The invention relates to the field of vacuum ion pumps and particularly pertains to vacuum pumping applications with cryostat housed magnets.
Vacuum ion pumps have been well studied over a period of more than 40 years, are the subject of a vast literature and have proven to be successful commercial products for maintenance of high vacuum. A review of such apparatus may be found in M. Hablanian, High Vacuum Technology: A Practical Guide, pp. 360-372, (Marcel Dekker, Inc., 1990).
Although ion pumping apparatus is not limited to a particular geometry, the vacuum ion pump ordinarily includes an anode formed from an array of close packed, axially symmetric (tubular) metal elements and a cathode surface normal to the axis of the tubular array and spaced apart from the anode. (“Tubular” includes hexagonal, rectangular, circular, square and other cross sections for the purposes of this work.) The cathode(s) and anode(s) are disposed within a hermetic sealed housing. A schematic representation of a single elementary pumping cell of prior art is illustrated in
The superconducting magnet that is a central component of modern NMR (including magnetic resonance imaging) apparatus is housed in an axially symmetric cryostat having a room temperature bore aligned along the axis of the magnet. A typical cryostat may include one or more heat shields and one or more reservoirs containing a liquefied gas (cryogen), the latter being vented to atmosphere or arranged in an appropriate closed cycle arrangement. The heat shields and reservoirs are contained in a vacuum insulated environment to thermally isolate these components from the ambient environment. The magnet is carefully designed to create a region of extremely homogeneous or (controlled gradient) magnetic field along the bore. This is a dipole field with the field distribution substantially confined to a central region, then flaring out at the ends of the bore to close by return from flared-out end region to the opposite flared-out end region. The field proximate to the ends of the bore flares out, that is, the field disperses radially and curves around the exterior of the magnet windings to return via the other end of the magnet windings. The field is thus diminished in density in the region external to the magnet windings and is termed a “fringe”, or “stray” field. While these magnets are designed with means to protect the inner region of the bore from outside magnetic influences and to minimize the magnetic field in the environment of the magnet, it is sufficient to observe that there is a substantial fringe field in the environs of the magnet. Because these magnets are capable of very high field intensity, the fringe fields, even when minimal, are far from negligible in proximity to the magnet/cryostat. The fringe field of such apparatus is a source of possible damage or anomalous behavior to some instrumentation, credit cards and possibly dangerous to human beings with pacemakers or defibrillators implanted.
It is apparent that the thermal performance of the cryostat is adversely affected if the gas pressure in the evacuated region should rise to the point that such higher pressure supports non-radiative mechanisms for heat transport, compromising the function of the cryostat. Conventional arrangements to maintain satisfactory vacuum conditions in such apparatus typically include a “getter” or sorbing agent within the vacuum space to trap gases.
Superconducting magnets are often identified with magnetic resonance instruments, a major application thereof. In the particular case of NMR apparatus, a homogeneous magnetic field is a basic requirement. The NMR phenomena is also critically dependent upon an RF probe coil (or coils) for exciting resonance and for coupling the resonance fields to sensitive detecting apparatus. A very much improved signal-to-noise ratio (SNR) is achievable for NMR data acquired with the RF coil maintained at low temperature. Several factors contribute to this temperature dependence of the signal. It is understood that cooling the RF coil to the superconducting phase transition of the coil conductor represents one goal, but it is still quite desirable to operate the RF coil at depressed temperatures that need not reach as far as a superconducting transition. Accordingly, there is a need for thermal isolation of a cooled coil, and enclosure in vacuo is necessary.
