MORA计划中透明轴对称离子阱的几何优化
Geometry optimisation of a transparent axisymmetric ion trap for the MORA project
摘要：
在MORA（Matter's Origin from the Radio Activity of Captured and Oriented ions）项目的框架下，设计了一个透明的轴对称射频离子阱（MORATrap），用于测量激光极化离子核$β-$衰变中的三重关联参数$D$。陷阱的设计灵感来自LPCTrap几何形状，在2005年至2013年期间在GANIL运行。在实际的（非理想的）保罗阱中，四极电势不是完美的，导致离子运动的不稳定性，并因此影响总体捕获效率。本文提出了一种数值方法，旨在优化的几何形状的陷阱。它被应用到MORATrap，以提高捕获效率，并扩大轴向透明立体角相比，LPCTrap。在整个优化过程中，采用基于边界元法的静电场求解器进行了电势和电场的数值计算。优化包括在最小化的目标函数（健身功能）取决于高阶多极的潜力。最后，系统地改变阱的尺寸和电极的位移被施加到调查的潜在质量的几何效应。
英文摘要：
In the frame of the project MORA (Matter's Origin from the Radio Activity of trapped and oriented ions), a transparent axially symmetric radio-frequency ion trap (MORATrap) was designed in order to measure the triple correlation parameter $D$ in nuclear $β-$decay of laser-polarised ions. The trap design was inspired from the LPCTrap geometry, operated at GANIL from 2005 to 2013. In a real (non-ideal) Paul trap, the quadrupole electric potential is not perfect leading to instabilities in ion motion and therefore affecting the overall trapping efficiency. This paper presents a numerical method aiming to optimise the geometry of a trap. It is applied to MORATrap in order to improve the trapping efficiency and to enlarge the axial transparent solid angle compared to LPCTrap. In the whole optimisation process, numerical computation of electric potential and field was carried out using an electrostatic solver based on boundary element method (BEM). The optimisation consisted in minimising an objective function (fitness function) depending on higher order multipoles of the potential. Finally, systematic changes of trap dimensions and electrode displacements were applied to investigate geometrical effects on the potential quality.
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