S.J. Kilpatrick

First-wall conditioning for enhanced confinement discharges and the DT experiments in TFTR

The conditioning techniques applied to the TFTR first-wall configuration that will be in place for the DT experiments in 1990–1991 are reviewed. Of primary interest is the helium conditioning procedure that was developed to control hydrogenic recycling from the graphite, inner-wall bumper limiter. Operation of TFTR over the plasma density range for gas-fueled ohmic plasmas, ne = (2–5) × 1019m−3, typically results in hydrogenic recycling coefficients near unity. The use of the helium conditioning procedure produced recycling coefficients as low as 0.5, and decreased the minimum ohmic plasma density to ne = 0.5 × 1019m−3 at IP = 0.8 MA. Low density ohmic target plasmas with low recycling conditions are prerequisite conditions for the enhanced confinement (e.g., “supershot”), neutral-beam-heated discharges observed in TFTR during 1986–1987, which is the primary mode being considered for study in the DT experiments. The recycling changes induced by the helium conditioning procedure are believed to be the result of a plasma pumping effect in the graphite induced by He and C ion desorption of hydrogenic species from the near-surface (< 20 nm) layer of the limiter. The capacity of the conditioned limiter to pump gas-fueled, pellet-fueled, and neutral-beam-fueled discharges is compared. The helium conditioning technique is also beneficial for isotopic exchange and for minimizing the in-vessel tritium inventory.

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.

Overview of TFTR transport studies

A review of TFTR plasma transport studies is presented. Parallel transport and the confinement of suprathermal ions are found to be relatively well described by theory. Cross-field transport of the thermal plasma, however, is anomalous with the momentum diffusivity being comparable to the ion thermal diffusivity and larger than the electron thermal diffusivity in neutral beam heated discharges. Perturbative experiments have studied nonlinear dependencies in the transport coefficients and examined the role of possible nonlocal phenomena. The underlying turbulence has been studied using microwave scattering, beam emission spectroscopy and microwave reflectometry over a much broader range in k perpendicular to than previously possible. Results indicate the existence of large-wavelength fluctuations correlated with enhanced transport.

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...

Initial limiter and getter operation in TFTR

Abstract During the recent ohmic heating experiments on TFTR, the movable limiter array, preliminary inner bumper limiter, and prototype ZrAl alloy bulk getter surface pumping system were brought into operation. This paper summarizes the operational experience and plasma characteristics obtained with these components. The near-term upgrades of these systems are also discussed.

Status and plans for TFTR

AbstractRecent research on TFTR has emphasized optimization of performance in deuterium plasmas, transport studies and studies of energetic ion and fusion product physics in preparation for the D-T experiments that will commence in July of 1993. TFTR has achieved full hardware design parameters, and the best TFTR discharges in deuterium are projected to QDT of 0.3 to 0.5.The physics phenomena that will be studied during the D-T phase will include: tritium particle confinement and fueling, ICRF heating with tritium, species scaling with tritium, collective alpha-particle instabilities, alpha heating of the plasma and helium ash buildup. It is important for the fusion program that these physics issues be addressed to identify regimes of benign alpha behavior, and to develop techniques to actively stabilize or control instabilities driver by collective alpha effects.

High power neutral beam heating experiments on TFTR with balanced and unbalanced momemtum input

New long-pulse ion sources have been employed to extend the neutral beam pulse on TFTR from 0.5 sec to 2.0 sec. This made it possible to study the long-term evolution of supershots at constant current and to perform experiments in which the plasma current was ramped up during the heating pulse. Experiments were conducted with co and counter injection as well as with nearly balanced injection of deuterium beams up to a total power of 20 MW. The best results, i.e., central ion temperatures Tio > 25 keV and neo τE Tio values of 3 × 1020 keV sec m-3, were obtained with nearly balanced injection. The central toroidal plasma rotation velocity scales in a linear-offset fashion with beam power and density. The scaling of the inferred global momentum confinement time with plasma parameters is inconsistent with the predictions of the neoclassical theory of gyroviscous damping. An interesting plasma regime with properties similar to the H-mode has been observed for limiter plasmas with edge qa just above 3 and 2.5.

High- Q plasmas in the TFTR tokamak

In the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Fusion 26, 11 (1984)], the highest neutron source strength Sn and D–D fusion power gain QDD are realized in the neutral‐beam‐fueled and heated ‘‘supershot’’ regime that occurs after extensive wall conditioning to minimize recycling. For the best supershots, Sn increases approximately as P1.8b. The highest‐Q shots are characterized by high Te (up to 12 keV), Ti (up to 34 keV), and stored energy (up to 4.7 MJ), highly peaked density profiles, broad Te profiles, and lower Zeff. Replacement of critical areas of the graphite limiter tiles with carbon‐fiber composite tiles and improved alignment with the plasma have mitigated the ‘‘carbon bloom.’’ Wall conditioning by lithium pellet injection prior to the beam pulse reduces carbon influx and particle recycling. Empirically, QDD increases with decreasing pre‐injection carbon radiation, and increases strongly with density peakedness [ne(0)/〈ne〉] during the beam pulse. To date, the best fusion resu...

TFTR confinement results

The characteristics of plasma operation on the axisymmetric inner toroidal limiter in TFTR are described. After conditioning, plasmas with low metal content and low zeff are obtained with this limiter. There is no substantial increase in zeff with total input power during neutral beam injection. Compared to operation on the outer blade limiter, additional gas is required to fuel plasmas on the inner limiter. Injection of D pellets increased the plasma density substantially and produced energy confinement times up to 0.5 s in ohmically heated plasmas. The four neutral beam lines have injected up to 13.5 MW total power into the plasma for 0.5 s with up to 90 kV accelerating voltage. The scaling of the plasma stored energy was studied as a function of the input power, plasma current and plasma density. In the range 1.4 to 2.2 MA, the overall and incremental confinement times for both the total and thermal stored energies increase with plasma current at fixed density. There appears to be a weak negative scaling of the total stored energy with density at high injection power.

Effect of the boundary plasma on TFTR ohmic discharges

The role of the boundary plasma in determining the power and particle balance of tokamak discharges is discussed. Detailed boundary plasma measurements of edge density, edge temperature, deuterium influx and carbon influx are reported from ohmically heated TFTR discharges over a range of plasma densities. The experimental results are compared with predictions from a simple zero-dimensional model based on power and particle balance. Reasonable agreement is obtained. More detailed impurity modelling is performed with the LIM impurity production and transport code. The complementary modelling approaches reveal, amongst other results, the important role of the sputtering yield at the limiter in determining the central effective charge of the discharge, the ability of a densified boundary plasma to screen impurities from the central plasma and the importance of cross-field particle transport to the TFTR limiter. The authors demonstrate that, under the conditions of this experiment, the behaviour of the boundary plasma and its effect on the central plasma appear to be explicable using rather simple considerations of power and particle balance