P. L. Walstrom

• PhD in physics
• Los Alamos National Laboratory

The past two decades have yielded several new measurements and reanalysis of older measurements of the neutron lifetime. These have led to a 4.4 standard deviation discrepancy between the most precise measurements of the neutron decay rate producing protons in cold neutron beams and the most precise lifetime measured in neutron storage experiments....

The past two decades have yielded several new measurements and reanalysis of older measurements of the neutron lifetime. These have led to a 4.4 standard deviation discrepancy between the most precise measurements of the neutron decay rate producing protons in cold neutron beams and the most precise lifetime measured in neutron storage experiments....

The past two decades have yielded several new measurements and reanalyses of older measurements of the neutron lifetime. These have led to a 4.4 standard deviation discrepancy between the most precise measurements of the neutron decay rate producing protons in cold neutron beams and the lifetime measured in neutron storage experiments. Measurements...

We report an improved measurement of the free neutron lifetime τ_n using the UCNτ apparatus at the Los Alamos Neutron Science Center. We count a total of approximately 38 × 10⁶ surviving ultracold neutrons (UCNs) after storing in UCNτ’s magnetogravitational trap over two data acquisition campaigns in 2017 and 2018. We extract τ_n from three blinded...

We report an improved measurement of the free neutron lifetime $\tau_{n}$ using the UCN$\tau$ apparatus at the Los Alamos Neutron Science Center. We counted a total of approximately $38\times10^{6}$ surviving ultracold neutrons (UCN) after storing in UCN$\tau$'s magneto-gravitational trap over two data acquisition campaigns in 2017 and 2018. We ext...

Charged particle radiography with 800 MeV protons has been used for decades at LANL and developed around the world to study dynamic material properties. Recently, charged particle radiography has been demonstrated with the use of high-energy electrons. Because of the difference in the mass of the electron compared to the mass of the proton, the rad...

The neutron is the simplest nuclear system that can be used to probe the structure of the weak interaction and search for physics beyond the standard model. Measurements of neutron lifetime and β-decay correlation coefficients with precisions of 0.02% and 0.1%, respectively, would allow for stringent constraints on new physics. The UCNτ experiment...

In the UCNτ experiment, ultracold neutrons (UCN) are confined by magnetic fields and the Earth's gravitational field. Field-trapping mitigates the problem of UCN loss on material surfaces, which caused the largest correction in prior neutron experiments using material bottles. However, the neutron dynamics in field traps differ qualitatively from t...

A new boron-coated CCD camera is described for direct detection of ultracold neutrons (UCN) through the capture reactions $^{10}$B (n,$\alpha$0$\gamma$)$^7$Li (6%) and $^{10}$B(n,$\alpha$1$\gamma$)$^7$Li (94%). The experiments, which extend earlier works using a boron-coated ZnS:Ag scintillator, are based on direct detections of the neutron-capture...

The neutron is the simplest nuclear system that can be used to probe the structure of the weak interaction and search for physics beyond the standard model. Measurements of neutron lifetime and β-decay correlation coefficients with precisions of 0.02% and 0.1%, respectively, would allow for stringent constraints on new physics. The UCNτ experiment...

In the UCN{\tau} experiment, ultracold neutrons (UCN) are confined by magnetic fields and the Earth's gravitational field. Field-trapping mitigates the problem of UCN loss on material surfaces, which caused the largest correction in prior neutron experiments using material bottles. However, the neutron dynamics in field traps differ qualitatively f...

High energy electrons have been used to investigate an extension of transmission electron microscopy. This technique, transmission high energy electron microscopy (THEEM), provides two additional capabilities to electron microscopy. First, high energy electrons are more penetrating than low energy electrons, and thus, they are able to image through...

How long does a neutron live? Unlike the proton, whose lifetime is longer than the age of the universe, a free neutron decays with a lifetime of about 15 minutes. Measuring the exact lifetime of neutrons is surprisingly tricky; putting them in a container and monitoring their decay can lead to errors because some neutrons will be lost owing to inte...

The precise value of the mean neutron lifetime, $\tau_n$, plays an important role in nuclear and particle physics and cosmology. It is a key input for predicting the ratio of protons to helium atoms in the primordial universe and is used to search for new physics beyond the Standard Model of particle physics. There is a 3.9 standard deviation discr...

