During the early phase of the carbon dioxide experiment, we had many discussions that helped me understand the experimental techniques I was using. The experimental work with the copper chloride laser benefited from the support provided by Mr. Frank Linton for technical assistance provided over the years and for his expertise in preparing the illustrations at such short notice.
One problem limiting the development of the CuCl laser is the decrease in output power with increases in tube temperature above 400°C. The aim of the project was to measure the peak radiation temperature (assumed to be related to the average energy of electrons) in the laser discharge as a function of the tube temperature. Using the above result, we postulated that this sudden increase is a result of Penning ionization of the Cu atoms.
A microwave cavity (3 GHz) was used to measure the radial average electron density ne and the electron-neutral collision frequency in the laser discharge. The results show that for the conditions studied, the dissociation as a function of electron density is uniquely determined by the STP partial flow rate of co2, regardless of the amount of N2 and/or He present.
INTRODUCTION TO COPPER VAPOR LASERS
In atomic systems based on this mechanism, most of the excitation energy is normally spent in the excitation of the first resonance level. Calculation of the quantum efficiency of these lasers places an upper limit of 38% in the case of copper atoms. The effects of the partial pressure of copper atoms and the discharge current cannot be isolated because the current itself affects the evaporation of the metallic copper.
The simpler operation of the double-pulsed copper chloride laser therefore makes it attractive for the investigation of the laser mechanism. At the laser's operating temperature (400°C), copper chloride is largely in the solid phase. Since typical operating temperatures for the 1 aser are 1 ow this 1 evening 1, the amount of monomer present in the laser medium is assumed to be negligible.
However, a large number of metastable copper atoms in the lower levels of the laser transi-. It is the lifetime of the lower metastable level that places an upper limit on the repetition rate in continuously pulsed lasers using pure copper, copper chloride, or copper iodide.
TEMPERATURE , °C
METHODS OF MEASURING AVERAGE ELECTRON ENERGY
Many of the useful properties of plasmas are due to the interaction of the electrons with the other particles in the plasma. The current fl m'li ng to the probe is measured as a function of the applied voltage. Under the most favorable conditions, the potential and the velocity distribution of electrons of the undisturbed plasma in the immediate vicinity of the probe can be derived from properties.
In theory, the time resolution of this system should only be limited by the detector and the time constant of the excitation process. From a diagnostic point of view, the measurement of thermal (incoherent) radiation provides information about the electron temperature of the plasma (Ref. 2.8). The expected signal levels (low watts) for glow discharges make direct detection of the emission impractical.
At this point, the signal received from both input branches is of equal magnitude. If the gate signal is synchronized with the plasma event, then a point-by-point comparison of the transient signal with the reference source can be achieved.
APPARATUS AND INSTRUMENTATION
One was placed next to the reservoir of copper chloride while the other monitored the temperature of the tube in the active area of the laser. Of course, this is not exactly the temperature of the medium in the laser. The circuit consists of a charging system (25 KV, 25 rna), a capacitor (which is continuously charged) and a high voltage quick switch for each of the two pulses.
This is achieved with a copper fitting (Fig. 3.4) in which a waveguide section crosses the axis of the laser tube at a ten degree angle. This bandwidth was determined by noting that all other instruments in the system had bandwidths greater than that of the preamplifier in the microwave mixer, which was 12 MHz. Thus, the output signal strength can be correlated with the radiation temperature under the same geometric and electronic conditions.
The signal delay time in the circuit between the input of the microwave mixer to the input of the box motor averager is measured using the scheme depicted in Figure 3.5. The high voltage pulses break down the discharge tube, causing the electron density in the tube to increase rapidly.
EXPERIMENTAL RESULTS
This schedule is described along with graphs of the peak radiation temperature (during the second pulse) versus the laser tube temperature for helium and neon, with and without copper chloride in the system. The data obtained show a definite effect on the radiation temperature of the presence of copper chloride vapors in the system. It is believed that with this procedure all the impurities were driven away from the copper chloride powder, since the use of distilled copper chloride (as opposed to the commercially available one) did not make any appreciable difference in the behavior of the laser.
During operation of the power supply, radiometric readings of the radiation temperature were taken during the second pulse (the first pulse dissociates the copper chloride, the second excites the dissociated copper atoms). Thus, it was possible to obtain a set of radiation temperature curves as a function of time (Figure 4.1).
Te ELECTRON TEMPERATURE DISCHARGE CURRENT
Data were taken on the first three days, while the fourth day was dedicated to testing the instruments and. The peak of the radiation temperature, shown in Figure 4.1, roughly coincides with the laser emission under all conditions. Peak radiation temperatures as a function of laser tube temperature for pure helium (no copper chloride present) and neon are shown in Figures 4.2 and 4.3.
LASER TUBE TEMPERATURE, °C
LAS E R TUBE TEMPERATURE, °C
The maximum radiation temperature as a function of the laser tube temperature for mixtures of copper chloride and buffer gases helium or neon is shown in Figures 4.4 and 4.5, respectively. Both curves show a minimum electron temperature at the laser temperature of approximately 400°C, which is also the optimal temperature for maximum laser yield for the experimental setup used.
LAS ANT: CuCI BUFFER GAS: Ne
PULSE DELAY: 5'5 fLSec
DISCUSSION OF THE RESULTS
INTRODUCTION TO CHARACTERISTICS OF CARBON DIOXIDE LASES In this chapter a brief review of co. Gases present in the co2 laser mixtures can have different effects on the operation of the laser. Based on these values, an estimate of the electric field in the pipe can be made.
The electron density dependence of carbon dioxide dissociation appeared to be nearly independent of diameter. 8.3.1) regarding dissolution in mixtures without water vapor~. dissociation was independent of the total pressure in the tube; and was inversely dependent on the STP flow rate. 8.6) dissociation results cited in the literature are not accompanied by electron density measurements.
Oco 2 and K given by equation (10.13)
The resulting curve appeared to be the same under all conditions studied in which water vapor was not present. Tyte and Sage (Ref. 10.10), using a microwave radiometer, found the following results for the electron radiation temperature. Finally, the preceding argument~, although very rough approximations of physical processes in the plasma, gives plausible qualitative explanations for the dissociation behavior occurring in.
Rather than being the final word on the subject, they suggest future areas of exploration, both theoretical and experimental 1. Bell, “t~ass-Spectrometric Study of Ionic Species Present Under Oxidation of CO and the Decomposition of co2.
CHAPTER XI
RADIATION TEMPERATURE
