WO2006097720A1

WO2006097720A1 - Sample support with tilting and rotating means for cryogenic applications

Info

Publication number: WO2006097720A1
Application number: PCT/GB2006/000923
Authority: WO
Inventors: Robert Kurt William Haselwimmer
Original assignee: Cambridge Magnetic Refrigeration, Ltd.
Priority date: 2005-03-16
Filing date: 2006-03-15
Publication date: 2006-09-21
Also published as: GB0718021D0; GB2438784A; GB2438784B; GB0505393D0; GB2424198A

Abstract

A cryogenic support comprising: a first platform extending in a first direction between first and second opposite sides and rotatable about a first axis substantially parallel to the first direction; a first rotator attached to the first side of the first platform for supporting the first platform and selectively rotating the first platform about the first axis; and a support member for rotatably supporting the second side of the first platform for rotation about the first axis. The rotator may be a piezoelectric rotator and additional rotators may be provided to allow rotation of additional sample platforms about a plurality of axes. A rotatable metal-to-metal connection may be used between the support member and the sample platform.

Description

SAMPLE SUPPORT WITH TILTING AND ROTATING MEANS FOR CRYOGENIC APPLICATIONS

The present invention relates to an apparatus for cryogenic temperature research. In particular, the invention relates to a cryogenic support, which may be used to mount components or samples to be measured and/or control their orientation. By cryogenic temperatures, what is meant is temperatures comparable to or lower than the temperature of common cryogenic liquids such as liquid helium.

Cryogenic measurement systems are often designed so that components or samples to be measured are in good thermal contact with the lowest temperature part of the cooling system: the so-called "low temperature stage". The low temperature stage itself may be attached to a measurement probe and inserted into a cryostat containing a bath of a liquid cryogen such as liquid helium. This provides the basis for cooling down to around the boiling point of the cryogen in question (around 4.2 K for liquid helium). To achieve further cooling, different mechanisms are required and a number of alternative approaches are well known in the field. These include those forming the basis of the so-called dilution refrigerator, the adiabatic demagnetisation refrigerator, and simple pumped cryogen systems. Depending on the type of fridge in question, additional components will be required in the measurement probe, in the cryostat region and/or in the outside laboratory (e.g. pumping apparatus and control systems). The nature of the cooling provided by a particular type of fridge varies greatly.

Some, such as the dilution refrigerator, are capable of applying a continuous cooling power while others, such as the adiabatic demagnetisation fridge, work by cooling an element of relatively high heat capacity in a "single shot" and thereafter holding the low temperature stage near a lowest temperature by efficient thermal isolation. It may be of interest to study how the low temperature physical properties of a sample vary in an applied magnetic field. Often, this magnetic field dependence will depend on the orientation of the magnetic field relative to crystallographic axes of the sample.

Due to practical constraints, the orientation of a magnetic field relative to the inserted measurement probe is usually fixed and usually parallel to a longitudinal axis of the sample probe. In the case where the sample orientation is fixed with respect to the sample probe, angular-dependent magnetic field measurements are only possible by repeatedly removing the probe from the measurement cryostat, manually reorienting the sample with respect to the probe and returning the probe to the measurement cryostat. This procedure is time consuming and expensive, because it is necessary to warm up the entire probe to room temperature and cool it down again between each set of measurements at different angles.

Another situation in which it may be desirable to rotate a sample held at very low temperatures is in the field of electron microscope imaging. In this application, samples may be cooled in order to reduce vibration and/or to minimise sample damage caused by the incident electron beam. Rotation of the sample in the beam allows angular dependent images to be resolved, which can be used to produce a three dimensional representation of the sample.

Because of the extreme environmental parameters involved, in particular the low temperatures (which may be as low as a few millikelvin) and, where present, the high magnetic fields (possibly several tesla or more), it is difficult to design reliable and effective rotation mechanisms for rotating the cooled sample.

It is an object of the present invention to overcome or at least reduce these problems.

According to an embodiment of the invention, there is provided a cryogenic support comprising: a first platform extending in a first direction between first and second opposite sides and rotatable about a first axis substantially parallel to the first direction; a first rotator attached to the first side of the first platform for supporting the first platform and selectively rotating the first platform about the first axis; and a support member for rotatably supporting the second side of the first platform for rotation about the first axis, wherein the first rotator is a motor operable when held at cryogenic temperatures.

The support member of this arrangement provides a means by which adverse torque (i.e. torque that is not parallel to the first axis) arising from the weight of the first platform and attached components can be alleviated. For example, the support member can transfer some of the weight of the platform to a base structure rather than have all of the weight and torque on elements of the first rotator. In this way, even when the first platform extends horizontally, the efficient operation of the first rotator is not compromised. This greatly facilitates the inclusion of an enlarged sample area on the platform (which enables more measurements to be carried out simultaneously, thus reducing costs). The design also facilitates the inclusion of an integral motor within the first rotator (such that the motor, platform and support member can all be located at the low temperature stage), the mechanism of which may be sensitive to adverse torque. This avoids having to provide a mechanical coupling between the cryogenic support and an external motor positioned at a higher temperature stage. The result is that the cryogenic support can be thermally isolated more effectively. As is discussed in more detail below, the support member also provides an alternative heat-sinking path between the first platform and the base structure (or other mounting member).

