Patent Grant
11499536
The present invention relates to the field of piston pumps, and more specifically, to piston pumps employing cylindrical, magnetically-coupled drive mechanisms.
Experiments based on noble elements such as gaseous or liquid argon or xenon utilize the ionization and scintillation properties of the target materials to detect radiation-induced recoils. A requirement for high light and charge yields is to reduce electronegative impurities well below the ppb1 level. To achieve this, the target material is continuously circulated in the gas phase through a purifier and returned to the detector. Additionally, the low backgrounds necessary dictate low-Rn-emanation rates from all components that contact the gas.
Detectors based on noble elements have become widespread in many applications such as Compton telescopes, ionization calorimeters, neutrino-less double-beta-decay searches, and direct dark matter detection experiments. Radiation-induced recoils in the detector medium produce scintillation and ionization signals that are read out by photosensors or charge sensors. As these detectors become larger, the propagation of the light and charge must improve to reach the required threshold and energy resolution, necessitating an increased demand on gas-purifying systems. Similarly, backgrounds from radioactive impurities like Rn must be minimized to reach high sensitivity to rare events.
The operational specifications are dominated by the need to drift electrons over lengths 1 m and greater. Electronegative elements like O2 and H2O are continually introduced to the detector material by out-gassing of detector components. As these, and other electronegative impurities, impede charge and light propagation, they are continuously removed, usually by pumping the detector material in the gaseous phase through a heated metal getter, then returning it to the detector. In the case of XENON1T, an O2 equivalent concentration in the xenon below 1 ppb is required to drift charge over a 1 m scale without appreciable electron attenuation via attachment to impurities.
The other aspect of detector purity involves Rn mitigation, which is predominantly handled by careful selection of materials with low Rn-emanation rates. The gas handling and purification systems, including the pumps, are a key contributor to the internal Rn background. Incorporating radiopurity screening in a collaborative effort with pump manufacturers provides some level of success, reaching Rn emanation rates of a few mBq, but further Rn reduction by an order of magnitude is still needed.
At the same time, since detectors become larger, the requirements on pumps increase in kind. Larger detectors need a higher purity to reach the same level of charge attenuation. This is coupled with the fact that there is more material to clean, which necessitates a pump with significantly improved performance to allow high throughput at a pressure differential greater than 1 bar.
Since commercial pumps often introduce electronegative impurities from lubricants on internal components or through small air leaks, and are not designed to meet the radiopurity requirements, custom-built pumps are an advantageous alternative. The present technology is directed to a new pump developed based on a magnetically-coupled piston. In some embodiments, it is used in a hermetically sealed low-Rn-emanating vessel and delivers high performance for noble gases, reaching more than 210 standard liters per minute (slpm) with argon and more than 170 slpm with xenon while maintaining a compression of up to 1.9 bar. Embodiments of the present technology demonstrating its capability for noble gas detectors and other applications requiring high standards of gas purity.
Accordingly, objects of the present technology include a pump that features a larger effective volume in combination with an enhanced magnetic gradient based on alternating polarity to allow O(kN) coupling forces between internal and exterior magnets. In some embodiments, such a pump reaches flow rates at O(100) slpm at a pressure differential up to 2 bar. Some embodiments have potential application in the XENON dark matter project and the nEXO neutrino-less double-beta-decay experiment. In one embodiment, the pump body has a length of 520 mm and an inner diameter of 127 mm.
According to another embodiment of the present technology, a magnetically coupled piston pump including a piston, a pump body, and a set of drive magnets is provided. The piston includes a set of piston magnets. Each piston magnet has poles arranged along a longitudinal axis of the piston and each piston magnet is arranged in the piston so that the orientation of the poles of each piston magnet is the opposite of the orientation of each adjacent piston magnet. The piston is disposed in and is permitted to move within the pump body. The set of drive magnets is arranged outside the pump body. Each drive magnet corresponds to a piston magnet and each drive magnet has poles arranged along the longitudinal axis. Each drive magnet is arranged so that the orientation of the poles is opposite the orientation of the poles of the piston magnet to which it corresponds.
In some embodiments, the pump further includes a drive unit coupled to the drive magnets for moving the drive magnets along the longitudinal axis.
In some embodiments, the pump further includes a first flange attached to a first end of the pump body, and a second flange attached to the second end of the pump body. The first and second flanges each have inlet and outlet ports that permit fluid to enter and exit the pump body. In some embodiments, the fluid is argon gas. In other embodiments, the fluid is xenon gas.
In some embodiments, the pump further includes a cooling system for regulating the temperature within the pump.
In some embodiments, the set of piston magnets includes at least two permanent magnets, and the set of drive magnets includes at least two permanent magnets. In other embodiments, the set of piston magnets and the set of drive magnets each include three permanent magnets.
