X. Bonnin

High quality MPACVD diamond single crystal growth: high microwave power density regime

The growth of monocrystalline diamond films of electronic quality and large thickness (>few hundreds of microns) is an important issue in particular for high-power electronics. In this paper, we will describe the different key parameters necessary to reach this objective. First, we will examine the deposition process and establish that only microwave assisted diamond deposition plasma reactors can achieve the optimal growth conditions for the efficient generation of the precursor species to diamond growth. Next, we will consider the influence of the monocrystalline diamond substrate orientation and quality on the growth of the epitaxial layer, especially when the deposited material thickness exceeds 100 µm. The need to use a specific pre-treatment procedure of the substrate before the growth and its impact will also be discussed. Finally we will look at the growth conditions themselves and assess the influence of the process parameters, such as the substrate temperature, the methane concentration, the microwave power density and the eventual presence of nitrogen in the gas phase, on both the morphology and quality of the films on the one hand and the growth rate on the other hand. For this, we will introduce the concept of supersaturation and comment on its evolution as a function of the process parameters.

Plasma?surface interaction, scrape-off layer and divertor physics: implications for ITER

Recent research in scrape-off layer (SOL) and divertor physics is reviewed; new and existing data from a variety of experiments have been used to make cross-experiment comparisons with implications for further research and ITER. Studies of the region near the separatrix have addressed the relationship of profiles to turbulence as well as the scaling of the parallel power flow. Enhanced low-field side radial transport is implicated as driving parallel flows to the inboard side. The medium-n nature of edge localized modes (ELMs) has been elucidated and new measurements have determined that they carry ~10?20% of the ELM energy to the far SOL with implications for ITER limiters and the upper divertor. The predicted divertor power loads for ITER disruptions are reduced while those to main chamber plasma facing components (PFCs) increase. Disruption mitigation through massive gas puffing is successful at reducing PFC heat loads. New estimates of ITER tritium retention have shown tile sides to play a significant role; tritium cleanup may be necessary every few days to weeks. ITERs use of mixed materials gives rise to a reduction of surface melting temperatures and chemical sputtering. Advances in modelling of the ITER divertor and flows have enhanced the capability to match experimental data and predict ITER performance.

Disruption studies in ASDEX Upgrade in view of ITER

Experiments on ASDEX Upgrade and other tokamaks have shown that the magnitude of mechanical forces and thermal loads during disruptions can be significantly reduced by raising the plasma density with massive injection of noble gases. This method should be applicable to ITER too. Nevertheless, the suppression of the runaway electron (RE) avalanche requires a much larger (two order of magnitude) density rise. This paper reports on recent experiments aimed at increasing the plasma density towards the critical value, needed for the collisional suppression of REs. An effective electron density equal to 24% of the critical density has been reached after injection of 3.3?bar?l of neon. However, the resultant large plasma density is very poloidally and toroidally asymmetric; this implies that several valves distributed around the plasma periphery become necessary at this level of massive gas injection to ensure a homogeneous density distribution.

Modelling of electric fields in tokamak edge plasma and L-H transition

The fluid simulation of a divertor tokamak edge plasma by the B2-SOLPS5.0 transport code gives the dependence of the radial electric field on the local and global plasma parameters. The shear of the radial electric field, which is responsible for the transition to an improved confinement regime, is a linear function of a local ion temperature and the local average toroidal velocity and is inversely proportional to the toroidal magnetic field. The scaling for the L-H transition threshold agrees with the experimental H-mode scaling of ASDEX Upgrade. The radial electric field shows no bifurcation and is close to the neoclassical electric field with the toroidal rotation contribution determined by the radial anomalous transport of the toroidal momentum. The fine structure of the electric field at the separatrix and its dependence on the toroidal magnetic field inversion is analysed.

Subroutines for some plasma surface interaction processes: physical sputtering, chemical erosion, radiation enhanced sublimation, backscattering and thermal evaporation ☆

A suite of FORTRAN subroutines/functions to generate data using empirical formulas for physical sputtering of mono-atomic targets for any elemental incident ion (atom), chemical erosion of graphite, Radiation Enhanced Sublimation (RES) of graphite, the number and energy backscattering coefficients for any elemental incident ion (atom) on a compound target and thermal evaporation of graphite is presented. Since chemical erosion, RES and thermal evaporation depend on the surface temperature of graphite, a subroutine implementing the 1-D heat diffusion equation to determine the temperature of any plasma-facing graphite surface is implemented. As an example to illustrate the use of these subroutines/functions, a simple model for the erosion of a plasma-facing surface, consisting of a simple collisionless sheath model, a 1-dimensional steady state heat diffusion model and 0-dimensional steady state particle balance at the target is developed and a sample listing of the program is presented.