The support member may comprise a hollow portion for providing access for electrical connections to the first platform (or other electrically driven elements of the cryogenic support). The hollow portion acts to protect electrical wires enclosed therein and helps to ensure electrical reliability of the cryogenic support and devices attached to the cryogenic support.

The engagement between the first platform and the support member may be made via an axle member associated with the first platform. The axle member, which may have rotational symmetry around the axis of the first rotator, may be arranged to rotate within an opening in the support member. An alternative arrangement is to have a support member with a pin that is oriented substantially parallel to the axis of the first rotator and is arranged to engage in a hollow cone formed in or attached to the first platform. In this way, the pin may support the first platform in a rotatable manner. Both of the above arrangements have the advantage of allowing 360° rotation (or more). However, the former arrangement involving the axle member has a particular advantage in that the axle member may be adapted (by providing a hollow opening, for example) to allow electrical wiring and/or a remote drive mechanism to pass through it.

The support member may comprise an opening to receive the axle member. The engaging portion may fit loosely or be held in place under the effects of gravity. Preferably, the support member comprises clamping members and an urging device to press the clamping members against the axle member. Forcing the clamping members onto the axle member ensures a stable rotation of the first platform and the combination of axle and clamping members may help to ensure angular stability during measurements (perturbating rotational forces may originate from external magnetic fields and/or imperfections in the first rotator, for example). The technique also improves the ease with which heat can flow across the platform-support member interface and therefore improves the capability of the support member to act as a heat- sinking pathway for the platform. The force with which the clamping members are urged onto the axle member may be tuned so as to provide an appropriate balance between thermal and mechanical properties. Urging forces that are too low will lead to undesirably low thermal conductance while excessively large forces will make rotation more difficult due to increased friction.

The urging device may comprise one or more elastic members removably coupled between the clamping members so as to press them against the axle member. This arrangement has the advantage that it allows the clamping members to be separated easily, which may facilitate assembly and disassembly of the device. In addition, tuning the clamping force to the desired strength can be achieved more easily because individual elastic members can be exchanged quickly for elastic members of different strength and/or positioned differently relative to the clamping members. This latter parameter may particularly vary the clamping force when the coupling positions vary relative to a hinge or coupling point between separate clamping members. The support member and/or the axle member may be formed from a metallic material or a metal-insulator composite. For example, beryllium copper or phosphor bronze may be used, both of which have the advantage of being non-magnetic, and may additionally have layers of good low temperature conductors, such as copper, aluminium, or gold coated onto them to improve the their thermal conductance. Alternatively, where a metal-insulator composite is chosen, sheets comprising an insulating layer, such as plastic or glass-fibre, combined with elements of highly thermal conducting material, such as copper or gold, may be used. Composites with properties similar to printed circuit boards have the advantage of being particularly easily machined (for example, routed or milled). The layered structure also provides more scope for control over which portions of the structure are to be strongly electrically and/or thermally good conducting. The contact area between the clamping members and the axle member may also be controlled, for example, in combination with the urging force, so as better to achieve the abovementioned balance between high thermal conductance and low friction. Preferably, an inner portion of the clamping members may be provided with a sharpened profile. This arrangement provides a reduced contact area that provides a higher pressure for a given urging force. The higher pressure tends to favour high thermal conductance across the interface. For example, localised welding may occur between the material of the clamping members and the axle member. Additionally and/or alternatively, low thermal conductivity surface layers (caused by oxidation, for example) may be pierced. At the same time, the reduced contact area tends to reduce the magnitude of frictional forces associated with a given urging force. The use of a sharpened profile (i.e. with a wedge-shaped cross-section) is advantageous because the clamping members can be made generally more bulky for a given contact area. Clamping members with a uniform cross-section (i.e. such that the thickness does not decrease substantially towards the edge) would be likely to have lower structural rigidity for the same contact area, and would also wear more quickly. The wedge-shaped profile may also be more efficient for piercing impurity layers.

Preferably, the clamping members are arranged to make a metal-to-metal contact with said axle member over at least a portion of the contact area between the clamping members and the axle member. The metal-to-metal contact provides a good thermal connection while contact area and clamping force tuning can be used as described above to achieve the necessary mechanical/frictional properties.

