Abstract
A doublet of achromatic quadrupole lenses has been tested as a probe-forming lens for high-energy ions. Each lens has length 6 cm and diameter 14 cm, and contains a magnetic quadrupole of maximum pole tip field strength 1.0 T and bore 3.40 mm, and an electric quadrupole of maximum pole potential 3.0 kV and bore 1.05 mm. Maximum achromatic operating energy is calculated to be 29 MeV for protons and 410 keV for singly charged antimony ions. To test the doublet, backscattering of 600 keV protons is measured from a tungsten wire as the beam is deflected by dipole excitation of the second lens. In preliminary measurements, a chromatic aberration coefficient of less than 1.2 cm and beam width of 4 μm are found, compared to 20 cm and 4 μm for operation with no electric quadrupole voltage. If their parasitic aberrations can be reduced, lenses of this type offer prospects for over coming the problems of chromatic aberration which limit the resolution of microprobes used for proton-induced X-ray emission or Rutherford backscattering measurements, as well as of kilovolt energy systems utilizing field ionization sources. Experimental methods of characterizing and compensating parasitic aberrations of first and second order will be presented.
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