We find V,ff = 150 + 10 ml for HD, while the physical dead volume was 10 j= 1 ml ( f = 40 ml/min). Hence for H D and all more soluble gases, V,ff will depend mainly on the volume of oil, Veil, and on the solubility of the gas in the oil, as long as the gas collector is operated with a relatively small volume. Then Veff = VoilCoi~/Cgss, and since our Veil = 2,600 ml, our results imply COil/Cga3 = 53. = for H D in pump oil a t 70 “C. This may be compared t o X~O = 89.10-3 reported for Hz in transformer oil (3). Other
(3) “International Critical Tables of Numerical Data, Physics, Chemistry, and Technology, Vol 111.” E. W. Washburn, Ed., McGraw-Hill Book Company, New York, N.Y., 1928, pp 255, 262-268,
relevant solubilities are: Ns, 118; 0 2 , 192; CO, 198; CHI, 212, units of 10-3 in transformer oil a t 80 ”C (3). To minimize Veff, one should select the smallest pump consistent with the flow requirements. The Precision Scientific Model 25 should have Veri for H D of not more than 60 ml a t flows up t o 150 ml/min when equipped with a conical exhaust gas collector. The simple conical design works better t o reduce physical dead volume than several more elaborate devices we tried, and the reduction in effective dead volume is well worth the effort. A simpler o r more effective modification appears unlikely. RECEIVED for review August 16, 1971. Accepted October 14, 1971.
Evaluation of an Attenuator for Improved Operation of Electrodeless Discharge Lamps R. M. Dagnall and M. D. Silvester Department of Chemistry, Imperial College of Science and Technology, London, S W7 2A Y , UK DEALLY ALL the charge material in a microwave excited electrodeless discharge lamp (EDL) should be in the vapor phase during operation. However, this is rarely true and frequently small amounts of solid are present. This solid material changes position during the operation of the EDL and causes a variation in the coupling efficiency of the microwave cavity, thus changing the level of reflected microwave power. In certain instances, gross changes in reflected power can affect the actual power output of the magnetron. This can cause a further change in the discharge conditions of the EDL resulting in further changes in the position of the solid material within the EDL. This process usually produces a cyclic variation in the intensity of the radiation output of the EDL. An attenuator connected between the magnetron and the microwave cavity will diminish these effects by attenuating the reflected microwave power as well as the forward microwave power. In consequence, an increase in the stability of operation of the EDL will be achieved. Also, certain electrodeless discharge lamps, oiz. Hg, As, and Se,
Lamp type Cd Zn PbI2 TIC1
Hg Se
Wavelength of measurement, nm 228.8 213.9 283.3 276.7 253.7 196.1
Table I. Effect of Attenuator on Lamp Operating Conditions No attenuator Attenuator Power, Short term Drift, Warm-up Power, Short term Drift, %/hr time, min watts noise, Zlhr watts noise, Z 35 25 25 25 18 20
3.1 3.3 4.2 2.8 5.0 6.4
-2.0 -2.2 -1.8 -2.0 -3.2 -3.6
require low operating microwave powers (20-30 watts) and a t such low powers the magnetron output is often unstable. The use of an attenuator with these lamps will increase the required microwave output of the magnetron generator and hence give an increase in the stability of the magnetron output. RESULTS AND DISCUSSION
The short term noise and the drift (long term noise) of the intensity of resonance radiation emitted from several EDL’s were measured with and without the attenuator (Table I) using 204
a Servoscribe potentiometric recorder with a time constant of 1 second. The short term noise was defined as the peak to peak variation of the signal, measured over a two-minute period, and the drift was defined as the percentage change in the signal per hour. In these measurements the time taken before the intensity reached a steady value (warm-up time) was measured also. The attenuator decreased the short term noise, as predicted, but only by some 14%, except for those EDL’s which operate at very low power ratings (i.e.,
