Speaker
Description
One of the most significant applications of plasma-based hydrogen negative ion (H-) sources is the production of high current, large area H-beams, which are used for the neural beam injection (NBI) system of a fusion plasma reactor like ITER. Such beams demand high density, uniform plasma (~10^12/cm^3 with variation limited to 10%) with low electron temperature (≤ 1 eV) generated over a large area. Presently, to meet such requirement, RF-ICP discharges are used, that however utilizes high powers (Prf~800 kW). In the present work, an alternate novel plasma source, namely Compact Electron Cyclotron Resonance (ECR) Plasma Source (CEPS, patentee: Plasma Lab, IIT Delhi) is described that aims to produce similar plasma in a large expansion chamber with low input power to improve the energy efficiency. The CEPS (dia: 0.09 m, length: 0.11 m) is coaxially attached to the top dome (marked z = 0) of the chamber (dia ~ 1.0 m, height ~ 1.0 m). It is a compact and portable source that uses axially poled NdFeB permanent ring magnets encapsulating CEPS’s source section. The magnetic field structure is fairly complex inside CEPS with an ECR surface lying within it. In the expansion chamber, the magnets produce a diverging field that decays exponentially along the axis. Experiments were conducted with minimum μ-wave power ≈ 400W and at hydrogen gas pressure of 1-5 mTorr. The large volume plasma was characterized with cylindrical Langmuir probes, placed at different positions, in both axial as well as radial direction. Axial direction measurements revealed high plasma density of ~2×10^11/cm^3 near to the source (z = 5 - 10 cm), reducing to ~ 6 × 10^10/cm^3 over the axial span of 60 cm towards the downstream. On axis electron temperature (Te) near the source was recorded to be ~ 55 eV which drops to ~ 5 eV within 5 cm span towards the downstream and further reduced to ~ 2eV at z = 60 cm. Radial profiles at different planes of the expansion chamber indicated the presence of moderately dense and uniform plasma with a low Te ~ 0.5 - 3 eV in the downstream over 80 cm dia. Noticeable improvement in radial uniformity is also observed, as one moves from up-stream region (~ 30 %) i.e., near to the CEPS source to downstream regions (~15 %) but at the expense of a fall in the plasma density. The low Te and the uniformity are in favor of large area surface production of H- which is prone to destruction in a high Te environment due to its low electron affinity (0.75 eV). Moreover, the pertaining condition is also encouraging for volumetric production of H-via the 2-step dissociative attachment (DA) process, where one needs high Te (~20 eV like in upstream) for excited H2 formation and low temperature ( 1 eV in downstream) for H-production. Work is now underway to estimate the H-density at the downstream plane using a particle balance model.
