D.K. Mansfield

The effect of lithium surface coatings on plasma performance in the National Spherical Torus Experiment

National Spherical Torus Experiment [which M. Ono et al., Nucl. Fusion 40, 557 (2000)] high-power divertor plasma experiments have shown, for the first time, that benefits from lithium coatings applied to plasma facing components found previously in limited plasmas can occur also in high-power diverted configurations. Lithium coatings were applied with pellets injected into helium discharges, and also with an oven that directed a collimated stream of lithium vapor toward the graphite tiles of the lower center stack and divertor. Lithium oven depositions from a few milligrams to 1g have been applied between discharges. Benefits from the lithium coatings were sometimes, but not always, seen. These benefits sometimes included decreases in plasma density, inductive flux consumption, and edge-localized mode occurrence, and increases in electron temperature, ion temperature, energy confinement, and periods of edge and magnetohydrodynamic quiescence. In addition, reductions in lower divertor D, C, and O luminosi...

Attainment of high confinement in neutral beam heated divertor discharges in the PDX tokamak

Abstract The PDX divertor configuration has recently been converted from an open to a closed geometry to inhibit the return of neutral gas from the divertor region to the main chamber. Since then, operation in a regime with high energy confinement in neutral beam heated discharges (ASDEX H-mode) has been routine over a wide range of operating conditions. These H-mode discharges are characterized by a sudden drop in divertor density and H α emission and a spontaneous rise in main chamber plasma density during neutral beam injection. The confinement time is found to scale nearly linearly with plasma current, but can be degraded due either to the presence of edge instabilities or heavy gas puffing. Detailed Thomson scattering temperature profiles show high values of T c near the plasma edge (∼ 450 eV) with sharp radial gradients (∼ 400 eV/cm) near the separatrix. Density profiles are broad and also exhibit steep gradients close to the separatrix.

Study on H-mode access at low density with lower hybrid current drive and lithium-wall coatings on the EAST superconducting tokamak

The first high-confinement mode (H-mode) with type-III edge localized modes at an H factor of HIPB98(y,2) ∼ 1 has been obtained with about 1 MW lower hybrid wave power on the EAST superconducting tokamak. The first H-mode plasma appeared after wall conditioning by lithium (Li) evaporation before plasma breakdown and the real-time injection of fine Li powder into the plasma edge. The threshold power for H-mode access follows the international tokamak scaling even in the low density range and a threshold in density has been identified. With increasing accumulation of deposited Li the H-mode duration was gradually extended up to 3.6 s corresponding to ∼30 confinement times, limited only by currently attainable durations of the plasma current flat top. Finally, it was observed that neutral density near the lower X-point was progressively reduced by a factor of 4 with increasing Li accumulation, which is considered the main mechanism for the H-mode power threshold reduction by the Li wall coatings. (Some figures in this article are in colour only in the electronic version)

Nondimensional transport scaling in the Tokamak Fusion Test Reactor: Is tokamak transport Bohm or gyro-Bohm?

General plasma physics principles state that power flow Q(r) through a magnetic surface in a tokamak should scale as Q(r)= {32π2Rr3Te2cu2009nea/[eB (a2−r2)2]} F(ρ*,β,ν*,r/a,q,s,r/R,...) where the arguments of F are local, nondimensional plasma parameters and nondimensional gradients. This paper reports an experimental determination of how F varies with normalized gyroradius ρ*≡(2TeMi)1/2c/eBa and collisionality ν*≡(R/r)3/2qRνe(me/ 2Te)1/2 for discharges prepared so that other nondimensional parameters remain close to constant. Tokamak Fusion Test Reactor (TFTR) [D. M. Meade et al., in Plasma Physics and Controlled Nuclear Fusion Research, 1990, Proceedings of the 13th International Conference, Washington (International Atomic Energy Agency, Vienna, 1991), Vol. 1, p. 9] L‐mode data show F to be independent of ρ* and numerically small, corresponding to Bohm scaling with a small multiplicative constant. By contrast, most theories predict gyro‐Bohm scaling: F∝ρ*. Bohm scaling implies that the largest scale size f...

Wall conditioning with impurity pellet injection on TFTR

Solid lithium and boron pellets have been injected into TFTR plasmas to improve plasma performance by coating the graphite inner wall bumper limiter with a small amount of lower Z pellet material, which reduces the influx of carbon from the walls and reduces the edge electron density. This new wall conditioning technique has been applied successfully when continued He conditioning discharges, which are normally used for wall conditioning, no longer significantly reduce the carbon and deuterium influxes. The results show that both Li and B pellets significantly improve wall conditioning and lead to 15–20% improvements in supershot plasma performance when injected ≥1 s prior to neutral beam injection in supershot target plasmas. Neutral beam penetration calculations indicate that the lower edge densities resulting from Li or B pellet wall conditioning lead to improved beam penetration. Sputtering yield calculations confirm that the addition of small amounts of Li on a graphite target can significantly reduce the C sputtering yield.