We have followed up on our previous measurements of upscattering of ultracold neutrons (UCNs) from a series of gases by making measurements of total cross sections on the following gases hydrogen, ethane, methane, isobutene, n-butane, ethylene, water vapor, propane, neopentane, isopropyl alcohol, and ^3He. The values of these cross sections are imp...

Charged particle radiography has been developed in the past decade to provide high-resolution, muti-frame flash radiography of dynamic systems. This development has focused on protonradiography utilizing 11 MeV to 50 GeV protons for a wide range of measurements. Recently, these techniques are being applied to the use of high energy electrons for ap...

The neutron lifetime is important in understanding the production of light nuclei in the first minutes after the big bang and it provides basic information on the charged weak current of the standard model of particle physics. Two different methods have been used to measure the neutron lifetime: disappearance measurements using bottled ultracold ne...

The neutron lifetime is important in understanding the production of light nuclei in the first minutes after the big bang and it provides basic information on the charged weak current of the standard model of particle physics. Two different methods have been used to measure the neutron lifetime: disappearance measurements using bottled ultracold ne...

We present measurements of the upscattering cross sections of ultracold neutrons (UCNs) from room-temperature hydrogen, deuterium, neon, argon, xenon, C4H10, CF4, and air. The values of these cross sections are important for estimating the loss rate of trapped neutrons due to residual gas and are therefore of importance for neutron lifetime measure...

An investigation into the appropriate materials for use as a diffuser foil in electron radiography was undertaken in GEANT4. Simulations were run using various refractory materials to determine a material of appropriate Z number such that energy loss is minimal. The plotted results of angular spread and energy spread are shown. It is concluded that...

The UCN$\tau$ experiment is designed to measure the lifetime $\tau_{n}$ of the free neutron by trapping ultracold neutrons (UCN) in a magneto-gravitational trap. An asymmetric bowl-shaped NdFeB magnet Halbach array confines low-field-seeking UCN within the apparatus, and a set of electromagnetic coils in a toroidal geometry provide a background "ho...

In general, generation of charged-particle transfer maps for conventional iron-pole-piece dipole magnets to third and higher order requires a model for the midplane field profile and its transverse derivatives (soft-edge model) to high order and numerical integration of map coefficients. An exact treatment of the problem for a particular magnet req...

A prerequisite to studying ultracold chemistry is achieving a sufficient collision frequency as facilitated by creating a high number density. In this poster we show that some of the basic concepts and components of particle injectors and accumulators used in accelerators can be exploited to generate dense ensembles of ultracold magnetic particles,...

A conceptual design of a 3-GeV linac upgrade that would enable enhanced proton radiography at the Los Alamos Neutron Science Center (LANSCE) is presented. The upgrade is based on the use of superconducting accelerating cavities to increase the present LANSCE linac output energy from 800 MeV to 3 GeV. The LANSCE linac currently provides negative hyd...

A large class of cold neutral atoms and molecules is subject to magneticfield-gradient forces. In the presence of a field, hyperfine atomic states are split intoseveral Zeeman levels. The slopes of these curves vs. field are the effective magneticmoments. By means of optical pumping in a field, Zeeman states of neutral lithium atoms andCaH molecule...

The feasibility of D-3He reactor plasma conditions in a tokamak of the NET/INTOR class is investigated. It is found that, depending on the energy confinement scaling law, energy breakeven can be achieved in NET without significant modification of its design. Significant improvement in Q (ratio of fusion power to injected power) can be achieved by r...

Three-dimensional field distributions from realistic beamline elements can be obtained only by measurement or by numerical solution of a boundary-value problem. In numerical charged-particle map generation, fields along a reference trajectory are differentiated multiple times. Any attempt to differentiate directly such field data multiple times is...

We discuss an innovative low-dose approach for detecting shielded strategic nuclear materials (SNM) based on measuring the energy-loss of energetic protons penetrating an object.

We present a conceptual design for an experiment to measure the neutron lifetime with a relative accuracy of 10−4. The measurement will be performed by observing the decay rate of a sample of ultra-cold neutrons (UCN) confined in vacuum in a magneto-gravitational trap. The UCN sources under development are expected to produce stored UCN densities o...

