M. Ulrickson

Effects of boronization of the first wall in TFTR

Abstract A plasma-enhanced CVD process was used to deposit thin (40–100 nm), amorphous carbon/boron films onto the first wall of TFTR. A series of plasma experiments was performed to test the effect of this first-wall modification on low-Z impurity (carbon, oxygen) behavior, suppression of metallic impurities, post-disruption recovery, hydrogenic recycling, and plasma density limits. The analysis of the plasma performance following the first few boronization attempts on TFTR shows: (1) suppression by a factor of 3-1.5 of carbon impurities immediately after boronization, although this suppression is short-lived (~10 shots); (2) long term suppression of oxygen and metallic impurities by a factor of two; (3) significant suppression of oxygen impurity emission following disruptions and during pulse discharge cleaning; and (4) no significant change in hydrogenic recycling properties in comparison to the pre-existing carbon first wall.

Conditioning of the graphite bumper limiter for enhanced confinement discharges in TFTR

A strong pumping effect was observed with plasma operation on the toroidal graphite bumper limiter in TFTR. The pumping effect was induced by conditioning the limiter with a short series (10-20) of low density deuterium or helium initiated discharges. The decay constant for gas fuelled Ohmic discharges was reduced from before conditioning to a minimum of after conditioning, which corresponds to a reduction in the global recycling coefficient from about 100% to less than 50%. Coincident with the low recycling conditions, low current neutral beam fuelled discharges showed global energy confinement times which were enhanced by a factor of two over the values obtained with an unconditioned limiter. Two models are proposed for the observed pumping effects: (1) a depletion model, based on pumping of hydrogenic species in the near-surface region of the limiter after depletion of the normally saturated surface layer by (carbon and helium) ion induced desorption; and (2) a co-deposition model, based on adsorption of hydrogenic species in carbon films formed by material sputtered from the limiter during conditioning.

Enhanced carbon influx into TFTR supershots

Under some conditions, a very large influx of carbon into TFTR occurs during neutral beam injection into low recycling plasmas (the supershot regime). These carbon blooms result in serious degradation of plasma parameters. The sources of this carbon have been identified as hot spots on the TFTR bumper limiter at or near the last closed flux surface. Two separate temperature thresholds have been identified. One threshold, at about 1650°C, is consistent with radiation enhanced sublimation (RES). The other, at about 2300°C, appears to be thermal sublimation of carbon from the limiter. The carbon influx can be quantitatively accounted for by taking laboratory values for RES rates, making reasonable assumptions about the extent of the blooming area and assuming unity carbon recycling at the limiter. Such high carbon recycling is expected, and it is shown that, in target plasmas at least, it is observed on TFTR. The sources of the carbon blooms are sites which have either loosely attached fragments of limiter material (caused by damage) or surfaces that are nearly perpendicular to the magnetic field lines. Such surfaces may have local power depositions two orders of magnitude higher than usual. The TFTR team modified the limiter during the opening of winter 1989–1990. The modifications greatly reduced the number and magnitude of the blooms, so that they are no longer a problem.

Characterization of deposition and erosion of the TFTR bumper limiter and wall

Abstract To understand material transport by the plasma in the TFTR tokamak, graphite bumper limiter tiles and metal surfaces have been studied. Detailed measurements of the TFTR inner bumper limiter POCO ™ AXF-5Q graphite tiles indicate areas of net erosion and areas of net deposition. These areas are poloidally asymmetric and on the scale of a bay (l/20th of the torus) repeat regularly toroidally. Finer scale measurements indicate that there are subtle variations in the interaction of the plasma with the wall. Furthermore, a study of the correlation of limiter deposits with the type of discharges in TFTR indicates that the material composition depth distribution was determined by the tokamak operational history. In particular, the relative amounts of carbon, hydrogen, oxygen, and metal in the deposits changed over time, reflecting plasma impurity levels. The outer wall of the vessel was not exposed to direct plasma flux and does not show evidence of erosion.

