Improved Particle Confinement Research Articles

Relaxation phenomena induced by edge biasing experiments in the CASTOR tokamak

In this paper we present the first experimental results obtained in the CASTOR tokamak by using a segmented biasing electrode, which is composed of five segments radially separated by 3 mm. The basic idea of choosing such configuration was to allow a spatial distribution of the biasing voltage in the radial direction. In the described experiments, one or more selected segments can be biased up to +300V, while the remaining segments can be either grounded or floating. Two rake probes measure the edge radial profiles of the floating potential and the ion saturation current at the top and low field side of the torus. A Gundestrup probe, located at the top of the torus, measures the parallel and perpendicular Mach numbers together with the local electron temperature and density. A clear and reproducible transition to a regime with improved particle confinement is routinely observed, if the electrode is inserted deep enough into the plasma (r/a ≈ 0.5) and if one or two segments are biased up to +250V (the remaining segments are floating). The steepening of the radial profiles of the plasma density and potential demonstrate the formation of a transport barrier just inside the last closed flux surface. Fast relaxations of the edge profiles, with a frequency of about 10 kHz or higher, are observed when the value of the average radial electric field within the barrier reaches values of about 20 kV/m and the density gradient increases up to a factor 3 with respect to the ohmic phase. A detailed analysis of the spatial-temporal behavior of these relaxations is presented.

Lower hybrid current drive experiments with graphite limiters in the HT-7 superconducting tokamak

Recent progress of lower hybrid (LH) experiments with new graphite limiters configuration in the HT-7 tokamak is presented. The lower hybrid current drive (LHCD) efficiency can be determined by fitting based on experimental data. Improved particle confinement was observed via LHCD ( P LHW > 300 kW ) characterized by the particle confinement time τ p increased about 1.56 times. It is found that runaways are suppressed during loop voltage is decreasing at the flat-top phase of LH discharges. The main limitations of pulse length are presented in long-pulse experiments with new limiter configuration.

Comparison of limiter and emissive electrode bias on the tokamak ISTTOK

Biasing experiments have been performed on the tokamak ISTTOK with both a moving limiter and an emissive electrode. We have observed that large currents (>15A) can be drawn at negative applied voltage by both localized limiter and emissive electrode bias, leading to significant modifications in the edge plasma potential profile and to an improvement in particle confinement. However, compared with the localized limiter, the emissive electrode has the advantage of perturbing significantly less the plasma column. Furthermore, its use leads to the formation of stronger radial electric fields and consequently to a much larger improvement in particle confinement.

Improved confinement events triggered by emissive electrode biasing on the tokamak ISTTOK

Edge polarization experiments were carried out on ISTTOK using an emissive electrode. The dependence of the bias current on the radial electric field, measured by a radial array of Langmuir probes, is in qualitative agreement with the expression derived by Stringer for the transitional region between plateau and collisional transport regimes (Stringer T.E. 1993 Nucl. Fusion 33 1249). Improved particle confinement is clearly observed for negative bias associated with a large radial electric field. In some cases, a double-peaked profile of the radial electric field is observed just after biasing, evolving afterwards to a single-peaked profile as confinement improves. These types of profiles may be related to the multiple solutions of the non-linear equation for the radial electric field, recently put forward by Kasuya et al (2003 Nucl. Fusion 43 244). Above a certain threshold of the bias current stronger improved confinement events are observed, during short periods, for both bias polarities.

The influence of neutral particles on boundary turbulence and confinement in the HT-7 tokamak

An effective technique for wall conditioning by injecting an ion cyclotron resonant frequency wave, RF coating, was developed in the HT-7 tokamak. Under different wall conditions, namely before and after lithium/silicon coating, the boundary plasma behaviour was investigated using a reciprocating Langmuir probe system. On reduction of impurity radiation, strong shears of the radial electric field and the poloidal velocity accompanied by turbulence suppression and de-correlation were observed in the edge region of the coated wall plasma. This led to a reduction in edge transport and improvement in plasma confinement. An enhanced shear of the radial electric field was observed on increasing the electron density, which may account for the improvement in particle confinement. The results suggest a close link between wall conditions and boundary plasma. The features of ionization and radiation driven turbulence, T̃e/Te∼ñe/ne and θ_T̃eñe∼π, had been found in the plasma edge region. The interplay between atomic processes in the boundary plasma and the formation of the radial electric field shear may be responsible for this behaviour.