The heat-sinking path provided by the support member ensures that components or samples to be measured that are mounted on the platform can be cooled effectively to the required temperature of the low temperature stage. Additionally, any heat generated by elements attached to the first platform (during electrical measurements and/or rotation, for example), can be conducted away in an efficient manner. Preferably, a continuous metallic thermal pathway is provided from at least a portion of the metal -to-metal contact through the support member to the base structure or mounting. Additionally and/or alternatively, a continuous metallic thermal pathway may be provided between at least a portion of the metal- to-metal contact and one or more heat- sinking pads/tracks arranged on the first platform. In this way, a good thermal pathway may be achieved between the heat-sinking pads/tracks and the base structure or mounting (and, thereby, the low temperature stage). Components or samples to be measured on the first platform can be conveniently heat-sinked by making thermal connection with one or more of the heat-sinking pads/tracks. According to an embodiment of the invention, the first rotator is a piezoelectric motor and comprises means by which an electrical signal can be applied thereto to control the angle of rotation of the first platform. This arrangement has advantages over systems where the motor is positioned at a higher temperature stage and mechanical connections used to turn a rotating member connected to the platform. For example, only fine electrical wires need to be fed from high temperature stages in the measurement apparatus to the low temperature platform. In contrast to mechanical couplings, these fine wires can be arranged so as to conduct heat only very weakly and therefore need not significantly compromise either the maximum time for which the sample area can be held below a target "base temperature", or the lowest base temperature achievable by the system. Furthermore, because the signal carrying wires do not comprise any moving parts, frictional dissipation of heat is not a problem. The fine wires may be formed from material that is superconducting at operating temperatures of the fridge and within certain magnetic field ranges. Superconducting wires conduct electricity extremely well and heat extremely poorly.

The improved thermal isolation and controlled heat generation associated with the piezoelectric motor arrangement discussed above may improve the efficiency with which the low temperature stage, cryogenic support and, therefore, the components or samples to be measured may be cooled. As mentioned, this may lead to an improved base temperature for the fridge (i.e. the lowest temperature that can be reached) as the best efficiency of cooling is achieved when all parts of the apparatus can be kept in thermal equilibrium during the demagnetisation cooling process.

However, the advantage may be less pronounced for certain types of fridge. For example, compensation of the kind of heat loads that may arise where mechanical connections are made out of the low temperature stage may be possible (although expensive) and/or tolerable in a dilution-type refrigerator, or similar system, where the cooling power is relatively high and can be maintained on a continuous basis for long periods of time. However, in systems based on adiabatic demagnetisation, and other single-shot fridges, additional heat loads of such magnitude are likely to be devastating because they restrict not only the base temperature but also the refrigerator "hold time", which determines the time for which continuous low temperature measurements can be made. In other respects, adiabatic demagnetisation systems have a number of advantages over the alternative fridge mechanisms discussed above. For example, they are intrinsically low vibration because they do not require pumps, have rapid turnaround and are simple to operate and automate.

The piezoelectric motor design may also dissipate less heat than conventional compact motor designs and/or be capable of achieving a higher angular resolution. In addition, the piezo-type motors can also be used in a slow 'scan' mode where small, reproducible displacements, or rotations, can be achieved by the application of controlled DC voltages.

The piezoelectric rotator may comprise a first portion rigidly fixable with respect to a base structure and a second portion that is rotatable about the first axis with respect to the first portion. The first platform may be rigidly connected to the second portion of the rotator. According to a particular embodiment, the first portion of the first rotator may comprise one or more piezoelectric stacks and means for applying an electric field to one or more of the piezoelectric stacks in order to control the length thereof, the piezoelectric stacks being positioned so as to apply a torque about the first axis to the second portion of the first rotator. In particular, the second portion of the first rotator may be arranged to grip a first portion of a transmission rod, the transmission rod having an elongated form in a direction substantially parallel to the first axis and being arranged to interact with the piezoelectric stacks. For example, the transmission rod may be arranged to interact with the piezoelectric stacks by means of paddles extending from a second portion of the transmission rod. The transmission rod may be gripped by the second portion of the first rotator with a force such that: when the transmission rod is made to rotate at a first angular speed, the transmission rod and the second portion rotate at substantially the same angular speed as each other; and when the transmission rod is made to rotate at a second angular speed, greater in magnitude than the first angular speed, the transmission rod rotates at a substantially greater speed than the second portion. According to an embodiment, the angular displacement of the second portion of the first rotator can be controlled by applying a control signal to the piezoelectric stacks that causes the transmission rod to cycle between a state of rotation substantially at the first angular speed and a state of rotation substantially at the second angular speed but in the opposite sense, the total angular displacement being determined by the displacement per cycle and the number of such cycles.

This type of piezo-rotator has the major advantage for a low temperature measurement that the slipping surface (where the transmission rod comes into contact with the second portion of the rotator) can be almost entirely enclosed, and can therefore be protected from contamination by water vapour or finger grease, which could otherwise freeze the rotator solid when cooled to low temperature.

