Tiberiu Minea

Plasma diagnostics for understanding the plasma–surface interaction in HiPIMS discharges: a review

The physical and chemical aspects of plasma–surface interaction in high-power impulse magnetron sputtering (HiPIMS) discharges are overviewed. The data obtained by various plasma diagnostic methods representing the important sputtering discharge regions, namely the cathode vicinity, plasma bulk, and substrate vicinity, are reported. After a detailed introduction to the problem and description of the plasma characterization methods suitable for pulsed magnetron discharge analysis, an overview of the recent plasma diagnostics achievements in both non-reactive and reactive HiPIMS discharges is presented. Finally, the conclusions and perspectives suggesting possible directions and research strategies for increasing our knowledge in this domain are given.

Room temperature synthesis of carbon nanofibers containing nitrogen by plasma-enhanced chemical vapor deposition

This letter reports low-pressure, room-temperature growth of carbon nanofibers containing nitrogen by plasma chemical vapor deposition arrangement. By alternating pure acetylene plasma and afterglow pure nitrogen high dense plasma, a fine control of the fibers growth kinetic is obtained. This layer-by-layer deposition technique takes advantage of nitrogen chemical etching effects during the growth of nitrogen-doped carbon nanofibers.

Argon metastables in HiPIMS: time-resolved tunable diode-laser diagnostics

Time-resolved tunable diode-laser absorption spectroscopy measurements were performed on the argon metastable (Ar-m) level 3s(2)3p(5)(P-2(3/2)degrees)4s excited at 801.478 nm, in the dense plasma r ...

Understanding deposition rate loss in high power impulse magnetron sputtering: I. Ionization-driven electric fields

The lower deposition rate for high power impulse magnetron sputtering (HiPIMS) compared with direct current magnetron sputtering for the same average power is often reported as a drawback. The often invoked reason is back-attraction of ionized sputtered material to the target due to a substantial negative potential profile, sometimes called an extended presheath, from the location of ionization toward the cathode. Recent studies in HiPIMS devices, using floating-emitting and swept-Langmuir probes, show that such extended potential profiles do exist, and that the electric fields Ez directed toward the target can be strong enough to seriously reduce ion transport to the substrate. However, they also show that the potential drops involved can vary by up to an order of magnitude from case to case. There is a clear need to understand the underlying mechanisms and identify the key discharge variables that can be used for minimizing the back-attraction. We here present a combined theoretical and experimental analysis of the problem of electric fields Ez in the ionization region part of HiPIMS discharges, and their effect on the transport of ionized sputtered material. In particular, we have investigated the possibility of a ?sweet spot? in parameter space in which the back-attraction of ionized sputtered material is low. It is concluded that a sweet spot might possibly exist for some carefully optimized discharges, but probably in a rather narrow window of parameters. As a measure of how far a discharge is from such a window, a Townsend product ?Townsend is proposed. A parametric analysis of ?Townsend shows that the search for a sweet spot is complicated by the fact that contradictory demands appear for several of the externally controllable parameters such as high/low working gas pressure, short/long pulse length, high/low pulse power and high/low magnetic field strength.

Spokes and charged particle transport in HiPIMS magnetrons

Two separate scientific communities are shown to have studied one common phenomenon, azimuthally rotating dense plasma structures, also called spokes, in pulsed-power E × B discharges, starting from quite different approaches. The first body of work is motivated by fundamental plasma science and concerns a phenomenon called the critical ionization velocity, CIV, while the other body of work is motivated by the applied plasma science of high power impulse magnetron sputtering (HiPIMS). Here we make use of this situation by applying experimental observations, and theoretical analysis, from the CIV literature to HiPIMS discharges. For a practical example, we take data from observed spokes in HiPIMS discharges and focus on their role in charged particle transport, and in electron energization. We also touch upon the closely related questions of how they channel the cross-B discharge current, how they maintain their internal potential structure and how they influence the energy spectrum of the ions? New particle-in-cell Monte Carlo collisional simulations that shed light on the azimuthal drift and expansion of the spokes are also presented.

