Impurity Ion Temperature Research Articles

Monte Carlo Modelling of Impurity Atom and Ion Transport Near A Limiter

AbstractA Monte Carlo code, LIM (Limiter Impurity), is used to track impurity atoms and ions toroidally/radially in the vicinity of a limiter. As an illustration of the use of LIM, two cases are analysed: physical sputtering and sublimation of carbon from the limiter. For the same total impurity production from the limiter, physical sputtering is found to be approximately three times more efficient at contaminating the plasma centre as sublimation. A number of further quantities, which can be compared with experiment, are computed: (a) the toroidal/radial shapes of the impurity clouds of successive ionization states, (b) the average impurity ion temperature in each cloud, (c) the ratio of the different charge states of the impurities returning to the limiter, their spatial distribution and impact energy, and (d) total radiated power.

TFTR diagnostic neutral beam

A neutral beam has been designed and fabricated to aid in the measurement of ion temperatures of hot, dense tokamak fusion test reactor (TFTR) plasmas. The injected fast neutral atoms offer a twofold benefit: (1) Penetration to the plasma center considerably increases charge exchange neutralization of thermal plasma ions, leading to a localized enhancement of the thermal neutrals efflux into charge exchange neutral analyzers; and (2) charge exchange recombination of fully stripped intrinsic impurity ions, C or O, with fast neutrals allows Doppler broadening and shift measurements of the emitted light to determine both the impurity ion temperature and toroidal rotation with unbalanced beam heating. The design and construction of the beam and those features of the beam unique to its function as a diagnostic tool for TFTR will be presented.

Zero-dimensional energy balance modelling of the CTX spheromak experiment

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-Härm 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.

Spectroscopic investigation of a reversed field pinch operated without limiters

Spectroscopic measurements on the ZT-40M reversed field pinch [submitted to Fusion Tech.] discharge are reported. Specifically, the impurity content, Zeff, recycling behavior, ion temperature, and propagation of magnetohydrodynamic modes in the edge are addressed. The importance of the plasma–wall interaction in ZT-40M, operated without limiters, is demonstrated.

Ion energy measurements on a reversed-field pinch experiment using Doppler broadening

Ion energies have been measured on the ZT-40M reversed-field pinch experiment using Doppler broadening of various impurity lines in the vacuum UV and ultraviolet range. The impurity ion temperatures are expected to relax quickly to the deuteron temperature according to the classical relaxation times. These ion temperatures as measured using O+6 (1623 Å) exceed the electron temperatures for plasma currents greater than 300 kA and fill pressures ≤2.0 mTorr. These investigations were carried out over a wide range of peak plasma currents 120 ≤Iφ<450 kA, with central electron densities 0.4≤ne≤1.2×1014 cm−3 yielding ion temperatures up to ∼500 eV. A zero-dimensional model using an approximation for the ion heating rate by the current driven electrostatic ion-cyclotron instability has been used to make comparison with the measured ion temperatures. This mechanism appears plausible but other mechanisms may be equally likely.

Determination of plasma-ion velocity distribution via charge-exchange recombination spectroscopy

Spectroscopy of line radiation from plasma impurity ions excited by charge-exchange recombination reactions with energetic neutral beam atoms is rapidly becoming recognized as a powerful technique for measuring ion temperature, bulk plasma motion, impurity transport, and more exotic phenomena such as fast alpha particle distributions. In particular, this diagnostic offers the capability of obtaining space- and time-resolved ion temperature and toroidal plasma rotation profiles with relatively simple optical systems. Cascade-corrected excitation rate coefficients for use in both fully stripped impurity density studies and ion temperature measurements have been calculated to the principal ..delta..n = 1 transitions of He+, C/sup 5 +/, and O/sup 7 +/ with neutral beam energies of 5 to 100 keV/amu. A fiber optically coupled spectrometer system has been used on PDX to measure visible He/sup +/ radiation excited by charge exchange. Central ion temperatures up to 2.4 keV and toroidal rotation speeds up to 1.5 x 10/sup 7/ cm/s were observed in diverted discharges with P/sub INJ/ less than or equal to 3.0 MW.

