution and is almost the same as the calculation. Initially, we suspected that the stainless steel forming the chamber of the cell may not be completely non-magnetic, so the magnetic permeability of the flange made of the same material was measured and its magnetic permeability was found to be 101001. Obviously, it was not caused by experiments. Theoretical calculations are far from the reasons. Finally, the following experiment was done. Remove the mirror, under the action of the circularly polarized traveling wave field, as previously mentioned, only a dark line at the zero point of the magnetic field can be seen. Two parallel thin rays are incident from the direction perpendicular to the beam for calibration. Then place a permanent magnet on the outside of the cavity coaxially with the coil. Move the magnet to the vicinity of the quartz window. Obviously, the dark line can be clearly moved. It can be moved to coincide with another calibration ray. The distance to move is equal to the distance between the two calibrating rays. . Record the position of the permanent magnet at this time. If you remove the magnet and measure the strength of the magnetic field at the same distance on the ground using the millet tilometer (the magnetic background is deducted), the difference in magnetic field strength between the two calibrating rays can be obtained. Divided by the distance, it is both a magnetic field gradient.
Using this method, when the coil current is 2A, the magnetic field gradient is 10G/cm, which is obviously smaller than the results obtained by sweeping fluorescence method and the theoretical calculation results. After careful analysis, it was found that considering the non-linear magnetization of the sealing material between the stainless steel cylinder and the quartz glass, the above seemingly contradictory results can be explained. First, the theoretical calculation assumes a magnetic permeability of 1, which is reasonable for coils, coil bobbins (aluminum materials) and non-magnetic stainless steel cavities. However, the permeability of the sealing material is not 1 and its magnetization is not linear. It is not surprising that the actually generated magnetic field is stronger than the calculated value after the coil is turned on. At the same time, when we note the non-linear magnetization of the sealing material, the gradient of the magnetic field measured by moving the dark line with an applied magnetic field is also smaller than the theoretically calculated value. The position of the dark line O is the position of the magnetic zero. Assume that the dark line moves from O to Oc after the applied magnetic field, and moves $L. The entire external magnetic field caused by the permanent magnet and the magnetic medium is $B at Oc, then the original four-pole magnetic field The gradient is $B/$L, where $B=$B0 (1 L). Where $B0 is the strength of the permanent magnet itself at point Oc and L is the susceptibility. When we took the permanent magnet off, it was actually measured at the same distance as $B0$B0$LL1, making $B0=$B1 L
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