J. M. Potter

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.

Radio Frequency Quadrupole Accelerating Structure Research at los Alamos

The results of studies of radio frequency quadrupole accelerating structures are presented. Measurements indicate that this is a promising structure for transporting and accelerating ion beams in the energy region of 50 KeV to 2 MeV.

CW Side-Coupled Linac for the los Alamos/NBS Racetrack Microtron

A 2.7-m side-coupled linac has been built as part of the 5-MeV injector for the cw room-temperature racetrack microtron (RTM) being constructed in collaboration with the National Bureau of Standards (NBS). The linac is designed to accelerate the electron beam from 1 to 5 MeV with an accelerating gradient of 1.5 MeV/m. Fabrication of the structure started October 4, 1982 and was completed February 28, 1983, when it was tested with a cw power level of 82 kW. The structure has an effective shunt impedance (ZT2) of 82.5 M¿/m. No change in field distribution was detected at any power level. The operating frequency is 2380 MHz.

Rapidly Tuned Buncher Structure for the los Alamos Proton Storage Ring (PSR)

In the PSRs short-bunch operating mode, accumulated beam currents are intense and change rapidly. The resonant frequency of the 503.125-MHz buncher used in this mode must be rapidly adjusted through a 100-kHz range to maintain the correct 90° phase relation between cavity voltage and beam current. Modulation rates are up to 3 kHz/¿s. Each structure consists of two side-coupled buncher cavities, resonantly coupled to a ferrite-loaded tuner cavity. The needed frequency change ¿f in the buncher cells is produced by a 50 × ¿f change in the tuner, accomplished by varying a magnetic field applied to the ferrite perpendicular to the rf magnetic field. Fast modulation of this bias is provided by a low-inductance ferritecore magnet excited by a special function generator. The resonantly coupled multicavity structure configuration allows buncher and tuner cells to be independently optimized for their specific functions. This paper describes the buncher design, ferrite selection, and test results from a prototype ferrite-loaded tuner cavity. The tests have demonstrated the tuning schemes feasibility, showing that the necessary 5-MHz range can be attained with only 12% of the tuner cell filled with ferrite, and that losses in the ferrite are small throughout this frequency interval.

Resonantly Coupled Accelerating Structures for High-Current Proton Linacs

The results of model studies and an equivalent circuit analysis of the operation of a chain of cavities in the /g=P//2 mode is given. Various /g=p//2 mode structures are described and the performance of models of these structures is discussed. Results of numerical solutions to an equivalent circuit applicable to the problem are also given, which include frequency error tolerances and coupling tolerances, for typical cases of interest. Multiple drive of a long resonator chain is also discussed.

Progress on the NBS-LANL CW Microtron

The NBS-LANL racetrack microtron (RTM) currently under construction at the National Bureau of Standards is a demonstration accelerator to determine the feasibility of, and to develop the technology necessary for building high-energy, high-current, continuous beam (CW) electron accelerators using beam recirculation through room temperature rf accelerating structures. Parameters of the RTM are: injection energy - 5 MeV; energy gain per pass - 12 MeV; number of passes - 15 or 16; final beam energy - 185-197 MeV; maximum current - 550 ¿A; rf frequency - 2380 MHz. At present, the electron gun and 100 keV beam transport line are operational, and most other major subsystems are in the construction or installation phase. Exceptions are the rf structure (under development), the 5 MeV beam transport line (in engineering design), and the extraction beam line (in conceptual design). Our studies of the original candidate accelerating structure, the disk-and-washer, have led to the discovery of beam steering modes which render this structure unsuitable for the RTM without at least substantial further development beyond the scope of the project. The most promising alternate for meeting the design goal of CW operation at 1.5 MeV/m is the side-coupled structure. A shunt impedance of 80 M¿/m has been measured in a test section of side-coupled structure at 2380 MHz, adequate cooling has been designed, and a 2.7 m long section of this design is under construction. The electron optics of the RTM have been studied in detail.

A Spiral-Resonator Radio-Frequency Quadrupole Accelerator Structure

A radio-frequency quadrupole (RFQ) structure operating at low frequency has been developed for possible use in accelerators for heavy ion fusion or Tokamak plasma heating. The structure uses a series of shunt spiral inductors placed periodically along the electrodes of an electric quadrupole to achieve resonance at the desired frequency. A 1.2-m-long model has been constructed for low-power testing. The model resonates near 12 MHz and has radial dimensions that are reduced by a factor of 15 compared to the commonly used four-vane resonator.

Operating Characteristics of a 2.0-MeV RFQ

A second radio-frequency quadrupole (RFQ) accelerator has been designed, constructed and operated at Los Alamos National Laboratory. The accelerators design parameters represent a major extension from the original Los Alamos RFQ, with the new accelerator being 2.5 times as long, having three times the output energy, and with 2.5 times the current limit. The new accelerators operating characteristics were studied for 3 months before disassembly to incorporate design modifications. Results are discussed.

A Long Distance CAMAC Branch via Data-Link and Microprogrammed Branch Driver

An unusual solution to the problem of remote operation of CAMAC data systems has been implemented at the Clinton P. Anderson Meson Physics Facility (LAMPF). Data-link communications from a remote CAMAC system, controlled only by a Microprogrammed Branch Driver (MBD), to another CAMAC system coupled to a digital computer, can effectively extend the CAMAC branch highway to several hundred meters.

Electrical Behavior of Long Linac Tanks and a New Tank-Coupling Scheme

The electrical behavior of long linac tanks can be represented by the behavior of a finite chain of coupled harmonic oscillators.1 A theory is given of chains of oscillators of two kinds, which is appropriate for semi-resonant or resonant coupling structures.2 The perturbation theory given previously is extended to predict the effects of errors in the coupling cells and in the main cells.2 Some advantages of ?/2 mode operation of the double chain are discussed. A new accelerating scheme is presented in which the coupling structures are resonant and the tanks are resonantly coupled. The theory, numerical calculations, and measurements on laboratory models3 all indicate that tolerances on power amplifier voltage and phase can be relaxed by as much as a factor of five.