The combination of an in-situ piezoelectric-powered first rotator with a first platform that is rotatably supported by a support member is particularly effective because the support member removes adverse torques and/or protects against accidental knocking or other interference to which the piezoelectric-type motors tend to be highly susceptible (one reason for this is the brittle nature of the piezoelectric material). More generally, the support member helps to maintain a constant and well-defined factional association at low temperatures between the axle and platform, which is crucial for optimal performance. Li particular, the support member ensures efficient operation of the first rotator even when the load on the first platform is high and/or when the first platform is elongated due to its torque supporting action. The above combination also facilitates addition of a second rotator (which may be relatively heavy) to the first platform, the second rotator being configured to rotate about a second axis different from the first axis and thus to provide a second platform that can be rotated independently about the first and second axes. The second rotator preferably comprises a piezoelectric motor, which may be the same as or similar to that used for the first rotator. Where a heat-sinking path is provided via the support member, heat generated by the second rotator can be removed efficiently by this path without having to pass through the first rotator (piezo-electric material is necessarily insulating and therefore a bad conductor of heat). Although the first and second axes may be chosen arbitrarily, it is preferable to arrange them to be perpendicular to each other so that the components or samples to be measured can be oriented most efficiently with respect to an external magnetic field. Preferably, where the magnetic field is arranged to be vertical, the first axis may be horizontal and the second axis vertical. Preferably, the second platform comprises a planar portion defined by a normal vector substantially parallel to the second axis. The second platform may be formed as part of the face of the second rotator, for example. In this arrangement, the weight of elements attached to the first platform provides an adverse torque that is compensated by the support member while the weight of elements attached to the second platform acts downwards through the second rotator, thus providing no adverse torque, or, if the weight is distributed asymmetrically on the second platform, with respect to the second axis, a limited adverse torque. When arranged in this way, the first and second rotators can orient components or samples to be measured in any desired way relative to the applied magnetic field (assuming the field is vertical and rotation of the entire structure about the z-axis has no effect).

According to an alternative embodiment of the invention, the cryogenic support may further comprise a remote measurements arm extending along the first axis substantially beyond the support member and configured to rotate with the first platform, wherein the remote measurements arm comprises a third platform at or near a distal end thereof, which is configured to rotate about an axis parallel to the second axis by means of a coupling between the second rotator and the third platform. Alternatively or additionally, the cryogen support may further comprise a remote measurements arm extending along the first axis through and substantially beyond the first rotator and configured to rotate with the first platform, wherein the remote measurements arm comprises a third (or fourth, if another remote measurements arm is included with its own rotating platform) platform at or near a distal end thereof, which is configured to rotate about an axis parallel to the second axis by means of a coupling with the second rotator. The above arrangements allow samples to be inserted into extremely confined measurement volumes and be rotated in-situ about axes parallel to the first and second axes of the first and second rotators. For example, this arrangement may be used for the rotation of samples in the confined sample region between the poles of the main electromagnetic lenses in an electron microscope - here the sample would be in a magnetic field and in an electron beam and could be rotated in order to perform 3D electron beam tomography. The coupling may preferably be provided by a thread (which may be a conducting wire, for example, which could be used to help heat-sink the third platform) that is wound around a spindle connected to the second rotator at one end, and wound around a groove in a lateral side of the third platform at the other. The coupling may be arranged to pass through a longitudinal opening in the axle member. This arrangement minimizes the critical lateral dimension of the remote measurements arm. This also allows the sample to be located in a region of field, electron or neutron flux, or some other environment, in which it would be impossible or inadvisable also to locate the piezo-motors.

According to embodiments of the invention, the support member and/or axle member is/are formed from one or more of the following: a highly conducting metallic material and a layered metal-insulator composite. For example, the highly conducting metallic material may be copper or aluminium, and may additionally have a gold coating to reduce surface oxidation and improve the members thermal conductance. Where a metal-insulator composite is used, this may include, printed circuit board, or a material with similar physical properties, which would have the advantage of being easily machinable (various machines have been developed specifically for the purpose of machining printed circuit board for use in other contexts). The axle member could be made from a material particularly suited for the type of environment the sample platform will be exposed to. For example, it may be made from aluminium, which is relatively transparent to neutrons, when in a neutron environment, or it may be made from a good thermal conductor when it is required to discharge significant amounts of heat where the platform is exposed to an extended incident radiation or electron flux.

According to the present invention, there is also provided a cryogenic support as defined above when in use at cryogenic temperatures.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which: - Figure 1 a is a perspective view of a cryogenic support comprising a first rotator, a first platform, an axle member and an axle support member according to an embodiment of the invention;

Figure Ib illustrates a cross-section of a support member with hollow pathways for wires; - Figure 2 depicts a sectional plan view of a portion of a piezoelectric motor for use in a rotator according to an embodiment of the invention;

Figure 3 is a sectional side view of a piezoelectric motor for use in a rotator according to an embodiment of the invention;

Figure 4 depicts a transmission rod of the piezoelectric motor comprising paddles coupled to piezoelectric stacks according to an embodiment of the invention;

Figure 5 shows a perspective view of a cryogenic support comprising a first rotator and a second rotator according to an embodiment of the invention; and

Figure 6 depicts a perspective view of a first rotator and a second rotator, wherein the second rotator is coupled to a remote measurement arm.

