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=============================================================== Glossary Part 19: Terms beginning with "S" FREQUENTLY USED TERMS IN CONVENTIONAL FUSION RESEARCH AND PLASMA PHYSICS Edited by Robert F. Heeter, firstname.lastname@example.org Guide to Categories: * = plasma/fusion/energy vocabulary & = basic physics vocabulary > = device type or machine name # = name of a constant or variable ! = scientists @ = acronym % = labs & political organizations $ = unit of measurement The list of Acknowledgements is in Part 0 (intro). ================================================================== SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS @ (?) SHIVA: see entry under Shiva, below. @ SI: Systeme Internationale; see SI Units @ SM: Symmetric Mirror @ SN: Single Null; see entry for Divertor @ SNL: Sandia National Laboratories; see entry @ SOL: Scrape-Off Layer; see entry @ STM: Symmetric Tandem Mirror experiment. $ Sv: Sievert; see entry * Safety Factor: (q) The number of times a field line goes around a torus "the long way" for each time around "the short way". In a tokamak, this number is typically near unity in the center of the plasma and between two and 6 or 8 at the edge. So-called because it helps to determine the degree of stability the plasma has against certain instabilities. The safety factor is the inverse of the rotational transform, and can be expressed mathematically as q = (r * Bt )/(R * Bp), where r and R are the minor and major radii of the torus, and Bt and Bp are the toroidal and poloidal magnetic fields. ! Sakharov, Andrei: Russian physicist; among other achievements, he is credited with the initial design of the tokamak. % Sandia National Laboratories: Located in Albuquerque, New Mexico. Another large DOE laboratory; has PBFA-II (Particle Beam Fusion Accelerator, an ICF device) and some pinch devices. Some divisions located in Livermore (Sandia-Livermore). * Sawtooth: When a tokamak runs with enough current to achieve q < 1 on the magnetic axis, the plasma parameters (n, T, B) oscillate with a "sawtooth" waveform. The oscillation is localized to a region roughly within the q=1 surface, and arises from internal MHD effects. Confinement is degraded within the sawtooth region. * Scaling Laws: These are mathematical rules explaining how variation in one quantity affects variations in other quantities. For instance, in a tokamak reactor it's generally believed that energy confinement depends on the size of the device and the strength of the magnetic field, but the precise nature of the dependence is not fully understood, so empirical "scaling laws" are tested to see what the dependence is. Scaling laws are useful for extrapolating from parameter regimes where the mathematical relationships between the various quantities are known, into unexplored regimes. & Scattering: The deflection of one particle as a result of collisions with other particles or with waves. See also Elastic. * Schlieren Method: An optical technique that detects density gradients occuring in a fluid flow. In its simplest form, light from a slit is collimated by a lens and focused onto a knife edge by a second lens; the flow pattern is placed between the two lenses, and the resulting diffraction pattern is observed on a screen or photographic film placed behind the knife edge. * Scientific Feasibility: Fusion will be considered scientifically feasible when (a) experiments are done which reach scientific breakeven-type plasma conditions (see entry on breakeven), and (b) the experimental results suggest that the approach can be "scaled up" into a power-producing system. Tokamak fusion reactors are closing in on (a), and tokamak researchers think (b) holds as well, so they are designing a power-producing machine (ITER) to demonstrate net energy production from tokamak fusion. Inertial confinement is also approaching this point. * Scrape-Off Layer (SOL): [from Art Carlson] Outer layer of a plasma which is affected ("scraped off") by a divertor or limiter. That is, the outer layer of a magnetically confined plasma (ca. 2 cm thick) where the field lines penetrate a material surface (limiter or divertor plate) rather than close upon themselves. This region defines the outer limit of the plasma because any plasma crossing into the SOL is rapidly lost since transport along the field is much faster than that across the field. That is, particles follow these field lines into the material surface and are lost from the plasma. * Screw Pinch: A variant on the theta pinch, in which axial currents (as in a z pinch, but less intense) produce a poloidal magnetic field (in addition to the usual longitudinal field), thus making a corkscrew field configuration. See also theta pinch, z pinch, pinch device. * Second-stability Region: A high pressure region where the plasma becomes stable to the pressure-gradient-driven ballooning ballooning instability. The plasma is stable in the limit of small pressure gradients, becomes unstable at