The design of the compound lens, used to focus the beam, is based on the one used for our electrostaticly focused microbeam. However, in order to simplify the PMM operation we have elected to use permanent magnets to construct the lens as opposed to the electrostatic lenses. The use of permanent magnets eliminates the need for bulky power supplies and cooling systems required by other types of ion lenses in addition to allowing a tighter configuration (and therefore better optical properties) than common electromagnetic lenses. Magnet strength is adjusted by moving rare earth magnets in and out of a shaped yoke as seen in the figure.
Two permanent magnet quadrupole triplets have been purchased from STI Optronics. The optimized lens, shown here, consists of two outer 4.25 cm long magnetic quadrupoles and an 8.5 cm long center quadrupole with inter-quadrupole gaps of 1.67 cm and a bore of radius 6.35 mm. It should be noted that such a small bore radius is rather difficult to obtain with standard electromagnets.
Just prior to the decommissioning of the RARAF Van de Graaff accelerator in June 2005, we had attained a beam spot size of 20 µm. As a first step, the beam was imaged at the focal plane of the first quadrupole triplet, using a commercial CCD chip. The observed spot size of 50 x 150 µm was in good agreement with that expected from simulations.
As a second step, the beam diameter was measured at the endstation using the knife edge technique. The spot size was tuned by adjusting all magnets while maintaining Russian symmetry - in particular we tried to keep quadrupoles 1, 3, 4 and 6 at the same strength (A) and quadrupoles 2 and 5 at the same strength (B). The figure shows the theoretical and measured spot size and shape at the end station. While the general trends are very similar in both cases, the smallest spot size obtained experimentally was only 20 µm in diameter, two times larger than the theoretical prediction. Simulations have shown that this is probably due to residual high-order fields in the quadrupoles or due to misalignment.
In 2006, the magnetic quadrupole system had to be removed for the construction of the laboratories on the third floor and was reassembled in early 2007. Without adjusting the magnets, we measured a beam spot size of 20 microns, demonstrating the robustness of this design. Following the replacement of the scattering foil with a phase space sweeper and smaller aperture we have obtained a beam spot approximately 8 µm in diameter.
The magnetostatic lens is in routine use.
For more details see:
Testing the stand-alone microbeam at Columbia University. Garty G., Ross G.J., Bigelow A., Randers-Pehrson G. and Brenner D.J. Radiat. Prot. Dosim. 122:292-296, 2006.
A single-particle / single-cell microbeam based on an isotopic alpha source. Ross G.J., et al. Nucl. Instrum. Meth. B. 231:207-211, 2005.
A microbeam irradiator without an accelerator. Garty G., et al. Nucl. Instrum. Meth. B. 241:392-396, 2005.