Electrical rotation of quadrupole lenses

Abstract

Applying voltage to the electrically isolated magnetic poles of a quadrupole lens produces two coincident lenses having principal sections inclined at 45°. The principal section of the combined lens is then inclined at an angle which depends on the relative electric and magnetic strengths. This method of rotation requires fewer poles than other methods and generates only a small 12-pole perturbing field. A lens of this type having bore 3.4 mm and pole tip field strength of 0.5 T, when used to focus 2 MeV He + ions, will have its principal section rotated 0.57° by a pole voltage of 73 V. The two lenses in a quadrupole doublet may be rigidly attached to one another. Parasitic first-order aberrations are not produced by small translations and tilts of the axes of the individual quadrupoles with respect to each other, nor by small angular and translational misalignment of the doublet axis with respect to other optical components. However an angular displacement of one of the lenses about the common axis does blur the image. A ray deviation of 3 μm per degree of misalignment is estimated in a typical case. Electrical rotation may be used to compensate any such fixed mechanical misalignment.