Particle Confinement Time Research Articles

Experimental relation between particle and energy confinement in reversed-field pinches.

Confinement studies have been performed on the high-aspect-ratio, high-current-density Extrap-T1 reversed-field pinch. Experimental evidence is presented that the scaling of the particle confinement time is not dominated by the dynamo activity. This results in an anticorrelation between particle confinement time and energy confinement time, which becomes apparent in this experiment, as the ratio of Spitzer to total input power is varied over the wide range 0.4--0.8.

Particle transport studies for single-null divertor discharges in DIII-D

In this paper an investigation of the particle confinement for beam-heated single-null discharges in the open divertor configuration of Doublet III-D (DIII-D) [E. J. Doyle et al., Phys. Fluids B 3, 2300 (1991)] is described. Results are based on a Monte Carlo neutral transport model with a relatively simple plasma model that utilizes experimental data on density, temperature, and heat flux profiles in the edge plasma. For a typical discharge, it is found that the particle confinement time in the quiescent H-mode phase is only about a factor of 2 larger than during the L-mode phase, an increase comparable to the energy confinement time increase. For both H-mode and L-mode phases the particle confinement time is about a factor of 4 larger than the energy confinement time. It is also found that the core plasma fueling rate is higher in the H mode due to the increased transparency of a thinner scrape-off layer. The longer particle confinement time and the increased fueling rate both contribute to the observed density rise during the quiescent period following the L–H transition. Flux surface-averaged transport modeling of the time evolution for the core plasma density profile during H mode suggests that a strong inward particle pinch is necessary near the separatrix.

Particle and energy confinement bifurcation in tokamaks

A bifurcation in the particle and energy confinement properties of a tokamak plasma, with properties similar to the experimentally observed L-mode to H-mode transition, is shown to follow from a simple model for the transport. The basic assumptions are that the edge turbulence is suppressed by sheared E×B flow, and the radial ion pressure gradient is approximately balanced by the radial electric field. The particle and thermal diffusivities are assumed to be given by simple nonlinear functions of the radial electric field shear, which is related to the density and pressure gradients. The steady-state density and pressure profiles are found to have large gradient regions near the plasma edge when the product of edge energy flux and particle flux exceeds a threshold value, which corresponds to the H-mode threshold. The particle and energy confinement times are significantly larger when this threshold is exceeded. The confinement times exhibit hysteresis, corresponding to a dependence of quasiequilibrium states on the time history. The width of the edge layer where the gradients are large is determined mainly by the particle source profile, which is assumed to be concentrated near the plasma edge. This edge layer width increases slowly with increasing edge heat flux or heating power, as observed experimentally.

Plasma profile modifications using magnetic field perturbations in the ATF torsatron

Effects of deliberately introduced magnetic field perturbations on plasma conditions have been studied in the Advanced Toroidal Facility (ATF) torsatron. Magnetic dipole coils, located above the plasma at two positions 180 degrees apart, created a perturbation Bz0/Bϕ0 ⩽ 1.4%, which produced a large l=1/2 island. Two modes of plasma operation were employed: (1) constant gas feed allowing the electron density to vary and (2) constant density with the gas feedback controlled. Without density control, the density and the stored energy were approximately halved. With density control, the stored energy decreased only slightly and the potential increased in the vicinity of the island. It was found that both with and without density control, the pressure profile narrowed and the particle confinement was lowered substantially when the perturbations were applied. Thus, field perturbations, provide an external control on the pressure profile and the particle confinement time

ITER edge-plasma conditions versus pump configuration

Two-dimensional fluid simulations of ITER double-null divertor scrape-off-layer plasma conditions have been restricted to examining a single outboard divertor plate with up/down symmetry assumed. The present work evaluates the effect of pumping at only one plate on particle flow patterns and other parameters of interest. Pumping only at one plate results in reduced sheath temperatures at both plates but an increased heat flux at the pumped plate. The physics assumptions for separatrix density (n{sub SEP}={l angle}n{r angle}/3.5) and for radial particle diffusivity (D=0.66m{sup 2}/s) used in the simulation of ITER edge plasma result in particle throughputs two orders of magnitude greater than that required for acceptable fusion-product ash removal rates. The particle confinement time, however, is an order of magnitude shorter than the transport energy confinement time, {tau}{sub E}. Plasmas (D=0.04m{sup 2}/s) which would have {tau}{sub p} {approximately} {tau}{sub E} are evaluated and found to have unacceptably high plasma temperatures and heat flux at the plate. Ash removal rates are still acceptable. A plasma recycle coefficient of R=0.965 reduces the particle throughput by a factor of 2.8 below the no-recycle case.

