Gradient Accelerator Research Articles

Accelerators for charged particle therapy: PAMELA and related issues

Cancer is a dreadful disease that will affect one in three people at some point in their life; radiotherapy is used in more than half of all cancer treatment, and contributes about 40% to the successful treatment of cancer. Charged Particle Therapy uses protons and other light ions to deliver the lethal dose to the tumor while being relatively sparing of healthy tissue and, because of the finite range of the particles, is able to avoid giving any dose to vital organs. While there are adequate technologies currently available to deliver the required energies and fluxes, the two main technologies (cyclotrons and synchrotrons) have limitations. PAMELA (the Particle Accelerator for MEdicaLApplications) uses the newly-developed non-scaling Fixed Field Alternating Gradient accelerator concepts to deliver therapeutically relevant beams. The status of the development of the PAMELA conceptual design is discussed.

Superconducting properties of experimental YBCO coils for FFAG accelerator magnets

A project to develop fundamental technologies for accelerator magnets using high-Tc coated conductors is currently in progress. The primary applications of this project are fixed field alternating gradient (FFAG) accelerators for carbon cancer therapy systems and accelerator-driven subcritical reactors. Several types of superconducting coils for FFAG accelerators have been conceptually designed. These coils have complicated shapes, including a negative-bend part or a three-dimensional bent part. One of the aims of the project is to establish winding technologies for complicated coil shapes using coated conductors. To demonstrate winding technologies for YBa2Cu3O7-x (YBCO) coils, small test coils having a negative-bend part or a three-dimensional bent part were designed and fabricated according to the present design of the FFAG magnet. The outside dimensions of the negative-bend test coil were 460 mm long and 190 mm wide, and the radius of curvature of the negative-bend part was 442 mm. The outside dimensions of the three-dimensional test coil were 380 mm long and 280 mm wide, and the radius of curvature of the mandrel of the three-dimensional coil was 700 mm. The test coils were wound using YBCO coated conductors with a length of about 100 m and were then impregnated with epoxy resin. The coils were placed in liquid nitrogen and excited to measure their V-I characteristics. From the V-I characteristics throughout a voltage range down to 10−9 V/cm, the V-I characteristics before and after impregnation were approximately the same, demonstrating that the superconducting properties were not degraded.

Reconstruction of lattice parameters and beam momentum distribution from turn-by-turn beam position monitor readings in circular accelerators

In high chromaticity circular accelerators, rapid decoherence of the betatron motion of a particle beam can make the measurement of lattice and bunch values, such as Courant-Snyder parameters and betatron amplitude, difficult. A method for reconstructing the momentum distribution of a beam from beam position measurements is presented. Further analysis of the same beam position monitor data allows estimates to be made of the Courant-Snyder parameters and the amplitude of coherent betatron oscillation of the beam. The methods are tested through application to data taken on the linear nonscaling fixed field alternating gradient accelerator, EMMA.

Harmonic ratcheting for fast acceleration

A major challenge in the design of rf cavities for the acceleration of medium-energy charged ions is the need to rapidly sweep the radio frequency over a large range. From low-power medical synchrotrons to high-power accelerator driven subcritical reactor systems, and from fixed focus alternating gradient accelerators to rapid cycling synchrotrons, there is a strong need for more efficient, and faster, acceleration of protons and light ions in the semirelativistic range of hundreds of MeV/u. A conventional way to achieve a large, rapid frequency sweep (perhaps over a range of a factor of 6) is to use custom-designed ferrite-loaded cavities. Ferrite rings enable the precise tuning of the resonant frequency of a cavity, through the control of the incremental permeability that is possible by introducing a pseudoconstant azimuthal magnetic field. However, rapid changes over large permeability ranges incur anomalous behavior such as the ``Q-loss'' and ``f-dot'' loss phenomena that limit performance while requiring high bias currents. Notwithstanding the incomplete understanding of these phenomena, they can be ameliorated by introducing a ``harmonic ratcheting'' acceleration scheme in which two or more rf cavities take turns accelerating the beam---one turns on when the other turns off, at different harmonics---so that the radio frequency can be constrained to remain in a smaller range. Harmonic ratcheting also has straightforward performance advantages, depending on the particular parameter set at hand. In some typical cases it is possible to halve the length of the cavities, or to double the effective gap voltage, or to double the repetition rate. This paper discusses and quantifies the advantages of harmonic ratcheting in general. Simulation results for the particular case of a rapid cycling medical synchrotron ratcheting from harmonic number 9 to 2 show that stability and performance criteria are met even when realistic engineering details are taken into consideration.