We describe some of the accomplishments of the Beam Dynamics portion of the SciDAC Accelerator Science and Technology project. During the course of the project, our beam dynamics software has evolved from the era of different codes for each physical effect to the era of hybrid codes combining start-of-the-art implementations for multiple physical e...

SciDAC has had a major impact on computational beam dynamics and the design of particle accelerators. Particle accelerators—which account for half of the facilities in the DOE Office of Science Facilities for the Future of Science 20 Year Outlook—are crucial for US scientific, industrial, and economic competitiveness. Thanks to SciDAC, accelerator...

A deterministic forward model that includes the essential physics of all of the elements of the radiographic chain is required to extract volume density distributions and material interface surface positions from proton transmission radiographs by iterative non-linear fitting of parametric forward models. An essential part of the forward model is a...

With the introduction of continuous-energy heavy charged particle transport in MCNP5, the need for tracking charged particles in a magnetic field becomes increasingly important. Two methods for including magnetic field effects on charged particles are included in the proton transport version of the code. The first technique utilises transfer maps p...

Continuously varying surface and volume source-density distributions are used to model magnetic fields inside of cylindrical volumes. From these distributions, a package of subroutines computes on-axis generalized gradients and their derivatives at arbitrary points on the magnet axis for input to the numerical map-generating subroutines of the Lie-...

The recently completed 100-MeV H{sup +} Isotope Production Facility (IPF) at the LANSCE will provide radioisotopes for medical research and diagnosis, for basic research and for commercial use. A change to the LANSCE accelerator facility allowed for the installation of the IPF. Three components make up the LANSCE accelerator: an injector that accel...

The Advanced Hydrotest Facility (AHF), under study by LANL, utilizes large-bore superconducting quadrupole magnets to image protons for radiography of fast events. The lens system uses two types of quadrupoles: a large bore (48-cm beam aperture) for wide field of view imaging and a smaller bore (23 cm aperture) for higher resolution images. The gra...

The Advanced Hydrotest Facility, under study by LANL, uses large-bore superconducting quadrupole magnets. In the paper we discuss the conceptual design of such quadrupoles using active shielding. The magnets are specified to achieve gradients of up to 24 T/m with a 28-cm warm bore and to have 0.01% field quality. Concepts for quench protection and...

Performance requirements, design concepts, and test results for a prototype multipulse kicker modulator based on solid-state switches and a voltage-adding transformer topology are described. Tape-wound cores are stacked to form the transformer primary windings and a cylindrical pipe that passes through the circular inner diameters of the cores serv...

The ability to extract particle beam bunches from a ring accelerator in arbitrary order can greatly extend an accelerator's capabilities and applications. A prototype solid-state kicker pulser capable of generating asynchronous bursts of 50 kV pulses has been designed and tested into a 50 Ω load. The pulser features fast rise and fall times and is...

In order to exploit the full potential of the Advanced Hydrodynamic Facility (AHF) facility to produce a time sequence of proton transmission radiographs throughout the dynamic event, a kicker/modulator for extraction from the 50 GeV ring that is capable of generating a string of 25 pulse pairs at arbitrary times within a total time duration of 100...

In order to exploit the full potential of the Advanced Hydrotest Facility (AHF) facility to produce a time sequence of proton transmission radiographs throughout the dynamic event, a kicker/modulator for extraction from the 50 GeV ring that is capable of generating a string of 25 pulse pairs at arbitrary times within a total time duration of 100 mi...

The passage of a charged particle through a region of nonvanishing electromagnetic fields (e.g., a bending magnet, multipole magnet, spectrometer, electrostatic lens, electromagnetic velocity separator, etc.) can be described by a transfer map. In the magnetic case, computation of this map in canonical coordinates requires a knowledge of the vector...

The Advanced Hydrotest Facility is to provide a time sequence of multiple radiographic images using a proton beam produced by a 50-GeV synchrotron. We give an overview of the approaches devised to produce up to twelve simultaneous radiographic images of an object by a sequence of beam-transport lines. The required distribution system has undergone...

