GB2351549A - Liquified gas cryostat - Google Patents

Abstract

A liquified gas cryostat has an inner wall (3) to contain the liquified gas (4) and an outer wall (2) spaced therefrom. The space between the walls is evacuated and includes a metallized multi-layer insulation (5) formed of a woven fabric coated with a metallic layer to create discrete, self defined areas of metallisation. The fabric may be of polyester. The metallic is chosen from gold or aluminium. The matallic layer may be coated on both sides of the woven fabric so long as there are discontinuities, but the layer is preferably coated on one side only of the woven material.

Description

2351549 LIOUTFIED GAS CRYOSTAT The present invention relates to a

liquified gas cryastat and 5 particularly to liquid helium cryostats.

In a conventional magnetic resonance inaging (MRI) system, which operates at a static field strength of approximately 1 Tesla, and a corresponding Larmar frequency 42.5 MHZ, most of the noise which degrades the final image quality is caused by eddy current losses in'the sample (patient). These losses can be treated as an effective additional series resistance in the receiver coil, R,,,,, at the sample temperature, scales with the 4 -h or 5'-' power of the sample radius and with the sauare of the operating frequency of t-he MR1 system.

In a low field system, operating below approximately 0.1 Tesla, the sample losses can become less significant than the intrinsic losses in the receiver coil, particularly if a surface coil of small dimensions is used. The coil is tuned to the NY-R (Larmor) frequency with a capacitor. The receiver coil and its tuning capacitance have a total intrinsic a temperature T,,;, resistance which is riaintained at (normally room temperature). R,,,, generates an r.m.s. noise 25 voltage given by:- V, = v4k- where k.2 is BaltzmannIs constant and.6f is the measurement bandwidth. The signal to noise ratio (SNR) of an NMR receiver can be expressed in terns of the coil and sample parameters as: - SNR - -- ') 3, + 57 (2) 2 where ch is the Larmar frequency of the NMR system, which is proportional to the main f ield strength, and B, is a parameter which describes the magnetic coupling between the sarple and the receiver coil (D.I. Hoult and P.C. Lauterbur, J. Magn. 5 Reson. 34, 425 433 (1979)).

If it is the case that the coil is the dominant system noise source it follows from Eqn. 2 that by reducing its temperature and resistance the system SNR can be improved. There is considerable interest at present in the development and use of surface coils fabr-icated from high temperature (High-T, superconducting materials). These are cooled below their superconducting transition temperature (T,) with liquid nitrogen at 770K. Liquid nitrogen is easy to handle and can -at made from expanded 15 be held in a very simple cryost _ polystyrene foam. The results obtained at 77 K with High-T, coils are often disappointing because although the coil's noise is reduced, the mreamplifier which follows the coil in the receiver chain may generaze too rauch noise in itself to realise the improvement. High-T,, coils have a further disadvantage that, at present, thev must be fabricated -o offer the best epitaxially on a ceramic substrate in order t performance. There is not yet a highT, wire with good radio frequency (RF) characteristics which can easily be wound into suitable coil shapes.

Further improvements in coil performance can be obtained by using a low-T, superconductor such as niobium. This is a refractory metal and is easily formed into coils of any required shape. Its low transition temnerature of approximately 9 K requires that it be cooled in liquid helium at 4. 2 K. From Eqn. 1 it is apparent from the temperature ratio that the r.m.s. noise voltage produced by a resistance a"L-- 4.2 K should be a factor of approximately 4.3 less than that produced by the same resistance at 77 K. Although superconducting coils are used, the capacitors used to tune them will always give rise to some resistance which reduces 3 on cooling, so the expected improvement factor is greater than the temperature ratio suggests. At such low temperatures a very high performance amplifier is required to match.to the low noise coil. For many years the SQUID (Superconducting Quantum interference Device) has been used as a low noise preamplifier for solid-state NMR experiments. The SQUID is the most sensitive magnetic field detector yet devised (it is the only device, for example, capable of detecting the magneto-encephalogram. (MEG) - the minute magnetic signals jO generated by brain activity).

