WO2015189805A1

WO2015189805A1 - Adjustable magnet undulator

Info

Publication number: WO2015189805A1
Application number: PCT/IB2015/054435
Authority: WO
Inventors: Bengt Mathias HAMBERG
Original assignee: Hamberg Bengt Mathias
Priority date: 2014-06-13
Filing date: 2015-06-11
Publication date: 2015-12-17

Abstract

The present invention relates to an undulator for affecting electron beams in for example synchrotrons and free electron lasers. Permanent magnet structures are mounted in individually adjustable pair structures. Such structures are stacked in rotatable discs. This makes it possible to produce a wide range of undulator settings including normal linear with different period length and with a mixture of periods, helical with different helix angle. Furthermore each such magnet pair the distance can be adjusted allowing for individual magnetic flux intensity change in every disc. Also the relative angle of each magnet pair can be adjusted. This facilitate longitudinal as well as transverse tapering including the helical case of the magnetic field in the central passage through the structure.

Description

Adjustable Magnet Undulator

Field of the Invention

The present invention relates to an undulator for affecting electron beams in for example synchrotrons and free electron lasers. In particular the invention relates to an undulator comprising a plurality of dipole magnet pairs.

Background:

An undulator is an insertion device from high-energy physics and usually part of a larger installation, a synchrotron storage ring or free electron laser. Typical use is for creating photon radiation used for various investigations in medicine, biology, crystallography, chemistry, physics and other scientific areas. It can also be used for charged particle beam manipulation together with or without external radiation sources in order to manipulate the particle beam. Other names for essentially the same device is wiggler and or twister, hereinafter the term undulator is used for all such devices. It consists of a periodic structure of dipole magnets. The static magnetic field is alternating along the length of the undulator with a wavelength X_u. Electrons traversing the periodic magnet structure are forced to undergo oscillations and thus to radiate energy.

Typical depicted magnet principle for an undulator (as north and south magnetic poles) is displayed in Fig. 1. Due to the change of direction of the field (up-down-up-down...) the electrons passing through it will start to have a wavy trajectory (undulate).

Different types of undulators are in use: In the so called Apple type of undulators the permanent magnets are placed in a Halbach arrays (the direction of the magnetic field is indicated by arrows in Fig. 2) and the upper and lower parts of the undulator are separated into two halves that can be shifted with respect to each other as seen in Fig. 2. Top left: Linear vertical polarization; Top right: Circular polarization; Bottom left: Linear vertical polarization; Bottom right: Variable linear polarization.

A further type of Undulator is the helical, for example based on superconducting technology, with two spirally formed superconducting magnets.

Undulators are typically tuned by adjusting the separation between the upper and lower magnet array, in-between which the electron beam is running as schematically illustrated in Fig. 3.

Other adjustments can be horizontal tapering, where the undulator gap is changing along the electron travelling pathway, as schematically illustrated in Fig. 4.

Yet another adjustment is transversal tapering adjustment. With current technology this is made uniformly for the whole undulator section as schematically illustrated in Fig. 5, where figure is looking into the electron beams direction.

Variants of the undulator types presented are disclosed in US5099217, US4764743 and US4862128, utilizing rotatable disks of fixed rigid magnetic circuits in order to facilitate an overall helical magnetic field in the center pathway throughout the undulator which is referred to as "constant gap cladded twister". Since the gap of each magnetic circuit is constant, the magnitude of the magnetic field in the center of each disk shaped magnetic circuit is constant. Furthermore the shape and complexity of each disc like magnetic circuit is challenging from manufacturing point of view so that each disc likely will become slightly different from the other and lack the ability to be adjusted. The devices have limitations such as fixed magnetic flux in each magnetic circuit and in the very complex structure of the magnetic circuits, rendering in difficulties to manufacture them at a reasonable cost.

Summary of the invention

The present invention aims at obviating the aforementioned disadvantages and failings of previously known undulators, and at providing an improved undulator.

The undulator according to the present invention is adapted to influence an electron beam and arranged to be centered around the beam axis of the electron beam. The undulator comprises at least two rotation discs rotatable around the beam axis of the electron beam, wherein each rotation disc comprises a pair of a first permanent magnet structure and a second permanent magnet structure. The first and second permanent magnet structures are individually adjustable with regards both to their relative position and their respective position on the rotation disc.

