CONDICIONES DEL RENDIMIENTO ACADÉMICO

4.2.1 Electrical Factors Influencing Discharge Stability

It is well known that adequate preionisation is essential for the formation of a high pressure glow discharge and that to maintain the glow discharge for as long as possible certain criteria concerning electron number density and homogeneity must be met. These criteria and the methods of preionisation currently in use are covered fully in chapter 1.

Chapter 4

from spark arrays combined with various masks results in a glow discharge forming only in the regions of high preionisation electron density. In regions of low or no preionisation either a filamentary discharge or no discharge at all forms. Similar results were obtained by S Sumida et a l, (8), using X-ray preionisation and by Taylor, (9), using a KrF laser beam to preionise a XeCl laser by shining along the optical axis of the main laser. The advantage of this second method is that the preionisation spatial distribution can easily be precisely defined. Interestingly it shows that in the direction transverse to the current flow it is possible to produce regions of uniform and filamentary discharge. This implies that inhomogeneities in the initial preionisation influence the nature of the glow discharge and that localised preionisation deficiencies can persist throughout the duration of the discharge. It is not clear however how these discharge non-uniformities drive instabilities that result in discharge collapse. Taylor, (9), has noted that filamentary instabilities tend to emerge at the sharp boundaries of glow discharge regions corresponding to the edges of preionised and unpreionised regions. The explanation provided points to the strong preionisation electron density gradient and the resultant local electric field transverse to the applied electric field. Making the boundaries between the preionised and unpreionised regions less sharp resulted in improved performance.

Another factor that has been shown to influence glow discharge stability is the electrical pump power density. While the optimum power deposition rate for efficient lasing in XeCl* is believed to be between 300 and 700 kWcm-3, (10), very high rates of specific energy extraction have been achieved at pump power densities in excess* of 10 MWcra'3, (11). Although pumping efficiency in kinetic terms is reduced at high pumping rates the major drawback with this technique is the short pulse duration, -40 ns FWHM, believed to be due to premature discharge collapse.

A detailed study of discharge stability and energy loading in the laser gas is given by Taylor, (9), and shows that filament growth leading to discharge collapse progresses at

* Estimated from figures given in paper

a faster rate with higher discharge current densities. For the particular system described the rate of filament growth proceeds linearly with increasing current density at current densities less than 200 Acm-2 and less than linearly thereafter. This saturation effect at higher current densities is explained by Taylor as possibly being due to halogen depletion effects; the role of halogen donors in stability will be discussed subsequently.

Poor discharge stability resulting in short duration laser pulses has been confirmed by other workers, (12), although direct comparisons are hard to make because of other differing factors such as different discharge geometries between machines. In general, from analysis of other workers results, long pulse durations tend to be obtained from systems with low pump power deposition rates, which is not altogether surprising.

The laser used for the experiments described in this thesis had a relatively short gain length of only 25 cm implying that for optimization in terms of output energy the small signal gain needs to be higher than in systems with longer gain lengths. The small signal gain coefficient is increased by pumping the laser at higher pump power densities, because of the increased upper state formation rate, and consequentially the laser output energy will be optimized at a relatively high pumping rate. The connection between high pumping rates and discharge instability helps to explain why short gain length systems tend to have shorter duration optical pulses. Despite the parametric survey of operating conditions carried out in chapter 2, with the laser system performance optimized for pulse energy it was difficult to obtain pulse durations much in excess of 100 ns FWHM. In terms of specific energy extraction typical outputs were in the region of 3 Joules per litre which is respectable in comparison with other systems. For comparison the long pulse duration experiments carried out by Taylor and Leopold, (13), in which the pump power rate was deliberately kept low yielded specific energy extractions of at best 1.5 Joules per litre and much lower for the longer duration pulses.

Chapter 4 4.2.2 Halogen Donor Depletion model of Instability

It is well known that the presence of the highly electronegative halogen donor in an excimer gas mix strongly influences discharge stability. It is generally believed that the presence of some form of electron attaching species is necessary to sustain a stable discharge by balancing electron production with a loss mechanism in order to produce a steady state. Without the electron loss processes due to halogen donor dissociatve

attachment it is believed that electron production mechanisms such as ionisation of rare gas excited metastables would cause the discharge current to rapidly increase, leading to the formation of high conductivity localised filaments and eventually full scale arcs.

An instability mechanism for an electron beam sustained KrF* laser was first proposed by Daugherty et al, (14), The theory states that without the electron loss mechanism provided by the dissociative attachment of electrons with F2, electron production can accelerate by ionisation of Kr* metastables to Kr+ ions. The ionisation potential of Kr* is obviously much smaller than Kr atoms and the reaction rate constant is orders of magnitude larger. The electronegative halogen donor molecules therefore play an important role in preventing the electron density from increasing out of control and the consequent formation of filamentary channels leading to discharge collapse. In this way increasing halogen donor concentration should lead to more stable discharges.

It was discovered experimentally however that increasing the halogen donor concentration tends to hasten the onset of filamentary instabilities in contradiction to the theory of Daugherty. A related mechanism by which halogen donors could increase the rate of discharge collapse, known as the local halogen depletion model, was first published by C. E. Webb et al, (15).

It was believed that localised inhomogeneities in the electron distribution could bum-up the F2 causing a similarly localised failure of the F2 to limit the the electron growth rate. By this positive feedback mechanism electron production can grow unchecked in a small region leading to the formation of a filament and eventual discharge collapse. An analogous process was proposed for the other rare gas halide lasers.

A more rigourous mathematical treatment of this mechanism is given by Coutts and Webb, (16), for the XeCl* system. Starting from the electron continuity equation:

(d/dt)ne= riekml [Xe*] - kane [HCl]t 4.1

where kmi and k^ are ionisation and dissociation attachment constants respectively and assuming all electron production to be due to ionisation of Xe* metastables an index of the glow phase duration, t, can be derived. If T is defined as the time taken for the initial glow discharge electron density, <neo>, to double then: