R. W. Hamm

Performance Characteristics of a 425-MHz RFQ Linac

A radio-frequency quadrupole (RFQ) focused proton linac has been developed and successfully tested at the Los Alamos Scientific Laboratory (LASL) for the purpose of evaluating its performance and applicability as a low-beta accelerator. The geometry of the structure was designed to accept a 100-keV beam, focus, bunch, and accelerate it to 640 keV in 1.1 m with a high-capture efficiency and minimum emittance growth. The accelerator test facility includes an injector, low-energy transport section for transverse matching, and a high-energy transport section for analysis of the beam properties. The accelerator cavity is exited through a manifold powered by a 425-MHz klystron. Diagnostic instrumentation was prepared to facilitate operation of the accelerator and to analyze its performance. Measurements of the beam properties are presented and compared with the expected properties resulting from numerical calculations of the beam dynamics.

Atomic Physics with Relativistic Beams

The availability of a beam of H- ions with near-luminal velocity has enabled us to investigate photon-atom interactions at photon energies and electric field strengths presently beyond the scope of ordinary laboratory techniques. See Fig. 1. We have surveyed the entire photodetachment spectrum of H- from threshold for single electron detachment to well above the threshold for two-electron detachment. Embedded in the one-electron photodetachment continuum of H- are a series of doubly excited states, termed “resonances” since they are unstable against autodetachment into the continuum. The two most striking of these lie near the threshold for the production of H° (n = 2): a very narrow state just below the threshold, the “Feshbach” resonance, and a broader state lying just above the n = 2 threshold, the “shape” resonance. By means of Stark mixing, we have been able to use these 1P states to uncover nearby 1S and 1D states. Just below the threshold for H° (n = 3) production, we have found a prominent dip in the photodetachment cross section. This “Feshbach”-type state is accompanied by a narrower recursion lying between it and the n = 3 threshold. The single electron Open image in new window Fig. 40. We are studying a System moving with 84% the speed of light. Time is dilated and lengths are contracted by 1.853. photodetachment total cross section above n = 3 shows no further distinctive features upon examination with our current resolution, although there is some evidence for structure below n = 4.

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 ~

A High-Current Four-Beam Xenon Ion Source for Heavy-Ion Fusion

10 million.

A Compact 250-kV Injector System for PIGMI

The growing interest in inertial confinement fusion using heavy ions has elicited from the Los Alamos Scientific Laboratory a proposal to use a multi-channel radiofrequency quadrupole (RFQ) structure for the initial stage of the heavy-ion accelerator. The RFQ would have 4 channels in each module and each channel would accelerate 25 mA of Xe+1. Based on experiments with xenon beam production with a high current duoPIGatron source at Chalk River Nuclear Laboratories, a 245 keV 4-beam xenon injector has been designed for this 4-channel RFQ. The injector is of modular design with 4 small independent plasma sources mounted in a 10 cm square array on a common combined extraction and acceleration column. The electrodes have 4 separate sets of apertures and each channel produces a 29 mA beam for injection into its corresponding RFQ channel. This paper presents a conceptual design for the injector, code calculations for the column electrode design and results of a preliminary test carried out to verify the feasibility of the concept.

Photodetachment of Relativistic Ions

A 250-kV proton injector to be used in the development of a linac suitable for medical applications has been constructed. This injector utilizes a spherical Pierce geometry to produce a converging beam. A gas insulated accelerating column is cantilevered on a grounded vacuum system, with a separate high voltage equipment dome connected to a 300-kV Cockcroft-Walton power supply. The injector can be operated locally or remotely, with the remote control accomplished by a microprocessor system linked to a central control minicomputer. This injector has been designed as a low-cost compact system. The design details and the data obtained during initial operation are presented.

Time-Resolved Beam Energy Measurements at LAMPF

A series of fundamental laser ion beam experiments has been made feasible by the high-quality, relativistic (ß = 0.842) H- ion beam available at the Clinton P. Anderson Meson Physics Facility (LAMPF). The relativistic Doppler shift of the light from an ordinary ultraviolet laser provides what is, in effect, a continuously tunable vacuum-ultraviolet laser in the rest frame of the moving ions. The Lorentz transformation of a modest laboratory magnetic field provides an electric field of several megavolts/centimeter. The latest results of our photo-detachment work with H- beams and our spectroscopic work with H0 beams are presented. Our plans for future work are discussed.

Observation of two-electron photoionization of the H/sup -/ ion near threshold

A narrow atomic photodetachment resonance is used to measure the LAMPF beam energy. Energy and time resolution are adequate to permit the use of this method in studying transient changes in accelerated beam energy.