Zoom quadrupole focusing systems producing an image of an object

摘要

The main difference between quadrupole and axisymmetric charge particle beam focusing systems is that quadrupole systems do not produce a regular image of an object like axisymmetric systems. Because of the special symmetry in geometry and electric or magnetic fields along the axis, only the Russian quadruplet (RQ) can produce an image, however, for each geometry and given maximum field gradient of the RQ, the number of experimentally attainable focal distances in one system is limited to one or two. A new quadrupole focusing system with the same symmetry as the RQ is now under development at the Louisiana Accelerator Center that produces an image and has the possibility to vary the focal distance over a wide range by changing the excitation of the middle lenses. As a result, the magnification may be changed from 10~(-3) to 10~(+3) so that this versatile design can be adapted for several applications of high-energy focusing ion beam (HEFIB) systems, including nuclear nanoprobe, scanning microscope or transmission microscope. The configuration of this lens system consists of six quadrupole lenses and although these lenses can be combined either as magnetic (RMS) or electrostatic (RES) sextuplets, the Louisiana Accelerator Center is presently constructing an RMS system due to operational requirements. The zoom capability of the RMS is numerically investigated and the smallest beam spot size and appropriate optimal parameters of the RMS are presented for different emittances and compared with the corresponding parameters of the RQ.