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Optimized Spectrometer Design
Scientists at Lawrence Livermore National Laboratory collaborated with Princeton Plasma Physics Laboratory to design a novel x-ray crystal spectrometer. The device provides high-resolution measurements of a challenging feature of high-energy-density (HED) matter produced by National Ignition Facility (NIF) experiments. The work is featured in the May 10, 2021, online edition of Review of Scientific Instruments.
A spectrometer built earlier for NIF measures profiles of key parameters such as ion and electron temperatures in large volumes of hot plasmas magnetically confined in doughnut-shaped tokamak fusion devices to facilitate fusion reactions. NIF’s laser-produced HED plasmas, by contrast, are tiny, point-like substances. The newly developed multi-optics high-resolution absorption x-ray spectrometer (HiRAXS) uses crystals for extended x-ray absorption fine structure (EXAFS) experiments examining copper, tantalum, and lead. X-ray energy increases and signal noise decreases from copper to tantalum to lead, motivating efforts to optimize spectrometer design.
“Experiments at NIF that measure the EXAFS spectrum at high x-ray energies have had low signals,” says Marilyn Schneider, Livermore’s Radiative Properties group lead and a co-author of the paper. “The HiRAXS design concentrates the low signal and increases the signal-to-noise ratio while maintaining the high resolution required for observing EXAFS.” Co-author Yuan Ping adds, “The new design enables us to meet strict requirements for EXAFS measurements to prove the thermal state of highly compressed, high-atomic-number materials.”
Contact: Marilyn Schneider (925) 422-0725 (schneider5@llnl.gov).
Quantum Computing Stability
Livermore physicist Jonathan DuBois and collaborators proved that quantum computing errors are tied to cosmic rays. Understanding the cause of errors and how quantum circuits react to them is required to build a functioning quantum system and move closer to achieving the potential of quantum computing.
The team’s findings, published June 16, 2021, in Nature, indicated that fluctuations in the electrical charge of quantum bits (qubits) can be highly correlated rather than independent and random as assumed in earlier approaches to error correction. Bursts of energy from outside the system, such as cosmic rays, can affect nearby qubits by creating a blast of high-energy electrons, potentially heating the quantum device’s substrate and disrupting the qubits. When particle impact occurs, electrons scatter and produce high-energy vibrations and heat, further altering the electrical field and the thermal and vibrational environment around the qubits. As a result, correlated errors span the system and simultaneously affect performance.
To view the disruptions, researchers sent radio frequency signals into a four-qubit system. By measuring the excitation spectrum and performing spectroscopy, the team saw the qubits “flip” from one quantum state to another at the same time in response to changes in the charge environment. The team concluded that mitigation strategies must be developed to protect quantum systems from correlated errors due to cosmic rays and other particle impacts. Says DuBois, “We are focused on understanding the dynamics of these microscopic explosions and developing ways to absorb the energy before the delicate states inside quantum computing devices are disrupted.” Research partners included the University of Wisconsin-Madison, Fermi National Accelerator Laboratory, Google, Stanford University, Kavli Institute for Cosmological Physics, Institute of Nuclear Physics Sezione di Roma, and Sorbonne Université.
Contact: Jonathan DuBois (925) 422-1406 (dubois9@llnl.gov).
Evolution of Liquid and Solid Microjets
Lawrence Livermore National Laboratory scientists tested and confirmed predictions of a computational study investigating the effect of melting on shock-driven metal microjets. Livermore scientist David Bober sought to test a surprising trend reported in the 2020 simulations. “The computational study predicted that melting the base material might not always lead to a dramatic increase in the mass of material ejected from a surface feature, which goes against the conventional wisdom,” he says. Bober and Livermore scientists Kyle Mackay, Minta Akin, and Fady Najjar co-authored a manuscript detailing their experimental results in the July 2021 Journal of Applied Physics.
In the experiment, the team applied a fast-moving projectile to a tin plate, initiating a fluid-like jet of tin into the path of an intense x-ray beam. Images using x rays and an array of high-speed cameras enabled the team to calculate the jet’s mass and velocity, confirming the 2020 simulations. Colleagues at Argonne National Laboratory’s Advanced Photon Source Dynamic Compression Sector contributed to the success of the experiment.
The study of microjets supports research in broader jetting and ejecta processes that occur throughout condensed matter shock physics, from explosives to asteroid impact. Future shots have been planned to explore the phenomena. “We are on a path to improving ejecta models by detailing the physics that happen around the melt transition,” says Bober.
Contact: David Bober (925) 422-3725 (bober1@llnl.gov).
