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Permanent Magnet Microbeam

Permanent Magnet Microbeam

pmmThe permanent magnet microbeam (PMM), under development at RARAF, presents an alternative approach to microbeam design. Instead of focusing the ion beam using electromagnetic or electrostatic lenses, this system uses permanent magnets, which require no power supplies.

4He ions from the accelerator are focused using a compound magnetic lens consisting of two quadrupole triplets. The first triplet is placed 2 m above the object aperture, with a second (identical) triplet placed 2 m above the focal plane of the first. Since each triplet does not have identical demagnifications in the x and y axes, the two lenses are rotated by 90º in the x-y plane so that a circular beam spot is obtained (Russian Symmetry). The object aperture was initially covered with a 1.8-μm thick Al scattering foil (since replaced by a phase space sweeper), used to eliminate any correspondence between angle and position for the particles in the beam. A limiting aperture is placed before the first triplet and inside the second triplet to reject ions which have very large aberrations.

The cells to be irradiated are placed at the image plane of the compound lens. The PMM is mounted on the original microbeam endstation consisting of a microscope with a particle detector mounted on one of the objective lenses and an x-y stage positioned by stepping motors.

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Magnetostatic Lens



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.

Beam Tests

pmmJust 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.

pmmAs 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.

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Radiological Research Accelerator Facility Nevis Laboratories
P.O. Box 21, 136 S. Broadway, Irvington, N.Y. 10533