Analysis of the temperature influence on Langmuir probe measurements on the basis of gyrofluid simulations

The influence of the temperature and its fluctuations on the ion saturation current and the floating potential, which are typical quantities measured by Langmuir probes in the turbulent edge region of fusion plasmas, is analysed by global nonlinear gyrofluid simulations for two exemplary parameter regimes. The numerical simulation facilitates a direct access to densities, temperatures and the plasma potential at different radial positions around the separatrix. This allows a comparison between raw data and the calculated ion saturation current and floating potential within the simulation. Calculations of the fluctuation-induced radial particle flux and its statistical properties reveal significant differences to the actual values at all radial positions of the simulation domain, if the floating potential and the temperature averaged density inferred from the ion saturation current is used.Comment: Submitted to Plasma Physics and Controlled Fusio

Absence of Debye Sheaths Due to Secondary Electron Emission

A bounded plasma where the electrons impacting the walls produce more than one secondary on average is studied via particle-in-cell simulation. It is found that no classical Debye sheath or space-charge limited sheath exists. Ions are not drawn to the walls and electrons are not repelled. Hence the plasma electrons travel unobstructed to the walls, causing extreme particle and energy fluxes. Each wall has a positive charge, forming a small potential barrier or "inverse sheath" that pulls some secondaries back to the wall to maintain the zero current condition.Comment: 4 pages, 3 Figure

The digital mirror Langmuir probe: Field programmable gate array implementation of real-time Langmuir probe biasing

High bandwidth, high spatial resolution measurements of electron temperature, density, and plasma potential are valuable for resolving turbulence in the boundary plasma of tokamaks. While conventional Langmuir probes can provide such measurements, either their temporal or spatial resolution is limited: the former by the sweep rate necessary for obtaining I-V characteristics and the latter by the need to use multiple electrodes, as is the case in triple and double probe configurations. The Mirror Langmuir Probe (MLP) bias technique overcomes these limitations by rapidly switching the voltage on a single electrode cycling between three bias states, each dynamically optimized for the local plasma conditions. The MLP system on Alcator C-Mod used analog circuitry to perform this function, measuring Te, VF, and Isat at 1.1 MSPS. Recently, a new prototype digital MLP controller has been implemented on a Red Pitaya Field Programmable Gate Array (FPGA) board which reproduces the functionality of the original controller and performs all data acquisition. There is also the potential to provide the plasma parameters externally for use with feedback control systems. The use of FPGA technology means the system is readily customizable at a fraction of the development time and implementation cost. A second Red Pitaya was used to test the MLP by simulating the current response of a physical probe using C-Mod experimental measurements. This project is available as a git repository to facilitate extensibility (e.g., real-time control outputs and more voltage states) and scalability through collaboration

Transport and Deposition of 13c From Methane Injection into Detached H-Mode Plasmas in DIII-D

Experiments are described which examine the transport and deposition of carbon entering the main plasma scrape-off layer in DIII-D. {sup 13}CH{sub 4} was injected from a toroidally symmetric source into the crown of lower single-null detached ELMy H-mode plasmas. {sup 13}C deposition, mapped by nuclear reaction analysis of tiles, was high at the inner divertor but absent at the outer divertor, as found previously for low density L-mode plasmas. This asymmetry indicates that ionized carbon is swept towards the inner divertor by a fast flow in the scrape-off layer. In the private flux region between inner and outer strike points, carbon deposition was low for L-mode but high for the H-mode plasmas. OEDGE modeling reproduces observed deposition patterns and indicates that neutral carbon dominates deposition in the divertor from detached H-mode plasmas