some intermediate pressure, and then becomes stable again at still higher pressures. Tokamaks operating in the second-stability region would be more attractive because the higher pressures (beta) would provide more fusion reactivity per unit volume of plasma, allowing smaller reactors to be built. * Separatrix: [from Art Carlson] In a divertor tokamak (and some other configurations), the last closed flux surface (see entry) is formed not by inserting an object (limiter) but by manipulating the magnetic field, so that some field lines take a topologically different route (through the divertor, rather than simply around the central plasma). The boundary between the two types of field lines is called the separatrix. * Sheared Flow: Fluid flow where the magnitude of the fluid velocity changes along a direction perpedicular to the direction of the fluid flow. (Freeway traffic often exhibits sheared flow in that traffic in the "fast lane" moves more rapidly than traffic in the slow lane with the exits...) Sheared flow typically correlates with reduced transport and enhanced confinement. (This definition is rather informal and may not be fully technically correct - R.F. Heeter) * Shear Fields: As used in plasma physics, this refers to magnetic fields having a rotational transform (or, alternatively, safety factor) that changes with radius (e.g., in the stellarator concept, magnetic fields that increase in pitch with distance from the magnetic axis.) * Sheath: See Debye Sheath > Shiva: 20-beam Nd-glass fusion laser facility at LLNL. Was completed in 1977 and used for target irradiation experiments until mid-1981. Succeeded by Nova. * Shock Heating: Heating produced by the impact of a shock wave. * Shock Tube: A gas-filled tube used in plasma physics to quickly ionize a gas. A capacitor bank charged to a high voltage is discharged into the gas at one tube end to ionize and heat the gas, producing a shock wave that may be studied as it travels down the tube. * Shock Wave: Wave produced (e.g., in a gas or plasma) as a result of a sudden violent disturbance. To produce a shock wave in a given region, the disturbance must take place in a shorter time than the time required for sound waves to traverse the region. $ Sievert: International unit for radiation dose. One Sievert equals 100 rem (see entry for rem); average per-capita exposure is about 0.3 Sv, primarily from natural background (see entry) and medical x-rays. * Shock Heating: The heating produced by the impact of a shock wave. * Shock Wave: Wave produced as a result of a sudden, violent disturbance which occurs in a particular region faster than sound waves can traverse the region. * Shot: Fusion jargon for the production of a (short-lived) plasma. In the early days, plasmas were produced by the "discharge" of capacitor banks, which (frequently) made a BANG. A modern tokamak produces a few dozen "shots" per day, each lasting a few seconds and, if nothing goes wrong, inaudible. See also: capacitor, tokamak (Arthur Carlson, email@example.com) $ SI Units: (also known as MKS, MKSA Units) System of measurement in which the fundamental units are meters, kilometers, seconds, and the ampere. $ Sievert: Unit of absorbed radiation dose equivalent to 100 rem. (see also rem, rad, Gray) The sievert is based on the Gray in the same way that the rem is based on the rad, I believe. & Solenoid: Cylindrical coil of wire which, when current flows through it, acts as an electromagnet. For long solenoids with many turns, the magnetic field inside the center is nearly uniform. * Solid Breeder: Solid lithium-bearing compounds, usually ceramics such as Li2O and LiAlO2, which might be used in the blanket of a D-T fusion reactor to produce ("breed") additional tritium fuel from the n + Li => He + T (+n) reactions. * Solid State Laser: A laser using a transparent substance (crystalline or glass) as the active medium, doped to provide the energy states necessary for lasing. The pumping mechanism is the radiation from a powerful light source, such as a flashlamp. The ruby, Nd-YAG, and Nd:glass lasers are solid-state lasers. * Solitons: Stable, shape-preserving and localized solutions of nonlinear classical field equations. Of recent interest as possible models of extended elementary particles. * Sound Waves: See entries on compression waves, waves. * Space Frame or Spaceframe: Three-dimensional "optical bench" that holds laser components stable from vibrational and thermal excursions. * Spallation: See radiation damage, surface. * Spatial Filter: Device consisting of a lens pair and a pinhole aperture stop. Intensity fluctuations over the spatial extent of a laser beam are removed by passing the focused beam through the aperture stop. The pinhole must be placed in a vacuum to prevent air breakdown by the focused beam. These filters are used to counter the effects caused by self-focusing. > Spherator: Single-ring multipole device with an additional current-carrying rod perpendicular to the ring axis. > Spheromak: [from Art Carlson] A