In one embodiment of the invention each permanent magnet structure is provided with first adjustment means arranged at one end of the permanent magnet structure and a second adjustment means arranged at the opposite end of the permanent magnet structure, so that the first and second permanent magnet structures can be positioned at an angle relative each other. The adjustment means are preferably ball screws, that optionally are motor driven.

According to a further embodiment are the rotation disc is provided with a position sensors arranged to indicate the positions of the permanent magnet structures, preferably a plurality of sensors so that the relative position as well as angles can be indicated.

According to one embodiment the permanent magnet structures are accommodated in in cassettes and the adjustment means are arranged to interact with the cassettes.

Brief description of the drawings

A more complete understanding of the abovementioned and other features and advantages of the present invention will be apparent from the following detailed description of preferred embodiments in conjunction with the appended drawings, wherein:

Fig. 1 is a schematic presentation of a prior art undulator;

Fig. 2 is a schematic presentation of a prior art undulator in different configurations;

Fig. 3 is a schematic presentation of a tuning of an undulator;

Fig. 4 is a schematic presentation of a tuning of an undulator;

Fig. 5 is a schematic presentation of a tuning of an undulator;

Fig. 6 is a schematic presentation of a rotation disc according to the present invention;

Fig. 7 is a schematic presentation of a rotation disc according to the present invention;

Fig. 8 is a schematic presentation of a disc stack according to the present invention;

Fig. 9 is a schematic presentation of cog wheels according to the present invention; Fig. 10 is a schematic presentation of different tapering according to the present invention;

Fig. 11 is a schematic presentation of magnetic configurations of the permanent magnetic structures according to the present invention;

Figs. 12a, 12b are a schematic presentation of cassette according to the present invention;

Fig. 13 is a schematic presentation of a configuration of the undulator according to the

present invention;

Fig. 14 is a schematic presentation of a configuration of the undulator according to the

present invention;

Fig. 15 is a schematic presentation of a configuration of the undulator according to the

present invention;

Fig. 16 is a graph of a simulation set-up of the undulator according to the present invention;

Fig. 17 is a graph of a simulation outcome of the undulator according to the present invention;

Fig. 18 is a graph of a simulation outcome of the undulator according to the present invention;

Fig. 19 is a graph of a simulation outcome of the undulator according to the present invention; Fig. 20 is a graph of a simulation outcome of the undulator according to the present invention;

Fig. 21 is a graph of a simulation set-up of the undulator according to the present invention;

Fig. 22 is a graph of a simulation outcome of the undulator according to the present invention;

Fig. 23 is a graph of a simulation outcome of the undulator according to the present invention;

Detailed description

In the undulator according to the present invention magnets are mounted in stacked rotating discs. Each disc comprises a magnet-couple, or magnetic structure pair, where their separations towards each other and the beamline can be adjusted independently. Since the magnet couples are built into a rotating disc the rotation around the beam axis can be adjusted arbitrary compared to the other discs. Therefore normal linearly polarized light can be produced as well as circularly polarized light. Furthermore, depending on the rotation angle between each disc, the helix angle can be changed. Additionally tapering of the magnetic field can be achieved in longitudinal and transversal direction. Tapering effects can also be achieved for helical cases.

The undulator according to the present invention comprises rotation discs 101, depicted in a center cross section view, Fig. 6, and a front view, in Fig. 7.

The disc is provided with an opening 107 arranged to receive the electron beam during operation. Arranged on the rotation disc 101 are at least a first and a second a permanent magnet structures 104:1 and 104:2 that each comprises at least a one permanent magnet and together form a magnet pair. The first and second permanent magnet structures are preferably bar-shaped with an elongated side facing the opening 107 of the rotation disc 101. The permanent magnet structures are connected to the rotation disc via adjustment means 103, so that the position of one magnet can be adjusted relative the other permanent magnet structure in the magnet pair. Preferably the adjustment means are arranged to both adjust position the magnet and to tilt the magnet relative a thought normal to the electron beam. This is preferably realized by a pair of ball screws, preferably driven with motors 105, arranged to engage with the permanent magnet structures 104 in close proximity to respective end of the permanent magnet structure. Alternatives to ball screws, and meant to be included in the term ball screws are different types of threading based positioning devices such as lead screws and roller screws. The position of the magnet structures 104, is provided by position sensors 102, preferably encoders. The position sensors are preferably arranged on each short end of each permanent magnet structure. Alternatively other types of position sensors may be used and may be arranged in different ways relative the respective permanent magnet structure as long as it is possible to determine the position and angle of the permanent magnet structure relative the electron beam through the rotational disc.