Transport analysis of divertor plasma in JT-60U

A simple divertor model has been developed in order to analyze the divertor transport based on experimental data. The fluid equations are solved numerically in one dimension along the magnetic field lines. In contrast to the conventional divertor code, the boundary conditions are given by experimentally determined plasma parameters at the divertor plate, which are measured by Langmuir probes. The neutral particle transport is simulated in two dimensions by a Monte Carlo code. The interaction between the plasma and the neutral particles is solved self-consistently using an iterative procedure; the plasma parameters converge in a few iterations. The little computational time needed facilitates a systematic analysis of the divertor transport. By applying this model to the JT-60U divertor plasmas, the particle confinement time in the main plasma and the heat diffusivity in the scrape-off layer are evaluated.

Hydrogen recycling and its control on HL-1 tokamak

Particle recycling significantly affects the present operation of the HL-1 tokamak device. Recycling behavior and its effect on particle confinement are surveyed under different operational conditions in this paper. Global hydrogen recycling coefficient, R , ranges from 0.8 to 0.9. The fuelling ratio is quite low (20–30%). Various techniques of recycling control have been adopted to improve particle confinement, and consequently, to extend the operating regime. The optimized conditioning procedure, vessel bakeout at 180–200°C concurrent with ac (50 Hz) discharge cleaning with H 2 +Kr mixed gas, produces R as low as 0.5–0.6. Wall modification by titanium film saturated with H 2 , carbonization or inert gas puffing enhances the wall fuelling and increases the time constant of the density decay τ p * by a factor of 3, where τ p * = τ p /(1− R ) and τ p is the particle confinement time. H α emission near the limiter shows strong density and plasma current scaling (α n e 1.5 / I p 2 ). Two relative steps of discharge control during current ramp-up and flat-top phase were taken. The pump limiter decreases R by 13%. The highest density, 1.08×10 20 m −3 , is achieved and the Murakami parameter is raised up to 4.66×10 19 m −2 T −1 by using the above mentioned techniques.

Particle fluxes and impurities in EXTRAP T1 from surface probes and spectroscopy

EXTRAP T1 is a toroidal device which can be operated as a reversed field pinch, or with an externally applied octupole field as an external ring trap. The device has a stainless steel vessel and has been operated with hydrogen, helium and deuterium plasmas. In the present paper, a series of similar RFP discharges in deuterium have been studied, using surface probes and visible, H α and VUV spectroscopy. Data from H α spectroscopy and surface probes lead to the conclusion that the particle confinement time was τ p ≈ 20 μs and that the total flux density of particles to the wall was ∼2×10 19 D cm −2 s −1 . Moreover, it can be inferred that the flux to the wall consists predominantly of neutrals, with energy 30 eV or less, but includes a high energy component of mainly ions, with energy distribution corresponding to kT i ≈100 eV. The deposition rate of stainless steel components on probes at the wall position is ∼0.5% of the deuterium flux, and is consistent with uniform physical sputtering. The conditions during RFP operation with clean walls are compared with other modes of operation and with operation with contaminated surfaces.

E×B transport in the DIII-D boundary plasma

We have measured the electrostatic turbulence and associated particle transport in the DIII-D boundary plasma using a fast reciprocating Langmuir probe array located on the outboard midplane. Both the normalized rms fluctuation levels (density and floating potential) and the fluctuation-driven particle transport are altered by the L-H transition in the SOL. At the separatrix, the density fluctuation level is reduced a factor of 2, consistent with reflectometry results. There is a corresponding decrease in the turbulent particle flux. Deeper in the SOL, the turbulent particle transport in H-mode exceeds the L-mode value. The perpendicular diffusion coefficient D ⊥ and particle confinement time τ p have been estimated, assuming that the transport is purely turbulent and uniform on a flux surface. We find D ⊥ =0.7 D B (L) and 0.04 D B (ELM-free H), and τ p =54 ms (L) and 480 ms (ELM-free H).

Plasma wall particle balance in Tore Supra

A comprehensive study of the particle balance between the carbon wall and the plasma is presented. One finds that the effective particle content of the wall which governs the plasma equilibrium density departs from the deposited number of particles. This effect is dominant for the fully desaturated wall. A scaling law of the plasma density in terms of the wall effective particle content has been obtained. Moreover, the experimental data allows to estimate the plasma particle confinement time. Values ranging from 0.2 to 0.5 s are found depending on the density. An analytical functional dependence of the particle confinement time is obtained.

