EP4346338A1 - Système accélérateur de particules à géométrie de champ magnétique fractale - Google Patents

Système accélérateur de particules à géométrie de champ magnétique fractale Download PDF

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Publication number
EP4346338A1
EP4346338A1 EP23200620.5A EP23200620A EP4346338A1 EP 4346338 A1 EP4346338 A1 EP 4346338A1 EP 23200620 A EP23200620 A EP 23200620A EP 4346338 A1 EP4346338 A1 EP 4346338A1
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Prior art keywords
field
magnetic field
recited
magnet
focusing
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EP23200620.5A
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German (de)
English (en)
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Malek Haj Tahar
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    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
    • H05H13/00—Magnetic resonance accelerators; Cyclotrons
    • H05H13/08—Alternating-gradient magnetic resonance accelerators
    • H05H13/085—Fixed-field alternating gradient accelerators [FFAG]
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
    • H05H7/00—Details of devices of the types covered by groups H05H9/00, H05H11/00, H05H13/00
    • H05H7/04—Magnet systems, e.g. undulators, wigglers; Energisation thereof
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
    • H05H7/00—Details of devices of the types covered by groups H05H9/00, H05H11/00, H05H13/00
    • H05H7/04—Magnet systems, e.g. undulators, wigglers; Energisation thereof
    • H05H2007/043—Magnet systems, e.g. undulators, wigglers; Energisation thereof for beam focusing
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
    • H05H7/00—Details of devices of the types covered by groups H05H9/00, H05H11/00, H05H13/00
    • H05H7/04—Magnet systems, e.g. undulators, wigglers; Energisation thereof
    • H05H2007/045—Magnet systems, e.g. undulators, wigglers; Energisation thereof for beam bending

