V. Danilov

Nonlinear accelerator lattices with one and two analytic invariants

Integrable systems appeared in physics long ago at the onset of classical dynamics with examples being Keplers and other famous problems. Unfortunately, the majority of nonlinear problems turned out to be nonintegrable. In accelerator terms, any 2D nonlinear nonintegrable mapping produces chaotic motion and a complex network of stable and unstable resonances. Nevertheless, in the proximity of an integrable system the full volume of such a chaotic network is small. Thus, the integrable nonlinear motion in accelerators has the potential to introduce a large betatron tune spread to suppress instabilities and to mitigate the effects of space charge and magnetic field errors. To create such an accelerator lattice one has to find magnetic and electric field combinations leading to a stable integrable motion. This paper presents families of lattices with one invariant where bounded motion can be easily created in large volumes of the phase space. In addition, it presents 3 families of integrable nonlinear accelerator lattices, realizable with longitudinal-coordinate-dependent magnetic or electric fields with the stable nonlinear motion, which can be solved in terms of separable variables.

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.

Multipacting on the trailing edge of proton beam bunches in the PSR and SNS

The Proton Storage Ring (PSR) in Los Alamos has a fast intensity-limiting instability, which may result from an electron cloud interaction with the circulating proton beam leading to a transverse mode coupling instability. The most probable mechanism of the electron creation is multipacting. Though the effect depends on many parameters, a model is presented which predicts a large electron creation in the vacuum chamber. A comparison of this effect between the PSR in Los Alamos and the Spallation Neutron Source (SNS) in Oak Ridge is given. In addition, several possibilities to reduce multipactor are discussed.

Transverse impedance implementation in ORBIT

The transverse stability and broadening of the beam turn out to be the most problematic issues in the Spallation Neutron Source ring operation. The particle simulation code ORBIT, initially written for space charge and halo growth studies, now is able to cover a broad spectrum of the accumulator ring problems. Here we present an implementation of a collective force simulation due, to impedance elements in the vacuum chamber of the ring.

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.

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.

Techniques for Measurement and Correction of the SNS Accumulator Ring Optics

The Spallation Neutron Source (SNS) Accumulator Ring will reach peak intensities of 1.5×1014protons/pulse through multi-turn charge-exchange injection. Accumulation of these unprecedented beam intensities must be accomplished while maintaining extremely low losses (less than 1 W/m). It is anticipated that the understanding and control of the ring optics will be important for achieving these low loss rates. We describe our plans for measuring and correcting the optical functions of the accumulator ring lattice.

Exploration of beam fault scenarios for the Spallation Neutron Source target

The Spallation Neutron Source (SNS) accelerator systems will provide a 1 GeV, 1.44 MW proton beam to a liquid mercury target for neutron production. In order to ensure adequate lifetime of the target system components, requirements on several beam parameters must be maintained. A series of error studies was performed to explore credible fault scenarios which can potentially violate the various beam-on-target parameters. The response of the beam-on-target parameters to errors associated with the phase-space painting process in the ring and field setpoint errors in all the ring-to-target beam transport line elements were explored and will be presented. The plan for ensuring beam-on-target parameters will also be described.

Calculations of electron accumulation in the SNS storage ring

This is a study to determine what kind of materials could be used in the Spallation Neutron Source ring to prevent electron accumulation due to secondary emission. The calculations presented are simple but based on experimental facts. Practical recommendations for the accumulator ring are considered.

Beam instabilities in Very Large Hadron Collider

The Very Large Hadron Collider (VLHC) is a superconducting proton-proton collider with approximately 100 TeV cm and approximately 10/sup 34/ s/sup -1/ cm/sup -2/ luminosity. Currently, beam dynamics in this future accelerator is the subject of intensive studies within the framework of the US-wide VLHC R&D program. This presentation summarizes recent developments in the field. Besides general discussion on relevant VLHC parameters, we consider various beam instabilities and ways to avoid them. Finally, we outline possibilities for theoretical and experimental R&D.