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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Effects of shock strength on shock turbulence interaction

Direct numerical simulation (DNS) and linear analysis (LIA) of isotropic turbulence interacting with a shock wave are performed for several upstream shock normal Mach numbers (M(sub 1)). Turbulence kinetic energy (TKE) is amplified across the shock wave, but this amplification tends to saturate beyond M(sub 1) = 3.0. TKE amplification and Reynolds stress anisotropy obtained in DNS are consistent with LIA predictions. Rapid evolution of TKE immediate downstream of the shock wave persists for all shock strengths and is attributed to the transfer between kinetic and potential modes of turbulence energy through acoustic fluctuations. Changes in energy spectra and various length scales across the shock wave are predicted by LIA, which is consistent with DNS results. Most turbulence length scales decrease across the shock. Dissipation length scale (rho-bar q(exp 3) / epsilon), however, increases slightly for shock waves with M(sub 1) less than 1.65. Fluctuations in thermodynamic variables behind the shock wave stay nearly isentropic for M(sub 1) less than 1.2 and deviate significantly from isentropy for the stronger shock waves due to large entropy fluctuation generated through the interaction.

Lee, Sangsan↗
DOE OSTI2021

Beam Envelope Stability in an Advanced Linear Induction Accelerator

A new linear induction accelerator (LIA) is under development for multipulse flash radiography. Because it has substantially more cells than present LIAs, higher magnetic focusing fields are needed to suppress beam breakup (BBU). It is, therefore, more susceptible to the parametric beam envelope instability (PEI), which has an instability threshold that has usually been typified by the vacuum phase advance per cell exceeding some large fraction of π . Here we derive a threshold criterion for PEI that depends not only on the magnetic field, but also on the beam space charge and emittance. A tune designed to suppress BBU in Scorpius is shown to be stable to the PEI according to this criterion, and also by the lack of emittance growth in particle-in-cell (PIC) code simulations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

DARHT Axis 1 Time-Resolved Injector Energy Measurement

The Dual-Axis Radiographic Hydrodynamic Test (DARHT) facility provides flash radiography capabilities using electron Linear Induction Accelerators (LIA’s). The strict requirements for flash radiography require a detailed understanding of the LIA’s performance, including precision measurements of the injected electron beam energy. The DARHT Axis 1 injector of produces a 3-4 MeV, 1-2 kA, 80-ns-FWHM electron beam. Injector capacitive monitors (EVACSUM) are summed to give the injector beam energy. Calibration of EVACSUM was last done in 1999 and is needed. In addition, the flatness of the injector drive voltage is controlled by a peaking capacitor in a Blumlien that is charged by the prime power tank. Time resolved measurements are used to optimize the value of this capacitor.

47 OTHER INSTRUMENTATION↗
DOE OSTI2020

Scorpius injector emittance measurements with solenoid scans

A classic scan solenoid can establish that the injector meets emittance requirements. Incorporating an aperture plate can validate predictions of much lower emittance. Measurements of beam parameters on the injector and ITS can inform predictions for the full LIA. Injector beam data is needed to tune the full LIA for initial turn-on and operation. Emittance measurements on the Scorpius injector are difficult because the beam is space-charge dominated. Emittance measurements on Scorpius injector and ITS will need large beam size to prevent overheating.

43 PARTICLE ACCELERATORS↗
DOE OSTI2020

Optimization of DARHT Axis 1 Injector Voltage

The Dual-Axis Radiographic Hydrodynamic Test (DARHT) facility provides flash radiography capabilities using two electron Linear Induction Accelerators (LIA’s). Axis-1 of DARHT produces a 20- MeV, 2-kA, 80-ns-FWHM electron beam. The strict requirements for flash radiography require a detailed understanding of the LIA’s performance, including precision measurements of the injector electron beam energy. The technique for time resolved measurement of the electron beam energy using electron Permanent Magnet Spectrometer (ePMSpec) has been developed. The electron energy then is used to infer the voltage produced by the injector pulsed power. The injector pulsed power of DARHT I consists of a 1.5-MV, glycol-insulated Blumlein that is pulsecharged by a step-up transformer and switched by four, laser-triggered spark gaps. A series of increasing impedance transmission lines are used to transform the output voltage of the Blumlein to a maximum of 3.8MV at the diode. Figure 1a shows a graphic of the Axis-1 prime power tank, Blumlein and downstream transmission lines. The injector pulse power is designed to produce voltage on a velvet cathode located in the vacuum vessel. The specifications of the voltage-pulse flat-top over 60ns is +/-1%. A key element in the high-voltage circuit is the ethylene glycol Blumlein. The outer volume, adjacent to the laser triggered switches of the Blumlein, is a peaking capacitor. This is an independent volume that contains a mixture of ethylene glycol and water to make it an adjustable capacitor of 380 to 750 pF.

