Hexapole Research Articles

Possibility of Yerevan synchrotron magnet structure transformation

We study a possibility of improving the YerPhI synchrotron by changing the magnet structure of accelerating ring so that the beam dynamics preserves its stability. The proposed changes in magnet structure allow one to get free intervals of sufficient length between the magnets which will be used for installing the system of multiturn injection and ejection of the beam as well as for installing a “cooling” system for the ion beam acceleration. Besides, additional focusing or defocusing magnets may be installed in the formed long free intervals for more effective control of the “working point” location, as well as the sextupole magnets may be installed for control of the chromaticity of the beam.

Superconducting magnets for the RAON electron cyclotron resonance ion source

The RAON linear accelerator of Rare Isotope Science Project has been developed since 2011, and the superconducting magnet for ECRIS was designed. The RAON ECR ion source was considered as a 3rd generation source. The fully superconducting magnet has been designed for operating using 28 GHz radio frequency. The RAON ECRIS operates in a minimum B field configuration which means that a magnetic sextupole field for radial confinement is superimposed with a magnetic mirror field for axial confinement. The highest field strength reaches 3.5 T on axis and 2 T at the plasma chamber wall for operating frequency up to 28 GHz. In this paper, the design results are presented of optimized superconducting magnet consisting of four solenoids and sextupole. The prototype magnet for ECRIS was fabricated and tested to verify the feasibility of the design. On the basis of test results, a fully superconducting magnet will be fabricated and tested.

Transport of 3D space charge dominated beams

In this paper we present the theoretical analysis and the computer code design for the intense pulsed beam transport. Intense beam dynamics is a very important issue in low-energy high-current accelerators and beam transport systems. This problem affects beam transmission and beam qualities. Therefore, it attracts the attention of the accelerator physicists worldwide. The analysis and calculation for the intense beam dynamics are very complicated, because the state of particle motion is dominated not only by the applied electromagnetic fields, but also by the beam-induced electromagnetic fields (self-fields). Moreover, the self fields are related to the beam dimensions and particle distributions. So, it is very difficult to get the self-consistent solutions of particle motion analytically. For this reason, we combine the Lie algebraic method and the particle in cell (PIC) scheme together to simulate intense 3D beam transport. With the Lie algebraic method we analyze the particle nonlinear trajectories in the applied electromagnetic fields up to third order approximation, and with the PIC algorithm we calculate the space charge effects to the particle motion. Based on the theoretical analysis, we have developed a computer code, which calculates beam transport systems consisting of electrostatic lenses, electrostatic accelerating columns, solenoid lenses, magnetic and electric quadruples, magnetic sextupoles, octopuses and different kinds of electromagnetic analyzers. The optimization calculations and the graphic display for the calculated results are provided by the code.

Gauge potential for neutral atoms in a magnetic quadrupole field

We consider a spinor Bose-Einstein condensate of neutral atoms in a magnetic quadrupole field and investigate the gauge potential derived from the spatially-varying magnetic field. We discuss the critical condition for a singular, quantized vortex state being energetically favored in the spinor condensate and find that it is necessary under experimentally realistic conditions to use atoms with hyperfine spin F > 1 and/or a more spatially-varying magnetic field, e.g., hexapole or octopole fields. Finally, we show that a spinor condensate in a ring geometry is beneficial for observing the gauge potential effects.

Steric effect in O2 sticking on Al(111): preference for parallel geometry.

We present experimental evidence that the sticking probability of O2 on Al(111) depends strongly on the alignment of an O2 molecule relative to the surface. The steric effect was measured with an aligned O2 beam prepared by a hexapole magnet. It has been found that, at translational energies <0.2 eV, O2 sticking occurs predominantly when the molecular axis is parallel to the surface, and that the dissociation barrier at the perpendicular geometry is ∼ 0.1 eV higher than at the parallel geometry. The present results indicate that the abstraction process, which would occur at the perpendicular geometry, is a minor event at low translational energies.

