Atomic and molecular spectra emitted by normal liquid ⁴He excited by corona discharge

The liquid ⁴He at fixed temperature 4.2 K and different pressures up to 8 MPa was excited by corona discharge of both negative and positive polarity. Emission of He I atomic lines and He₂ molecular bands are observed. In negative corona the lines spectra show a distinct blue-shift and line-broadening, which becomes stronger with the pressure increasing. The rotational structure of molecular bands is resolved at pressures (0.1–0.2) MPa. The blue shift of the Q-branch maximum at different pressures was observed. Rotational temperature of 900 K has been estimated for the d³Σ⁺u-b³Πg molecular band. A positive corona was realized on a point anode for fewer radii of the electrode and larger voltage than in the negative corona. Electric currents in both negative and positive corona differ weakly. Spectral analysis of the radiation from the positive corona shows qualitative differences of spectral features of these discharges. The spectra observed in the positive corona have marked nonsymmetric shape. The asymmetric atomic and molecular spectra show an increased intensity of their long-length (red) wings

Cathode sheath formation of corona discharge in liquid helium

5ème Conférence de la Société Française d'Electrostatique Grenobl

Spectroscopic study of the negative corona discharge in liquid and supercritical 4He

International audienc

Experimental and theoretical characterization of the long-range interaction between He*(3s) and He(1s)

International audienceThe long-range interaction between S-{1,S-3} He*(3s) and S-1 He(1s) was studied in bulk liquid helium by fluorescence measurements and by combined theoretical electronic structure and bosonic density functional theory calculations. The excited He* atoms were produced in the liquid by corona discharge with subsequent impact excitation instigated by hot electrons from the discharge. The long-range contribution to the repulsive "hump" near 5 angstrom in the S-{1,S-3} He-S-1 He potentials was interrogated by monitoring He* S-{1,S-3}(3s) -> P-{1,P-3}(2p) fluorescence profile characteristics as a function of external pressure between 0.1 and 3.5 MPa. Fluorescence line shifts and widths as a function of pressure were extracted from the experimental data and compared to the theoretical predictions, establishing that the nascent He* atoms reside in a bubble state within the liquid. It was observed that the experimental data could only be consistently reproduced if the excited He* atoms emit in a less-dense environment as compared with the rest of the bulk liquid

New spectroscopic study of liquid helium

International audienc

Spectroscopic study of the negative corona discharge in liquid helium

International audienc

Spectroscopic study of negative corona discharge in liquid helium

International audienc

Ionization phenomena in liquid helium by the corona discharge

5ème Conférence de la Société Française d'Electrostatique Grenobl

Modelling electroluminescence in liquid argon

We present Monte-Carlo simulations of electron transport through liquid argon motivated by our recent observation of electroluminescence light emanating from a thick gaseous electron multiplier (THGEM) in a liquid argon volume. All known elastic and inelastic reaction cross-sections have been accounted for, providing electroluminescence light yield predictions for arbitrary electrostatic fields. This study concludes that the large field gradients needed to produce electroluminescence cannot be accounted for by straightforward electrostatic field calculations based on ideal THGEM holes, suggesting that further experimental investigations are required

Mechanisms of impulse breakdown in liquid: the role of Joule heating and formation of gas cavities

The impulse dielectric behaviour of insulating liquids is of significant interest for researchers and engineers working in the field of design, construction and operation of pulsed power systems. Analysis of the literature data on transformer oils shows that potentially there are several different physical processes which could be responsible for dielectric breakdown by sub-microsecond and microsecond impulses. While for short, sub-microsecond impulses ionisation (plasma streamer) is likely to be the main breakdown mechanism, for longer impulses, thermal effects associated with Joule heating start to play an important role. The present paper is provides a theoretical analysis of the latter mechanism in dielectric liquids of different degrees of purity stressed with high voltage impulses with duration sufficient to cause local heating, evaporation and formation of pre-breakdown gas bubbles. The proposed model is based on the assumption that dielectric breakdown is developed through percolation channels of gas bubbles and the criterion of formation of these percolation chains is obtained. In order to test the developed model, the breakdown field-time characteristics have been calculated for the liquid with chemical composition close to that of transformer oils but with known thermodynamic characteristics (n-hexane). Its dielectric strength has been obtained as a function of externally applied pressure and temperature. The analytical results show a good agreement when compared with the experimental data available in the literature