R.N. Ridlon

Performance of the Cygnus X-ray source

Summary form only given. Cygnus is a radiographic X-ray source developed for support of the Sub-Critical Experiments Program at the Nevada Test Site. Major requirements for this application are: a dramatically reduced spot size as compared to both Government Laboratory and existing commercial alternatives, layout flexibility, and reliability. Cygnus incorporates proven pulsed power technology (Marx Generator, Pulse Forming Line, Water Transmission Line, and Inductive Voltage Adder sub-components) to drive a high voltage vacuum diode. In the case of Cygnus, a relatively new approach (the rod pinch diode) is employed to achieve a small source diameter. Design specifications are: 2.25 MeV peak energy, < 1 min source diameter, and 5-10 rads dose at 1 meter. The pulsed power and system architecture design plan has been previously presented (Weidenheimer et al., 2001). The first set of Cygnus shots are now underway and are geared to verification of electrical parameters and, therefore, use a large area diode configuration offering increased shot rate as compared to that of the rod pinch diode. Later tests incorporate the rod pinch diode and will concentrate on X-ray production with time resolved measurements of X-ray dose and spot size. In this work we present results of initial operation in terms of electrical and radiation parameters. In addition, the issues associated with static and time resolved radiographs may be included. Performance of the pulse power system is being evaluated by comparison of measured to design output parameters. This is accomplished by comparison of multiple voltage and current measurements throughout the system with various circuit model codes such as MicroCAP and T-Line.

Performance of the Cygnus x-ray source

Cygnus is a radiographic x-ray source developed for support of the Sub-Critical Experiments Program at the Nevada Test Site. Major requirements for this application are: a dramatically reduced spot size as compared to both Government Laboratory and existing commercial alternatives, layout flexibility, and reliability. Cygnus incorporates proven pulsed power technology (Marx Generator, Pulse Forming Line, Water Transmission Line, and Inductive Voltage Adder sub-components) to drive a high voltage vacuum diode. In the case of Cygnus, a relatively new approach (the rod pinch diode [1]) is employed to achieve a small source diameter. Design specifications are: 2.25 MeV endpoint energy, &#60; 1 mm source diameter, and >3 rads dose at 1 meter. The pulsed power and system architecture design plan has been previously presented [2]. The first set of Cygnus shots were geared to verification of electrical parameters and, therefore, used a large area diode configuration offering increased shot rate as compared to that of the rod pinch diode. In this paper we present results of initial rod pinch operation in terms of electrical and radiation parameters.

Rex, A 5-MW Pulsed-power Source For Driving High-brightness Electron Berm Diodes

The Relativistic Electron-beam Experiment, or REX accelerator, is a pulsed-power source capable of driving a 100-ohm load at 5 MV, 50 kA, 45 ns (FWHM) with less than a 10-ns rise and 15-ns fall time. This paper describes the pulsed-power modifications, modeling, and extensive measurements on REX to allow it to drive high impedance (100s of ohms) diode loads with a shaped voltage pulse. A major component of REX is the 1.83-m diam x 25.4-cm-thick Lucite insulator with embedded grading rings that separates the output oil transmission line from the vacuum vessel that containing the re-entrant anode and cathode assemblies. A radially tailored, liquid-based resistor provides a stiff voltage source that is insensitive to small variations of the diode current and, in addition, optimizes the electric field stress across the vacuum side of the insulator. The high-current operation of REX employs both multichannel peaking and point-plane diverter switches. This mode reduces the prepulse to less than 2 kV and the postpulse to less than 5% of the energy delivered to the load. Pulse shaping for the present diode load is done through two L-C transmission line filters and a tapered, glycol- based line adjacent to the water PFL and output switch. This has allowed REX to drive a diode producing a 4-MV, 4.5-kA, 55-ns flat-top electron beam with a normalized Lapostolle emittance of 0.96 mm-rad corresponding to a beam brightness in excess of 4.4x10/sup 8/ A/m/sup 2/-rad/sup 2/[1,2].

Electron-beam generation, transport, and transverse oscillation experiments using the REX injector

The REX machine at LANL (Los Alamos National Laboratory) is being used as a prototype to generate a 4-MV, 4.5-kA, 55-ns flat-top electron beam as a source for injection into a linear induction accelerator of the 16-MeV dual-axis radiographic hydrotest facility. The pulsed-power source drives a planar velvet cathode producing a beam that is accelerated through a foilless anode aperture and transported by an air core magnetic lens for injection into the first of 48 linear induction cells. Extensive measurements of the time-resolved (<1-ns) properties of the beam using a streak camera and high-speed electronic diagnostics have been made. These parameters include beam current, voltage, current density, emittance, and transverse beam motion. The effective cathode temperature is 117 eV, corresponding to a Lapostolle emittance of 0.96 mm-rad.<<ETX>>

A 4-megavolt, 5-kiloampere pulsed-power high-brightness electron beam source

The REX machine at Los Alamos National Laboratory is being used to generate a 4-MV, 5-kA, 50-ns electron beam. The beam is produced by a planar velvet cathode, accelerated through a foilless anode aperture, and transported by an air core magnetic lens. Extensive measurements of the time-resolved (<1 ns) properties of the beam using a streak camera and high-speed electronic diagnostics have been made. These parameters include beam current, voltage, current density, and emittance for a single cathode-anode configuration. Results indicate beam brightness in excess of 10/sup 8/A/m/sup 2/-rad/sup 2/. Numerical simulations of the experiment have been performed with ISIS, a time-dependent particle-in-cell code, and are in good agreement with the measurements. This electron beam source is being considered as an injector to a linear induction accelerator and can be operated at higher voltages and currents than a stand-alone flash X-ray machine.<<ETX>>

