Gauss Magnetic Fields Research Articles

Giant collimated gamma-ray flashes

Bright sources of high energy electromagnetic radiation are widely employed in fundamental research as well as in industry and medicine. This steadily growing interest motivated the construction of several facilities aiming at the realisation of sources of intense X- and gamma-ray pulses. To date, free electron lasers and synchrotrons provide intense sources of photons with energies up to 10-100 keV. Facilities under construction based on incoherent Compton back scattering of an optical laser pulse off an electron beam are expected to yield photon beams with energy up to 19.5 MeV and peak brilliance in the range 10$^{20}$-10$^{23}$ photons s$^{-1}$ mrad$^{-2}$ mm$^{-2}$ per 0.1% bandwidth. Here, we demonstrate a novel mechanism based on the strongly amplified synchrotron emission which occurs when a sufficiently dense electron beam interacts with a millimetre thickness solid target. For electron beam densities exceeding approximately $3\times10^{19}\text{ cm$^{-3}$}$ filamentation instability occurs with the self-generation of 10$^{7}$-10$^{8}$ gauss magnetic fields where the electrons of the beam are trapped. This results into a giant amplification of synchrotron emission with the production of collimated gamma-ray pulses with peak brilliance above $10^{25}$ photons s$^{-1}$ mrad$^{-2}$ mm$^{-2}$ per 0.1% bandwidth and photon energies ranging from 200 keV up to several hundreds MeV. These findings pave the way to compact, high-repetition-rate (kHz) sources of short (30 fs), collimated (mrad) and high flux ($>10^{12}$ photons/s) gamma-ray pulses.

Deep probing of the photospheric sunspot penumbra: no evidence of field-free gaps

Some models for the topology of the magnetic field in sunspot penumbrae predict the existence of field-free or dynamically weak-field regions in the deep Photosphere. To confirm or rule out the existence of weak-field regions in the deepest photospheric layers of the penumbra. The magnetic field at $\log\tau_5=0$ is investigated by means of inversions of spectropolarimetric data of two different sunspots located very close to disk center with a spatial resolution of approximately 0.4-0.45 arcsec. The data have been recorded using the GRIS instrument attached to the 1.5-meters GREGOR solar telescope at El Teide observatory. It includes three Fe I lines around 1565 nm, whose sensitivity to the magnetic field peaks at half a pressure-scale-height deeper than the sensitivity of the widely used Fe I spectral line pair at 630 nm. Prior to the inversion, the data is corrected for the effects of scattered light using a deconvolution method with several point spread functions. At $\log\tau_5=0$ we find no evidence for the existence of regions with dynamically weak ($B<500$ Gauss) magnetic fields in sunspot penumbrae. This result is much more reliable than previous investigations done with Fe I lines at 630 nm. Moreover, the result is independent of the number of nodes employed in the inversion, and also independent of the point spread function used to deconvolve the data, and does not depend on the amount of straylight (i.e. wide-angle scattered light) considered.

Extreme increase in atomic transition probability of the Cs D_2 line in strong magnetic fields under selective reflection

Selective reflection of 852-nm laser radiation from the interface between cesium vapor and the sapphire window of a 30-micrometer-thick microcell was used to record an extreme increase in the probability of the Fg=3&#x2192;Fe=5 transitions associated with the Cs-atom D2 lines in magnetic fields with inductions ranging from 300 to 3200 Gauss. We showed that a group of seven transitions Fg=3, mF=&#x2212;3, &#x2212;2, &#x2212;1, 0, +1, +2, +3&#x2192;Fe=5, mF=&#x2212;2, &#x2212;1, 0, +1, +2, +3, +4 was formed in accordance with the selection rules &#x0394;mF=+1 for &#x03C3;+-circularly-polarized radiation. These seven transitions have much higher probabilities in 500–1000 Gauss magnetic fields, with three of the transitions having probabilities higher than all of the other transitions originating from the level Fg=3. In magnetic fields with induction greater than 3000 Gauss, this group of seven transitions at the high-frequency end of the spectrum is completely separate from the group of transitions with Fg=3&#x2192;Fe=4. Comparison of the frequencies and probabilities of these seven atomic transitions with those for the transitions Fg=3&#x2192;Fe=5 showed good agreement between theory and experiment. We also discuss potential practical applications for these transitions.

Constraining primordial vector mode from B-mode polarization

The B-mode polarization spectrum of the Cosmic Microwave Background (CMB) may be the smoking gun of not only the primordial tensor mode but also of the primordial vector mode. If there exist nonzero vector-mode metric perturbations in the early Universe, they are known to be supported by anisotropic stress fluctuations of free-streaming particles such as neutrinos, and to create characteristic signatures on both the CMB temperature, E-mode, and B-mode polarization anisotropies. We place constraints on the properties of the primordial vector mode characterized by the vector-to-scalar ratio rv and the spectral index nv of the vector-shear power spectrum, from the Planck and BICEP2 B-mode data. We find that, for scale-invariant initial spectra, the ΛCDM model including the vector mode fits the data better than the model including the tensor mode. The difference in χ2 between the vector and tensor models is Δχ2 = 3.294, because, on large scales the vector mode generates smaller temperature fluctuations than the tensor mode, which is preferred for the data. In contrast, the tensor mode can fit the data set equally well if we allow a significantly blue-tilted spectrum. We find that the best-fitting tensor mode has a large blue tilt and leads to an indistinct reionization bump on larger angular scales. The slightly red-tilted vector mode supported by the current data set can also create \U0001d4aa(10-22)-Gauss magnetic fields at cosmological recombination. Our constraints should motivate research that considers models of the early Universe that involve the vector mode.

