Lorentz Drift Research Articles

Single magnetic white dwarfs with Balmer emission lines: a small class with consistent physical characteristics as possible signposts for close-in planetary companions

ABSTRACT We report the identification of SDSS J121929.45+471522.8 as the third apparently isolated magnetic (B ≃ 18.5 ± 1.0 MG) white dwarf exhibiting Zeeman-split Balmer emission lines. The star shows coherent variability at optical wavelengths with an amplitude of ≃0.03 mag and a period of 15.26 h, which we interpret as the spin period of the white dwarf. Modelling the spectral energy distribution and Gaia parallax, we derive a white dwarf temperature of 7500 ± 148 K, a mass of 0.649 ± 0.022 M⊙, and a cooling age of 1.5 ± 0.1 Gyr, as well as an upper limit on the temperature of a sub-stellar or giant planet companion of ≃250 K. The physical properties of this white dwarf match very closely those of the other two magnetic white dwarfs showing Balmer emission lines: GD356 and SDSS J125230.93−023417.7. We argue that, considering the growing evidence for planets and planetesimals on close orbits around white dwarfs, the unipolar inductor model provides a plausible scenario to explain the characteristics of this small class of stars. The tight clustering of the three stars in cooling age suggests a common mechanism switching the unipolar inductor on and off. Whereas Lorentz drift naturally limits the lifetime of the inductor phase, the relatively late onset of the line emission along the white dwarf cooling sequence remains unexplained.

Survivability of radio-loud planetary cores orbiting white dwarfs

ABSTRACT The discovery of the intact metallic planetary core fragment orbiting the white dwarf SDSS J1228+1040 within one Solar radius highlights the possibility of detecting larger, unfragmented conducting cores around magnetic white dwarfs through radio emission. Previous models of this decades-old idea focused on determining survivability of the cores based on their inward Lorentz drift towards the star. However, gravitational tides may represent an equal or dominant force. Here, we couple both effects by assuming a Maxwell rheological model and performing simulations over the entire range of observable white dwarf magnetic field strengths (103–109 G) and their potential atmospheric electrical conductivities (10−1–104 S m−1) in order to more accurately constrain survivability lifetimes. This force coupling allows us to better pinpoint the physical and orbital parameters which allow planetary cores to survive for over a Gyr, maximizing the possibility that they can be detected. The most robust survivors showcase high dynamic viscosities (≳1024 Pa s) and orbit within kG-level magnetic fields.

Lorentz drift compensation in high harmonic generation in the soft and hard X-ray regions of the spectrum.

We present a semi-classical study of the effects of the Lorentz force on electrons during high harmonic generation in the soft and hard X-ray regions driven by near- and mid-infrared lasers with wavelengths from 0.8 to 20 μm, and at intensities below 1015 W/cm2. The transverse extent of the longitudinal Lorentz drift is compared for both Gaussian focus and waveguide geometries. Both geometries exhibit a longitudinal electric field component that cancels the magnetic Lorentz drift in some regions of the focus, once each full optical cycle. We show that the Lorentz force contributes a super-Gaussian scaling which acts in addition to the dominant high harmonic flux scaling of λ-(5-6) due to quantum diffusion. We predict that the high harmonic yield will be reduced for driving wavelengths > 6 μm, and that the presence of dynamic spatial mode asymmetries results in the generation of both even and odd harmonic orders. Remarkably, we show that under realistic conditions, the recollision process can be controlled and does not shut off completely even for wavelengths >10 μm and recollision energies greater than 15 keV.

Radiation from free electrons in a laser focus at 10^18 W/cm^2: modeling of photon yields and required focal conditions

In support of an experiment designed to measure the strength of radiation scattered from low-density free electrons in an intense laser focus, we model a variety of physical parameters that impact the rate of scattered photons. We employ a classical model to characterize duration of electron exposure to high-intensity laser light in a situation where the electrons are driven by strong ponderomotive gradients. Free electrons are modeled as being donated by low-density helium, which undergoes strong-field ionization early in the pulse or during a prepulse. When exposed to relativistic intensities, free electrons experience a Lorentz drift that causes redshifting of the scattered 800 nm light. This redshift can be used as a signature to discern light scattered from the more intense regions of the focus. We characterize the focal volume of initial positions leading to significant redshifting, given a peak intensity of 2×1018 W/cm2. Under this scenario, the beam waist needs to be larger than several wavelengths for a pulse duration of 35 fs. We compute the rate of redshifted scattered photons from an ensemble of electrons distributed throughout the focus and relate the result to the scattered-photon rate of a single electron. We also estimate to what extent the ionization process may produce unwanted light in the redshifted spectral region.

Negative ion drift and diffusion in a TPC near 1 bar

Drift velocity and longitudinal diffusion measurements are reported for a Negative Ion TPC (NITPC) operating with Helium + CS 2 gas mixtures at total pressures from 160 to 700 torr. Longitudinal diffusion at the thermal-limit was observed for drift fields up to at least 700 V/cm in all gas mixtures tested. The results are of particular interest in connection with mechanical simplification of the DRIFT direction-sensitive Dark Matter search, and for high energy physics experiments in which a low- Z , low density, gaseous tracking detector with no appreciable Lorentz drift is needed for operation in very high magnetic fields.

A Monte-Carlo study of a precision magnetic spectrometer with planar geometry

The possibility of reconstructing the true mean initial energy at the origin employing a detector having a linear dependence of mean energy losses on path length in its components is demonstrated. A Monte-Carlo study of a magnetic spectrometer with planar drift chambers orthogonal to the magnetic field direction has been done. Simulations were carried out for positrons in the range 20–50 MeV in a magnetic field B = 2 T and DME as the drift gas. It is shown that 2 keV reconstruction accuracy of a mean energy at the origin can be achieved without using any specific information about the value of energy loss of positrons in the detector. A suggested “sliding” technique provides track reconstruction with a reliable confidence level by eliminating most large kinks. Application of 1 cos θ energy loss dependence to continuous spectrum reconstruction is studied. The influence of the Lorentz drift angle and positive ion effects on reconstruction accuracy has been analyzed. It is shown that though under certain circumstances the Lorentz drift effect can cause significant shifts of a mean reconstructed energy and distortions of the continuous spectrum, these distortions become negligible for a distributed source or one averaged over 2π azimuthal angle range and a continuous flat spectrum can be reconstructed with total statistics up to 10 12 in 4π solid angle without systematic error.

Magnetodiode transients

The usual eigenfunction representation of photoconductive decay in thin semiconductor samples is generalized to cover double injection as well as Lorentz drift effects.

Plasma fluctuations in crossed electric and magnetic fields

An experimental study has been made of the high-amplitude electrical fluctuations associated with a gas discharge in a strong magnetic field.Hydrogen, helium, argon, and nitrogen gas were used at pressures from 0.3 to 100 micronsThe power spectrum on plasma probes was investigated for various values of magnetic field, gas pressure, and power input to the discharge.For the range of frequencies studied (0.5 to 4000 megacycles per second), the spectrum shows a continuous high level that increases in amplitude with decreasing pressures at all frequencies and increases with increasing magnetic field at low frequencies.A sharp dip in the spectrum was observed near the ion cyclotron frequency for discharges in hydrogen and helium. The velocity and direction of propagation for the low frequency fluctuations were determined by cross-correlating the potential variations on two neighboring probes. These fluctuations appear to propagate in the direction of the electron drift and at a velocity somewhat less than the Lorentz drift velocity.