Showing papers by "Princeton Plasma Physics Laboratory published in 1985"

Structure of edge-plasma turbulence in the Caltech tokamak

Abstract: Measurements of the spatial structure of tokamak edge density fluctuations have been made using compact probe arrays in the Caltech tokamak. The results indicate that the structure of in the tokamak edge is characterized by very short auto-correlation times and short cross-correlation lengths, i.e. that the edge density fluctuations consist of small-scale 'turbulence'. The space-time structure of this turbulence is consistent with several recent non-linear edge instability models. The structure of the local electrostatic potential is measured to be qualitatively similar to that of , and local cross-correlation between and indicates large outward radial particle transport similar to predictions of the edge instability models.

Angular distribution of the bremsstrahlung emission during lower hybrid current drive on PLT

Abstract: The bremsstrahlung emission from the PLT tokamak during lower-hybrid current drive has been measured as a function of angle between the magnetic field and the emission direction. The emission is peaked strongly in the forward direction, indicating a strong anisotropy of the electron velocity distribution. The data demonstrate the existence of a nearly flat tail of the velocity distribution, which extends out to approximately 500 keV and which is interpreted as the plateau created by Landau damping of the lower-hybrid waves.

Scaling of density fluctuations in PDX

Abstract: The scaling of turbulent density fluctuations with drift-wave characteristics has been examined in the PDX tokamak using scattering of 2 mm microwaves. The authors have measured how the frequency spectra, the k⊥ spectra and the magnitude of the fluctuations vary with toroidal field, neutral beam heating power, plasma current, position and confinement regime in PDX. Since turbulence is a possible cause of anomalous heat loss in PDX and other tokamaks, the scaling of the turbulence is compared with energy confinement scalings. In many cases, an increased fluctuation level is correlated with decreased energy confinement. In one case, the fluctuation level does not change as the confinement properties of the plasma change. The scaling of the experimental fluctuation levels is compared with various drift-wave parameters. It is found that the fluctuation level scales as the mixing-length limit, n/n ~ l/k⊥Ln, in some sets of data, but not in other sets.

Improving current generation in a tokamak by electron cyclotron waves

Abstract: Current drive by electron cyclotron heating of the far end of a lower-hybrid-sustained electron tail is investigated. The current density profile is determined by means of the kinetic theory and the ray-tracing code for the PLT and the Alcator C parameters. It is shown that the value of the ratio W/I for the combined effects of lower hybrid and electron cyclotron wave absorption can be improved compared to that of the present lower hybrid experiments. Suitable shaping of the current density profiles can be achieved by adjusting the wave-launching parameters.

Zero-dimensional energy balance modelling of the CTX spheromak experiment

Abstract: A zero-dimensional time-dependent energy balance model is used to explore the energy loss mechanisms of the CTX spheromak experiment at Los Alamos National Laboratory. A coupled set of model equations representing electron, ion, neutral, and impurity particle balance, electron and ion temperature, and magnetic field decay, are solved from initial values and the results compared to the time behaviour of experimentally measured average densities, temperature, and magnetic field. The energy balance model considers all the major atomic physics processes, especially the effects of radiation from a non-equilibrium distribution of impurity charge states evolving in time. The model includes the effects of a strong neutral-particle source which replaces by ionization the plasma being lost by a short particle confinement time. The neutral source is required in experiments to prevent the sudden termination of the discharge associated with low densities. A major new conclusion is that all the data from resistively decaying spheromaks can be effectively modelled, when the flux loss effects of a resistive flux conserver are also included, using plasma resistivity increased by a factor of 3.2 ? 0.6 over the Spitzer-H?rm value evaluated with the volume-average temperature. This factor appears to be constant for all discharges at all times. The analysis has determined that the power density associated with the particle replacement is the most important loss in the warm, non-radiation-dominated CTX spheromaks.