Research has been conducted into using low-T, receiver coils with SQUID amplifiers to improve the SNR of low field NY-R (MRI) performed on room temperature samples positioned outside the cryostat, for example, in Phys. Med. Biol. U 2133-21-37 (1992) and in I= trans. Appl. Supercon 5. 3218-3221 (1995) (both to H. C. Seton et al). A further relevant publication is "A 4.2 K receiver coil and SQUID Amplifier used to im.prove the SNR ol- magnetic resonance images of the human arm, in Meas.Sci-Technol., 8, 198-207, 1997; by H.C. Seton et al. Unlike liquid nitrogen, liquid helium requires specialised handling and the design of cryostats to contain detectors provides challenging engineering and design problems. Relatively sophisticated insulation techniques are required to ensure that a cryostat's liquid helium hold-time is acceptable.

A commercially manufactured cryostat (Biomagnetic Technologies Inc.) intended for use in biomagnetism experiments available. This cryostat normally has a hold time of 4.5 days; the time required for a fill ofliquid helium (6 litres) to evaporate completely.

The cryostat is a double walled dewar vessel, with the space between the walls evacuated to elimrinate gas conduction to the liquid helium volume. The walls are fabricated from glass reinforced plastic (g. r.p.) to minimize eddy current losses.

4 In addition, approximately 30 layers of multilayer insulation (MLI) typically of aluminized Mylar@ are placed between the walls to reduce the radiative heat flux. The thin auminium layer on this material has a very low emissivity and can be regarded as a heat reflector.

The rate of radiative heat transfer between a hotter surface at temperature Th,, (i.e. the outside wall of a cryostat at room temperature, 30P K) and a cooler surface at T,,.Id (i.e.

the inside wall of a cryostat at liquid helium temperature, 4.2 K)-Js given by Stefan's Law which can be stated as:- c e A (Th,,4-Tcojd 4 (3) where o, is Stefan's constant, o is the emissivity of the surfaces and A is the area over which the radiative heat f N layers of MLI are interposed transfer is taking place. If between the two walls at T.,,, and T,-I,, the radiative heat transfer given by Eqn. 3 is reduced by a factor of 1/ (M+1).

The lavers of MLI reach thermal eauilibrium mainly by radiative heat transfer and by conduction within a layer. The efficiency of the MLI layers can be greatly improved by inserting a third surface, called a radiation shield, or heat shunt, between the outer and inner walls at an intermediate temperature T,j,,11-,- This shield can be cooled either by contact with a liquid nitrogen reservoir (at 77 K) or a cryocooler, or by being thermally anchored to a poin't on the tube venting the helium gas, sometimes called the cryostat 'neck, evolved as the liquid helium. boils off. The "cold end" of the vent tube is at a temperature near that of liquid helium (4.2 K) - This rises along the tube's length almost to room t - -emperature at the top of the cryostat so, in principle, any shield temnerature in this range can be obtained by correct choice of anchoring position along the neck. The shield acts by intercepting the radiant. heat f lux from, the outside wall of the cryostat (reduced by any intervening MLI layers) and conducting this heat to its anchor point on the venting tube.

This heat is now removed from the radiant flux into the liquid helium volume. The helium volume at temperature T,,1d is now presented with a surf ace at T,,h,ld rather then at Th,, and Eqn. 2 shows that if the shield temperature were 77 K the radiant flux would be reduced by approximately 230 times. However, part of this heat is returned to the helium volume because one must also take accounlL of the additional heating of the venting tube by the heat shunt which will increase the conductive heat f lux to the liquid helium. Either copper or aluminium is used to make radiation shields in a conventional cryostat, since each material has high thermal conductivity in the useful temperature region of 60-150 K (see Figure 1); Unfortunately the electrical conductivity of both these metals is also very high at low temperatures and gives rise to eddy is current losses. Phys. Med. Biol. (1992) Vol. = No. 11 P21332137 reveals the detection of magnetic resonance signals at 425 kHz and a cryostat arrangement for use therewith.

Because. magnetic fields are detected from outside the crvosta",--, the signals must pass through a vacuum gap and any insulation it contains. The conventional untuned SQUID magnetometer detector has a uniform response to magnetic.fields -Lorom d.c. to a frequency determined by a roll-off filter in the input circuit (usually a few tens of kHz).

Since the input circuit is superconducting, the detector sensitivity is largely governed by the SQUID's white noise level which, expressed as an equivalent flux noise, is typically 5,u00/HzP2.

, where (D. is the f lux quantum, equal to h/2e. The magnetic field sensitivity depends on this SQUID 30 noise, the SQUID's input coil inductance, input coil coupling coefficient and the pick-up coil geometry. A typical magnetometer exhibits a field sensitivity of approximately 5 f T/Hz1. m-he cryostat eddy current losses generate additional f requencyindependent noise in this type of untuned, 35 superconducting detector. A suitable cryostat design for this det-ect-or ensures that the noise level due to eddy current losses is below approximately 2 fT/Hz1 so tLhat it is less than 6 the SQUID noise.

