When completed, the DARHT-II linear induction accelerator (LIA) will produce a 2 kA, 18 MeV electron beam with more than 1500 ns current/energy “flat-top.” In initial tests DARHT-II has already accelerated beams with current pulse lengths from 500 ns to 1200 ns full-width at half maximum (FWHM) with more than 1.2 kA peak current and 12.5 MeV peak energy. Experiments will soon begin with a ∼ 1600 ns flat-top pulse, but with reduced current and energy. These pulse lengths are all significantly longer than any other multi-MeV LIA, and they define a novel regime for high-current beam dynamics, especially with regard to beam stability. Although the initial tests demonstrated the robustness of the DARHT-II LIA to BBU, the < 1200 ns FWHM pulse lengths were too short to test the predicted protection against ion-hose instability. The present experiments are designed to resolve these and other beam-dynamics issues with a ∼ 1600 ns pulse length beam.

The DARHT-II accelerator produces an 18-MeV, 2-kA, 2-μs electron beam pulse. After the accelerator, the pulse is delivered to the final focus on an x-ray producing target via a beam transport section called the Downstream Transport. Ions produced due to beam ionization of residual gases in the Downstream Transport can affect the beam dynamics. Ions generated by the head of the pulse will cause modification of space-charge forces at the tail of the pulse so that the beam head and tail will have different beam envelopes. They may also induce ion-hose instability at the tail of the pulse. If these effects are significant, the focusing requirements of beam head and tail at the final focus will become very different. The focusing of the complete beam pulse will be time dependent and difficult to achieve, leading to less efficient x-ray production. In this paper, we will describe the results of our calculations of these ion effects at different residual-gas pressure levels. Our goal is to determine the maximum residual-gas pressure allowable in DARHT-II Downstream Transport such that the required final beam focus is achievable over the entire beam pulse under these deleterious ion effects.

The DARHT-II linear-induction accelerator has been successfully operated at 1.2-1.3 kA and 12.5-12.7 MeV to demonstrate the production and acceleration of an electron beam. Beam pulse lengths for these experiments were varied from 0.5 mu s to 1.2 mu s full-width half-maximum. A low-frequency inductance-capacitance (LC) oscillation of diode voltage and current resulted in an oscillation of the beam position through interaction with an accidental (static) magnetic dipole in the diode region. There was no growth in the amplitude of this oscillation after propagating more than 44 m through the accelerator, and there was no loss of beam current that could be measured. The results of these initial experiments are presented in this paper.

Progress was made during the past decade towards a better understanding of halo formation caused by beam mismatch in high-intensity beams. To test these ideas an experiment was carried out at Los Alamos with proton beams in a 52-quadrupole focusing channel. Rms emittances and beam widths were obtained from measured beam profiles for comparison with the maximum emittance-growth predictions of a free-energy model and the maximum halo-amplitude predictions of a particle-core model. The experimental results are also compared with multiparticle simulations. In this paper we will present the experimental results and discuss the implications with respect to the validity of both the models and the simulations.

The second axis of the Dual Axis Radiographic Hydro-Test (DARRT) facility will provide up to four short (< 150 ns) radiation pulses for flash radiography of high-explosive driven implosion experiments[1]. To accomplish this the DARHT-II linear induction accelerator (LIA) will produce a 2-kA electron beam with 18-MeV kinetic energy, constant to within +/-0.5% for 2mus. A fast kicker will cleave four short pulses out of the 2mus flattop, with the bulk of the beam diverted into a dump. The short pulses will then be transported to the final-focus magnet, and focused onto a tantalum target for conversion to bremsstrahlung pulses for radiography. DARHT-II is a collaborative effort between the Los Alamos, Lawrence Livermore, and Lawrence Berkeley National Laboratories of the University of California.