In some embodiments, the piston and drive magnets are ring magnets.
In some embodiments, each drive magnet of the set of drive magnets includes a plurality of bar magnets attached to a frame and arranged in a cylindrical configuration.
In some embodiments, the pump body is hermetically sealed and the piston is sealed against an interior wall of the pump body with a plurality of gaskets formed of ultra-high molecular weight polyethylene.
In some embodiments, the piston magnets and the drive magnets are configured to have a coupling strength of about 790N to about 3500N. In other embodiments, the piston magnets and the drive magnets are configured to have a coupling strength of about 3500N.
In some embodiments, adjacent magnets of the set of piston magnets and the set of drive magnets are evenly spaced along the longitudinal axis. In other embodiments, the set of piston magnets and the set of drive magnets have about 100 mm spacing between adjacent magnets along the longitudinal axis.
In some embodiments, the pump further includes a gap of about 8.5 mm transverse to the longitudinal axis between the set of piston magnets and the set of drive magnets.
According to yet another embodiment of the present technology, a magnetically coupled drive mechanism including a piston, at least three drive magnets arranged outside the piston, and a drive unit is provided. The piston includes at least three piston magnets. Each piston magnet has poles arranged along a longitudinal axis of the piston, and each piston magnet is arranged in the piston so that the orientation of the poles of each piston magnet is the opposite of the orientation of each adjacent piston magnet. Each drive magnet corresponds to a piston magnet. Each drive magnet has poles arranged along the longitudinal axis, and each drive magnet is arranged so that the orientation of the poles is opposite the orientation of the poles of the corresponding piston magnet. The drive unit is coupled to the drive magnets for moving the drive magnets along the longitudinal axis.
In some embodiments, the piston and drive magnets are ring magnets. In other embodiments, each of the drive magnets includes a plurality of bar magnets attached to a frame and arranged in a cylindrical configuration.
In some embodiments, the piston is configured to be disposed in and permitted to move within a cylindrical pump body, and the drive unit and drive magnets are arranged outside the cylindrical pump body.
Further objects and features of the present technology will be apparent from the below description and attached drawing figures.
As used herein when describing a measureable numerical value, the term “about” includes the specific numerical value and a reasonable range encompassing the specific numerical value to account for, e.g., errors inherent in the measuring equipment used and in the humans handling such equipment. The reasonable range can be ±5%, or even ±10% of the specific numerical value. Thus, for example, a recitation of “about 100N” could reasonably include the range of 90-110N without departing from the spirit and scope of the present technology.
To drive a piston using coupling of permanent magnets, some embodiments of the present technology include an optimized magnet configuration. In some embodiments, the pressure specifications on this pump are around a factor of 2 higher than those of previously designed pumps, such as the EXO-200 pump. This, combined with the flow increase, requires improved magnetic coupling.
The embodiment shown in
In some embodiments, a variety of magnet configurations and the corresponding coupling forces were compared. Based on the availability of commercial magnets, several parameters were fixed and used as constraints in the simulations. The piston magnets 5 considered had a field strength of 1.32 T and were solid cylinders with a diameter of do=120 mm. The drive ring magnets 6 had the same field strength of 1.32 T and had inner and outer diameters that ranged from 130 mm to 200 mm. The length of the magnet pairs L and the spacings s along the longitudinal axis 7 of the pump body 3 between adjacent magnet pairs were varied, as was the number of magnet pairs used.
In some embodiments, the variation of the number and length of the magnets was performed simultaneously to directly compare the difference between the same magnet mass in different arrangements. For example, two magnet pairs of a given length have the same mass as four magnet pairs that are half as long. In these embodiments, magnet lengths L of 25 mm, 51 mm, and 76 mm were considered (labeled 12, 13, and 14, respectively), with the number of magnet pairs ranging from three to five and an inter-magnet spacing of s=10 mm. The drive magnets 6 used in these simulations had an inner diameter of Di=140 mm and an outer diameter of Do=200 mm. The resulting coupling force for several exemplary embodiments is shown as a function of piston displacement in
In some embodiments, the spacing s along the longitudinal axis 7 between adjacent magnet pairs was analyzed. In these embodiments, two magnet pairs were used with piston magnets 5 measuring L=20 mm in length and drive magnets 6 having an inner diameter Di=140 mm and an outer diameter Do=200 mm. The spacing s was then varied from 2 mm to 20 mm. As shown in
In some embodiments, the optimal inner and outer diameters were determined by experimenting on the size of the drive magnets 6. For determining the optimal outer diameter, the inner diameter was fixed at Di=140 mm, and the outer diameter Do was varied from 150 mm to 200 mm. As shown in
For determining the optimal inner diameter Di, the thickness of the drive magnet 6 was fixed at 20 mm. All other parameters were fixed to the same values as for the outer diameter experiments, and the inner diameter Di was varied from 130 mm to 160 mm.
