Sextupole Field Research Articles

Design methods of the spherical quadrupole magnets and sextupole magnets

In high-energy physics, medical treatments, and space detections, quadrupole field distributions and sextupole field distributions are often required over a large spherical volume. Traditionally, these fields can be generated with cylindrical magnets. In comparison with cylindrical magnets, the spherical magnets have the advantages of smaller values of inductances, lower energy storage, and less magnetic flux leakage. To satisfy these special fields requirements, this paper describes the design methods of spherical magnets. The basic method derives from the target field approach that has been used to solve many of the inverse electromagnetic problems. Applying this approach, this paper has accomplished the design of spherical quadrupole magnets and sextupole magnets. In engineering and manufacture, the above calculation results are hard to utilize directly because of the complex magnet structures and, thus, high fabrication costs. After redesign and optimization on the above calculation results, this paper has also completed the construction of a modified spherical quadrupole magnet that has constant current distributions. This novel type of quadrupole magnet can generate nearly linear gradient fields and is easy to fabricate in engineering. Simulation and test results of this magnet are also presented.

Advanced superconducting electron cyclotron resonance ion source SECRAL: Design, construction, and the first test result

Superconducting electron cyclotron resonance (ECR) ion source with advanced design in Lanzhou (SECRAL) is a next generation ECR ion source and aims for developing a very compact superconducting ECR ion source with a structure and high performances for highly charged ion-beam production. The ion source was designed to be operated at 18GHz at initial operation and finally will be extended to 28GHz. The superconducting magnet confinement configuration of the ion source consists of three axial solenoid coils and six sextupole coils with a cold iron structure as field booster and clamping. At full excitation, this magnet assembly can produce peak mirror fields on the axis of 3.6T at injection, 2.2T at extraction, and a radial sextupole field of 2.0T at plasma chamber wall. What is different from the traditional design, such as LBNL VENUS and LNS SERSE, is that the three axial solenoid coils are located inside of the sextupole bore in order to reduce the interaction forces between the sextupole coils and the solenoid coils. SECRAL may open the way for building a compact and high-performance 18–28GHz superconducting ECR ion source. Very preliminary commissioning results are promising. Detailed design, construction issues and very preliminary test results of the ion source at 18GHz are presented.

Novel Design for a Superconducting Multipole Corrector Magnet

A novel concept of a superconducting, multipole corrector magnet is presented. This magnet is assembled from 12 identical racetrack-type coils powered by separate power supplies, and can generate any combination of dipole, quadrupole, and sextupole magnetic fields. In the case of normal dipole, quadrupole, and sextupole fields the total field is symmetrical relative to the magnet median plane and there are only five, powered separately coil groups. The new type of multipole corrector magnet, which has been proposed for BTeV project at Fermilab has the following advantages: universal configuration, simple manufacture, and a high mechanical stability. The results of the magnetic design, including the field quality and magnetic forces in comparison with known shell-type superconducting correctors, are presented.

Sextupole Winding in a Dipole Magnet Gap for a Storage Ring

In special synchrotrons a controllable sextupole field is required in the bending dipole magnets. One of them is NewSUBARU, a 1.5 GeV synchrotron light source. It has inverse dipole (bending) magnets to control the linear momentum compaction factor. It has to have a controllable sextupole at the position of the inverse bend for an independent control of its horizontal chromaticity and its higher order momentum compaction factor while keeping enough dynamic aperture. The inverse bend is a normal conducting C-type magnet whose bending angle, maximum field strength, yoke length and magnet gap are - 8 degrees, 1.554 T, 444 mm and 40 mm, respectively. The height of vacuum chamber in it is 28 mm, smaller than that in a normal bending magnet, while the magnet gaps are the same. This is because the vertical beta function at the inverse dipole is smaller. A pair of water-cooled coil sheets, which produce a sextupole field, are set at the space between the chamber and the pole face. Its thickness, the maximum current and the maximum sextupole field are 4.4 mm, 154 A and 70 T/m/sup 2/, respectively.

