1969 sigel thinsolidhydrogenfoils sciinstr garching,germany

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Production of thin solid-hydrogen foils for use as targets in high vacuum

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1969 J. Phys. E: Sci. Instrum. 2 187

(http://iopscience.iop.org/0022-3735/2/2/316)

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Journal of Scientific Instruments (Journal of Physics E) 1969 Series 2 Volume 2

Produc t ion of th in sol id-hydrogenfo i ls fo r use as targets in h ighvacuum

R Sigel, H Krause and S WitkowskiInstitut fur Plasmaphysik, Garching b. Munchen, GermanyMS received 16 September 1968Abstract Thin solid-hydrogen disks with a lifetime ofabout 10 min were produced by freezing a liquid-hydrogenfilm spread over a hole in a liquid-helium-cooledcondensation plate. The transparent self-supporting disks

have a diameter of 2 mm, and they diminish in thicknessfrom initially about 1 mm to zero as a result of slowvaporization. The method described here enables materialswhich are gaseous under normal conditions to be producedas solid foils in high vacuum. This can be done in quicksuccession by means of the simple apparatus described.Such foils can be used, for example, as targets for producingplasma by laser light.

1 IntroductionSome special physical investigations are only possible if thematerial involved can be spread out in high vacuum in theform of a foil. The production of self-supporting, solid foilsis difficult, however, especially with materials which aregaseous at room temperature. Such gases have first to becooled to low temperatures before they can attain the liquidand solid states. Furthermore, careful thermal insulation isnecessary to prevent a premature return of the condensedmatter to the gaseous state, thus making it more complicatedto handle.

At present, there is great interest in the field of plasmaphysics in producing solid hydrogen targets, because it ishoped to obtain hot plasmas with a high degree of purity byvaporizing and ionizing solid hydrogen by means of pulselasers. A good way of investigating the heating process is toplace a thin disk of solid hydrogen in the laser focus (Sigelet al. 1968).

This paper describes a technique for producing solidhydrogen foils in the form of round disks. In principle thismethod can also be applied to other gases. The basic idea isto spread a film of liquid gas by virtue of its surface tensionover the hole of a condensation plate and cool it carefullyso that it freezes without optical inhomogeneities.2 Behaviour of a solid hydrogen disk in a vacuum vesselBefore describing how the disks are produced (3), we shallfirst study some theoretical aspects of the problem. Let usconsider a solid hydrogen disk which is placed in a vacuumvessel and has no thermal contact with its support. The timebehaviour of the disk shall be described by the two equations

?= wv wdtdU d dT dm

dt dt---t - t (mcT) mc cTwhere U is the internal energy, WV the energy loss per unittime due to vaporization (cal s-l), WS the energy gain perunit time due to radiation from the surrounding vessel(cal s-I), T the temperature, m the mass of the disk and cthe specific heat per unit mass (assumed time independent).The gain of energy and mass due to condensation is neglected,the reasons being given in 92(v).

(i) Firstly, the heat gain due to radiation is neglectedWs=O).With

dmW v = h t (3)where .1 is the heat of sublimation per unit mass, equations(1) and (2) yield

dm cdTm A-cT-

which has the solutionm - l-cTomo A-cT

4)

where mo is the initial mass and TO he initial temperature(TO< 1 3 . 9 5 ~or hydrogen).

For T+O, m tends to the finite mass m m. Because cToQ-1(and, in addition, c-0 for T-tO), it holds that mPNmo.Therefore, if a piece of solid hydrogen is placed in a highvacuum, there will initially be pronounced vaporization onits surface, depending on its temperature and vapour pressure(the vapour pressure of solid hydrogen being 54 torr at13.95 ~).This causes the hydrogen to cool. The internalenergy of the hydrogen is only sufficient, however, to vaporizea small fraction of its mass, and so the mass of the hydrogenis almost completely conserved.(ii) At the surface of the disk free evaporation into the vacuumtakes place. But so that the rate of decrease of mass may becalculated approximately, the disk may be considered as inequilibrium with its own vapour. In this case the number ofevaporating molecules equals the number of condensingmolecules per unit time and can therefore be calculated fromthe known equilibrium vapour pressure curve ~ D ( T )f thesubstance. This consideration gives for the rate of decreaseof mass of the disk

where u is the condensation coefficient, F the total surfaceof the disk, R the gas constant, M the molecular weight, the

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R Sigel, H Kvause and S Witkowskinumber of particles per unit volume, the mean thermalvelocity of the molecules and p the mass of a molecule.This yields a differential equation for the time dependence ofthe temperature

7)dT- ucFM112(il- c T ) ~ D ( T )d t mc(2rrRT l 2Under the conditions cT


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Production of thin solid-hydrogen foils for use as ta rgets n high vacuumspeed S chosen for the pump for the purpose of obtaining alow final pressure ( 2(iv)) is of the order of 1000 s-1 thecond ition (13) is readily satisfied an d th e estimates in 2(i)-(iv)are valid.

