219 research outputs found
Local magnetic divertor for control of the plasma-limiter interaction in a tokamak
An experiment is described in which plasma flow to a tokamak limiter is controlled through the
use of a local toroidal divertor coil mounted inside the limiter itself. This coil produces a local
perturbed field B_C approximately equal to the local unperturbed toroidal field B_T ≃ 3 kG, such
that when B_C adds to B_T the field lines move into the limiter and the local plasma flow to it
increases by a factor as great as 1.6, and when B_C subtracts from B_T the field lines move away
from the limiter and the local plasma flow to it decreases by as much as a factor of 4. A simple
theoretical model is used to interpret these results. Since these changes occur without significantly
affecting global plasma confinement, such a control scheme may be useful for optimizing the
performance of pumped limiters
Scientific Payload for an Interstellar Probe Mission
NASA's Interstellar Probe Mission will be the first spacecraft specifically designed to explore the outer solar system, pass through the boundaries of the heliosphere, and sample the nearby interstellar medium. During the spring of 1999, NASA's Interstellar Probe Science and Technology Definition Team* developed a concept for a mission that will travel to 200- 400 AU using solar-sail propulsion. The principal scientific goals would be to explore the outer solar system, explore the structure of the heliosphere and its interaction with the interstellar medium, and explore the nature of the interstellar medium itself. These studies
would be carried out by a -150 kg spacecraft carrying a scientific payload designed to make comprehensive measurements of heliospheric and interstellar plasma, fields, energetic particles, neutral gas, and dust. We discuss the scientific goals and strawman payload for this
mission
Effects of a Local Interstellar Magnetic Field on Voyager 1 and 2 Observations
We show that that an interstellar magnetic field can produce a north/south
asymmetry in solar wind termination shock. Using Voyager 1 and 2 measurements,
we suggest that the angle between the interstellar wind velocity and
magnetic field is . The distortion of the
shock is such that termination shock particles could stream outward along the
spiral interplanetary magnetic field connecting Voyager 1 to the shock when the
spacecraft was within of the shock. The shock distortion is larger
in the southern hemisphere, and Voyager 2 could be connected to the shock when
it is within of the shock, but with particles from the shock
streaming inward along the field. Tighter constraints on the interstellar
magnetic field should be possible when Voyager 2 crosses the shock in the next
several years.Comment: 12 pages, 5 figure
Evolution of a Streamer-Blowout CME as Observed by Imagers on Parker Solar Probe and the Solar Terrestrial Relations Observatory
Context: On 26-27 January 2020, the wide-field imager WISPR on Parker Solar
Probe (PSP) observed a coronal mass ejection (CME) from a distance of
approximately 30 solar radii as it passed through the instrument's 95 degree
field-of-view, providing an unprecedented view of the flux rope morphology of
the CME's internal structure. The same CME was seen by STEREO, beginning on 25
January.
Aims: Our goal was to understand the origin and determine the trajectory of
this CME.
Methods: We analyzed data from three well-placed spacecraft: Parker Solar
Probe (PSP), Solar Terrestrial Relations Observatory-Ahead (STEREO-A), and
Solar Dynamics Observatory (SDO). The CME trajectory was determined using the
method described in Liewer et al. (2020) and verified using simultaneous images
of the CME propagation from STEREO-A. The fortuitous alignment with STEREO-A
also provided views of coronal activity leading up to the eruption.
Observations from SDO, in conjunction with potential magnetic field models of
the corona, were used to analyze the coronal magnetic evolution for the three
days leading up to the flux rope ejection from the corona on 25 January.
Results: We found that the 25 January CME is likely the end result of a slow
magnetic flux rope eruption that began on 23 January and was observed by
STEREO-A/Extreme Ultraviolet Imager (EUVI). Analysis of these observations
suggest that the flux rope was apparently constrained in the corona for more
than a day before its final ejection on 25 January. STEREO-A/COR2 observations
of swelling and brightening of the overlying streamer for several hours prior
to eruption on January 25 led us to classify this as a streamer-blowout CME.
The analysis of the SDO data suggests that restructuring of the coronal
magnetic fields caused by an emerging active region led to the final ejection
of the flux rope.Comment: 12 pages, 12 figures. Accepted in A&
Magnetic Effects Change Our View of the Heliosheath
There is currently a controversy as to whether Voyager 1 has already crossed
the Termination Shock, the first boundary of the Heliosphere. The region
between the Termination Shock and the Heliopause, the Helisheath, is one of the
most unknown regions theoretically. In the Heliosheath magnetic effects are
crucial, as the solar magnetic field is compressed at the Termination Shock by
the slowing flow. Recently, our simulations showed that the Heliosheath
presents remarkable dynamics, with turbulent flows and the presence of a jet
flow at the current sheet that is unstable due to magnetohydrodynamic
instabilities \cite{opher,opher1}. In this paper we review these recent
results, and present an additional simulation with constant neutral atom
background. In this case the jet is still present but with reduced intensity.
Further study, e.g., including neutrals and the tilt of the solar rotation from
the magnetic axis, is required before we can definitively address how the
Heliosheath behaves. Already we can say that this region presents remarkable
dynamics, with turbulent flows, indicating that the Heliosheath might be very
different from what we previously thought.Comment: 6 pages, 5 figures, to appear in IGPP 3rd Annual International
Astrophysics Conference, "PHYSICS OF THE OUTER HELIOSPHERE
A 2D Electromagnetic PIC Code for Distributed Memory Parallel Computers
The two dimensional electrostatic plasma particle in cell (PIC) code described an [1] has been upgraded to a 2D electromagnetic PIC code running on the Caltech/JPL Mark IIIfp and the Intel iPSC/860 parallel MIMD computers. The code solves the complete time dependent Maxwell’s equations where the plasma responses, i.e., the charge and current density in the plasma, are evaluated by advancing in time the trajectories of ~ 10^6 particles in their self-consistent electromagnetic field. The field equations are solved in Fourier space. Parallelisation is achieved through domain decomposition in real and Fourier space. Results from a simulation showing a two-dimensional Alfèn wave filamentation instability are shown; these are the first simulations of this 2D Alfèn wave decay process
Temperature fluctuations and heat transport in the edge regions of a tokamak
Electron temperature fluctuations have been investigated in the edge region of the Caltech research tokamak [S. J. Zweben and R. W. Gould, Nucl. Fusion 25, 171 (1985)], and an upper limit to this fluctuation level was found at Te/Te <~ 15%. This measurement, together with previous measurements of density and electric and magnetic field fluctuations, allows a unique comparison of the heat transport resulting from three basic turbulent mechanisms: (1) heat flux from the particle flux resulting from microscopic density and electric field fluctuations; (2) thermal conduction resulting from microscopic temperature and electric field fluctuations; and (3) thermal conduction resulting from microscopic magnetic field fluctuations. The measurements indicate that, in the edge regions, the electron heat transport caused by the measured turbulence-induced particle flux is comparable to or greater than that caused by the thermal conduction associated with the electron temperature and electric field fluctuations, and is significantly greater than that resulting from the measured magnetic fluctuations. This electron heat loss caused by the plasma turbulence is found to be an important electron energy loss mechanism in the edge regions
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