Patent Application
20220159883
Cold and ultra-cold matter physics (e.g., optical traps, magneto-optical traps (MOTs), ion traps, laser cooling, and Bose-Einstein Condensates) has spurred demand for compact high vacuum (HV) and ultra-high vacuum (UHV, e.g., from about 10−9 torr to about 10−13 torr) systems. At these pressures, the mean free path of a gas molecule is on the order of 40 kilometers (km), so gas molecules typically collide with chamber walls many times before colliding with each other. For this reason, almost all interactions take place on chamber walls and other surfaces within a UHV chamber.
Several vacuum technologies may be used together to establish a UHV. For example, a UHV cell may be baked at high temperatures to release particles prior to establishing UHV. Various pumping technologies can be used to establish UHV. However, a UHV can degrade as particles are introduced intentionally (e.g., as part of an experiment) or unintentionally (e.g., by effusion from or diffusing through vacuum cell walls), so an ongoing pumping technology may be needed to maintain a UHV.
In contrast to other common UHV pumps, such as turbomolecular pumps and diffusion pumps, ion pumps have no moving parts and use no oil. They are therefore clean, need little maintenance, and produce little or no vibrations. Accordingly, ion pumps are currently the most desirable and mature technology for actively maintaining UHV in a compact cell.
A typical ion pump makes use of a Penning trap constituted by an electric field and a magnetic field. The electric field gives rise to free electrons at a cathode and accelerates them toward an anode. A cross product of the magnetic field with the current associated with the accelerating electrons produces a force orthogonal to the electron path. This force diverts the electrons so that they form a swirling cloud. The resulting cloud of swirling electrons ionizes incident molecules, which are then accelerated by the electric fields so that they impact surfaces of getter material, to which the ions are adsorbed. In addition, some molecules, e.g., of hydrogen and noble gases, most significantly, helium, may be absorbed by the getter material. In a “sputter ion pump”, getter material may be liberated (“sputtered”) from the getter surface and then re-deposited, burying sorbed molecules and renewing the getter surface.
In accordance with the present invention, a magnetic-field shield includes one or more permanent magnets along with ferro-magnetic material so that magnetic fields that would otherwise extend beyond the shield are retained in the ferro-magnetic material. As a result, objects outside the shielding can be better protected from magnetic fields established in the interior of the shield.
Efforts are underway to make more compact UHV systems. UHV systems tend to be incorporated in other systems, the dimensions of which may scale with the size of the UHV system. Smaller UHV systems enable the incorporating systems to be more portable and less expensive. However, as UHV systems become smaller, ion pumps used to maintain UHV conditions become closer to the UHV cell, and the magnetic fields associated with the ion pump can adversely affect delicate fields (e.g., those associated with magneto-optical traps) in the UHV cell. While magnetic shielding around the ion pump magnets can be used help isolate them from the UHV cell, there is typically some leakage. The present invention minimizes this leakage so as to improve the effectiveness of compact UHV systems.
Magnetic shields are shown both with and without a drive magnet set of one or more permanent magnets in
Magnetic shield 150 includes a ferromagnetic enclosure 152 with a drive magnet 154 arranged to tighten the paths of magnetic flux 110 and 112 such that magnetic flux 112 is retained within the ferro-magnetic enclosure 152 of magnetic shield 150. As a result, magnetic flux density 50 mm away due to magnets 102 and 104 is reduced to approximately 5% of that associated with unenhanced shield 100. While magnetic shield 150 includes a single drive magnet, other embodiments use plural drive magnets.
A UHV system 200 is shown in
Ion pump 204 is shown in greater detail in
Power supply 306 applies a voltage differential, e.g., 5-6 kilovolts (kV) direct current (DC), between anode 302 and cathodes 304. This voltage differential draws electrons from cathodes 304 toward anode 302. Magnetic field 106 (
A challenge addressed by the present invention is to prevent magnetic fields produced by ion-pump magnets 102 and 104 from disturbing processes in chamber 202 (
The effectiveness of shield 150 is indicated by the graphs 510, 530, and 550 of
Graph 130 represents the magnetic flux density at the MOT when shield 100 (with no drive magnets) is used. At x=0, the magnetic flux density is 1.55×10-1 G or 155 milliGauss (mG). Graph 150 represents the magnetic flux density at magneto-optical trap (MOT) 208 (
Graph 600 of
Herein, “ion pump” refers to any system that removes mobile molecules (including monatomic molecules) from a local (incomplete) vacuum by:
1) ionizing the molecules to yield ions; and 2) immobilizing the ions by sorbing (adsorbing or absorbing) them to a “getter” material. Herein, “molecule” refers to the smallest particle in a chemical element or compound that has the chemical properties of that element or compound. Herein, a ferro-magnetic enclosure defines an interior and exterior even in cases where the enclosure is incomplete in that it is “interrupted”, e.g., to provide a channel to an ion pump.
Herein, any art labeled “prior art”, if any, is admitted prior art; any art not labeled “prior art”, if any, is not admitted prior art. The illustrated embodiments, variations thereupon and modifications thereto are provided for by the present invention, the scope of which is defined by the following claims.
| Number | Date | Country | |
|---|---|---|---|
| 63113281 | Nov 2020 | US |