Definitions

  • the present invention concerns particle accelerator systems and especially, but not exclusively, power amplifiers producing high-current and high-energy beams at the megawatt (MW) level and beyond.
  • MW megawatt
  • cyclotrons are more advantageous given that their footprint is lower than linacs and given their better energy efficiency.
  • cyclotrons are designed to operate in a Continuous Wave (CW) mode by continuously injecting the beam bunches which are accelerated in an isochronous regime to the extraction device. This mode of operation requires a rapid increase of the magnetic field with the radius of the beam which leads to the loss of vertical focusing and to the reduced orbit separation at extraction.
  • CW Continuous Wave
  • the ratio of the beam size to the orbit separation at extraction determines the maximum beam that can be extracted with low losses and therefore the maximum achievable power from the cyclotron accelerator.
  • Another limitation is that cyclotrons struggle to achieve energies beyond the GeV level due to the weak focusing and isochronism problem.
  • Applicant has identified a need for a design of a strong focusing isochronous fixed field accelerator (FFA) enabling enough turn separation to extract the highest possible beam currents.
  • Strong focusing allows the device to reach the highest energies beyond the cyclotrons reach (typically ⁇ 1 GeV for protons) and to confine the beam bunches to smaller sizes.
  • the isochronous regime enables the continuous injection of bunches into the ring and allows to rapidly accelerate the beam in order to overcome resonance crossing issues which might lead to beam losses and unavoidable interlocks of the machine.
  • Fixed field magnets were chosen because such magnets are easier to operate, and allow the use of superconducting magnets, which reach higher magnetic field strength (thus controlling the orbits of more energetic particles in smaller spaces).
  • the proposed design also provides a single turn extraction, no stripper at extraction, and less activation problems.
  • a new degree of freedom is introduced in the design of the magnet, which is based on the fractal geometry. This enables to enhance the turn separation and to reduce the beam size at extraction compared to state-of-the-art fixed field accelerator concepts.
  • a system according to the invention comprises:
  • the fixed field charged particle accelerator can be a non-scaling fixed field charged particle accelerator.
  • Some embodiments of the invention are described below in the context of accelerating protons from 75 million electron volts (MeV) to 1775 MeV.
  • the kinetic energy of the ion is expressed in electron volts, the amount of kinetic energy imparted to one electron by a voltage difference of one volt.
  • the invention is not limited to this context. In other embodiments, other magnets of greater or lesser strength are used, allowing other ions of greater or lesser charges to be maintained up to higher or lower energies in accelerators of the same or different size.
  • a Fixed-Field alternating gradient Accelerator is a circular particle accelerator on which development was started in the early 1950s, and that can be characterized by its time-independent magnetic fields (fixed-field, like in a cyclotron) and the use of strong focusing (alternating gradient, like in a synchrotron).
  • time-independent magnetic fields fixed-field, like in a cyclotron
  • strong focusing alternating gradient, like in a synchrotron.
  • FIG. 1A is a block diagram that illustrates an orbital plane of an example fixed field alternating gradient charged particle accelerator.
  • the curved trapezoidal figures are the footprint of the fixed field magnets 13, i.e., magnets that do not change their magnetic field strength with time.
  • the magnetic field is directed perpendicular to the page and into the page for a positively charged ion, such as a proton, to move clockwise (or a negatively charged ion to move counterclockwise).
  • the magnets 13 serve only to maintain the particles in a focused beam that is forced to circulate in 360 degree turns.
  • the particles follow a straight path between magnets and then are turned, focused (F) and defocused (D) by encountering the spatially alternating magnetic field (in the pattern DFD) at each magnet 13, then again go straight as they leave the field of the magnet 13.
  • the particle thus experiences an alternating gradient magnetic field, hence the name.
  • the magnets have greater strength at greater radius so that they can turn a greater energy particle the same angular amount.
  • a cyclotron as shown in FIG. 1B , there is no alternation of the gradient and the magnetic field increases radially in order to keep the revolution time of the particles constant.
  • a cyclotron is a weak focusing accelerator in comparison to a FFA accelerator.
  • the particles are accelerated in a radio frequency cavity (RF cavity 12) by the application of an electric field timed to attract the charged particles as they approach and to repel the charged particles as they recede from the RF cavity.
  • RF cavity 12 radio frequency cavity
  • the particle After each acceleration, the particle has increased its momentum (and hence its energy) and thus takes more distance to turn the 360 degrees.
  • the next orbit 14 is further from the center of the orbits.
  • Each successive orbit is further out as the kinetic energy of the ions increases.
  • the FFA accelerator is designed to output particles of only a certain energy at the maximum radius, represented by the extraction ray.
  • FIG. 2 is a block diagram that illustrates an orbital (X,Y) plane, also called horizontal plane, of an example fixed field accelerator with fractal magnet geometry 20, according to an embodiment.
  • This system 20 includes a charged particle linear accelerator module, such as RF cavity 24, and four fixed field magnet assemblies 23a, 23b, 23c and 23d either referenced as assembly 23 in an exemplary embodiment.
  • the fixed field magnet assembly 23 is configured to control the orbits of the pulse in the device by turning a moving charged particle 90 degrees within a first plane (the X,Y orbital plane of FIG. 2 ).
  • Each assembly 23 includes FFA shaped magnets for which a strength Bz on the X,Y plane of a magnetic field perpendicular to the X,Y plane varies non-linearly along a radial direction from the reference point, which is not a dipole magnet; Such a magnetic field is split into self-similar structures at higher and higher radii.
  • the iterated function to generate such a fractal field map is also shown whereby the alternating gradient focusing structure is split into self-similar structures to enable stronger focusing for more energetic particles.
  • assembly 23 may resemble a tree-like fractal. For instance, when the beam accelerates from orbit 1 to orbit N, the focusing structure develops from a DFD (D for Defocusing and F for Focusing) to a DFDFD. This generates more wiggled orbits with modified path lengths.
  • DFD D for Defocusing and F for Focusing
  • FIG.3B is a block diagram that illustrates an orbital plane of an isochronous FFA with fractal magnet geometry, according to another embodiment.
