Received 28 February 2011DOI:https://doi.org/10.1103/PhysRevLett.106.109903© 2011 American Physical Society

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.

To produce four short X-ray pulses for radiography, the second-axis of the Dual Axis Radiographic Hydrodynamic Test facility (DARHT-II) will use a fast kicker to select current pulses out of the 2-/spl mu/s duration beam provided by the accelerator. Beam motion during the kicker voltage switching could lead to dilution of the time integrated beam spot and make the spot elliptical. A large elliptical X-ray source produced by those beams would degrade the resolution and make radiographic analysis difficult. We have developed a tuning strategy to eliminate the spot size dilution, and tested the strategy successfully on ETA-II with the DARHT-II kicker hardware.

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.

Performance of x-ray radiography facilities requires focusing the electron beams to sub-millimeter spots on the x-ray converters. Ions extracted from a converter by impact of a high intensity beam can partially neutralize the beam space charge and change the final focusing system. We will discuss these ion effects and mitigation.

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.

Summary form only given, as follows. In order to have confidence in the radiation converter target design for DARHT-II using LASNEX and our equation of state (EOS) for Ta, we need to compare the LASNEX simulations with experimental results from ETA-II and laser target experiments. ETA-II experiments include the following measurements: integrated line density, plasma expansion velocity, temperature requirement for hole formation, thermal images of the plasma plume, and foil heating. We will show that agreement between LASNEX simulations and experiments on ETA-II and laser-target experiments is good for essentially all the measurements obtained so far for solid Ta targets. This provides us confidence in the design of the DARHT-II radiation converter target using LASNEX and our eos for Ta. We will also report the comparison of simulations of the latest ETA-II foamed target experiments.

The ETA-II linear induction accelerator is used to drive a microwave free-electron laser (FEL). Corkscrew motion, which previously limited performance, has been reduced by: (1) an improved pulse distribution system which reduces energy sweep, (2) improved magnetic alignment achieved with a stretched wire alignment technique (SWAT), and (3) a unique magnetic tuning algorithm. Experiments have been carried out on a 20-cell version of ETA-II operating at 1500 A and 2.7 MeV. The measured transverse beam motion is less than 0.5 mm for 40 ns of the pulse, an improvement of a factor of 2 to 3 over previous results. Details of the computerized tuning procedure, estimates of the corkscrew phase, and relevance of these results to future FEL experiments are presented.<>

ETA-II resumed operation in the Fall of 1990 with the injector and first two ten-cell accelerating blocks and nominal electron beam parameters of 1500 A, 2.5 MeV, and 70-ns pulse width at 1 Hz PRF. The beam brightness diagnostics consisted of a Cerenkov foil view port and a pepper-pot emittance diagnostic. The Cerenkov foil experiment was used to determine the beam energy at the accelerator exit. The pepper-pot emittance diagnostic was used to determine the whole beam brightness. The brightness as a function of beam radius and time within the beam pulse was also measured. The brightness is defined as the ratio of the beam current within a given radius to the normalized four-dimensional volume occupied by particles within that radius.< >

The ETA-II linear induction accelerator has pulse power and beam diagnostic sensors distributed throughout the system. The accelerator consists of an injector, 20 accelerator cells arranged in two ten-cell blocks, and a transport section leading to an energy analyzer. In total there are approximately 120 beam diagnostic channels and 32 pulse power signals which are recorded on six Tektronix 7912AD oscilloscopes. The analysis, display, and interpretation of these data were done using systems for scientific visualization and a simple user interface. Results from several measurements are presented, showing how the diagnostics and system are utilized.< >

A flexible linear induction accelerator model (LIAM) was developed to predict both beam centroid position and the beam envelope. LIAM requires on-axis magnetic profiles and is designed to easily handle overlapping fields from multiple elements. Currently, LIAM includes solenoids, dipole steering magnets, and accelerating gaps. Other magnetic elements can be easily incorporated into LIAM due to its object-oriented design. LIAM is written in the C programming language and computes fast enough on current workstations to be used in the control room as a tuning and diagnostic aid. Combined with a non-linear least squares package, LIAM has been used to estimate beam energy at various locations within the ETA-II accelerator.<>

The ETA-II linear induction accelerator utilizes four pulse power conditioning chains. Magnetic pulse compression modulators (MAG1-Ds) form the last state of each chain. The upgrading and characterizing of the power conditioning chain on a high-average-power test stand (HAPTS) is reported. On the HAPTS, the pulse-to-pulse amplitude stability has been improved to less than 0.7% (one sigma) and the random jitter to 3-5 about a systematic timing variation. A description is presented of the work to achieve the desired performance level of the MAG1-D to allow high average power operation of ETA-II.<>

An expert system has been developed that acts as an intelligent assistant for tuning particle beam accelerators. This system is called MAESTRO-Model and Expert System Tuning Resource for Operators. MAESTRO maintains a knowledge base of the accelerator containing not only the interconnections of the beamline components, but also their physical attributes such as measured magnetic tilts, offsets, and field profiles. MAESTRO incorporates particle trajectory and beam envelope models which are coupled to the knowledge base permitting large number of real-time orbit and envelope calculations in the control-room environment. This capability has been used (1) to implement a tuning algorithm for minimizing transverse beam motion, (2) to produce a beam waist with arbitrary radius at the entrance to a brightness diagnostic, and (3) to measure beam energy along the accelerator by fitting orbits to focusing and steering sweeps.<>

The ETA-II electron beam will be used to drive a high power microwave frequency FEL for plasma heating experiments. For maximum FEL output power the beam energy at the entrance to the wiggler should be within +or-1% of the wiggler resonance value. In initial operations the ETA-II beam energy stayed within this range for a maximum time of less than 13 ns. Much of the energy variation was due to the design of the pulsed power feeds to the accelerator induction cells. A new multicable pulsed power feed design was tested in a shortened version of ETA-II where it extended the time during which the beam energy stayed within the +1% limits to greater than 40 ns. These design changes are now being incorporated into the full accelerator.<>