D. 0. Cowan
and R. P. DeTorna The Johns Hopkins University Boltimore, Maryland 21218
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I
Flash Lamp Design and Construction
The application of flash lamps has proven to be extremely useful in the areas of photochemistry, free radical chemistry, spectroscopy, chemical kinetics, and laser pumping (1). The recent wide spread interest in organic dye laser studies ( 2 ) and the recent use of flash photolysis in the junior physical chemistry laboratory to study iodine atom recombination (5,4) prompts us to report on the design and construction of a simple, inexpensive, reusable, and adaptable flash lamp (Fig. 1).
1.110"
Figwe 1.
TO..
Florhlomp design.
' Cajon Company, 32550 Old South Miles Road, Solon, Ohio 44139. Ultra-torr fittings are avtvrtilable in various sizes in brass or type 316 stainless steel. All sizes ensure 8. vacuum seal with only a. finger-tight connection and can be used with tubing of diameter '/I. in. smaller than the nominal size of the fitting. in. The particular fitting employed here is a No. BUT-66BT X 3/s in. reducing union.
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The lamp is constructed from two Cajon Uha-Torrl reducing unions which provide an O-ring vacuum seal for a fused quartz envelope and two stainless steel electrodes. The electrodes are constructed from type 303 stainless steel rod (3/s in. diameter) cut to an appropriate length. One of the electrodes is drilled through along the rod axis to facilitate gas filling. (This drilling operation can be eliminated by employing thiclc-walled stainless steel tubing.) Quartz tubing in millimeter sizes (12 mm for the lamp described here) is suitable for the lamp envelope; however, to ensure mechanical rigidity with the 0.6 in. i.d. of the Cajon fitting we have wrapped the quartz tube with Teflon tape to build up the diameter along the area where it fits into the nut and ferrule. Care must be taken to keep the section sealed by the O-ring exposed. This procedure is unnecessary if fractional size quartz tubing is available. To aid initial breakdown of the gas under applied voltage, the grounded electrode is treated with concentrated KOH solution, and gently flamed to dryness prior to final lamp assembly (5). A convenient quick connect-disconnect glass-to-metal seal for attaching the lamp to the vacuum line is a standard Cajon union. The lamp is evacuated to high vacuum ( 5lo-= torr) and then filled with Argon (20-50 torr). It is flashed about ten times to out gas the electrodes, re-evacuated and then refilled for general use. A typical time-intensity profile for a 6-in. (electrode spacing) lamp is displayed in Figure 2. Energy for this
Figure 2. O d l o t c o p e display showing light intensity or a function o f time for the 6 in. lamp. Vertical axis, relative intensity; Horizontal oxis time 1 2 0 sec ~ per major divirionl.
particular output is derived from a 5.21 NFcapacitor charged to 8 kV and is switched directly into the lamp via an ignitron. Under these conditions profiles for a 3 and 6 in. flashtube were identical in shape and duration. Irradiometric measurements in the spectral region between 375 and 700 nm employing an EG&G Model 580 radiometer showed that 8-15% of the total energy stored in the capcitor appeared as light o u t p ~ t . ~ Flashtube life was >I00 flashes per filling and no appreciable electrode wear was observed. The parasitic inductance of the capacitor employed here, -700 nH, increases the time constant for rapid power dissipation considerably. Low inductance capacitors should be used to optimize this condition, i.e., to approach a critically damped discharge. Analysis of the current pulse with and without the flashtube implied negligible inductance for these lamns. This efficiency depends on driving circuit performance as well as flashlamp parameters. DTheultimate limit is the maximum energy a given flashtuhe can discharge in a single flash without fragmentation.
I t is seen that this lamp design features a quick finger-tight vacuum and mechanical seal. Worn electrodes may be reconditioned (by polishing with emery paper), and erosion spoiled envelopes may be replaced in a matter of minutes. Little or no machining is required to construct the lamp. Alteration of lamp length and bore (using a different size Cajon fitting) to meet the requirements of a given experiment is easily achieved. I n this regard it is important not to exceed the ultimate limit3 of the lamp (6, 7). A useful orderof-magnitude design equation is given by the empirical relation for linear fused quartz Xenon lamps (6,7). where V (joules) is the uldimate limit; L (inches), the electrode spacing; D (mm), the bore diameter; and T(msec), the duration of the currcnt pulse. The design of a new lamp should employ a discharge energy less than 50% of its ultimate limit. This criterion will ensure reasonable life (> 100 flashes for each filling) and prevent lamp explosion. Literature Cited ( 1 ) P o m m G.. i n "Technique of Organic Chemistry.'. V I I I , pxrt 11, . L . , LEWIB,E. 6 . . A N D WEIBBBERCER, A , ) Inter(Bdilors: F m ~ s s 6. science Publisllem (division of John wiley & Sons, Ino.). Nerv York. 1062
(2) S c x i ~ ~P. n ,P., Angcm. Chem.Infernot.Edit., 9 , 9 (1970). (3) YAMANABHI, B. A N D NOW*=, A . W . , J. EDUO.. 4 5 , 705 ( 1 9 6 8 ) . (4) B L m e , d. A , , BURNS, G., A N D CHAW., S. K., J. CHEY.EDUC.,4 6 , 7 4 5
6..
CXBU.
119691. . ~.~
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(5) G L ~R.. W., Ph.D. Thesis, Vsnderbilt University (1968). Part 11, p. A*,.
"New Developments in Eleotronic Plhslkubes," Teohniosl Publieation. E.G. & G.. Ino.. 1965. ( 7 ) EDGERTON, H: E., DON;=. J.'H.. AND JAMESON. P. W.. "Xenon Piash V .G. A,. AND T r a = ~ ~ * n . Lamp Limits of Operation," i n D E G ~ A A J. P . , (Edilavs). Procrcdinw of the Sirth Inlcrnational Congram on HighSpeed Photoomphy. \Tillink, XI. 0. Tieek, and Zoon, N. V . Haarlem, 1963, p. 143.
(6) Goaes, J. H..
Volume 48, Number 2, February 1971
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