Intense heavy ion beam production from IMP LECR3 and construction progress of a superconducting ECR ion source SECRAL

Intense heavy ion beams have been produced from IMP 14.5 GHz LECR3 by optimization of the ion source conditions and transmission efficiency. Highly charged stable beams, such as 325 eμA of Ar11+, 95 eμA of Xe26+, 7 eμA of Xe30+, 140 eμA of Fe13+, and 75 eμA of Ni12+, were obtained by 14.5 GHz rf power 800–1000 W. Furthermore, an advanced superconducting ECR ion source named SECRAL is being constructed. SECRAL is designed to operate at rf frequency 18–28 GHz with axial mirror magnetic fields 4.0 T at injection, 2.2 T at extraction, and sextupole field 2.0 T at the plasma chamber wall. The unique feature of this superconducting ECR source is that the sextupole is located outside of the three axial solenoid coils to reduce the interaction force and make the source more compact. Fabrications of the superconducting coils, cryostat, beam transmission line, and other components are almost completed. Tests of the superconducting magnet with sextupole and solenoid coils are under way.

Optimum collimator shape and maximum emittance for submicron focusing of ion beams. Determination of the probe-forming system resolution limit

Improvements in the spatial resolution of instrumentation using focused ion beams are connected with further reduction of the beam spot size, while the beam intensity is kept sufficiently high. In search for probe-forming systems (PFS) of new generation it is important to determine the lower theoretical resolution limit for each system. In this case the lower resolution limit can be taken to be quality-factor of the system. As a general approach permitting the resolution limit to be found the authors propose the method of the maximum beam emittance. By proper selection of the object and angular slit dimensions and their positions with respect to the axis of the PFS, one attains the maximum beam emittance for a prescribed beam spot size. Calculations involving only demagnification and aberration coefficients allow a comparison to be made between different PFS irrespective of their dimensions and focusing elements employed. Investigations into conventional magnetic quadrupole PFS show that parasitic sextupole field components present in the lenses can lead to the asymmetric position of the angular slit and thus, to asymmetric position of the beam along the optical line.

Resolution limit of probe-forming systems with magnetic quadrupole lens triplets and quadruplets

Over the past decade, in MeV ion beam microanalysis efforts to achieve a spatial resolution better than 0.1 μm with a beam current of ∼100 pA have been connected with microprobes of new generation where the probe is formed by means of separated magnetic quadrupole lens structures [1]. However, as was pointed out in [2], no dramatic improvements in spatial resolution have been produced so far. For better understanding of the situation the authors carried out theoretical studies of multiparameter sets of probe-forming systems based on separated triplets and quadruplets of magnetic quadrupole lenses. Comparisons were made between the highest current values attained at different systems for a given beam spot size. The maximum parasitic sextupole and octupole field components were found whose contributions to spot broadening are tolerable. It is shown that the use of modern electrostatic accelerators [3] and precision magnetic quadrupole lenses [4,5] makes it possible to eliminate the effect of chromatic aberrations and second- and third-order parasitic aberrations resulting from distortions of the quadrupole lens symmetry. Therefore probe-forming systems with triplets and quadruplets of magnetic quadrupole lenses have a lower theoretical spatial resolution limit which is restricted mainly by intrinsic spherical third-order aberrations in state-of-the-art microprobes.

ECR ion sources at the Institute of Modern Physics: From classical to fully superconducting device