Operational Status of PF-Ring and PF-AR after the Earthquake

In 2011, two SR sources of KEK, PF-ring and PF-AR, needed to change the operation schedule because of the unprecedented earthquake on March 11. Though the injector linac and the storage rings suffered a serious damage, temporary recovery was accomplished quickly and the trial operation started in May. The regular user operation could be resumed in October 2011. In the restoration work after the earthquake, some old vacuum components were removed from PF-ring. This work fortunately brought an effect of settling the quadrupole-mode longitudinal instability. For the top-up injection of PF-ring, the pulsed sextupole magnet has been used instead of the conventional kicker magnets since 2011. The hybrid-fill mode in place of the single-bunch mode has become available. Recently, the 10-Hz orbit switching for the tandem circularly polarized undulators has been developed for the user operation.

EFFICIENT CONTROL OF ACCELERATOR MAPS

Recently, the Hamiltonian Control Theory was used in [Boreux et al., 2012] to increase the dynamic aperture of a ring particle accelerator having a localized thin sextupole magnet. In this paper, these results are extended by proving that a simplified version of the obtained general control term leads to significant improvements of the dynamic aperture of the uncontrolled model. In addition, the dynamics of flat beams based on the same accelerator model can be significantly improved by a reduced controlled term applied in only one degree of freedom.

Design study of an in-flight projectile fragment separator for rare isotope beams

Radioactive isotope(RI) beams are used to investigate the characteristics of unstable nuclei. Fragment separators, which have large angular and energy acceptances, were required to obtain high RI beam intensity. Careful design is required due to the large high order aberrations induced by the large aperture magnets, which are used to collect rare isotopes obtained from a high energy primary heavy-ion beam hitting a target [1]. In our design study, a high energy 12C primary beam was used to produce neutron rich medium mass heavy ions such as 9Li. Mirror symmetric optics provides smaller high order aberration and thus a design study of a mirror symmetric in-flight projectile fragment separator was performed to obtain large angular and energy acceptances. We investigated the optimal material and thickness of the target for the production of a 9Li beam. Based on the simulation, a beryllium target was selected to give a large yield with a smaller energy spread of the secondary beam. We also investigated the optimal thickness of the aluminum energy degrader. The selections of the target material and thickness were investigated by using the code LISE++.After optimization of the material and the thickness of the target, we performed a design study of the optics of the in-flight separator for a high resolution with high acceptance. The designed optics of the in-flight separator consists of the four normal conducting quadrupole triplets, three sextupoles and two normal conducting dipoles. The horizontal and vertical angular acceptances of the designed separator are 40 mrad and 70 mrad, respectively. The separator has a mass resolution of 640 when the object size is taken to be 1 mm. The correction of the second order aberration in the designed optics was performed by three sextupole magnets. The path length of the designed separator is 20.183 m. The optics design and the high order aberrations were investigated by using the code ORBIT.

Transformation of the Transverse Beam Intensity Distribution by Sextupole Focusing in a Transport Line

We investigate the transformation of the transverse intensity distribution of a charged-particle beam focused by one or two sextupole magnets in a beam transport line. It is expected, from the equation of transverse motion, that the beam is deflected toward one direction and thus deformed due to the second-order force of the sextupole magnet. Such a sextupole-induced deformation of the beam distribution has been studied analytically in detail. The beam centroid displacement and the change of the beam size are determined using the first-order and second-order moments of the intensity distribution function. In order to verify the theoretical consideration, numerical simulations and experiments were performed at the cyclotron facility of Japan Atomic Energy Agency. We experimentally demonstrate that an ion beam with a uniform transverse distribution can be formed using two sextupole magnets.

Alignment Effect of N2(A3Σu+) in the Energy Transfer Reaction of Aligned N2(A3Σu+) + NO(X2Π) → NO(A2Σ+) + N2(X1Σg+)

Steric effect in the energy transfer reaction of N(2)(A(3)Σ(u)(+)) + NO(X(2)Π) → NO(A(2)Σ(+)) + N(2)(X(1)Σ(g)(+)) has been studied under crossed beam conditions at a collision energy of ~0.07 eV by using an aligned N(2)(A(3)Σ(u)(+)) beam prepared by a magnetic hexapole. The emission intensity of NO(A(2)Σ(+)) has been measured as a function of the magnetic orientation field direction (i.e., alignment of N(2)(A(3)Σ(u)(+))) in the collision frame. A significant alignment effect on the energy transfer probability is observed. The shape of the steric opacity function turns out to be most reactive at the oblique configuration of N(2)(A(3)Σ(u)(+)) with an orientation angle of γ(v(R)) ~ 45° with respect to the relative velocity vector (v(R)), which has a good correlation with the spatial distribution of the 2pπ(g)* molecular orbital of N(2)(A(3)Σ(u)(+)). We propose the electron exchange mechanism in which the energy transfer probability is dominantly controlled by the orbital overlap between N(2)(2pπ(g)*) and NO(6σ).

