Handbook of Modern Coating Technologies

Focusing elements

The physical principles underlying most FEs include creation of an axisymmetric external electromagnetic field. This means that a particle traveling along the axis is unaffected by the field, and therefore does not change its direction at the exit from the FE. There are two types of axial symmetry, axial and quadrupole, realized in SNMP FEs. Results of theoretical and experimental studies of various FEs with these types of a field symmetry are reported in monograph [66], where some "exotic” FEs are considered among others, such as plasma and coaxial lenses. However, magnetic axially symmetric lenses based on superconducting sole­noids and systems of magnetic and electrostatic quadrupole lenses are used most commonly, because they ensure the highest resolution at the current stage of SNMP development.

Peculiarities of the application of superconducting solenoids in the capacity of FEs in SNMPs are due to certain properties of an axisymmetric magnetic field. A stigmatic image is produced upon changing a single parameter (current strength in solenoid coils). The adjust­ment depends on four positioning parameters, namely, transverse displacements (x, y), rota­tions in the yawing plane (x0z), and pitch (y0z), whereas the alignment of an MQL triplet requires 16 parameters to be controlled. Spherical aberrations are 5—10 times smaller, and chromatic aberrations are comparable to those in quadrupole PFSs. An important drawback of these lenses is their high operating costs due to liquid-helium cooling and the depen­dence of improving resolution parameters on the magnetic field induction, which is currently limited by the properties of the superconducting materials.

FEs with quadrupole field symmetry are most commonly used as active FEs in modern SNMPs. Modernization of quadrupoles for the improvement of the resolving power of SNMP facilities dates back to the first successful application of an MQL in the PFS of MeV-energy ion beams. General tendencies in the development of precision quadrupole lenses ensue from the necessity of creating PFSs with a small working distance, because short-focus ion- optical systems have higher reduction factors at moderate aberrations. A reduction of both the working distance and the focal length is associated with an increase in the optical power of quadrupole lenses. It follows from relations (2) and (3) that for fixed beam parameters the optical power of the quadrupoles is directly proportional to the magnetic induction or the pole potential and the effective lens length, and inversely proportional to the lens aperture radius. Despite such a simple dependence, all these parameters have limitations as regards their favorable changes. By way of example, an increase in the effective length is unrelated to the working distance, but leads to a greater focal length. The possibility of increasing mag­netic induction depends on both the degree of saturation of a given material and the geome­try of the pole tips. Potential growth in an electrostatic quadrupole is limited by vacuum breakdown conditions. A decrease in the aperture necessitates a change in pole geometry that, in turn, causes parasitic multipole field components to grow due to the deviation from the hyperbolic profile and the appearance of local pole saturation zones at the sites of the closest approach of the poles.

An equally important factor is the absence of physical mechanisms for precise mounting of an einzel MQL aligned with the beam axis. The transverse lens plane in existing micro­probe facilities is placed perpendicular to the laser beam. However, the axis of the light beam does not necessarily coincide with the ion beam axis due to the presence of scattered magnetic fields in the laboratory that cause distortion of the rectilinear beam axis. Therefore in PFSs with magnetic quadrupoles aligned separately on the optical pathway the lenses need to be coupled into doublets.

The following steps in upgrading magnetic quadrupoles have been taken over the 30-year

history of microprobe operation:

• the passage from the cylindrical to hyperbolic shape of pole tips allowed diminishing parasitic higher order multipole field components;
• the manufacturing technology of nonseparable OM-52 lenses was developed and brought into commercial practice by Oxford Microbeams (http://www.microbeams.co.uk) [76]

(Fig. 5—6B) using electroerosion metal processing with the accuracy of mutual arrangement of pole tips of ~2 pm;this permitted removing parasitic sextupole and octupole field components induced by breaking the quadrupole lens symmetry;

• new designs of magnetic circuits with protruded pole tips (OM-52) and indentations in the magnetic circuit for the placement of ion detectors (CSIRO-GEMOC) [77] ( 5—6C) were proposed in conjunction with integrated doublets for use in distributed PFSs whose yoke and poles are made from a single piece of magnetically soft material (IAP NASU)

[62] (Fig. 5—6A);these were used to decrease the working distance (from the outer boundary of the effective field of the last lens to the target plane);

• studies on the utilization of magnetic materials with a narrow hysteresis loop for manufacturing magnetic circuits and lens pole tips; and

(B)

(C)

FIGURE 5-6 Magnetic quadrupole lenses applied in the beam-focusing systems of nuclear microprobes: (A) integrated doublet (IAP NASU) [76], (B) OM-52 lens (Oxford Microbeam) [76], and (C) CSIRO-GEMOC lens [62,77].

• substantiation of the use of magnetic quadrupoles with superconducting coils to focus ~20-MeV ions [78].