Robert A. Jameson

Beam-Intensity Limitations in Linear Accelerators

Recent demand for high-intensity beams of various particles has renewed interest in the investigation of beam current and beam quality limits in linear RF and induction accelerators and beam-transport channels. Previous theoretical work is reviewed, and new work on beam matching and stability is outlined. There is a real need for extending the theory to handle the time evolution of beam emittance; some present work toward this goal is described. The role of physical constraints in channel intensity limitation is emphasized. Work on optimizing channel performance, particularly at low particle velocities, has resulted in major technological advances. The opportunities for combining such channels into arrays are discussed.

Design for low beam loss in accelerators for intense neutron source applications

Control of beam loss in intense ion linacs involves keeping beam spill below parts in 10/sup -5/-10/sup -8m by preventing total beam size from extending to the limiting apertures. Starting from good RMS design practices, new analysis of the machine architecture is described in terms of space-charge effects on the machine tune, free-energy constraint, and halo-producing mechanisms. It is shown that halos are produced by the time- (or position-) varying nature of common linac aspects (such as misalignment, mismatching, acceleration, and construction techniques) through collective core/single-particle interaction dynamics plus resonances.<<ETX>>

The Radio-Frequency Quadrupole: General Properties and Specific Applications

The radio-frequency quadrupole (RFQ) linac structure is being developed for the acceleration of low-velocity ions. Recent experimental tests have confirmed its expected performance and have led to an increased interest in a wide range of possible applications. We review the general properties of RFQ accelerators and present beam dynamics simulation results for their use in a variety of accelerating systems. These include the low-beta sections of the Fusion Materials Irradiation Test Accelerator, a 200-MHz proton linear accelerator, and a xenon accelerator for heavy ion fusion.

High-Intensity Deuteron Linear Accelerator (FMIT)

For fusion reactors to become operational, one of the many problems to be solved is to find materials able to withstand the intense bombardment of 14-MeV neutrons released by the fusion process. The development of alloys less likely to become damaged by this neutron bombardment will require years of work, making it desirable to begin studies in parallel with other aspects of fusion power generators. The Fusion Materials Irradiation Test (FMIT) Facility, to be built at the Hanford Engineering Development Laboratory (HEDL), Richland, Washington, will provide a high neutron flux and a neutron energy spectrum representative of fusion reactor conditions in volumes adequate to screen and qualify samples of candidate fusion reactor materials. FMITs design goal is to provide an irradiation test volume of 10 cm3 at a neutron flux of 1015 n/cm2-s, and 500 cm3 at a flux of 1014 n/cm2-s. This will not allow testing of actual components, but samples in the most intense flux region can be subjected to accelerated life testing, accumulating in one year the total number of neutrons seen by a fusion reactor in 10-20 years of operation. To produce the neutrons, a 100-mA, 35-MeV deuteron beam will be directed onto a 2-cm-thick, 600-gpm curtain of liquid lithium metal, which strips the deuterons and allows the remaining neutrons to continue on to the test samples. The deuterons will be produced by the largest component of the facility, a high-intensity, continuously operating linear accelerator (Linac).

Accelerators address nuclear waste problems

If the world is to continue using nuclear-generated electricity, the problem of radioactive waste disposal must be addressed. Permanent storage of long-lived waste from nuclear power stations is an issue that generates public concern, scientific uncertainty and political pressures. One alternative is to investigate ways of reducing the intensity and lifetime of the radioactivity of the waste so that it can be buried safely almost anywhere.

High-Brightness RF Linear Accelerators

Soon after electrons and ions were discovered, production of practical generators of particle beams began, and a succession of machines were invented that could produce more energetic and more intense beams. Progress on the energy frontier is often charted from the 1930s in the form of the Livingston Chart, Fig. 1, showing that particle accelerator energy has increased by a factor of about 25 every 10 years. The corresponding cost per million electron volts has decreased by about a factor of 16 per decade (Lawson, 1982). The physics principles on which all of these devices work were deduced long ago; the energy increase were possible because of cost reductions from thorough exploitation of parameters, engineering perfection, systems integration, and advanced manufacturing methods (Voss, 1982).

Feedforward Control of Accelerator RF Fields

A new and very effective technique for accelerator field control is demonstrated, using a feedforward control loop to complement a standard feedback controller. The accelerated beam current envelope, which acts as a load disturbance on the accelerator field amplitude and phase, is detected upstream from the module to be controlled. Due to differences in propagation velocity in the accelerator and external cables, true anticipating control is possible by feeding the current signal forward to the controller. In tests with full beam loading (22%) in the first 201.25 MHz tank at LAMPF, peak amplitude error was reduced to 0.4% and settling time to 20 ?sec at beam turn-on.

Recent Improvements in Beam Diagnostic Instrumentation

In high-current machines, such as LAMPF and the envisioned Hanford Engineering Development Laboratory (HEDL) Fusion Materials Irradiation Test (FMIT) Facility linac, hands-on maintenance is desired. Beam spill must be kept extremely low; therefore, attention must be given to beam fringes (tails). Beam matching to the structure becomes increasingly important. We describe equipment capable of accurately measuring transverse beam profiles over a range spanning more than four orders of magnitude and longitudinal phase profiles over ranges spanning more than three orders of magnitude. Errors in 100-MeV transverse emittance measurements are explored and experimental emittance measurements made with three different methods are compared. Advantages of one nondestructive method are developed.

Four-Dimensional Beam Tomography

A computer code has been developed to reconstruct the 4-D transverse phase-space distribution of an accelerator beam from a set of linear profiles measured at different angles at three or more stations along the beam line. The code was applied to wirescan data obtained on the low-intensity H- beam of the LAMPF accelerator. A 4-D reconstruction was obtained from 10 wire-scan profiles; 2-D projections of the reconstruction agree fairly well with slit-and-collector measurements of the horizontal and vertical emittance distributions.

The Application of a Digital Computer to the Control and Monitoring of a Proton Linear Accelerator

Preliminary design studies have been carried out on the control system for an 800-Mev proton linear accelerator based on the use of an on-line digital computer as the main controlling element. The accelerator system is divided into fifty-two subsections or modules. A module has complete control of all the equipment associated with it. Selected data and monitoring signals are transmitted on a time-shared basis to a central control computer. The functions which the computer performs in the control of the accelerator may be classified into Operational Control, Operational Supervision, and Data Monitoring and Handling. The operator gives instructions to the computer through interrupt lines. Commands to the modules and displays for the operator time-share a common channel in the computer output system. Synchronization of the input and output data flow is controlled by the computer through a timing unit. A comparison has been made of the computer-based control system with a possible alternate system which does not use a computer. The computer replaces much of the control and data handling equipment required by the alternate system. As a result, the costs of the two systems are comparable. However, the computer-based system is preferable since it has many functional advantages over the alternate system.