Electron cyclotron resonance (ECR) ion sources are used for cyclotron complex and atomic physics research at the Institute of Modern Physics (IMP). Intense beams of highly charged gaseous and metallic ions could be produced by the IMP 14.5 GHz ECR ion source (LECR2-Lanzhou Electron Cyclotron Resonance Ion Source No. 2). A particular emphasis has been put on the production of metallic ion beams recently. Metallic ion beams of Mg, Ca, Fe, Ni, Cu, Zn, and Pb were tested at the IMP 14.5 GHz ECR ion source (LECR2) to improve beam intensities and long-term stability. A new ECR ion source (Lanzhou Electron Cyclotron Resonance Ion Source No. 3), an upgraded version of the IMP 14.5 GHz ECR (LECR2) but with double-frequency wave heating (10 GHz+14.5 GHz), is under commissioning. The preliminary results of this new source will be presented. To satisfy the requirements of the heavy ion cooling storage ring that is under construction at IMP, a fully superconducting ECR ion source (Lanzhou Electron Cyclotron Resonance Ion Source with Superconducting Coils) with a completely new structure is being developed for the production of intense heavy ion beams of very high charge states, such as Xe30+ and U40+. The superconducting magnet consists of three axial solenoid coils and six saddle-curved sextupole coils with a cold iron structure as field booster and clamp. At full excitation, this magnet assembly will produce maximum peak fields on axis of 4.0 T at the injection, 2.2 T at the extraction region, and a radial sextupole field of 2.0 T at plasma chamber wall. The design features and status of this new superconducting ECR ion source will be presented.

Beam commissioning and achieved performance of SPring-8 storage ring phase-2 lattice with four magnet-free long straight sections

We had installed four magnet-free long straight sections (LSSs) of 27 m in the SPring-8 storage ring in the summer 2000 in spite that a lattice modification breaks high symmetry of the ring optics. This is because by the potentiality of the LSSs we can not only increase the brilliance of photon beams but also develop innovative light sources. The most serious problem for introducing the LSSs was that the lattice modification markedly deteriorates the stability of electron beams at a large amplitude or a large momentum deviation. To overcome this, a concept of “Quasi-Transparent Matching of sextupole fields” was proposed. This concept simultaneously manages both the symmetry restoration of sextupole fields and the local compensation of chromaticities. Owing to the reasonable lattice design and to the preparation with the precise ring model, the beam commissioning was successfully accomplished for about 3 weeks, achieving the beam performance which almost equals to that in the phase-1 lattice without the LSSs. A long undulator was also installed and it has been used in routine user operation. This success is the first step to challenge the new ideas growing up into future synchrotron radiation sources in the SPring-8 storage ring.

Correction of the persistent current effect in Nb/sub 3/Sn dipole magnets

The paper describes a method and results of simulation of persistent current effect in high field Nb/sub 3/Sn dipole magnets being developed for the future hadron colliders. Simple and effective techniques of passive correction of the persistent current effect in superconducting accelerator magnets are proposed. Using of these techniques allows a significant reduction of sextupole and decapole field components induced by persistent currents in a coil.

Persistent current effects in BSCCO common coil dipoles

A series of one-meter long racetrack-shaped windings has been fabricated from BSCCO tape conductors obtained from four manufacturers. Two coils were built from each conductor type and tested in the "common coil" dipole configuration in liquid helium. The effect of the remnant magnetization currents determined by measuring the residual dipole sextupole fields after cycling the magnets to progressively higher currents. Two coil sets have been measured and the results are compared to those obtained from a Nb/sub 3/Sn ribbon magnet of the same geometry.

Construction of the Superconducting ECR Ion Source Venus

A new, very high magnetic field superconducting ECR ion source, VENUS, is under development at the LBNL 88-Inch Cyclotron. It will boost the maximum energies and intensities for heavy ions from the cyclotron particularly for ions with mass greater than 60. It will also serve as R&D ion source for the proposed Rare Isotope Accelerator (RIA) project in the US, which requires up to 10 pµA of U30+. The superconducting magnet structure consists of three solenoids and six racetrack coils with iron poles forming the sextupole. The coils are designed to generate a 4 T axial mirror field at injection and 3 T at extraction and a radial sextupole field of 2.4 T at the plasma chamber wall. Test results of the magnet coils, which exceeded design requirements with minimum training, are presented. The magnet assembly with its cryostat will be enclosed by an iron shield and therefore must be designed to withstand any possible forces between coils and iron, which can be as high as 3.4 · 105 N (35,000 kgf). The low energy beam transport line (LEBT) and mass analyzing system of the ion source is designed to transport a proton- equivalent current of 25 mA at 20 kV extraction voltage. The design of the ion source and LEBT will be discussed.

