Contains reports on two research projects

Coyle, J. E.

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XVIII. SHOP NOTESA. LIGHT-BEAM CORRELATORThis optical instrument. shown in Fig. XVIII-I. was designed and built for S. Gruberand G. Bekefi. of the Plasma Physics Group. to measure and correlate light from aplasma source.Fig. XVIII-I. The light-beam correlator.The light from the plasma source is picked up by two photomultiplier tubes and thefluctuations of light intensity are observed. The tubes are mounted in light-tight boxes.each having a barrel with a hole, 0.010 inch in diameter.An optically ground. half-silvered mirror is mounted at a 45 0 angle above the tubes.299(XVIII. SHOP NOTES)LENS BARRELi . 1o OPENINC-,/LIGHT931- PHOTO MULTIPLIERpiinh V43/Fig. XVIII-2. Photomultiplier assembly.The mirror allows the two beams to become superimposed.The photomultiplier assembly is illustrated in Fig. XVIII-2. One of two phototubesis fixed in a vertical position. The other is mounted horizontally on a movable carriage.The carriage, which has a travel of 3 inches, allows measurements to be taken at anypoint in this distance.The movement of the carriage is achieved by a 1-inch micrometer fitted with anextension containing three grooves exactly 1 inch apart; this arrangement allows a fullinch of measurement because each groove is engaged and held by a stop ring.The carriage moves on three hardened steel balls placed in pyramid fashion to allowa smooth movement on a hardened and hand-lapped surface.The plunger of the micrometer is connected to the carriage by a floating Teflonbushing to avoid interference with the movement of the carriage which results from asudden vibration or jarring of the micrometer head.J. E. CoyleB. VACUUM CUTOFF SWITCHThe thermocouple vacuum gauge has long been in existence as a means of detectingsmall leaks in a vacuum system. It operates in the range 10 .- 500 .. and will detect a300(XVIII. SHOP NOTES)Fig. XVIII-3. The vacuum cutoff switch.minor leak within that range; it allows the pump to maintain a partial vacuum until theleak is discovered. However, when a large leak occurs and this range is exceeded theforepump may sustain serious damage.J. J. McCarthy, of the Plasma Physics Group, suggested the need for a vacuumcutoff switch that would operate in the range beyond that of the thermocouple gauge, andin case of a larger leak would shut off the forepump and the whole system.The cutoff switch shown in the photograph of Fig. XVIII-3 has been designed and builtfor R. E. Whitney, of the Research Laboratory of Electronics, and is incorporated inthe hollow-cathode discharge system. A cross section drawing is shown in Fig. XVIII-4.301(XVIII. SHOP NOTES)CONTACTPRESSURE SELECTORPIN&-Fig. XVIII-4. Cross-section view of the vacuum cutoff switch.This vacuum cutoff switch was first calibrated by a manometer to operate in therange 1 cm Hg and higher; at any selected pressure within this range a microswitchis tripped and the system is shut off. A pressure selector ring compresses a spring ofpredetermined tension to adjust the outside pressure to that of the Sylphon bellows. Thedesired pressure is achieved by compressing the spring in one direction or releasing itin the other at a rate of 4 mm per revolution at the ring.The vacuum cutoff switch is used in conjunction with the thermocouple gauge, andwhen the gauge reaches the limit of its range in small-leak detection the vacuum cutoffswitch takes over to detect larger leaks.J. E. Coyle302

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http://dspace.mit.edu/bitstream/1721.1/53611/1/RLE_QPR_062_XVIII.pdf

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