Square Root Of Electron Density Research Articles

Simulation of toroidicity-induced Alfven eigenmode excited by energetic ions in HL-2A tokamak plasmas

The toroidicity-induced Alfven eigenmode (TAE) excited by energetic (fast) ions was first simulated using the GTC code for HL-2A experiments. The simulation results indicate that the fraction of energetic ions is about 3% in the experiments. The critical density of energetic ions to excite the TAE mode is around 0.023, which is in good agreement with the theoretical expectation of 0.026. The real frequency of TAE with toroidal mode number n = 3 is around 211 kHz and is inversely proportional to the square root of electron density, which is quantitatively in agreement with the experimental observation (Ding et al 2013 Nucl. Fusion 53 043015). The frequency is in resonance with the summations of toroidal precession frequency and transit frequency of passing energetic ions, indicating that resonance with passing energetic ions is dominant for the excitation of the modes in the experiment. It is also checked by the phase space structures of (strength of energetic particle distribution function) that the TAE modes are mainly induced by passing energetic ions in HL-2A. The growth rates of TAE modes increase with fast ion density and its gradient. In addition, lower n TAE modes, such as n = 1 can also be driven by energetic ions when off-axis heating with higher beam energy is employed in the experiment. The width of radial mode structures for lower n modes is usually wider than those for higher n modes. The perpendicular wave vector of the modes and Larmor radius of ions satisfy the relation . Finally, the polarization of the mode shows that the perturbed parallel electric field is much smaller than the electrostatic parallel electric field.

Introducing PROFESS 2.0: A parallelized, fully linear scaling program for orbital-free density functional theory calculations

Orbital-free density functional theory (OFDFT) is a first principles quantum mechanics method to find the ground-state energy of a system by variationally minimizing with respect to the electron density. No orbitals are used in the evaluation of the kinetic energy (unlike Kohn–Sham DFT), and the method scales nearly linearly with the size of the system. The PRinceton Orbital-Free Electronic Structure Software (PROFESS) uses OFDFT to model materials from the atomic scale to the mesoscale. This new version of PROFESS allows the study of larger systems with two significant changes: PROFESS is now parallelized, and the ion–electron and ion–ion terms scale quasilinearly, instead of quadratically as in PROFESS v1 (L. Hung and E.A. Carter, Chem. Phys. Lett. 475 (2009) 163). At the start of a run, PROFESS reads the various input files that describe the geometry of the system (ion positions and cell dimensions), the type of elements (defined by electron–ion pseudopotentials), the actions you want it to perform (minimize with respect to electron density and/or ion positions and/or cell lattice vectors), and the various options for the computation (such as which functionals you want it to use). Based on these inputs, PROFESS sets up a computation and performs the appropriate optimizations. Energies, forces, stresses, material geometries, and electron density configurations are some of the values that can be output throughout the optimization.

Radio Polarimetric Imaging of the Interstellar Medium: Magnetic Field and Diffuse Ionized Gas Structure near the W3/W4/W5/HB 3 Complex

We have used polarimetric imaging to study the magneto-ionic medium of the Galaxy, obtaining 1420 MHz images with an angular resolution of 1' over more than 40 deg2 of sky around the W3/W4/W5/HB3 H II region/SNR complex in the Perseus Arm. Features detected in polarization angle are imposed on the linearly polarized Galactic synchrotron background emission by Faraday rotation arising in foreground ionized gas having an emission measure as low as 1 cm−6 pc. Several new remarkable phenomena have been identified, including: mottled polarization arising from random fluctuations in a magneto-ionic screen that we identify with a medium in the Perseus Arm, probably in the vicinity of the H II regions themselves; depolarization arising from very high rotation measures (several times 103 rad m−2) and rotation measure gradients due to the dense, turbulent environs of the H II regions; highly ordered features spanning up to several degrees; and an extended influence of the H II regions beyond the boundaries defined by earlier observations. In particular, the effects of an extended, low-density ionized halo around the H II region W4 are evident, probably an example of the extended H II envelopes postulated as the origin of weak recombination-line emission detected from the Galactic ridge. Our polarization observations can be understood if the uniform magnetic field component in this envelope scales with the square-root of electron density and is 20 μG at the edge of the depolarized region around W4, although this is probably an overestimate since the random field component will have a significant effect.

Whistler measurements of electron density and magnetic field strength in the remote magnetosphere

Whistler theory, supported by experiment, has shown that a whistler trace contains two types of information, one on minimum gyrofrequency along the field-line path, and the other on electron density. The minimum gyrofrequency fHmin is determined by the relation fHmin≃2.4fnwhere fn is the frequency of minimum travel time, or nose frequency, of the whistler [Smith, 1960] (see examples in Figure 1). Information on electron density is obtained from measurement of travel time at the whistler nose, tn. This travel time is proportional to the integral of the square root of electron density along the path.