Investigating Wall Effects on Plasma Instabilities in the ROBIN Ion Source Using 2D3V PIC-MCC Simulations

This study investigates plasma asymmetry in ROBIN (RF- operated beam source in India for negative ion production) using a 2D-3V massively parallel hybrid Particle-in-Cell Monte Carlo Collision (PIC-MCC) model, focusing on the role of boundary walls under different operational scenarios.

When wall effects are not included in the simulation, the plasma near the extraction region stays mostly uniform. However, thin strip-like structures caused by instabilities appear in the Transverse Magnetic Filter (TMF) region. Conversely, including boundary walls introduces pronounced asymmetries in these structures. The TMF creates sharp gradients in density, temperature, and potential—these provide the energy needed for instabilities to grow. Without wall effects, the simulation shows the formation of double layers and non-Maxwellian ion energy distribution functions (IEDFs). In contrast, the presence of walls results in Maxwellian IEDFs and asymmetric electron energy distribution functions (EEDFs).

Fast Fourier Transform (FFT) spectrogram analysis identifies two dominant instability modes: ion cyclotron modes around 10⁵ Hz and lower hybrid modes near 10⁷ Hz, both exhibiting significant harmonic content. These instabilities and the resultant asymmetries have substantial effects on cross-field plasma transport in the TMF region. Overall, the findings underscore the critical role of boundary walls in shaping plasma behaviour within negative ion sources. Wall-induced modifications to distribution functions and instability characteristics directly impact the uniformity of plasma near the extraction plane and influence the quality of the extracted negative ion beam. The simulation results are in strong agreement with recent experimental observations, offering new insights into optimizing plasma sources for enhanced ion beam performance.

This work has been carried out using the HPC facilities at DA-IICT, IPR Gandhinagar.