A diagnostic was developed for the determination of temporal history of an X-ray spot. A pair of thin (0.5 mm) slits image the x-ray spot to a fast scintillator which is coupled to a fast detector, thus sampling a slice of the X-ray spot. Two other scintillators/detectors are used to determine the position of the spot and total forward dose. The slit signal is normalized to the dose and the resulting signal is analyzed to get the spot size. The position information is used to compensate for small changes due to spot motion and misalignment. The time resolution of the diagnostic is about 5 ns and measures spots from 0.5 mm to over 3 mm. The theory and equations used to calculate spot size and position are presented, as well as data. The calculations assume a symmetric, Gaussian spot. The spot data is generated by the ETA II accelerator, a 2kA, 5.2 MeV, 60 ns electron beam focused on a tantalum target. The spot generated is typically about 1 mm FWHM. Comparisons are made to an X-ray pinhole camera which images the X-ray spot (in 2D) at four time slices.

Linear induction accelerators used in X-ray radiography have single-pulse parameters of the order 20 MeV of electron beam energy, 2 kA of beam current, pulse lengths of 50-100 ns, and spot sizes of 1-2 mm. The thermal energy deposited in a bremsstrahlung converter target made of tantalum from such a pulse is /spl sim/80 kJ/cc, more than enough to bring the target material to a partially ionized state. The tail end of a single beam pulse, or any subsequent pulse in a multi-pulse train, undergoes a number of interactions with the target that can affect beam transport and radiographic performance. Positive ions extracted from the target plasma by the electron beam space charge can affect the beam focus and centroid stability. As the target expands on the inter-pulse time scale, the integrated line density of material decreases, eventually affecting the X-ray output of the system. If the target plume becomes sufficiently large, beam transport through it is affected by macroscopic charge and current neutralization effects and microscopic beam/plasma instability mechanisms. We will present a survey of some of these interactions, as well as some results of an extensive experimental and theoretical campaign to understand the practical amelioration of these effects, carried out at the ETA-II accelerator facility at the Lawrence Livermore National Laboratory.

Ions extracted from a solid surface or plasma by impact of an high intensity and high current electron beam can partially neutralize the beam space charge and change the focusing system. We have investigated ion emission computationally and experimentally. By matching PIC simulation results with available experimental data, our finding suggests that if a mix of ion species is available at the emitting surface, protons dominate the backstreaming ion effects, and that, unless there is surface flashover, ion emission is source limited. We have also investigated mitigation, such as e-beam cleaning, laser cleaning and ion trapping with a foil barrier. The temporal behavior of beam spot size with a foil barrier and a focusing scheme to improve foil barrier performance are discussed.

This paper presents the physics design of the DARHTII downstream system, which consists of a diagnostic beam stop, a fast, high-precision kicker system and the xray converter target assembly. The beamline configuration, the transverse resistive wall instability and the ion hose instability modelling are presented. We also discuss elimination of spot size dilution during kicker switching and implementation of the foil-barrier scheme to minimize the backstreaming ion focusing effects. Finally, we present the target converter’s configuration, and the simulated DARHT-II x-ray spot sizes and doses. Some experimental results, which support the physics design, will be also presented.

Desorption and subsequent ionization of the monolayers from the vacuum wall of an accelerator system can have a detrimental effect on the performance of the beam transport system. Ions extracted from the resultant plasma neutralize the spacecharge and dynamically perturb the net focusing forces within the beam. To study the effect, a transparent first foil, presumably with contaminants on the surface, intercepts the beam. Placing an imaging foil tens of centimeters downstream from the first foil allows observation of minor fluxuations in the envelope. Using conducting foil targets, we see no effect unless the beam radius is small enough to damage the foil. Non-conducting foils produce a strong effect.

The so-called roll bar measurement uses a heavy metal material, optically thick to x-rays, to form a shadow of the x-ray origination spot. This spot is where an energetic electron beam interacts with a high Z target. The material (the "roll bar") is slightly curved to avoid alignment problems. The roll bar is constructed and positioned so that the x-rays are shadowed in the horizontal and vertical directions, so information is obtained in two dimensions. If a beam profile is assumed (or measured by other means), the equivalent x-ray spot size can be calculated from the x-ray shadow cast by the roll bar. Thus the ellipticity of the beam can be calculated, assuming the ellipse of the x-ray spot is aligned with the roll bar. The data is recorded using a scintillator and gated camera. Data will be presented from measurements using the ETA II induction LINAC. The accuracy of the measurement is checked using small elliptical targets.