43 PARTICLE ACCELERATORS↗
DOE OSTI2021

Beam Stability Overview [Slides]

Radiographic spot size is affected by many factors, including beam motion due to instabilities. Instabilities and effects that can influence the radiation-source spot size are being evaluated for the Scorpius flash-radiography accelerator. The Scorpius LIA design has a direct influence on beam stability. Many features of the design were specifically incorporated to mitigate instability. The major beam dynamics concerns for high-current LIAs are being evaluated for the long, 102-cell Scorpius design. For these results, all tools available were used including experimental data, analytic theory, and physics simulation codes.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

First Results from Diamagnetic Loop Measurements of the DARHT-I Electron Beam [Slides]

Diamagnetic-loop (DML) measurements can inform efforts to improve radiographic resolution. DML is non-invasive, so time-resolved data is available on every shot while tuning or executing a hydrotest. The time-resolved beam size can be deduced from the DML data. LIA beam-transport dynamics affecting beam size can be monitored while testing mitigation measures (e.g., beam halo suppression). Time-resolved beam size at final focus provides immediate information about spot size enlargement due to blur, and effectiveness of mitigation efforts. DML measures magnetic flux produced by a rotating beam, so it also enables monitoring of Larmor emittance that can enlarge the spot size. Beam rotation adds in quadrature with emittance, hence “Larmor emittance." Observing zero bias-field DML flux monitors beam rotation resulting from imperfect nulling of flux linking the cathode, and/or broken LIA transport symmetry (e.g., steering, quads, etc.).

43 PARTICLE ACCELERATORS↗
DOE OSTI2023

Magnetic-Field Diffusion Effects in Beam Position Monitors III: Application to DARHT-II Beam Data

Beam position monitors (BPMs) provide time-resolved measurements of the current and centroid position of high-current electron beams in linear induction accelerators (LIAs). One of the types of detectors used in BPMs is the B-dot loop, which generates a signal from the EMF due to the time varying magnetic flux through the loop. If some of the boundaries of the loop are composed of thick metal walls with finite conductivity, the resulting signal must be corrected for the magnetic field diffusion into the metal. The theoretically predicted flux due to diffusion is in remarkable agreement with experimental measurements. Although accurate BPM measurements of beam current require correction of magnetic field diffusion, accurate measurement of beam position requires no correction. In this note, we present experimental validation of current and position results from a prototype detector employing finite conductivity sensing areas, based on experiments on the DARHT-II LIA.

43 PARTICLE ACCELERATORS↗
DOE OSTI2023

Cathode Side-emission Mitigation for Linear Induction Accelerators

Linear induction accelerators (LIAs) are one of the main technologies used in creating flash x-rays used for diagnosing hydrodynamic experiments. The quality of the data produced in these experiments is dependent on the dose of x-rays generated via bremsstrahlung radiation. There are several electron beam quantities that effect the amount of dose extracted from the bremsstrahlung radiation, such as beam instabilities, corkscrew motion, and beam emittance. Injector design is key in delivering well-behaved electron beams to the accelerator, and design choices include components such as anode-cathode (AK) gap dimensions based on required voltage and current, shroud shaping, and cathode type and composition. Thermionic cathodes have shown reliable performance in electron emission, however, if enough of the cathode side is subject to high-enough electric field values, electrons can be emitted. This issue is more likely to happen when the cathode is treated with materials that have lower work functions, which is common for cathodes used in LIAs. These extra electrons can work their way through the rest of the beam, inducing high non-linearities in phase space which can increase the emittance of this beam. In this paper we outline conducted trade-space studies where we examined different shaping parameters of the cathode shroud design to decrease the effect side electrons may have on the beam emittance. Particle trajectories are determined by electric potential contours across the face of the cathode shroud component, and so we hypothesize that the proper shaping of the cathode-shroud interface can tune the electric field to mitigate the effect of side electrons. Electron beam transport is simulated down the length of the injector and the beam quality is diagnosed by studying the emittance as a function of position. Acceptable emittance increase is recorded and noted to meet radiographic requirements.

