Surface Insulation for Cryogenic FluidsRichard J. Allen Citation: Review of Scientific Instruments 31, 203 (1960); doi: 10.1063/1.1716927 View online: http://dx.doi.org/10.1063/1.1716927 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/31/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Cryogenic insulation standard data and methodologies AIP Conf. Proc. 1573, 463 (2014); 10.1063/1.4860737 Wrapped multilayer insulation for cryogenic piping AIP Conf. Proc. 1434, 1293 (2012); 10.1063/1.4707053 ROBUST MULTILAYER INSULATION FOR CRYOGENIC SYSTEMS AIP Conf. Proc. 985, 1359 (2008); 10.1063/1.2908494 Demonstration of Microsphere Insulation in Cryogenic Vessels AIP Conf. Proc. 823, 1351 (2006); 10.1063/1.2202555 Inception of Cavitation in the Flow of Cryogenic Fluids J. Appl. Phys. 38, 883 (1967); 10.1063/1.1709431

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NOTES 203

,., ·in c:: .,

1.0

1: -0.5 .~

" 0; a:

o 200 400 600 800 1000 Lens Current (Amperes)

FIG. 3. The variation of intensity at a point two meters from the rear of the wire lens vs the current through the lens.

The variation of intensity with lens current is shown in Fig. 3. The position of the maximum agrees with that calculated for 100 Mev / c particles.

Photographs on x-ray film (behind! in. of steel) at the focal spot showed an image of a 2-in. diam source which was approximately 3 in. high by 4 in. wide. Similar meas­urements were made with the scintillation probe. A smaller source (! in. diam) gives about the same size image, indi­cating that the deviations from the ideal field are limiting the "image" size of a point source to the order of the spacing between conductors.

Compared to the standard quadrupole the wire lens has the advantages of using no iron (less size, weight, and no saturation effects) and of obtaining a given focusing power with much less stored energy. A close-spaced symmetric quadrupole triplet of the same over-all length required fields approximately ten times larger for the same focusing power. The wire lens has smaller fringing fields and hence might be useful near accelerators.

On the other hand the imperfect focusing and opacity may be a serious limitation on the use of the wire lens in complex optical systems, except as a terminal element.

* This work is supported in part through funds provided by the U. S. Atomic Energy Commission, the Office of Naval Research, and the Air Force Office of Scientific Research.

1 W. K. H. Panofsky and W. R. Baker, Rev. Sci. Instr. 21, 445 (1950).

Surface Insulation for Cryogenic Fluids RICHARD J. ALLEN

Research and Development Section, The Martin Company, Bal.timore 3, Maryland

(Received November 16, 1959; and in final form,December 10, 1959)

T HE use of inflated polystyrene beads has been found to be an effective insulation for the surface of low­

temperature fluids. While useful for temperatures up to 100°C, they have been particularly advantageous with cryogenic fluids.

The individual polystyrene beads have an inflated diam­eter averaging about t in. They are white in color and quite similar in appearance and density to several inflated dry breakfast cereals. The commercially available1 beads are inflated to a density of about 2 Ib/ft3 by heating in an oven at 300°F for 2 min. A slight sprinkling of water before heating aids in expansion.

The beads, floating on the fluid surface, greatly reduce heat influx through radiation. Applied in a layer 1 to 2 in. thick, they appreciably lengthen the path for gaseous con­duction and convection. Their low thermal conductivity causes little increase in conduction loss. The beads are reusable and, being relatively small and floating, they conform to the surface contour well. The beads permit addition of fluid to the container without removing any radiation covers otherwise employed, as the fluid may be poured through the beads with negligible loss.

Since the beads float, they give an indication of the fluid level without requiring removal of radiation covers. It was found that dyeing a few of the beads a bright color aided in determining liquid level in areas of restricted visibility.

The beads have proven particularly useful with liquid­nitrogen cold traps in vacuum systems. They also have been used to reduce surface radiation and to indicate liquid level on liquid helium.

1 Dow Chemical Company, Midland, Michigan.

Modified Boat Design for Rapid Zone Purification

L. R. JOHNSON AND WILLIAM ZIMMERMAN, III

Crystal. Branch, Solid State Division, U. S. Naval. Research Laboratory, Washington, D. C.

(Received July 2, 1959; and in final form, December 7,1959)

SINCE Pfannl first introduced the idea of zone purifica­tion, many papers have been written both on the

theory and application of this process to the purification of salts, intermetallic compounds and elements, and on various modifications of the process.

