R. W. Garnett

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.

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.

A modified space charge routine for high intensity bunched beams

Abstract In 1991 a space charge calculation for bunched beam with a three-dimensional ellipsoid was proposed, replacing the usual SCHEFF routines. It removes the cylindrical symmetry required in SCHEFF and avoids the point to point interaction computation, whose number of simulation points is limited. This routine has now been improved with the introduction of two or three ellipsoids giving a good representation of the complex non-symmetrical form of the bunch (unlike the 3-d ellipsoidal assumption). The ellipsoidal density distributions are computed with a new method, avoiding the difficulty encountered near the centre (the axis in 2-d problems) by the previous method. It also provides a check of the ellipsoidal symmetry for each part of the distribution. Finally, the Fourier analysis reported in 1991 has been replaced by a very convenient Hermite expansion, which gives a simple but accurate representation of practical distributions. Comparisons with other space charge routines have been made, particularly with the ones applying other techniques such as SCHEFF. Introduced in the versatile beam dynamics code DYNAC, it should provide a good tool for the study of the various parameters responsible for the halo formation in high intensity linacs. Improvements of the method are under development by the authors. These improvements, which might lead to a new step in space charge computations, are however beyond the scope of this article.

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.

Conceptual design of a low-/spl beta/ SC proton linac

In this paper we discuss the conceptual design of a low-/spl beta/ superconducting proton linac based on multi-gap spoke resonator structures. We have demonstrated the feasibility of using superconducting accelerating structures throughout a proton linac for high-peak current applications. The injection energy for this linac is assumed to be 6.7 MeV, which equals the output energy of the CW RFQ built for the Low-Energy Demonstration Accelerator now operating at Los Alamos. The beam is accelerated to 109 MeV using multi-gap spoke resonators. Both 2-gap and 3-gap cavities are used in three accelerating sections with geometric-/spl beta/ values, of 0.175, 0.2, and 0.34. Higher beam energies can be achieved by transitioning to elliptical superconducting cavities to further accelerate the beam. Preliminary beam dynamics simulation results are shown and discussed.

Linear accelerator for tritium production

For many years now, Los Alamos National Laboratory has been working to develop a conceptual design of a facility for accelerator production of tritium (APT). The APT accelerator will produce high energy protons which will bombard a heavy metal target, resulting in the production of large numbers of spallation neutrons. These neutrons will be captured by a low‐Z target to produce tritium. This paper describes the latest design of a room‐temperature, 1.0 GeV, 100 mA, cw proton accelerator for tritium production. The potential advantages of using superconducting cavities in the high‐energy section of the linac are also discussed and a comparison is made with the baseline room‐temperature accelerator.

High-resolution parallel particle-in-cell simulations of beam dynamics in the spallation neutron source linac

Abstract In this paper, we present results of using high-performance parallel computers to simulate beam dynamics in an early design of the Spallation Neutron Source (SNS) linac. These are among the most detailed linac simulations ever performed. The simulations have been performed using up to 500 million macroparticles, which is close to the number of particles in the physical system. The simulations are fully three-dimensional, and utilize RF cavity field data from modeling over 400 RF cavities. Furthermore, they use an improved model of the beam dynamics within the cavities. Traditionally, small-scale two-dimensional simulations have been performed on PCs or workstations. While such simulations are sufficient for rapid design and for predicting root mean square properties of the beam, large-scale simulations are essential for modeling the tails of the beam. The large-scale parallel simulation results presented here represent a three order of magnitude improvement in simulation capability, in terms of problem size and speed of execution, compared with typical two-dimensional serial simulations. In this paper we will show how large-scale simulations can be used to predict the extent of the beam halo and facilitate design decisions related to the choice of beam pipe aperture.

Space-charge calculation for bunched beams with 3-D ellipsoidal symmetry

A method for calculating 3-D space-charge forces are developed that is suitable for bunched beams of either ions or relativistic electrons. The method is based on the analytical relations between charge-density and electric fields for a distribution with 3-D ellipsoidal symmetry in real space. At each step a Fourier-series representation is used for the smooth particle-density function obtained from the distribution of the macroparticles being tracked through the elements of the system. The resulting smooth electric fields reduce the problem of noise from artificial collisions, associated with small numbers of interacting macroparticles. Example calculations are shown for comparison with other methods.<<ETX>>

Commissioning plan for a high-current proton linac

High-power proton linacs (E>500 MeV) are potentially useful for transmutation applications, such as the production of tritium, In production applications, high availability is essential. Achieving high availability requires an accelerator design that simplifies maintenance and accommodates commissioning procedures designed to minimize tune-up time. These are worthwhile goals for any accelerator, but the high beam powers (170 MW) and heavy beam loading of the Accelerator Production of Tritium (APT) linac introduce significant new challenges. This paper describes the commissioning plan, as developed to date.

Simulation studies of the LAMPF proton linac

The work presented here is an extension of our previous work. We have attempted to do a more complete simulation by including modeling of the low-energy beam transport (LEBT). No measurements of the longitudinal structure of the beam, except phase-scans, are performed at LAMPF. Transverse measurements include slit and collector emittance measurements, and wire scans to determine beam size and centroids. Comparison of simulations to beam loss data suggest that the primary causes of beam spill are incomplete longitudinal capture and the lack of longitudinal matching. Measurements to support these claims are not presently made at LAMPF. However, agreement between measurement and simulation for the transverse beam properties and transmissions serve to benchmark the simulations.