compact torus with comparable toroidal and poloidal magnetic fields. The plasma is roughly spherical and is usually surrounded by a close-fitting conducting shell or cage. Both the poloidal and toroidal magnetic fields are generated by plasma currents. There are no toroidal field coils "linking" the plasma through the central plasma axis. External force is supplied by poloidal field coils outside the plasma separatrix. The resulting configuration is approximately a force-free magnetic field. The spheromak machine geometry can be simpler than a tokamak, but the close-fitting wall is a source of impurities and the current cannot be inductively driven. After early experiments failed to achieve a reasonable beta, interest has ebbed. The spheromak can also be considered as the low-aspect-ratio limit of the tokamak. See also: compact torus. * Spin-Polarized Fusion: A method to enhance nuclear fusion reaction rates in some fusion fuels by polarizing the nuclear spins. ! Spitzer, Lyman: Early Princeton Fusion Scientist; astrophysicist who first proposed orbiting space telescope; inventor of the stellarator. & Spontaneous Emission: Radiation randomly emitted by excited atoms or ions. Contrast with stimulated emission. * Sputtering: Process by which atoms are ejected from a solid surface by bombardment with plasma particles. See entry for "Radiation Damage, Surface." * Stability: characteristic of some types of equilibrium states; see equilibrium. > Starfire Tokamak: A conceptual design study of a modular tokamak reactor that operates in a steady-state condition while using conventional power-generating systems. & Stark Effect: The effect an electric field has on the spectral lines emitted from excited atoms. The effect may arise from externally-applied electric fields, from internal fields due to the presence of neighboring ions or atoms (pressure), or from the electric field associated with the Lorentz (v cross B) force (motional stark effect). Spectroscopic measurements of plasmas using the pressure-based and motional Stark effects are useful for diagnostic purposes. > Stellarator: (adapted from Herman) Device invented by Lyman Spitzer for the containment of a plasma inside a racetrack-shaped (sometimes a figure-8) tube. The plasma is contained by a magnetic field created by helical windings around the tube. More generally, a toroidal sort of device that attempts to average out particle drifts that would otherwise take plasma to the walls of the vacuum vessel by imposing a given amount of helicity to the toroidal field lines. "A toroidal plasma configuration, which, unlike a tokamak, is not axially symmetric. The poloidal fields necessary for confinement are produced by external coils (rather than a current in the plasma), either helical coils in addition to plane toroidal field coils, or out-of-plane toroidal field coils (pioneered in Germany on Wendelstein 7-AS). The stellarator is generally considered to be the most serious alternative to the tokamak. Since the concept is inherently steady state, it would not have the tokamak's problems with thermal and mechanical cycling, current drive, and disruptions." -- Arthur Carlson, firstname.lastname@example.org & Stimulated Emission: Radiation coherently emitted by excited ions when driven by a passing light wave and the appropriate transition wavelength. "Laser" means Light Amplification by Stimulated Emission of Radiation; it occurs when there is a population inversion between the upper and lower energy states of the transition, such that stimulated emission can dominate excitation. Stimulated emission is coherent and codirectional with the stimulating wave, and the rate of stimulated emission is proportional to the intensity of the stimulating wave. * Strong (Nuclear) Force: * Sturm-Liouville Problem: The general problem of solving a linear differential equation of order 2n, together with 2n boundary conditions; also known as the eigenvalue problem. * Superconductor: A type of electrical conductor that permits a current to flow with zero resistance. Without superconducting coils, a toroidal magnetic-confinement fusion reactor would not be possible, because too much energy would be required to maintain the magnetic fields against resistive energy losses in the coil conductors. * Suydam Stability Criterion: A criterion for the stability of modes localized arbitrarily close to a mode-rational surface (see rational surface) in a circular cylindrical geometry. & Synchrotron Radiation: (Also known as cyclotron radiation.) Electromagnetic energy radiated from a charged particle moving in a curved orbit (typically in a magnetic field), due to the acceleration required to change the direction of the particle's velocity. See also bremsstrahlung. * Symmetry axis: [from Art Carlson] The straight line (usually vertical) through the center of a configuration, when the configuration is symmetric to all (axisymmetric, like the tokamak) or some (periodic, like the stellarator) rotations about this line. Usually the z-axis.