Furthering fundamental physics by studying plasma

An image to illustrate
Fig. 1 Image of High Power Impulse plasma (> 10 kW/cm2)

Professor Tiberiu Minea outlines plasma as the reaction of matter receiving high energy and describes studies taking place at the Laboratory of Physics of Gases and Plasmas.

The Laboratory of Physics of Gases and Plasmas (LPGP) is a fundamental research unit focused on the physics of the gaseous state of matter, mainly ionised, but also activated by external energy. On the one hand, it studies hot and dense matter issued from high energy beams (laser or particles) and its interaction with a target (gaseous or solid), but it also studies the elementary processes involved in gas ionisation and energy transfer.

On the other, its research tackles current issues in areas such as bio-medicine, energy conversion, space transportation, thin films, etc. As its name suggests, since it was founded in 1965 LPGP has been closely related to laboratory studies of plasma, either totally ionised to produce energy such as fusion (inertial or magnetically confined) or partially ionised gases – cold plasmas – which are also often related to technological developments.

LPGP is a joint unit of National Council for Scientific Research (CNRS) and University Paris-Sud/Paris-Saclay. The laboratory’s physicists (of which there are about 30) are CNRS researchers or professors with share-time research activities. LPGP is involved in educational activities at all levels – Bachelor, Master’s, and engineering schools – with about 20 M2 students and PhDs per year.

In the field of laboratory plasma and related processing, the LPGP is partner of two Laboratoire d’Excellence of University Paris-Saclay: PALM (Physics of Atom, Light and Matter) and LaSIPS (Laboratory of Science for Engineering of Paris-Saclay). These two research areas also define the main activities at the LPGP:

• The fundamental research aiming to better understand the ionised state of matter, including the associated states (atoms, molecules, excited states, photons and their interaction with the matter, from high energy to thermal species) and, based on this deep understanding
• The wide domain of industrial applications of discharges, answering such societal topics as energy production/conversion, environment, nanometrology, security, bio-medicine, transport, micro- and nanoelectronics, innovative materials etc.

Plasma-laser acceleration

Laser-driven acceleration of electrons in plasmas is a promising approach for the development of compact accelerators at relativistic energies. Generated electron beams have unique properties and exit the plasma as ultra-short bunches with kA current.

The ITFIP team of LPGP is actively involved in the development of multi-stage laser plasma accelerators, through theory, simulations and experiments, with the goal to demonstrate controlled and scalable acceleration to high energy. This includes contributions to the physics of electron injection, trapping and acceleration in plasma waves. Tailoring the plasma profile is particularly crucial both for optimising the properties of electron sources, and for achieving high quality intense laser guiding over distances much larger than the diffraction length.

The expertise of the team extends from atomic physics in plasmas to non-linear laser plasma interaction, optics and radiation generation and propagation in dispersive media.

The ITFIP team is leading national (GdR APPEL) and international groups: the plasma development working group in the EuPRAXIA project, an activity preparing the conceptual design of a 5 GeV accelerator based on advanced plasma technology that will maintain Europe at the forefront of laser based advanced accelerators concept. The ITFIP team is part of the ARIES project, integrating accelerator activity in Europe and reaching out to advanced technology as well as the most advanced applications for society.

Challenging long term perspectives are addressed within the ALEGRO study group, bringing together the international community working on advanced acceleration techniques to propose alternative options to accelerate electrons or positron to energies relevant for high energy physics, from hundreds of GeV to multi-TeV.

Cold plasmas for high energy

Non-equilibrium discharges and related phenomena such as thermo-field or laser assisted electron emission are of central importance for high energy physics. These topics are addressed at LPGP, mainly via numerical modelling, in relation to charged particles (positive and negative ions or electrons) and their extraction from the plasma, the gas attenuation of synchrotron radiation, very high (~1MV) DC voltage holding for ITER, high power switches, metal additive manufacturing etc.

Jointly, TMP-DS and ITFIP teams aim to develop a capillary micro-wave micro-discharge to guide the laser radiation and improve the energy transfer in plasma-laser acceleration.

Kinetics and Plasma-jets

The physics of non-equilibrium discharges at (near-)atmospheric pressure and their applications constitute the core of the research activities of the DIREBIO team.
One focus is on the kinetics of organic molecules, hydrocarbons (HC) or Volatile Organic Compounds (VOCs), diluted in atmospheric gas mixtures, aiming at the reduction of polluting emissions (industry, transport, etc.) and the triggering of combustion. The objectives are to obtain and validate the basic data concerning the collisional processes and the elementary reactivity involved in their degradation by pulsed discharge plasma. Precisely, it concerns the description of the conversion kinetics of these organics for different types of discharges, producing either a perfectly homogeneous plasma (photo-triggered discharge), or a diffuse plasma (corona discharge), or a filamentary plasma (dielectric barrier discharge).

