A new design of an electrostatic quadrupole quadruplet focusing lens system for low MeV multi-ion micro-beam

The progress in the understanding of ion induced damage of cellular matrix has lagged behind the development of techniques in heavy ion therapy (HIT). In order to fill this gap, we have initiated an inter-university effort for the construction of a new sub-micron ion beam to perform fine controlled radiobiological studies with a variety of ions (with energies up to 4.5 MeV/charge(q)). In this work, we present our new design of an electrostatic quadrupole quadruplet (EQQ) lens system that will be used for the production of the sub-micron beam. Its demagnification factor (D_f) is 33 at a working distance (D_w) of 98 mm for 3MeV/q ion beams with an emittance of 0.2 μm-mrad at the object aperture. Compared to the EQQ tested under similar input beam conditions at the Columbia University (CU) micro beam¹, our D_f is 8 times higher at comparable D_w. Our design achieves a comparable focusing performance to the separated EQ triplet (EQT) lens system¹ (D_f ~ 38 and D_{w ~} 126 mm), but with a compact lens design and fewer quadrupoles. Our study also shows that EQQ has chromatic aberration very similar to and better spherical aberration than that of EQT. The simulations were done using SIMION 8.1® and GICOSY that were bench marked by reproducing the measured beam parameters of the CU micro beam¹.