When the FXR machine was first tuned on the 1980s, a minimal amount of diagnostics was available and consisted mostly of power monitors. During the accelerator upgrade, additional beam diagnostics were added. The sensor upgrades included beam bugs (resistive wall beam motion sensors) and high-frequency B-dot. Even with this suite of measurement tools, tuning was difficult. For the current double-pulse upgrade, beam transport is a more complex problem-the beam characteristics must be measured better. Streak and framing cameras, which measure beam size and motions, are being added. Characterization of the beam along the entire accelerator is expected and other techniques are also evaluated. Each sensor has limitations and only provides a piece of the puzzle. Besides providing more beam data, the set of diagnostics used should be broad enough so results can be cross validated. Results are also compared to theoretical calculations and computer models, and successes and difficulties are reported.

Summary form only given, as follows. We have constructed a 30 diameter plasma source chamber to explore the problems associated with large-area inductively coupled plasma (ICP) sources with a view towards sources useful for 400 mm semiconductor wafer processing. Our initial source design experiments use a 25 diameter planar inductive coil driven at 13.56 MHz. Plasma data is taken in Ar and N/sub 2/ over the pressure range 3-50 mTorr with powers up to 2000 W. Diagnostics include Langmuir probes, B dot probes, and optical emission spectroscopy. Electrical circuit measurements are compared with results from commercial EM modeling codes. Initial indications are that uniform plasmas suitable for 400 mm wafer processing are attainable.

We have investigated experimentally the propagation of a relativistic electron beam through an array of parallel conducting wires. Theory and particle simulation predict such an array will provide both charge and current neutralization, allowing beam transport above the drift tube limit. We injected a 60ns, l7kA (12OA/cm/sup 2/ )pulse of 1.4 MeV electrons into an array of 1 m long wires spaced 1 cm apart, filling a hexagon 15 cm across. Arrays have been tested with 12 mil diameter copper wire, 3 mil stainless steel wire, and 12 mil copper wires terminating on an insulated, segmented beam dump. B probes and streak camera data show that 67% of the current is transported in the case of the stainless steel array. The copper wire array transported electrons for 20ns only. The beam is injected with a 250 mrad divergence, and the transported beam has a divergence of less than 100 mrad. Follow-up experiments are to use thinner wires to improve both the propagation and divergence of the beam.

The so-called roll bar measurement uses a heavy metal material, optically thick to x-rays, to form a shadow of the x-ray origination spot. This spot is where an energetic electron beam interacts with a high Z target. The material (the "roll bar") is slightly curved to avoid alignment problems. The roll bar is constructed and positioned so that the x-rays are shadowed in the horizontal and vertical directions, so information is obtained in two dimensions. If a beam profile is assumed (or measured by other means), the equivalent x-ray spot size can be calculated from the x-ray shadow cast by the roll bar. Thus the ellipticity of the beam can be calculated, assuming the ellipse of the x-ray spot is aligned with the roll bar. The data is recorded using a scintillator and gated camera. Data will be presented from measurements using the ETA II induction LINAC. The accuracy of the measurement is checked using small elliptical targets.

The so-called roll bar measurement uses a heavy metal material, optically thick to x-rays, to form a shadow of the x-ray origination spot. This spot is where an energetic electron beam interacts with a high Z target. The material (the " roll bar ") is slightly curved to avoid alignment problems. The roll bar is constructed and positioned so that the x-rays are shadowed in the horizontal and vertical directions, so information is obtained in two dimensions. If a beam profile is assumed (or measured by other means), the equivalent x-ray spot size can be calculated from the x-ray shadow cast by the roll bar. Thus the ellipticity of the beam can be calculated, assuming the ellipse of the x-ray spot is aligned with the roll bar. The data is recorded using a scintillator and gated camera. Data will be presented from measurements using the ETA II induction LINAC. The accuracy of the measurement is checked using small elliptical targets.