Modification of the plasma edge properties during divertor plate biasing experiments on TdeV

A laser-ablated fast particle diagnostic has been used for the measurement of n e (r) from the wall to 50 mm inside the TdeV separatrix. Divertor plate biasing has been found to modify substantially the n e ( r ) profile. The e-folding length at the separatrix, ν n , has been evaluated as a function of the biasing potential, and the results suggest a modification of the radial transport as a function of the radial electric field. The “LaBombard model” using different diffusivities ( D ÷ ) and mobilities (μ ÷ ) for ions and electrons cannot reproduce the dependence of ν n as a function of biasing. A simple model using the same D ÷ and μ ÷ for electrons and ions has been developed which gives a reasonable fit to the experimental measurements for D ÷ =3.2 m 2 /s and μ ÷ =0.039 m 2 /s V. Also, the particle confinement time τ p has been estimated with the help of the model, suggesting that negative biasing of the divertor plates increases τ p .

Particle balance and heat balance in JT-60U

Profiles of the ion saturation current ( I s + and the electron temperature ( T e ) on the divertor plates are measured by the use of a poloidal Langmuir probe array in JT-60U hydrogen ohmic discharges. Total particle and heat fluxes to the divertor plates are calculated by integrating the components of I s + and I s + T e perpendicular to the divertor plate. The results are presented in relation to the main plasma parameters such as ¯n e , B t , and the distance of X-point from the divertor plates. The particle flux and ionized neutral influx, which is deduced from the H α emission signals from the plasma boundary, are in fairly good agreement, within experimental errors. The particle confinement time, τ P SOL , including SOL effects is derived from the measured influx, and it decreases with ¯n e in the relatively high density regime ( ¯n e ≥1.3×10 19 m −3 ). However, the deterioration depends on the direction of B t . The power balance between the heat flux and radiated power is discussed.

Helium Accumulation Effects Using a Benchmarked Zero-Dimensional Model

Multidimensional transport simulations using the BALDUR code have been used to examine the correlation between the effective global helium particle confinement time and the edge recycling coefficient with profile effects included. This provides a way to benchmark the widely used zero-dimensional model so that a first-order correction for these effects is possible. It is shown that the predictions of the effect of recycling on helium confinement using the benchmarked zero-dimensional correlation agrees fairly well with the BALDUR results.

AC discharge carbonization in HL-1 tokamak

A wall surface carbonization experiment was carried out with an ac discharge on the HL-1 tokamak and homogeneous carbon films were deposited. An empirical criterion for the carbon layer formation has been presented, taking into account the ratio of the partial pressures of carbon oxide and O−H species. The wall condition discharge just after carbonization with Kr is better than that with He for depleting hydrogen and oxygen within the carbon layer. An analysis of the layers deposited on INCONEL GH39 samples, which were exposed to the carbonization process, regarding chemical composition on the surface and in-depth profiles, interface formation and thickness, has been made with SEM, AES and SIMS. Impurity contents in the plasma during the tokamak discharge are measured by the analysis of resonance line intensities and QMA, metal contents are reduced to 20%. The hydrogen particle confinement time τp and recycling coefficient R increased by about 20% compared with those before carbonization.

Experiments with a biased pump limiter on the HL-1 tokamak

Experiments with a biased pump limiter of positive potential have been performed on the HL-1 tokamak. The line averaged density ¯ne increases by 50%. The Dα (or Hα) light from the plasma edge decreases a little (10%). The particle flux signal taken at the plasma edge decreases by 50%. The OVI signal at the edge increases by 60% and the loop voltage increases by 20%. The particle flux signal in the throat decreases by 60%. The particle confinement time τp* increases by 50–100%, the energy confinement time τE increases by 20–40%. But the CIII, CrI lines near the neutralizer plate increase for both positive and negative biasing. Generally, effects with negative biasing are much less pronounced than with positive biasing. For example, the line averaged density ne increases by 25% only with a negative biasing of 200 V.