Figure Ia shows a perspective view of a cryogenic support according to an embodiment of the invention. The support may be connectable to a base structure 100, which in turn is connectable to, or forms part of, a measurement probe to be inserted into a cryogenic measurement device. The base structure may be in good thermal contact with, or form part of, the low temperature stage of the probe. In the example shown, a connection is made using suitably selected screws in combination with screw holes 101. As mentioned above, where a magnetic field is to be applied to a sample mounted on the cryogenic support, the orientation of the field is most easily arranged to be parallel to the axis of the probe, hi the cryogenic probe shown, the base structure 100 is configured to be attached to the probe in such a way that axis Z is aligned with a principle longitudinal axis of the probe. As will become clear below, this orientation allows the sample to be rotated in the magnetic field in a particularly useful way. A platform 104 is provided on which a sample may be mounted and electrical connections (where appropriate) made. The platform 104 in the embodiment shown is arranged to be substantially planar and to be connected to a portion of a rotator 102. The rotator 112 has a first portion rigidly connected to an extension 103 of the base structure 100 and a second portion that can rotate relative to the first portion about an axis X. It is to this second rotating portion that the platform 104 is attached, hi the embodiment shown, the axis X is arranged to be perpendicular to the axis Z, but axis X may also be arranged at an oblique angle to angle Z without departing from the scope of the invention. The width of the platform 104 and the separation of the rotator 102 from the base plate of the base structure 100 is ideally chosen so that the platform can be rotated through 360°. In the case where the apparatus is to be used to measure a substantially planar sample, which is mounted parallel to the planar portion of the platform 104, rotation of the platform 104 between a position parallel to the base plate of the base structure 100 and perpendicular to the base plate of the base structure 100 corresponds to changing the magnetic field orientation from being perfectly out-of-plane to perfectly in-plane.

According to an embodiment of the invention, the rotator 102 is of a piezoelectric type, a rotation being obtained by varying an electric field applied to one or more piezoelectric stacks, arranged so that changes to their length will cause the second portion of the rotator to rotate. Some of the advantages of this type of rotator have been discussed above. Figure 2 shows a cross section of the second portion of the rotator 102 (the portion to which the platform is to be connected). The portion comprises an upper element 200 and a lower element 202 which are pressed together to grip a transmission rod 220 by screws 212 and 214, forcibly engaged with threads 216 and 218 against a resistance provided by springs 208 and 210. Lateral openings 204 and 206 provide access to the screws 212 and 214 in order that the screws 212 and 214 can be tightened or loosened to vary a clamping force on the transmission rod 220. The transmission rod 220 extends longitudinally along an axis pointing out of the page in the orientation depicted and acts to transmit a turning force generated by piezoelectric means to the second portion of the rotator 102.

According to this embodiment of the invention, the transmission rod is fixed with respect to a first portion of the rotator 102 which, in turn, is fixed relative to the base structure 100 via extension 103.

Figure 3 shows an example arrangement of how the second portion of the rotator 102 may be arranged relative to the first portion of the rotator 300. The view depicted is a side cross-sectional view, with openings 222 and 224 showing where the shafts of screws 212 and 214 would pass. The axis of rotation of the transmission rod 220 is along axis X as shown. The lower portion of the transmission rod, as shown in the figure, engages with the piezoelectric actuation means via paddles 306. The paddles 306 are rigidly connected to the transmission rod 220 and to piezoelectric stacks 302 and 304.

Figure 4 depicts a top plan view of the transmission rod paddle and piezoelectric stack assembly. The length of the piezoelectric stacks 302 and 304 may be controlled by applying an electric field perpendicular to the stack elements 402 and 404. The electric field may be established by applying a control signal to piezoelectric actuators 406 and 408 which, in turn, apply a potential difference between electrodes 405 and 407 respectively. The control signal may be generated from user instructions via piezoelectric rotator controller 410, which may be implemented on a computer, for example, at room temperature.

Continuous rotation in one direction is achieved by a stick and slip mechanism. According to this mechanism, the clamping force applied to the transmission rod 220 is controlled so that when the transmission rod is made to rotate in one direction at a relatively slow first angular velocity, the transmission rod 220 is substantially gripped by the second portion of the rotator 102 in such a way that both the transmission rod 220 and the second portion of the rotator 102 rotates substantially in time with each other and through substantially the same total angle. In contrast, when the transmission rod is made to rotate at a second higher angular velocity, slippage occurs between the transmission rod 220 and the second portion of the rotator 102 in such a way that the transmission rod 220 rotates substantially without a corresponding rotation of the second portion of the rotator 102. A net rotation of the second portion of the rotator 102 relative to the base structure 100, can thus be achieved by rotating the transmission rod 220 at the slow gripping speed in one direction and then returning the transmission rod 220 to the start position by rotating at the faster slipping speed in the opposite direction. A continuous rotation can thus be achieved by performing this cycle repeatedly. The angular resolution available in such a rotation mechanism is determined by the angle through which the second portion of the rotator 102 moves in each cycle.

