Core Density Profile Research Articles

Core reflectometer density profile measurements on DIII-D

Measurements of core (normalized flux coordinate ρ⩽0.3–0.4) electron density profiles have been obtained on the DIII-D tokamak via a reflectometer system operating in the ordinary polarization (O mode). This system was developed and implemented for two primary reasons: first, there is a pressing need for core density profile measurements on DIII-D in plasmas with core transport barriers and centrally peaked profiles, and second, the measurements reported here further extend the demonstrated range of conditions in which reflectometry can provide reliable density profile measurements. As a test of the ability of reflectometry to measure such core profiles an existing DIII-D broadband frequency swept (FMCW) reflectometer system was converted to use O-mode propagation. This test system covers 50–75 GHz, corresponding to densities of 3.1–7.0×1019 m−3. Using this system, core access has been investigated in medium density discharges. Results demonstrate that adequate signals, and hence, accurate profiles, can in fact be obtained to the plasma center in discharges with centrally peaked profiles.

Bulk density calibration for X-ray tomographic analyses of marine sediments

A calibration procedure is presented for generating bulk density profiles of intact marine sediment cores using X-ray computed tomography (CT). Developed using both artificial and natural marine sediment samples, the correlation is strong with an R2 value of 0.98. Similar experiments with carbonate sediments were also strongly linear, but offset from the terrigenous curve, illustrating the influence of sediment chemistry on scanner response. Comparison between newly computed densities for both sediment types with those from individually calibrated cores during our previous CT studies reveals good correspondence between the data sets.

Performance of Alcator C-Mod core Thomson scattering system

Design of the Alcator C-Mod Thomson scattering (TS) diagnostic is discussed and the results of the measurements are presented. The TS system has six spatial channels with observation volumes evenly distributed between the midplane and the edge of the plasma. Each channel is capable of measuring the electron density in the range Ne=5×1019–5×1021 m−3 and temperature from Te=200 eV to 10 keV. A 30 Hz, 1.5 J per pulse Nd-YAG laser is employed allowing the measurements of evolution of Te and Ne profiles during plasma shot. A laser beam position control and feedback system provides for the beam alignment stability and reliable electron density measurements. Examples of the core density and temperature profiles measured at different stages of the plasma evolution are discussed.

Excited states in superfluid4He: a Monte Carlo approach

We present diffusion Monte Carlo calculations of the phonon-roton spectrum and the vortex excitation in liquid4He. The sign problem associated with the excited state is solved in the framework of the fixed node approximation. Within this approximation it is shown that DMC improves in a significant way the well known variational predictions for the phonon-roton spectrum. Results for the core energy and density profile of a 2D vortex are also presented with an efficient method to work out the infinite extension of the phase present in the Feynman wave function. Finally we show how so the fixed node bounds are improved towards the exact values when the nodal surface is released.

Toroidal gyrokinetic particle simulations of core fluctuations and transport

The generation of low frequency electrostatic fluctuations and the formation of anomalous particle and heat diffusion in the core region of tokamak plasmas is investigated using gyrokinetic particle-in-cell simulations in toroidal geometry. We consider the case of H-mode tokamak plasmas characterized by nearly flat core density profiles. The principle source of fluctuations include ion temperature gradient (ITG)-driven instabilities and ITG-dissipative trapped electron modes. We compare the evolution of both branches of instability and find that the electron diffusivity can be larger than the ion thermal diffusivity in the latter case. An externally imposed radial electric field, which gives rise to sheared poloidal flows, is demonstrated to reduce the radial correlation length of the turbulent transport and hence reduce the anomalous thermal flux.

Nondestructive density determination on marine sediment cores from gamma-ray attenuation measurements

We describe an improved nondestructive technique for density measurement based on gamma-ray absorption that permits construction of high-resolution density profiles of marine sediment cores. The system is capable of resolving vertical density variations with a spatial resolution of 4 mm and absolute accuracy of ± 1%. Comparison with conventional density determination on samples confirms the absolute accuracy. In addition to standard vertical profiling, a core rotation option is included to measure density as a function of rotation angle. This allows the quantification of the variability of density within a layer and detection of inhomogenities like icerafted debris or biogenic structures.

Computing core oscillation eigenperiods for the rotating Earth: a test of the subseismic approximation

The liquid outer core (OC) density profile of the preliminary reference Earth model (PREM) is modified to have various values of the Brunt-Väisälä frequency N in order to study long-period gravity modes modified by rotation. The eigenperiods of gravity modes of both non-rotating and rotating, self-gravitating, stably and spherically stratified, compressible OC models with rigid-fixed boundaries are obtained by solving the exact two-potential description (TPD) of core dynamics and the subseismic wave equation (SSWE) respectively, so as to test the subseismic approximation (SSA). Comparison of the resulting eigenperiods shows the high accuracy of the SSA: (1) the eigenperiods computed from the SSWE differ from those calculated from the TPD by at most 0.8% for a non-rotating core with 2π N = 6 h ; (2) the weaker the stability of the OC, the smaller the difference; (3) the more complex the spatial variation of the eigenfunction, the smaller the difference; (4) the SSA gives the eigenperiods more accurately for a rotating Earth than for a non-rotating model. The results also show that the SSA always shortens the eigenperiods. Convergence of the truncated series of trial functions in the Galerkin method used in the calculations is carefully checked to guarantee reliability of the numerical results.

