Peter Ostroumov

Multiple-charge beam dynamics in an ion linac

There is demand for the construction of a medium-energy ion linear accelerator based on superconducting rf (SRF) technology. It must be capable of producing several hundred kilowatts of CW beams ranging from protons to uranium. A considerable amount of power is required in order to generate intense beams of rare isotopes for subsequent acceleration. At present, however, the beam power available for the heavier ions would be limited by ion source performance. To overcome this limit, we have studied the possibility of accelerating multiple-charge-state (multi-Q) beams through a linac. We show that such operation is made feasible by the large transverse and longitudinal acceptance which can be obtained in a linac using superconducting cavities. Multi-Q operation provides not only a substantial increase in beam current, but also enables the use of two strippers, thus reducing the size of linac required. Since the superconducting (SC) linac operates in CW mode, space charge effects are essentially eliminated except in the ECR/RFQ region. Therefore an effective emittance growth due to the multi-charge beam acceleration can be minimized.

Front End Design of A Multi-GeV H-Minus Linac

The proposed 8-GeV driver at FNAL [1] is based on ∼ 430 independently phased SC resonators. Significant cost savings are expected by using an rf power fan-out from high-power klystrons to multiple cavities. Successful development of superconducting (SC) multi-spoke resonators operating at ∼ 345-350 MHz provides a strong basis for their application in the front end of multi-GeV linear accelerators. Such a front-end operating at 325 MHz would enable direct transition to high-gradient 1300 MHz SC TESLA-style cavities at ∼ 400 MeV. The proposed front end consists of 5 sections: a conventional RFQ, room-temperature (RT) cross-bar H-type (CH) cavities, single-, double-and triple-spoke superconducting resonators. It is effective to use short RT CH-cavities between the RFQ and SC sections in the energy range 3-10 MeV as is discussed below.

A post-accelerator for the US rare isotope accelerator facility

Abstract The proposed rare isotope accelerator (RIA) facility includes a post-accelerator for rare isotopes (RIB linac) which must produce high-quality beams of radioactive ions over the full mass range, including uranium, at energies above the Coulomb barrier, and have high transmission and efficiency. The latter requires the RIB linac to accept at injection ions in the 1+ charge state. A concept for such a post accelerator suitable for ions up to mass 132 has been previously described [1] . This paper presents a modified concept which extends the mass range to uranium. A high resolution separator for purifying beams at the isobaric level precedes the RIB linac. The mass filtering process will provide high purity beams while preserving transmission. For most cases a resolution of about m/ Δ m=20 000 is adequate at mass A=100 to obtain a separation between isobars of mass excess difference of 5 MeV. The design for a device capable of purifying beams at the isobaric level includes calculations up to fifth order. The RIB linac will utilize existing superconducting heavy-ion linac technology for all but a small portion of the accelerator system. The exceptional piece, a very-low-charge-state injector section needed for just the first few MV of the RIB accelerator, consists of a pre-buncher followed by several sections of cw, normally-conducting RFQ. Two stages of charge stripping are provided: helium gas stripping at energies of a few keV/u, and additional foil stripping at ∼680 keV/u for the heavier ions. In extending the mass range to uranium, however, for best efficiency the helium gas stripping must be performed at different energies for different mass ions. We present numerical simulations of the beam dynamics of a design for the complete RIB linac which provides for several stripping options and uses cost-effective solenoid focusing elements in the drift-tube linac.

Development of electron beam ion source charge breeder for rare isotopes at Californium Rare Isotope Breeder Upgrade.

Recently, the Californium Rare Isotope Breeder Upgrade (CARIBU) to the Argonne Tandem Linac Accelerator System (ATLAS) was commissioned and became available for production of rare isotopes. Currently, an electron cyclotron resonance ion source is used as a charge breeder for CARIBU beams. To further increase the intensity and improve the purity of neutron-rich ion beams accelerated by ATLAS, we are developing a high-efficiency charge breeder for CARIBU based on an electron beam ion source (EBIS). The CARIBU EBIS charge breeder will utilize the state-of-the-art EBIS technology recently developed at Brookhaven National Laboratory (BNL). The electron beam current density in the CARIBU EBIS trap will be significantly higher than that in existing operational charge-state breeders based on the EBIS concept. The design of the CARIBU EBIS charge breeder is nearly complete. Long-lead components of the EBIS such as a 6-T superconducting solenoid and an electron gun have been ordered with the delivery schedule in the fall of 2011. Measurements of expected breeding efficiency using the BNL Test EBIS have been performed using a Cs(+) surface ionization ion source for external injection in pulsed mode. In these experiments we have achieved ∼70% injection∕extraction efficiency and breeding efficiency into the most abundant charge state of ∼17%.