One of the principal difficulties which arose in the puri­fication of many semiconductor materials by straight zone purification resulted from the characteristic expansion, on freezing, of these materials, which caused a migration of the charge to that end of the boat which was the last to freeze.2 It was shown by Pfann, Jain and Reneker, and others2,3 that by using an inclined furnace, this migration or piling up of the charge could be obviated. It has become customary to remove the charge after several passes and cut off the ends in which the impurities have concentrated, then to use the purer center sections for a second series of

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204 NOTES

OVERFLOW

CHARGE

DIRECTION OF

TRAVEL ....

FIG. 1. Modified boat.

passes through the purifier, thus approaching the ultimate purity.

At this Laboratory, instead of tilting the furnace, we have for the last six years taken advantage of this migra­tion to increase the efficiency, to expedite and simplify the zone purification process by forcing the last drop or two of liquid into an overflow trap, i.e., "to allow matter transport to cause an overflow of liquid at the ingot end."4 (See Fig. 1.) As the charge in the boat migrates and piles up at the end of the boat in which the overflow trap is located, the freezing of each succeeding zone at the end of its travel forces a small amount of the liquid to leave the main charge and land in the overflow trap. Thus the concen­tration of those impurities with a segregation factor of less than 1.0 is reduced automatically in the final zone on each pass, which increases the ultimate purification possible for a given charge and impurity concentration and allows for a much more rapid purification. The dimensions and design of the boat (Fig. 1) are those which we have found to be most convenient and allow for a charge of 200 to 300 g.

It has been demonstrated that one filling of the overflow trap with either germanium or one of the III-V compounds (i.e., removal of 10% of the bulk of the ingot) will require 10 to 15 single-zone passes and results in an average purity equivalent to that obtained after cutting off the last 20-25% of an ingot after 30 to 40 single-zone passes.

1 W. G. Pfann, Trans. Am. Inst. Mining Met. Engrs. 194, 747 (1952).

2 W. G. Pfann, Trans. Am. Inst. Mining Met. Engrs. 197, 1441 (1C?53).

3 A. L. Jain and D. H. Reneker, Ninth Annual Rept. to ONR, Institute for the Study of Metals, University of Chicago, 79 ff (1955-56); Rene Graf, Metaux Corrosion-Industries, No. 364. 463-75, December, 1955; H. A. Schell, Z. Metallk. 46, 58 (1955).

4 W. G. Pfann, Zone Melting (John Wiley & Sons, Inc., New York, 1958), p. 45.

Wide Range Marginal Oscillator for Operating Nuclear Resonance Probes through

Flexible Cable* L. Buss AND L. BOGART

High-Energy Physics Laboratory, Stanford University, Stanford, California

(Received October 23, 1959; and in finalform, November 16, 1959)

WHILE working on instrumentation for mapping the magnetic fields in the storage rings of a "colliding

beams" experiment,! it became evident that the use of nuclear magnetic resonance in making field strength meas­urements would be facilitated greatly by the insertion of several feet of coaxial cable between the probe and the oscillator. Frequencies in the range 47 to 55 Mc were re­quired, and, because of their simplicity, resonance absorp­tion methods were preferred. Since none of the many cir­cuits published in the last decade seemed adequate, it wa~ decided to develop an oscillator capable of doing the job.

The desirable features of a Colpitts oscillator, similar to that of Hopkins,2 were grafted onto a Franklin oscillator, similar to that of Pound and Knight.3 A condenser and choke were provided to keep audio off the oscillator grid; the second grid was grounded, and oscillation level changes were taken from the first plate. Pickup was reduced by heating the filaments with dc from the plate supply, the current drain being only about 50 rna.

The resulting simple circuit (Fig. 1) proved to have

,

I 100!(

, ~-1h

ALL RESI~,TORS 'WATT UNlfSS NOTEO

ALL CAPACITORS IN MICROFARAOS UNLESS NOTED

ISOK SIGNAL

OUT

o.~

FIG. 1. Oscillator arrangement for high-frequency measurements.

excellent frequency stability, high signal-to-noise ratios without a special filament supply, and a wide range of stable oscillation. These properties, plus the outstanding flexibility of the tool due to its simplicity, seemed to justify the addition of one more item to an already voluminous literature.

The arrangement shown in Fig. 1 has been used to ob­serve proton resonance in a field of 11 000 gauss (47Mc) with 3 ft of flexible cable (RG-7/u) between oscillator and probe. Signal-to-noise ratios were about 30 to 1 using! cc

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