Compensation of Aberrations in Achromatic Quadrupole Doublets

An achromatic quadrupole lens contains an electric and a magnetic quadrupole lens operated at the proper field ratio. Using a doublet of achromatic lenses in which a quadrupole voltage may be applied to the magnetic poles of one of the lenses in order to rotate its principal section, we have eliminated simultaneously what may be called three first order aberrations of a quadrupole doublet. These are chromatic aberration, rotational misorientation, and focus. By adding suitably chosen small potentials to the quadrupole excitation of an electric lens, 2nd order parasitic aberration may be compensated. Experimental procedure for compensation is described. The procedure appears capable of improvement, and further work is warranted. Currently a beam of 600 keV protons with a width of 0.6 micron is obtained.