There are a number of possible reasons why prior art systems have preferred not to place the motor for the rotator at the low temperature stage. For example, most conventional motors generate a considerable amount of heat, which may disrupt the cooling power of the fridge if it is allowed to leak directly into the low temperature stage. Furthermore, motors may contribute strongly to the level of background electrical noise, which may reduce the accuracy with which sample properties may be measured. There are also practical considerations, such as the amount of physical space available; this is often restricted in the region of the low temperature stage. The most obvious solution to these problems is to position the motor at a higher temperature stage, where there is generally more room, and where it can be isolated more easily, both from a mechanical and electrical point of view, from the low temperature stage. However many of these transfer mechanisms suffer from the effects of backlash and rotation limited to only a few turns.

In addition, in applications under high magnetic field, there are significant restrictions on the type of materials that can be inserted into (or near) the high field region.

Systems involving permanent magnets are likely to be particularly unsuitable, and even systems based on electromagnets are likely to cause problems. Supercoducting motors have been made, but they have significant frictional heating, even when there is no commutation and it is hard to get low speeds without using gears that introduce their own effects of frictional heating and backlash.

As mentioned above, it is expensive and time consuming to warm a measurement probe from its operating temperature up to room temperature and to cool it back down again. Therefore, it is advantageous to provide a system in which the frequency of such procedures can be minimized. One way in which this can be achieved is by providing a platform on which a plurality of samples can be arranged simultaneously. In this way, studies on several different samples can be carried out either simultaneously or one after another without having to remove the probe in order to switch samples. It is therefore generally preferable to provide a platform 104 that is as large as possible. However, where a magnetic field is to be applied, there may be a significant lateral limits to the platform dimensions due to the narrowness of the magnet bore within which it must be inserted. A further problem is that the weight of the enlarged platform can cause problems for the rotation mechanism that is employed, particularly where the stage is elongated in a direction pointing away from the rotator (such that a significant torque exists about the connection point between the platform and the portion of the rotator to which it is attached. These problems are minimal in systems that use a rotator coupled to a motor located elsewhere due to the design freedom that is available for such a rotator. However, where rotation is powered by a mechanism at low temperatures, design freedom is severely restricted, with the result that the sideways torque provided by the weight of the platform acting about a fixation point in the rotator 102 can severely hamper the efficiency of operation of the rotator 102. This adverse torque can either cause the rotator 102 to work inefficiently, for example by providing increased internal friction, or can affect the reliability and longevity of the rotator 102. Measurements of large magnetic samples in a magnetic field can also produce significant forces and torques on the driven platform.

According to an embodiment of the invention, the above problems can be at least partially circumvented by providing the platform with an axle 108 and supporting the axle 108 with an axle support member 110 which in turn is supported by the base structure 100 (see Figures Ia, 5 and 6, for example). There are a number of special difficulties with this kind of arrangement that would have no direct counterpart in an analogous arrangement for use at room temperature. In particular, special considerations are required when designing the coupling between the axle support member 110 and the axle member 108. The axle member 108 needs to be able to rotate as freely as possible with respect to the axle support member 110 without generating too much heat by friction but at the same time being robust enough to tolerate a certain minimum number of rotations before failure.

A (normally) separate problem is that of sample "heat-sinking". Heat-sinking is vital to controlling the temperature of samples and, in the present context, is achieved by providing a high thermal conductivity connection between the sample and the lowest temperature part of the fridge (the low temperature stage). In a dilution refrigerator, the lowest temperature part of the fridge will be the so-called mixing chamber. For an adiabatic de-magnetisation refrigerator, the lowest temperature part of the fridge will be the material that has been demagnetised in order to create the cooling power. For the embodiment shown in Figure 1, the first stage in achieving good heat sinking for the samples is to heat sink the base structure 100. This is easily achieved by clamping the base structure 100 to a metallic block that forms part of, or is itself in good thermal contact with, the low temperature stage. The problem then reduces to how to achieve efficient thermal contact between the base structure 100 and the samples on the platform 104. One possibility is to provide a metallic path from the extension 103 through the rotator 102 to the platform 104. However, this arrangement is complicated firstly by the fact that the rotator consists of two portions that rotate relative to each other, and secondly by the fact that • the rotator 102 is powered electrically, which restricts how metallic parts can be distributed within the rotation mechanism.

According to an embodiment of the invention, an alternative approach is used that exploits the axle support member 110 and axle 108. Although non-conducting materials such as PTFE may be considered more suitable for providing a low temperature, low friction connection between the axle support member 110 and axle 108, the present embodiment is based on the realisation that an adequately low friction coupling can be achieved using a metal-to-metal connection in which the contact area between the axle support member 110 and the axle 108 is controlled. In order for heat to conduct efficiently across the boundary between the axle support member 110 and the axle 108, however, a certain force needs to be maintained between the two members. This is achieved according to the present embodiment on the one hand by providing sharpened edges to the portion of the axle support member (the clamping members 111) in contact with the axle 108 and, on the other hand, by providing a controlled clamping force. The controlled clamping force is achieved by shaping the axle support member 110 in the form of a spring, such that the clamping members 111 urge inwards against the axle member 108. The combination of urging force and contact area needs to be controlled carefully to achieve a desirable balance between low friction properties and high thermal conductance. The material used for the axle support member 110 and axle 108 are also important parameters in this respect. In general softer materials will achieve the same quality of thermal connection with a lower clamping force. Different materials may be chosen for one or more of the axle support member 110, the clamping members 111 and the axle member 108 as discussed above. In addition, one or more of these components may be coated in a coating material such as gold plating.

