Search Results (15 total)

Numerical simulations of H^- stripping losses from blackbody radiation, electromagnetic fields, and residual gas have been implemented into the beam dynamics code TRACK. Estimates of the stripping losses along two high-intensity H^- linacs are presented: the Spallation Neutron Source linac currently being operated at Oak Ridge National Laboratory and an 8 GeV superconducting linac currently being designed at Fermi National Accelerator Laboratory.

To meet the beam power requirements for high-intensity ion linacs being proposed for rare isotope beam production and other nuclear physics applications, we have developed an injector system to extract, accelerate, analyze, and recombine multiple charge states of any heavy-ion beam. The injector consists of an electron cyclotron resonance ion source, a 100 kV platform and an achromatic low-energy beam transport system. Two charge states of bismuth-209 (20+ and 21+) were successfully accelerated by the high-voltage platform potential, separated and perfectly recombined in the transverse phase space with 100% transmission. The perfect recombination and maximum transmission of the cw beam are essential for smooth injection into a subsequent rf accelerator.

This paper presents designs for four types of very-low-velocity superconducting (SC) accelerating cavity capable of providing several MV of accelerating potential per cavity, and suitable for particle velocities in the range 0.006<v/c<0.06. Superconducting TEM-class cavities have been widely applied to cw acceleration of ion beams. SC linacs can be formed as an array of independently phased cavities, enabling a variable velocity profile to maximize the output energy for each of a number of different ion species. Several laboratories in the U.S. and Europe are planning exotic beam facilities based on SC linacs. The cavity designs presented here are intended for the front end of such linacs, particularly for the postacceleration of rare isotopes of low charge state. Several types of SC cavities have been developed recently to cover particle velocities above 0.06c. Superconducting four-gap quarter-wave resonators for velocities 0.008<β=v/c<0.05 were developed about two decades ago and have been successfully operated at the ATLAS SC linac at Argonne National Laboratory. Since that time, progress in simulation tools, cavity fabrication, and processing have increased SC cavity gradients by a factor of 3–4. This paper applies these tools to optimize the design of a four-gap quarter-wave resonator for exotic beam facilities and other low-velocity applications.

Beam acceleration in the International Linear Collider (ILC) will be provided by 9-cell 1300 MHz superconducting (SC) cavities. The cavities are designed for effective acceleration of charged particles moving with the speed of light and are operated on π-mode to provide a maximum accelerating gradient. A significant research and development effort has been devoted to develop ILC SC technology and its rf system which resulted in excellent performance of ILC cavities. Therefore, the proposed 8-GeV proton driver in Fermilab is based on ILC cavities above ∼1.2  GeV. The efficiency of proton beam acceleration by ILC cavities drops fast for lower velocities and it was proposed to develop squeezed ILC-type (S-ILC) cavities operating at 1300 MHz and designed for β_G=0.81, geometrical beta, to accelerate protons or H^- from ∼420  MeV to 1.2 GeV. This paper discusses the possibility of avoiding the development of new β_G=0.81 cavities by operating ILC cavities on 8 / 9π-mode of standing wave oscillations.

The design and operation support of hadron (proton and heavy-ion) linear accelerators require substantial use of beam dynamics simulation tools. The beam dynamics code TRACK has been originally developed at Argonne National Laboratory (ANL) to fulfill the special requirements of the rare isotope accelerator (RIA) accelerator systems. From the beginning, the code has been developed to make it useful in the three stages of a linear accelerator project, namely, the design, commissioning, and operation of the machine. To realize this concept, the code has unique features such as end-to-end simulations from the ion source to the final beam destination and automatic procedures for tuning of a multiple charge state heavy-ion beam. The TRACK code has become a general beam dynamics code for hadron linacs and has found wide applications worldwide. Until recently, the code has remained serial except for a simple parallelization used for the simulation of multiple seeds to study the machine errors. To speed up computation, the TRACK Poisson solver has been parallelized. This paper discusses different parallel models for solving the Poisson equation with the primary goal to extend the scalability of the code onto 1024 and more processors of the new generation of supercomputers known as BlueGene (BG/L). Domain decomposition techniques have been adapted and incorporated into the parallel version of the TRACK code. To demonstrate the new capabilities of the parallelized TRACK code, the dynamics of a 45 mA proton beam represented by 10⁸ particles has been simulated through the 325 MHz radio frequency quadrupole and initial accelerator section of the proposed FNAL proton driver. The results show the benefits and advantages of large-scale parallel computing in beam dynamics simulations.