The rotation disc 101 provide rigidity of the structure and acts as element for attaching the other parts to. Arranged around the rotation disc 101 are rotation cogs 106, which make it possible to rotate the wheel with external cogs. Such rotation is preferably possible with plus minus 185 degrees for each wheel making it possible to have cable connections into each motor and encoder.

A cog wheel 301, is connected to the cogs 106, of each rotation disc 101 as schematically illustrated in a front view in Fig. 9. Optionally such cog wheel can be mounted on each side of the disc in order to create a couple and therefore delivering a more pure momentum to the rotation disc.

A plurality of discs are stacked in front of each other forming a disc stack 202, as illustrated in the cross sectional side view in Fig. 8. Optionally bearings are arranged in between the rotation discs 101. In Fig. 8 and 9, it is illustrated how a disc is mounted with bearings 302, holding it in position.

Through each bearing a rigid axis 303, will be mounted that goes through the bearings for each disc securing the full disc stack. The rigid axis is be secured in undulator end-plates 201, arranged on each side of the disc stack providing rigid reference surfaces. The full disc setup structure 203 comprising the undulator end-plates 201 and the disc stack 202 can be secured on to a girder. The axis 303, will also be supported from surrounding girder structure to keep level.

Since encoders 102 and ball screws 103 are mounted on each side of the permanent magnetic structure 104 the relative angle of the permanent magnetic structure pair can be determined and adjusted independently. This allow for transverse tapering of the undulator. Three such tapering scenarios are illustrated in Fig 10 for three permanent magnet structure pairs, left tapering on one side, middle no tapering and right, tapering in the other direction. Regarding the permanent magnetic structure 104, it is important to note that adjusting a relative angle (tapering) of the couple in a disc requires slight freedom of movement in radial direction as well as angular direction relative to the ball screws 103.

Longitudinal tapering is achieved by for each disc decrease (or increase) the permanent magnet structure 104, pair separation slightly throughout the undulator.

The permanent magnetic structures 104, is preferably mounted in a cassette 601, as depicted in Fig. 12 a-b. Front view (a) and bottom view (b). The cassette main body 601 accommodates tat least one permanent magnet structure 104 which preferably is attached by clamping. Holes in the main body of the cassette allow for rotation 602-603 and sliding 602 of the attachment structure to the ball screws 103. Place for ball screws 103, will be made in the cassette body 601 to allow traveling up and down along ball screws. Mechanical couplings 604 arranged to engage with the encoders 102, may also extend from the cassette body 601. By getting the encoder position on each end of the permanent magnet structure 104, it is possible to get a well determined position including tapering angle.

The cassette 601 may accommodate two or more permanent magnets to amplify the magnetic flux at the beam position. This is schematically illustrated inln Fig. 11, wherein the magnetic flux direction is indicated for illustrative permanent magnetic structure pairs 104. Such structures could for simplicity be made up of single permanent magnets, Fig. 10. These magnetic structures can optionally be constructed as for example a simple triple combination of directed permanent magnets to amplify the magnetic flux at the beam position, Fig. 11. The supporting structures, including the rotation discs 101, end-plates 201, axis 303 and cassette main body 601, should be made of a material with high toughness, relatively small thermal expansion and non-magnetic Titanium or titanium alloys are suitable choice of materials for the supporting structures. The permanent magnets are preferably of high performance magnetic materials typically used in existing undulators such as neodymium based alloys.

After assembly of the full undulator, detailed commissioning will follow. In this commissioning standard methods will be used for magnetic characterization of the undulator structure.