Determining an appropriate clamping force or range of clamping forces may be done experimentally, a search being performed to determine forces that give the lowest stage thermal relaxation time, but without the cost of high frictional heating when the stage is in operation. The optimal force(s) may be set when the time taken for the stage to return, after cooling, to a small temperature above its starting temperature is found to be minimised.

As shown, the clamping members 111 are shown as forming an integral part of the support member 110 and are passively sprung. In this arrangement, changes to the clamping force may be achieved by varying the geometry and material of the portion of the axle support member 110 that is shaped in order to urge the clamping members 111 onto the axle member 108. However, alternative arrangements are possible without departing from the scope of the invention. For example, the support member 110 may comprise a plurality of physically separable clamping members (which need not be elastic), which are linked by separate extended elastic members that pull the clamping members towards each other and onto the axle member 108. Alternatively, the separate clamping members may be urged together by other mechanisms, for example magnetic forces or screws. The clamping members may be hinged or otherwise rotatably coupled together. The clamping force may be varied according to these alternative embodiments in a variety of ways. For example, where separate elastic linking members are used, their position relative to the clamping members (and any hinge linking the clamping members) may be varied. Alternatively and/or additionally, the elastic strength of the linking members may be varied. The separate linking members may be arranged to be easily separable from the clamping members to facilitate assembly and disassembly of the apparatus, and to allow efficient optimisation of the clamping force.

As a further development (see Figure Ib) the support member 110 can be adapted to comprise one or more hollow pathways (105) through which electrical wires can be fed through to the first platform (and/or elements connected to the first platform) from external power sources or electronics equipment, for example. Mounting wires in this way can improve reliability by protecting the wires from damage in this region (both directly and by virtue of keeping the wires safely out of the way of moving elements of the cryogenic support and/or unintended disturbance by a user). In the example shown in Figure Ib, two circular pathways 105 are shown in one of the legs of the support member 100. However, other configuration may equally be used, including variations in both the number and shape of the hollow pathways (105).

Figure 5 depicts an embodiment of the invention wherein the platform 104 has been adapted to incorporate a cradle 502 for a second rotator 504. The primary sample mounting area according to this embodiment is on the surface of the second rotator 504 and provides means for rotating a sample about two axes: that of the first rotator and that of the second rotator. The second rotator 504 may operate according to the same principle as the first rotator 102, i.e. may be powered by a piezoelectric motor, itself attached to the first platform, and provided with a portion that rotates in response to an applied electrical signal about an axis different from that of the first rotator. Because the angle of rotation of each of the rotators 102 and 504 depends only a control signal applied thereto, each rotator may be rotated independently of the other. This independence of operation provides a flexibility that is not possible in designs using remote motors and coupling mechanisms to rotatable components at the platform. This embodiment allows a sample mounted on the second rotator 504 to be oriented exactly according to a user's requirements. For example, in the case where an isotropic crystalline material is being measured, with crystallographic axes A, B and C, the magnetic field can be aligned precisely to each one of these axes without removing the sample from the cryostat for remounting between measurements. With a single rotation mechanism, it is only possible to align with respect to two orthogonal axes.

The addition of the second rotator 504 is made particularly advantageous by the provision of the axle support member 110 and axle member 108 because the additional weight of the second rotator 104 need not be excessively detrimental to the performance of the first rotator 102. In addition, the heat sinking mechanism provided when the axle member to axle support member connection is made via a metal-to-metal connection as described above, provides an effective means for removing heat generated in the rotation mechanism of the second rotator 504. The arrangement also reduces vibration (especially important for the electron microscope application, that would otherwise allow the thin remote measurements arm to vibrate with large amplitude), and prevents the sample stage being inclined away from the rotation axis (such as could happen due to flexing of the sample stage under differential thermal contraction or differential surface machining.)

Figure 6 depicts an embodiment of the invention comprising a first rotator 102, a second rotator 504 and a remote measurement arm 600. A third platform 608 is located at a distal end of the remote measurement 600 and is configured to rotate about an axis Z' via a coupling 606 to a spindle 604 mounted on the second rotator 504.

