Unveiling the role of dielectric trap states on capacitively coupled radio-frequency plasma discharge: dynamic charging behaviors

The influence of charge trap states in the dielectric boundary material on capacitively coupled radio-frequency (RF) plasma discharge is investigated with theory and particle-in-cell/Monte Carlo collision simulation. It is found that the trap states of the wall material manipulated discharge properties mainly through the varying ion-induced secondary electron emission (SEE) coefficient in response to dynamic surface charges accumulated within the solid boundary. A comprehensive SEE model considering surface charging is established first, which incorporates the valence band electron distribution, electron trap density, and charge trapping through Auger neutralization and de-excitation. Theoretical analysis is carried out to reveal the effects of trap states on sheath solution, stability, plasma density and temperature, particle and power balance, etc. The theoretical work is supported by simulation results, showing the reduction of the mean RF sheath potential as charging-dependent emission coefficient increases. As the gas pressure increases, a shift of the maximum ionization rate from the bulk plasma center to the plasma-sheath interface is observed, which is also influenced by the trap states of the electrode material where the shift happens at a lower pressure with traps considered. In addition, charge traps are proven to be helpful for creating asymmetric plasma discharges with geometrically symmetric structures; such an effect is more pronounced in gamma-mode discharges.

A search for heavy long-lived staus in the LHCb detector at √s = 7 and 8 TeV

Thi Viet Nga La

The Large Hadron Collider (LHC) has been producing pp collisions at 7 and 8 TeV since 2010 and promises a new era of discoveries in particle physics. One of its experiments, the Large Hadron Collider beauty (LHCb) experiment, was constructed to study CP violation in the B meson system. In addition to B physics, new Physics beyond the Standard Model can also be searched for at this single-arm forward spectrometer. With the different sub-detectors and the high resolution of the tracking system, the LHCb detector has the ability to search for heavy, long-lived and charged particles, which are predicted by extensions of the Standard Model. One of these extensions, the minimal Gauge Mediated Supersymmetry Breaking (mGMSB), proposes such a particle, named stau (τ~) - the SUSY bosonic counterpart of the heavy lepton tau (τ). The theory proposes that the staus may be pair-produced in pp collisions or in the decays of heavier particles, and have only electromagnetic interactions with the atoms of the medium like the muons. Therefore, we expect that at the energy of the LHC these particles can be produced if they do exist and that we have a chance to discover them at LHCb, as well as at the other experiments of the LHC. This thesis is dedicated to the search for stau pairs produced in pp collisions at the centre-of-mass energies √s = 7 and 8 TeV in the LHCb detector. For this purpose, we generated the stau pairs with seven different particle masses ranging from 124 to 309 GeV/c2 and simulated their path through the LHCb detector, as well as their muon background from the decays Z0, γ∗ → μ+μ−. Based on the results from the simulation, a set of cuts are then defined to select the stau pairs. Some muon pairs at high energies will also pass the selection cuts. Thus, to separate the stau pairs from the muon pairs, the Neural Network technique has been used. A first Neural Network has been used to distinguish the stau tracks from the muon tracks using their signals left in the sub-detectors: the VELO silicon detector, the electromagnetic calorimeter, the hadron calorimeter and the RICH detectors. Then, two methods to select the stau pairs have been developed: the first one is based on the product of the two responses from the first Neural Network (NN1) for the two tracks, the second one employs a second Neural Network to separate the stau pairs from the muon pairs by using the above product of the two NN1 responses and the invariant mass of pair. Finally, a favourable region for the staus finding has been defined and the expected numbers of stau and muon pairs in this region have been evaluated. The training of the Neural Network has been achieved with the Monte Carlo variables, then the trained Neural Network has been used to classify the data. The data used in our work were collected by the LHCb experiment in 2011 and 2012 and correspond to integrated luminosities of 1 fb−1 at √s = 7 TeV and of 2 fb−1 at √s = 8 TeV. No significant excess of signal has been observed. Upper limits at 95% CL on the cross section for stau pair production in pp collisions at √s = 7 and 8 TeV have been computed by using the profile likelihood method, which is derived from the well known Feldman and Cousins method.