Simulations of constant poloidal beta experiments on ZT‐40M using a one‐dimensional multispecies transport model

A one‐dimensional (1‐D) multispecies transport model for the reversed‐field pinch plasma, which includes neutral deuterium and a coronal nonequilibrium model for impurities, is used to simulate a set of ZT‐40M [Fusion Technol. 10, 307 (1986)] experiments. In these experiments, the radiation fraction is varied from 15% to 95% by use of krypton seeding, while the plasma operating parameters Iφ and ne are constrained to within a small variation from a fixed operating point. In effect, these experiments maintain an almost constant βθe on axis for the range of radiation fractions. The simulations match experimental estimates of particle, energy, and nonradiative confinement times, and of the electron poloidal beta. The results show that the anomalous thermal transport needed to reproduce the observed experimental behavior, i.e., constant βθe on axis, can be modeled by use of a small amount of parallel thermal conduction. Such a model phenomenologically accounts for anomalous heat transport due to fluctuations or stochastic field line diffusion. This model is also used for parametric studies on the design conditions of ZTH [Bull. Am. Phys. Soc. 31, 1546 (1984)]. To achieve the ZTH design values it is necessary for particle and thermal diffusivities to improve by a factor of 10–15 over ZT‐40M.

Evaluation of Minimum Neutron Source Strength Required for Ignition of D-T Fuel

Minimum thermal and cold neutron source strength necessary for the thermonuclear ignition of D-T fuel using the energy released in exothermic 3 He(n, p)T reaction is determined for representative tokamak parameters, when the tokamak inner walls are lined with good thermal neutron reflectors. It is found, that D-T ignition is possible with the help of currently available neutron sources if the particle and energy confinement times exceed tens of seconds. If thermal or cold neutron sources having strength greater than 10 23 s -1 can be constructed, then D-T ignition is possible even in plasmas having energy and particle confinement times in the range of one tenth of a second. Although the focus here is on tokamak fusion reactors the results are applicable to other type of reactors as well.

High-power magnetic-compression heating of field-reversed configurations

Magnetic compression heating experiments at the 1 GW level on field-reversed configuration (FRC) compact toroid plasmas are reported. FRC’s formed in a tapered theta-pinch coil have been translated into a single-turn compression coil, where the external magnetic field is slowly raised up to seven times its initial value. Significant electron and ion heating consistent with the expected B4/5 adiabatic scaling is observed, despite significant particle diffusion, which is enhanced during compression. The n=2 rotational instability is enhanced during compression, but has been controlled to an extent by the application of an external quadrupole field. The particle and flux confinement times, τN and τφ, remain approximately equal and decrease roughly with the square of the plasma radius R during compression, implying a constant nonclassical field-null resistivity. The observed τN and τφ magnitudes and scalings are compared with classical and anomalous transport theories, and existing empirical models. Particle diffusion dominates the energy confinement, accounting for three-fourths of the total losses. Upper bounds on the electron thermal diffusivities are estimated.

Impurity transport in the Advanced toroidal facility

The spectral line emissions from impurities injected using the laser ablation technique have been modelled in order to deduce impurity transport coefficients in the Advanced Toroidal Facility (ATF). The transport is found to be mainly diffusive, with little or no convective component. To reproduce the observed line emission it is necessary to postulate a radially dependent diffusion coefficient. For plasmas heated by electron cyclotron frequency waves the impurity transport is best fitted with a diffusion coefficient of 1000 cm2/s in the centre, increasing to 5000 cm2/s at the plasma edge. The global impurity particle confinement time for these plasmas is 65 ms and is considerably larger than the corresponding energy confinement time. Plasmas heated by high energy neutral particle beams display different impurity transport, with the best fit being a diffusion coefficient that is peaked on axis (5000 cm2/s) and that decreases towards larger minor radii (500 cm2/s at the plasma boundary). These transport coefficients have been successfully used to simulate the evolution of intrinsic impurities as well as laser ablated impurities in ATF

Stationary Nuclear Burning of Plasmas Consisting of Lighter Than 6Li Nuclei

The problem of self-sustained stationary burning of plasmas consisting of lighter than 6Li nuclei is investigated. It is found that among all the possible binary reactions between the nuclei 1H, 2H, 3H, 3He, and 4He, only 2H-2H, 2H-3H, and 2H-3He reactions are frequent and energetic enough to sustain stationary burning. The stationary burning can occur at two temperatures. Whereas the burning at the lower temperature is unstable, that at the higher temperature is asymptotically stable. It is also found that the maximum temperature at which stationary burning occurs does not depend on the absolute values of the particle and energy confinement times τ and τE, but rather on their ratio τ/τE.