The apparatus under discussion may be regarded as including as many as two separate cryostat structures. One of these is the housing (cryostat) surrounding the actual magnet windings and the other is the cryo-housing or dewar of a low temperature RF coil. The interior regions of these cryostats include spaces maintained at extremely low pressure to provide thermal insulation. In the example of a common magnet cryostat, one or more cryogens boil slowly from respective cryogen reservoirs thereby removing heat from the local environment and the resulting vapor escapes to the atmosphere through a necktube. Closed cycle systems are also available to provide refrigeration apparatus to remove heat from the magnet windings and radiation shields without loss of the cryogen(s) or, without the need for cryogen(s) at all. In any case, the degree of thermal isolation from ambient environment requires satisfactory vacuum for the cryostat interior between the cryostat outer walls and the cryogen reservoir. An RF coil cryostat, or dewar, similarly employs a vacuum jacket containing a refrigerating mechanism removing heat from a cold heatsink (“cold head”) cooled by cryogen circulated therethrough. The RF coil thermally communicates with the heatsink. In prior art, the dewar is maintained at the desired low pressure by a mechanical pump communicating with this vacuum jacket. It is also known to employ a sorb to sequester gases evolving (outgassing) from the walls of the jacket and other components located therein. Mechanical pumps pose a problem in that the vibration spectrum must be carefully suppressed to prevent introduction of artifacts into the NMR spectrum. Sorbs do not enjoy a very long useful life and must be renewed or re-activated.
It is also known that dewar housed RF coils for NMR studies are a source of pressure dependent RF noise triggered by high power RF pulses. This effect is described in U.S. Pat. No. 4,240,033, commonly assigned. That work taught moderation of the effect to some degree by insertion of a material to serve as a baffle for reducing the probability of collisions involving ions generated in the dewar. The effect is thus attributed to breakdown of residual gasses in the vacuum space of the RF coil dewar.
The present invention, in a first embodiment, achieves vacuum ion pumping in and close to a magnet cryostat with a vacuum ion pump that utilizes the fringe field of the cryostat-housed magnet. Such vacuum ion pump represents a less expensive and less weighty pump than prior art vacuum ion pumps. In one embodiment the elements of the pump form internal structure within the space to be evacuated, such as within the evacuated volume of the magnet cryostat, disposed in a region thereof where a significant component of the fringe field is oriented substantially parallel to the axis of the anodes. In a variation of this embodiment, the pump is located outside the main structure of the magnet cryostat with gas communication to the evacuated region, but in sufficient proximity to regions of the fringe field where the orientation and intensity is sufficient for vacuum ion pump operation. In yet another embodiment, another evacuated volume, in close proximity to the cryostat structure but independent thereof, is maintained at a desired vacuum. An example of the latter embodiment is a dewar enclosing the cooled RF coil of an NMR apparatus where such dewar is located in the bore of the cryostat and the supporting apparatus housing a suitable cathode/anode array is necessarily located proximate one end of that bore intercepting a substantial fringing field of the magnet. The latter embodiment is especially advantaged over conventional mechanical pumping for this purpose because the issue of vibration isolation from such mechanical pump is avoided, and the particular pumping properties of the vacuum ion pump aid in suppression of RF noise in the acquisition of NMR data.
a shows a portion of an NMR spectrum of a sample at a selected high value of the probe dewar pressure.
b is the same NMR spectral region as
The geometry of the cryostat typically includes a bore 40 for insertion of additional investigative apparatus or sample handling at ambient temperature. Such magnets are very frequently employed for NMR studies. Superconducting magnets are designed to produce extremely homogeneous magnetic field intensity within the bore 40 but the field distribution includes a portion external to the bore, representative magnetic field “lines” 60, 60′ of constant magnitude being shown. It is understood that the magnetic field is characterized by a continuous spatial distribution. Outside of the central region of the bore 40, the field is referenced as “fringe field” having direction and intensity that depends upon the spatial region of interest. Many such superconducting magnets include windings specifically designed to attenuate the fringe fields in regions external to the magnet or to shield the interior of the magnet bore 40 from magnetic influences arising externally. As a practical matter, the fringe field within and proximate to the cryostat remain substantial, at least in certain such regions. For example, a typical (unshielded) 4.12 Tesla magnet exhibits a fringe field magnitude of about 1200 gauss at the lower cryostat housing surface and about 6 inches off axis. The fringe field distribution is a property of the specific magnet design and varies rapidly with position. Fortunately, functional ion pump operation is not unusually sensitive to magnetic field intensity and direction beyond certain minimal requirements.
