ORNL, Oak Ridge, Tennessee, USA
A review of the Spallation Neutron Source (SNS) linac beam dynamics is presented. It describes transverse and longitudinal beam optics, losses, activation, and comparison between the initial design and the existing accelerator. The SNS linac consists of normal conducting and superconducting parts. The peculiarities in operations with the superconducting part of the SNS linac (SCL), estimations of total losses in SCL, the possible mechanisms of these losses, and the progress in the transverse matching are discussed.
ORNL, Oak Ridge, Tennessee, USA
The 248-meter Spallation Neutron Source (SNS) accumulator ring has accumulated up to 1.55·1014, 1 GeV protons. At this intensity, space charge effects contribute significantly to the beam dynamics. Here we present observations of space charge effects in the SNS ring, with emphasis on space charge effects and e-p instabilities.
UMD, College Park, Maryland, USA
LBNL, Berkeley, California, USA
The Neutralized Drift Compression Experiment (NDCX-I) generates high intensity ion beams to explore Warm Dense Matter physics. A ~150 kV, ~500 ns ramped voltage pulse is applied to a ~300 keV, 5-10 μs, 25 mA K+ ion beam across a single induction gap. The velocity modulated beam compresses longitudinally during ballistic transport along a space-charge-neutralizing plasma transport line, resulting in ~3A peak current with ~2-3 ns pulse durations (FWHM) at the target plane. Transverse final focusing is accomplished with a ~8 T, 10 cm long pulsed solenoid magnet. Time-dependent focusing in the induction gap, and chromatic aberrations in the final focus optics limit the peak fluence at the target plane for the compressed beam pulse. We report on time-dependent phase space measurements of the compressed pulse in the ballistic transport beamline, and measurement of the time-dependent radial impulses derived from the interaction of the beam and the induction gap voltage. We present results of start-to-end simulations to benchmark the experiments. Fast correction strategies are discussed with application to both NDCX-I and to the new NDCX-II accelerator.
LANL, Los Alamos, New Mexico, USA
UMD, College Park, Maryland, USA
The University of Maryland Electron Ring (UMER) is a machine designed to study high-intensity beam physics. With the application of axial fields to the bunch ends, we are able to keep a beam with an injected tune shift of 1.0, bunched over multiple turns. This is feasible with the application of tailored fields to optimally match the space-charge self-fields while minimizing the excitation of longitudinal space-charge waves. With this scheme, we have been able to extend the number of turns at the University of Maryland Electron Ring (UMER) by a factor of ten. Without the use of longitudinal focusing, head and tail effects begin to dominate, especially with the higher current beams. Time resolved measurements of the peak correlated energy spread have shown in some cases a change in the overall spread of 1.8% for the 0.6 mA beam, from the injected beam energy.
MIT, Cambridge, Massachusetts, USA
Adiabatic thermal equilibrium is an important state of a charged-particle beam. The rigid-rotor thermal beam equilibrium in a uniform magnetic focusing field is established. The equivalent kinetic and warm-fluid theories of adiabatic thermal beam equilibrium in a periodic solenoidal magnetic focusing field are discussed. Good agreement between theories and experiment is found. The warm-fluid theory of adiabatic thermal beam equilibrium in an alternating-gradient quadrupole magnetic focusing field is discussed. For the periodic solenoidal magnetic focusing field, charged-particle dynamics in the adiabatic thermal beam equilibrium are studied numerically and compared with those in the Kapchinskij-Vladimirskij (KV) type beam equilibrium. Numerical evidence is presented, indicating almost complete elimination of chaotic particle motion in the adiabatic thermal beam equilibrium.