Modelling of hydrogen isotope inventory in mixed materials including porous deposited layers in fusion devices

Hydrogen isotope inventory (HII) is a key issue for fusion devices such as ITER. Simultaneous use of Be, W and C as the wall material for different parts of plasma-facing components (PFCs) will bring in material mixing issues, which compound that of hydrogen isotope retention. To simulate the hydrogen inventory in the PFCs, we have developed a flexible standalone model called HIIPC (Hydrogen Isotope Inventory Processes Code). The particlebalance-based model for reaction–diffusion and HII in metal and porous media (mainly carbon and co-deposited layers) is presented, coupled with a heating model which can calculate the temperature distribution. Some sample results are given to illustrate the model’s capabilities and show good qualitative agreement with the experiment. (Some figures may appear in colour only in the online journal)

Discrepancy between modelled and measured radial electric fields in the scrape-off layer of divertor tokamaks: a challenge for 2D fluid codes?

Examination of radial electric field (E,.) profiles in the scrape-off layer (SOL) of ASDEX Upgrade (AUG) and JET revealed large discrepancies between 2D fluid edge modelling and experiment. Experimental profiles of plasma potential (V-p) in the outer (low field) side of the plasma, obtained with reciprocating Langmuir probes, decay radially with electron temperature, T-e, with the -eE(r)/del T-e ratio being > 1.5. In contrast, code simulated E-r are fairly low in most of the SOL (compared with -del T-e/e). Modelling with kinetic treatment of neutrals and drifts was performed using the SOLPS code for AUG cases and EDGE2D-Nimbus for JET cases. Mismatches between modelled and experimental E-r may be caused by the recently established tendency for the SOLPS code to underestimate T-e in the divertor of AUG. It was attributed to non-locality of parallel transport of supra-thermal, heat-carrying electrons originating upstream of the divertor, which are usually only weakly collisional and can penetrate, with few collisions, to the target. Ratios -eE(r)/del T-e obtained from the probe measurements in JET are of order 1.6, while in AUG, JT-60U and TCV they are of order 3. Such high values point to the possibility of fast electrons contributing, apart from target heat fluxes, also to the formation of the Debye sheath. The problem of the underestimation of E-r. in the codes must be closely related with the well-known problem of the underestimation of those parts of parallel ion flows in the SOL that are influenced by the toroidal field direction. It was demonstrated earlier that parallel ion flow at the outer midplane is dominated by the ion Pfirsch-Schluter flow, which in turn is partly driven by the radial electric field. The T-e and E-r discrepancies, as well as discrepancies between simulated and experimental parallel ion flows, raise a question of the validity of fluid codes for the plasma edge modelling and prompt the inclusion of kinetic effects into present-day 2D fluid codes which assume strong collisionality.

Potentials and currents in the edge tokamak plasma: simplified approach and comparison with two-dimensional modelling

A method of reduction of the two-dimensional equation for the potential to a one-dimensional ordinary equation is suggested and implemented in the B2SOLPS5.0 transport code to simulate a divertor tokamak. The one-dimensional version, which gives similar results as the full two-dimensional version, provides better understanding of the role of various mechanisms of perpendicular conductivity and reduces computational time. The scheme of poloidal (parallel) current calculation is presented and compared to the two-dimensional code results both for the core region and for the scrape-off layer.

SOLPS modeling of lithium transport in the scrape-off layer during real-time lithium injection on EAST

Edge fluid?plasma/kinetic?neutral SOLPS [1, 2] modeling of lithium (Li) transport and its effect on the edge plasma during real-time Li injection H-mode discharge on the EAST tokamak are analysed in this work. Since Li has strong chemical activity, deuterium (D) recycling is suppressed by a Li coated plasma-facing wall. By comparing the simulated edge plasma parameters between the no Li case and the Li injection case, it is found that both of the D atom and molecule densities in the divertor region are reduced with the Li injection. It is also found that most of the radiated power is radiated in the divertor. The simulation provides and analyzes the distributions of each Li ion charge state, and the evolution of Li impurity distribution. The simulation shows that the Li+ prefers to accumulate on the high-field side than on the low-field side, which is in qualitative agreement with the experimental measurements on EAST. The possible reason for the Li+ preferential accumulation is discussed in this study.

Impact of E x B drifts on the distribution of impurities in the Tokamak plasma edge

Abstract The impurity transport in the edge plasma of a divertor tokamak is simulated by means of B2-SOLPS5.0 2D multi-fluid code where diamagnetic and E → × B → drifts for all species are taken into account. It is demonstrated that these drifts to large extent determine the poloidal distribution of impurities near the separatrix and hence penetration of impurities to the core region. The influence of the drifts on the low ionized impurity distribution in the divertor region is more modest.