A review of particle-solid processes pertinent to modelling plasma-wall interactions is presented, and sets of recommended data are given. Analytic formulas are used where possible; otherwise, data are presented in the form of tables and graphs. The incident particles considered are e−, H, D, T, He, C, O, and selfions. The materials include the metals aluminum, beryllium, copper, molybdenum, stainless steel, titanium, and tungsten and the nonmetals carbon and TiC. The processes covered are light ion reflection, hydrogen and helium trapping and detrapping, desorption, evaporation, sputtering, chemical effects in sputtering, blistering caused by implantation of helium and hydrogen, secondary electron emission by electrons and particles, and arcing.

To study the effect of edge neutrals on the low-to-high confinement transition threshold, a broad range of plasma discharges has been analyzed. These discharges vary by gas puffing and pumping rates, position of the X point, and line-averaged density. It is shown that the determination of the neutral density (or neutral pressure) in the scrape-off layer (SOL) can give a misleading indication of the neutral population inside the separatrix. An increase of neutral density in the SOL creates an increase of plasma density that, in turn, increases the opacity to the neutrals and results in reduced neutral penetration. At a constant magnetic field, the transition power divided by the density appears to be a function of a single parameter measuring the neutrals effect. From this analysis, this parameter cannot be uniquely identified. For instance, it may be the radial decay length of the neutral profile or the charge-exchange damping rate at about r/a≈0.95. A similar correlation exists between these neutral parameters and local plasma parameters such as electron and ion temperature. This indicates that a missing parameter linked to the neutrals exists in the power threshold scaling laws.

The first encouraging experiments demonstrating direct, explicit control of the He 2+ density in a tokamak plasma have been performed in the TEXTOR tokamak with the Advanced Limiter Test-II pump limiter. Helium is injected in a short gas puff from the outside of the plasma, is observed to reach the plasma core, and then is readily removed from the plasma. An exhaust efficiency of -8% is obtained. Active charge-exchange spectroscopy is used to study the exhaust and transport of He 2 * within the plasma, and the density evolution is modeled with a diffusive-convective transport code.PACS numbers: 52.25.Fi, 34.70.+e, 52.55.Fa, 52.70.Kz In future burning fusion devices, helium (He) ash must be continuously removed from the core to prevent dilution of the deuterium-tritium (D-T) fuel and concomitant quenching of the burn. Thus, He ash removal is fundamental to the operation of any fusion reactor, since the rate at which the a-particle by-products of the fusion reaction are purged from the core plasma will determine the pulse length available before the burn is quenched. For proposed steady-state tokamaks, such as the International Thermonuclear Experimental Reactor (ITER), continuous purging of the He ash is essential. Recent estimates 1 show that newly created He ions must be removed within 7 to 15 energy confinement times to maintain continuous reactor operation.The recycling of injected He from the wall and the lack of direct diagnostic capability to measure He concentrations have complicated previous efforts to determine the He removal rate. 2 " 4 Estimating this rate requires knowledge of the recycling coefficient, which can only be determined indirectly and depends on the wall conditioning status and history of the device. The experiments reported here are the first in which direct, explicit removal of injected He has been demonstrated. This is made possible by the Advanced Limiter Test-II (ALT-II) system, a toroidal belt limiter 5-7 that uses turbomolecular pumps (TMPs). Most existing particle exhaust schemes use gettering materials or cryopumping systems, which do not pump He. We simulate the presence of recycled He ash in a tokamak by puffing concentrations of 3%-5% He (relative to n e ) into the TEXTOR plasma just before or during neutral-beam injection (NBI). The transport of the He into the plasma core and its subsequent pump-out phase using the ALT-II system are followed with spectroscopic techniques by observing the He in three locations: the plasma core, the plasma edge at the ALT-II limiter, and the ALT-II pumping duct. By combining the results from these measurements, the exhaust efficiency for the He found in the plasma core is obtained during ALT-II pumping.In the plasma core, the He density is measured by charge-exchange excitation (CXE) spectroscopy, in combination with NBI. Measuring spatially and temporally resolved ion temperatures and absolute densities using CXE line intensities is a well-established technique on many tokamaks. 8 " 11 We use CXE spectroscopy to obtain the local He 2+ densi...