42 ENGINEERING↗
DOE OSTI2023

Reflections

Flash radiography is a much used diagnostic technique for high-explosively driven hydrodynamic experiments. Two linear-induction electron accelerators (LIAs) at the Los Alamos Dual Axis Radiography of Hydrodynamic Tests (DARHT) facility provide the source spots for point projection radiography of exceptionally large and dense experiments. In these LIAs, the beam acceleration is provided by high-voltage pulses applied to a number of inductively isolated gaps. The high-voltage pulses are produced by remotely located pulsedpower generators and delivered to the gaps by long highvoltage cables. For Scorpius, these pulse generators are called line replacement units (LRUs).

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
NASA NTRS1991

Direct numerical simulation and analysis of shock turbulence interaction

Two kinds of linear analysis, rapid distortion theory (RDT) and linear interaction analysis (LIA), were used to investigate the effects of a shock wave on turbulence. Direct numerical simulations of two-dimensional isotropic turbulence interaction with a normal shock were also performed. The results from RDT and LIA are in good agreement for weak shock waves, where the effects of shock front curvature and shock front unsteadiness are not significant in producing vorticity. The linear analyses predict wavenumber-dependent amplification of the upstream one-dimensional energy spectrum, leading to turbulence scale length scale decrease through the interaction. Instantaneous vorticity fields show that vortical structures are enhanced while they are compressed in the shock normal direction. Entrophy amplfication through the shock wave compares favorably with the results of linear analyses.

Lee, Sangsan↗
NASA NTRS2005

Investigating Long-term Behavior of Outlet Glaciers in Greenland

Repeat surveys by airborne laser altimetry in the 1990s have revealed significant thinning of outlet glaciers draining the interior of the Greenland Ice Sheet, with thinning rates up to several meters per year. To fully appreciate the significance of these recent glacier changes, the magnitude of retreat and surface lowering must be placed within the broader context of the retreat since the Last Glacial Maximum and, more significantly, of the retreat following the temporary glacier advance during the Little Ice Age (LIA). The LIA maximum stand is marked by trimlines, sharp boundaries between recently deglacifated unvegetated rocks, and vegetated surfaces at higher elevations. The objective of this project was to demonstrate the use of remote sensing data to map these trimlines and other glacial geomorphologic features.

Csatho, Beata↗
NASA NTRS2016

Larch Forests of Middle Siberia: Long-Term Trends in Fire Return Intervals

Fire history within the northern larch forests of Central Siberia was studied (65+degN). Fires within this area are predominantly caused by lightning strikes rather than human activity. Mean fire return intervals (FRIs) were found to be 112 +/- 49 years (based on firescars) and 106 +/- 36 years (based on firescars and tree natality dates). FRIs were increased with latitude increase and observed to be about 80 years at 64N, about 200 years near the Arctic Circle and about 300 years nearby the northern range limit of larch stands (approx.71+degN). Northward FRIs increase correlated with incoming solar radiation (r = -0.95). Post- Little Ice Age (LIA) warming (after 1850) caused approximately a doubling of fire events (in comparison with a similar period during LIA). The data obtained support a hypothesis of climate-induced fire frequency increase. Keywords Fire ecology Fire history Fire frequency Siberian wildfires Larch forests Climate change

Fire ecology↗
DOE OSTI2020

Steering an intense relativistic electron beam in a linear induction accelerator

The best electron beam transport through a linear induction accelerator (LIA) is achieved when the beam is well centered on the magnetic axis of the focusing solenoids. Since the beam may be injected offset from, or at an angle to, the centerline, dipole magnets are usually provided as a means to steer the beam and center it. Steering may be accomplished by trial and error, but this is very time consuming, especially for accelerators with a low repetition rate for beam position measurements and dipole adjustments. With this in mind, this article presents a steering method requiring a minimal number of measurements and adjustments to position the beam at any desired location, including positioning on the centerline.