In this paper, we discuss the specific issues related to the design of the fast kicker systems for high intensity proton accelerators. To address these issues in the preliminary design stage can be critical since the fast kicker systems affect the machine lattice structure and overall design parameters. Main topics include system architecture, desi...

In this paper, we will discuss the specific issues related to the design of the Fast Kicker Systems for high intensity proton accelerators. To address these issues in the preliminary design stage can be critical since the fast kicker systems affect the machine lattice structure and overall design parameters. Main topics will include system architec...

This article reviews the status of an experiment to study the neutron spin-electron angular correlation with the Los Alamos Ultra-Cold Neutron (UCN) source. The experiment will generate UCNs from a novel solid deuterium, spallation source, and polarize them in a solenoid magnetic field. The experiment spectrometer will consist of a neutron decay re...

The inductance of the vacuum chamber of the Proton Storage Ring at Los Alamos National Laboratory was intentionally increased by the introduction of ferrite rings to counteract the longitudinal space-charge effect of the intense beam. The magnetic permeability of the ferrite could be adjusted by introducing current into solenoids wound around the f...

With the nuclear weapons program moving to Science Based Stockpile Stewardship (SBSS), new diagnostic techniques are needed to replace weapons testing. Proton Radiography is being developed within the SBSS program as one such tool. It is analogous to transmission X-ray radiography, but uses protons instead of photons. Proton Radiography has high pe...

Two prototype nonlinear multipole magnets have been designed for use in the 800-MeV beam test of the APT beam-expansion concept at LANSCE. The iron-dominated magnets each consist of three independent coils, two for producing a predominately octupole field with a tunable duodecapole component, and one for canceling the residual quadrupole field. Two...

An upgrade program for increasing the stored beam current in the proton storage ring is presently under way. A part of the upgrade is the design and installation of a four-magnet beam-bumping system used for phase-space painting and minimizing interaction of the stored beam with the injection stripper foil. This paper describes the bump-magnet syst...

In proton radiography, high-performance magnetic quadrupole lenses are required for point-to-point imaging of the proton-illuminated object on to the detector plane. In order to reduce chromatic aberrations, the length of the imaging system must be minimized. This requirement, along with the large field of view needed, leads to use of large-bore, r...

This is the final report of a one-year, Laboratory Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). The North Port Target Facility proposal includes a 100-MeV beam line to be built at the LANSCE accelerator. In developing cost and schedule estimates for this proposal, the greatest uncertainties are asso...

A 50 GeV accelerator complex for dynamic proton radiography, including a linac, synchrotron, and multiple isochronous beamlines is described, and critical technology development is outlined

An upgrade program for increasing the stored beam current in the Proton Storage Ring is presently under way. A part of the upgrade is the design and installation of a four-magnet beam-bumping system used for phase-space painting and minimizing interaction of the stored beam with the injection stripper foil. This paper describes the bump-magnet syst...

Multi-turn injection of H^- by foil stripping into an accumulator ring produces metastable neutrals that contribute to machine activation. Placing the foil in a magnetic field has been shown to greatly reduce such losses. However, the electrons associated with the H ^- repeatedly and destructively pass through the foil under the...

The dipole-sheet formalism can be used to describe both currents on arbitrary surfaces and magnets made of magnetized material. For current sheets, a fictitious normal dipole density corresponds to the stream function for currents. Currents derived in this way are automatically divergence-free. The stream function provides a natural way of finding...

This first volume of the five volume set begins with a CPDR overview and then details the PF magnet system, manufacturing R&D, Westinghouse R&D, the central solenoid, the PF 5 ring coil, the PF 6/7 ring coil, quality assurance, and the system design description.

The TPX magnet preliminary design effort is summarized. Key results and accomplishments during preliminary design and supporting R&D are discussed, including conductor development, quench detection, TF and PF magnet design, conductor bending and forming, reaction heat treating, helium stubs, and winding pack insulation

The Westinghouse team has extended the Lawrence Livermore National Laboratory advanced conceptual design for the TPX PF magnets through preliminary design. This is the first time superconducting PF magnets have been designed for application in a tokamak. Particular challenges were encountered and solved in developing the coil insulation system, wel...