Although shown with the remote measurement arm 600 extending away from the first rotator 102 and past the support member 110, an axial opening may be provided in the first rotator and the remote measurements arm (or a second remote measurements arm) may extend in this direction instead (or in addition), through and behind (in the orientation shown) the first rotator. The coupling 606, may for example be a flexible thread provided with frictional properties such as to grip the spindle 604 and a corresponding lateral portion of the third platform 608. A remote measurement arm of this type may be useful in applications where space is extremely limited but in which it is nevertheless advantageous to be able to rotate the sample with respect to two non-parallel axes. It may be used, for example, for rotation of a sample in the beam of an electron microscope for collecting a series of projections at different angles that can be recombined (using tomographic techniques, for example) to produce a complete three dimensional image with very high spatial resolution. Here the sample is cooled in order to reduce vibration and to minimise the sample damage due to electron beam damage. Very low vibration of the sample is required in order to obtain the maximum resolution, but this is where the piezo- based rotators, which have zero-backlash and can be rigidly held are particularly appropriate for use as a driving element, because there is no freedom to move when the rotator is not being driven. In the above description, the angle of the first rotator 102 relative to a primary longitudinal axis of the measurement probe has been depicted as perpendicular. This is envisaged as being the most likely implementation, but the rotator 102 may equally be mounted at an oblique angle relative to the longitudinal axis of the measurement probe. Similarly the axis of rotation of the second rotator 504 may be arranged at any non-zero angle with respect to the angle of rotation of the first rotator 102, and need not necessarily be parallel to the longitudinal axis of the measurement probe.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

Claims

1. A cryogenic support comprising: a first platform extending in a first direction between first and second opposite sides and rotatable about a first axis substantially parallel to the'first direction; a first rotator attached to the first side of the first platform for supporting the first platform and selectively rotating the first platform about the first axis; and a support member for rotatably supporting the second side of the first platform for rotation about the first axis, wherein the first rotator is a motor operable when held at cryogenic temperatures.

2. A cryogenic support according to claim 1 , wherein at least one of said support member and said first rotator are connectable to a base structure.

3. A cryogenic support according to claim 1 or 2, wherein said support member comprises a hollow portion for providing access for electrical connections to said first platform.

4. A cryogenic support according to claim 1, 2 or 3, wherein the first platform has an axle member that engages with the support member.

5. A cryogenic support according to claim 4, wherein the support member comprises clamping members and an urging device to press the clamping members against the axle member.

6. A cryogenic support according to claim 5, wherein the urging device comprises at least one elastic member removably coupled between the clamping members so as to press the clamping members against the axle member.

7. A cryogenic support according to claim 5 or 6, wherein an inner portion of the clamping members is provided with a sharpened profile.

8. A cryogenic support according to claim 5, 6 or 7, wherein the clamping members are arranged to make a metal-to-metal contact with the axle member over at least a portion of the contact area between the clamping members and the axle member.

9. A cryogenic support according to claim 8 when dependent on claim 2, wherein a continuous metallic thermal pathway is provided from at least a portion of the metal-to-metal contact through the support member and, when connected, to the base structure.

10. A cryogenic support according to claim 8 or 9, wherein a continuous metallic thermal pathway is provided between at least a portion of the metal-to-metal contact and one or more heat sinking pads or tracks arranged on the first platform.

11. A cryogenic support according to any one of the preceding claims, wherein the first rotator is a piezoelectric motor comprising means by which an electrical signal can be applied thereto to control the angle of rotation of the first platform.

12. A cryogenic support according to any one of the preceding claims, wherein a second rotator, configured to rotate a second platform about a second axis, which is not parallel to the first axis, is attached to the first platform.

13. A cryogenic support according to claim 12, wherein the second rotator is a piezoelectric motor comprising means by which an electrical signal can be applied thereto to control the angle of rotation of the second platform.

14. A cryogenic support according to claim 12 or 13, wherein the second axis is perpendicular to the first axis.

15. A cryogenic support according to any one of claims 12 to 14, further comprising a remote measurements arm extending along the first axis substantially beyond the support member and configured to rotate with the first platform, wherein the remote measurements arm comprises a third platform at or near a distal end thereof, which is configured to rotate about an axis parallel to the second axis by means of a coupling with the second rotator.

16. A cryogen support according to any one of claims 12 to 15, further comprising a remote measurements arm extending along the first axis through and substantially beyond the first rotator and configured to rotate with the first platform, wherein the remote measurements arm comprises a third platform at or near a distal end thereof, which is configured to rotate about an axis parallel to the second axis by means of a coupling with the second rotator.

17. A cryogenic support according to claim 15 or 16, wherein the coupling is provided by a thread, the thread being wound around a spindle connected to the second rotator at one end, and wound around a groove in a lateral side of the third platform at the other.

18. A cryogenic support according to claim 15, 16 or 17, wherein the coupling passes through a longitudinal opening in the axle member.

19. A cryogenic support according to any one of the preceding claims, wherein the support member and/or the axle member is/are formed from a metal-insulator layered composite.

20. A cryogenic probe comprising a cryogenic support according to any one of the preceding claims.

21. A cryogenic probe for use in an adiabatic demagnetisation refrigerator comprising a cryogenic support according to any one of the preceding claims.

22. An adiabatic demagnetisation measurement system comprising a cryogenic support according to any one of the preceding claims.

23. An electron microscope having a cryogenic support according to any one of claims 15 to 19.

24. A cryogenic support substantially as hereinbefore described with reference to and/or as illustrated by any one of the accompanying drawings.