Roger P. Shurter

Ion Source Development for the los Alamos Heavy Ion Fusion Injector

A multi-beam injector is being designed and built at Los Alamos for the U.S. Heavy Ion Fusion Program [1]. As part of this program, development of an aluminum-spark, pulsed plasma source is being carried out. Faraday cup diagnostics are used to study current emission and to map the current profile. An aluminum oxide scintillator with photographic film is used in conjunction with a pepper-pot to obtain time integrated emittance values.

Performance improvements with advanced design foils in high-current electron beam diodes

A design for the vacuum/pressure barrier of an electron-beam diode ready to be fielded on a large krypton-fluoride excimer laser is described. The barrier is a composite foil, fabricated from carbon fibers, Kapton-membrane, epoxy, and copper foil. This composite foil has advantages over more traditional metal foils, exhibiting particularly high tensile strength and a high modulus of elasticity. Other important properties of these composites for use in KrF excimer laser applications include: high electron transmission with low loss to scattering, chemical compatibility with fluorine, low porosity, and low reflectivity in the ultraviolet. The mechanical properties of the composite foil allow the design of support structures (hibachis) which incorporate larger openings than are possible with metal foils with similar electron transmission characteristics. >

Design Status of Heavy Ion Injector Program

Design and development of a sixteen beam, heavy ion injector is in progress at Los Alamos National Laboratory (LANL) to demonstrate the injector technology for the High Temperature Experiment (HTE) proposed by Lawrence Berkeley Laboratory. The injector design provides for individual ion sources mounted to a support plate defining the sixteen beam array. The beamlets are electrostatically accelerated through a series of electrodes inside an evacuated (10-7 torr) high voltage (HV) accelerating column. The column consists of two 28-inch diameter insulator modules made of 85 percent A1203 ceramic rings brazed to niobium feedthrough rings to which the electrodes are mechanically attached. Field shaping is used to minimize electron avalanche induced flashover along the inside surface of the ceramic rings. The column is self-supporting and is cantilevered from one end of the containment vessel. A brazed assembly was chosen to provide the required bond strength and high vacuum capability. The HV pulsed power supply is a 2MV Marx generator cantilevered from the opposite end of the containment vessel. The stainless steel pressure vessel (PV) contains a 65 psig mixture of SF6(30%) and nitrogen (70%) to provide the electrical insulation.

A wide-acceptance Compton spectrometer for spectral characterization of a medical x-ray source

Accurate knowledge of the x-ray spectra used in medical treatment and radiography is important for dose calculations and material decomposition analysis. Indirect measurements via transmission through materials are possible. However, such spectra are challenging to measure directly due to the high photon fluxes. One method of direct measurement is via a Compton spectrometer (CS) method. In this approach, the x-rays are converted to a much lower flux of electrons via Compton scattering on a converter foil (typically beryllium or aluminum). The electrons are then momentum selected by bending in a magnetic field. With tight angular acceptance of electrons into the magnet of ~ 1 deg, there is a linear correlation between incident photon energy and electron position recorded on an image plate. Here we present measurements of Bremsstrahlung spectrum from a medical therapy machine, a Scanditronix M22 Microtron. Spectra with energy endpoints from 6 to 20 MeV are directly measured, using a CS with a wide energy range from 0.5 to 20 MeV. We discuss the sensitivity of the device and the effects of converter material and collimation on the accuracy of the reconstructed spectra. Approaches toward improving the sensitivity, including the use of coded apertures, and potential future applications to characterization of spectra are also discussed.

Measuring x-ray spectra of flash radiographic sources

A Compton spectrometer has been re-commissioned for measurements of flash radiographic sources. The determination of the energy spectrum of these sources is difficult due to the high count rates and short nature of the pulses (~50 ns). The spectrometer is a 300 kg neodymium-iron magnet which measures spectra in the <1 MeV to 20 MeV energy range. Incoming x-rays are collimated into a narrow beam incident on a converter foil. The ejected Compton electrons are collimated so that the forward-directed electrons enter the magnetic field region of the spectrometer. The position of the electrons at the magnet’s focal plane is a function of their momentum, allowing the x-ray spectrum to be reconstructed. Recent measurements of flash sources are presented.

Beam rotation and shear in a large electron beam diode

The time-averaged electron-beam current distribution of one of the electron guns of the large Aperture Module (LAM) of the Aurora laser was measured as part of a larger set of experiments designed to study the electron beam transport to and energy deposition in the LAM laser chamber. The radiograms made on the center line of the LAM laser chamber while the laser chamber was at vacuum pressure demonstrated several of the expected results. The beam was relatively uniform over the aperture, with the exception of shadows cast by the diode anode wires, the Hibachi ribs, and the Hibachi support structure. At a depth of 50 cm into the laser chamber, the self-magnetic field of the beam produced a shear in the top and bottom edges of 15 cm. At the same depth the applied magnetic field caused a rotation of the entire beam profile of about 3 degrees . >

Development and characterization of diamond film and compound metal surface high current photocathodes

High current photocathodes operating in vacuum environments as high as 8/spl times/E-5 torr are being developed at Los Alamos for use in a new generation of linear induction accelerators. We report quantum efficiencies in wide bandgap semiconductors, pure metals and compound metal surface photocathode materials illuminated by ultraviolet laser radiation.

Progress on the Los Alamos heavy-ion injector

Heavy‐ion fusion using an induction linac requires injection of multiple high‐current beams from a pulsed electrostatic accelerator at as high a voltage as practical. Los Alamos National Laboratory is developing a 16‐beam, 2‐MeV, pulsed electrostatic accelerator for Al+ ions. The ion source will use a pulsed metal vapor arc plasma. A biased grid wil control plasma flux into the ion extraction region. This source has achieved a normalized emittance of en<3⋅10−7π‐m‐rad with Al+ ions. An 800 kV Marx prototype with a laser fired diverter is being assembled. The ceramic accelerating column sections have been brazed and leak tested. Voltage hold off on a brazed sample was more than doubled by selective removal of the Ticusil braze fillet extending along the ceramic. A scaled test module held 250 kV for 50 μs, giving confidence that the full module can hold 175 kV per section. The pressure vessel should be received in June 1986. High‐voltage testing of a 1 MV column will begin by early 1987.