On the stability of an argon plasmajet.

Abstract

I the course of recent experiments in the Imperial College argon plasmajet,some observations were made of the stability and steadiness of the gas flow, which may perhaps be relevant to the current controversy in the literature. The plasma stream was investigated spectroscopically, using a Bausch and Lomb dual grating spectrograph converted to photoelectric operation. The spectral lines due to neutral argon at 4300 and 4182 A were examined, employing two RCA 1P21 photomultipliers. In the first group of experiments, the two photomultiplier anode currents were displayed separately on a dual beam oscilloscope. , The intensity contribution due to continuum background is negligible under these conditions, and it is easily shown that the plasma excitation temperature is a simple logarithmic function of the ratio of the line intensities. An oscillogram of such an experiment is included in Fig. 1, which also shows the corresponding time-varying excitation temperature. This run was conducted at a nondimensional enthalpy H/RTQ of 121, and the argon mass flow rate was 0.15 Ib/min, which was constant for all the experiments reported in this paper. The output trace is typical of the jet running under apparently reasonably steady conditions. The oscillation in the intensity of each spectral line is about ±20% peak to peak, whereas the variation in temperature as derived from the intensity ratio, shown in the lower part of the figure, is about ±30% peak to peak. The frequency is 300 cps, deriving from the power supply ripple. The exact form of the temperature variation curve is difficult to determine, owing to the lack of smoothness in the oscilloscope traces and to the possibility of a slight phase difference between the two signals resulting from unequal anode line impedances. The significance of these observations will be commented upon below. A second useful method of diagnosis is one of observation of the oscillations or instabilities of the jet, which may be of a nonperiodic nature. By long-term we mean a period of seconds or perhaps minutes during which an aerodynamic test may be conducted in the facility. It is important to know how much effect gross changes in the brightness of the plasma stream, visible to the naked eye, have upon temperature. For this purpose a special facility of the Tektronix 502 oscilloscope was invoked, whereby one spectral line signal may be connected to the vertical deflection plates of the cathode Y&J tube, and the other to the horizontal deflection plates. Figure 2 includes such an oscillogram, taken over a period of 50 sec at H/RTQ = 195, and recorded on the usual Polaroid 3000 film. Attempts to record such traces on photographic plates proved unsuccessful with limited photographic equipment, owing to the extreme faintness of the moving light spot. Oscillograms were, however, recorded on Polaroid type 146-L projection film, which permitted densitometry, the calibration being imposed by a step filter placed in front of the oscilloscope screen. These measurements were of necessity ver}^ crude, owing to the lack of control over the development process, but they served to determine approximately a typical contour of the relative intensity of the as a function of its spread. The intensity of the blob is clearly proportional to the probability of the light spot being found at that station at any time. The excitation temperature is a direct