USTC/NSRL, Hefei, Anhui, People's Republic of China
Fermilab, Batavia, USA
ANL, Argonne, USA
* V. Lebedev, "Intrabeam Stripping in H− Linacs", LINAC2010
** P. Ostroumov, "TRACK, The Beam Dynamics Code", PAC2005
Fermilab, Batavia, USA
Fermilab, Batavia, USA
NSU, Novosibirsk, Russia
Fermilab, Batavia, USA
Fermilab, Batavia, USA
LBNL, Berkeley, California, USA
LBNL, Berkeley, California, USA
High charge-state heavy-ion beams are of interest and used for a number of accelerator applications. Some accelerators produce the beams downstream of the ion source by stripping bound electrons from the ions as they pass through a foil or gas. In other accelerator systems, ions of charge state >1 are produced directly in the ion source. Heavy-ion inertial fusion (HIF) would benefit from low-emittance, high current ion beams with charge state >1. For these accelerators, the desired dimensionless perveance upon extraction from the emitter is ~0.001, and the electrical current of the beam pulse is ~ 1 A. For accelerator applications where high charge state and very high current are desired, space charge effects might present unique challenges. For example, in a stripper, the separation of charge states might create significant nonlinear space-charge forces which would impact the beam brightness. We will report on the particle-in-cell simulation of the formation of such beams for HIF, and review the possible technical approaches.
LLNL, Livermore, California, USA
LBNL, Berkeley, California, USA
A one-dimensional Vlasov-Poisson model for sheet beams is presented to provide a simple framework for analysis of space-charge effects. Centroid and rms envelope equations including image charge effects are derived and reasonable parameter equivalences with commonly employed 2D transverse models of unbunched beams are established. This sheet beam model is applied to analyze several problems of fundamental interest. First, a sheet beam thermal equilibrium distribution in a continuous focusing channel is constructed and shown to have analogous properties to two- and three-dimensional thermal equilibrium models in terms of the equilibrium structure and Deybe screening properties. Second, the simpler formulation for sheet beams is exploited to explicitly calculate the distribution of particle oscillation frequencies within a thermal equilibrium beam. It is shown that as space-charge intensity increases, the frequency distribution becomes broad which suggesting robust stability properties for beams with strong space-charge.
Northern Illinois University, DeKalb, Illinois, USA
ANL, Argonne, USA
Higher wakefield gradients can be achieved by increasing the total beam charge which is passed through a dielectric-loaded structure and by reducing the transverse size of the beam. Currently, the Argonne AWA photoinjector operates with electron bunches of up to 100 nC and the goal is to raise the total beam charge to about 1000 nC and to improve the beam focusing to a few 100's microns transverse spot size. The increase of the beam charge can be done by superimposing electron bunches that fill up several consecutive RF buckets. Although the energy stored in a single 7-cell linac is by design large the multi-bunch operation with short bunch trains (~10 ns) is still plagued by large energy spread due to significant beam loading effects. In this paper we present a technique intended to reduce the energy spread for a high charge bunch train by properly choosing the time delay between consecutive bunches. The simulations show that the energy spread can be lowered to about 2.8% from about 6.0% for a 10-bunch train of total charge 1000 nC and kinetic energy of about 70 MeV.
UMD, College Park, Maryland, USA
We apply the Venturini-Reiser envelope-dispersion equations* to a continuous beam in a uniform focusing/bending lattice to study the combined effects of linear dispersion and space charge. Within this simple model we investigate the scaling of average dispersion and the effects on beam dimensions; we also introduce a generalization of the space-charge intensity parameter and apply it to the University of Maryland Electron Ring (UMER) and other machines. In addition, we present results of calculations to test the smooth approximation by solving the Venturini-Reiser original equations and also through simulations with the code ELEGANT.
*M. Venturini and M. Reiser, Phys. Rev. Lett. 81, 1, p. 96, 6 July 1998
UMD, College Park, Maryland, USA
The shift in betatron tunes as a function of space charge has been studied in many accelerators and storage rings. Because of its low operating energy (10 keV, γ = 1.02) and wide range of beam currents (0.6 to 100 mA, corresponding respectively to predicted incoherent tune shifts of 1.2 to 5.2), the University of Maryland electron ring (UMER) provides a unique opportunity to study space charge driven tune shifts over a wide parameter space. Comparisons of predictions and measurements are presented, including a discussion of special factors such as the magnetic penetration of the vacuum chamber walls.