Philippe Teste

R&D around a photoneutralizer-based NBI system (Siphore) in view of a DEMO Tokamak steady state fusion reactor

ince 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 D0 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.

Cathode Surface Morphology Effects on Field Emission: Vacuum Breakdown Creation of Field Emitters

The effect of stainless steel cathode surface morphology on field electronic emission in high vacuum (~10-5 Pa) is studied. The surface rugosity is shown to affect the emission intensity; high-aspect ratio surface features lead to locally enhanced electric field, thereby increasing the field electronic emission. A breakdown in the vacuum, caused either by impact of charged dust particles or other impurities, or by overheating and vaporization of field emitters, can lead to cratering on the cathode surface. This cratering drastically changes the local surface rugosity, enhancing the local field and leads to greater emission intensity. Treatment of the cathode surface by glow discharge at higher pressure is known to reduce field emission by ionic bombardment and sputtering of emission sites; an image of such a glow-type discharge in the same apparatus used to study field emission is presented.

Influence of ambient gas pressure and carbon adsorption on dark current emission from a cathode

Electronic field emission current (dark current) from surfaces under vacuum at high field strengths can be reduced by the injection of gas into the ambient volume. A possible reversible mechanism responsible for this gas effect is proposed. The mechanism involves the formation of nanoscale emitter structures by polymerization of hydrocarbon contamination with low-flux ion bombardment at low pressure, and the destruction of these structures by high flux ion bombardment at sufficiently high pressure. Experimental evidence, in particular, x-ray photoelectron spectroscopy analysis of the electrode, is provided in support of this mechanism. Density functional theory calculations are presented to show that the morphology of the carbon layer, not its chemical composition, is the important parameter influencing dark current levels.

Controlled electron emission and vacuum breakdown with nanosecond pulses

Vacuum electron sources exploiting field emission are generally operated in direct current (DC) mode. The development of nanosecond and sub-nanosecond pulsed power supplies facilitates the emission of compact bunches of electrons of high density. The breakdown level is taken as the highest value of the voltage avoiding the thermo-emission instability. The effect of such ultra-fast pulses on the breakdown voltage and the emitted electron current is discussed as a result of the thermo-emission modelling applied to a significant protrusion. It is found that pulsing very rapidly the vacuum breakdown occurs at higher voltage values than for the DC case, because it rises faster than the heat diffusion. In addition, the electron emission current increases significantly regardless of the theoretical approach is used. A comparative study of this theoretical work is discussed for several different forms of the protrusion (elliptic and hyperbolic) and different metals (hence varying the melting point), particularly refractory (tungsten) versus conductor (titanium). Pulsed mode operation can provide an increase on breakdown voltage (up to 18%) and a significant increase (up to 330%) of the electron extracted current due to its high non-linear dependency with the voltage, for the case for the case with a hyperbolic protrusion.

Modeling of the electron emission by picosecond lasers under an intense electric field in vacuum

Electron emission induced by picosecond laser from solid surfaces placed under an intense electric field has been modeled. The results show an important difference between the electrons temperature (5500 K) and the phonons temperature (850 K). In these conditions, the Fermi-Dirac distribution depends of the electron temperature, while the thermo-field emission becomes effective for temperatures well below the fusion of the metal.

Contribution to partial discharge analysis in inverter-fed motor windings for automotive application

The work presented here is part of a larger study concerning the analysis of electrical stress and degradation phenomena of electrical insulation of low voltage machines fed by Pulse Width Modulation (PWM) controlled inverter. The goal is to assess the risk of partial discharge (PD) in stator windings of rotating machines. In this objective, two complementary approaches are adopted: electrostatic simulations using Finite Element Method (FEM) in 2D approximation, and experimental measurements. The model seems to be a good tool allowing an approximation of PD occurrence in a given wire type. In the experimental part of this study, a simple and efficient method to detecting PD under PWM controlled voltage is presented.