A new procedure has been developed for automatic longitudinal tuning of a multiple-charge-state heavy-ion beam. It uses a matrix-based code to track the beam centroids and Twiss parameters of the individual charge state beams and a minimization code to minimize a goal function by adjusting the synchronous phases and field levels in the accelerating cavities. The procedure has been successfully tested in the case of the Rare Isotope Accelerator driver linac and proven to improve an original manual tune by significantly reducing beam losses. The procedure was also applied for fast retuning of the linac after one or more cavity failure and restoring the beam with limited beam loss.

The proposed Rare Isotope Accelerator (RIA) Facility, an innovative exotic-beam facility for the production of high-quality beams of short-lived isotopes, consists of a fully superconducting 1.4 GV driver linac and a 140 MV postaccelerator. To produce sufficient intensities of secondary beams the driver linac will provide 400 kW primary beams of any ion from hydrogen to uranium. Because of the high intensity of the primary beams the beam losses must be minimized to avoid radioactivation of the accelerator equipment. To keep the power deposited by the particles lost on the accelerator structures below 1   W/m, the relative beam losses per unit length should be less than 10^-5, especially along the high-energy section of the linac. A new beam dynamics simulation code TRACK has been developed and used for beam loss studies in the RIA driver linac. In the TRACK code, ions are tracked through the three-dimensional electromagnetic fields of every element of the linac starting from the electron cyclotron resonance (ECR) ion source to the production target. The simulation starts with a multicomponent dc ion beam extracted from the ECR. The space charge forces are included in the simulations. They are especially important in the front end of the driver linac. Beam losses are studied by tracking a large number of particles (up to 10⁶) through the whole linac considering all sources of error such us element misalignments, rf field errors, and stripper thickness fluctuations. For each configuration of the linac, multiple sets of error values have been randomly generated and used in the calculations. The results are then combined to calculate important beam parameters, estimate beam losses, and characterize the corresponding linac configuration. To track a large number of particles for a comprehensive number of error sets (up to 500), the code TRACK was parallelized and run on the Jazz computer cluster at ANL.

The applicability of superconducting TEM-class spoke cavities to high-energy ion linacs is discussed, and detailed designs for two TEM-class, triple-spoke-loaded superconducting niobium resonant cavities are presented. The 345 MHz cavities have a velocity range of 0.4<β<0.75 and a beam aperture of 4 cm. Spoke-loaded cavities offer several advantages compared with the higher-frequency elliptical-cell cavities that are currently being developed for this range of particle velocities. The proposed triple-spoke cavities can provide broader velocity acceptance, more accelerating voltage per cavity, reduced heat-load operation at 4.2 K, and increased longitudinal acceptance through the high-energy section. Application to the proposed U.S. rare-isotope accelerator driver linac is discussed in detail.

Beam-bunch arrival time has been measured for the first time by operating superconducting cavities, normally part of the linac accelerator array, in a bunch-detecting mode. The very high Q of the superconducting cavities provides high sensitivity and allows for phase-detecting low-current beams. In detecting mode, the resonator is operated at a very low field level comparable to the field induced by the bunched beam. Because of this, the rf field in the cavity is a superposition of a “pure” (or reference) rf and the beam-induced signal. A new method of circular phase rotation (CPR), allowing extraction of the beam phase information from the composite rf field was developed. Arrival time phase determination with CPR is better than 1° (at 48 MHz) for a beam current of 100 nA. The electronics design is described and experimental data are presented.