The tuned input circuit that is used with SQUID detectors permits magnetic field sensitivities of below 0.1 fT/Hz' and so the-cryostat noise must be reduced below this level. The ultimate detector sensitivity is set mainly by Johnson noise due to losses in the capacitor used to tune the input circuit to the required Larmor frequency, with only minor contributions from the SQUID amplifier's noise source. For this circuit, cryostat losses appear as an additional resistance which sca1es with the square of the Larmor frequency. When such a detector is used at a high frequency, these losses can reduce the circuit Q-factor dramatically and generate noise which exceeds that due to the intrinsic losses.

is Therefore special measures are required to reduce cryostat RF losses when tuned detector coils are used.

It has prgi-v-Jously been found necessary -,c remove part of the insulation. surrounding the end of the cryastat (which is formed into a narrow "tail") because the metal content of the MLI and the radiation shield gives rise to eddy current losses. These losses destroy any benefit gained from using a tuned detection coil cooled to such low temperatures.

It should be noted that cryostat manufacturers have already partially addressed the problem of eddy current losses.

Rather than being made out of an unbroken copper cylinder, the shield in a typical commercial cryastat is formed from electrically insulated strips or wires of aluminium or copper.

These are set lengthways into a g.r.p. tube. This construction ensures that the radiant heat incident an the shield is conducted efficiently up the length of the cryostat (it is not necessary to have good thermal conductivity circumferentially), bult. that the areas of any electrically 35. conducting paths are kept to a minimum, since it is these which give rise to RF (eddy current) losses. Similarly, the metallisation on the MLI layers has been broken up into areas 7 of approximately 3 CM2 to cut down the area of any conducting paths. These measures reduce cryostat noise to an acceptable level for use with untuned nagnetometers,. but the noise becomes excessive when tuned coils are used at high.5 frequencies.

The present invention provides a new type of liquid helium cryostat with a cooled shield that exhibits low losses at radio frequencies. 10 -ion therefore there According to a f-irs'. feature of the invent is provided a liquified gas cryostat comprising:- an evacuated housing having inner and outer. walls provided is with multi- layer insulation and a cooled radiation shield, said shield being continuous over the areas of the inner wall c the housing juxtaposed to zhe Jntended level of IC-he licu.,1'.fied gas.

The -Invention is characterized in that the radiation shield is formed of an electrical -ins,.iia-.--r with high thermal conductivity but negligible electrical conductivity in the temperature range of intended use. The insulator may be selected from a sintered ceramic material, sapphire or diamond 25 composite powder.

The sintered ceramic material may be alunina (A1103), aluminum nitride (A1N) or silicon carbide (SiC) for example. -' The liquified gas may be nitrogen or -preferably helium.

The radiation shield is preferably operatively connected at or towards its intended upper end by means of a heat exchange s--rin which interconnects the upper portions of the shield with the inner wall- of the housing. This heat exchange strip 35 may be made of copper or aluminium and may be in the form of a continuous or discontinuous annulus. The radiation shield may alternatively be thermally isolated from the cryostat neck and cooled by a cryo-cooler to extend the cryogen lifetime over that possible with the vapour-cooled shield.

-The multi-layer insulation is metallized and is trqated to provide an arrangement such that the metal layer is in discrete areas that do not exceed 2 mm by 2 mm.

in a preferred form of the invention, the insulation is preferably formed of a woven fabric, for example, a woven polyester fabric thinly coated with a metallized layer of gold or aluminium. Discontinuities in the metallisation arise because the thin metal,coating is applied to a woven surface. Each time one thread crosses another, there is a "masked" region, one thread wide, which is not metallized. This metallized layer can be coated on both sides so long as there are discontinuities, but the layer is pre-,ferably coated on one side only of the woven material. The individual elements of the metallized layer may have an average size oil approximately 500gr, by 20gm. indeed, areas of netallisation as small as 10;.zm x 30ogm have been produced easily and cheaply by neans cl this technique. This -orovides a self-defined, highly uniform, low eddy current loss, reflective insulating material for use as superinsulattion in crvostats. Although polyester woven filaments a'I-e suggested, any smooth woven filament with a low vacuum outgassing rate is suitable.