In preferred embodiments, the piston 2 has a set of three rings of permanent neodymium magnets 5, each having a strength of 1.32 T, a length L of 20 mm, an inner diameter di of 80 mm, and an outer diameter do of 120 mm. The magnetization of the rings is oriented along the longitudinal axis 7 of the pump body 3, but with alternating direction, as described above. In some embodiments, the magnets 5 are supported by a custom non-magnetic aluminum support structure that maintains a 10 mm gap between each ring magnet. Piston 2 is preferably contained in stainless steel cylinder 19 having a length of 155 mm and outer diameter of 125 mm, which is hermetically sealed via laser welding to ensure no contact between the fluid and the magnets 5, as shown in
In some embodiments, the gasket design differs to create a larger sealing contact between the piston 2 and the interior wall of the pump body 3. The gasket design in these embodiments is optimized to minimize the dead volume between the end cap of the pump body 3 and the end of the piston 2, thus accomplishing a more complete exhaust of fluid in each stroke. This improves performance and reduces adiabatic heating of fluid that remains inside the pump body 3.
In some embodiments, the fluid used in pump 1 is a gas. Preferably, the gas is a pure noble gas, such as argon or xenon. Because the gaskets 20 are directly in contact with the pure noble gas, the gasket material must have low rates of outgassing, Rn emanation, and physical wear. UHMWPE has been shown to perform adequately in these regards, and thus is used in preferred embodiments. By utilizing a vertical orientation of the pump body 3, a more symmetrical alignment of the piston 2 is achieved, which further reduces wear rate.
Preferably, the piston 2 is magnetically coupled to a set of drive ring magnets 6 located outside the pump body 3. In some embodiments, each of the drive ring magnets 6 are constructed of a plurality of permanent neodymium bar magnets 21, each having a strength of 1.38 T measuring 20 mm×10 mm×20 mm in a cylindrical arrangement, with the 10 mm dimension tangent to the circumference. In some embodiments, the drive ring magnets 6 are supported by a custom nonmagnetic aluminum frame 22, as shown in
In preferred embodiments, the piston 2 and drive unit 4 have three pairs of corresponding ring magnets 5/6 configured to yield a maximum coupling force of about 3500N. This coupling strength corresponds to a pressure difference of about 2.7 bar across the piston 2. Thus, in some embodiments, magnets 5/6 yield a maximum coupling force in the range of about 790N to about 3500N.
Preferably, the pump 1 is mounted vertically. In some embodiments, the drive unit 4 is driven with a linear drive composed of an electric cylinder (SEW, CMS) that is powered by a frequency converter (SEW, MDX 61B). In some embodiments, the converter uses a Modbus connection, allowing operation via an external slow control environment.
In some embodiments, the pump 1 also includes a cooling system to avoid demagnetization of the neodymium magnets 5/6 at critical temperatures higher than 70° C. during continuous operation. As shown in
In some embodiments, temperatures are monitored by three PT1000 temperature sensors in combination with an internal temperature sensor of the linear drive. Preferably, TT1 is installed at the gas inlet, TT2 is installed at the gas outlet, and TT3 is installed inside the pump body 3 just below the top flange 15. TT3 is important because it is mounted at the point of highest fluid compression, and thus, at the point of highest temperature.
In some embodiments, pressure sensors PT1 and PT2 measure the pressures before and after the pump body 3. The differential pressure across the pump 1 is then defined as ΔPpump=PT2−PT1 and measures the pressure difference between the inlet and outlet of the flanges 15. In some embodiments, PT3 and PT4 are located next to the compression volumes of the pump body 3 and can therefore measure pressure differential across the piston 2 as ΔPpiston=PT3−PT4, or the pressure above the top of the piston 2 minus the pressure below the bottom of the piston 2. With this definition, a positive ΔPpiston corresponds to an upward stroke, while negative values correspond to a downward stroke. Handvalve HV3 is preferably closed during normal operation to isolate the two ends of the pump body 3, and is only opened for cleaning under vacuum.
In some embodiments, during standard operation, if ΔPpiston exceeds the critical decoupling pressure of ΔPcrit=2.7 bar, when force of the fluid against the piston 2 plus the friction of the gaskets 20 against the interior wall of the pump body 3 is greater than the magnetic coupling force, the piston 2 will decouple from the drive unit 4. As shown in
In some embodiments, tests were performed to monitor the temperature evolution during operation using argon at an inlet pressure of (1.45+/−0.05) bar, a flow of (103+/−4) slpm, and a ΔPpump of (0.76+/−0.05) bar.