Simulations of alpha parameters in a TFTR DT supershot with high fusion power

A TFTR supershot with a plasma current of 2.5 MA, a neutral beam heating power of 33.7 MW and a peak DT fusion power of 7.5 MW is studied using the TRANSP plasma analysis code. Simulations of alpha parameters such as the alpha heating, pressure and distributions in energy and v1/v are given. The effects of toroidal ripple and mixing of the fast alpha particles during the sawteeth observed after the neutral beam injection phase are modelled. The distributions of alpha particles on the outer midplane are peaked near forward and backward v1/v. Ripple losses deplete the distributions in the vicinity of v1/v=-0.2. Sawtooth mixing of fast alpha particles is computed to reduce their central density and broaden their width in energy

Multichannel far-infrared laser interferometer for electron density measurements on the tokamak fusion test reactor

A ten-channel far-infrared laser interferometer which is routinely used to measure the spatial and temporal behavior of the electron density profile on the tokamak fusion test reactor is described, and representative results are presented. This system has been designed for remote operation in the very hostile environment of a fusion reactor. The possible expansion of the system to include polarimetric measurements is briefly outlined.

Plasma-material interactions in TFTR

This paper presents a summary of plasma-material interactions which influence the operation of TFTR with high current (≤ 2.2 MA) ohmically heated, and high-power (∼ 10 MW) neutral-beam heated plasmas. The conditioning procedures which are applied routinely to the first-wall hardware are reviewed. Fueling characteristics during gas, pellet, and neutral-beam fueling are described. Recycling coefficients near unity are observed for most gas fueled discharges. Gas fueled discharges after helium discharge conditioning of the toroidal bumper limiter, and discharges fueled by neutral beams and pellets, show R<1. In the vicinity of the gas fueled density limit (at ne = 5–6 × 1019 m−3) values of Zeff are ≦1.5. Increases in Zeff of ≦1 have been observed with neutral beam heating of 10 MW. The primary low Z impurity is carbon with concentrations decreasing from ∼10% to <1% with increasing ne. Oxygen densities tend to increase with ne, and at the ohmic plasma density limit oxygen and carbon concentrations are comparable. Chromium getter experiments and He2+/D+ plasma comparisons indicate that the limiter is the primary source of carbon and that the vessel wall is a significant source of the oxygen impurity. Metallic impurities, consisting of the vacuum vessel metals (Ni, Fe, Cr) have significant (∼10−4 ne) concentrations only at low plasma densities (ne <1019 m−3). The primary source of metallic impurities is most likely ion sputtering from metals deposited on the carbon limiter surface.

Transient electron heat diffusivity obtained from trace impurity injection on TFTR

A new method for obtaining a transient (pulse) electron heat diffusivity (χep) in the radial region 0.38 < r/a < 0.56 in TFTR L mode discharges is presented. Small electron temperature perturbations were caused by single bursts of injected impurities which radiated and cooled the plasma edge. A case of iron injection by laser ablation was found to be more definitive than a supporting helium gas puff case. In this new cold pulse method, the authors concentrate on modelling just the electron temperature perturbations, tracked with electron cyclotron emission diagnostics, and on being able to justify separation of the perturbations in space and time from the cooling source. This χep is obtained for these two cases to be χep = (6.0 m2/s ± 35%) ~ 4χe (power balance), which is consistent with but more definitive than results from other studies that are more susceptible to ambiguities in the source profile

Correlations of heat and momentum transport in the TFTR tokamak

Measurements of the toroidal rotation speed vφ(r) driven by neutral beam injection in tokamak plasmas and, in particular, simultaneous profile measurements of vφ, Ti, Te, and ne, have provided new insights into the nature of anomalous transport in tokamaks. Low‐recycling plasmas heated with unidirectional neutral beam injection exhibit a strong correlation among the local diffusivities, χφ≊χi>χe. Recent measurements have confirmed similar behavior in broad‐density L‐mode plasmas. These results are consistent with the conjecture that electrostatic turbulence is the dominant transport mechanism in the tokamak fusion test reactor tokamak (TFTR) [Phys. Rev. Lett. 58, 1004 (1987)], and are inconsistent with predictions both from test‐particle models of strong magnetic turbulence and from ripple transport. Toroidal rotation speed measurements in peaked‐density TFTR ‘‘supershots’’ with partially unbalanced beam injection indicate that momentum transport decreases as the density profile becomes more peaked. In hi...