Dual Axis Radiographic Hydrodynamic Test Research Articles

Calibration of two compact permanent magnet spectrometers for high current electron linear induction accelerators.

In this work, two compact, permanent magnet, electron spectrometers have been built to measure the electron beam energy at the Dual Axis Radiographic Hydrodynamic Test facility. Using H- and OH- anions, the spectrometers were calibrated at the Special Technologies Laboratory in Santa Barbara, California (USA). The spectrometers were mounted on a custom drift tube that allows the magnet assemblies to be translated, which increases the path length of the electrons traveling through the magnetic field and therefore increases the upper bound of the measurable electron kinetic energy. The measurable range of electron kinetic energies is between 2.8 MeV-4.1 MeV for the first spectrometer and 14.1 MeV-21.1 MeV for the second spectrometer, with an overall measurement uncertainty of 0.32%.

Quantifying spot size reduction of a 1.8 kA electron beam for flash radiography

The spot size of Axis-I at the Dual Axis Radiographic Hydrodynamic Test facility was reduced by 15.5% by including a small diameter drift tube that acts to aperture the outer diameter of the electron beam. Comparing the measured values to both analytic calculations and results from a particle-in-cell model shows that one-third to one-half of the spot size reduction is due to a drop in beam emittance. We infer that one-half to two-thirds of the spot-size reduction is due to a reduction in beam-target interactions. Sources of emittance growth and the scaling of the final focal spot size with emittance and solenoid aberrations are also presented.

Direct measurement of the image displacement instability in a linear induction accelerator

The image displacement instability (IDI) has been measured on the 20 MeV Axis I of the dual axis radiographic hydrodynamic test facility and compared to theory. A 0.23 kA electron beam was accelerated across 64 gaps in a low solenoid focusing field, and the position of the beam centroid was measured to 34.3 meters downstream from the cathode. One beam dynamics code was used to model the IDI from first principles, while another code characterized the effects of the resistive wall instability and the beam break-up (BBU) instability. Although the BBU instability was not found to influence the IDI, it appears that the IDI influences the BBU. Because the BBU theory does not fully account for the dependence on beam position for coupling to cavity transverse magnetic modes, the effect of the IDI is missing from the BBU theory. This becomes of particular concern to users of linear induction accelerators operating in or near low magnetic guide fields tunes.

Beam Breakup in an Advanced Linear Induction Accelerator

Two linear induction accelerators (LIAs) have been in operation for a number of years at the Los Alamos Dual Axis Radiographic Hydrodynamic Test (DARHT) facility. A new multipulse LIA is being developed. We have computationally investigated the beam breakup (BBU) instability in this advanced LIA. In particular, we have explored the consequences of the choice of beam injector energy and the grouping of LIA cells. We find that within the limited range of options presently under consideration for the LIA architecture, there is little adverse effect on the BBU growth. The computational tool that we used for this investigation was the beam dynamics code linear accelerator model for DARHT (LAMDA). To confirm that LAMDA was appropriate for this task, we first validated it through comparisons with the experimental BBU data acquired on the DARHT accelerators.

Ultrafast camera’s early history

For the record, scientists at Los Alamos National Laboratory developed systems that use a streak tube and image compression to record ultrafast two-dimensional movies of transient events some 30 years ago11. C. T. Mottershead, IEEE Trans. Nucl. Sci. 29, 900 (1982). https://doi.org/10.1109/TNS.1982.4335986,22. M. Wilke, N. S. P. King, N. Gray, D. Johnson, D. Esquibel, P. Nedrow, S. Ishiwata, Proc. SPIE 0566, 185 (1986). https://doi.org/10.1117/12.949787 (Physics Today, February 2015, page 12). A cylindrical lens focuses the light from a 2D scene into a 1D line, which is actually a tomographic projection integral containing unfiltered information from the entire image. Four such time-dependent projections of a rapidly evolving scene, one every 45 degrees, were fed into coherent fiber-optic ribbons and sent some 25 m to the photocathode of a streak tube. The fiber-optic ribbons, filtered by both wavelength and mode, achieved subnanosecond time resolution. A digital video system recorded the resulting spacetime streak-tube image, with each scan line containing a time sample of the four projections.In the data analysis, a modified version of G. Minerbo’s maximum entropy tomography algorithm33. G. Minerbo, Comput. Graphics Image Process. 10, 48 (1979). https://doi.org/10.1016/0146-664X(79)90034-0,44. C. T. Mottershead, in Maximum Entropy and Bayesian Methods: Santa Fe, New Mexico, U.S.A., 1985, K. M. Hanson, R. N. Silver, eds., Kluwer Academic (1996), p. 425. 978-94-010-6284-8 was used to reconstruct a frame of the output movie from each scan line of the streak-tube image. A later variant of the algorithm produced a 3D spacetime tomographic reconstruction using a time-integrated 2D image of the same scene, exposed for the duration of the streak-tube sweep, as a tomographic projection along the time axis. The reconstructed movie solution is then constrained to add up to the time exposure.More recently, workers at the Dual Axis Radiographic Hydrodynamic Test Facility at Los Alamos successfully used several streak-tomography systems as near-real-time beam diagnostics in the commissioning of the facility’s high-current pulsed electron accelerator.55. H. Bender, C. Carlson, D. Frayer, D. Johnson, K. Jones, A. Meidinger, C. Ekdahl, Rev. Sci. Instrum. 78, 013301 (2007); https://doi.org/10.1063/1.2409770 D. K. Frayer, D. Johnson, C. Ekdahl, in Proceedings of 2010 Beam Instrumentation Workshop, TUPSM012, available at http://accelconf.web.cern.ch/accelconf/BIW2010/papers/tupsm012.pdf.REFERENCESSection:ChooseTop of pageREFERENCES <<1. C. T. Mottershead, IEEE Trans. Nucl. Sci. 29, 900 (1982). https://doi.org/10.1109/TNS.1982.4335986, Google ScholarCrossref2. M. Wilke, N. S. P. King, N. Gray, D. Johnson, D. Esquibel, P. Nedrow, S. Ishiwata, Proc. SPIE 0566, 185 (1986). https://doi.org/10.1117/12.949787, Google ScholarCrossref3. G. Minerbo, Comput. Graphics Image Process. 10, 48 (1979). https://doi.org/10.1016/0146-664X(79)90034-0, Google ScholarCrossref4. C. T. Mottershead, in Maximum Entropy and Bayesian Methods: Santa Fe, New Mexico, U.S.A., 1985, K. M. Hanson, R. N. Silver, eds., Kluwer Academic (1996), p. 425. 978-94-010-6284-8, Google ScholarCrossref5. H. Bender, C. Carlson, D. Frayer, D. Johnson, K. Jones, A. Meidinger, C. Ekdahl, Rev. Sci. Instrum. 78, 013301 (2007); https://doi.org/10.1063/1.2409770 , Google ScholarCrossrefD. K. Frayer, D. Johnson, C. Ekdahl, in Proceedings of 2010 Beam Instrumentation Workshop, TUPSM012, available at http://accelconf.web.cern.ch/accelconf/BIW2010/papers/tupsm012.pdf. , Google Scholar© 2015 American Institute of Physics.

