Velocity measurements in plasmas: Issues and prospects.
Résumé
The simultaneous measurements of various parameters (temperature, velocity, density, pressure) in different locations of a flow give the opportunity to complete one-shot mapping of plasma flows. Techniques were developed in high velocity cold flows and the application to plasmas includes some issues. Several techniques will be presented (Doppler LIF, FLEET, KTV) but a focus will be made on the VENOM tagging technique.
The principle is to carry out a first excitation of NO by a 226 nm laser pulse (γ 0-0 band) in order to obtain a first fluorescence reference and to produce some vibrationally excited molecules. Those molecules, convected by the flow, are then probed by a second laser pulse at 224 nm (γ 1-1 band) to obtain a second fluorescence reference. Knowing the delay between both laser pulses and the distance between both fluorescence references gives the local flow velocity.
A simple four-level model neglecting rotational and vibrational energy transfers for short times model allows to understand the population dynamics.
At 3000 K, the same calculation shows that the NO(X,v=1) population is almost not increased by the first laser excitation. The explanation is to be found in the presence of native NO(X,v=1) in the plasma due to the thermal excitation. That native population tends to shift the equilibrium and avoids the overpopulation.
A closer look to the ratio between NO(X,v=1) at the beginning of the experiment and after the first laser excitation demonstrate that the population increase can hardly exceed 5% at T=3000 K.