A study of dynamic Lorentz force detuning of 650 MHz βg=0.9 superconducting radiofrequency cavity

The small bandwidth of superconducting cavities makes the study of dynamic Lorentz force detuning and its compensation indispensable in case of pulsed mode operation of high gradient accelerators. In this paper, we present the study of this detuning and also propose an optimized design for five cell 650MHz βg=0.9 elliptic superconducting cavities, which will be used in the high energy section of the 1GeV H− linear accelerator for the proposed Indian spallation neutron source project, by suitably inserting the inter-cell stiffeners. The paper presents a sequential design methodology which starts with study of static Lorentz force detuning and tunability; and progresses to find out the structural modes and related dynamic detuning values by performing transient structural dynamics calculations. The developed methodology is general in nature and can be used for a three dimensional model of any geometry. The work will be useful for optimizing the design against dynamic Lorentz force detuning of superconducting radiofrequency cavities of any shape.

Physical design of FEL injector based on the performance-enhanced EC-ITC RF gun

To meet the requirements of high performance THz-FEL (Free Electron Laser), a compact scheme of FEL injector was proposed. A thermionic cathode was chosen to emit electrons instead of a photo-cathode with its complex structure and high cost. The effective bunch charge was improved to ∼200 pC by adopting an enhanced EC-ITC (External Cathode Independently Tunable Cells) RF gun to extract micro-bunches; back bombardment effects were almost eliminated as well. Constant gradient accelerator structures were designed to improve energy to ∼14 MeV, while the focusing system was applied for emittance suppressing and bunch state maintenance. The physical design and beam dynamics of the key components for the FEL injector were analyzed. Furthermore, start-to-end simulations with multi-pulses were performed using homemade MATLAB and Parmela. The results show that continual high brightness electron bunches with a low energy spread and emittance could be obtained stably.

Controlling nonlinear longitudinal space charge oscillations for high peak current bunch train generation

The evolution of picosecond modulations of the longitudinal profile of an electron beam generated in an rf photoinjector is analyzed and optimized with the goal of obtaining high peak current electron bunch trains at very high frequencies ($\ensuremath{\ge}\mathrm{THz}$). Taking advantage of nonlinear longitudinal space charge forces, it is found that more than 500 A peak current 1 THz bunch trains can be generated using a standard 1.6 cell SLAC/UCLA/BNL rf gun. Postacceleration is used to freeze the longitudinal phase space dynamics after one half plasma oscillation. Applications range from tunable narrow bandwidth THz radiation generation to drivers for high frequency high gradient accelerators.

Conception design of helium ion FFAG accelerator with induction accelerating cavity

In the recent decades of particle accelerator R&D area, the fixed field alternating gradient (FFAG) accelerator has become a highlight for some advantages of its higher beam intensity and lower cost, although there are still some technical challenges. In this paper, the FFAG accelerator is adopted to accelerate a helium ion beam on the one hand for the study of helium embrittlement on fusion reactor envelope material and on the other hand for promoting the conception research and design of the FFAG accelerator and exploring the possibility of developing high power FFAG accelerators. The conventional period focusing unit of the helium ion FFAG accelerator and three-dimensional model of the large aperture combinatorial magnet by OPERA-TOSCA are given. For low energy and low revolution frequency, induction acceleration is proposed to replace conventional radio frequency (RF) acceleration for the helium ion FFAG accelerator, which avoids the potential breakdown of the acceleration field caused by the wake field and improves the acceleration repetition frequency to gain higher beam intensity. The main parameters and three-dimensional model of induction cavity are given. Two special constraint waveforms are proposed to refrain from particle accelerating time slip (ΔT) caused by accelerating voltage drop of flat top and energy deviation. The particle longitudinal motion in two waveforms is simulated.

Vertical orbit excursion fixed field alternating gradient accelerators

Fixed field alternating gradient (FFAG) accelerators with vertical orbit excursion (VFFAGs) provide a promising alternative design for rings with fixed-field superconducting magnets. They have a vertical magnetic field component that increases with height in the vertical aperture, yielding a skew quadrupole focusing structure. Scaling-type VFFAGs are found with fixed tunes and no intrinsic limitation on momentum range. This paper presents the first multiparticle tracking of such machines. Proton driver rings to accelerate the 800 MeV beam from the ISIS synchrotron are presented, in terms of both magnet field geometry and longitudinal behavior during acceleration with space charge. The 12 GeV ring produces an output power of at least 2.18 MW. Possible applications of VFFAGs to waste transmutation, hadron therapy, and energy-recovery electron accelerators are also discussed.

