Preliminary Investigations on Detection of Hydrogen Negative Ions at the Downstream of a Large Volume ECR Plasma Source

Extraction of H - ions from plasma and their implementation in various applications such as plasma-based material processing and, most importantly, in fusion science for heating and diagnosis of thermonuclear fusion plasmas has led to significant research and development in the field of Hydrogen (H 2 ) plasmas and negative hydrogen ion sources. The Plasma Lab, IIT Delhi which has been associated with development of a large volume Hydrogen plasma source for the past few years, has obtained the optimal downstream plasma parameters for large area production of H - ions that can be extracted for fusion applications. This paper presents preliminary studies related to the detection of negative hydrogen ions in the large-volume plasma system using a quadrupole molecular beam mass spectrometer (MBMS). Experiments were performed in a cylindrical expansion chamber with a height and diameter of 1 m. Hydrogen plasma was produced using a compact ECR microwave plasma source mounted onto the top dome of the chamber. The magnetic field configuration of the source section (cylindrical, diameter Φ = 91 mm and length, l = 110 mm) has the field strength decaying exponentially into the plasma expansion chamber of 1 m diameter. In this work, plasma was characterized under different operating conditions of microwave power (400–600 W) and gas pressure (1–3 mTorr) using a Langmuir probe that was fabricated in-house. The low-pressure plasma sampling probe of the MBMS was inserted into the expansion chamber through one of the radial ports located at z = 37.5 cm plane where z is the axial distance from the source mouth. Uniform plasma was found to be produced at that plane with density n e ∼ 6 × 10 10 cm -3 and low electron temperature T e ∼ 1 eV favouring volume mode H - production. Apart from H - detection, the MBMS can be also tuned accordingly for the detection of positive ions (H + , H 2 + , H 3 + ) and neutral (RGA mode) particles present inside the expansion chamber. Results indicate significant presence of H - ions whose intensity decreases at higher power and pressure due to dominant loss channels. The presence of low energy peak in the H - energy distribution confirms their production through the volume mode. At higher power, the low energy peak shifted towards higher energy. Correlation between plasma parameters and detected intensities are presented.