Laser induced fluorescence studies of a hydrogen plasma in a small tokamak

Abstract

Plasma properties such as the density of the various species present in the plasma and the temperature of these species are of importance in the understanding of plasma behaviour. Many techniques have been developed to measure these plasma parameters. This thesis is concerned with one such technique, namely laser induced fluorescence, (LIF). In LIF a laser is tuned to the atomic spectral transition of interest. This excites atoms from the lower to the upper energy level of the transition thus increasing the population density of the upper level. The enhancement of the upper level population density gives rise to an enhancement of the spontaneous emission from this level which can be observed against the normal (no laser excitation) spontaneous emission from the plasma.

Unlike conventional spectroscopic diagnostic techniques [Hu65] the HF signal gives information about only that volume of plasma. at the intersection of the laser beam and the optical axis of the detection system, thus providing a local measurement. If the laser power is sufficiently large the transition being pumped will be saturated: that is the ratio of population density of the lower and upper levels of the transition will be driven to the ratio of their statistical weights. This simplifies analysis of the data as the fluorescence is then independent of the laser power. Another advantage of saturation is that the enhancement of the upper level population density, and hence the LIF signal, is at its maximum when the transition is saturated.

Because the population density of any one level is coupled by collisional and radiative processes to the population density of other levels a change in population density of one level, caused for example by laser excitation, leads to changes in the population density of other levels. Any change in population density of a level leads to a change in the spontaneous emissions from that level so that LIF signals can be observed on transitions other than the atomic transition to which the laser is tuned. The processes taking place in the plasma which cause transitions between these levels can be investigated by monitoring the LIF signal from the pumped level and also from an adjacent level.

In order to predict the intensity of spontaneous emission from the plasma it is necessary to determine the population density of each of the electron energy levels in the atoms which are in the plasma. If the plasma is in thermal equilibrium then the plasma is uniform and population densities are given by the Boltzman equation. If there are temperature gradients and other inhomogeneities in a plasma it may still contain regions which are in local equilibrium. For a plasma in Local Thermodynamic Equilibrium (LTE) the densities in specific quantum states are those pertaining to a system in complete thermodynamic equilibrium which has the same total mass density, temperature and chemical composition as the actual system[Gr64 ]. A plasma will be in LTE if the collisional processes dominate the radiative processes in determining population densities. If the plasma is in neither LTE nor thermal equilibrium the population density may be found using a Collisional—Radiative Model(CRM). In a CRM all the collisional and radiative processes which cause a change in the population densities of the energy levels are taken into account. CRM’S will be discussed at length later in this thesis.