Lifetimes and Oscillator Strengths for the Resonance Transitions in Cu-Like Iodine, I XXV

Abstract: Using the beam-foil time-of-flight technique we have measured decay curves for 4s 2S1/2 -4p 2P1/2, 3/2 and 4p 2P1/2, 3/2 -4d 2D3/2, 5/2 transitions in Cu-like iodine, I XXV. Accurate experimental lifetimes of the 4p levels have been obtained after careful cascade corrections using the ANDC technique. The results are 46.7 ± 2 ps (2P1/2) and 23.1 ± 1 ps (2P3/2). These values are in good agreement with relativistic Hartree-Fock calculations. An up-to-date comparison between experimental and theoretical oscillator strengths is given for the whole copper sequence.

Charge-exchange measurements of beam ion thermalization in MHD-quiescent plasmas in the Poloidal Divertor Experiment

Abstract: The horizontally scanning, multi-angle charge-exchange analyser on the Poloidal Divertor Experiment (PDX) was used to study the beam ion slowing-down process with high-power perpendicular injection. Measurements were made over a wide range in toroidal field (8 kG

The TFTR Lithium Blanket Module program

Abstract: The Lithium Blanket Module (LBM) is an approximately 80×80×80 cm cubic module, representative of a helium-cooled lithium oxide fusion reactor blanket module, that will be installed on the TFTR (Tokamak Fusion Test Reactor) in late 1986. The principal objective of the LBM Program is to perform a series of neutron transport and tritium-breeding measurements throughout the LBM when it is exposed to the TFTR toroidal fusion neutron source, and to compare these data with the predictions of Monte Carlo (MCNP) neutronics codes. The LBM consists of 920 2.5-cm diameter breeder rods constructed of lithium oxide (Li2O) pellets housed in thin-walled stainless steel tubes. Procedures for mass-producing 25,000 Li2O pellets with satisfactory reproducibility were developed using purified Li2O powder, and fabrication of all the breeder rods was completed in early 1985. Tritium assay methods were investigated experimentally using both small lithium metal samples and LBM-type pellets. This work demonstrated that the thermal extraction method will be satisfactory for accurate evaluation of the minute concentrations of tritium expected in the LBM pellets (0.1–1 nCi/g).

Two-Chamber Model for Divertors with Plasma Recycling

Abstract: To model particle and heat-loss terms at the edge of a tokamak with a divertor or pumped limiter, a simple two-chamber formuluation of the scrapeoff has been constructed by integrating the fluid equations, including sources, along open field lines. The model is then solved for a wide range of density and temperature conditions in the scrapeoff, using geometrical parameters typical of the poloidal divertor in the poloidal divertor experiment (PDX). The solutions characterize four divertor operating conditions for beam-heated plasmas: plugged, unplugged, blowthrough, and blowback.

TFTR Plasma Feedback Systems

Abstract: The Tokamak Fusion Test Reactor employs feedback control systems for four plasma parameters, i.e. for plasma current, for plasma major radius, for plasma vertical position, and for plasma density. The plasma current is controlled by adjusting the rate of change of current in the Ohmic Heating (OH) coil system. Plasma current is continuously sensed by a Rogowski coil and its associated electronics; the error between it and a preprogrammed reference plasma current history is operated upon by a ''proportional-plusintegral-plus-derivative'' (PID) control algorithm and combined with various feedforward terms, to generate compensating commands to the phase-controlled thyristor rectifiers which drive current through the OH coils. The plasma position is controlled by adjusting the currents in Equilibrium Field and Horizontal Field coil systems, which respectively determine the vertical and radial external magnetic fields producing J X B forces on the plasma current. The plasma major radius position and vertical position, sensed by ''B /sub theta/ '' and ''B /sub rho/ '' magnetic flux pickup coils with their associated electronics, are controlled toward preprogrammed reference histories by allowing PID and feedforward control algorithms to generate commands to the EF and HF coil power supplies. Plasma density is controlled by adjusting the amount ofmore » gas injected into the vacuum vessel. Time-varying gains are used to combine lineaveraged plasma density measurements from a microwave interferometer plasma diagnostic system with vacuum vessel pressure measurements from ion gauges, with various other measurements, and with preprogrammed reference histories, to determine commands to piezoelectric gas injection valves.« less