Material behavior and materials problems in TFTR

Abstract This paper reviews the experience with first-wall materials in TFTR over a 20 month period of operatiori during 1985–1987. Experience with the axisymmetric inner wall limiter, constructed of graphite tiles, is described, including the necessary conditioning procedures needed for impurity and particle control of high power (≤ 20 MW) neutral injection experiments. The thermal effects in disruptions have been quantified and no significant damage to the bumper limiter has occurred as a result of disruptions. Carbon and metal impurity redeposition effects have been quantified through surface analysis of wall samples. Estimates of the tritium retention in the graphite limiter tiles and redeposited carbon films have been made, based on analysis of deuterium retention in removed graphite tiles and wall samples. New limiter structures have been designed using a 2D carbon/carbon ( C C ) composite material for RF antenna protection. Laboratory tests of the important thermal, mechanical, and vacuum properties of C C materials are described. Finally, the last series of experiments in TFTR with in-situ Zr Al surface pumps are discussed. Problems with Zr Al embrittlement have led to the removal of the getter material from the in-torus environment.

Plasma fluxes to surfaces for an oblique magnetic field

The poloidal and toroidal spatial distributions of D α , He I and C II emission have been obtained in the vicinity of the TFTR bumper limiter and are compared with models of ion flow to the surface. The distributions are found not to agree with a model (the “cosine” model) which determines the incident flux density using only the parallel fluxes in the scrape-off layer and the projected area of the surface perpendicular to the field lines. In particular, the cosine model is not able to explain the significant fluxes observed at locations on the surface which are oblique to the magnetic field. It is further shown that these fluxes cannot be explained by the finite Larmor radii of impinging ions. Finally, it is demonstrated, with the use of Monte Carlo codes, that the distributions can be explained by including both parallel and cross-field transport onto the limiter surface.

Operating experiences with the TFTR escaping alpha detectors

This paper reviews the operating experiences obtained with a set of scintillator‐based escaping fast ion detectors which have been used successfully for several years on the TFTR tokamak. There have been several operational problems which need to be resolved before these detectors are used to measure 3.5 MeV DT alphas in 1993. The main problem has been overheating by edge plasma heat flux for large major radius plasmas, when the detectors were not shadowed by the adjacent limiter. Other problems have been due to runaway electron‐induced x‐ray flux and scintillator and foil damage.

Dynamics and energy flow in a disrupting tokamak plasma

Abstract This article contains a description of our effort to apply a recently developed tokamak disruption modeling code, DSTAR, to describe in some detail the dynamic evolution of a disrupting plasma in the TFTR experiment. The physical phenomena modeled in DSTAR, and two important new extensions of this model, those of hyper-resistivity and a plasma halo, are reviewed in this paper. We also present some data taken during one of the most rapid disruptions in TFTR and present the results of comparison of this data with predictions from the DSTAR code.

TFTR tritium inventory analysis

Abstract The Tokamak Fusion Test Reactor (TFTR) is scheduled to begin Due5f8T operation in 1990 with the on-site tritium inventory limited to 5 grams. The physics and chemistry of the in-vessel tritium inventory will impact safety concerns, and also the entire operating schedule of the tokamak. We have investigated plasma-material interaction processes that will affect this first tritium-fueled tokamak. Tritium inventory estimates for TFTR are derived from: (1) laboratory simulation, (2) in-situ plasma measurements, (3) post-run surface analysis, and (4) modeling. This paper presents the results of these investigations, the derivation of a tritium inventory estimate and its uncertainties, and a discussion of its impact. A particular discharge-by-discharge operating schedule has been developed and evaluated. The major source of in-vessel tritium inventory will be codeposition of tritium and eroded carbon onto surfaces. We find that the on-site limit may be approached unless specific inventory reduction techniques are invoked, e.g., discharge cleaning.

Constraints on escaping alpha particle detectors for ignited tokamaks

Several modifications to existing escaping alpha scintillation detectors in TFTR will be needed before they could be used on ignited tokamaks such as CIT or ITER. The main difficulties are the large heat flux at the desired detector locations and the accumulated radiation damage to the scintillator itself. Constraints imposed by these problems can probably be overcome by using remotely movable (and removable) detectors.