The nuclear science community considers the construction of the Rare Isotope Accelerator (RIA) facility as a top priority. The RIA includes a 1.4 GV superconducting linac for production of 400 kW cw heavy ion beams. The initial acceleration of heavy ions delivered from an electron cyclotron resonance ion source can be effectively performed by a 57.5 MHz 4-m long room temperature RFQ. The principal specifications of the RFQ are (i) formation of extremely low longitudinal emittance, (ii) stable operation over a wide range of voltage for acceleration of various ion species needed for RIA operation, and (iii) simultaneous acceleration of two-charge states of uranium ions. cw operation of an accelerating structure leads to a number of requirements for the resonators such as high shunt impedance, efficient water cooling of all parts of the resonant cavity, mechanical stability together with precise alignment, reliable rf contacts, a stable operating mode, and fine tuning of the resonant frequency during operation. To satisfy these requirements a new resonant structure has been developed. This paper discusses the beam dynamics and electrodynamics design of the RFQ cavity, as well as some aspects of the mechanical design of the low-frequency cw RFQ.

The Rare Isotope Accelerator (RIA) facility project includes a cw 1.4 GeV driver linac and a 100 MV postaccelerator both based on superconducting (SC) cavities operating at frequencies from 48 to 805 MHz. In these linacs more than 99% of the total voltage is provided by SC cavities. An initial acceleration is provided by room temperature radio frequency quadrupoles. The driver linac is designed for acceleration of any ion species, from protons up to 900 MeV to uranium up to 400 MeV/u. The novel feature of the driver linac is an acceleration of multiple charge-state heavy-ion beams in order to achieve 400 kW beam power. This paper presents design features of a medium-energy SC heavy-ion linac taking the RIA driver linac as an example. The dynamics of single and multiple charge-state beams are detailed, including the effects of possible errors in rf field parameters and misalignments of transverse focusing elements. The important design considerations of such linac are presented. Several new conceptual solutions in beam dynamics in SC accelerating structures for heavy-ion applications are discussed.

Superconducting cavities presently used for acceleration of ions in velocity range ∼0.01c to 0.3c (where c is the speed of light) are based on quarter-wave resonators. Currently there are several design proposals in nuclear physics laboratories for application of this type of cavity for acceleration of light and heavy ions. The operating frequencies of the cavities range from ∼50 to 360 MHz to satisfy various specifications. Electrodynamics studies of the field distributions in the beam-cavity interaction area indicate appreciable dipole components of both electric and magnetic fields, especially for higher-frequency cavities. The dipole fields induce beam steering, which is a strong function of rf phase and which couples the longitudinal and transverse motion. This can result in growth in the transverse emittance of the beam. In this paper, we propose two possible methods for the correction of such dynamic beam-steering effects in quarter-wave resonators. We analyze and discuss the correction methods for the particular examples of two quarter-wave resonators operating at 57.5 and 115 MHz designed for the driver linac of the Rare Isotope Accelerator facility.

The possibility of simultaneously accelerating particles with a range of charge-to-mass ratios ( ∼20%) to the same energy is proposed and demonstrated for a superconducting linac. Uranium ions stripped in a foil with eight charge states have been accelerated through a portion of the ATLAS linac from 286 to 690 MeV, with 94% of the injected uranium in the accelerated beam. Emittance of the resultant beam has been measured and the energy spread was 1.3% compared to 0.4% for a single charge state. This development has immediate application to the high-intensity acceleration of heavy ions that are limited by ion-source intensities, such as the proposed Rare Isotope Accelerator Facility.

An advanced facility for the production of nuclei far from stability could be based on a high-power driver accelerator providing ion beams over the full mass range from protons to uranium. A beam power of several hundred kilowatts is highly desirable for this application. At present, however, the beam power available for the heavier ions would be limited by ion source capabilities. A simple and cost-effective method to enhance the available beam current would be to accelerate multiple charge states through a superconducting ion linac. This paper presents results of numerical simulation of multiple charge state beams through a 1.3 GeV ion linac, the design of which is based on current state-of-the-art superconducting elements. The dynamics of multiple charge state beams are detailed, including the effects of possible errors in rf field parameters and misalignments of transverse focusing elements. The results indicate that operation with multiple charge state beams is not only feasible but straightforward and can increase the beam current by a factor of 3 or more.