The dipole-sheet formalism can be used to describe both cylindrical current-sheet multipole magnets and cylindrical-bore magnets made up of permanent magnet blocks. For current sheets, the formalism provides a natural way of finding a finite set of turns that approximate a continuous distribution. The formalism is especially useful in accelerator a...

The Superconducting RF Cavity Development Program at Los Alamos has designed, fabricated, and tested single-cell niobium cavities at 3 GHz and 805 MHz. This work is being done in preparation for procuring and testing a multicell niobium cavity. The multicell cavity is designed to accelerate protons at β=0.9; initial tests will be without beam. Prog...

A compact high-field superconducting quadrupole magnet was designed and built with poles made of the rare-earth metal holmium. The magnet is intended for use in superconducting coupled-cavity linear accelerators where compact high-field quadrupoles are needed, but where the use of permanent magnets is ruled out because of trapped-flux losses. The m...

We have developed a new capability to compute third and fifth order Lie algebraic transfer maps for a family of realistic multipole magnets, including dipoles. The general Hamiltonian is expanded symbolically to arbitrary order. The vector potential off axis, for a given multipole symmetry, is determined from the appropriate magnetic field gradient...

In this paper, we present a review of a workshop on small-period wiggler and undulator designs held at Los Alamos National Laboratory on April 13, 1989.^The wiggler designs are based on the following mechanisms: microwave fields, electromagnetic coils, miniature permanent magnets, current sheets, superconductive coils, and iron-free pulsed wire.^9...

Steady-state temperature profiles in turbulent He II forced flow in a heated tube in the temperature range 1.6–2.0 K are measured and compared with calculated profiles based on the two-fluid model with vortex lines and mutual friction. To make measured and calculated profiles agree, a vortex line density component that is created by normal fluid tu...

Thermocouples are probably the most widely used temperature sensors in science and industry but perhaps also the most often misunderstood. The basic distinction between an ordinary resistance thermometer, in which the sensing element is localized, and a thermocouple circuit, in which potentials vary along the wires themselves, is most clearly shown...

The feasibility of investigating D-He3 reactor plasma conditions in a tokamak of the NET/INTOR class was investigated. Depending on the energy confinement scaling law, energy breakeven may be achieved without significant modification to the NET design. The best results are for the more optimistic ASDEX H-mode scaling law. Kaye-Goldston scaling with...

The temperature rise in highly turbulent He II flowing in tubing was measured in the temperature range 1.6-2.1 K. The effect of internal convection heat transport on the predicted temperature profiles is calculated from the two-fluid model with mutual friction. The model predictions are in good agreement with the measurements, provided that the pre...

Pressure drop experiments in highly turbulent He II flow were performed in flow loops driven by either a centrifugal pump or a single-stroke bellows pump. Pressure drops in straight tubing, coiled tubing, bellows sections, valves and Venturi flow meters were measured over a range of flow rates and temperatures. Our pressure drop data are in general...

Pressure drops in highly turbulent He II flow were measured in round tubes, valves, and Venturi flowmeters. Results are in good agreement with single-phase flow correlations for classical fluids. The temperature rise in flow in a round tube was measured, and found to agree well with predictions for isenthalpic expansion. Cavitation was observed in...

The conceptual design of the Space Orbiting Advanced Fusion Power Reactor (SOAR) achieved a specific mass of approximately 0.5 kg/kWe at an electric power level of 1000 MWe. That design is summarized and scaling the power output in order to assess alternative applications of the concept is examined. Critical issues that arose in the first phase of...

Twisted coils, ie., coils for which windings do not lie in a plane, are used widely in plasma confinement devices, especially those in the general class of stellerator-type devices. In the design of these devicesad hoc expressions for the positions of turns and the outer envelope of the windings are usually assumed. However, these expressions do no...

Siemens KPY pressure sensors are evaluated for use at liquid helium temperatures. Sensor output for KPY-12 (2 bar range) sensors is ≈2.5 times greater at liquid helium temperature than at room temperature. Sensor output sensitivity varies by more than +-20% from sensor to sensor at both room and liquid helium temperatures. Sensor calibration in the...

Radiation effects in superconducting magnets of fusion reactors are analyzed and experimental data reviewed. The interaction between the different radiation effects and impact on reactor design is discussed. The need for experimental data with higher irradiation levels is assessed.

