D. A. Swenson

RF Quadrupole Beam Dynamics

A method has been developed to analyze the beam dynamics of the radio frequency quadrupole accelerating structure. Calculations show that this structure can accept a dc beam at low velocity, bunch it with high capture efficiency, and accelerate it to a velocity suitable for injection into a drift tube linac.

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.

A Single-Cavity Double-Frequency Buncher

A single-cavity buncher has been developed that resonates at both the fundamental and twice the fundamental frequency to form a more nearly ideal bunching voltage waveform in the gap. The cavity utilizes the TM020-like mode as the first harmonic of the fundamental TM010-like mode. Field distributions on or near the axis, which are seen by the beam, are essentially identical for the two modes. Many beam bunching applications require two bunchers with the harmonic buncher being physically as close as possible to the fundamental frequency buncher - the buncher described here accomplishes this property with a single cavity and excitation of two modes. Calculated parameters for cavity designs with a fundamental frequency of 0.45 GHz are presented for different cavity lengths which represent a range of interest for accelerators and rf tubes. Means of tuning and fabrication are described. A geometry chosen for PIGMI is described in more detail.

Low-Energy Linac Structure for Pigmi

The higher radio frequency (450 MHz) and lower injection energy (250 keV) of the PIGMI (Pion Generator for Medical Irradiations) linac design seriously compound the problem of beam containment in the first few meters of the structure. The conventional quadrupole-focused, drift-tube linac represents the best solution for beam energies above 8 MeV, but because of the small space available for quadrupoles in the PIGMI designs, cannot provide the required focusing at lower energies. A satisfactory solution to this focusing problem has been found based on pure alternating phase focusing for the first few MeV, followed by a smooth transition to a pure permanent magnet quadrupole-focused structure at 8 MeV. The structure and its calculated performance are described.

An Optimized Design for PIGMI

PIGMI (Pion Generator for Medical Irradiations) is a compact linear proton accelerator design, optimized for pion production and cancer treatment use in a hospital environment. Technology developed during a four-year PIGMI Prototype experimental program allows the design of smaller, less expensive, and more reliable proton linacs. A new type of low-energy accelerating structure, the radio-frequency quadrupole (RFQ) has been tested; it produces an exceptionally good-quality beam and allows the use of a simple 30-kV injector. Average axial electric-field gradients of over 9 MV/m have been demonstrated in a drift-tube linac (DTL) structure. Experimental work is underway to test the disk-and-washer (DAW) structure, another new type of accelerating structure for use in the high-energy coupled-cavity linac (CCL). Sufficient experimental and developmental progress has been made to closely define an actual PIGMI. It will consist of a 30-kV injector, an RFQ linac to a proton energy of 2.5 MeV, a DTL linac to 125 MeV, and a CCL linac to the final energy of 650 MeV. The total length of the accelerator is 133 meters. The RFQ and DTL will be driven by a single 440-MHz klystron; the CCL will be driven by six 1320-MHz klystrons. The peak beam current is 28 mA. The beam pulse length is 60 ps ¿s at a 60-Hz repetition rate, resulting in a 100-¿A average beam current. The total cost of the accelerator is estimated to be ~

The NBS-LASL CW Microtron

10 million.

Choice of Geometry for the Alvarez Section of the FMIT Deuteron Linac

The NBS-LASL racetrack microtron (RTM) is a joint research project of the National Bureau of Standards and the Los Alamos Scientific Laboratory. The project goals are to determine the feasibility of, and develop the necessary technology for building high-energy, high-current, continuous-beam (cw) electron accelerators using beam recirculation and room-temperature rf accelerating structures. To achieve these goals, a demonstration accelerator will be designed, constructed, and tested. Parameters of the demonstration RTM are: injection energy - 5 MeV; energy gain per pass - 12 MeV; number of passes - 15; final beam energy - 185 MeV; maximum current 550 ¿A. One 450 kW cw klystron operating at 2380 MHz will supply rf power to both the injector linac and the main accelerating section of the RTM. The disk and washer standing wave rf structure being developed at LASL will be used. SUPERFISH calculations indicate that an effective shunt impedance (ZT2) of about 100 M¿/m can be obtained. Thus, rf power dissipation of 25 kW/m results in an energy gain of more than 1.5 MeV/m. Accelerators of this type should be attractive for many applications. At beam energies above about 50 MeV, an RTM should be considerably cheaper to build and operate than a conventional pulsed rf linac of the same maximum energy and time-average beam power. In addition, the RTM provides superior beam quality and a continuous beam which is essential for nuclear physics experiments requiring time-coincidence measurements between emitted particles.

High Energy Accelerating Structures for High Gradient Proton Linac Applications

The Hanford Fusion Materials Irradiation Test (FMIT) 35-MeV linac is intended to produce neutrons for materials studies. Its operation will be virtually continuous for at least 20 years. Such operation implies that the accelerator design must be conservative to avoid excessive downtime and that the accelerator be economical of power. The initial design of the linac drift-tube section was preceded by more than 800 SUPERFISH computer runs. SUPERFISH allows the selection of a geometry for the FMIT machine that has very nearly the maximum value of ZT2 consistent with Kilpatricks criterion. This maximum value insures that power costs will be as low as possible while keeping the maximum surface fields at a level conservative enough to prevent sparking. The optimization procedure is examined to show how the best geometry is achieved. The drift-tube section will operate at 80 MHz with an average 1.4 MV/m gradient. As presently conceived, it will consist of two tanks, one 248 cm in diameter and about 18.-m long and the other 240 cm in diameter and about 14.-m long. The number of drift-tubes will be 73 if a stable phase of -30° is chosen.

Proton Injector for CW‐Mode Linear Accelerators

The high-energy part of a proton linac, following a drift tube section, accelerates protons and H- ions of energies above 150 MeV. High efficiency and high gradients in the accelerating structure considered for this part of a proton linac are the objectives of this study. Several known and improved structures working at 1350 MHz were optimized for maximum shunt impedance. The study was performed with the extensive use of a computer code -- SUPERFISH. The theoretical results of this study are presented.

Beam-Cavity Interaction Measurements in a DAW Structure

Numerous applications exist for CW linear accelerators with final energies in the 0.5 to 4.0 MeV proton energy range. Typical proton current at the linac output energy is 20 mA. An important subsystem for the accelerator facility is a reliable dc mode proton injector. We present here design and laboratory results for a dc, 25-keV, 30-mA proton injector. The proton source is a 2.45-GHz microwave hydrogen ion source which operates with an 875-G axial magnetic field. Low emittance, high proton fraction (>85%), beams have been demonstrated from this source. The injector uses a novel dual-solenoid magnet for matching the injector beam into a radio frequency quadrupole (RFQ) linear accelerator. Recently, a dc ion-source development program has given up to 30 mA beam current. The dual solenoid is a compact and simple design utilizing tape-wound, edge-cooled coils. The low-energy beam transport design as well as 25-keV beam matching calculations to an RFQ will also be presented.