DARHT-II, the second axis of the Dual-Axis Radiographic Hydrodynamics Test Facility, is being commissioned. DARHT-II is a linear induction accelerator producing 2-microsecond electron beam pulses at 20 MeV and 2 kA. These 2-microsecond pulses will be chopped into four short pulses to produce time resolved X-ray images. Radiographic application requires the DARHT-II beam to have excellent beam quality, and it is important to study various beam effects that may cause quality degradation of a DARHT-II beam. One of the beam dynamic effects under study is "corkscrew" motion. For corkscrew motion, the beam centroid is deflected off axis due to misalignments of the solenoid magnets. The deflection depends on the beam energy variation, which is expected to vary by plusmn0.5% during the "flat-top" part of a beam pulse. Such chromatic aberration will result in broadening of beam spot size. In this paper, we will report simulation results of our study of corkscrew motion in DARHT-II. Sensitivities of beam spot size to various accelerator parameters and the strategy for minimizing corkscrew motion will be described. Measured magnet misalignment is used in the simulation

We present results from an experimental study of the beam halo in a high-current 6.7-MeV proton beam propagating through a 52-quadrupole periodic-focusing channel. The gradients of the first four quadrupoles were independently adjusted to match or mismatch the injected beam. Emittances and beamwidths were obtained from measured profiles for comparisons with maximum emittance-growth predictions of a free-energy model and maximum halo-amplitude predictions of a particle-core model. The experimental results support both models and the present theoretical picture of halo formation.

In 1997, Los Alamos National Laboratory (LANL) initiated a development program for high-current, superconducting, proton-linac technology to build prototype components of this linac and demonstrate the feasibility. Seven hundred-MHz niobium cavities with elliptical shapes are being built, as well as power couplers to transfer high RF power to these cavities. The cavities and power couplers will be integrated in cryostats as linac cryomodules. This paper describes the linac design and the status of the development program.

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

High-current proton linacs are promising sources of neutrons for material processing and research applications. Recently, a linac design that makes use of a combination of normal-conducting (NC) and superconducting (SC) linac technologies has been proposed for the US Accelerator Production of Tritium (APT) Project. As a result, a multi-year engineering development and demonstration (ED&D) program is underway. In this paper, we will describe the design and merits of the NC/SC hybrid approach. The scope, technology issues, and present status of the ED&D Program, and the participation of industry will also be described.

The Accelerator Production of Tritium (APT) Project is investigating using a superconducting linac for the high-energy portion of the accelerator. As this accelerator would be used to accelerate a high-current (100-mA) CW proton beam up to 1700 MeV, it is important to determine the effects of stray-beam impingement on the superconducting properties of 700-MHz niobium cavities. To accomplish this, two 3000-MHz elliptical niobium cavities were placed in a cryostat, cooled to nominally 2 K in sub-atmospheric liquid helium, and irradiated with 798-MeV protons at up to 490-nA average current. The elliptically shaped beam passed through the equatorial regions of both cavities in order to maximize sensitivity to any changes in the superconducting surface resistance. Over the course of the experiment, 6x10(16) protons were passed through the cavities. After irradiation, the cavities were warmed to 250 K, then recooled to investigate the effects of a room-temperature annealing cycle on the superconducting properties of the irradiated cavities. A detailed description of the experiment and the results shall be presented. These results are important to employing superconducting RF technology to future high-intensity proton accelerators for use in research and transmutation technologies.

As part of the linac design for the accelerator production of tritium (APT) project, we are assembling the first approximately 20 MeV portion of this cw proton accelerator. Primary objective of this low-energy demonstration accelerator (LEDA) is to verify the design codes, gain fabrication knowledge, understand LEDA's beam operation, and be able to better predict costs and operational availability for the full 1700 MeV APT accelerator. This paper provides an updated report on this past year's progress that includes beam tests of the 75 keV injector, fabrication of the 6.7 MeV radio-frequency quadrupole (RFQ), preparation of the facility, procurement and assembly of the rf system, and detailed design and documentation of many pieces of support equipment. First tests with the 6.7 MeV, 100 mA, cw beam from the RFQ are scheduled for late 1998. References are given to many detailed papers on LEDA at this conference.