Development of Saddle-Shaped Coils Using Coated Conductors for Accelerator Magnets

A project to develop fundamental technologies for accelerator magnets using high-T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> superconductors is currently underway. The primary applications in this project are fixed field alternating gradient (FFAG) accelerators for carbon cancer therapy systems and accelerator-driven subcritical reactors. One of the challenging issues in this study is to accomplish the complicated coil shape required for the accelerator magnets using YBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> (YBCO) coated conductors. An asymmetric, three-dimensional winding structure has been designed for the main ring of an FFAG accelerator, but the anisotropic bending flexibility of coated conductors is constrained because of their tape-like shape. To demonstrate a suitable winding technology for a three-dimensional coil using coated conductors, a prototype winding machine with a two-axis motion mechanism was constructed. Based on the present design of the FFAG magnet, a saddle-shaped coil wound on a curved surface with a radius of 700 mm was designed. A 100-turn coil was wound by the winding machine using an approximately 90 m length of 3 mm-wide YBCO coated conductor made by the IBAD-MOCVD method. The coil was impregnated with epoxy resin, and the voltage-current ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> ) characteristics were evaluated in a liquid nitrogen environment. From the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> characteristics in an electric field region of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> V/cm, an index value of above 20 was obtained, which means that the superconducting properties were not degraded.

Progress of Research and Development of Fundamental Technologies for Accelerator Magnets Using Coated Conductors

A project to develop the fundamental technologies for accelerator magnets using coated conductors is in progress. A coil-dominated magnet and an iron-dominated magnet were designed, based on the conceptual design of spiral sector fixed field alternating gradient accelerator for carbon cancer therapy and that for accelerator-driven subcritical reactor, respectively. The required winding technologies were clarified through designing the magnets. The R&D of winding technologies for coils with three-dimensional shape and those with negative bend have been carried out. The influence of the magnetization of coated conductors on the field quality of magnets was studied experimentally.

Conceptual design of a nonscaling fixed field alternating gradient accelerator for protons and carbon ions for charged particle therapy

The conceptual design for a nonscaling fixed field alternating gradient accelerator suitable for charged particle therapy (the use of protons and other light ions to treat some forms of cancer) is described.49 MoreReceived 23 November 2012DOI:https://doi.org/10.1103/PhysRevSTAB.16.030101This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical Society

Straight Scaling FFAG line

Recent developments in scaling fixed field alternating gradient (FFAG) accelerators have opened new ways for lattice design, with straight sections. An experiment to study zero-chromatic straight sections is in progress at KURRI.

EMMA

EMMA stands for Electron Model for Many Applications and is expected to demonstrate the novel concept of a non-scaling Fixed Field Alternating Gradient (FFAG) accelerator. A non-scaling FFAG accelerator was originally invented to accelerate muons up to a few tens of GeV energy range although it is now hoped to have a wider variety of applications such as a proton driver for Accelerator Driven Sub-critical Reactor (ADSR). We will report the recent developments of the EMMA experiment, with more emphasis on acceleration of a muon beam.

EXPERIMENTAL ANALYSES OF SPALLATION NEUTRONS GENERATED BY 100 MEV PROTONS AT THE KYOTO UNIVERSITY CRITICAL ASSEMBLY

Neutron spectrum analyses of spallation neutrons are conducted in the accelerator-driven system (ADS) facility at the Kyoto University Critical Assembly (KUCA). High-energy protons (100 MeV) obtained from the fixed field alternating gradient accelerator are injected onto a tungsten target, whereby the spallation neutrons are generated. For neutronic characteristics of spallation neutrons, the reaction rates and the continuous energy distribution of spallation neutrons are measured by the foil activation method and by an organic liquid scintillator, respectively. Numerical calculations are executed by MCNPX with JENDL/HE-2007 and ENDF/B-VI libraries to evaluate the reaction rates of activation foils (bismuth and indium) set at the target and the continuous energy distribution of spallation neutrons set in front of the target. For the reaction rates by the foil activation method, the C/E values between the experiments and the calculations are found around a relative difference of 10%, except for some reactions. For continuous energy distribution by the organic liquid scintillator, the spallation neutrons are observed up to 45 MeV. From these results, the neutron spectrum information on the spallation neutrons generated at the target are attained successfully in injecting 100 MeV protons onto the tungsten target.

