Christopher K. Allen

XAL Application Programming Structure

XAL is an application programming framework used at the Spallation Neutron Source (SNS) project in Oak Ridge. It is written in Java, and provides users with a hierarchal view of the accelerator. Features include database configuration of the accelerator structure, an online envelope model that is configurable from design or live machine values, an application framework for quickstart GUI development, a scripting interface for algorithm development, and a common toolkit for shared resources. To date, about 25 applications have been written, many of which are used extensively in the SNS beam commissioning activities. The XAL framework and example applications will be discussed.

Experimental study of proton-beam halo induced by beam mismatch in LEDA

We report measurements of transverse beam halo in mismatched proton beams in a 52-quadrupole FODO transport channel following the 6.7-MeV LEDA RFQ. Beam profiles in both transverse planes are measured using beam-profile diagnostic devices that consist of a movable carbon filament for measurement of the dense beam core, and scraper plates for the halo measurement. The gradients of the first four quadrupoles can be independently adjusted to mismatch the RFQ output beam into the beam-transport channel.

XAL online model enhancements for J-PARC commissioning and operation

The XAL application development environment has been installed as a part of the control system for the Japan Proton Accelerator Research Center (J-PARC) in Tokai, Japan. XAL was initially developed at the spallation neutron source (SNS) and has been described at length in previous conference proceedings. Included in XAL is an online model for doing quick physics simulations. We outline the upgrades and enhancements to the XAL online model necessary for accurate simulation of the J- PARC linac and transport system.

Measurements of halo generation for a proton beam in a FODO channel

An experimental effort has been undertaken to investigate the production of halo particles in a proton beam having significant space charge forces. The LEDA RFQ was used to inject a pulsed 6.7 MeV 15-75 mA beam into a linear FODO channel. Four matching quads at the input of this 52-quadrupole transport line were used to generate specific mismatch oscillations, believed to be a key mechanism in the generation of beam halo. A suite of diagnostics that provide beam profile measurements over a wide dynamic range enabled a detailed comparison of measurements with theoretical models.

Optimal beam matching in particle accelerators via extremum seeking

The matching problem for a low energy transport system in a charged particle accelerator is approached using the extremum seeking feedback method for non-model based optimization. The beam dynamics are modeled using the KV (Kapchinsky-Vladimirsky) envelope equations. Extremum seeking is employed for the lens tuning in the beam matching system. Numerical simulations illustrate the effectiveness of this approach

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.

The Spallation Neutron Source Beam Commissioning and Initial Operations

The Spallation Neutron Source (SNS) accelerator delivers a one mega-Watt beam to a mercury target to produce neutrons used for neutron scattering materials research. It delivers ~ 1 GeV protons in short (< 1 us) pulses at 60 Hz. At an average power of ~ one mega-Watt, it is the highest-powered pulsed proton accelerator. The accelerator includes the first use of superconducting RF acceleration for a pulsed protons at this energy. The storage ring used to create the short time structure has record peak particle per pulse intensity. Beam commissioning took place in a staged manner during the construction phase of SNS. After the construction, neutron production operations began within a few months, and one mega-Watt operation was achieved within three years. The methods used to commission the beam and the experiences during initial operation are discussed.

Unified Beam Control System of J-PARC Linac

A beam control system has been developed for the J-PARC linac to achieve strict design goals of beam performance and beam loss suppression in the high-intensity H- beams. An emerging concept of unified beam control has been adopted in the system, where a unified data source, standardized control units, a unified online model, and uniform high-level application frameworks have been designed and implemented. For precise beam controls, various beam tuning algorithms have been developed and tested. Design goals of beam performance, beam stability, and beam loss suppression have been achieved in the linac.

Optimized Beam Matching Using Extremum Seeking

The matching problem for a low energy transport system is approached from a control theoretical viewpoint. The beam dynamics are modeled using the KV (Kapchinskij-Vladimirskij) envelope equations. Multi-Parameter Extremum Seeking, a real-time non-model based optimization technique, is considered in this work for the lens tuning in the beam matching system. Numerical simulations illustrate the effectiveness of this approach.