Crosstalk Issues of Vicinity Magnets Studied With a Novel Rotating-Coil System

Taiwan Photon Source (TPS) is a low-emittance synchrotron radiation factory. The lattice magnets are located compactly within the limited space in the storage ring. The mutual proximity of the magnets induces field crosstalk and influences the dynamic aperture of the electron beam. The field becomes distorted, induced by the crosstalk within not only the iron yoke but also the edges of the magnets. Precise measurements with rotating-coil systems were conducted to characterize the integral field quality of the magnets; a new method to detect the center of magnets is presented. Simulation with TOSCA software was undertaken for comparison with these experimental results. We report the field distortions induced by crosstalk on the basis of our measurements and simulations. A misalignment effect between the quadrupole and sextupole magnets is discussed.

Variable Permanent Magnet Multipoles

Permanent magnet quadrupoles (PMQ) and sextupoles (PMSx) have been fabricated and tested. Their effective strengths are variable. The PMQ has been developed as a final focus magnet for a linear collider. There is enough space to put Gluckstern's 5-ring singlet structures in each incoming beam line. The magnetic center offsets and higher harmonic components can be adjusted by adjusting the positions of the magnet pieces with screws on holders. The measured integrated gradient can be reduced down to less than 1% of the maximum strength. The PMSx has been developed to focus pulsed cold neutrons for a small angle neutron scattering (SANS) experiment. The strength is modulated by a so called double ring structure, where the inner ring is fixed and the outer ring is rotated. A torque canceller can reduce the torque to rotate the outer ring by one third of its original magnitude. The focal length of the fabricated triplet lens is less than 50 cm for Very Cold Neutrons (VCN). It is demonstrated that VCN-SANS can shorten a SANS beamline, which usually has a few tens of meters, down to 2~3 meters.

Magnetic Design of the Advanced Light Source Harmonic Sextupoles

Forty-eight harmonic sextupole magnets with integrated dipole correctors and skew quadrupole coils will be introduced in the Advanced Light Source Storage Ring. These new magnets are required to allow the ALS to provide the 40 beamline users with higher photon beam brightness (factor of 2 or 3). Introducing new combined-function magnets in an existing storage ring is a challenge due to the limited space available and a balance had to be found between magnet performance and spatial constraints. Consequently four different magnet designs were required. The calculation and simulation results obtained for each design as well as the impact of the different design choices on the magnetic performance are developed in this paper.

Magnetic Field Measurements of RCS Dipole for CSNS

The China Spallation Neutron Source (CSNS) is an accelerator-based multidisciplinary user facility. The CSNS facility consists of an 80 MeV H- linear accelerator (1inac), a 1.6 GeV rapid cycling synchrotron (RCS), beam transport lines, a target station, and three sets of instruments with auxiliary equipments. The RCS consists of 24 dipole magnets, 48 quadrupole magnets, 16 sextupole magnets and some corrector magnets. The magnets of the RCS are excited with a repetition frequency of 25 Hz AC bias DC. The magnetic field measurements of the dipole prototype under DC and 25 Hz AC bias DC were carried out at the Institute of High Energy Physics (IHEP). The integral field distribution, harmonic components, transfer function were measured. The measurement results are presented in this paper.

Improvement of extraction efficiency from a slow extraction synchrotron by applying the constant spiral step condition

An operating condition for high extraction efficiency of a synchrotron with a slow extraction method is presented. With this condition, the separation between a particle injected into the electrostatic septum and a particle circulating in the synchrotron (the spiral step) becomes independent of the momentum of the particle (the constant spiral step condition). In a synchrotron with the Hardt condition, the spiral step varies according to the momentum of the extracted particle. So the extraction efficiency of the circulating beam with large momentum deviation becomes low because of the beam loss at the entrance of the electrostatic septum (ES). In a synchrotron with the constant spiral step condition, the beam loss at the entrance of the ES decreases. Therefore, the extraction efficiency of the synchrotron with the constant spiral step condition can become higher than that with the Hardt condition despite the loss of particles with large momentum deviation in the middle of the ES. Analogous to the Hardt condition, setting the horizontal chromaticity of the synchrotron to an appropriate value provides the constant spiral step condition. Particle tracking simulation results indicate that the extraction efficiency of the synchrotron with the constant spiral step condition is slightly higher than that with the Hardt condition. Adequate extraction efficiency is achieved with the horizontal chromaticity set between the Hardt condition and the constant spiral step condition. The magnetic strength of the sextupole magnets in the synchrotron should be set high enough to get horizontal chromaticity of the ring at any value in this range. Furthermore, it is desirable to design a synchrotron so that the constant spiral step condition becomes close to the Hardt condition.