Fidelity of a Time-Resolved Imaging Diagnostic for Electron Beam Profiles

An optical tomographic diagnostic instrument has been fielded at the Dual-Axis Radiographic Hydrodynamic Test Facility at Los Alamos National Laboratory. Four optical lines of sight create projections of an image of an electron beam on a Cerenkov target, which are relayed via optical fiber to streak cameras. From these projections, a reconstruction algorithm creates time histories of the beam’s cross section. The instrument was fielded during and after facility commissioning, and tomographic reconstructions reported beam parameters. Results from reconstructions and analysis are noted.

Beam-energy-spread minimization using cell-timing optimization

Beam energy spread, and related beam motion, increase the difficulty in tuning for multipulse radiographic experiments at the dual-axis radiographic hydrodynamic test facility's axis-II linear induction accelerator (LIA). In this article, we describe an optimization method to reduce the energy spread by adjusting the timing of the cell voltages (both unloaded and loaded), either advancing or retarding, such that the injector voltage and summed cell voltages in the LIA result in a flatter energy profile. We developed a nonlinear optimization routine which accepts as inputs the 74 cell-voltage, injector voltage, and beam current waveforms. It optimizes cell timing per user-selected groups of cells and outputs timing adjustments, one for each of the selected groups. To verify the theory, we acquired and present data for both unloaded and loaded cell-timing optimizations. For the unloaded cells, the preoptimization baseline energy spread was reduced by 34% and 31% for two shots as compared to baseline. For the loaded-cell case, the measured energy spread was reduced by 49% compared to baseline.

A Multi-Frame, Megahertz CCD Imager

The Los Alamos National Laboratory's Dual Axis Radiographic Hydrodynamic Test Facility (DARHT) generates flash radiographs of explosive experiments using two linear induction electron accelerators situated at right angles. The DARHT second axis accelerator generates an 18-MeV, 2 kA, 2 musec electron beam which is converted or ldquochoppedrdquo into four individual pulses ranging from 20 to 100 nsec in length at 2 MHz frequency. The individual electron beam pulses are down-converted by a segmented lutetium oxyorthosilicate scintillator, creating four visible light flashes, to image explosively driven events. To record these events, a high efficiency, high speed, imager has been fabricated which is capable of framing rates of 2 MHz. This device utilizes a 512 times 512 pixel charge coupled device (CCD) with a 25 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> active area, and incorporates an electronic shutter technology designed for back-illuminated CCD's, making this the largest and fastest back-illuminated CCD in the world. Characterizing an imager capable of this frame rate presents unique challenges. High speed LED drivers and intense radioactive sources are needed to perform basic measurements. We investigate properties normally associated with single-frame CCD's such as read noise, gain, full-well capacity, detective quantum efficiency (DQE), sensitivity, and linearity. In addition, we investigate several properties associated with the imager's multi-frame operation such as transient frame response and frame-to-frame isolation while contrasting our measurement techniques and results with more conventional devices.

Multiple pulse electron beam converter design for high power radiography

The typical response of the x-ray converter material to the passage of a high-powered relativistic electron beam is vaporization and rapid dispersal. The effect of this dispersal on subsequent pulses for multi-pulse radiography is the collective effects on the propagation of the electron beam through the expanding plasma and the reduced number of electron to photon interactions. Thus, for the dual-axis radiographic hydrodynamic test facility, the converter material must either be replaced or confined long enough to accommodate the entire pulse train. Typically the 1-mm-thick high Z and full density converter material is chosen to give peak dose and minimum radiographic spot. For repeated pulses we propose a modified converter, constructed of either low density, high Z material in the form of foam or of foils spaced over ten times the axial thickness of the standard 1 mm converter. The converter material is confined within a tube to impede outward motion in radius outside the beam interaction region. We report single-pulse experiments which measure the dose and spot size produced by the modified converter and compare them to similar measurements made by the standard converter. For multiple pulses over a microsecond time scale, we calculate the radial and axial hydrodynamic flow to study the material reflux into the converter volume and the resultant density decrease as the electron beam energy is deposited. Both the electron transport through the expanding low density plasma and beam in the higher density material are modeled. The x-ray source dose and spot size are calculated to evaluate the impact of the changing converter material density distribution on the radiographic spot size and dose. The results indicate that a multiple-pulse converter design for three or four high-power beam pulses is feasible.