Simulation of the discharge propagation in a capillary tube in air at atmospheric pressure

International audienceThis paper presents simulations of an air plasma discharge at atmospheric pressure initiated by a needle anode set inside a dielectric capillary tube. We have studied the influence of the tube inner radius and its relative permittivity ε r on the discharge structure and dynamics. As a reference, we have used a relative permittivity ε r = 1 to study only the influence of the cylindrical constraint of the tube on the discharge. For a tube radius of 100 µm and ε r = 1, we have shown that the discharge fills the tube during its propagation and is rather homogeneous behind the discharge front. When the radius of the tube is in the range 300 to 600 µm, the discharge structure is tubular with peak values of electric field and electron density close to the dielectric surface. When the radius of the tube is larger than 700 µm, the tube has no influence on the discharge which propagates axially. For a tube radius of 100 µm, when ε r increases from 1 to 10, the discharge structure becomes tubular. We have noted that the velocity of propagation of the discharge in the tube increases when the front is more homogeneous and then, the discharge velocity increases with the decrease of the tube radius and ε r. Then, we have compared the relative influence of the value of tube radius and ε r on the discharge characteristics. Our simulations indicate that the geometrical constraint of the cylindrical tube has more influence than the value of ε r on the discharge structure and dynamics. Finally, we have studied the influence of photoemission processes on the discharge structure by varying the photoemission coefficient. As expected, we have shown that photoemission, as it increases the number of secondary electrons close to the dielectric surface, promotes the tubular structure of the discharge

Teachers' beliefs about multilingualism : findings from Q method research

This study explores teachers’ beliefs about multilingualism and multilingual students in Swedish primary schools. The aim is to support a better-informed discussion about teachers’ decision- making in linguistically diverse classrooms in the European nation- state. The use of Q method combines qualitative and quantitative data analyses. Q material applied in the present study provided the participants with all the necessary language to describe their beliefs. Two Q sets of statements – one regarding the understanding of the phenomenon and the other concerning suggested pedagogical responses in relation to current multilingualism – were constructed using a variety of sources. The participants are forty teachers, predominantly female, employed at three different primary schools in southern Sweden. Applying inverted factor analysis and abductive interpretation, three sets of teachers’ beliefs emerged and descriptions represent the teachers’ complex views about multilingualism in the classroom. Overall, teachers’ beliefs are rather welcoming towards multilingualism and multilingual students and recent concepts with growing acceptance in literature, such as translanguaging, are well accepted. However, sceptical views, often based on monolingual ideologies are present and are likely to pose challenges for the implementation of pluralistic policies. This study contributes to an open debate about benefits and challenges of current multilingualism in education