Magnet system for an ECR ion source

A superconducting magnet assembly has been built for an ECR (Electron Cyclotron Resonance) ion source at the 88-inch cyclotron at LBL. Three 34-cm ID solenoids provide axial plasma confinement and a sextupole assembly in the solenoid bore provides radial stability. Two large solenoids are spaced 50 cm. Apart with a smaller opposing solenoid between. The sextupole assembly is 92 cm long with winding inner diameter of 20 cm. And outer diameter of 27.2 cm. The design goal is to achieve a field on axis of 4 T and 3 T at the mirrors with 0.4 T between and a sextupole field of 2.0 T at 15-cm diameter in the confinement volume. Each solenoid uses rectangular conductor wish copper/SC ratio of 4; the three coils are wet-wound on a one-piece aluminum bobbin with aluminum banding for radial support. The sextupole uses rectangular conductor with copper/SC ratio of 3. Each of the 6 coils is wet-wound with filled epoxy on a metal pole; the ends of the pole are aluminum and the central 34-cm is iron to augment the sextupole field. The six coils are assembled on a 20-cm-OD stainless steel tube with a 1.4-cm thick 30.0-cm OD aluminum tube over the assembly for structural support. Thin metal bladders are expanded azimuthally between each coil and axially at tire ends to pre-load the assembly. The sextupole assembly fits inside the solenoid bobbin, which provides support for the magnetic forces. The magnet exceeds design requirements with minimum training.

Design and measurement of the SLS booster combined function magnets

The booster bending magnets for the Swiss Light Source (SLS) at PSI include dipole, quadrupole and sextupole field components. The two types of magnet, focusing and defocusing, are laminated for operation at 3 Hz in the booster synchrotron, The 2D pole profile has been optimised with the smooth shimming method developed by the PSI Magnet Section. The 3D fringe field effects have also been computed. The prototype magnets were manufactured with removable pole ends to allow various end cuts to be measured. The prototypes were magnetically measured at the standard PSI measurement facility. The analysis, performed using the mid-plane field maps, confirmed the good field profile quality and helped determine the final pole end cuts for the magnet series,.

The design of a compact cryogen-free superconducting wavelength shifter for synchrotron radiation

A compact three-pole superconducting magnet with a warm bore for electron orbit has been designed to serve as a wavelength shifter for Taiwan synchrotron radiation light sources. The wavelength shifter will have a warm beam duct, which is 600 mm in length, 100 mm in width and 20 mm in high. Three pairs of racetrack superconducting coils were designed to create a central field and to adjust the integral field close to zero. A maximum magnetic field of 6.5 Tesla at the central pole will be generated. To avoid induced currents, all six coils are connected electrically in series and powered by a high current power supply. Two low current trim power supplies are used to adjust currents on the two side pairs of coils in order to eliminate the first and the second magnetic field integrals. The shape of pole pieces and superconducting coils are specially designed to minimize the higher harmonic terms of the magnetic field, especially the sextupole field component. Both the superconducting coils and the iron yokes are cooled by a single 1.5 W Gifford-McMahon cryo-cooler. The normal operation of the 6.5 T wavelength shifter will be cryogen free.

Characterisation of lens aberrations and parasitic fields using beam rocking

Beam rocking has been used on both the Oxford high-excitation triplet and the Melbourne Russian quadruplet to study lens aberrations such as multipole fields. A focused MeV proton beam is rocked over a fine mesh grid and the resulting beam displacement is mapped. Distorted patterns are produced in the presence of multipole fields originating either in the lenses or elsewhere due to stray fields. The minimisation of parasitic multipole fields is not only required in order to focus ion microprobes to small spot sizes, but is also important for the production of proper angular channelling scans using beam rocking since such fields distort an otherwise linear scan. In the absence of sextupole and higher order multipole fields, symmetric patterns are produced, exhibiting only spherical aberration. The patterns obtained experimentally are interpreted with the aid of simulations produced using an ion optics ray-tracing program.