Figure 2 Schematic diagram of the vacuum and coolingsystems

Figure 3 Cryogenic device for producing thin solid-hydrogen disks

3 Apparatus and production of diskThe disks are produced by liquefying and freezing gaseoushydrogen by means of liquid helium in a cooling deviceoperating on the vaporizer principle described by Klipping1961). Because of the low heat capacity of such an apparatus,production of a disk can be started at any time and interruptedwith only small loss of helium.Figure 2 shows how the liquid helium flows out of a storagetank through a transfer line in to th e cryogenic device. Afterleaving the device, the helium cools a copper cylinder in thehigh vacuum chamber which acts as a cryopump for theresidual gases. It is then heated up to room temperature andcompressed by a rotary pump. The helium flow can be variedby manual operation of a valve, thus allowing the tem peratureof the copper condensation plate to be regulated. Th e vacuumvessel is evacuated by a diffusion pum p (2000 s l for air).Figure 3 shows the cryogenic device for producing thesolid-hydrogen disks. Th e cryogenic appar atus is mounted o na single flange, and this whole set-up is placed on top of thevacuum vessel. In this system, the disk is accessible from anyside. The disks are produced in the slit hole of a coppercondensation plate. The condensation plate is screwed to acopper block (with a vapour pressure gauge attached) whichis thermally insulated by a thin-walled stainless-steel tubeand cooled by a liquid-helium coil. The volume around thecondensation plate is separated from the high vacuum by aremovable bell glass during production of a hydrogen disk.The hydrogen gas to be liquefied is filled into this volume.The volume under the glass and the filling pressure are suchthat a liquid film spreads on th e copper plate without formingdrops.When the device is cooled, it may be observed through thebell glass that this film of liquid hydrogen also covers the slithole in the condensation plate, owing to surface tension. Asthe cooling process advances, it can be observed that thehydrogen freezes at first on the plate, i.e. the solid-liquid

Figure 4 A solid-hydrogen disk in high vacuum; diameterof the hole, 2 mm

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R Sigel H Krause and S Witkowskiinterface shifts slowly from the margin to the middle of thehole in the plate. C areful regulation of the helium flow ensuresice formation without optical inhomogeneities. Once all ofthe hydrogen is frozen, the vapour pressure in the condensa-tion cham ber begins to decrease. The bell glass is removed ata vapour pressure of several torr. The rest of the hydrogenflows into the vacuum. A transparent hydrogen disk remainsin the hole of the plate. While the disk exists, a vacuum of10 pto rr is maintained by the pump.Figure 4 shows a photograph of such a disk. The diameter ofthe hole is 2 m m an d the thickness of the copper plate is1 mm. I t can be seen that th e hydrogen film originally coveringthe plate has vanished. This film vaporizes immediately onremoval of the bell glass because it has direct thermal contactwith the copper, whereas the disk in the hole touches thecopper plate only at a very few points and remains intact.This fact justifies neglecting the heat transfer fro m the bound-ary of the disk, as described in 92. The disk thickness thendiminishes from initially about 1mm to zero at a constantdiameter. Both of the surfaces thereby become increasinglyflat. After about 10min the disk vanishes. The thickness canbe measured from the side by a long-focus microscope. Forthis purpose, the copper plate is slit from the hole to themargin. A new disk can readily be produced by lifting thebell glass and replenishing with gaseous hydrogen. Less tha nof liquid helium is needed for one disk.AcknowledgmentsThis work was performed under the terms of the agreementon association between the Institut fur Plasmaphysik andEuratom.ReferencesKlipping G 1961 Kultefechnik 13 250-2Scott R B 1963 Cryogenic Engineering (Princeton: VanNostrand) p. 298Sigel R Biichl K Mulser P and Witkowski S 1968 Phys.Lett. 26A 498-9

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1969 sigel thinsolidhydrogenfoils sciinstr garching,germany

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