| Glossary of Terms |
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|---|---|---|
Term |
Definition |
Source |
| Plasma |
"A quasineutral gas of charged and neutral particles which exhibits collective behavior" | F.F. Chen, Introduction to Plasma Physics and Controlled Fusion |
Quasineutral |
A plasma is approximately neutral in any region because electrons tend to surround any local concentrations of positive charge (Debye shielding) | F.F. Chen, Introduction to Plasma Physics and Controlled Fusion |
Collective behavior |
Plasma dynamics are dominated by electromagentic forces rather than by collisions between neutral particles |
F.F. Chen, Introduction to Plasma Physics and Controlled Fusion |
Local Thermodynamic Equilibrium (LTE) |
A state in which collisional excitation and de-excitation are the dominant atomic processes. The electron density must be high enough that an ion in an excited state has a greater chance of returning to the ground state through collisional de-excitation than through spontaneous emission. LTE is distinguished from thermodynamic equilibrium in that the temperature need not be the same everywhere, and the spectrum is not a pure blackbody. | A. Thorne, U. Litzén, and S. Johansson, Spectrophysics: Principles and Applications |
| A state in which the electron density is too low for LTE, so that upward atomic transitions are assumed to be caused by collisions between electrons and ions and downward transitions occur by spontaneous emission. | A. Thorne, U. Litzén, and S. Johansson, Spectrophysics: Principles and Applications | |
Echelle |
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Blazing |
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Spheromak |
The donut-shaped plasma structure with toroidal and poloidal magnetic fields that is produced in SSX | Slava's thesis |
Flux conservers |
Cylindrical copper containers within the SSX vacuum chamber that act as conducting boundaries to contain the plasma | Jerome's thesis |
Magnetic Reconnection |
A phenomenon that occurs when the magnetic field lines from two colliding plasma structures change their topology rapidly and merge, releasing some of the stored magnetic energy. | Jerome's thesis |
Stuffing coil |
During spheromak formation in SSX, current through these coils creates a magnetic field that is dragged along by the plasma as it leaves the gun and then reconnects to form the poloidal field of the spheromak. | |
| Magnetohydro- |
A theory that uses a combination of fluid mechanics and classical electrodynamics to describe plasmas. In ideal MHD, the plasma is assumed to be a perfectly conducting fluid (no resistivity). | Jerome's thesis |
Plasma Kinetic Theory |
A theory used to analyze plasmas that relies on statistical mechanics. Useful for low-density plasmas where infrequent collisions between particles make MHD invalid. | Jerome's thesis |
Magnetized Ions |
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Alfvén's theorem / Frozen-in flux theorem |
In an ideal MHD plasma, the magnetic flux through a closed loop moving with the plasma is constant in time (the field lines are "frozen into" the plasma). This theorem is violated during magnetic reconnection. | Jerome's thesis |
Field-Reversed Configuration (FRC) |
A plasma configuration that results from the merging of two spheromaks with opposing helicities. The toroidal magnetic fields of the spheromaks annihilate during reconnection, so an ideal FRC has only a poloidal field. | Abram's thesis |
Photomultiplier tube (PMT) |
A device that converts the energy of a small number of photons into a measurable signal. Incoming photons strike the photocathode, causing electrons to be emitted through the photoelectric effect. These electrons are accelerated through a potential difference and strike a metal surface, which emits more electrons. The repetition of this process leads to a large amplification of the original signal. | |
Vacuum Ultraviolet (VUV) monochromator |
An SSX diagnostic that measures the flux of photons emitted in a specific narrow wavelength band between 50 nm and 560 nm in real time. | Slava's thesis |
Magnetic Reynolds number (RM) |
A dimensionless quantity describing the relative importance of the effect of convection versus diffusion on the magnetic field in a plasma. | Jerome's thesis |
Lundquist number (S) |
A special case of the Reynolds number when the velocity in the formula for RM is equal to the Alfvén speed. It measures how well the magnetic field lines are frozen into the plasma (in ideal MHD S is infinite) | Jerome's thesis |
Debye shielding |
There are no large-scale electromagnetic fields in plasmas, because electrons tend to drift around positive ions, and they shield one another's electrical influence from the rest of the plasma. | Jerome's thesis |
Internal measurements |
Measurements by probes placed within the plasma--these produce precise measurements but only represent a small fraction of the total volume of the inhomogeneous plasma. They may also perturb the plasma and interfere with its properties | Slava's thesis |
External measurements |
Measurements made by detecting photons and energized ions emitted by the plasma--these can only produce volume or line-of-sight averaged values, so they integrate over any localized plasma structures | Slava's thesis |
Ion Doppler spectroscopy (IDS) |
An external, photon-based plasma diagnostic that allows one to infer ion flow velocity from emission line Doppler shifts and ion temperature from emission line Doppler broadening. | |
Soft x-ray detector (SXR) |
An external, photon-based SSX diagnostic that uses four photodiodes filtered by thin metal films to capture broad spectral information with high time resolution. | |
PrismSPECT |
A commercial software application that simulates radiative processes in plasmas, produces detailed spectra, and provides a collection of tools for analysis of properties such as ionization balances and line intensities. | |
Dynamo |
A mechanism that amplifies or sustains magnetic fields in electrically conducting media such as plasmas or liquid metals. | Eric G. Blackman and Hantao Ji, "Laboratory Plasma Dynamos..." |
Toroidal |
Refers to magnetic field lines or currents that trace out circles around the axis of a torus (e.g. a spheromak or tokamak) | Abram's thesis |
Poloidal |
Refers to magnetic field lines or currents that pass through the center of a torus (i.e. through the donut hole). | Abram's thesis |
This page was last updated on 10/12/06