Development of 18 GHz electron cyclotron resonance ion source with high-voltage extraction configuration

An 18 GHz ECR ion source with high-voltage extraction configuration (NIRS-HEC) has been developed for the Heavy Ion Medical Accelerator in Chiba (HIMAC), National Institute of Radiological Sciences (NIRS). NIRS-HEC has a high extraction voltage of up to 60 kV in order to reduce the unwanted space charge effects at the extraction region. NIRS-HEC also has a strong magnetic field. A radial size of the plasma chamber is made as small as 40 mm, so that a sextupole field strength of 11 kG and an axial field strength of 12 kG are easily obtained. The experimental data show that a high extraction voltage is effective to realize the high beam intensity when the extraction current density is high. It is consistent with the computer calculations of the FUGUN code and TRAPCAD code.

Status of 2.45 GHz compact National Institute of Radiological Sciences electron cyclotron resonance ion source

A 2.45 GHz compact National Institute of Radiological Sciences electron cyclotron resonance (NIRS-ECR) ion source has been developed for heavy ion medical accelerator in Chiba at NIRS. A new ECR source is expected to produce C2+ ions of more than 160 eμA for the high-energy heavy-ion cancer treatment. An ECR-type ion source is adopted because of its excellent characteristics of long lifetime and easy operation. The compact ECR ion source is 15 cm in diameter, 20 cm in length, and about 20 kg in weight. A set of permanent magnets was adopted to generate both the axial mirror and the radial sextupole fields. Several gas materials, He, CO2, CH4, N2, and Ne, have been tested. The present performance for C2+ ions, however, is 15 eμA and far below the medical requirements. The estimated vacuum in the plasma chamber is around 5.0×10−5 Torr. A much better vacuum pressure is desired to produce C2+ ions of more than 100 eμA. The ECR plasma, however, cannot be kept stable under such a high vacuum pressure. As another method to get the stable plasma, the different microwave injection was tested with a microwave antenna.

Development of the third generation electron cyclotron resonance ion source

The LBNL third generation electron cyclotron resonance (ECR) ion source has progressed from a concept to the fabrication of a full scale prototype superconducting magnet structure. This new ECR ion source will combine the recent ECR ion source techniques that significantly enhance the production of high charge state ions. The design includes a plasma chamber made from aluminum to provide additional cold electrons, three separate microwave feeds to allow multiple-frequency plasma heating (at 10, 14, and 18 GHz or at 6, 10, and 14 GHz) and very high magnetic mirror fields. The design calls for mirror fields of 4 T at injection and 3 T at extraction and for a radial field strength at the wall of 2.4 T. The prototype superconducting magnet structure which consists of three solenoid coils and six race track coils with iron poles forming the sextupole has been tested in a vertical Dewar. After training, the sextupole magnet reached 105% of its design current with the solenoids off. With the solenoids operating at approximately 70% of their full design field, the sextupole coils operated at 95% of the design value which corresponds to a sextupole field strength at the plasma wall of more than 2.1 T.

A superconducting spectrometer magnet system for the detection of heavy isotopes

LASSY is a proposed large angle gas filled spectrometer for detecting and analyzing heavy nuclei generated when a heavy ion beam (such as fully stripped argon) collides with an actinide thin film target (such as curium). With luck, superheavy nuclei, up to atomic number 114 to 116, produced at the target. In order to improve acceptance and resolution of the shortest lived nuclei, the length of the magnet system must be as short as possible. The LASSY spectrometer consists of a 4 to 6 T superconducting magnet that produces 2.5 Tm of bending (bend radius induction product) with a bend angle of 135 degrees or more. Particles generated at the target (and there decay products) must be focused on an electronic detector less than 3 meters from the target, The resulting spectrometer magnet is quite complex with sections that focus as well as bend the particles from the target. This report presents two possible designs for a large superconducting bending magnet that is a pure bending magnet in the center with defocusing bending sections at the ends of the magnet. This paper shows how one might build bending magnet that is in sections, with each section having a tunable quadrupole and sextupole field in addition to the dipole field.