Showing papers by "Princeton Plasma Physics Laboratory published in 1980"

Parametric decay into ion cyclotron waves and drift waves in multi-ion-species plasma

Abstract: Parametric decay processes near the ion cyclotron frequency are investigated experimentally and theoretically in multi‐ion species plasmas. The relevant theoretical dispersion relation of the parametric coupling is derived, including the ion drift motion. Experimental data obtained in the Princeton L‐4 device verify these theoretical predictions in some detail. In a helium‐neon plasma, the relative ion drift motion excites electrostatic ion cyclotron waves (the kinetic ion‐ion hybrid mode) when ω0≳ΩHe +ΩNe. In a region of large density gradient, the ion drift motion also excites low‐frequency drift waves when ω0≳ΩHe+ω*. The experimental data are found to agree well with the theory. The relevance of these processes to ion cyclotron heating of fusion plasmas is discussed.

Experimental evidence of charge-exchange recombination of highly ionized iron and titanium in Princeton large torus

Abstract: The observed behavior of the emissivitives of boronlike Fe xxii, lithiumlike Fe xxiv and Ti xx, and the heliumlike Fe xxv ions in the Princeton large torus tokamak during high-power neutral (${\mathrm{H}}^{0}$ or ${\mathrm{D}}^{0}$) beam heating is described. A substantial lowering of the dominant ionization state in the center of the discharge, while the electron temperature is rising, is attributed primarily to increased recombination rate of the ions through charge exchange with neutral hydrogen. This interpretation is supported by the different space and time behavior or the lithiumlike and boronlike ions of comparable ionization potentials, and by comparisons of neutral beam heating of the plasma with ion cyclotron resonance heating, which does not appreciably change the neutral hydrogen concentration. The observations are compared with approximate zero-dimensional model calculations, using experimental plasma conditions and estimated charge-exchange rates.

Axisymmetric instability in a non-circular tokamak: experiment and theory

Abstract: The stability of dee, inverse-dee and square-cross-section plasmas to axisymmetric modes has been investigated experimentally in Tokapole II, a tokamak with a four-null poloidal divertor. Experimental results are closely compared with predictions of two numerical stability codes: the PEST code (ideal MHD, linear stability) adapted to tokapole geometry and a code which follows the non-linear evolution of shapes similar to tokapole equilibria. Experimentally, there exists an optimal initial vertical position for the plasma for which both dees will experience a small oscillation with the restoring force provided by non-linear passive feedback. Slight vertical displacement of the initial position causes, however, both dees to be unstable to a non-rigid vertical motion. The central-magnetic-axis displacement grows exponentially with a growth time ~103 poloidal Alfven times or on the order of the plasma L/R time. Contrarily, the square is always vertically stable. Experimental poloidal flux plots are produced directly from internal-magnetic-probe measurements. The PEST code, ignoring passive feedback, predicts all equilibria to be vertically unstable with the square having the slowest growth. With passive feedback, all are stable. Thus experiment and code agree that the square is the most stable shape, but experiment indicates that passive feedback is partially defeated by finite resistivity. In both code and experiment, square-like equilibria exhibit a relatively harmless horizontal instability.

Ignition of an overheated, underdense, fusioning tokamak plasma

Abstract: Methods of igniting an 'overheated' but 'underdense' D-T plasma core with a cold plasma blanket are investigated by using a simple two-zone model with a variety of transport scaling laws, and also using a one-dimensional transport code. The power consumption of neutral-beam injectors required to produce ignition can be reduced if the underdense core plasma is heated to temperatures much higher than the final equilibrium ignition values, followed by fuelling from a cold plasma blanket. The reduction in power consumption varies from a modest saving if energy confinement improves rapidly with increasing density in the reactor regime, to a significant saving if energy confinement improves slowly with increasing density. It is also found that the allowed impurity concentration in the initial hot core can be higher than normally permitted for ignition provided that the blanket is free from impurities.

A survey of the Tokamak fusion test reactor (TFTR) feedback control design

Abstract: The TFTR is an experimental Tokamak fusion reactor being built by Princeton's Plasma Physics Laboratory for the Department of Energy. Its primary function is to demonstrate that the power that can be produced in a Deuterium Tritium reaction will exceed the power required to sustain the reaction. This will be the first Tokamak device that uses Tritium and has a large enough power density to achieve a break even in energy production. To achieve the power density requires high temperatures which are achieved by causing the current in the plasma to very rapidly increase. The high currents cause heating by the atomic losses. This ohmic heating is not sufficient, however, to achieve the required temperatures. Thus a source of high energy neutral deuterium atoms is "injected" into the plasma to cause further heating. This neutral beam heating is further supplemented with a rapid motion of the plasma (on the order of 50 m/s) to cause compression and adiabatic heating. Each of these heating effects, combined with the problem of initiating a plasma at the start of the reaction, imposes severe control problems because plasma current, position, and temperature are interrelated quantitites. The Plasma Current and Plasma Position feedback control were developed using linear optimal control theory. This paper surveys the results that guarantee when the control system is implemented on the actual machine, the control will be satisfactory. To this end, simulation and design models with eddy currents, plasma current diffusion, and plasma heating have been developed. The resulting control designs are shown to be very insensitive to modeling uncertainties.