Challenge to Functional, Efficient, and Compact Accelerator Systems using High &lt;i&gt;T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; Superconductors

Application of high Tc superconductors to accelerator magnets provides the following benefits. It reduces power consumption compared to copper magnets, it allows simple operation and less governmental regulation compared to low Tc superconductors because of the applicability of cryocoolers, and it is potentially more stable thermally, when operated at high temperatures where the specific heats of materials are large. An R&D project focusing on fundamental technologies for accelerator magnets using coated conductors is in progress, funded by the Japan Science and Technology Agency from January, 2010. Stage I of the project was completed successfully in March, 2012, and the project moved to the four-year Stage II in April, 2012. We aim to realize application of high Tc superconductors to magnets of strong-focusing accelerators for carbon cancer therapy or accelerator-driven subcritical reactors (ADSR). In Stage I, we conceptually designed a couple of fixed-field alternating gradient (FFAG) accelerators for carbon cancer therapy and for ADSR as well as a beam transport line for a carbon cancer therapy facility. Based on the conceptual designs of accelerators, coil-dominated magnets and iron-dominated magnets wound with coated conductors were designed. The required winding technologies were clarified through designing these magnets, and the R&D of winding technologies for coils with three-dimensional shape and those with negative bend have been carried out. The influence of the magnetization of coated conductors on the field quality of magnets was studied experimentally. The influence of the neutron radiation on the conductivity of aluminum and that of copper was also studied.

Conceptual design of multi-targets for accelerator-driven system experiments with 100-MeV protons

The Kyoto University Research Reactor Institute is engaged in research on an accelerator-driven system (ADS) using the fixed field alternating gradient accelerator at the Kyoto University Critical Assembly. With the aim of achieving the high-energy neutrons anticipated in the actual ADS experiments, we propose the use of a new target concept (the multi-target) in this study. The conceptual design of the multi-target is numerically carried out, and the high-energy neutrons generated by the injection of high-energy protons at the multi-target are successfully measured using a liquid organic scintillator in the experiments. The conceptual design and effectiveness of the multi-target is confirmed by the obtained neutron spectrum and neutron yield in both the experimental and calculated results.

Experimental study of resonance crossing with a Paul trap

The effect of resonance crossing on beam stability is studied systematically by employing a novel tabletop experimental tool and a multiparticle simulation code. A large number of ions are confined in a compact linear Paul trap to reproduce the collective beam behavior. We can prove that the ion plasma in the trap is physically equivalent to a charged-particle beam propagating through a strong focusing channel. The plasma confinement force is quickly ramped such that the trap operating point traverses linear and nonlinear resonance stop bands. Assuming a nonscaling fixed field alternating gradient accelerator composed of many identical FODO cells, we measure how much ion losses occur under diverse conditions. It is experimentally and numerically demonstrated that too slow resonance crossing leads to significant ion losses as expected. Particular attention must be paid to the linear coherent resonance excited at a quarter-integer tune. When the beam intensity is high, this type of linear stop band can seriously affect the beam quality even for rather fast resonance crossing. A scaling law is given of the emittance growth caused by the quarter-integer resonance crossing.

Straight scaling FFAG beam line

Fixed field alternating gradient (FFAG) accelerators are recently subject to a strong revival. They are usually designed in a circular shape; however, it would be an asset to guide particles with no overall bend in this type of accelerator. An analytical development of a straight FFAG cell which keeps zero-chromaticity is presented here. A magnetic field law is thus obtained, called “straight scaling law”, and an experiment has been conducted to confirm this zero-chromatic law. A straight scaling FFAG prototype has been designed and manufactured, and horizontal phase advances of two different energies are measured. Results are analyzed to clarify the straight scaling law.

Inverse free electron laser accelerator for advanced light sources

We discuss the inverse free electron laser (IFEL) scheme as a compact high gradient accelerator solution for driving advanced light sources such as a soft x-ray free electron laser amplifier or an inverse Compton scattering based gamma-ray source. In particular, we present a series of new developments aimed at improving the design of future IFEL accelerators. These include a new procedure to optimize the choice of the undulator tapering, a new concept for prebunching which greatly improves the fraction of trapped particles and the final energy spread, and a self-consistent study of beam loading effects which leads to an energy-efficient high laser-to-beam power conversion.