Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Reexamination Certificate
2001-03-30
2003-06-03
Donovan, Lincoln (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
C335S302000
Reexamination Certificate
active
06573817
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to variable-strength multipole beamline magnets, and more specifically, to a beamline magnet that permits the adjustment of not only the field strength but also the magnetic centerline.
BACKGROUND OF THE INVENTION
A number of techniques are available for producing variable-strength magnets. They are especially useful for bending, focusing, and higher-order control of beams in charged particle accelerators. Most charged particle beam accelerators use magnets to control the beam. This is especially true for high-energy accelerators, i.e., relativistic particle accelerators. The magnets affect the beam in ways that are mathematically similar, but not identical, to how optical lenses and mirrors affect an optical beam. In the present description, devices based on pseudo-optical properties of magnets are called beamline magnets.
Common beamline magnets are dipoles, quadrupoles, and sextupoles. Dipoles change the direction of the beam as well as provide some focusing or defocusing, like a light pipe with lenses. Quadrupoles focus the beam like a lens. Sextupoles can be used to correct certain types of aberrations. More generally, a beamline magnet with a plurality of poles, including dipoles, quadrupoles, and sextupoles, is termed a multipole magnet. For example, an octupole that uses eight poles is also a multipole magnet, which is suitable for correcting higher-order distortions of the beam.
Many beamline magnets are electromagnets. In these devices ordinary or superconducting coils are wound around specially shaped poles to generate the desired magnetic field. Adjusting the current passing through the coil(s) controls the magnetic field strength. This has the desirable property that the pole shape controls the field quality. The coils simply supply the magnetomotive force needed to generate the field. Room temperature coils usually need cooling to dissipate the heat generated by the finite resistance of the coils. This is accomplished by using fans, cooling channels, or liquid-cooled copper tubing for forming the coils. When copper tubing is used to form the coils, deionized water is circulated within the tubing while the current flows through the copper. There are a number of limitations to electromagnets. One is that expensive electrical power and additional plumbing are needed to operate these magnets. In addition, an electromagnet has a size limitation because the current densities, with which the power dissipation scales quadratically, are inversely proportional to the magnets' linear dimension. Thus, smaller electromagnets need to use reduced currents to avoid cooling problems, and cannot have strong fields.
A second, less common type of beamline magnet is made by arrangements of specially shaped magnets. These devices use special arrangements of magnets without poles to produce the desired fields. Sample magnets of this type can be found in U.S. Pat. No. 4,355,236 to Holsinger and U.S. Pat. Nos. 4,429,229 and 4,538,130 to Gluckstern. In these devices, the magnetic field strength is adjusted by rotating rings or disks of magnets. Because of the absence of poles, the magnetic fields of the individual magnets superimpose on each other, which makes analysis of their performance much easier. These magnets also have the advantage that they do not require power supplies to generate currents in the coils or plumbing for cooling the coils as in the electromagnets. However, the field quality produced by these magnets is inferior to that produced by electromagnets. Any mechanical imperfection of the magnets or magnetization nonuniformity degrades the magnetic field quality.
A third type of beamline magnet uses poles to produce a high-quality field like the one produced by an electromagnet, but uses permanent magnets in place of the coils used in an electromagnet. A sample device of this type can be found in U.S. Pat. No. 4,549,155 to Halbach, wherein the field strength is adjusted by rotating magnets. The rotation of magnets, however, causes the field strength to vary nonlinearly and sinusoidally as a function of a rotating angle, which makes it difficult to adjust the field strength with high precision. Another example of the type of beamline magnet using poles and permanent magnets can be found in U.S. Pat. No. 2,883,569 to Kaiser et al. In this patent, a flux shunt selectively slides over a portion of a cylindrical magnet to short out a varying amount of the magnetic field. This design, though, is intrinsically less efficient because there is a major magnetic flux leakage path between pairs of poles. In addition, this design also produces a nonlinear field adjustment, which is not desirable for high-precision strength adjustment. Yet another example of this type of beamline magnet uses cylindrical magnets that are individually rotated about their axes of symmetry. For these designs, there is one rotating magnet for each pole. The field strength is varied by adjusting the angular position of each magnet with respect to each pole. As before, this style of magnet produces a sinusoidal variation in the magnetic field strength and it is difficult to remove backlash in the rotational system to achieve precise adjustment of the field strength. In addition, many applications require a field strength setting (&Dgr;B/B) of {fraction (1/10000)} (0.01%). This implies extremely fine angular resolution: the angular encoders need to have resolutions of {fraction (1/50000)} radians, or approximately 300,000 encoder ticks in 360 degrees, which would be extremely difficult to obtain, if not impossible.
A need exists for a beamline magnet which does not require power supplies or plumbing, and yet produces a high-quality field. Preferably, such a beamline magnet is capable of achieving nonsinusoidal field strength adjustment to allow for high precision adjustment.
SUMMARY OF THE INVENTION
The present invention provides a multipole beamline magnet that is capable of selectively adjusting magnetic field strength and a magnetic centerline. Specifically, the beamline magnet includes a plurality of stationary poles formed of ferromagnetic material and one or more permanent magnets that are disposed between the plurality of stationary poles. Each of the permanent magnets supplies magnetomotive force to two adjacent stationary poles, so that the poles produce a magnetic field in a central space defined by the poles. A mechanical axis of the beamline magnet extends through the central space perpendicularly to the plane defined by the magnets and the poles. The beamline magnet further includes a linear drive for moving the permanent magnet(s) along radial lines perpendicularly to the mechanical axis, i.e., radially inward or outward with respect to the mechanical axis. Thus constructed, the beamline magnet produces a high-quality field using its stationary poles, and further allows for precise adjustment of the magnetic field strength and the magnetic centerline by collectively or selectively moving the permanent magnets.
In accordance with one aspect of the invention, the beamline magnet further includes a pair of nonmagnetic end caps that are provided to sandwich the poles and the magnets. In one embodiment, at least one of the end caps defines one or more guide channels for movably mounting the one or more permanent magnets, respectively. The guide channels are provided for greater control of the linear movement of the magnets.
In accordance with another aspect of the invention, the beamline magnet further includes a pair of ferromagnetic shield plates mounted on the nonmagnetic end caps, to thereby sandwich the nonmagnetic end caps, which in turn sandwich the poles and the magnets. The shield plates are used to effectively eliminate magnetic interactions between the beamline magnet and nearby instruments or other beamline magnets.
In accordance with yet another aspect of the invention, the beamline magnet further includes a magnetic field sensor arranged to determine the strength of the magnetic field in the central space defined by the stationary
Donovan Lincoln
Johnson Kindness PLLC
O'Connor Christensen
STI Optronics Inc.
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