EPFL2013

Etude de l'origine et de la croissance de particules submicrométriques dans les plasmas radiofréquence réactifs

Christian Deschenaux

The formation of sub-micron sized particulates has been studied in a low pressure (0.1 mbar) radio-frequency (13.56 MHz) capacitively coupled plasma discharge. Particles are studied in situ by infrared absorption spectroscopy, laser illumination and Cavity Ring Down Absorption Spectroscopy. Transmission Electron Microscopy was applied ex situ for particulates collected during the plasma. We propose that two principal mechanisms may be involved in the formation of these particulates: formation and trapping of negative ions in the gaseous phase, and the recycling of material deposited on the electrodes during the discharge, by film erosion or delamination. The relative importance of these two mechanisms was studied in silane, methane, ethylene and acetylene plasmas. The growth of the particulates in silane plasmas, monitored with Infrared and Cavity Ring Down Absorption Spectroscopy, allowed us to distinguish an increasing volume growth phase followed by an agglomeration phase at constant volume, in agreement with the literature. A slight powder and film recycling effect is detected as the experiments are repeated, as observed by an increasing particle size. The hydrogen content of powder formed in silane plasmas was estimated in situ by infrared absorption spectroscopy and changes in the infrared absorption spectrum could be observed for silane diluted with hydrogen and argon. In particular, the porous structure of the powder was observed from the high content of surface related vibration bonds relative to bulk related bonds. Photoluminescence was detected for the particles at 2.15eV that could possibly be attributed to the crystalline structure of the particles. Infrared Absorption Spectroscopy and mass spectrometry applied in situ showed that acetylenic compounds are preferentially produced in methane, ethylene and acetylene plasmas and that the hydrogen content of the other species produced in these plasmas depends on the initial gas hydrogen content. Negative ions were evidenced in these plasmas and a polymerization reaction is proposed between acetylene and C2nHx- negative ions to explain the formation of particle's precursors. The formation of aromatic structures is suggested for n>3. The different relative abundance of C2Hx- ions is proposed to explain the different powder production rates with different initial gases or different fluxes. Hydrogen may inhibit the formation of high-mass negative ions and particles. Fast growth of spherical amorphous carbon particles (∼100s) was observed for a low pressure (0.1 mbar) acetylene plasma. The sp3 structure was confirmed in situ by infrared absorption spectroscopy. Slow growth (∼1000s) of irregularly shaped particles is observed for methane plasmas at the same pressure and power. Particle heterogeneity was studied with Transmission Electron Microscopy and showed graphitic onions, multiwalled nanotubes, flakes and agglomerates attributed to the erosion or the delamination of the deposited carbon layer. The influence of the recycling of the deposited layer on the powder formation was observed in situ from enhanced particle formation as the experiments were repeated. A similar behavior was observed for discharges containing mixtures of hexamethyldisiloxane, oxygen and helium. Keeping in mind that the control of powder homogeneity implies a control over the reactor contamination and its history, we conclude that an important part of the particulates formed in the discharges studied depends on the gas phase reactions, determined by the plasma chemistry.

EPFL2002

Modelling of sawtooth-induced fast ion transport in positive and negative triangularity in TCV

Marcelo Baquero Ruiz, Basil Duval, Alexander Karpushov, Antoine Pierre Emmanuel Alexis Merle, Dmytry Mykytchuk, Mario Podesta, Olivier Sauter, Lorenzo Stipani, Duccio Testa, Matteo Vallar

Internal kinks are a common magneto hydro-dynamic (MHD) instability observed in tokamak operation when the q profile in the plasma core is close to unity. This MHD instability impacts both the transport of the bulk plasma (current, particle and energy transport) and minority species, such as fast ions. In tokamak a configuration variable (TCV) (R (0)/a = 0.88 m/0.25 m) the fast ion population is generated in the plasma by neutral beam tangential injection of energies up to 28 keV. TCV features 16 active shaping coils permitting a great flexibility in plasma shape, including negative triangularity (delta) configurations that show surprisingly high confinement. This study focuses on the transport of fast ions induced by sawteeth, by comparing two triangularity cases and simulation results with experimental data. Comparison of two equilibria with opposite delta shows that the fast ion drifts are larger for delta < 0. Furthermore, the sawtooth-induced transport in this case is larger than delta > 0 in similar conditions. Comparison with experimental data confirms the dominance of the modification of thermal kinetic profiles following the sawtooth crash in explaining drops in the neutron rates and fast ion D-alpha signals. Additional fast ion diffusion, however, improves the interpretation of the experimental data. For delta < 0, the amplitude of the perturbation better representing the experimental data is larger. Finally, an exploratory study for 50 keV particles (soon available in TCV) shows that the situation does not worsen for such particles.