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Kirk T. McDonald

Publications and source records attributed to Kirk T. McDonald.

At least 19 recordsLinked to original sources
arXiv2009

Undulator-Based Production of Polarized Positrons

Full exploitation of the physics potential of a future International Linear Collider will require the use of polarized electron and positron beams. Experiment E166 at the Stanford Linear Accelerator Center (SLAC) has demonstrated a scheme in which an electron beam passes through a helical undulator to generate photons (whose first-harmonic spectrum extended to 7.9MeV) with circular polarization, which are then converted in a thin target to generate longitudinally polarized positrons and electrons. The experiment was carried out with a one-meter-long, 400-period, pulsed helical undulator in the Final Focus Test Beam (FFTB) operated at 46.6GeV. Measurements of the positron polarization have been performed at five positron energies from 4.5 to 7.5MeV. In addition, the electron polarization has been determined at 6.7MeV, and the effect of operating the undulator with a ferrofluid was also investigated. To compare the measurements with expectations, detailed simulations were made with an upgraded version of Geant4 that includes the dominant polarization-dependent interactions of electrons, positrons, and photons with matter. The measurements agree with calculations, corresponding to 80% polarization for positrons near 6MeV and 90% for electrons near 7MeV.

physics.ins-det↗
arXiv2005

Can a "Hidden Variable'' Quantum Theory Evade the "No-Cloning" Theorem?

If YES, then we can look forward to physical realization of superluminal communication, as the original considerations of the ``no-cloning'' theorem were motivated in part as an explanation of why certain schemes for superluminal signaling cannot work. If NO, then it would seem that some aspects of the ``hidden'' variables must be ``intrinsically hidden'', i.e., ``unknowable'', such that ``hidden-variable'' theories belong more to the ``idealist'' than to the ``realist'' school of thought. I pose this question without proposing a definite answer. I am unaware of any commentary on this topic during these 23 years since the formulation of the ``no-cloning'' theorem, but I would be pleased to be enlightened by more knowledgeable readers.

quant-ph↗
arXiv2003

Isotropic Radiators

We give two examples of antennas with isotropic radiation patterns. Because these involve elliptically polarized radiation, they evade the "hairy-ball theorem" that suggests isotropic radiation would be impossible.

physics.class-ph↗
arXiv2003

Gaussian Laser Beams via Oblate Spheroidal Waves

We deduce a set of circularly polarized Gaussian laser beam modes via a separation-of-variables solution to the Helmholtz wave equation in oblate spheriodal coordinates. On transforming to cylindrical coordinates these become the well-known Gaussian-Laguerre modes.

physics.class-ph↗
arXiv2003

An Electrostatic Wave

In general, Maxwell's equations require that a wave of electric field be accompanied by a wave of magnetic field, and vice versa. However, it is possible to have a plane wave in a dielectric medium with electric field E parallel to the wave vector k (a longitudinal wave) with no time-dependent magnetic field provided the electric displacement D is zero. We give an example from plasma physics: the so-called Bernstein wave.

physics.plasm-ph↗
arXiv2003

Magnetostatic Spin Waves

In general, Maxwell's equations require that a wave of magnetic field be accompanied by a wave of electric field, and vice versa. In magnetic media it is possible to have waves of magnetization with negligible electric field. We discuss an example of this based on ferromagnetic spin waves.

physics.class-ph↗
arXiv2003

Methods of Calculating Forces on Rigid Magnetic Media

Despite 180 years of theory on magnetism, it appears that the practice of calculating forces on magnetic media is ambiguous, as illustrated by a recent article by Casperson. Potentially troubling issues include: Which field B or H should be used? Should the total field be used, or only the external field? And if the latter, what is meant by "external"? Can/should effects of magnetization currents and/or fictitious magnetic poles be included? What is the force on only a portion of a circuit? We review several well-known approaches to magnetic force calculations on elements of rigid circuits, and find it very helpful to use an explicit example to compare and contrast the various methods.

physics.class-ph↗
arXiv2003

Hidden Momentum in a Coaxial Cable

As an illustration of the concept of "hidden" mechanical momentum, we consider a coaxial cable one end of which is connected to a battery and the other to a load resistor. The nonzero electromagnetic momentum of this configuration is balanced by the slight difference in the mechanical momenta of the conduction electrons in the inner and outer conductors.

physics.class-ph↗
arXiv2003

A Capacitor Paradox

Consider two capacitors in parallel, but initially isolated by a switch such that one is charged and the other not. After the switch is closed, the charges redistribute themselves in such a way that the final electrostatic energy is less than the initial. We show that the "missing" energy has been radiated away by including an appropriate form of the radiation action in an equivalent circuit analysis.

physics.class-ph↗
arXiv2003

Uncertainties in the Measurement of the Momentum and Position of an Electron

It has been suggested that the uncertainty in the measurement of a particle's momentum could be made arbitrarily small by observing the particle at two ends of an arbitrarily long flight path. However, consideration of the nature of the detection process shows that the usual limits of the uncertainty principle hold independent of the length of the flight path.

quant-ph↗
arXiv2003

Electron Bubbles in Liquid Helium

When an electron (or positronium atom) is injected into liquid helium with nearly zero energy, a bubble quickly forms around it. This phenomenon (which also occurs in liquid hydrogen, liquid neon and possibly in solid helium) lowers the mobility of the electron to a value similar to that for a positive ion. We estimate the radius of the bubble at zero pressure and temperature based on the zero point energy of the electron. If the liquid is held in a state of negative pressure, the bubble will expand beyond the radius at zero pressure. We also estimate the negative pressure such that a bubble once formed will grow without limit.

quant-ph↗
arXiv2002

The Rolling Motion of a Disk on a Horizontal Plane

Recent interest in the old problem of the motion of a coin spinning on a tabletop has focused on mechanisms of dissipation of energy as the angle alpha of the coin to the table decreases, while the angular velocity Omega of the point of contact increases. Following a review of the general equations of motion of a thin disk rolling without slipping on a horizontal surface, we present results of simple experiment on the time dependence of the motion that indicate the dominant dissipative power loss to be proportional to the Omega^2 up to and including the last observable cycle.

physics.class-ph↗
arXiv2000

Negative Group Velocity

The group velocity for pulses in an optical medium can be negative at frequencies between those of a pair of laser-pumped spectral lines. The gain medium then can amplify the leading edge of a pulse resulting in a time advance of the pulse when it exits the medium, as has been recently demonstrated in the laboratory. This effect has been called superluminal, but, as a classical analysis shows, it cannot result in signal propgation at speeds greater than that of light in vacuum.

physics.optics↗
arXiv2000

Classical Radiation Processes in the Weizsacker-Williams Approximation

The main features of radiation by relativistic electrons are well approximated in the Weizsacker-Williams method of virtual quanta. This method is best known for its application to radiation during elementary particle collisions, but is equally useful in describing "classical" radiation emitted during the interaction of a single relativistic electron with an extended system, such as synchrotron radiation, undulator radiation, transition radiation and Cerenkov radiation.

physics.class-ph↗
arXiv2000

Circular Orbits Inside the Sphere of Death

A wheel or sphere rolling without slipping on the inside of a sphere in a uniform gravitational field can have stable circular orbits that lie wholly above the "equator", while a particle sliding freely cannot.

physics.class-ph↗