arXiv subjects

T. Allison

Publications and source records attributed to T. Allison.

4 recordsLinked to original sources

arXiv2024

The Continuous Electron Beam Accelerator Facility at 12 GeV

This review paper describes the energy-upgraded CEBAF accelerator. This superconducting linac has achieved 12 GeV beam energy by adding 11 new high-performance cryomodules containing eighty-eight superconducting cavities that have operated CW at an average accelerating gradient of 20 MV/m. After reviewing the attributes and performance of the previous 6 GeV CEBAF accelerator, we discuss the upgraded CEBAF accelerator system in detail with particular attention paid to the new beam acceleration systems. In addition to doubling the acceleration in each linac, the upgrade included improving the beam recirculation magnets, adding more helium cooling capacity to allow the newly installed modules to run cold, adding a new experimental hall, and improving numerous other accelerator components. We review several of the techniques deployed to operate and analyze the accelerator performance, and document system operating experience and performance. In the final portion of the document, we present much of the current planning regarding projects to improve accelerator performance and enhance operating margins, and our plans for ensuring CEBAF operates reliably into the future. For the benefit of potential users of CEBAF, the performance and quality measures for beam delivered to each of the experimental halls is summarized in the appendix.

physics.acc-ph↗

arXiv2017

Spatiotemporal modelling of sea duck abundance: implications for marine spatial planning

Effective marine spatial plans require information on the distribution and abundance of biological resources that are potentially vulnerable to anthropogenic change. In North America, spatially-explicit abundance estimates of marine birds are necessary to assess potential impacts from planned offshore wind energy developments (OWED). Sea ducks are particularly relevant in this context as populations of most North American species are below historic levels and European studies suggest OWEDs. We modelled species occupancy using a generalized additive model and conditional abundance with generalized additive models for location, scale, and shape; the models were subsequently combined to estimate unconditional abundance. We demonstrate this flexible, model-based approach using sea ducks (Common Eider [Somateria mollissima], Black Scoter [Melanitta americana], Surf Scoter [M. perspicillata], White-winged Scoter [M. deglandi], and Long-tailed Duck [Clangula hyemalis]) in Nantucket Sound, Massachusetts, USA, which supports some of the largest concentrations of wintering sea ducks in eastern North America and where a 454-MW OWED is proposed. Our approach to species distribution and abundance modelling offers several useful features including (1) the ability to model all conditional distribution parameters as a function of covariates, (2) integrated variable reduction and selection among many covariates, (3) integrated model selection, and (4) the efficient incorporation of smooth effects to capture spatiotemporal trends poorly explained by other covariates. This modelling approach should prove useful for marine spatial planners in siting OWEDs while considering key habitats and areas potentially vulnerable to anthropogenic stressors. Moreover, the approach is equally suitable for terrestrial or aquatic systems.

stat.AP↗

arXiv2015

The Q_weak Experimental Apparatus

The Jefferson Lab Q_weak experiment determined the weak charge of the proton by measuring the parity-violating elastic scattering asymmetry of longitudinally polarized electrons from an unpolarized liquid hydrogen target at small momentum transfer. A custom apparatus was designed for this experiment to meet the technical challenges presented by the smallest and most precise ${\vec{e}}$p asymmetry ever measured. Technical milestones were achieved at Jefferson Lab in target power, beam current, beam helicity reversal rate, polarimetry, detected rates, and control of helicity-correlated beam properties. The experiment employed 180 microA of 89% longitudinally polarized electrons whose helicity was reversed 960 times per second. The electrons were accelerated to 1.16 GeV and directed to a beamline with extensive instrumentation to measure helicity-correlated beam properties that can induce false asymmetries. Moller and Compton polarimetry were used to measure the electron beam polarization to better than 1%. The electron beam was incident on a 34.4 cm liquid hydrogen target. After passing through a triple collimator system, scattered electrons between 5.8 degrees and 11.6 degrees were bent in the toroidal magnetic field of a resistive copper-coil magnet. The electrons inside this acceptance were focused onto eight fused silica Cerenkov detectors arrayed symmetrically around the beam axis. A total scattered electron rate of about 7 GHz was incident on the detector array. The detectors were read out in integrating mode by custom-built low-noise pre-amplifiers and 18-bit sampling ADC modules. The momentum transfer Q^2 = 0.025 GeV^2 was determined using dedicated low-current (~100 pA) measurements with a set of drift chambers before (and a set of drift chambers and trigger scintillation counters after) the toroidal magnet.

physics.ins-det↗

arXiv2001

Versatile Data Acquisition and Controls for Epics Using Vme-Based Fpgas

Field-Programmable Gate Arrays (FPGAs) have provided Thomas Jefferson National Accelerator Facility (Jefferson Lab) with versatile VME-based data acquisition and control interfaces with minimal development times. FPGA designs have been used to interface to VME and provide control logic for numerous systems. The building blocks of these logic designs can be tailored to the individual needs of each system and provide system operators with read-backs and controls via a VME interface to an EPICS based computer. This versatility allows the system developer to choose components and define operating parameters and options that are not readily available commercially. Jefferson Lab has begun developing standard FPGA libraries that result in quick turn around times and inexpensive designs.

cs.AR↗