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High voltage electrical discharges in water are of increasing interest for the degradation of organic compounds and destruction of biological species. The present study reports measurements of the rates of molecular hydrogen, molecular oxygen, and hydrogen peroxide formation in a pulsed positive needle-plane corona-like electrical discharge in water. In experiments for various solution conductivities, applied voltages, and discharge powers, the ratio of the molar rate of production of hydrogen:hydrogen peroxide:oxygen was approximately 4:2:1. The highest observed rate of hydrogen production was 1.3 μmol/s at discharge power of 37 W (or 0.25 g of H2/kW·h) at solution conductivity 50 μS/cm. The G-value for hydrogen production was 0.17 molecule of H2/100 eV, falling in the range of that found in the radiation chemistry literature (∼0−0.5 molecule of H2/100 eV, depending on scavenger concentration). A global reaction is proposed to add to existing kinetic models for the simulation of reactive species production in electrical discharge in water.

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Since the signature of the ITER treaty in 2006, a new research programme targeting the emergence of a new generation of Neutral Beam (NB) system for the future fusion reactor (DEMO Tokamak) has been underway between several laboratories in Europe. The specifications required to operate a NB system on DEMO are very demanding: the system has to provide plasma heating, current drive and plasma control at a very high level of power (up to 150 MW) and energy (1 or 2 MeV), including high performances in term of wall-plug efficiency (η > 60%), high availability and reliability. To this aim, a novel NB concept based on the photodetachment of the energetic negative ion beam is under study. The keystone of this new concept is the achievement of a photoneutralizer where a high power photon flux (~3 MW) generated within a Fabry Perot cavity will overlap, cross and partially photodetach the intense negative ion beam accelerated at high energy (1 or 2 MeV). The aspect ratio of the beam-line (source, accelerator, etc.) is specifically designed to maximize the overlap of the photon beam with the ion beam. It is shown that such a photoneutralized based NB system would have the capability to provide several tens of MW of D 0 per beam line with a wall-plug efficiency higher than 60%. A feasibility study of the concept has been launched between different laboratories to address the different physics aspects, i.e., negative ion source, plasma modelling, ion accelerator simulation, photoneutralization and high voltage holding under vacuum. The paper describes the present status of the project and the main achievements of the developments in laboratories.