D. Manos

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.

Edge turbulence measurements in TFTR

Abstract The edge turbulence in TFTR is characterized by several diagnostics. Langmuir probes and D-alpha imaging have detected large amplitude, small-scale, broadband density fluctuations in the scrape-off region near the wall. Broadband fluctuations with a similar frequency spectrum are detected by small-angle microwave scattering and magnetic pickup loops. Increases in the turbulence level are seen during neutral beam injection. Some preliminary analysis of this data is presented.

Experiments on TFTR supershot plasmas

Improvements to the TFTR limiter have extended the threshold for carbon blooms (an uncontrolled massive influx of carbon) to greater than 32 MW for 1 s so that blooms seldom occur in present TFTR supershot experiments. As a result of the progression from strong blooms to modest blooms to no blooms, improvements in confinement could be correlated with the occurrence of a carbon bloom in the plasma which immediately preceded the supershot. It is speculated that the carbon influx during a carbon bloom results in a limiter surface which has a slightly reduced self-sputtering yield for the subsequent discharge. The influence on the supershot plasma seems similar to phenomena obtained by conditioning with lithium pellets.

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.

Ohmic and neutral beam heated detached plasmas on TFTR

Abstract Detached plasmas have been proposed as a means of distributing power losses uniformly on walls in ITER and other reactor scenarios. Ohmically and neutral beam injection (NBI) heated detached plasma studies are being carried out on TFTR in order to help determine the practicality of this concept. NBI heated detached plasmas with up to 8.5 MW of beam power have been maintained detached for up to 300 ms. A confinement related delay in the radiated power is observed at low beam power. H-mode-like events have been observed for several beam heated detached plasmas and radiative emissivities as high as 0.8 MW/m 3 have been realized. These results are especially of interest to ITER and other reactor design studies.

Midplane measurements of MeV ion confinement in TFTR

A new detector has been designed and installed on TFTR for studying the confinement of MeV ions, especially for measuring the losses due to the toroidal field ripple. It is located just below the outer midplane where the peak in ripple‐induced losses is expected. The detector consists of a scintillator [ZnS(Ag)] and collimating apertures mounted on a radially movable probe. New design features of this detector are presented along with some of the first results.

The PLT rotating pumped limiter

Abstract A limiter with a specially contoured front face and the ability to rotate during tokamak discharges has been installed in a PLT pump duct. These features have been selected to handle the unique particle removal and heat load requirements of ICRF heating and lower-hybrid current-drive experiments. The limiter has been conditioned and commissioned in an ion-beam test stand by irradiation with 1 MW power, 200 ms duration beams of 40 keV hydrogen ions. Operation in PLT during ohmic discharges has proven the ability of the limiter to reduce localized heating caused by energetic electron bombardment and to remove about 2% of the ions lost to the PLT walls and limiters.

Noble gas pumping by the TFTR graphite limiter

The TFTR limiter is graphite. It is possible to decrease the amount of D trapped in the limiter by making plasma discharges with gas injected only as a prefill. When the limiter is deuterium depleted well enough by this technique for supershots (with R d ∼0.5–0.6) it pumps noble gases. The effective confinement time τ p * ranges from ≤1 s for He down to 0.08 s for Kr. These noble gases escape from the limiter slowly after the run. The partial pressures of Ar and Ne decay with time constants of several hours, and the data for Ar show evidence of two trapping sites.

Power and particle balance during neutral beam injection in TFTR

Abstract Detailed boundary plasma measurements on TFTR have been made during a NBI power scan in the range Ptot = 1–20 MW in the L-mode regime. The behaviour of the plasma density 〈ne〉, radiated power Pred, carbon and deuterium fluxes Γc, ΓD, and Zeff can be summarized as: (1) 〈ne〉 ∝P1/2tot, (2)Prad, Γc, ΓD ∝ Ptot, (3)Zeff ≈ constant. These results obtained during NBI heating in TFTR are very similar to recent JET observations during ICRF heating and suggest that the form of heating plays little role in the behaviour of the discharge parameters. In the case of NBI in L-mode discharges it is shown that central fuelling plays a minor role in the particle balance of the discharge. More important is the NBI role in the power balance. Both the TFTR data during NBI and the JET data during ICRH are consistent with a simple model of particle and power balance in which the particle sources originate primarily at the graphite limiter.

Studies of the edge plasma of RF heated PLT discharges

Studies of RF heated PLT discharges have been performed using Langmuir probes and combined electrostatic calorimeter probes. In addition to the usual conductive and convective losses in the edge, evidence is found for the presence of a substantial group of fast ions having energies in the ≈105 eV range. Such ions are present in the cases of H+ minority and H+ 2nd harmonic heating, however, no such group has been observed in 3He minority heating. The data are presented with calculations to estimate the velocity space distributions of these ions.