TRACE 3-D is an interactive beam-transport code for bunched beams that includes accelerating elements and linear space-charge forces. It has been integrated with an improved GUI (graphic user interface) based on the Shell for Particle Accelerator Related Codes. Recent modifications to the code include centroid tracking and an improved beam description consisting of a set of beam slices, each having its own 6D centroid and sigma matrix. This allows us to study some nonlinear effects, such as wakefields, that are related to the variation of the beam bunch along the longitudinal direction.

During a Workshop on Gamma-Gamma Colliders in Lawrence Berkeley Laboratory, it was pointed out that an 1-/spl mu/m laser that can produce 1-J, 1-ps pulses at a few hundred hertz is required. With high-power scalability and ease of formatting, an FEL can be a promising candidate for such a laser. We propose an FEL scheme based on chirped-pulsed regenerative amplification to achieve this high peak-power laser. The l-ps pulse of a solid-state laser will be stretched, amplified, and recompressed to achieve the high peak power. The system is relatively simple and consists of mostly components that have already been demonstrated. This paper will describe the proposal and the important issues of such a scheme.

TRACE 3-D is an interactive code that calculates and displays the envelopes of a bunched beam through a user-defined transport system. Accelerating elements and linear space-charge forces are included. The beam is described by a 6-D sigma matrix of second moments. We have extended the capabilities of this code to include effects, such as wakefields, related to the variation of the beam bunch in the longitudinal direction. This nonlinear capability was implemented by adding centroid tracking and describing the beam by a collection of slices, each described by a 6-D centroid and sigma matrix. External forces, space-charge forces, and wakefields act on the collection of beam slices. Results are presented in terms of an overall sigma matrix, computed by combining the slice distributions. The new TRACE 3-D has been integrated with an improved graphic user interface (GUI) based on the Shell for Particle Accelerator Related Codes. This new approach to modeling wakefields demonstrates the flexibility of extending the capabilities of moment codes to handle important physical effects, and the rapid incorporation of the new capabilities into the graphic interface illustrates the ease of customizing the new GUl. The wakefield model and features of the new interface are presented.

A SCRF (superconducting RF linac) has been developed for a high-intensity proton linac which will be used as the driver for neutron sources. This design is conservative, using current SCRF technologies. As well as lowering operating cost, the design offers performance advantages in availability, beam loss, and upgradability, which are important for the application as a neutron source.

Improvement of beam availability is a prime objective of the present LANSCE (Los Alamos Neutron Scattering Center) Upgrade. A RAMI (reliability, availability, maintainability, and inspectability) program is being developed to identify the most cost-effective improvements to achieve the availability goal. The beam-delivery system is divided into subsystems appropriate for the modeling of availability. The availability of each subsystem is determined from operation data and assessment of individual component designs. These availability data are incorporated in an availability model to predict the benefit of improvement projects to achieve cost benefit prioritization. Examination of the data also identifies a comprehensive list of factors affecting availability. A good understanding of these factors using root-cause analysis is essential for availability improvement. In this paper, we will describe the RAMI program and the development of the availability model

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.

The proton linac for APT (Accelerator Production of Tritium) will produce a nominal CW beam power of 130 MW at 1300 MeV. Two designs are currently under consideration. Reference design is composed entirely of normal-conducting (NC) copper accelerating structures, while an advanced-technology design employs superconducting Nb cavities above 217 MeV. Front-end accelerator for both concepts is a 100-mA NC linac. In this paper, the two APT linac designs are described and compared in terms of key factors, including power efficiency, beam loss control, machine availability and flexibility, and construction and operating costs.

Funnels are a key component of accelerator structures proposed for transmutation technologies. In addition to conventional accelerator elements, specialized rf‐cavities are needed for these structures. Simulations were done to obtain their electromagnetic field distribution and to minimize the rf‐induced heat loads. Using these results a structural and thermal analysis of these cavities was performed to insure their reliability at high average power and to determine their cooling requirements. For one cavity the thermal expansion data in return was used to estimate the thermal detuning.