Leak detection with liquid helium

Nuclear Instruments and Methods 177 (1980) 481484 0 North-Holland Publishmg Company

LEAK DETECTION WITH LIQUID HELIUM * M.L. MALLORY and H.W LAUMER Mmhtgan State Umvemty,

Qdotron

Laboratov,

East Lanmg,

Mchlgan

48824,

USA

Received 13 May 1980

A method for approxunately locatmg ultra-small leaks m a superconductmg cyclotron magnet cryostat has been developed usmg hquld hehum. A method for locatmg ultra-small leaks usmg cold helium gas (<20 K) has been reported. The cold gas method requues a means of varymg the return path of the hqmd hehum boll-off gas to the refngerator. The mam part of the MSU cryostat IS maccessible to temperature variations due to gas flow changes, which prnnarlly occur at the top of the cryostat. In the main body of the cryostat there IS the option of rmmg and lowering the hquld hehum level, and it has been discovered that the location of the leak is clearly defined as it IS covered (uncovered) by hquid hehum.

1. Introduction

utllizmg hquld hehum for detecting these leaks is described.. In February, 1980, the cryostat was taken apart and a leak was found by using conventional techniques at room temperature, which verified our interpretation of the liquid helium data.

A superconductmg cyclotron is presently under construction at wchgan State University. The cyclotron has been constructed m several stages, with testing cycles between them. In particular, the first construction stage was the testing of the superconducting toll and magnet configuration Durmg the first operation of the cryostat it was discovered that helium leaked into the cryostat vacuum Jacket from the hquld helium container. These leaks were very small and did not hamper the operation of the corl when a small diffusion pump was used on the vacuum Jacket Normally, these leaks would have been ignored, but the opportunity of getting at the cryostat agam, since it was necessary to take apart the cryostat to install the beam extraction channel, has led to about 3 years of accumulating leak data and the devising of vanous tests to help locate leak pantlons. Leaks have been dlscovered m the top of the cryostat by the cold gas technique [l]. These were accessible and have been fixed, urlth the result that the enigmatic behavior of the liquid level at the cryostat top has abated. Leaks m the lower part of the cryostat are unaffected by temperature (flow) changes created at the cryostat top and it is impossible to find them by that method. In the followmg section a technique

2. Leak measurements Figure 1 is a schematic drawmg showmg the vanous devices used m locating and monitoring the leaks in the hehum cryostat. The devices used are vacuum lonizatlon gauge, helium mass spectrometer, liquid helium level gauges and platinum resistance thermometers Fig. 2 shows the rate of nse of the pressure in the cryostat vacuum Jacket after the dlffusion pump valve was closed, the cryostat was completely filled with liquid hehum. The rate of rise gves a leak rate of about 1 X lo-” g/s of helium, where room temperature helium gas is assumed to be present in the vacuum Jacket. At this leak rate, an increase m the heat load of the cryostat 1s detected about 24 h after closmg the dlffuslon pump valve. It is also assumed that this rate of rise m the pressure 1s due only to helium gas (all other gases are cryopumped) coming from the liquid helium contamer Measurements of the helium leak rate as a function of temperature of the cryostat are shown m fig. 3, where it 1s expected that only leaks from the hehum container would vary as the temperature changes. The expenmental data show a change of about 3 magm-

* Tlus matenal 1s based upon work supported by the National Science Foundation

under Grant No. PHY 78-22696. 481

ML Mallory, H W Laumer /Leak detectton with kquld He

482 Llquld Hellurn Level Gayges

r

h

1

/

Ionlzatlon Gauge

\ Llquld Helium Refrigerator

Cryostal Vacuum Jacket

Helium Mass Spectrometer

i

Diffusion Pump

MechanIcal Pump

Liquid Helium Supply Dewar

Fig. 1. A schematic drawmg of the superconducting cyclotron cryostat showing varrous devrces used 111detectmg the locatron of ultra-small leaks in the hehum container.

tudes in the leak rate as the cryostat is cooled to about 4.8 K. Eq. (1) [2] is the mass flow per unit time (m/t) for a capillary tube of length I, diameter II, pressure drop P, gas density p and vrscosrty /.I.

for m fig. 3 is the change of the helium pressure m the cryostat from 1.8 to 1.2 atm during cool-down. r

I

I

I

I I

Single

m p nPa4 _=-t p 81 *

Phose Llquld

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-7.

(1) I

The caprllary mass flow for hehum as a functron of temperature IS also shown in fig. 3, normahzed to the expenmental value at room temperature, and reproduces the observed experimental leak rate change farrly well. Frg. 4 shows the temperature difference between two drstant pomts m the cryostat as it IS cooled and hence the temperature drscrepancy in fig. 3 can be attributed to not knowing the precise temperature at the leak locatron. Also unaccounted

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F

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L

Leak

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300

I 250

Magnet

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(hrr)

Ftg. 2. The cryostat pressure rate of rise ISshown and ts equal to a mass flow of about 1 X 10 -lo g/s (5.6 x lo-’ std cc/s) of hehum. A small diffusion pump readily pumps the helmm from the cryostat and allows operation of the coil wtth no detectable increase on the cryostat heat load.

I 200

Rate

I 150

Temperature

I 100

I 50

(K )

!-_ 4 5 Llqutd Hellurn

I

Frg. 3. The measured increase m the helium concentratron m the cryostat vacuum jacket as the superconductmg coil IS cooled to hquid helium temperature is shown. A theoretical leak rate curve as a fun&on of temperatue is also shown and compares favorably wrth expenmental results. The theoretical s&e-phase hquid leak rate ISlarger than the gaseous leak rate, whereas the experrmental leak rate IS found to drop below the gaseous leak rate when the cryostat 1s full of hqurd hehum.

