Showing papers by "Princeton Plasma Physics Laboratory published in 1979"

Numerical studies of current generation by radio‐frequency traveling waves

Abstract: By injecting radio‐frequency traveling waves into a tokamak, continuous toroidal electron currents may be generated. This process is studied by numerically solving the two‐dimensional Fokker–Planck equation with an added quasi‐linear term. The results are compared with the one‐dimensional analytic treatment of Fisch, which predicted a reduced plasma resistivity when high‐phase‐velocity waves are employed. It is shown that two‐dimensional velocity space effects, while retaining the predicted scaling, further reduce the ratio of power dissipated to current generated by about 40%. These effects enhance the attractiveness of steady‐state tokamak reactors utilizing this method of current generation.

Alpha transport and blistering in tokamaks

Abstract: The particle flux and angular distribution of 3.5 MeV alpha particles impinging on the first wall from uncontained banana orbits in an axisymmetric tokamak reactor have been calculated. The resulting helium concentration profiles in the first wall can give rise to surface exfoliation under specified conditions. The major mitigating factor is the simultaneous surface recession due to sputtering by the D-T charge-exchange neutral flux. For the parameters used in these calculations blistering in high-sputtering-rate materials such as beryllium is unlikely, whereas in low-sputtering-rate materials such as niobium helium-induced surface deformation is quite probable.

Vacuum-Ultraviolet Continua from Rare-Gas Ion-Excitation of Solids

Abstract: A wide range of targets was bombarded with rare gas ions, mostly at 100 keV. Strong continua were observed in the 8-12eV (100-160 nm) range. They are due to the decay of bound-excited rare gas molecules (excimers). The dose dependence of the continuum intensity was measured. Also, experiments were performed in which targets previously implanted with one rare gas were impacted with another. Both the dose dependence and the mixed-projectile work indicate that the continuum arises from interactions involving implanted species. It may be possible to use excimer continuum intensity measurements to determine the depth distribution of implanted rare gas atoms.

Particle beam systems in plasma diagnostics

Abstract: SUMMARY We discuss the use of active particle beam systems in plasma diagnostics. The evolution of these systems over the last two decades has been impressive and has contributed to the measurement of a large fraction of those plasma quantities which are necessary for an understanding of plasma behavior. For the sake of completeness, an attempt will be made to cover the principles behind most of these systems. The main emphasis of the lecture will, for limited space, be devoted to a few of the more recent innovations in particle beam systems which have made significant contributions to plasma quantities that were hitherto rather difficult to measure. This will be systems for the measurement of plasma potential, and current and impurity profiles.

Bypass operation by cyclic firing of the bridge thyristors.

Abstract: By passing of a thyristor bridge is being used in large convertor systems such as in h.v.d.c. transmission lines for fault conditions. In normal voltage-control operation, an increase in the total power factor of the whole convertor system can be achieved by relieving a few bridges from current by using external bypass valves. However, a fully rated external bypass valve is relatively expensive because it has to carry continuously the full direct current. A novel way of continuous operation of the bridge thyristors in a cyclic firing method is described in the paper. This method allows the designer to eliminate the external bypass valve and still get the bypass mode of operation.

Ion Beam Analysis of Surface Modifications in Tokamaks

Abstract: This paper briefly reviews the use of ion beam techniques to study surface modification of samples exposed to tokamak plasma discharges. Primary emphasis is given to a continuing study of samples exposed at the first wall of the Princeton Large Torus (PLT) tokamak throughout its operating life. Both impurity flow and hydrogen recycle effects are investigated. Laboratory hydrogen implantations at 200-400 eV suggest a method to determine the mean ion temperature at the wall from the hydrogen saturation trapping dependences in Si samples.

Determination of plasma ion temperatures by analysis of charge-exchange neutrals

Abstract: SUMMARY We discuss the principles and techniques by which analysis of the fast neutrals produced by charge exchange in the plasma volume can be used to determine the plasma ion temperature The fast neutral flux emerging from the plasma interior declines sharply with increasing density and minor radius due to both a drop in the neutral density within the plasma interior as well as attenuation of the outgoing flux Methods for artificially enhancing the neutral density with spatial resolution adequate for ion temperature profile measurements are considered Neutral beam heating experiments have necessitated fast neutral measuring systems containing both momentum and energy analysis in order to distinguish the thermal plasma from the slowing down distribution of beam particles This has enabled measurements on PLT of the thermal species, eg, D+, to be extended into the energy range where the beam particles H+ are orders of magnitude more abundant

ICRF heating in PLT

Abstract: Fundamental properties of the modes excited in PLT are being investigated to permit optimization of ICRF heating. In particular, we report here on successful mode tracking through active frequency programming on the plasma density and magnetic field. Also, the damping characteristics in the vicinity of Ω = ω/ωci = 2 are presented and their implications with respect to heating the electrons and ions of PLT discussed.

Fokker-planck/transport analyses of fusion plasmas in contemporary beam-driven tokamaks

Abstract: The properties of deuterium plasmas in experimental tokamaks heated and fueled by intense neutral-beam injection are evaluated with a Fokker-Planck/radial transport code coupled with a Monte Carlo neutrals treatment. Illustrative results are presented for the Poloidal Divertor Experiment at PPPL as a function of beam power and plasma recycling coefficient, R/sub c/. When P/sub beam/ = 8 MW at E/sub b/ = 60 keV, and R/sub c/ = 0.2, then approximately 0.5, (/sup 2///sub 3/ ) = 22 keV approximately 6, and the D-D neutron intensity is 10/sup 16/ n/sec.

Evolution of neutral-beam-driven current in tokamak plasmas

Abstract: Neutral-beam heating and current induction have been analyzed with a one-dimensional model that includes a Fokker-Planck analysis of the circulating ion current, the time and space dependence of the electron ''return'' current, and the steady electron shielding current including electron trapping effects With 12 MW of 40 keV H/sup 0/ injected tangentially into a 25-cm radius PLT plasma with = 20 x 10/sup 13/ cm/sup -3/, the maximum beam-driven current is about 100 kA and results in a 024-V reduction in loop voltage Reversed loop voltages should be observable in low density plasmas in ISX-B and TFTR, which would provide an unambiguous detection of the beam-driven current When the radial profiles of beam-driven current are less peaked than the ohmic-current profile, the profiles of the ''safety factor'' q should be more stable to n = 1, m = 1 and n = 1, m = 2 kink modes, if the total plasma current is kept constant

Twenty-Million Watt Vacuum Tube and Test Results

Abstract: The development of this tube represents an advancement in the state of the art for gridded tubes in terms of plate dissipation, voltage holdoff, and switched power at the pulse widths involved. Prior to this development,switch tube ratings have been limited to approximately 100 kilovolts holdoff voltage with a plate dissipation of several hundred kilowatts. These ratings applied typically to pulse widths of several milliseconds. The S94000E is rated for 200 kilovolts holdoff voltage and a plate dissipation of two million watts at five seconds pulse width. Load or output currents up to 125 amperes are available, with a tube voltage drop during the pulse in regulator use of typically seven to thirty kilovolts.