The techniques of differential geometry have been applied to the problem of predicting the shape of thick twisted coil windings as successive turns and layers of turns are applied to a winding form. The explicit expressions for the required Christoffel symbols for parallel surfaces are derived in terms of the starting surface parameterization. Expr...

Radiation effects in superconducting magnets of fusion reactors are analyzed and experimental data reviewed. The interaction between the different radiation effects and impact on reactor design is discussed. The need for experimental data with higher irradiation levels is assessed.

The techniques of differential geometry have been applied to the problem of predicting the shape of thick twisted coil windings as successive turns and layers of turns are applied to a winding form. The explicit expressions for the required Christoffel symbols for parallel surfaces are derived in terms of the starting surface parameterization. Expr...

The radiation limits for fusion reactor magnets have a direct impact on the cost of electricity. For example, reducing the inboard shield by 1 cm saves up to $3 million in the Tokamak Fusion Core Experiment cost. The magnet components most sensitive to radiation damage are the superconductor, stabilizer, and insulators. Nuclear heating in the magne...

The Large Coil Task (LCT) is a collaboration between the US, Euratom, Japan, and Switzerland for the production and testing of 2.5 × 3.5 m bore, superconducting 8-T magnets. The definitive tests in the design configuration, the six coils arrayed in a compact torus, will begin in 1985. Partial-array tests are being done in 1984. In January the initi...

The superconducting toroidal field coils that are being tested in the Large Coil Test Facility (LCTF) are heavily instrumented. General Electric coil, a lead wire of an internal sensor became shorted across an estimated three or four turns of the pancake winding. This short occurred during the final stages of the winding fabrication and was not acc...

A unique quench detection system was designed for the International Fusion Superconducting Magnet Test Facility (IFSMTF), where a simultaneous test of six large superconducting toroidal field magnets will be carried out. The scheme was based on analog subtraction of self and neighboring pickup winding voltage from the coil voltage to yield a compen...

Two types of heat sinks for cryogenic applications have been made with commercial technology. Both are described and analysed.

When a superconducting magnet with a shorted turn or turns is rapidly discharged, the changing magnetic flux can drive large currents through the short. These currents can lead to local temperature rises, to melting of the conductor and failure of insulation, and finally to destruction of the magnet. It is, therefore, highly desirable to detect and...

The present data show that the worst case steady-state stability in the GE/LCT magnet windings is at a horizontal conductor orientation. The heat transfer improves with inclination of the conductor from horizontal. Calculations show that for these small regions normal zones will recover by cold-end conduction from the inclined conductor on either e...

The design of a cryogenic helium pump for circulating liquid helium in a magnet and the design of a test loop for measuring the pump performance in terms of mass flow vs pump head at various pump speeds are described. A commercial cryogenic helium pump was tested successfully. Despite flaws in the demountable connections, the piston pump itself has...

A survey of commercially available strain gauges potentially suitable for testing of superconducting magnets is presented. Strain measurement errors caused by magnetic fields and temperature changes are discussed. Previously unpublished data for the magnetic field induced error for weldable half-bridge strain gauges are given, along with the result...

In the design of the superconducting conductor for the Large Coil Program (LCP) test coil, which is a large magnet cooled by pool boiling in liquid helium and operated with its bore axis horizontal, it is necessary to know the heat transfer capabilities of the conductor as a function of orientation (i.e., location along the coil perimeter) in the h...

A strain-gage signal-conditioning system, providing wide-band noise rejection and isolation from high voltages that occur during emergency coil discharges, has been developed and tested. The multichannel system combines double-shielded transformers, neutralizing networks, and bandpass filters (with commercial 3-kHz carrier amplifier modules to isol...

The design of the instrumentation system for the Large Coil Test Facility is described. Sensors are divided into two categories: coil diagnostic sensors, installed in the test coils; and facility sensors, installed in the various systems of the test facility in order to monitor their performance. After signal conditioning, data from the 'fast' chan...

The effects of high-beta operation of ISX-B were evaluated by installing a new poloidal field (PF) coil set which subjects the coils to higher out-of-plane loads. It was found that the iron core affects the load distribution; the forces were calculated using the GFUN-3D code which takes the iron core effects into account. The internal load distribu...