The Slichter modes of the rotating Earth: a test of the subseismic approximation

SUMMARY The eigenperiods of the principal translational (Slichter) mode of free oscillation of the solid inner core (split into a triplet by the Earth's rotation) are computed by a variational principle based on the subseismic approximation, neglecting ellipticity but taking Coriolis coupling into account to high order in the liquid core. First-order Coriolis coupling is retained in the solid parts of the Earth to compute their dynamical response (represented by load Love numbers) to disturbances of the outer core. A simple analytical model explains the presence of a low-frequency resonance in the Love numbers of degree 1 for the inner core boundary, which renders them highly frequency dependent near the Slichter eigenfrequencies. For PREM, with the outer core density profile slightly modified so as to be exactly neutral, the Slichter period neglecting Earth's rotation is 5.4119 hr. Fifth-order Coriolis coupling in the outer core is more than adequate to ensure convergence of the computed eigenperiods: we obtain 5.9697 hr, 5.3017 hr and 4.7594 hr for the prograde equatorial, axial and retrograde equatorial modes respectively. We draw attention to some shortcomings of the subseismic approximation encountered in this study.

Limits on right-handed interactions from SN 1987A observations.

We consider the emission of light (${m}_{{\ensuremath{\nu}}_{R}}\phantom{\rule{0ex}{0ex}}\ensuremath{\lesssim}$ 10 MeV right-handed neutrinos from supernova 1987A via right-handed charged- and neutral-current interactions. By requiring that the neutronization process ${e}_{R}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\nu}}_{R}n$ does not carry away most of the energy that can be radiated by the supernova, ${E}_{T}\phantom{\rule{0ex}{0ex}}\ensuremath{\lesssim}\phantom{\rule{0ex}{0ex}}$ (4-5) \ifmmode\times\else\texttimes\fi{} ${10}^{53}$ ergs, we exclude the range of values for the right-handed $W$-boson mass, (3.7-6.7)${\mathrm{M}}_{{\mathrm{W}}_{\mathrm{L}}}$ \ensuremath{\le} ${\mathrm{M}}_{{\mathrm{W}}_{\mathrm{R}}}$ \ensuremath{\le} (280-500)${\mathrm{M}}_{{\mathrm{W}}_{\mathrm{L}}}$ for vanishing ${\mathrm{W}}_{\mathrm{L}}$ --- ${\mathrm{W}}_{\mathrm{R}}$ mixing parameter \ensuremath{\zeta}, and for \ensuremath{\zeta} \ensuremath{\ne} 0 we get (0.4-1.2) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}5}$ \ensuremath{\le} ${\left[{\ensuremath{\zeta}}^{2}\phantom{\rule{0ex}{0ex}}+\phantom{\rule{0ex}{0ex}}({M}_{{W}_{L}}^{4}\phantom{\rule{0ex}{0ex}}/\phantom{\rule{0ex}{0ex}}{M}_{{W}_{R}}^{4})\right]}^{\frac{1}{2}}\phantom{\rule{0ex}{0ex}}\ensuremath{\le}(0.02\ensuremath{-}0.07)$. For right-handed $W$ bosons heavier than 300 ${M}_{{W}_{L}}$, a significant ${\ensuremath{\nu}}_{R}$ emission may arise from the neutral-current-induced process ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\overline{\ensuremath{\nu}}}_{R}{\ensuremath{\nu}}_{R}$, as well as $\mathrm{nn}\ensuremath{\rightarrow}\mathrm{nn}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ mediated by an extra gaugae boson $Z\ensuremath{'}$. In this case, for an appropriately defined effective mass ${M}_{N}$ for $Z\ensuremath{'}$, we find the excluded region $(2.4\ensuremath{-}4.3){M}_{{W}_{L}}<{M}_{N}<(30\ensuremath{-}85){M}_{{W}_{L}}$, mainly dependent on the core temperature and density profile.

Radial transport reduction in tandem mirrors using end-wall boundary conditions

Results from the Tandem Mirror Experiment-Upgrade (TMX-U) demonstrate that radial transport in the plasma volume can be affected by the boundary conditions at the end wall. Measurements in which end-wall plates, mapping to the plasma core, are switched from floating to grounded during plugging operation demonstrate that this floating increases the build-up rate of the central cell plasma (by increasing the radial confinement time), steepens the core density profile, and affects the plasma throughout the entire cross section.