APRECIACIONES DEL AUTOR AL PROCESO INMEDIATO

It is suspected that water vapour and its products, created by the action of the microwave discharge in the TR cell and the discharge at the keep-alive electrode are absorbed by the cell body. The total pressure of gas in the cell throughout the life of the cell is not known, nor are the partial pressures of the constituent gases. Maddix (1968) has measured the partial pressures of water vapour, oxygen and hydrogen, assuming that the argon partial pressure remains constant, for a TR cell containing argon and water vapour. The discharge was operational for 10 minutes, with 10 minutes recovery of the cell. The TR cells considered in this thesis have a lifetime of the order of hundreds of hours, however, and in this thesis .we are considering long-term changes in the gaseous constituents of the cells. A quartz pre-TR tube is attached to a gas filling station as shown in fig (4.10) and the tube is filled with known pressures of argon and water vapour. Quartz is chosen since it is effectively impermeable to argon and water vapour and effectively chmically inert (see section 6.8 on Surface Reactions in Chapter 6). Therefore the measured pressure in the pre-TR tube should accurately represent the pressure of gas admitted to the tube. The purpose of the series of experiments is to measure the intensities of the argon, hydrogen and oxygen spectral lines measured in the TR cell discharge for varying, known pressures of argon and water vapour

and to compare these results with those taken for the TR cells, for which the pressures of argon and water vapour present are not known (except at stages 1 and 5» hot exhaust and cold refill). Because of the differing geometries of the TR cell and the pre-TR tube, the breakdown power levels differ. Hence, the intensities of the spectral lines studied are measured at a range of power levels; 1.87 kW. 2.61 kW and 3.75 kW pulsed power, using a prf of 3 kHz and a pulse length of 1 microsecond, the same as were used for the TR cell measurements.

The intensities of the spectral lines in the discharges in the TR cell and the pre-TR tube are not compared directly; we compare the ratios of two lines. The ratios of the argon line at 6965 A to the line and to the oxygen line at 7772 A are calculated, for the results obtained using the pre-TR tube and the TR cells; also the ratio of the Ho< line to the oxygen line above-mentioned. Graphs are plotted of the above-mentioned ratios against pressure of water vapour for varying power inputs (see fig (4.11)). The pressure of argon in the TR cell is assumed to remain constant at 9.5 torr (the pressure added at filling) since argon is known to be absorbed very slowly by the metal and glass of the cell. The two sets of results are compared by noting that the water vapour pressure in the TR cell is 11 torr after hot exhaust and after cold refill. At the various stages throughout the manufacture and life of the cells, the water vapour content may be estimated and a mechanism proposed as to the effects of each stage of manufacture

4.6.2 Impurities in Pre-TR Tubes

Microwave-excited discharges in quartz pre-TR tubes containing a range of gases at a range of pressures have been studied. Table 4.2 gives a list of the gases studied and their pressures. Impurity gases are seen in the discharges of some of the pre-TR tubes; these gases have not been observed in the discharge in a TR cell. The impurity gases have probably been absorbed by the quartz during manufacture of the tube and subsequently released either under the action of the discharge or when the quartz is heated due to the discharge. In emission, band systems belonging to molecules that are not chemically stable, but which are formed in the discharge, often appear. Also, band systems of impurities appear frequently with a mudi greater relative intensity than their concentration would appear to warrant. If a tube has an air leak, then the bands of nitrogen are seen in the discharge. The impurity gases observed in the discharge are CO and also C^. a free radical normally only observed in electric discharges, being not chemically stable under normal conditions.

For nitrogen, the band systems observed in a microwave discharge are the First and Second Positive systems (see figs (4.12) to (4.14)). In the First Positive system- bands with values of v ’ (from equation (4.10)) increasing from 4 to 9 with the corresponding v" values of 1 to 6 are seen. All the bands of the Second Positive system as listed by Pearse and Gaydon (1976) are seen- down to 3943 A. According to Pearse and Gaydon, these bands

are the most readily seen in a discharge through air, such as a

leak in a discharge tube. The bands of the First and Second

Positive systems are degraded to shorter wavelengths.

Carbon monoxide is one of the gases seen as an impurity in the discharge of some pre-TR tubes. Many band systems have been recorded for CO, the Angstrom and Herzberg systems being visible in the microwave discharge. The bands observed in the Angstrom system are those with v' equal to zero and v ” between 0 and 3 (see equation (4.10)). The only band observed in the Herzberg system has v ’ equal to 0 and v ” equal to 4. Other bands are not present with sufficient intensity to be observed. According to Pearse and Gaydon, the material of a new discharge tube tends to produce enough CO to give the above-mentioned bands. In both the Angstrom and Herzberg systems the bands are degraded to (Sorter wavelengths (see fig (4.15)). No trace of CO was observed in the tubes containing argon or tritiated argon, but it was observed in the discharges of all the other tubes. The intensity of the bands was greatly reduced in the higher pressure tubes, probably due to the reduced partial pressure of CO present.

For Cg. the Swan band system is the only one observed in the microwave discharge in a pre-TR tube. According to Pearse and Gaydon, the bands of the Swan system have been readily observed in discharge tubes containing helium and carbon monoxide. The bands are degraded to shorter wavelengths, with a single head, and the band sequences are well marked (see fig (4.16)). The bands observed in the Swan system result frcm the electronic transitions

(0,1) to (4,5); (0,2) to (5,7); (1,0) to (4,3) and (0.0) and (1.1), where the v* value is listed first and the v” value is listed second. Other bands are not sufficiently intense to be observed in the microwave discharge. The bands of are only observed in the tubes containing argon, tritiated argon, krypton and tritiated krypton.