D.C. Moir

Simple laser‐driven, metal‐photocathode electron source

An ArF excimer laser was used to produce photoelectrons from common metal surfaces at modest vacuum. The photoelectrons are subsequently accelerated across an anode‐cathode gap. Both space‐charge and emission‐limited flow were examined. The temporal characteristics of the emission‐limited electron beam produced by this device are nearly identical to those of the laser pulse used to produce the photoemission. In the space‐charge‐limited case, a rise time of 3 ns was obtained. Maximum current densities of 70 A/cm2 were achieved.

Guiding of an electron beam from a rf accelerator by a laser-ionized channel

The first guiding of multiple pulses from a rf accelerator by an ultraviolet‐laser‐ionized channel in low‐pressure benzene has been demonstrated. In particular, two 5‐ns, 750‐A, 30‐MeV pulses separated by 20 ns were transported a maximum distance of 13.5 m. Detailed measurements of the temporal behavior of the beam as it propagated versus distance, laser‐channel parameters, and accelerator conditions were made.

High‐brightness electron‐beam generation and transport

Experimental results for a high‐brightness electron‐beam source are compared to results of numerical and analytic calculations. The 4‐MV, 5‐kA beam is generated from a velvet cathode and focused by a solenoidal magnet. Data for the diode impedance and the beam focusing length and spot size agree well with numerical simulations. The minimum spot size is consistent with a normalized Lapostolle emittance of 0.12 cm rad, yielding a brightness of 3.5×108 A/m2  rad2 . Transverse beam oscillations are observed in streak photographs and are thought to be caused by electromagnetic dipole modes in the diode cavity. The oscillation amplitude is significantly reduced by making the current rise more slowly.

Ion emission from solid surfaces induced by intense electron beam impact

Ions or ionized neutrals released from solid surfaces by electron beam impact can be accelerated and trapped in the beam potential causing beam disruption. Experiments have been performed on the DARHT-I accelerator (1.7 kA, 19.8 MeV, 60 ns) to study this phenomenon. The beam, focused to a range of diameters, was transmitted through thin targets made of various materials. The time evolution of the beam radial profile was measured downstream of the target. For low current density, the downstream-beam radial profile was time invariant as expected for a pure electron beam. At higher current density, the downstream beam was clearly disrupted during the pulse followed by a large-amplitude transverse centroid instability. Two-dimensional calculations using the Lsp particle-in-cell code show that if the space-charge-limiting ion current is allowed to flow after the target surface temperature increases by about 400 K, the main features of the experimental observations are replicated. Three-dimensional Lsp calculations show growth of the ion hose instability at a frequency close to that observed in the experiments.

Beam injector and transport calculations for ITS

The Integrated Test Stand at Los Alamos National Laboratory (LANL) is addressing issues in high-brightness electron beam generation, acceleration, and transport. The machine consists of a 3 kA, 3.5 MV injector, eight induction acceleration gaps, a drift section and a final-focus magnet which focuses the beam onto diagnostic targets. One of the goals of the program is to test and improve the predictive capability of numerical models. We have carried out detailed simulations of the diode and initial drift region with the particle-in-cell codes IVORY and SPROP, obtaining good agreement with experimental streak-camera data at several axial locations. Transport through the accelerating cells to the target 10 m from the cathode is modeled with the envelope codes LAMDA and XTR. The magnet settings for minimum spot-size are close to the experimental values. From the measured value of the minimum spot-size we infer a normalized Lapostolle emittance of about 0.2 cm-rad. We have characterized the sensitivity of the spot-size to variation in the machine parameters.

Radiation-induced transient darkening of optically transparent polymers

Results are presented for the radiation‐induced transient darkening of thin organic polymer films normally used as Cerenkov light emissions sources. The radiation source is a 27‐MeV, 10‐μC, 200‐ns electron beam generated by the PHERMEX accelerator. The typical dose for a single pulse is 5 Mrad. At this dose, the broadband time‐resolved percent transmission above 520 nm was measured for four common polymers: polyimide (Kapton‐H), polyethylene terephthalate (Mylar), cellulose acetate, and high‐density polyethylene. Kapton was found to darken the most and polyethylene darkened the least. The recovery time to normal transmission for Kapton was found to be greater than 10–20 μs. The radiation‐induced attenuation coefficient is shown to depend on electronic band energy separation. The results show that Kapton is not the material of choice for a Cerenkov light source.

Temporal response of a surface flashover on a velvet cathode in a relativistic diode

Surface flashover of a carbon fiber velvet cathode generates a discharge from which electrons are relativistically accelerated to γ ranging from 4.9 to 8.8 through a 17.8 cm diode. This discharge is assumed to be a hydrocarbon mixture. The principal objective of these experiments is to quantify the dynamics over the ∼100 ns pulse of the plasma discharge generated on the surface of the velvet cathode and across the anode-cathode (A-K) gap. A qualitative comparison of calculated and measured results is presented, which includes time resolved measurements with a photomultiplier tube and charge-coupled device images. In addition, initial visible spectroscopy measurements will also be presented confirming the ion species are dominated by hydrogen.

BBU gain measurements on the ITS 6-MeV, 4-kA linac

The amplification of transverse beam motion by the ITS eight-cell linear induction accelerator (LIA) was measured with a 4-kA beam at frequencies between 710- and 950-MHz. The 3.75-MeV injector beam was deflected with amplitudes of /spl sim/1 mrad by an RF-driven tunable cavity 84 cm before the linac. The centroid-position waveforms from calibrated B-dot loops were recorded with 5-GS/s digitizers at the linac entrance, exit, and after a further free drift of 108 cm. The FFT amplitudes of the waveforms were used to compute the gains. The code LAMDA, an envelope and BBU model for the linac, was used to interpret the gain curves, using the transverse impedances of the cells as adjustable parameters. The results compare favorably with cell impedances previously measured with a two-wire TSD method.

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>>

Upgrade of the PHERMEX accelerator

The PHERMEX (Pulsed High Energy Radiographic Machine-Emitting X-rays) electron beam accelerator at Los Alamos National Laboratory, a 50-MHz, 30-MV standing-wave device, typically operates with a 500-A, 600-kV injector. A higher-current injector is under consideration, and the authors have modeled a flat-cathode diode geometry which can deliver 1-1.5 kA. A three-coil field configuration has been designed to maintain low beam emittance in the diode region. It is shown that the existing two transport magnets are marginally capable of transporting a 1 kA beam to the first RF cavity. The authors examine the possibility of accelerating a 4-kA, 4-MV beam, which could be provided by an existing pulsed power machine, through the first RF cavity.<<ETX>>