C50 Cryomodule Magnetic Shielding Design and Analysis Gary G. Cheng and Edward F. Daly

Transcription

1 JLAB-TN C50 Cryomodule Magnetic Shielding Design and Analysis C50 Cryomodule Magnetic Shielding Design and Analysis Gary G. Cheng and Edward F. Daly Introduction The C50 cryomodule testing reveals that the quality factor, Q o, is low. It is strongly suspected that the original magnetic shielding is insufficient. Magnetic field survey is then conducted at room temperature by probing into the beam pipe. It is found that the magnetic flux densities in all directions are noticeably high, especially the axial (or longitudinal) component. This confirms the poor shielding performance of the exiting shields and a re-design of the magnetic shielding is necessary. This technical report is a summary of the design and analysis efforts made toward improving the shielding. The analysis methodologies are similar to those applied to C100 cryomodule magnetic shield analysis [1]. A new design featured with 0.04" Amumetal outer shield and 0.02" Cryoperm inner shield is suggested to replacing the existing shields. Design Considerations The primary factors that are addressed in the new shield design include: 1. Axial shielding: the existing double-cylinder magnetic shield design is found to be weak in attenuating the axial component of earth magnetic field. Field measurement and analysis show that the axial shielding factor can be as low as 1.6 (also refer to the summary table in next section). Transverse shielding factor is estimated to be much higher, say 30, for the existing shields. Therefore, the new design will focus on axial shielding. 2. Permeability: almost all high permeability shielding materials experience a significant reduction in permeability when temperature drops to cryogenic temperature [1], which is 4.2K for liquid helium cooled SRF cavity application. Amuneal s special shielding material, Cryoperm, is well-accepted as a high permeability alloy at cryogenic temperatures. The new design proposes using of Cryoperm for the inner shield. 3. Saturation Prevention: if some regions of the shield become saturated due to locally high magnetic field, the shielding performance will deteriorate dramatically. With magnetic sources and available material permeability being relatively constant, increasing shield thickness becomes the most effective way to avoid saturation. Sharp corners, openings, dentations, and so on, are all incentives of saturation. Efforts shall be made to minimize the number and areas of saturation prone zones, which can be judged from magnetic flux density plots from analysis. In general, the outer shield has a larger chance to be saturated than the inner shield. Saturation limit for Amumetal is around 8,000 Gauss and for Cryoperm, it is 9,000 Gauss. Axial Shielding Factor Estimation The axial shielding factor is calculated by setting up 2-D axisymmetric finite element models in ANSYS [1]. The following describes some common features of all 2-D models used: 1/1

2 JLAB-TN C50 Cryomodule Magnetic Shielding Design and Analysis Earth s magnetic field axial component is assumed to be 0.5 Gauss. Plane symmetry is utilized to reduce the 2-D axisymmetric model to be symmetric about the center plane perpendicular to beam axis. Surrounding air is 20 times longer than one half of the outer shield length and 15 times thicker than the outer diameter of the outer shield. The thicknesses of both inner and outer shields are discretized into 4 finite elements. Table 1 summarizes the axial shielding factors for 9 designs. Note that non-convergence of nonlinear analysis implies saturation. Explanations about the designs are given as follows: Design #1 is the baseline design that reflects the existing shield configuration. Magnetic field survey is conducted for the baseline design and measured axial component of leaked field is compared to linear analysis of design #1, see Fig.1. The low shielding factor is suspected to be the consequence of saturated outer shield with a thickness of t 2 = 0.01". Design #2 is based on the design #1. Nonlinear analysis using B-H curve of Amumetal is performed. It is found that when outer shield thickness of t 2 is 0.025" or greater, the nonlinear analysis of this type of double-cylinder shield design starts to converge and the outer shield is not saturated in a 2-D model. Design #3~#6, #8 are based on the same concept (the proposed new magnetic shielding configuration): thicken the outer Amumetal shield and replace the single long-cylinder inner shield with four 0.02" Cryoperm shield modules. From #3 to #6, the outer shield thickness is varied to investigate the influence on axial shielding factor. The targeted S a is 50. Note that the design #8 used linear analysis. Design #6 is the proposed new design. Design #7 and #9 share the same concept: add eight 0.02" Cryoperm shield modules inside the helium vessels. This concept is close to the magnetic shielding design for the C100 cryomodule [1]. However, it is apparently more costly to do. Transverse Shielding Factor Transverse shielding factor for design #1 is analyzed by a 3-D finite element model and formulas given in Amuneal s catalog. According to interactions with vendor [2], the nominal permeability of 60,000 for Amumetal could be discounted by as much as 75% in practical design. Therefore, μ 300K = 15,000 ~ 60,000. Also, there are scenarios that the shields are in room temperature for field measurements or inner shield cooled down in normal operation. At 4K, Amumetal s permeability is μ 4K = 3,775 [1]. Combinations of design parameters are studied and calculated transverse shielding factors are listed in Table 2. The low peak field validates linear analysis using constant µ s. The transverse shielding factor for the proposed new design is expected to be high. Since the proposed new design resembles C100 magnetic shielding design in a great extent, please refer to results in JLAB-TN for information. 2/2

