Numerical investigation of ion energy and angular distributions in a dc-biased H2 inductively coupled discharge

Abstract

The ion energy and angular distributions of H+, H2+, and H3+ impinging on an extraction aperture (3 mm in radius) have been investigated with a hybrid model of an inductively coupled H2 ion source. A dc is applied at the end of the cylinder discharge chamber, which pulls these three ions toward the extraction aperture. With an increase in the bias voltage, their ion angular distributions (IADs) become more symmetrical about the 0° axis and the full width at half-maximum (FWHM) of their IADs shrinks continuously. On the other hand, the ion-energy distributions (IEDs) of the three ions first form a single peak (Vbias< 50 V) and then have a bimodal structure at 50 V <Vbias< 290 V. Finally, they return to a single peak at high bias voltage (Vbias> 290 V). The interval between two IED peaks (ΔE) first increases to a maximum around 150 V and then decreases monotonously. This can be attributed to the variation of the ratio of the ion transit time through the sheath (τi) to the rf field period (τrf). Moreover, the FWHMs of IEDs of H+, H2+, and H3+ first increase to 2.62, 2.45, and 2.32 eV around Vbias = 150 V, respectively, and then decrease continuously with the bias voltage. Using a low rf power and low gas pressure may help to narrow the FWHM of the IEDs of the three ions. The hybrid model is verified by comparing the results from the simulation and experiment, and they exhibit a qualitative agreement. The results in this work could lead to deeper insights into the dependence of IADs and IEDs on the discharge parameters, which is important for realizing a monoenergetic and collimated ion beam in a proton or an ion accelerator.