  • the orbits are accelerated from 75 MeV to 1775 MeV such that the average magnetic field along each orbit is chosen to maintain a constant revolution frequency.
  • the beam is first focused by enhancing the splitting of the focusing structures D4DFD4DF DFDFDFDFDFDFD which increases the alternating gradient focusing effect.
  • the ratio of negative field to positive field is adjusted radially in order to pack together the closed orbits in the central region and create more separation at extraction. For instance, an orbit separation of 7cm at least can be achieved for the last couple of turns.
  • the shape of the magnetic field allowing this is shown in FIG. 3D .
  • the increased variation of the field acts on the average trajectory of the beam but also modifies the focusing effect.
  • the fractal splitting of the magnetic field can evolve in a random way as seen in FIG.4 . This can be utilized in order to enhance the turn separation at the location of the extraction device only.
  • the focusing part of the structure in Orbit 1 splits into an FDF structure.
  • the focusing part of the latter in Orbit 2 is subsequently split into another FDF structure.
  • ring accelerators One of the major concerns in ring accelerators is the crossing of the transverse resonances. The latter can lead to losses of the majority of ions in the beam. Thus, it is desirable to come up with a ring accelerator concept in which the crossing of the transverse resonances is very rapid to overcome its effect. Besides, a key requirement to extract the highest possible currents is to maintain the smallest beam sizes at extraction. This can be achieved by avoiding the loss of focusing during acceleration (as is the case in linear non-scaling FFA).
  • FFAs with fractal magnet geometry have the property that their focusing can be continuously adjusted by creating iterated functions at higher and higher radii that splits the initial focusing structure (i.e., at lower radii) into self-similar structures at higher radii.
  • the vertical component of the field of the magnet is expressed as in Equation 1.
  • B R ⁇ B 0 + B 1 R + B 2 R 2 + B 3 R 3 + B 4 R 4 ⁇ F R ⁇
  • B is the vertical (Z direction) component of the magnetic field in the median plane of the accelerator
  • R is the radial coordinate with respect to the center of the orbits
  • F ( R, ⁇ ) is a fringe field factor (also called a flutter function) that describes the azimuthal variation of the field of the magnet and which is not separable in radial and azimuthal coordinates.
  • the flutter function is based on the Enge model and is given as a piecewise function of the radius. Equation 2.
  • F R ⁇ 1 1 + e P 1 R ⁇ ⁇ 1 1 + e P 2 R ⁇
  • the polynomials P describe the fringe field falloff at the edge of the magnet; and, the subscript 1 indicates the entrance of the magnet, and the subscript 2 indicates the exit of the magnet.
  • the polynomials are given by Equations 3 and 4.
  • the magnetic field required obeys Maxwell equations and its amplitude depends on the footprint of the accelerator, the injection/extraction energies i.e., the momentum multiplication factor, as well as the amplitude of field reversal.
  • the use of superconducting magnet technology ensures that a wide range of optics can be explored.
  • a general approach to handle the design generation and optimization is of interest. Since the magnets are made of coils arranged to create the desired azimuthal and radial field variations, the magnetic field calculations can be performed by integrating the Biot-Savart law initially. A simple example relying on such a calculation is shown in FIG. 6A . The coils arrangement enabling this are displayed in FIG. 5A and 5B .
  • each loop consists of two coils tilted symmetrically with respect to the XY plane (median plane) such as they come closer together towards larger radii. This produces the observed field increase in X direction.
  • the current flow is clockwise/counter-clockwise.
  • the arrangement can be iterated by breaking the coils into self-similar structures at higher radii and applying the superposition principle. Furthermore, optimizing the location of the coils can have major effects on the fringing field of the structure as shown in FIG 6B where the coils are moved closer together in comparison with FIG. 6A .
  • the flutter function F is then modified in a way to represent the fractal geometry of the magnet and thus becomes Radial-dependent.
  • Enhancing the field variations by introducing higher order harmonics with positive and negative fields enhances the scalloping of the orbit, and also enables to increase the Alternating Gradient focusing.
  • Step 2 Use the tracking code ZGOUBI to track the particles in the field map. Accommodating the Maxwell equations allows to determine the magnetic field components out of the median plane.
  • Step 3 Use a fitting method to find the closed orbits for different energies and assess the level of isochronism achieved. If the revolution frequency changes with the energy by more than 1%, the average field increase is adjusted for both the Focusing and the Defocusing magnets as well as the shape of the flutter function. This is done gradually for larger and larger radii adjusting the amplitude of the flutter variation for a given radial span (e.g. , a 20 for R 20 ⁇ R ⁇ R 20 + ⁇ R )
  • Step 4 Ensure the stability of the particle trajectories in the transverse plane.
  • the number of betatron oscillations per turn is thus computed for each energy to determine the level of focusing achieved. Based on the scheme described in FIG. 3B , the number of oscillations per turn can Increase by a factor of four from low energy to high energy. This results from the enhancement of the Alternating Gradient focusing is such a scheme.
  • k R/B dB/dR which measures the increase of the magnetic field with the radius
  • FD ratio ratio between the focusing and defocusing field amplitudes.
  • indefinite article “a” or “an” is meant to indicate one or more of the item, element or step modified by the article.
  • a value is “about” another value if it is within a factor of two (twice or half) of the other value. While example ranges are given, unless otherwise clear from the context, any contained ranges are also intended in various embodiments. Thus, a range from 0 to 10 includes the range 1 to 4 in some embodiments.
  • the merit of the field maps is that, once the magnets are built, the simulated fieldmaps can be replaced with the measured ones to yield a realistic representation of the accelerator model.

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • Optics & Photonics (AREA)
  • Particle Accelerators (AREA)
EP23200620.5A 2022-09-29 2023-09-28 Système accélérateur de particules à géométrie de champ magnétique fractale Pending EP4346338A1 (fr)

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Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN109348609A (zh) * 2018-11-27 2019-02-15 中国原子能科学研究院 一种圆型加速器中实现高能等时性和工作路径稳定的方法

Patent Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN109348609A (zh) * 2018-11-27 2019-02-15 中国原子能科学研究院 一种圆型加速器中实现高能等时性和工作路径稳定的方法

Non-Patent Citations (14)

* Cited by examiner, † Cited by third party
Title
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