The field profile can be measured using a Hall probe at different gap settings and both on and off- electron beam axis. The field integrals can be measured with a stretched wire. The undulator can then be installed on a test bench with the Hall probe scanning the magnetic field in the gap between the magnet couples in each disc of the undulator. The undulator setup can be controlled by a standard PLC system. The measurements will aid in calibration of the setup and will be run multiple times for different configurations. Examples

Due to the flexibility of the construction one undulator can be used for many purposes.

Linearly polarized light can be produced when the magnet couples are aligned with interchanging magnetic field of each disc (up down up etc.), as schematically illustrated in Fig. 13 (figure seen from side view with electron beam in center).

Linearly polarized light of longer wavelengths can be produced by pairing rotating discs, as schematically illustrated in Fig. 14 (figure seen from side view with electron beam in center). Circularly polarized light can be produced when the discs are rotated such that an additional rotation angle of up to 90 degrees is implemented, as schematically illustrated in Fig. 15, where figure is depicted from front with electron beam in center. In the figure the rotation angle is called alpha and four magnet structure pairs are implemented.

Combinations of disc rotations can be implemented aiming for producing light with two wavelengths simultaneously or simply for particular phase space manipulation.

Simulations

The undulator setup was simulated with COMSOL doing FEM analysis. Benchmark studies were also made for an APPLE II undulator for comparison. In the simulation magnets were modeled as permanent magnets lying in pairs and all the magnets were placed in a sufficiently large box of air, as schematically illustrated in Fig. 16. The magnets are indicated as wired boxes.

For the linear case all the magnets will be aligned as in Fig. 13 and 16. A simulation outcome is illustrated in Fig. 17, where the flux intensity is shown in the center cross section with shading after intensity and the vector field along the undulator center axis is indicated with arrows. It is clear that the magnetic field is along the plane and interchanging as wanted.

Examples of how the magnets can be aligned for the helical case are found in Fig. 15, 18 and 19. Simulation outcomes are illustrated in Fig. 18, 19 and 20 for two different angle increase alpha between each disc. The flux intensity is shown in the center cross section with shading after intensity and the vector field along the undulator center axis is indicated with arrows. It is clear that the magnetic field is rotating around the center beam axis and interchanging as wanted. We note here that as expected the smaller angle alpha between each rotating disc, the better and more even the magnetic flux will spiral around the center beamline and the worst case scenario with alpha increase 90-degrees scenario is illustrated in Fig. 20.

For the benchmark studies simulations, as schematically illustrated in Fig. 21, 22 and 23, the magnets where arranged as in Fig. 2 top right scenario for circularly polarized light. It is clear that an obvious back-draw of the Apple II undulator is that it does not give a pure circular polarization, as schematically illustrated in Fig. 23 where the figure show the magnetic flux vectors indicated with arrows along the electron beam pathway in the center of the undulator.

Claims

1. An undulator adapted to influence charged particle beams and arranged to be centered around the beam axis of the particle beam, the undulator comprising at least two rotation discs rotatable around the beam axis of the particle beam, wherein each rotation disc comprises a pair of a first permanent magnet structure and a second permanent magnet structure and that the first and second permanent magnet structures are individually adjustable with regards both to their relative position and their respective position on the rotation disc.

2. The undulator according to claim 1, wherein each permanent magnet structure is provided with first adjustment means arranged at one end of the permanent magnet structure and a second adjustment means arranged at the opposite end of the permanent magnet structure, so that the first and second permanent magnet structures can be positioned at an angle relative each other.

3. The undulator according to claim 2, wherein the first and second adjustment means are ball screws.

4. The undulator according to claim 1, wherein each rotation disc is provided with a first

position sensor arranged to indicate the position of the first permanent magnet structure and a second position sensor arranged to indicate the position of the second permanent magnet structure.

5. The undulator according to claim 1, wherein at least one of the permanent magnet structure comprise at least two directed permanent magnets to amplify the magnetic flux at the particle beam position.

6. The undulator according to claim 2, wherein the permanent magnet structures are

accommodated in cassettes and the adjustment means are arranged to interact with the cassettes.

7. A method of operating the undulator according to claim 1, comprising individually adjusting the first and second permanent magnet structures with regards both to their relative position and their respective position on the rotation disc.