47 OTHER INSTRUMENTATION↗
DOE OSTI2020

Scorpius and The Integrated Test Stand [Slides]

Scorpius will provide (DARHT scale) flash-radiography of dynamic nuclear material. DARHT radiographs non-nuclear (surrogate) materials at a cadence of 8 to 10 hydrodynamic shots per year. Scorpius will radiograph nuclear material at an initial cadence of about 2 to 3 subcritical shots per year in a more challenging environment. Both machines are Linear Induction Accelerators (LIAs) and generate X-rays with high-current, relativistic electron beams. Their use is termed Flash-Radiography.

45 MILITARY TECHNOLOGY, WEAPONRY, AND NATIONAL DEF↗
DOE OSTI2020

DARHT Axis 1 Performance Results with and without a Blumlein Charge Unit

The Dual-Axis Radiographic Hydrodynamic Test (DARHT) facility provides flash radiography capabilities using two electron Linear Induction Accelerators (LIA’s). Axis 1 of DARHT produces a 20- MeV, 2-kA, 80-ns-FWHM electron beam. As DARHT Axis 1 enters its twentieth year of operation, the probability of Blumlein Charge Unit (BCU) failure has increased and the risk of a failure on the day of an explosive experiment has become more likely. Replacement time for a BCU is four to six hours. This report examines the radiographic and beam transport effect of operating the accelerator with a missing or reduced charge voltage BCU. The results are compared with normal, full machine operation. A prescription for alternate operation with missing or reduced charge voltage on BCU22 is given.

47 OTHER INSTRUMENTATION↗
DOE OSTI2021

Re-Analysis of DARHT Axis 2 S4 Magnet Sweep Measurements

The Dual-Axis Radiographic Hydrodynamic Test (DARHT) facility provides flash radiography capabilities using two electron Linear Induction Accelerators (LIA’s). Understanding fundamental properties of the electron beams is essential to optimizing spot size and dose for radiography. This work describes distribution measurements of a single-kicked electron beam (16.5MeV, 1.7kA, 60ns) at a location downstream at imaging station C on DARHT Axis 2. The beam at station C is focused by the S4 solenoid located upstream. The beam distribution is measured by imaging optical transition radiation (OTR) from electrons striking a 51-µm thick titanium foil. This data and analysis were originally published by Ekdahl. The results here are a re-analysis of the asymmetric halo emittance contribution as compared to the emittance resulting from the FWHM of the distribution. This is accomplished using xtr2 fits to the data which include non-linear magnetic field effects in S4. Also examined are the calculated S4 spherical aberration contributions to the emittance using xtr.

43 PARTICLE ACCELERATORS↗
DOE OSTI2021

Estimating Beam-Target Heating

At Los Alamos National Laboratory, two high-current linear induction accelerators (LIAs) are used to produce bremsstrahlung source spots for flash radiography of high-explosive driven hydrodynamic experiments at the Dual Axis Radiographic Hydrodynamic Test (DARHT) facility. Measurements of the electron-beam current density profile are valuable for understanding the beam dynamics in order to improve the quality of the radiography source spot. A technique commonly used at DARHT is to image the profile in Cerenkov or Optical Transition Radiation (OTR) light created by the beam striking a thin target inserted into the beam line. Target materials include aluminized dielectrics and titanium foils for OTR, and fused silica wafers for Cerenkov radiation. A practical complication with this technique is heating of the target by the electron beam. If the beam density is too great, the target can be destroyed. Moreover, even if the beam density is kept low enough to be nondestructive, the beam can heat the target to a high enough temperature to desorb gas from the surface. In that case, direct impact ionization of the gas by beam electrons can partially neutralize the beam, causing it to over-focus, thereby spoiling the data, if not destroying the target. The purpose of this note is to review some of the fundamental physics of electron beam heating in order to provide some elementary guidance for design of these imaging experiments to avoid overheating the target. Some specific examples for materials that we often use for imaging targets and beam-target experiments are provided.

43 PARTICLE ACCELERATORS↗