ML. Mallory, H W Luumer /Leak detectton wzthliquid He ‘s

I

z3

REFR;GERATOR

OFF I

HELIUM

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Fig. 4. The maxunum temperature difference detected m the

cryostat during cool-down. This known temperature difference can account for the major part of the discrepancy m fig. 3 between the theoretlcal and expenmental curves.

Fig. 3 clearly shows the increased sensltlvlty that is obtained from leak checking v&h cold helium gas. The single phase hquld hehum leak rate determined from eq. (1) (fig. 3) is about 2 times bigger than the single phase gaseous hehum leak rate at the hquld transition temperature. The expenmentally measured leak rate, on the other hand, 1s observed to decrease after fiImg the cryostat mth hquld helium. The explanation for this dscrepancy is presented below.

3. Liquid helium leak measurements The vertical posltlon of a leak 1s determmed by monitoring the hehum concentration m the vacuum Jacket with the hehum mass spectrometer and correlating these changes wth the liquid helium level height as measured by the superconductmg level gauges when liquid hehum was added to or was allowed to boll from the cryostat. Fig. 5 shows an example of the helium concentration m the vacuum Jacket as liquid hehum boils from the cryostat. On the left m fig. 5 the liquid level gauge indicates hquld hehum boiling at a constant rate while the helmm concentration remains constant At the me&an plane of the cryostat, the hquld level disappears from the top level gauge and then reappears on the lower level gauge, at the same time the hehum concentration has dramatically changed. It increases by about 2, then decreases lmearly as a function of liquid level height The inverse process of fig. 5 1s detected when filling the cryostat. But, the effect on the vacuum Jacket helium concentration is not as pronounced, possibly because the filling time (“4 h) 1s shorter than the

MASS

SPECTROMETER

,

'0

I

2 TIME

3 Ihrsl

4

5

‘30%

Fig. 5. The hehum concentration m the vacuum Jacket and the decreasing liquid level as a function of time are given. Above the median plane, the helium concentration is constant. At the median plane the helium concentration mcreases by about 2 and then decreases. TIus change in hehum concentration IS attnbuted to uncovering a leak channel m the cryostat by the hquid hehum.

time it takes to boil off (-30 h for the fulI cryostat). The data m fig. 5 has been interpreted as indlcatmg a leak at the median plane of the cryostat. The constant hehum concentration measured as the hqmd level drops to the median plane is taken as an mdlcatlon that no leaks are present above the median plane. The decreasing hehum concentration as the hquld level drops away from the median plane 1s interpreted to indicate that the gas temperature at the locatlon of the leak 1s increasing.

4. Liquid helium leak model The MSU cryostat uses liquid helium pool boiling. The temperature of the liquid helium m the cryostat is at the saturation pomt. It is assumed that saturated hquid helium enters the leak channel. As it flows through the leak channel, the pressure drops as shown m eq. (2) [3], where f 1s the Fanning fnction factor, u the fluid velocity, AX is channel length, and Dh IS the hydraulic diameter. The reduction in pressure causes the liquid hehum to be Ap = 2fu’(Ax)/D,,

(2)

superheated, v&h the consequence that some of the hquld must boll and a change from one-phase hehum to two-phase helium occurs in the leak channel. Therefore, the mass flow equation for two-phase helium [3] must be used and dependmg upon the quahty of the hehum, we can find a decrease m the mass flow versus the single-phase hehum gas at the

484

ML Mallory, H W Laumer /Leak detectlon with llquld He

transitron temperature. The change m helium concentratron as the leak channel 1s covered by hqurd hehum 1s therefore attrrbuted to a reductron m the mass flow caused by two-phase hehum. Thrs change m mass flow 1s used to ascertain the vertrcal locatron of the leak.

when the hqurd level drops below the coil partition. Thus, changes m the internal flow pattern may lead to changes m the helium leak rate and indicate false leak locations.

6. Conclusions 5. False signals The helium concentratron m the vacuum jacket 1s also sensitive to pressure changes in the cryostat and such pressure changes are unavordable when adjusting the hqurd hehum refrrgerator. Changes in helmm concentration due to refngerator valvmg operatrons are not correlated with the hqurd level gauge and can be eliminated by comparing several measurements. A very small decrease m the hehum concentration 1s always detected as the hehum level passes one location m the bottom half of the cryostat where a partrtron m the coil IS located; however, no leak was found at thrs level. As the liqurd helium level drops below thrs partition, the flow pattern of the boil-off gas must change. With pmpomtmg of the leak at the median plane, we can ‘account for thus change m helium concentratron, smce boll-off helium gas IS forced to flow past the median plane leak channel

We have described a means of locatmg ultra-small leaks wrth liquid helium. Using these methods, employing cold helium gas and liquid hehum, leak detection of greater than three magnitudes beyond the present state of the art at room temperature can be achieved. Secondly, the concepts of helium mass flow through capillanes appear to be workmg successfully at the mrcroscopx level

References [I] M.L Mallory and H.G. Blosser, Nucl. Instr and Meth. 155 (1978) 573. [ 21 J.L.M Polsemlle, MBmou& pr&ent& par divers savants 4 l’acadimle royale des sciences de l’mstltut de France, 9 433 (1846), Rheoloflcal Memoirs, 7 (1940). [ 31 G.G. Haselden, Cryogemc Fundamentals (Academic, London and New York, 1971).