Boundary plasma behavior under different wall conditions on HT-7 tokamak

The boundary plasma behaviors under different wall conditions on the HT-7 tokamak were investigated using a reciprocating Langmuir probe system. The ion cyclotron radio frequency (ICRF) boronization has become a routine wall conditioning technique on HT-7. After ICRF boronization, the turbulence de-correlation and radial electric field shear in the plasma edge region accompanied the strong reduction of impurity radiation, which resulted in the enhancement of edge transport barrier. The central chord average density scanning experiment had been carried out on HT-7. Accompanying the increase of density an evident increase of radial electric field shear could be found in the plasma edge region, which could be responsible for the turbulence de-correlation and improved particle confinement. The results presented here suggest a link between wall conditions and boundary plasma physics; especially interplay between atomic processes and turbulence through the formation of radial electric field shear in the plasma edge region.

Edge-core relationship of ELMy H-mode plasmas in JT-60U

The effects of the conductive heat flow on the energy transport concerning the temperature profile similarity have been analysed for (\\ref{eq1}) high triangularity and (2) Ar seeded ELMy H-mode plasmas on JT-60U. The temperature profiles of the plasma core on the logarithmic plot are shape preserving and independent of the varying pedestal height. As the conductive heat flux increases, the effective heat conductivity, χeff, adjusts itself to sustain a constant ∇(ln Ti) in the plasma core. In high triangularity discharges, highly improved energy confinement quality accompanied by high core temperature is obtained when the boundary temperature rises with an increase in βpol. On the other hand, the peaked density profiles in Ar seeded discharges allow the temperature at the pedestal shoulder to be higher relative to the cases without Ar gas injection, and the core temperature is also raised. As the conductive heat flux is reduced by increasing the radiation loss flux in Ar seeded H-mode plasmas, χeff decreases with a similar ∇(ln Ti) of the plasma core in the case without Ar gas injection. Improved particle confinement due to Ar gas puff raises the achievable density and contributes to the reduction in χeff.

Experiments and Studies of Edge Plasma Radial Transport and Confinement Property on the HL-1M Tokamak

This paper describes a Mach/Langmuir probe array with five pins and six pins, which can measure not only parallel flows and the flow perpendicular to the magnetic field but also the radial and the poloidal electric field Er and Eθ as well. Experimental measurements of the edge fluctuations, velocities of the toroidal, the poloidal flow and electric field have been carried out on both of SOL and the boundary region of HL-1M for Ohmic, biased H-mode, Lower Hybrid Current Drive (LHCD), Supersonic Molecular Beam Injection (MBI), Multi-shot Pellet Injection (MPI), Neutral Beam Injection (NBI), Ion Cyclotron Resonance Heating (ICRH) and Electric Cyclotron Resonance Heating (ECRH) discharges. The results show that the suppressions of the fluctuations are related to poloidal rotations produced by different discharge modes in the improved particle confinement property, simultaneously the change of the radial and poloidal electric field is generated and becomes more negative at the Tokamak plasma edge, and the sheared poloidal flow is related to the reduction in fluctuation level, and the poloidal velocity is mainly dominated by the E × B drift.

Driven plasma rotation in the Compact Auburn Torsatron

Poloidal plasma rotation has been driven in the Compact Auburn Torsatron (CAT) [R. F. Gandy et al., Fusion Technol. 18, 281 (1990)]. In these studies, a biased electrode is used to establish the rotation. Currents of up to 1 A are injected into hydrogen, helium, and argon plasmas from a biased electrode placed inside of the last closed magnetic flux surface of the CAT device. The response of the plasma to the injected current is measured with Langmuir, Mach, and emissive probes. Poloidal plasma velocities of up to 10 km/s in hydrogen plasmas are measured by the Mach probe. The plasma density and electron temperature increased during the driven rotation, and there are indications of improved particle confinement. The experimental measurements show generally good agreement with a model of plasma rotation by Coronado and Talmadge. Experiments also show that the sign of the injected current (i.e., the direction of the radial electric field) has a significant impact on the effect of the driven rotation. A reduction in the particle transport is observed only for radially inward electric fields.