Pulsed sextupole injection for Sweden’s new light source MAX IV

The MAX IV facility presently under construction will include two storage rings for the production of synchrotron radiation. Both rings will be operated at a constant 500 mA of stored current with top-up shots supplied by the MAX IV linac acting as a full-energy injector. Until recently, injection into both storage rings was designed using a conventional approach: a closed four-kicker injection bump brings the stored beam to the septum blade where the injected bunches are captured in a single turn. This method, although commonly found in third generation light sources, has significant disadvantages. Therefore, an alternative injection into the storage rings using pulsed multipoles has been investigated. This type of injection does not require an injection bump and has the potential to make top-up injection fully transparent to users. Design studies have been successfully completed and as a consequence it has been decided to replace the originally foreseen conventional injection scheme with pulsed multipole injection in both MAX IV storage rings. This paper reports on these studies, presents pulsed sextupole injection schemes, and summarizes requirements for the pulsers, sextupole magnets, and vacuum chambers.

Stretched wire measurement of multipole accelerator magnets

Theoretical aspects of stretched wire field integral measurements are investigated. A least square approach is used for the processing of the measured signal; it can be applied to any wire trajectory and it allows numerical correction of the wire position errors. Moreover, the signal from some multipoles can be compensated for in a way comparable to the ``bucked'' rotating coil method. This can be used for accurate measurements of field multipole errors. A stretched wire bench has been built at the European Synchrotron Radiation Facility. This bench has been used for measuring dipole, quadrupole, and sextupole magnets. The results obtained with various wire trajectories are compared. It is demonstrated that an accuracy of a few ${10}^{\ensuremath{-}4}$ of the main multipole can be obtained.

Development of DRAGON electron cyclotron resonance ion source at Institute of Modern Physics

A new room temperature electron cyclotron resonance (ECR) ion source, DRAGON, is under construction at IMP. DRAGON is designed to operate at microwaves of frequencies of 14.5-18 GHz. Its axial solenoid coils are cooled with evaporative medium to provide an axial magnetic mirror field of 2.5 T at the injection and 1.4 T at the extraction, respectively. In comparison to other conventional room temperature ECR ion sources, DRAGON has so far the largest bore plasma chamber of inner diameter of 126 mm with maximum radial fields of 1.4-1.5 T produced by a non-Halbach permanent sextupole magnet.

The magnet design for the HLS storage ring upgrade project

In order to improve the performance of the Hefei Light Source (HLS), in particular to get higher brilliance synchrotron radiation and increase the number of straight section insertion devices, an upgrade project called HLSII will be launched soon. The storage ring lattice, which has a double bend achromatic structure with four periods, comprises eight dipoles, 32 quadrupoles and 32 combined function sextupoles. The design and analysis of the magnets are shown in this paper, along with the optimization of the multipurpose combined function magnet, which consists of three magnets: skew quadrupole, horizontal dipole and vertical dipole, with the main sextupole magnet. This type of magnet is the first one that has been designed and used in China. The mechanical design and fabrication procedures for the magnets are also presented.

Measurement of the ground-state hyperfine splitting of antihydrogen

The ASACUSA collaboration at the Antiproton Decelerator of CERN is planning to measure the ground-state hyperfine splitting of antihydrogen using an atomic beam line, consisting of a cusp trap as a source of partially polarized antihydrogen atoms, a radiofrequency spin-flip cavity, a superconducting sextupole magnet as spin analyser, and an antihydrogen detector. This will be a measurement of the antiproton magnetic moment, and also a test of the CPT invariance. Monte Carlo simulations predict that the antihydrogen ground-state hyperfine splitting can be determined with a relative precision of ∼10−7. The first preliminary measurements of the hyperfine transitions will start in 2011.