A Langmuir multi-probe system for the characterization of atmospheric pressure arc plasmas

Abstract

The 'high-pressure' atmospheric (TIG) arc plasma is studied by means of a multi-Langmuir probe system. In order to determine the appropriate regime of operation, definitions of the plasma parameters for the description of the argon arc are considered and evaluations are presented. A description of the probe basic techniques is followed by an in-depth discussion of the different regimes of probe operation. The emphasis is put on atmospheric and flowing (arc) regimes. Probe sheath theories are compared and "Nonidealities" like cooling due to plasma-probe motion and probe emission mechanisms are then described.The extensive literature review reveals that the existing probe theories are inappropriate for a use in the TIG arc, because of 'high' pressure (atmospheric), broad range of ionization across the arc, flowing conditions, and ultimately, to the uncertainty about onset of Local Thermodynamical Equilibrium. The Langmuir probe system is built to operate in floating and biased conditions. The present work represents the first extensive investigation of electrostatic probes in arcs where the experimental difficulties and the primary observed quantities are presented in great detail. Analysis methodologies are introduced and experimental results are presented towards a unified picture of the resulting arc structure by comparison with data from emission spectroscopy. Results from different measurements are presented and comparison is made with data on TIG arcs present in literature. Probe obtained temperatures are lower than the values obtained from emission spectroscopy and this 'cooling' is attributed to electron-ion recombination. However, it is believed that probes can access temperatures regions not attainable by emission spectroscopy. Only axial electric potential and electric field are obtained because of the equipotential-probe requirement. Estimations of the sheath voltage and extension are obtained and a qualitative picture of the ion and electron current densities within the arc is given.on floating and plasma potentials and estimates of the sheath voltage are made, which are then compared with the results of Chapter 4. Chapter 13 deals with further 'electrical' parameters (current densities, electric field and electrical conductivity). In Chapter 14 several experimental results are presented together in order to gain a unified picture of the resulting arc structure and some observations about possible conduction structures within arcs are made. Finally Chapter 15, after a summary of the whole work, reports the conclusions obtained and suggests further investigations.

Investigation of low current gas tungsten arc welding using split anode calorimetry

“…Understanding these phenomena is important to improve the welding process control and applicability 1, 2 , and many investigations have focussed on the physical mechanisms 3,4 . Previous research has focussed on the current and power density distributions with weld currents >100 ampere arcs involving different experimental techniques such as spectroscopy [5][6][7][8][9][10][11][12][13] , non-invasive laser-scattering 2, [14][15][16] , arc pressure measurement [17][18][19][20] , Langmuir probe 21,22 and split anode calorimetry for GTAW 2,4,14,15,[23][24][25][26][27] and Plasma Arc Welding 28 . In addition, numerical modelling has been extensively used to simulate gas shielded tungsten arcs and their interaction with the work piece e 25,[29][30][31][32][33][34][35][36][37][38] .…”

Langmuir probe diagnostics of an atmospheric pressure, vortex–stabilized nitrogen plasma jet

“…15,16 In spite of the fact that the use of Langmuir probes was mostly restricted to low-pressure plasmas (where the colisional mean free path of charged particles is greater than the characteristic length of the probe and the perturbed region around it); they have been also employed to study high-pressure (collision-dominated) arcs. [17][18][19][20][21][22][23][24][25] In particular, the ion current branch of the probe characteristic curve was frequently employed in collision-dominated arcs, [17][18][19][20][21][22]24,25 although waveforms of the floating plasma potential signal were also analized. 23 However, the electrostatic probe's theory in atmospheric pressure plasmas is not well developed yet.…”

“…23 However, the electrostatic probe's theory in atmospheric pressure plasmas is not well developed yet. 15,[26][27][28][29] The Langmuir probe technique in atmospheric pressure arcs was used by Gick et al, 17 Fanara and Richardson, 18 and Fanara, 19,20 working with tungsten inert gas (TIG) arcs; and by Prevosto et al [21][22][23] working with high-energy density cutting arcs. In these studies, to avoid probe damage and electron emission from the probe surface, sweeping probes have been employed.…”

Characterization of Atmospheric Pressure Plasmas by a Gerdien Condenser

“…Langmuir probe technique requires that the ion production rate of plasma discharge to be sufficient so that the current intensity overcomes background noise. There have been attempts [2][3][4] to use Langmuir probes for atmospheric pressure plasmas, but the theory for atmospheric pressure plasma largely differs from the one for reduced pressure plasma.…”