E×B flow shear and enhanced confinement in the Madison Symmetric Torus reversed-field pinch

Strong E×B flow shear occurs in the edge of three types of enhanced confinement discharge in the Madison Symmetric Torus [Dexter et al., Fusion Technol. 19, 131 (1991)] reversed-field pinch. Measurements in standard (low confinement) discharges indicate that global magnetic fluctuations drive particle and energy transport in the plasma core, while electrostatic fluctuations drive particle transport in the plasma edge. This paper explores possible contributions of E×B flow shear to the reduction of both the magnetic and electrostatic fluctuations and, thus, the improved confinement. In one case, shear in the E×B flow occurs when the edge plasma is biased. Biased discharges exhibit changes in the edge electrostatic fluctuations and improved particle confinement. In two other cases, the flow shear emerges (1) when auxiliary current is driven in the edge and (2) spontaneously, following sawtooth crashes. Both edge electrostatic and global magnetic fluctuations are reduced in these discharges, and both particle and energy confinement improve.

The fluctuations and flow velocities measured with a Mach probe array on the HL-1M tokamak

This paper describes a Mach probe array with four pins which can measure not only parallel flows but the flow perpendicular to the magnetic field as well. Experimental measurements of the fluctuations and velocities of the toroidal and the poloidal flow have been carried out on both of SOL and the boundary region of HL-1M for ohmic, biased H-mode and LHCD discharges. The results show that the suppressions of the fluctuations are related to poloidal rotations produced by electrode biasing and LH waves injecting in the improved particle confinement modes.

Improved Particle Confinement Mode in the H-1 Heliac Plasma.

The first experimental observation of the sudden transition to the improved particle confinement mode in the H-1 heliac is reported and shows a clear dependence on magnetic configuration. In a lowtemperature plasma a transition to improved confinement followed by a twofold increase in the electron density is observed when the magnetic field exceeds a critical value Bcr .A t B B cr the transition occurs spontaneously (within 1 ms). This Bcr strongly decreases with increasing rotational transform. The improvement in confinement correlates with the suppression of the fluctuations and fluctuationinduced particle flux and with the increase in the radial electric field. [S0031-9007(96)01636-5]

Spectroscopic measurement of target plate erosion in the ASDEX Upgrade divertor

The erosion of the graphite divertor plates in the ASDEX Upgrade tokamak is measured spectroscopically. Spatial profiles of the D0 and C+ influxes across the outer target plate are determined from measured absolute line intensities. Plasma parameters (ne, Te) at the target, which are required to determine the appropriate photon emission efficiencies for these lines, are obtained from an in-vessel reciprocating Langmuir probe above the target plate. Yields for the erosion of the graphite by the incident D+ flux are determined from the ratio of the measured C+ to D0 fluxes. Over a range of moderate densities the measured yields of ⩽4% are explicable in terms of physical sputtering alone. Chemical sputtering by low energy Franck-Condon neutrals probably contributes, however, to the total erosion. At higher densities detachment of the plasma from the targets occurs owing to formation of a MARFE near the X point. Under these conditions localized physical sputtering of the targets ceases. The impurity level (Zeff) is, however, maintained following detachment, indicating a corresponding maintenance of carbon influx, perhaps due to chemical erosion of the total graphite surface and/or an improvement in particle confinement in the detached state

Fuel Source Isotopic Tailoring and Its Impact on International Thermonuclear Experimental Reactor Design, Operation, and Safety

Isotopic tailoring of the deuterium and tritium density profiles in fusion reactors can lead to reduced tritium inventory in plasma facing components and, therefore, improved safety considerations. The isotopic tailoring concept consists of utilizing a tritium-rich pellet source for core fueling and a deuterium-rich gas source for edge fueling. Because of the improved particle confinement associated with the deeper tritium core fueling component, comparable core densities of deuterium and tritium can be maintained even when the edge deuterium fuel source is much larger than the core tritium fuel source. The fuel composition of the edge and scrape-off plasmas as well as the isotope fractions in plasma facing components reflect the total throughput of all makeup fuel and are therefore deuterium-rich. This innovative fueling concept results in about a factor of two reduction in tritium inventory of the plasma facing components. The higher tritium burn fraction allows a significant reduction in tritium gas flows into and out of the vacuum vessel and, for fusion reactors, implies lower required tritium breeding ratios. 10 refs., 3 figs., 2 tabs.