3D modelling of negative ion extraction from a negative ion source

The development of a suitable negative ion source constitutes a crucial step in the construction of the neutral beam injector of ITER. To fulfil the ITER requirements in terms of heating and current drive, the negative ion source should deliver 40 A of D−. The achievement of such a source constitutes a technical and scientific challenge, and it requires a deeper understanding of the underlying physics of the source. The present knowledge of the ion extraction mechanism from the negative ion source is limited. It constitutes a complex problem that involves understanding the behaviour of magnetized plasma sheaths when negative ions and electrons are pulled out from the plasma. Moreover, due to the asymmetry induced by the crossed magnetic configuration used to filter the electrons, any realistic study of this problem must consider the three spatial dimensions. To address this problem in a realistic way, a 3D particles-in-cell electrostatic code specifically designed for this system was developed. The code uses a Cartesian coordinate system and it can deal with complex boundary geometry as it is the case of the extraction apertures (Hemsworth et al 2009 Nucl. Fusion 49 045006). The complex magnetic field that is applied to deflect electrons is also taken into account. This code, called ONIX, was used to investigate the plasma properties and the transport of negative ions and electrons close to a source extraction aperture. Results in the collisionless approach on the formation of the plasma meniscus and the screening of the extraction field by the plasma are presented here, as well as negative ions trajectories. Negative ion extraction efficiency from volume and surfaces is discussed.

Time resolved metal line profile by near-ultraviolet tunable diode laser absorption spectroscopy

Pulsed systems are extensively used to produce active species such as atoms, radicals, excited states, etc. The tunable diode laser absorption spectroscopy (TD-LAS) is successfully used to quantify the density of absorbing species, but especially for stationary or slow changing systems. The time resolved-direct absorption profile (TR-DAP) measurement method by TD-LAS, with time resolution of μs is proposed here as an extension of the regular use of diode laser absorption spectroscopy. The spectral narrowness of laser diodes, especially in the blue range (∼0.01 pm), combined with the nanosecond fast trigger of the magnetron pulsed plasma and long trace recording on the oscilloscope (period of second scale) permit the detection of the sputtered titanium metal evolution in the afterglow (∼ms). TR-DAP method can follow the time-dependence of the temperature (Doppler profile) and the density (deduced from the absorbance) of any medium and heavy species in a pulsed system.

On the anisotropy and thermalization of the metal sputtered atoms in a low-pressure magnetron discharge

Space-resolved velocity distributions of titanium atoms sputtered in a direct-current (dc) magnetron discharge working at low pressure (0.4 Pa) were investigated using the laser-diode–induced fluorescence technique. A blue-light laser diode, covering the Ti 3d24s2-3d2(3F)4s4p(1P0) transition λ0=398.289 nm, was used to excite neutral Ti atoms and to measure their Doppler absorption profile through the induced fluorescence signal. Taking advantage of the very narrow laser linewidth, energetic and thermalized atoms can be clearly distinguished. Moreover, flux distributions of sputtered atoms along the laser direction were deduced and discussed.

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.

Extracted current saturation in negative ion sources

The extraction of negatively charged particles from a negative ion source is one of the crucial issues in the development of the neutral beam injector system for future experimental reactor ITER. Full 3D electrostatic particle-in-cell Monte Carlo collision code—ONIX [S. Mochalskyy et al., Nucl. Fusion 50, 105011 (2010)]—is used to simulate the hydrogen plasma behaviour and the extracted particle features in the vicinity of the plasma grid, both sides of the aperture. It is found that the contribution to the extracted negative ion current of ions born in the volume is small compared with that of ions created at the plasma grid walls. The parametric study with respect to the rate of negative ions released from the walls shows an optimum rate. Beyond this optimum, a double layer builds-up by the negative ion charge density close to the grid aperture surface reducing thus extraction probability, and therefore the extracted current. The effect of the extraction potential and magnetic field magnitudes on the extraction is also discussed. Results are in good agreement with available experimental data.