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.
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 (xcex94B/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.
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 poles. The sensed magnetic field strength data may then be used to control the linear drive for selectively or collectively moving the permanent magnets.
In accordance with still another aspect of the invention, the beamline magnet further includes a beam position sensor arranged to sense the location of a charged particle beam in the central space defined by the stationary poles. The sensed beam position may then be used to control the linear drive for selectively or collectively moving the permanent magnets to adjust the magnetic field strength or magnetic centerline.
In accordance with still another aspect of the invention, the beamline magnet includes a means of passive temperature compensation for maintaining the magnetic field strength substantially constant regardless of any changes in the operating temperature. Specifically, ferromagnetic materials having a low Curie temperature are magnetically coupled to the permanent magnets in a parallel flux shunting configuration to compensate for temperature-dependent flux variation of the permanent magnets. At a low temperature, the permanent magnets are stronger than at a high temperature, and thus could supply more flux in the central space than at a high temperature. At a low temperature, though, the ferromagnetic materials shunt a larger fraction of the available flux away from the central space than they do at a high temperature. Consequently, the resulting flux in the central space is substantially the same at both low and high temperatures; at a low temperature, the magnets are stronger but more flux is shunted away from the central space, and at a high temperature, the magnets are weaker but less flux is shunted away from the central space. With proper choice of the ferromagnetic material, its dimensions and location, the magnetic field strength can be maintained at an essentially constant level despite changes in the operating temperature.
In accordance with still another aspect of the invention, the beamline magnet includes a means of passive temperature compensation to correct for thermally induced shifts of the magnetic centerline. Centerline shifts can be caused by various thermal reasons, for example, by thermal expansion or contraction of all the materials in the beamline magnet, temperature dependence of the magnetic properties of the permanent magnets, and temperature induced movement of a support platform on which the beamline magnet is mounted. According to the present invention, thermal compensation of centerline shift is achieved by coupling different amounts of temperature compensating material (i.e., ferromagnetic material having a low Curie temperature) on each magnet. With proper choice of the material, its dimensions and location, the magnetic centerline can be maintained at an essentially constant location despite changes in the operating temperature.
In accordance with still another aspect of the invention, the beamline magnet further includes electromagnetic corrector coils to make small adjustments to the magnetic centerline and/or the magnetic field strength. One or more corrector coils are strategically placed to selectively supply a predetermined amount and polarity of magnetomotive force to one or more stationary poles. Adjustment using the electromagnetic corrector coils is achieved by merely modifying wiring of, and the current passing through, the coils, and hence the adjustment is quick and precise. For fine-tuning the field strength and/or the magnetic centerline, electromagnetic adjustment may be more advantageous than the mechanical adjustment of the present invention using the linear movement of the permanent magnets.
In accordance with still another aspect of the invention, the beamline magnet includes a plurality of poles and a plurality of permanent magnets. The poles and the magnets may be provided in equal numbers, and may be arranged equiangularly over 360xc2x0. The poles may be made of various materials and in various shapes. All the poles in a beamline magnet may be fabricated the same, or differently from each other. Likewise, the permanent magnets may be made of various materials, in various shapes, and having various magnetization directions. All the permanent magnets in the beamline magnet may be fabricated the same or differently from each other. Furthermore, each of the permanent magnets may be formed of a plurality of submagnet portions having the same or different shapes or properties. The shapes and properties of each pole and each permanent magnet (or submagnet portion) are determined so as to produce the desired magnetic field distribution according to each application.
In accordance with still another aspect, the beamline magnet of the present invention further includes one or more stationary auxiliary magnets positioned between the central space defined by the poles and the one or more permanent magnets, respectively. In other words, the auxiliary magnets are arranged radially inward of the permanent magnets with respect to the mechanical axis. The auxiliary magnets remain fixed while the permanent magnets disposed radially outward of the auxiliary magnets are moved.