In some embodiments, maintain a constant ΔPpiston is non-trivial, due to the fact that the pressure rises slowly over the piston stroke. A linear driving profile (standard profile) that drives the piston 2 at a constant speed over the stroke is inefficient as the maximum ΔPpiston is reached very late in the stroke. This was demonstrated using xenon gas at an inlet pressure of (1.8+/−0.1) bar and measuring the maximum flow and pressure differential, as shown in
In some embodiments, further improvement via profile optimization is shown in
In some embodiments, to characterize the performance of the pump 1 quantitatively, measurements of flow vs. differential pressure were made using both xenon and argon at different inlet pressures. In some embodiments, the inlet pressure can be controlled by closing handvalve HV2 within the recirculation circuit stepwise, by varying the amount of the gas in the system, or by changing the linear drive velocity. Thereby, the pump performance can be tested for a wide operation range. Tests were made at a constant pump inlet pressure, as measured by PT1. For all measurements, the pump was operated at maximum ΔPpiston, which is the condition of maximal flow. Multiple measurements were then made of the flow and ΔPpump. The results of the flow and ΔPpump are averaged over several strokes of the pump, and are shown for argon in
For argon, flows exceeded the capacity of the mass flow controller, which has a maximum flow for argon of 211 slpm. This was achieved for several inlet pressures ranging from 1.2 bar to 2.6 bar. In particular, at a 2.6 bar inlet pressure, a maximum differential pressure of ΔPpump=1.85 bar was reached at a flow of 95 slpm. At the maximum flow allowed by the mass flow controller of 211 slpm, a differential pressure of 1.72 bar was obtained for the same inlet pressure. For xenon, a maximum flow of 171 slpm was achieved with a ΔPpump of 1.45 bar at an inlet pressure of 3.0 bar. The maximum differential pressure of ΔPpump=1.9 bar was reached with a flow of 45 slpm at the same inlet pressure of 3.0 bar.
In some embodiments, the steep flow vs ΔPpump curves for argon and xenon at high inlet pressure show that the pump 1 works essentially as a pressure amplifier in this performance regime, with the flow largely dominated by the impedance of the circuit. This is consistent with the fact that the pumping mechanism is effectively adiabatic compression, with the subsequent motion of the gas being passive flow through the impeding circuit. This feature is more extreme in the argon data. This, coupled with the lower flow rates of xenon compared to argon, is indicative of the increased difficulty of pumping a heavy gas like xenon. Due to this, performance with lighter gases like neon is expected to be even better than for argon.
In some embodiments, the relatively flat curves for both argon and xenon at low inlet pressure indicate a different performance regime. At inlet pressures below 1.2 bar for argon and 0.8 bar for xenon, the limited power of the linear drive was insufficient to drive the piston 2 fast enough to reach maximum ΔPpiston, resulting in a characteristically different flow to pressure relationship.
In some embodiments, performance improves with higher inlet pressures. The anti-correlation between flow and ΔPpump in other embodiments of this pump. Embodiments requiring a large pressure differential often include lower flow requirements, and those with high flow requirements often include low impedance circuits. Another important pump characteristic is long term stability with high performance, as this is required for many embodiments. To this end, a study of the stability in a high performance state was performed using xenon.
For some embodiments, such as for use in rare event experiments, the radon emanation of pumps is of crucial importance. To assess this aspect of the pump 1, a measurement was performed to determine the emanation rate of 222 Rn from the interior of the fully-assembled pump 1. A gas sample was extracted and measured following the method in G. Zuzel and H. Simgen, High sensitivity radon emanation measurements, Applied Radiation and Isotopes 67 (2009) (5), pp. 889-893, resulting in an emanation rate of (330±60)μBq, an order of magnitude cleaner than what has been achieved in commercial pumps. This meets the current radiopurity needs for low background environments. In other embodiments where further Rn reduction is necessary, surface treatments to minimize the surface area and Rn emanation are implemented.
Thus, some embodiments of the present technology relate to pump designs with a magnetically-coupled drive mechanism based on an alternating magnet configuration. Embodiments of the technology have particular use in noble gas applications. The complete isolation of the drive from the gas and the usage of only clean components in some embodiments yields a high purity, with a 222Rn emanation rate of (330±60) μBq. A stable performance of more than 210 slpm for argon and more than 170 slpm for xenon combined with a compression up to 1.9 bar makes the pump according to an exemplary embodiment a promising tool for many noble gas and high purity applications.
Although the technology has been described and illustrated with respect to exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions, and additions may be made therein and thereto, without parting from the spirit and scope of the present technology.
This application claims the benefit of U.S. Provisional Patent Application No. 62/796,798, filed Jan. 25, 2019, which is incorporated by reference as if disclosed herein in its entirety.
This invention was made with government support under grant #1719259 awarded by the National Science Foundation. The government has certain rights in the invention.
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