Interstellar turbulent magnetic field generation by plasma instabilities

The maximum magnetic field strength generated by Weibel-type plasma instabilities is estimated for typical conditions in the interstellar medium. The relevant kinetic dispersion relations are evaluated by conducting a parameter study both for Maxwellian and for suprathermal particle distributions showing that micro Gauss magnetic fields can be generated. It is shown that, depending on the streaming velocity and the plasma temperatures, either the longitudinal or a transverse instability will be dominant. In the presence of an ambient magnetic field, the filamentation instability is typically suppressed while the two-stream and the classic Weibel instability are retained.

Generation of electron-beam produced plasmas and applications to surface modification

NRL has developed a number of hollow cathodes to generate sheets of electrons culminating in a ‘Large Area Plasma Processing System’ (LAPPS) based on the electron-beam ionization process. Beam ionization is fairly independent of gas composition and produces low temperature plasma electrons (<0.5 eV in molecular gases) in high densities (10 9–10 12 cm −3). The present system consists of a pulsed planar plasma distribution generated by a magnetically collimated sheet of 2 kV electrons (<1 mA/cm 2) injected into a neutral background of processing gases (oxygen, nitrogen, sulfur hexafluoride, argon). Operating pressures range from 2–13 Pa with 150–165 Gauss magnetic fields for a highly localized plasma density of ∼10 11 cm −3. This plasma source demonstrated anisotropic removal rates of polymeric (photoresist) material and silicon with O 2 and Ar/O 2/SF 6 mixtures, respectively. In pure nitrogen, this same source showed a surface nitriding rate of ∼1 μm/h of plasma exposure time on austenitic stainless steel at 400 °C. Time-resolved in situ plasma diagnostics (Langmuir probes and mass spectrometry) of these pulsed plasmas are also shown to illustrate the general plasma characteristics.

Inhibitory effects of powerline-frequency (60-Hz) magnetic fields on pentylenetetrazol-induced seizures and mortality in rats

The possibility that exposure to powerline frequency (60-Hz) magnetic fields might affect the form or intensity of epileptic seizures, induced by administration of pentylenetetrazol (PTZ) in rats, was examined. Male adult rats were exposed to either 60-Hz magnetic fields with intensities of up to 1.85 gauss (185 microT) or to a sham field condition, for 1 h prior to injections of PTZ (45-75 mg/kg). The subsequent seizures were monitored and recorded on videotape and any subsequent mortalities were noted. Exposure to 60-Hz magnetic fields prior to administration of PTZ was found to significantly (P less than 0.005) reduce the lethality of the drug-induced seizures. The LD50 for the sham-exposed group was 65.88 mg/kg, whereas for the 60-Hz magnetic field-exposed rats, the LD50 was 85.33 mg/kg. In some experiments exposure to the 1.0 and 1.5 gauss magnetic fields also produced significant (P less than 0.05) reductions in seizure durations. These findings suggest that acute exposure to low intensity 60-Hz magnetic fields has an inhibitory effect on the lethality and expression of PTZ-induced seizures in rats. Some possible mechanisms, which could account for these observed effects of magnetic field exposure on seizures, are discussed.

An Inverse Compton Scattering Model for the Spectra of X-ray Pulsars

Many observations have been reported in the field of X-ray pulsars, but the mechanism for X-ray emission is not well understood. The X-ray spectra can not be simply described in terms of blackbody or thermal bremsstralung. The high-energy cutoff could be due to cyclotron absorption in high (≧1012 Gauss) magnetic fields. For the lower energy it can be fitted by a power law with energy index α.

Astronomy: Neutron stars in Nanjing

Astronomy: Neutron stars in Nanjing

The effect of magnetic field intensities on the oxygen consumption in Porcellio laevis latre (Oniscoidea, Isopoda).

The effects of the 5 gauss, 50 gauss and 500 gauss magnetic fields on the oxygen consumption in Porcellio laevis adults, at soil pH 3.8 and 7.0, were investigated in a series of laboratory experiments. The oxygen consumption was the highest at 5 gauss and pH 3.8, and lowest at 500 gauss and pH 7.0. Highest isopod mortality was recorded under all magnetic fields at pH 3.8, and lowest at pH 7.0. The significance of these findings is discussed.

The Zeeman effect in astrophysics

George E. Hale discovered the Zeeman effect in sunspot spectra in 1908, and derived the laws of sunspot polarity. Recent observations of the much weaker (1 to 50 gauss) magnetic fields of the solar photosphere, made with the solar magnetograph, are described. The bearing of these results on circulation in the sun and their consequences for the topology of the sun's magnetic field are mentioned, with reference to the solar cycle and to reversal of the sun's polar field. The problem of measuring the magnetic field of stars is outlined, with reference to instrumental requirements. Spectrographs and crystals are described, together with techniques of measurement of magnetic fields in sharp-line stellar spectra. Results of a systematic program of observations with the 100-inch and 200-inch telescopes are reviewed. All stellar magnetic fields are variable; some are periodic. The strongest yet measured is 34 000 gauss. The magnetic variations are usually related to variations in light, color, line width, and line intensity. Chemical abundance anomalies are common in magnetic stars, and this has led to much work on nuclear reactions in stellar atmospheres, but the phenomena have not been satisfactorily explained. The possible importance of element concentration through paramagnetic migration in strong magnetic field gradients, enhanced by optical pumping, is pointed out.