3 No. Design description * t 1 =0.014" Amumetal RT inner shield, t 2 = 0.010" Amumetal RT outer shield t 1 =0.014" Amumetal RT inner shield, t 2 = 0.025" Amumetal RT outer shield t 1 =0.02" Cryoperm capped inner shields, t 2 = 0.010" Amumetal RT outer shield t 1 = 0.02" Cryoperm capped inner shields, t 2 = 0.020" Amumetal RT outer shield t 1 = 0.02" Cryoperm capped inner shields, t 2 = 0.025" Amumetal RT outer shield t 1 = 0.02" Cryoperm capped inner shields, t 2 = 0.040" Amumetal RT outer shield t 1 = 0.014" Amumetal cryogenic middle shield, t 2 = 0.010" Amumetal RT outer shield, t 3 = 0.02" Cryoperm inner shield t 1 = 0.02" Cryoperm capped inner shields, t 2 = 0.010" Amumetal RT outer shield t 1 = 0.014" Amumetal cryogenic middle shield, t 2 = 0.010" Amumetal RT outer shield, t 3 = 0.02" Cryoperm inner shield Table 1: Summary of axial shielding factors for various designs Axial Shielding factor B H or Peak field on cavity surface, Sa Linear ** (Gauss) 1.6 Linear 4, B H 6,481 Memo (1) Nonlinear analysis cannot converge. (2) See Fig. 2 for shields layout. (1) t to avoid nonconvergence. (2) See Fig. 2 for shield layout & field B H 7,131 See Fig. 3 for shield layout B H 7,131 See Fig. 3 for shield layout B H 7,131 See Fig. 3 for shield layout B H 5, B H 7,131 See Fig. 3 for shield layout & field See Fig. 4 for shield layout & field Linear 7,406 See Fig. 3 for shield layout Linear 6,715 See Fig. 4 for shield layout * B-H means nonlinear analysis with nonlinear B-H curves. Refer to shield layout figures and definitions of thickness varaibles for design details. ** From the B-H curves, for Amumetal, when the peak magnetic flux density B > 4,000 Gauss, linear analysis with constant μ s is questionable. 3/3

4 Fig. 1 Calculated and measured magnetic flux density axial component 4/4

5 t 1 = 0.014", Amumetal inner shield t 2 = 0.025", Amumetal outer shield Fig. 2 Magnetic flux density (Tesla) in outer & inner shields with design #2 conditions, shields layout is good for both design #1 and #2 5/5

6 t 1 = 0.02", Cryoperm inner shield t 2 = 0.04", Amumetal outer shield Fig. 3 Magnetic flux density (Tesla) in outer & inner shields with design #6 conditions, shields layout is good for shields layout is good for designs #3, #4, #5, #6 & #8 6/6

7 t 2 = 0.01", Amumetal outer shield t 3 = 0.02", Cryoperm inner shield t 1 = 0.014", Amumetal middle shield Fig. 4 Magnetic flux density (Tesla) in outer & inner shields with design #7 conditions, shields layout is good for shields layout is good for both design #7 and #9 7/7

8 JLAB-TN C50 Cryomodule Magnetic Shielding Design and Analysis Table 2: Transverse shielding factor for double-cylinder designs (Inner shield thickness t 1 = 0.014", 0.5 Gauss external magnetic field) μ=15,000 μ 300K =15,000 µ4k=3,775 μ 300K =60,000 µ4k=3,775 Outer shield thickness t 2 =0.01" t 2 =0.02" t 2 =0.01" t 2 =0.02" t 2 =0.01" By formula D FEA Peak field (FEA), Gauss 1,951 1,096 2,005 1,114 2,357 REFERENCES [1]. G. Cheng, E. F. Daly, and W. R. Hicks, C100 Cryomodule Magnetic Shielding Finite Element Analysis, JLAB-TN [2]. Per communications with L. Maltin, Amuneal Manufacturing Corp., Philadelphia, PA. 8/8