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Ion energy measurements have been made in the scrape off layer of the Mega Amp Spherical Tokamak (MAST) using a midplane retarding field energy analyser (RFEA) in H-mode plasmas during the inter-edge localised mode (ELM) period and during type I and type III ELMs. During the inter-ELM period at distances of 3 to 8 cm from the last closed flux surface (LCFS), ion temperatures of 20 to 70 eV have been measured giving an ion to electron temperature ratio of 2 to 7 with a mean of 4. During type III ELMs, an ion temperature of 50 eV has been measured 3 to 6 cm from the LCFS which decreases to 30 eV at distances 11 to 16 cm from the LCFS. During type I ELMs, an ion temperature of 40 eV has been measured at a distance of 10 to 15 cm from the LCFS., Comment: 15 pages, 5 figures

New measurements of ion transport in the scrape-off layer (SOL) during inter-ELM periods are obtained from a retarding field analyzer (RFA) in the ASDEX Upgrade tokamak. The ion current density measured in the far SOL features large-amplitude bursts associated with turbulent plasma filaments passing the RFA. These filaments convect plasma to the wall with larger density than the background plasma density. The ions in the filament also have higher impact energies than the background ion impact energies. These observations indicate that a filament-wall interaction can lead to erosion of the first wall in the burning plasma tokamaks such as ITER (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The ball pen probe (BPP) technique is used successfully to make profile measurements of plasma potential, electron temperature and radial electric field on the Mega Amp Spherical Tokamak (MAST). The potential profile measured by the BPP is shown to significantly differ from the floating potential both in polarity and profile shape. By combining the BPP potential and the floating potential the electron temperature can be measured, which is compared with the Thomson scattering (TS) diagnostic. Excellent agreement between the two diagnostics is obtained when secondary electron emission is accounted for in the floating potential. From the BPP profile an estimate of the radial electric field is extracted which is shown to be of the order ~1kV/m and increases with plasma current. Corrections to the BPP measurement, constrained by the TS comparison, introduce uncertainty into the ER measurements. The uncertainty is most significant in the electric field well inside the separatrix. The electric field is used to estimate toroidal and poloidal rotation velocities from ExB motion. This paper further demonstrates the ability of the ball pen probe to make valuable and important measurements in the boundary plasma of a tokamak.

SOLPS simulations of MAST-U have been carried out to identify in more detail the physics and operational properties of novel divertor configurations such as Super-x divertor (SXD), in particular the physics of detachment. A well diagnosed L-mode discharge from MAST has been utilised to determine L-mode transport coefficients representative for MAST-U L-mode plasmas. Simulations show that under the same core plasma conditions, the MAST-U SXD is strongly detached whilst the conventional divertor (CD) is not (1 eV versus 20 eV at the divertor plate). The detachment and higher power losses (1.6×) in the SXD versus the attached CD lead to a factor of 25 reduction in the target power load and are attributed to changes in radial location of the target. An attached regime can be established for the SXD in L-mode for higher pumping speed and/or heating power. In contrast, the simulation predicts that the MAST-U CD requires 3× higher density or 4× reduced power than the SXD to detach. Comparing two versions of the SXD, each with a different amount of poloidal expansion in the region near the divertor plate, we find that the effect of additional poloidal flux expansion of the SXD on an already detached plasma is small for a change in flux expansion in volume by a factor of 2–3 (target temperature 0.7 eV versus 1.1 eV). The poloidal flux expansion re-arranges the radiation pattern with only a small increase in divertor power losses (1.06×) compared to changing from the CD to SXD topology. By artifically increasing the leakage from the divertor chamber, we confirmed that the tight closure of the divertor region leads to strong increases in neutral density with concomitant power losses.

We have conducted characterization of a scanning plasma enhanced chemical vapor deposition (PECVD) system for producing controlled non-uniform deposition or etching profiles. Both self-bias and plasma potential showed that the plasma conditions were disturbed significantly when the source was very close to the chamber wall but electron temperature and ion density were not affected significantly. It was found that a very thin but long tail of parasitic deposition was present over the entire large substrate. To eliminate the parasitic deposition an aperture (plasma guarding house) was constructed and was found to eliminate the parasitic plasma deposition. Deposited silicon nitride and silicon oxide thin films using the plasma guarding house in the scanning PECVD system showed very good optical properties similar to those obtained in conventional deposition methods. No multilayer structure was observed in TEM analysis on these films.

Knowledge of the ion temperature (Ti) is of key importance for determining heat fluxes to the divertor and plasma facing components, however data regarding this is limited compared to electron temperature (Te) data. Ti measurements at the divertor target, between edge-localised modes (inter-ELM) H-mode, have been made using a novel retarding field energy analyser (RFEA)., Comment: 17 pages, 6 figures

Knowing the ion temperature in the scrape-off layer (SOL) of tokamaks is of great importance for understanding the heat flux to plasma facing components. Few measurements are available for SOL ion temperatures compared with electron temperatures due to the relative complexity of the measurement. Two retarding field energy analysers (RFEAs) have been used in the mega amp spherical tokamak (MAST) to measure the ion temperature at both the midplane and the divertor in ohmic L-mode plasmas with a range of densities. Midplane SOL Ti was found to be higher than Te by a factor of about 2 or greater. Divertor Ti measurements at the target showed Ti???Te having considered the effect of plasma flows on the divertor RFEA measurements.

Plasma immersion ion implantation (PIII) is a surface modification technique in which an object is placed in a plasma and negatively pulse biased. For the treatment of insulators, a metal mesh can be placed around the object and pulse biased to minimize surface charging. In this paper, the effects of surface charging of an insulator on the ion flux and ion energy distribution (IED) during PIII are investigated both with and without a mesh present. IED measurements were made through a hole in a Mylar insulator using a retarding field energy analyser and compared with the results of a two-dimensional numerical simulation. Qualitative agreement was found between experimental and simulation results. With an insulator and no mesh, at the start of the pulse, the IED is similar to the case with no insulator, with a peak close to the maximum ion energy and ions with a spread of energies between zero and the maximum ion energy. During the pulse, the number of ions measured with lower energies increases. Simulations show that this is caused by surface charging of the insulator. With a mesh and insulator, compared with the no mesh case a larger flux of ions is measured during the early stages of the pulse but during the pulse the flux decreases and more low energy ions are measured. Simulations show that this is caused by surface charging changing the potential in the mesh region which affects the trajectories of incoming ions.

We report experimental results for time dependent plasma properties during microarcing in a rf plasma system. A cutoff discriminator bias voltage can be defined as a parameter representing plasma potential. The cutoff voltage during the microarcing does not exactly follow the dependence of floating potential. The plasma/ion density increases significantly after microarc initiation and then reduces slowly. We propose that a significant number of electrons enter the plasma in the initial period of the microarc and gain very high energy. These high energy electrons lose their energy through a relatively slower process of collisions while generating a significantly higher plasma density.