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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↗

Beam Breakup Simulations for the Scorpius Flash-Radiography Accelerator

Beam breakup (BBU) in linear induction accelerators (LIAs) used for flash radiography is problematic because the high-frequency beam motion can blur the source spot, thereby degrading resolution. Amplification of BBU depends directly on details of cell design and is suppressed in operation by the solenoidal magnetic field focusing on the electron beam. Therefore, much effort has gone into design of the cells and magnetic focusing for the new Scorpius radiography LIA. Here, in this article, we use computer simulations to demonstrate that BBU in Scorpius should be no more than in present radiography LIAs at the Los Alamos National Laboratory.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

Time-Resolved Beam Position Measurements for the Scorpius Multipulse Linear Induction Accelerator

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 a theoretical derivation and the experimental validation of this result.

43 PARTICLE ACCELERATORS↗

Analysis Background & Noise in Stretched Wire Alignment Technique Measurements

The Stretched-Wire Alignment Technique (SWAT) is one method of magnet alignment for linear induction accelerators. The applications of SWAT have been implemented for aligning solenoid magnets on the Scorpius linear induction accelerator which will be sited at the Nevada National Security Site and the Flash X-Ray (FXR) linear induction accelerator at Lawrence Livermore National Laboratory’s Contained Firing Facility. This article describes both systematic (repeatable) and random sources of background and noise as well as practical ways to eliminate or reduce them to acceptable levels. Systematic sources include reflections from wire ends, rapid sag due to ohmic heating of the wire, magnetic materials, and shot rate. Random sources include air currents, vibration of nearby equipment, mechanical stability of test equipment, and the instruments used to measure the wire motion. Mitigations include curve fitting and adaptive noise signal cancellation, and mechanical damping. Finite Element Analysis (FEA) was used to identify and resolve a repeatable wire vibration frequency interfering with the signal resolution. Two stretched wire alignment technique set ups from Sandia National Labs and Lawrence Livermore National Lab have shown background noise sources and ways of mitigating them by either analysis methods or change of mechanical configuration. Conclusions that were drawn included the severe sensitivity of the deflection to even small external interferences of the SWAT wire such that it requires attention to detail in mechanical set up and analysis.

Linear Inductive Accelerator↗

Photoneutron Production Using an Electron Linear Accelerator for Applications in Neutron Imaging

Photoneutron production is possible using an electron linear accelerator and a target capable of generating photonuclear reactions. A short pulse neutron source can be useful for neutron imaging dynamic experiments. This study is aimed at the feasibility of photoneutron production using a 20 MeV electron linear accelerator and tungsten and depleted uranium targets of various thicknesses. MCNP6 (Monte Carlo N-Particle) code will be used to develop a computational model to estimate total neutron yield, and this will be verified at the Idaho State University’s Accelerator Center. After verification of the neutron yield and energy spectra, an additional MCNP6 model will be developed to analyze the neutron imaging processes. This study will potentially prove it is possible to conduct multi-mode imaging experiments on the anticipated Scorpius electron linear accelerator at the Nevada National Security Sites.

43 PARTICLE ACCELERATORS↗

Scorpius Beam Physics Overview and a Few Remaining Concerns [Slides]

With so many new faces on board, I thought to take the opportunity to review some of the many beam physics issues that might cause problems after Scorpius is built and accelerating electron beams. Mitigation measures for all of these have been incorporated in the Scorpius design, wherever possible. In this talk, I’ll quickly review the physics issues that I feel are most problematic for Scorpius, based on my own experience, and that documented by other workers in the field of linear induction accelerators.

43 PARTICLE ACCELERATORS↗

Cathode to Target Simulations for Scorpius: I. Simulation Codes and Models

The new Scorpius linear induction electron accelerator is under development for multi-pulse flash radiography of large, explosively-driven hydrodynamic experiments. Beam physics from the cathode to the target was examined with computer simulations, including envelope, ray-trace, and particle-in cell (PIC) codes. Beam instabilities investigated included beam breakup (BBU), image displacement, diocotron, parametric envelope, ion hose, and the resistive wall instability. Beam corkscrew motion and emittance growth from beam mismatch were also studied. The results of these simulations is documented in a series of reports. In this report the computer codes and physical models used for these simulations are described. The conclusion of this study is that Scorpius will produce and accelerate a beam with radiographic quality equivalent to the present accelerators at Los Alamos National Laboratory if the same engineering standards and construction details are upheld.

43 PARTICLE ACCELERATORS↗

Current-Pulse Excitation of Beam Breakup in Scorpius

Perhaps the most dangerous instability for electron linear induction accelerators (LIA) is the beam breakup (BBU) instability. For flash-radiography LIAs like DARHT or Scorpius it is particularly troublesome, because low-level high-frequency BBU motion can blur the source spot. Theoretically, the number of e-foldings Γ of exponential BBU growth in an LIA is linearly proportional to beam current I, number of accelerating cells N, and transverse coupling impedance Z ⊥ , and inversely proportional to the strength of magnetic focusing B.

43 PARTICLE ACCELERATORS↗

Scorpius Injector Solenoid Magnet Alignment and Characterization

The Scorpius project consists of a linear induction accelerator (LIA) that will be built in the coming few years at the Nevada test site. Large radius solenoid magnets are used to focus and transport the electron beam through the injector of Scorpius. Typically, solenoid magnets are wrapped directly around the beam pipe. They have an inner radius of about 8 cm and weight approximately 170 lbs. The magnets for the Scorpius injector are much larger, the largest of which extends to an inner radius of 45 cm with a weight of 709 lbs. In addition, the magnet measurement effort is typically time-consuming, taking several months to complete. With 5 different designs of 52 solenoid magnets to measure in under a year, this is a challenging undertaking that requires a systems solution. Due to the size, variability, and quantity of solenoid magnets, as well as the compressed schedule, the process of measuring the solenoid magnets before integrating them into their respective assemblies and shipping them to Nevada is a challenging problem. There is a need for the injector magnets to be characterized and aligned in preparation for the commissioning of Scorpius. This will ensure that the injector magnet installation meets requirements.

42 ENGINEERING↗

Multi-Probe Radiography: For 2035 and Beyond

The Multi-Probe Radiography Project is investigating the use of multiple species of probe beams to provide more information from a radiography. Contextual radiographic, source, and measurements information is being provided through a series of four workshops at Los Alamos National Laboratory. The third workshop on Accelerator and Pulsed Power Technology is summarized in this report. The state of the large NNSA facilities such as Scorpius, DARHT, pRad, and Cygnus are presented. Future enhancements and innovative upgrades are also discussed.

43 PARTICLE ACCELERATORS↗