In
The separation of anode and cathode may be conveniently termed a line of separation. While the actual shape and relative orientation of these electrodes determines the actual electric field distribution between them at a given potential difference, it is sufficient for descriptive purposes to recognize a gross electric field direction along a line of separation. The anode and cathode are disposed to present a substantial component along the local fringe field direction. A substantially parallel orientation for these electric and magnetic (fringe) fields is desirable to produce a magnetically confined plasma discharge with axial extraction of positive ions from the tubular anode. The (average) angle to which the fields intersect will determine the efficiency of pumping.
The required pumping speed of the pump 10 is estimated from the cryostat dimensions and thermal design.
Q=Q0 e−t/τ
An estimate of the initial outgassing rate Q0 provides guidance for the requirements for the pumping element. Consider that the interior surface of the containment vessel remains an outgassing source, whereas the thermal shields and cryogen reservoirs are at very reduced temperatures and contribute negligibly to outgassing. These surfaces stabilize most residual gasses through the phenomena of crypumping. The principle contribution therefore remains the interior surface of the containment vessel. For stainless steel, outgassing has been quoted at 2×10−7 torr•liters/sec/cm2.
One commercially available vertical 600 MHz superconducting magnet cryostat has gross dimensions of about 1 meter (o.d.) by 1.4 meter high, of conventional construction as suggested above, and is typically evacuated to about 5×10−5 torr and sealed off. When filled with the LN2 and He cryogens the internal pressure is typically about 5×10−8 torr. From the (internal) surface area of the containment vessel and the quoted initial outgassing rate for stainless steel, the value of Q0 is estimated at 0.017 torr•liter/sec. As noted above the outgassing contribution to pump load decreases exponentially with time. A typical 8 liter per second pumping array therefore pumps at about 340 times the maximum (initial) outgassing rate at ambient temperature. Such a pumping array may be disposed as an integral cryostat component. A typical 8 liter/second ion pump array such as Varian part 87-900-064-01(A) is quoted to exhibit an operational life of 40,000 hours at about 10−6 torr. Except during the initial cryogen filling, the internal pressure is about 2 orders of magnitude lower pressure than quoted for this performance parameter. Accordingly, the operational lifetime for the pump is expected to exceed the useful lifetime for the magnet.
Returning to
In one example of this apparatus, the vacuum space of the dewar 70 represents a volume of about 1.5 liter. Communicating with this vacuum space, an 8 liter/sec ion pump (without a pump magnet) was disposed about 6 inches under a cryostat housing an unshielded 4 Tesla magnet and radially displaced by about 3 inches from the outer periphery of the bore 40 of the magnet. The intensity of the fringe field was found to vary by about 30% over the space occupied by the ion pump. Satisfactory vacuum conditions for thermal isolation were readily achieved. In operation over a period of weeks, ion pump current indicated a vacuum pressure of <10−6 torr.
Another benefit has been found for the inventive embodiment of cryo-dewar housed RF coils for NMR measurements. Turning now to
The reduction in noise contribution is readily recognized in comparison of noise phenomena corresponding to the high pressure datum A and the relatively low pressure datum B as shown in
The application described above for this embodiment particularly benefits from active pumping. It is believed that free hydrogen is produced from the dissociation of residual water vapor through collisions of energetic molecules deriving kinetic energy from the RF power pulses. The present invention provides the desired pumping without imposition of the magnetic field of an additional magnet, specific to the pump.
The exploitation of the fringe field of high field magnets for operation of vacuum ion pump apparatus is found to provide the necessary magnetic field for effective pump operation. In this manner there is no requirement for a separate, bulky and heavy ion pump magnet specific to ion pump operation. The invention is not limited to cryo-statically housed magnets: fringe fields of room temperature magnets in diverse applications may supply the magnetic fringe field environment suitable to ion pump operation in a vacuum vessel proximate said magnet. Although NMR apparatus has been discussed here, other analytic instruments requiring substantial magnetic fields will also benefit from the invention.
Although this invention has been described with reference to particular embodiments and examples, other modifications and variations will occur to those skilled in the art in view of the above teachings. It should be understood that, within the scope of the appended claims, this invention may be practiced otherwise than as specifically described.