In accordance with a further aspect, the beamline magnet of the present invention includes a ferromagnetic tuning shim. For example, the shim may be attached to the stationary auxiliary magnets, moving permanent magnets, poles, end magnets, or the nonmagnetic end caps. Shims serve to compensate for field errors produced due to imperfection in fabricating the permanent magnets and/or the poles.
The present invention further provides a method of selectively adjusting a magnetic field in a multipole beamline magnet. The method includes three steps. First, a plurality of stationary ferromagnetic poles are provided. Second, a plurality of permanent magnets are arranged between the plurality of stationary ferromagnetic poles, so that each of the permanent magnets supplies magnetomotive force to two adjacent stationary ferromagnetic poles. As a result, the stationary ferromagnetic poles produce a magnetic field in a central space defined by the stationary ferromagnetic poles. A mechanical axis of the beamline magnet is defined to extend through the central space, perpendicularly to the plane defined by the magnets and the poles. Finally, the one or more permanent magnets are moved perpendicularly to the mechanical axis.
The method may be applied in various ways to achieve the desired adjustment to the magnetic field, such as adjusting the field strength and the magnetic centerline. In a general case, the magnets are individually moved to selectively adjust the magnetic field strength and the magnetic centerline.
In a more special case, one may apply the method to adjust the strength of the magnetic field without changing the field distribution. This may be done, for example, by collectively moving all the permanent magnets in a radially inward or outward direction relative to the mechanical axis so as to uniformly increase or decrease the magnetic flux coupling to all the poles. The strength adjustment may be linear, thus allowing for high precision adjustment.
As another special case, one may adjust the magnetic centerline without changing the field strength. This may be done, for example, by moving a pair of opposing permanent magnets that are 180xc2x0 apart in one direction. Such movement merely translates (i.e., shifts in parallel) magnetic flux lines, and in effect linearly moves the magnetic centerline.
The present invention offers various advantages. First, the beamline magnet of the present invention does not require power supplies or plumbing, and yet produces a high-quality field due to the use of stationary poles. Second, the invention allows for linear adjustment of the field strength and the magnetic centerline, which in turn permits high precision adjustment of the field strength and the centerline. Third, in the present invention the magnets are moved linearly to make various adjustments, as opposed to being rotated, thus the precise adjustment of the magnets is made easier. This permits extremely accurate adjustments of the field strength (0.01%) and the magnetic centerline (microns) with commercially available linear encoders having 1-20 micron resolution. As discussed above, designs that use rotary motion typically require angular resolutions of approximately 300,000 encoder ticks in 360 degrees for 0.01% accuracy. This is not easily achieved with any commercial encoders.
Fourth, the present invention is versatile in permitting various adjustments of the magnetic field. For example, the present invention may be used to adjust the field strength without changing the magnetic centerline, or adjust (shift) the magnetic centerline without changing the field strength. Fifth, the versatile field adjustment capability described above may be readily applied to compensate for any errors in the magnetic properties of the beamline magnet (i.e., magnetic field strength, magnetic centerline, and magnetic field distribution) introduced during fabrication of the beamline magnet. For example, if the permanent magnets have differing strengths, then they can be moved linearly to compensate for the differences. If the magnetization direction of the permanent magnets is nonuniform, then the tuning shims can be used to compensate. Likewise, imperfections in the pole shapes or poles"" magnetization properties can be compensated for by combinations of linear motion of the permanent magnets and the use of ferromagnetic tuning shims. Furthermore, when electromagnetic corrector coils are provided, fine adjustments of the field strength or the magnetic centerline can be readily achieved by selectively wiring and passing a current thorough the coils. Thus, the present invention is highly tolerant to variations in the quality of the magnets and/or poles, thereby reducing the overall cost of manufacturing.
Lastly, the construction of the beamline magnet is such that it allows one to access the central space of the beamline magnet by removing one or more permanent magnets. This advantageously permits the beamline magnet to receive an electron beam sensor adjacent the central space for monitoring the behavior of the electron beam passing through the beamline magnet.