Impurity transport in the ASDEX edge plasma studied by a combination of passive and active probes with SIMS analysis

Radial profiles of impurity fluxes in the edge plasma of ASDEX are measured with collecting probes. Rutherford backscattering (RBS) and secondary ion mass spectrometry (SIMS) are used to analyse the collectors. The good agreement between the two methods indicates that the SIMS can be used as a new rapid analysis technique for large collectors. The impurity flux is found to decay exponentially with an e-folding length corresponding to the length of the flux tube in front of the probe, indicating that the flow is free streaming. Impurity ion temperatures are measured by means of an E × B analyser. A preliminary evaluation yields Fe ion temperatures of about 10 to 20 eV. Hence the cross field diffusion coefficient is estimated to be about 3 × 10 3 cm 2/s .

Effect of bias on trapping probes and bolometers for Tokamak edge diagnosis

Analysis is presented for the particle and energy flow to an electrically biasable probe immersed in the magnetic field of the boundary plasma of a Tokamak or other magnetic confinement device. The analysis is appropriate to the operation of the probe in the voltage-current (Langmuir) probe mode, the thermal power (bolometer) mode, hydrogenic ion trapping mode or impurity deposition mode. The formulation of the analysis permits the use of any cross-field diffusion model, i.e. classical, semi-classical, empirical. The principal conclusion of the analysis is that a probe should be operated simultaneously in at least two modes, e.g. Langmuir and bolometer modes. In this way it is possible to measure plasma density and electron and ion temperatures separately with several consistency checks also available. Under some circumstances it is also possible to measure the hydrogenic cross-field diffusion coefficient and impurity ion density, temperature and charge state. It is necessary to operate a bolometer in a biased mode in order to infer heat loads to non-floating surfaces, e.g. limiters and divertor targets, since heat flux is a strong function of the potential difference between the plasma and surface.

Determination of ion temperatures in the edge plasma from ion flux transmission of apertures

The flow of ions through an aperture perpendicular to a strong magnetic field has been calculated for a Maxwellian velocity distribution. The results obtained are needed for the interpretation of time resolved analysis of fluxes using the collecting probe technique and also for in situ mass, energy, and charge state analysis with spectrometers. Moreover, the energy dependent transmission of the aperture can be used to determine ion temperatures. Deuterium, carbon and metal impurity ion temperatures in the limiter shadow plasma of TFR 600 have been evaluated from measured ion flux transmission. For deuterium a temperature of 100 eV was found on the electron side of the collector. At the ion side the temperatures are higher by a factor of two. Impurity ion temperatures are ambiguous to the extent of the knowledge on the charge state. Assuming singly charged ions for Ni + a temperature of 10 eV on the ion side and 4 eV on the electron side was found during high density discharges with an Inconel limiter. In case of a carbon limiter the C + ion temperature on the ion side was about 35 eV.

The Silver/Silver Iodide/Silver Thermocell

Various thermodynamic expressions for the thermoelectric effects in solid electrolytes, including silver iodide, have been reported in the literature. No attempt has been made so far to account for the contraction phenomenon occurring in γ, β silver iodide during temperature increase. Considering that we are dealing actually with a thermomechanical electrical effect, a novel equation for the thermoelectric power of silver iodide was developed which appears to account quantitatively for the experimental data.The equation contains two terms. Discussion of the nature of these terms and their numerical values lead us to believe that the first term is identical with the heterogeneous thermoelectric effect and the second term with the homogeneous thermoelectric effect. These two effects were postulated and discussed by earlier investigators.The equation permits computation of both effects as well as computation of the total thermoelectric power as a function of impurity ion concentration and temperature from 0° to 140 °C. It appears that the heterogeneous thermoelectric effect is a function of the impurity ion concentration level. The levels in a commercial product of powdered silver iodide from which the sensor pellets were prepared were . These levels correspond to an averaged thermoelectric power of at room temperature.