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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].

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Abstiact-The magnetic lattice of the TeV electron superconducting linear accelerator (TESLA) will consist of superconducting quadrupoles for beam Pocusing and superconducting correction dipoles for beam steering, incorporated in the cryostats containing the superconducting cavities. This report describes the design of these magnets, presenting details S% the magnetic as well as the mechanical design.The measured characteristics of the TESLA Test Facility (TTF) quadrupoles and dipoles are compared to the results obtained from numerical computations.The TESLA Test Facility ( T T F ) located a t DESY, Hamburg, will include a 500-800 MeV linac. The main body of t,he linac consists of four cryomodules each containing right 1 mi long nine-cell superconducting cavities operating at 1.3 GHz with a base accelerating gradient of 15 MV/m. At the end of each 12.2 m long cryomodule, a superconducting magnet package will be installed. The magnet package, as shown in Fig. 1, consists of a superferric quadrupole doublet, powered in series; two pairs of superconducting dipole steering coils inside the quadrupole yoke bore; an RF beam position monitor (BPM) consisting of a pill-box RF cavity, rigidly connected t o the quadrupole yoke; a stainless steel beam pipe through the magnet bore, evacuated t o UHV, rigidly connected t o the BPM, to the quadrupole doublet and, through a bellow, t o the nearest cavity. T h e pipe also absorbs the unwanted higher order modes (HQM) from electromagnetic energy leaking out from the cavities. For this reason a cooling slceve around it is provided, in which 70 K He gas is circulated; Fig. 1. The superconducting magnet package. Dimensions in mm's 0 eight current leads for the quadrupole doublet and the dipole steering coils, enclosed in a special pipe running from the helium vessel to a flange on t,he cryostat vacuum vessel. T h e leads are cooled by cold He gas. At the cold end, the pipe begins at a connection box in which the current leads are connect,ed to the magnet coil ends.The magnet package will he housed in a stainless steel vessel and cooled by 4 K liquid helium.T h e assembly of the first cryomodule is expected to be completed by November 1995. Details of the overall design of the TTF have been described in a full report by TESLA Collaboration [l]. GENERAL DESIGNT h e limited space in the cryomodule a,nd t,lie demand for a cost efficient design both prefer the superferric, quadrupole magnet configuration. The quadrupoles must, have a relatively large aperture, with a radius of 56 inin, t o provide room for the HQM absorber, the annular space for the insulating vacuum, the inner helium vessel tube and the correction dipoles wound on it. The dipole windings can also only have a single winding layer due to the limited space.T h e length of the quadrupoles was determined by the Manuscript This work was supportedin part by the Research Institute of High available space. A yoke length of 150 m m was considered t o be sufficient. The straight sections of the coils are longer than the pole apertu...