A. Shishlo

The EPICS based virtual accelerator-concept and implementation

A virtual accelerator (VA) concept and an implementation founded on TRACE3D and PARMILA codes are presented. This virtual accelerator is suitable for accelerators with a control system based on EPICS and consists of the EPICS portable channel access server (PCAS), the EPICS client providing communication between a simulation model and PCAS, and the simulation model itself. The virtual accelerators for the SNS linac and experience in using these VAs are discussed.

Performance of SNS Front end and warm linac

The Spallation Neutron Source accelerator systems will deliver a 1.0 GeV, 1.4 MW proton beam to a liquid mercury target for neutron scattering research. The accelerator complex consists of an H-injector, capable of producing one-ms-long pulses at 60 Hz repetition rate with 38 mA peak current, a 1 GeV linear accelerator, an accumulator ring and associated transport lines. The 2.5 MeV beam from the Front End is accelerated to 86 MeV in the Drift Tube Linac, then to 185 MeV in a Coupled-Cavity Linac and finally to 1 GeV in the Superconducting Linac. With the completion of beam commissioning, the accelerator complex began operation in June 2006 and beam power is being gradually ramped up toward the design goal. Operational experience with the injector and linac will be presented including chopper performance, transverse emittance evolution along the linac, and the results of a beam loss study.

Development and Implementation of δT Procedure for the SNS Linac

The Δt procedure is a time of flight technique for setting the phases and amplitudes of accelerating fields in a multi-cavity linac. It was initially proposed and developed for the LAMPF linac in the early seventies [1,2] and since then has been used in several accelerators [3,4]. The SNS linac includes four CCL modules (Side Coupled Structure) operating at 805 MHz for the energy range from 86.8 MeV up to 185.6 MeV. The Δt procedure has been implemented for the SNS CCL linac and was used for initial beam commissioning of three CCL modules. A brief theory of the procedure, the results of the design parameter calculations and the experimental results of phase and amplitude set points are presented and discussed.

Laser stripping of H - beams: theory and experiments

Thin carbon foils are used as strippers for charge exchange injection into high intensity proton rings. However, the stripping foils become radioactive and produce uncontrolled beam loss, which is one of the main factors limiting beam power in high intensity proton rings. Recently, we presented a scheme for laser stripping an H- beam for the Spallation Neutron Source ring. First, H- atoms are converted to H0 by a magnetic field, then H0 atoms are excited from the ground state to the upper levels by a laser, and the excited states are converted to protons by a second magnetic field. In this paper we report on the first successful proof-of-principle demonstration of this scheme to give high efficiency (around 90%) conversion of H- beam into protons at SNS in Oak Ridge. In addition, future plans on building a practical laser stripping device are discussed.

SNS global database use in application programming

A global relational database is being assembled to track accelerator components for the Spallation Neutron Source (SNS). As part of this activity, beamline element information is stored for use in high level application programs. A hierarchal accelerator framework is generated from the database and used for initialization of a Java based programming infrastructure. From within this framework input files for beam simulation codes can be generated using either live machine values or design values. The database also includes global coordinates for beamline element alignment, and magnet measurement data. An overview of the table schema and relationships to tables used in other parts of the project are discussed.

Self-Consistent Electron-Cloud Simulation for Long Proton Bunches

The results of numerical self-consistent electron-cloud simulations are presented and compared with data from the Proton Storage Ring at LANL. The ORBIT code with a recently developed electron-cloud module has been used. The model includes fully coupled “proton bunch – electron cloud” dynamics, a multipacting process model with a realistic secondary emission surface model, and a realistic lattice and injection scheme. The growth rates of proton-bunch transverse instabilities were studied as functions of the beam intensity, RF cavity voltage, external magnetic fields, and number of interaction points between protons and electrons in the model.

A modular on-line simulator for model reference control of charged particle beams

We have implemented a particle beam simulation engine based on modern software engineering principles with intent that it be a convenient model reference for high-level control applications. The simulator is an autonomous subsystem of the high-level application framework XAL currently under development for the Spallation Neutron Source (SNS). It supports multiple simulation techniques (i.e., single particle, multi-particle, envelope, etc.), automatically synchronizes with operating accelerator hardware, and also supports off-line design studies. Moreover, since it is implemented using modern techniques in the Java language, it is portable across operating platforms, is maintainable, and upgradeable.

Application programming structure and physics applications

The Spallation Neutron Source (SNS) is using a Java based hierarchal framework for application program development. The framework is designed to provide an accelerator physics programming interface to the accelerator, called XAL. Much of the underlying interface to the EPICS control system is hidden from the user. Use of this framework allows writing of general-purpose applications that can be applied to various parts of the accelerator. Also, since the accelerator structure is initiated from a database, introduction of new beamline devices or signal modifications are immediately available for all XAL applications. Direct scripting interfaces are available for both Jython and Matlab, for rapid prototyping uses. Initial applications such as orbit difference, orbit correction and a general purpose diagnostic tool have been developed and tested with the SNS front end. The overall framework is described, and example applications are shown.

ORBIT: A Code for Collective Beam Dynamics in High‐Intensity Rings

We are developing a computer code, ORBIT, specifically for beam dynamics calculations in high-intensity rings. Our approach allows detailed simulation of realistic accelerator problems. ORBIT is a particle-in-cell tracking code that transports bunches of interacting particles through a series of nodes representing elements, effects, or diagnostics that occur in the accelerator lattice. At present, ORBIT contains detailed models for strip-foil injection, including painting and foil scattering; rf focusing and acceleration; transport through various magnetic elements; longitudinal and transverse impedances; longitudinal, transverse, and three-dimensional space charge forces; collimation and limiting apertures; and the calculation of many useful diagnostic quantities. ORBIT is an object-oriented code, written in C++ and utilizing a scripting interface for the convenience of the user. Ongoing improvements include the addition of a library of accelerator maps, BEAMLINE/MXYZPTLK, the introduction of a treatment of magnet errors and fringe fields; the conversion of the scripting interface to the standard scripting language, Python; and the parallelization of the computations using MPI. The ORBIT code is an open source, powerful, and convenient tool for studying beam dynamics in high-intensity rings.

High efficiency laser-assisted H − charge exchange for microsecond duration beams

Laser-assisted stripping is a novel approach to H- charge exchange that overcomes long-standing limitations associated with the traditional, foil-based method of producing high-intensity, time-structured beams of protons. This paper reports on the first successful demonstration of the laser stripping technique for microsecond duration beams. The experiment represents a factor of 1000 increase in the stripped pulse duration compared with the previous proof-of-principle demonstration. The central theme of the experiment is the implementation of methods to reduce the required average laser power such that high efficiency stripping can be accomplished for microsecond duration beams using conventional laser technology. The experiment was performed on the Spallation Neutron Source 1 GeV H- beam using a 1 MW peak power UV laser and resulted in ~95% stripping efficiency.