In recent years, important achievements have been accomplished concerning:

• The dissociation of HC and VOC by the excitation transfer process of the metastable electronic states of the nitrogen molecule, and the plasma-driven competition between the oxidation reactions and dissociation processes of the molecules
• The physics of the corona discharges created by very high voltage pulses (50 kV and more) with very fast growth front and very short duration (10ns) which generate a diffuse plasma, not completely homogeneous but not filamentary, in air at atmospheric pressure

Another issue is the physics of small-scale discharges and the generation of reactive species by so-called ‘micro-discharges’. The research focuses on: Micro Hollow Discharge Cathode (MHCD), Micro Cathode Sustained Discharge (MCSD), and ‘plasma jets’. Applications concern in particular the treatment of cancer cells (oncology), the detection of traces of weakly volatile molecules on surfaces (global safety), and the development of new materials (electronics).

Recently, the DIREBIO team also tackled ‘plasma jets’ in rare gases, helium or argon, and their interactions with liquid or solid surfaces. These jets are a particular type of micro-discharges that are drawn much attention. The spatio-temporal mapping of metastable excited state densities of atoms, at very small scales (10μm, ns), by infra-red laser absorption spectroscopy is certainly one of the keys to understand the energy transport in these jets.

Further, their application in oncology is a question of identifying the reactive species and reaction routes, in gas and in liquid, created by the plasma and its effect on the cell viability. As typical result, our findings has suggested that the efficiency of plasma treatment strongly depends on the combination of H2O2 and the NO2- anion in determined concentrations. We also showed that the interaction of the He plasma jet with the ambient air is required to generate anions in solution.

Plasma-based aerosol process are proposed to achieve targeted potentialities of nanotechnologies for materials (production and coating processes) and environment (diagnostic, filtration). Based on controlled properties of non-thermal atmospheric pressure plasma, new applications of electrical discharges are tested to establish proofs-of-concept, eventually patented. Using our plasma and aerosol characterisation platforms, products properties (nanoparticles and gases) are defined for different plasma energies and related ionising and reactive properties.

Materials
In the field of materials, plasma are used for the production of nanoparticles with tuneable properties (size, shape, composition and structure). These tailored nanoparticles first suspended in gases can then be coated for either integration in volume or collection on surface of so-formed nanomaterials. Both bottom-up and top-down strategies have been validated for the production of pure and composite core-shell nanoparticles from any material (Metals, MOx, semi-conductor, polymers) as well as SiOx and polymer films; the first by gas-to-particle conversion also called nucleation/condensation in Plasmas and the second by droplet evaporation using monodisperse Electro-sprays.

Environmental applications

For environmental applications, plasmas also apply to unipolar and to bipolar charging of aerosol (sub-micron sized particles in gas). The interest of LPGP’s plasma-chargers (two patents, 2012), initially funded by French topic-oriented institutes, is confirmed by European and Taiwanese SME findings and international collaborations.

In terms of societal impact, LPGP could now take part in international programmes on emerging applications of so-charged aerosols. Indeed, developments of plasma chargers are in progress worldwide for diagnostics (e.g. for asbestos fibres concentrations, size distribution of nanoparticles), for electro-processing/-patterning (coagulation for assembly or homogeneous/focused deposition for nanostructuration) and maybe more critical, for sustainable development of nanotechnologies, implying filtration of so-charged particles.

Low pressure and magnetised discharges

Magnetron and HiPIMS (High Power Impulse Magnetron Sputtering) are very convenient lab-scale ExB devices (together with Hall thrusters, drift tubes, etc.) for studying phenomena in magnetised plasmas.

Let us notice that the power densities at target above can exceed 1 kWcm-2 in HiPIMS, (see Figure 1) i.e., power values similar to the engine of an Ariane 5 rocket. It is not surprising to find plasma instabilities in the HiPIMS discharge, since any region of high electron density in a magnetic trap is prone to instabilities resulting in self-organisation and breaks in the symmetry. Hence, the TMP-DS team has recently showed, experimentally, theoretically and numerically, the relation between short (mm) and long (cm) scale instabilities which are involved in rotating dense plasma structures – called ‘spokes’ – further responsible for the acceleration of charged particle, either electron or ions.

HiPIMS is a novel plasma technique belonging to the class of Ionised Physical Vapour Deposition (IPVD) methods widely used for thin film deposition. HiPIMS plasmas exhibit a high ionisation fraction of both gas and sputtered material, leading to high kinetic ion energies, which result in thin films with superior quality compared to conventional techniques. This open an enormous field for tailoring the thin and ultra-thin film materials with specific properties (direct epitaxial growth, selective metastable phase deposition, etc.).

Microwave micro-discharges presents the particularity to provide high density plasma over a large range of pressures (1 mbar to 1 bar) in a continuous wave (CW) regime. They are very inhomogeneous discharges, with a (micro-)fluid behaviour along the capillary tubes (smallest inner diameter 50µm) but rather ballistic in the radial direction. Understanding these discharges is a challenge, particularly due to its large panel of applications in several fields like photonics (fibre laser), (micro-)source of active species (excited states, radicals, etc.), of charged particles (electron or positive/negative ions), but also for assisted combustion or localized surface treatment.

Professor Tiberiu Minea
Laboratoire de Physique des Gaz et des Plasmas
Université Paris-Sud
+33 01 69 15 66 54
tiberiu.minea@u-psud.fr
www.lpgp.u-psud.fr

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