The invention will now be described by way of illustration only with ref erence to figures i and 2 of the drawings, wherein Figure I shows a graph of the thermal conductivity of various materials, and Figure 2 shows a vertical cross-section through a liquid helium cryostat in accordance with Ithe nresent invention.

With reference to Figure 2, there is provided a double walled 35 dewar vessel housing I in whicil-i a space between the outer wall 2 and the inner wall 3 is evacuated via valve 7 to eliminate gas conduction into the liquid helium volume. The walls of 9 the dewar vessel 1 are fabricated from glass reinforced plastic (GR.P) to minimize eddy current losses and closed at their upper ends by a vacuum seal lo. Disposed within the evacuated space are a plurality of approximately 30 to 60 layers of aluminised nylar multi-layer insulation 5 to reduce heat flux. Generally there tend to by more layers adjacent the side of the shield to minimize liquified gas boil off, and fewer layers near and covering the base to minimize RF losses near the detection Coil. The thin aluminium layer on the mylar material has a very low emissivity and can be regarded as a heat reflector, but in accordance with the present invention should have discrete aluminised areas preferably of a size below 2 mm by 2 rm to prevent electrical conduction.

Disposed within the lower portion of the housing 1 is a radiation shield 6 formed of alumina ceramic. This is in this instance in the -form of a right cylinder with the botto-m secured such that the whole mortion of the inner wall 3 ovc.-the mortion which in use will be covered by liquid helium 4 is juxtaposed to the radia'--iz)n shield 6. The upper po--"L-io.,,i of Ithe radiation shield 6 is operatively interconnected by copper strips in the form of an annulus which extends between the outer ff ace of the inner wall 3 and the outer f ace of the top of the radiation shield 6 towards the neck 8 of the crvosta-'--.

The shield 6 thus takes the form of a tube with wall thickness of approximately 2 mm and a closed bottom end of the same thickness. This bottom end is machined as a separate piece (a 2 mm thick disc) and then glued to t-he tube with epoxy resin to form the closed end. The open end is then supported mechanically and fi--m.ly anchored to the cryostall= neck using strips of copper 11 fixed with epoxy resin. The strips form a heat exchange-- between the cold gas 4 boiling off from the liquid helium volume and the ceramic radiation shield 6. The presence of copper at the end of the cryostat neck 8 does no', give rise to any significantk eddy current losses in the - detector coil, and it will be appreciated that the position of the upper end of the copper strips defines the temperature of the shield which may be adjusted by altering the:elative position 9 of the strip with regard to the open rim of the S neck 8. The radiation shield may, alternatively, be thermally isolated from the cryostat neck and cooled by a cryo-cooler to extend the cryogen lifetime over that possible with the vaDour-cooled shield.

Plural shields may be used and, in these circumstances, a mixture of vapour and.cryocooling may be used if desired. Thus both or all shields may be vapour-cooled, both or all shields may be cryo-cooled or, depending upon how many shields are used, one or more may be vapour cooled and the -remainder is may be cryo-cooled, depending upon the desired operational faczors and the oerformance and cost requirements of the sys tem The arrangement shown in Figure 2 may be utilized with a SQUID 20 as described in Phys. Med. Biol. reference described above.

The cerami-c shield in accordance with the present invention is applicable to all types of low noise cryostats including those required for biormagnetism determ inations. The cryostat has a reduced eddy current loss even in the biomagnetic frequency range and so would permit more sensitive measurements to be made if a more sensitive SQUID became available. The cryostat of the invention cnly requires refilling with liquid helium at the same frequency as conventional low boil-off types.

The main area of use is in NMP, and MRI determination performed at room temperature an f or example patients. In particular a liquid helium temperature tuned superconducting surface coil coupled to - a SQUID detector operating in such a cryostat allows M images with high SNIR to be obtained at low f ield strength. This avoids the expensive requirement f or a high field imager and permits studies which can only be performed at low f ield strength to be performed satisfactorily. The invention therefore provides a cryostat comprising a ceramic

I radiation shield and in the alternative a cryostat comprising a metallized woven fabric insulator.

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Claims (3)

CLAIMS:-

1 A liquified gas cryostat comprising an evacuated housing having inner and outer walls provided with metallized multi- layer insulation; characterized in that the insulation is formed of a woven fabric coated with a metallic layer to create discrete, self defined areas of metallisation.

2. A cryostat according to claim 1 wherein said fabric is a polyester fabric.

3. A cryostat according to claim 1 or 2 wherein said metallic layer is chosen from gold or aluminium.