A Simple Modification of DC Current Septa to Reduce Current Density by Half

Circular accelerators typically have one injection and one extraction septum magnet. CEBAF is a recircuating electron linac which has a total of 27 DC current septa and one Lambertson. Current densities range from 28-48 A/mm 2 . Current sheet widths are 5-24 mm, turns count 5-24 and lengths 1000-3000 mm. A design exercise to increase the beam energy to 22 GeV is underway. Since doubling the current density in copper is not practical in the CEBAF layout a conductively cooled superconducting septum concept was examined. Putting the current sheet and its cryostat between the poles as in a standard current sheet septum would have required 90 mm pole gap. The poles were brought to 40 mm separation and the steel notched for the 90 mm cryostat. The field in the bore increased while the field outside the current sheet remained close to zero as in a conventional septum. Required current density dropped enough that a copper coil became possible. Two examples will be shown, the one discussed above and the modification of the 3000 mm septum with 0.92 T bore field.Comment: 10 pages, 11 figures, Prepared for submission to JINS

Simultaneous Optimization of the Cavity Heat Load and Trip Rates in Linacs Using a Genetic Algorithm

In this paper, a genetic algorithm-based optimization is used to simultaneously minimize two competing objectives guiding the operation of the Jefferson Lab\u27s Continuous Electron Beam Accelerator Facility linacs: cavity heat load and radio frequency cavity trip rates. The results represent a significant improvement to the standard linac energy management tool and thereby could lead to a more efficient Continuous Electron Beam Accelerator Facility configuration. This study also serves as a proof of principle of how a genetic algorithm can be used for optimizing other linac-based machines

Origin of Complexity in Hemoglobin Evolution

Most proteins associate into multimeric complexes with specific architectures, which often have functional properties such as cooperative ligand binding or allosteric regulation. No detailed knowledge is available about how any multimer and its functions arose during evolution. Here we use ancestral protein reconstruction and biophysical assays to elucidate the origins of vertebrate hemoglobin, a heterotetramer of paralogous α- and β-subunits that mediates respiratory oxygen transport and exchange by cooperatively binding oxygen with moderate affinity. We show that modern hemoglobin evolved from an ancient monomer and characterize the historical “missing link” through which the modern tetramer evolved—a noncooperative homodimer with high oxygen affinity that existed before the gene duplication that generated distinct α- and β-subunits. Reintroducing just two post-duplication historical substitutions into the ancestral protein is sufficient to cause strong tetramerization by creating favorable contacts with more ancient residues on the opposing subunit. These surface substitutions markedly reduce oxygen affinity and even confer cooperativity because an ancient linkage between the oxygen binding site and the multimerization interface was already an intrinsic feature of the protein’s structure. Our findings establish that evolution can produce new complex molecular structures and functions via simple genetic mechanisms that recruit existing biophysical features into higher-level architectures. The interfaces that hold molecular complexes together typically involve sterically tight, electrostatically complementary interactions among many amino acids. Similarly, allostery and cooperativity usually depend on numerous residues that connect surfaces to active sites. The acquisition of such complicated machinery would seem to require elaborate evolutionary pathways. The classical explanation of this process, by analogy to the evolution of morphological complexity, is that multimerization conferred or enhanced beneficial functions, allowing selection to drive the many substitutions required to build and optimize new interfaces. Whether this account accurately describes the evolution of any natural molecular complex requires a detailed reconstruction of the historical steps by which it evolved. Hemoglobin (Hb) is a useful model for this purpose, because the structural mechanisms that mediate its multimeric assembly, cooperative oxygen binding, and allosteric regulation are well established. Moreover, its subunits descend by duplication and divergence from the same ancestral proteins, so their history can be reconstructed in a single analysis. Despite considerable speculation, virtually nothing is known about the evolutionary origin of Hb’s heterotetrameric architecture and the functions that depend on it

The Solenoidal Large Intensity Device (SoLID) for JLab 12 GeV

The Solenoidal Large Intensity Device (SoLID) is a new experimental apparatus planned for Hall A at the Thomas Jefferson National Accelerator Facility (JLab). SoLID will combine large angular and momentum acceptance with the capability to handle very high data rates at high luminosity. With a slate of approved high-impact physics experiments, SoLID will push JLab to a new limit at the QCD intensity frontier that will exploit the full potential of its 12 GeV electron beam. In this paper, we present an overview of the rich physics program that can be realized with SoLID, which encompasses the tomography of the nucleon in 3-D momentum space from Semi-Inclusive Deep Inelastic Scattering (SIDIS), expanding the phase space in the search for new physics and novel hadronic effects in parity-violating DIS (PVDIS), a precision measurement of $J/\psi$ production at threshold that probes the gluon field and its contribution to the proton mass, tomography of the nucleon in combined coordinate and momentum space with deep exclusive reactions, and more. To meet the challenging requirements, the design of SoLID described here takes full advantage of recent progress in detector, data acquisition and computing technologies. In addition, we outline potential experiments beyond the currently approved program and discuss the physics that could be explored should upgrades of CEBAF become a reality in the future.Comment: This white paper for the SoLID program at Jefferson Lab was prepared in part as an input to the 2023 NSAC Long Range Planning exercise. To be submitted to J. Phys.

Dark sectors 2016 Workshop: community report

This report, based on the Dark Sectors workshop at SLAC in April 2016, summarizes the scientific importance of searches for dark sector dark matter and forces at masses beneath the weak-scale, the status of this broad international field, the important milestones motivating future exploration, and promising experimental opportunities to reach these milestones over the next 5-10 years

Preliminary Statistical Analysis of CEBAF's Cavity Pair Assembly Process Data

CEBAF has been collecting much data during the cavity pair assembly process. Some process data has been entered and analyzed during the last two years as part of our attempt to apply statistical process control methods. Analysis is presented here on mechanical tolerances achieved by the industrial fabricator of the CEBAF superconducting rf cavities (Siemens). Suggestions for tolerances obtainable in future procurements are made. Influence of cooldown conditions during vertical test on field emission onset gradient is discussed. An increase in the mean gradient of 2 MV/m was seen after a simple change in procedure