Biased H mode experiments in Phaedrus-T

Inserting a positively biased electrode to just inside the Phaedrus-T tokamak limiter results in typical H mode behaviour (i.e. Hα or Dα drop, density rise, increase in stored energy, profile steepening, and reduction of edge turbulence and radial transport) in deuterium, hydrogen and helium discharges. Hα or Dα emission suggests that the improvement in particle confinement with H mode is poloidally asymmetric, with the greatest improvement occurring on the low field side. The radial conductivity is examined and measured values are compared with theory

Study of electron density profile in the CASTOR tokamak using a lithium beam produced by diagnostic impulse arc source

A diagnostic impulse arc source of a beam of lithium atoms was used for the measurement of electron density profile in front of the lower hybrid grill on the CASTOR tokamak. It was shown that this profile is modified significantly during lower hybrid wave application if the regime with improved particle confinement is achieved.

Trapped particle losses in torsatrons

The confinement of helically trapped particles is studied in torsatrons of type ℰt > ℰl, in comparison with conventional torsatrons of the Heliotron-E type. The main emphasis is on the simulation of the lost trapped particle angular distribution on the last closed magnetic surface, as well as on the improvement of particle confinement due to the optimization of the vacuum magnetic configuration of a torsatron. Optimization criteria for a set of device parameters are proposed. The effect of toroidal satellite harmonics on trapped particle confinement is also studied

Impurity behavior in high-β plasmas in PBX-M

In PBX, an elongated, indented tokamak, an accumulation of impurities leading to strongly peaked radiation profiles was observed, most frequently during the H-mode. The modified device, PBX-M, features a close-fitting stabilizing metallic shell that covers over 85% of the plasma surface and has the capability to produce highly indented plasmas. Two distinct modes of operation were used to generate high-β equilibria in neutral beam heated discharges: 1) high current, highly indented high-βt plasmas, where the plasma was in close proximity to the stabilizing plates and 2) low current, lesser indented high-βp discharges where the plasma equilibrium shape was more oblate than the shell and the plasma was farther from the plates. In co-injected high-βt plasmas, radiative losses were typically 15–25% of the input power and impurity accumulation was mild even in discharges lasting up to 650 ms with 250 ms H-modes. In high-βp discharges, both co- and counter-injection was investigated. In discharges with tangential co-injection, total radiated power gradually peaked on axis, ultimately reaching 60% of the heating power. H-modes were frequently present and the high-Z impurity density profile was strongly peaked. In tangentially counter-injected discharges, the current profile modification led to the suppression of sawteeth and the near elimination of MHD activity outside the q=1 surface. The improved particle confinement led to high electron density, high impurity radiation levels and a thermal collapse; however, the concentration and accumulation of metallic impurities was not as severe as the co-injected case.

Fueling and gas evolution studies on the Tokamak de Varennes

Fueling and gas evolution have been investigated in the Tokamak de Varennes using time-resolved mass spectrometric gas analysis during and after the shot, gas flow measurements, and standard plasma diagnostics. The evolution of the H and impurity content, both in the plasma and the neutral gas, is used to compare the recycling behaviour of H 2 and CO. An accounting of the exhaust of H 2 and other gases after the shot is given, together with a discussion of the outgassing mechanisms. The scaling of the fueling efficiency with density and current has been studied and shows a remarkable increase at high current due to improved particle confinement and possibly an increased recycling coefficient.

Spatial structure of particle-orbit loss regions in l=2 helical systems

Particle orbits and loss regions in both configuration and velocity spaces are studied on the basis of adiabatic invariants and guiding-center drift equations. The boundary of the loss region in the pitch angle major radius plane (χ0-ρ) is determined from the condition of whether the drift surfaces for helically trapped and transition particles hit the limiter or not. Two typical loss boundaries on the χ0-ρ plane are obtained. One determines the boundary of absolute confinement where no particles are lost. The other gives the boundary of the confined region of deeply trapped particles with v∥=0. An analytical form describing the latter boundary is also derived. Effects of ripple modulation and electrostatic potential on these loss boundaries are discussed for the improvement of particle confinement.