In multi-beamlet negative ion accelerators for Neutral Beam Injectors, the transverse magnetic field necessary for suppressing the co-extracted electrons induces a deflection of the negative ion beamlets that must be corrected. For the design, particle-tracing simulation codes are used to compute ion trajectories and optical properties of the beamlets in the acceleration stage. In these codes, uniform boundary conditions are normally assumed for the ion current density distribution at the surface (called meniscus), where negative ions are extracted from the plasma and form beamlets, which are accelerated across the apertures of the accelerator grids. Recently, experimental campaigns dedicated to the accurate measurement of the beamlet deflection in the acceleration phase revealed higher deflection than foreseen by simulations. In this work, we demonstrate that an agreement with the experimental data can be obtained by incorporating in the numerical simulations a non-radially symmetric distribution of the ion current density extracted across the meniscus surface. In the first part, the asymmetry of the ion current density at the meniscus is studied in an empirical way, by analyzing and fitting the experimental results obtained with different operating parameters. In the second part, we show that the ion current asymmetry estimated by this procedure is well consistent with the flow pattern of H– ions calculated in the meniscus zone using a detailed Particle in Cell (PIC) model of the ion source in the presence of transverse magnetic field.

中文翻译：

---

负离子加速器等离子体弯液面提取电流密度不对称性的实验和数值研究

在用于中性束注入器的多子束负离子加速器中，抑制共同提取电子所需的横向磁场会引起必须校正的负离子子束偏转。在设计中，粒子追踪仿真代码用于计算加速阶段中子束的离子轨迹和光学特性。在这些代码中，表面（称为弯液面）的离子电流密度分布通常假定为均匀边界条件，其中从等离子体中提取负离子并形成子束，这些子束在加速器网格的孔径上加速。最近，致力于精确测量加速阶段小束偏转的实验活动显示出比模拟预测的偏转更高。在这项工作中，我们证明，通过在数值模拟中加入跨弯月面提取的离子电流密度的非径向对称分布，可以获得与实验数据的一致。在第一部分中，通过对不同操作参数下获得的实验结果进行分析和拟合，以经验的方式研究了弯液面处离子电流密度的不对称性。在第二部分中，我们展示了通过该程序估计的离子电流不对称性与使用离子源的详细粒子中粒子 (PIC) 模型在弯月面区域中计算的 H– 离子的流动模式非常一致，在横向磁场。