Pulsed 4-MeV electron injector with an excimer laser driven photocathode

The Relativistic Electron-Beam Experiment injector at the Los Alamos National Laboratory is used to generate a 4-MV pulse across an anode-cathode gap. A simple metal photocathode is illuminated by a pulsed excimer laser. Time-resolved measurements of current, voltage, and current density were made. The resulting quantum efficiencies are being used to obtain the required laser power for a multikiloampere, high-brightness electron gun to be used as an injector for a linear induction accelerator. It is shown that an aluminum photocathode capable of emitting 3.3 kA would require an ArF laser operating at 650 kW/cm/sup 2/. This would generate 72 A/cm/sup 2/, and the laser energy is well below the plasma threshold. To construct a DARHT (Dual Axis Radiographic Hydrotest Facility) injector (3.3 kA, 60 ns), a 1.75-J ArF laser would be required. This type of laser is within current electron-beam-pumped laser technology.<<ETX>>

Beam emittance diagnostic for the DARHT second axis injector

Low beam emittance is key to achieving the required spot size at the output focus of the DARHT Second Axis. The nominal electron beam parameters at the output of the injector are 2 kA, 4.6 MeV, 2-microsecond pulse width and an rms radius less than 1 cm. Emittance is measured by bringing the beam to a focus in which the emittance is a dominant influence in determining the spot size. The spot size is measured from Cerenkov or optical transition radiation (OTR) generated from a target intercepted by the beam. The current density in the focused DARHT beam would melt this target in less than 1/2 microsec. To prevent this we have designed a DC magnetic transport system that defocuses the beam on the emittance target to prevent overheating, and uses a 125-ns half period pulsed solenoid to selectively focus the beam for short times during the beam pulse. During the development of the fast-focusing portion of this diagnostic it has been determined that the focusing pulse must rapidly sweep through the focus at the target to an over-focused condition to avoid target damage due to overheating. The fast focus produces /spl sim/1 kilogauss field over an effective length of /spl sim/50 cm to bring the beam to a focus on the target. The fast focus field is generated with a 12-turn coil located inside the beam-transport vacuum chamber with the entire fast coil structure within the bore of a D.C. magnet. The pulsed coil diameter of /spl sim/15 cm is dictated by the return current path at the nominal vacuum wall. Since the drive system is to use 40 kV to 50 kV technology and much of the inductance is in the drive and feed circuit, the coil design has three 120 degree segments. The coil, driver and feed system design, as well as beam envelope calculations and target heating calculations are presented below. Operation of the OTR imaging system will be discussed in separate publication.

Time-resolved emittance measurements of an excimer-laser-driven metal photocathode

The velvet cathode material of a 4-MeV, electron beam injector was replaced with a material cathode and driven by an ArF excimer laser. Time-resolved measurements of photoelectron current and effective cathode temperature were made. A streak camera and B-dot (magnetic field) probes were the primary diagnostics for the experiment. The streak camera determined effective cathode temperature in a single spatial dimension. Time-integrated photography was used to observe plasma information in the cathode region. The results will be used to ascertain the feasibility of long-pulse (75-ns) beam generation for injection into a linear induction accelerator.<<ETX>>

Pulse-power-induced oscillations of the REX electron beam

The Relativistic Electron-Beam Experiment (REX) pulse power generator is used to produce a 4 MeV, 5 kA, 50 ns cathode. Multiple current rise times are used to examine pulse-power-induced beam oscillations. Diagnostics include a streak camera and high-speed electronic probes for monitoring current and voltage. Correlation between beam oscillations and TE-TM (transverse electric-transverse magnetic) cylindrical cavity modes in the anode-cathode (A-K) gap is examined. Electromagnetic excitation of the cylindrical A-K gap in the REX experiments was found to produce unacceptable levels of beam motion as a function of time. Correction of hardware asymmetries did not eliminate the problem; the problem was solved by tailoring the current rise time of the pulse power to minimize the coupling of the pulse power drive to the cavity.<<ETX>>

A 100-MW, electron-beam-pumped, argon fluoride laser for driving metal photo-cathodes

Summary form only given, as follows. A reliable and repeatable electron beam pumped ArF laser system to drive bare aluminum metal photo-cathodes is described. The system consists of a seven stage Marx generator charged to 65 kV/stage that in-turn pulse charges a 150-ns-long, 5.5-ohmm water PFL to voltages as high as 750 kV. The PFL utilizes a low-jitter, self-breaking SF/sub 6/ switch and is terminated in an adjustable radial resistor to match varying diode impedances and voltages from 250- to 375-kV. The optimized diode geometry consists of a flat velvet emitter (7.6 cm high /spl times/ 98 cm long) operated at 275 kV with an AK gap of 3.3 cm. The goal of attaining this 10-ohmm diode geometry at 275 kV was to match a sly developed thyratron based, 11-ohmm, 300-kV Blumlein driver. The velvet diode delivers 28 kA of current through a 3-mil Kapton foil into the 6.5-cm-diam by 100-cm-long laser gas region at 1500 torr. In order to achieve long foil lifetimes /spl sim/500 pulses, a pair of Helmholtz coils are used to guide, compress, and profile the electron beam. The output laser energy, power, and spatial profile from a stable resonator configuration have been shown to be reproducible to less than 5 percent over any 50 shot sequence without any adjustment to the system.