Magnetic shielding of an accelerator beam using passive ferromagnetic material

Abstract

A particle beam can be shielded from extcriial inagiletic fields by ferromagnetic shields, up to a ccrtaiii field strength depending 011 the saturation cliaracteristics of the. used shielding material. The liiriiftb of pabsive shielding, in terms of external flux denisitieb aiid local orielitations of the field, are studi d . Tliv alternative solutions, i.e. the cancelation of the extwna1 field by either actively powered coils or hy using a passive superconducting shield, are also discu5hecl. The active aiid passive shieldiiig can also be coirrbincd. As ai1 example thc proposed shielding of tlw HERA electron beam at thc HERA-N experiment site at DESY is described.The strength of the shielding effect depends 011 t,hr permeability of the used shielding material, tlir size and geometry of the shield as well a.s 012 t81ie shield wall thickness. Only for very simple forms of thr shield, assuriiinig a constant magnetic permeability of' the material ancl a homogeneous background field, the shieldiiig factor ,5' can be computed analytically [a].

Study on the Leakage Fields Prediction of a Static Ferromagnetic Shield in Consideration of Hysteresis and Residual Strain of Shielding Materials

“…The data measured in this deformed state are also used for the calculation to estimate the effect of the damage during the machining and assembly or installation of the constituent component in the shielding systems after their proper heat treatment. 6,10) The FEM calculations were performed on the single-layer cylindrical shell model as shown in Fig. 1, the dimensions of which are D ¼ 100 mm and d ¼ 25 mm with infinite length.…”

“…21,22) The numerical analyses of the static magnetic shielding are virtually performed with values of the permeability corresponding to the virgin magnetization behavior of a shell material without considering hysteresis. 2,3,10) Theoretical understandings of the static magnetic shield are fundamentally described with Maxwell's equations and their subordinate equation as follows:…”

“…That is to say, the estimated leakage field, H i j %0 , very near the surface of the shielding shell is equal to the magnetizing force of the shell material H H M and, at the center of the shielded space, H i j ¼=2 is =2 times that of H H M . 3,10) Then the leakage field can be determined by referring to eq. ( 6) as a value corresponding to the magnetizing force of the shell material under the magnetized condition during shielding.…”

“…The reason why the leakage field estimated by the ''with hysteresis'' calculation approaches a constant value with decreasing external field is derived from the permeability used in the FEM analysis given as eq. (10). In other words, when the shield works in the worst-case scenario of the magnetized condition where permeability is defined in the ''with hysteresis'' calculation, the leakage fields lower than the value corresponding to the coercivity of the shell material cannot be realized because the shell material can be magnetized only for H > H C .…”

“…In physical science experiments including the above big science projects, the ferromagnetic shields employing enclosures made from pure iron, Permalloys, or mild steel are usually applied for the required shielding due to ease of fabrication and on the basis of cost/performance. [6][7][8][9][10] A magnetic hysteresis is well known as a phenomenon characterizing ferromagnetic material. 11) It is obvious that the ferromagnetic shielding effectiveness is affected by the hysteresis of the shell material (material used for shielding shell) as its magnetized condition is varied depending on the previous state.…”