A parallel 3D Poisson solver for space charge simulation in cylindrical coordinates

Abstract This paper presents the development of a parallel three-dimensional Poisson solver in cylindrical coordinate system for the electrostatic potential of a charged particle beam in a circular tube. The Poisson solver uses Fourier expansions in the longitudinal and azimuthal directions, and Spectral Element discretization in the radial direction. A Dirichlet boundary condition is used on the cylinder wall, a natural boundary condition is used on the cylinder axis and a Dirichlet or periodic boundary condition is used in the longitudinal direction. A parallel 2D domain decomposition was implemented in the ( r , θ ) plane. This solver was incorporated into the parallel code PTRACK for beam dynamics simulations. Detailed benchmark results for the parallel solver and a beam dynamics simulation in a high-intensity proton LINAC are presented. When the transverse beam size is small relative to the aperture of the accelerator line, using the Poisson solver in a Cartesian coordinate system and a Cylindrical coordinate system produced similar results. When the transverse beam size is large or beam center located off-axis, the result from Poisson solver in Cartesian coordinate system is not accurate because different boundary condition used. While using the new solver, we can apply circular boundary condition easily and accurately for beam dynamic simulations in accelerator devices.

Advanced Beam-Dynamics Simulation Tools for RIA

We are developing multi-particle beam-dynamics simulation codes for RIA driver-linac simulations extending from the low-energy beam transport (LEBT) line to the end of the linac. These codes run on the NERSC parallel supercomputing platforms at LBNL, which allow us to run simulations with large numbers of macroparticles. The codes have the physics capabilities needed for RIA, including transport and acceleration of multiple-charge-state beams, beam-line elements such as high-voltage platforms within the linac, interdigital accelerating structures, charge-stripper foils, and capabilities for handling the effects of machine errors and other off-normal conditions. This year will mark the end of our project. In this paper we present the status of the work, describe some recent additions to the codes, and show some preliminary simulation results.

Design and test of a beam profile monitoring device for low intensity radioactive beams

Development efforts have gone into the construction and performance testing of a device that can be used to provide snap shot images of the beam profile. It is intended to function even at very low ion intensities, such as those expected from a rare-isotope accelerator. Intensity profiles and emittance analysis are among the most critical tools used for optimizing beam transport through accelerators. This article describes the design and performance of a beam image monitor. The device is sensitive at a wide dynamic range which spans from ∼102 to ∼1012 pps. With the advent of double-plane slits or a pepper pot plate, this system can be used to scan transverse emittance profiles in both the x–x′ and y–y′ phase space planes, simultaneously. Conventional diagnostic devices used for heavy ion accelerators generally require at least 109 pps intensity to carry out similar diagnostics, which is not practical when considering beams with very low intensities, such as rare isotope beams. Furthermore, the detection s...

HINS Linac Front End Focusing System R&D

This report summarizes current status of an R&D program to develop a focusing system for the front end of a superconducting RF linac. Superconducting solenoids will be used as focusing lenses in the low energy accelerating sections of the front end. The development of focusing lenses for the first accelerating section is in the production stage, and lens certification activities are in preparation at FNAL. The report contains information about the focusing lens design and performance, including solenoid, dipole corrector, and power leads, and about cryogenic system design and performance. It also describes the lens magnetic axis position measurement technique and discusses scope of an acceptance/certification process.

EBIS charge breeder for CARIBU

A high-efficiency charge breeder based on an Electron Beam Ion Source (EBIS) is being developed by the ANL Physics Division to increase the intensity and improve the purity of accelerated radioactive ion beams. A wide variety of low-energy neutron-rich ion beams are produced by the Californium Rare Isotope Breeder Upgrade (CARIBU) for the Argonne Tandem Linac Accelerator System (ATLAS). These beams will be charge-bred by an EBIS charge breeder to a charge-to-mass ratio (q/A) ≥ 1/7 and accelerated by ATLAS to energies of about 10 MeV/u. The assembly of the CARIBU EBIS charge breeder except the injection/extraction beam lines has been completed. This summer we started electron beam commissioning of the EBIS. The first results on electron beam extraction, transport from the electron gun to a high power electron collector are presented and discussed.

Fast and efficient charge breeding of the Californium rare isotope breeder upgrade electron beam ion source

The Electron Beam Ion Source (EBIS), developed to breed Californium Rare Isotope Breeder Upgrade (CARIBU) radioactive beams at Argonne Tandem Linac Accelerator System (ATLAS), is being tested off-line. A unique property of the EBIS is a combination of short breeding times, high repetition rates, and a large acceptance. Overall, we have implemented many innovative features during the design and construction of the CARIBU EBIS as compared to the existing EBIS breeders. The off-line charge breeding tests are being performed using a surface ionization source that produces singly charged cesium ions. The main goal of the off-line commissioning is to demonstrate stable operation of the EBIS at a 10 Hz repetition rate and a breeding efficiency into single charge state higher than 15%. These goals have been successfully achieved and exceeded. We have measured (20% ± 0.7%) breeding efficiency into the single charge state of 28+ cesium ions with the breeding time of 28 ms. In general, the current CARIBU EBIS operational parameters can provide charge breeding of any ions in the full mass range of periodic table with high efficiency, short breeding times, and sufficiently low charge-to-mass ratio, 1/6.3 for the heaviest masses, for further acceleration in ATLAS. In this paper, we discuss the parameters of the EBIS and the charge breeding results in a pulsed injection mode with repetition rates up to 10 Hz.