The TFTR Charge Exchange Diagnostic Software Systems

Abstract: This paper describes the applications software systems for computer control and monitoring of diagnostic hardware, and for data acquisition and analysis of the TFTR (Tokamak Fusion Test Reactor) Charge Exchange diagnostics. The TFTR Charge Exchange diagnostics are comprised of two autonomous systems, each consisting of up to six independent analyzer modules viewing the plasma at different angles and toroidal locations. Each system will have the capability of acquiring up to 2.5 megabytes of raw data for each shot. Users will have the capability of controlling all analyzers, and analyzing hydrogen mass species for up to ten analysis pulse time regions for multiple plasma shots. These features make the Charge Exchange systems among the largest diagnostic applications software systems on TFTR.

Measuring nanocurie quantities of tritium bred in metallic lithium and lithium oxide samples

Abstract: The LBM program requires that nanocurie quantities of tritium, bred in both lithium oxide pellets and lithium samples, be measured with an uncertainty not exceeding + or - 6%. Two methods of accurately measuring nanocurie quantities of tritium bred in LBM lithium oxide pellets and one method of accurately measuring nanocurie quantities of tritium bred in lithium samples are described. Potential errors associated with these tritium measurement techniques are also discussed.

Comprehensive design procedure and spreadsheet for internally cooled cabled superconductors

Abstract: A comprehensive procedure has been set up to consider together the various phenomena influencing the design of an Internally Cooled Cabled Superconductor (ICCS). Sensitivity studies done for TFCX show the relative order of importance of conduit structural behavior assumption, dump circuit design, and nuclear heating rate on the cavity current density. Although none of these is completely specified or known presently, the TFCX TF coil design goal of 3500 A/cm2at 10 Tesla appears achievable using moderately optimistic assumptions. The study was done in an automated manner using spreadsheet software on a personal computer, which proved to be extremely convenient and flexible.

Interface Language for Diagnostic Control

Abstract: The neutron multichannel collimator diagnostic on TFTR is run with the help of a keyboard-entered interface language. The language allows the user to interact with the real-time control and data analysis systems in a consistent and efficient manner. It uses a vocabulary that can be abbreviated into one character commands which the proficient user may concatenate into command words. This allows the user to progress quickly from a novice to an expert operating mode. A similar type interface language could be applied to many interactive applications accepting keyboard inputs.

Synchronous absolute calibration technique for in-situ calibration of the TFTR scanning millimeter wave radiometers

Abstract: A synchronous signal averaging technique for calibrating the three Tokamak Fusion Test Reactor (TFTR) radiometers while they operate in the frequency scanning mode, between 75–220 GHz, will be presented. Improvements in accuracy and data collection efficiency will be discussed.

Neutral beam orientation on the tokamak fusion test reactor.

Abstract: The efficiency of plasma heating and current drive expected in the Tokamak Fusion Test Reactor has been computed for various orientations of neutral injection beamlines Computer codes that model plasma transport and particle orbits have been used to compute power losses caused by ''shinethrough,'' charge-exchange, intersection of particle orbits with the limiter or wall, and toroidal field ripple, and to compute the effect of finite size particle orbits on penetration of injected beam particles Less readily quantifiable considerations such as impurity contamination and toroidal plasma rotation are discussed briefly

Neutral pumping rates for a next step tokamak ignition device

Abstract: Neutral pumping rates are calculated for pump-limiter and divertor options of a next step tokamak ignition device using a method that accounts for the coupled effects of neutral transport and plasma transport. For both pump limiters and divertors the plasma flow into the channel surrounding the neutralizer plate is greatly reduced by the neutral recycling. The fraction of this flow that is pumped can be large (>50%) but in general is dependent on the particular geometry and plasma conditions. It is estimated that pumping speeds greater than or approximately 10/sup 5/ L/s are adequate for the exhaust requirements in the pump-limiter and the divertor cases.