Simulations and experimental characterizations of a stationary plasma thruster are compared for four different wall materials to investigate near-wall conductivity (dielectric materials) and in-wall conductivity (conducting materials) in such a discharge. Using a one-dimensional transient fluid model that takes into account a possible electron temperature anisotropy, it is shown that electron-wall backscattering plays a crucial role by maintaining a relatively high electron temperature along the magnetic field lines which in turn drives large electron currents toward the walls. The large differences in discharge current observed experimentally for the dielectric materials are qualitatively recovered, confirming that near-wall conductivity results from the combined effects of secondary electron emission and electron backscattering. A clear correlation is found between the appearance of space charge saturation at the walls and a jump of the discharge current observed in experiments when varying the discharge voltage or the magnetic field. The anomalously high values of discharge current observed experimentally with graphite are also correctly recovered in simulations, which highlight a plasma short-circuiting effect resulting from in-wall currents.

Wall material effects in stationary plasma thrusters. II. Near-wall and in-wall conductivity

Effects of space-charge limitation have been considered for an analytical estimation of plasma potential with use of emissive probes, taking account of a correct expression of emission current under space-charge limited condition. The analytical results are obtained for the floating potential method of emissive probe and for the differential emissive probe method, both without and with an oscillating ac potential. In the case of the floating potential method, the operating condition is discussed for different plasma parameters at several probe temperatures. These results indicate that the floating potential of the emissive probe gives us the plasma potential to an accuracy of the order of the plasma electron temperature Te/e for a strong probe emission. In the case of the differential emissive probe method, the relations among space-charge effects, the ratio of thermoelectron emission current to plasma electron collection current, the plasma electron temperature, the probe temperature, and the amplitude o...

Effect of space-charge limited emission on measurements of plasma potential using emissive probes

/pdf/baseline-high-heat-flux-and-plasma-facing-materials-for-14l1toybrg.pdf

Baseline high heat flux and plasma facing materials for fusion

The sheaths that occur at surfaces in laboratory and space plasmas are reviewed with an emphasis on numerical models that can be solved with modest computational resources. The surfaces in plasma may be the interior walls of confinement devices or inserted probes. Fluid and kinetic models are presented in some detail, and particle-in-cell models are discussed briefly. The numerical methods find the spatial profile of the potential, the particle densities near the surfaces and the current to the surfaces. Maxwellian electrons and cold ions are assumed at the outset and subsequently the models are expanded to encompass (1) multiple electron populations, (2) multiple ion species, (3) finite ion temperature, (4) surfaces that emit electrons such as heated cathodes or emissive probes and (5) surfaces that emit plasma as in the Q-machine. These complications may produce nonmonotonic sheaths in which the first derivative of the potential changes sign or double layers in which the second derivative changes sign. The effect of charge-exchange collisions on ion losses to the wall and on ion current to probes is discussed, but models with collisions of electron are omitted. Some recent advances are discussed, including experiments that measure the ion distribution function in sheaths using laser-induced fluorescence, experiments and numerical models on sheaths with multiple ion species and computational models of sheaths surrounding objects in flowing plasma.

https://iopscience.iop.org/article/10.1088/0741-3335/55/9/093001/pdf

Sheaths in laboratory and space plasmas

The key remaining physics design issue for the ITER tungsten (W) divertor is the question of monoblock (MB) front surface shaping in the high heat flux target areas of the actively cooled targets. Engineering tolerance specifications impose a challenging maximum radial step between toroidally adjacent MBs of 0.3 mm. Assuming optical projection of the parallel heat loads, magnetic shadowing of these edges is required if quasi-steady state melting is to be avoided under certain conditions during burning plasma operation and transiently during edge localized mode (ELM) or disruption induced power loading. An experiment on JET in 2013 designed to investigate the consequences of transient W edge melting on ITER, found significant deficits in the edge power loads expected on the basis of simple geometric arguments, throwing doubt on the understanding of edge loading at glancing field line angles. As a result, a coordinated multi-experiment and simulation effort was initiated via the International Tokamak Physics Activity (ITPA) and through ITER contracts, aimed at improving the physics basis supporting a MB shaping decision from the point of view both of edge power loading and melt dynamics. This paper reports on the outcome of this activity, concluding first that the geometrical approximation for leading edge power loading on radially misaligned poloidal leading edges is indeed valid. On this basis, the behaviour of shaped and unshaped monoblock surfaces under stationary and transient loads, with and without melting, is compared in order to examine the consequences of melting, or power overload in context of the benefit, or not, of shaping. The paper concludes that MB top surface shaping is recommended to shadow poloidal gap edges in the high heat flux areas of the ITER divertor targets.

https://publikationen.bibliothek.kit.edu/1000076499/4840600

Physics conclusions in support of ITER W divertor monoblock shaping

We have derived an exact theoretical expression for the space-charge limited current from the solid surfaces adjacent to plasmas that is applicable for an arbitrary sheath voltage. Our expression shows that the space-charge limited current tends to saturate with the sheath voltage. This new formula is evaluated by 1-D Particle in Cell (PIC) simulation and experiment, and is in a good agreement with the simulation and experimental results. We have also obtained an analytical equation fitted to the new formula based on conventional Child-Langmuir formula by taking into account a more sophisticated dependence of the electrode potential and the plasma density through the effect of Debye shielding and a sheath expansion due to increased voltage across the sheath.

Space-Charge Limited Current from Plasma-Facing Material Surface

Plasma heat flow to material surfaces through sheaths is studied, taking several key physics factors into account. Electron emission from the surface, which breaks a thermal insulation of the sheath, is studied in both thermoelectron emission (TEE) and secondary electron emission (SEE), in which a correct expression under space charge limited condition is given for arbitrary sheath voltages. Nonlinear thermal bifurcation induced by electron emission is analyzed in the experiment and the theory. The local heat flow was found to be enhanced by a thermal contraction induced by cross-field potential variation in a plasma. An enhancement of SEE of hydrogen-absorbed graphite, and a suppression of SEE by the gyromotion of emitted electrons in obliquely incident magnetic field are identified. The effects of ion reflection on the surface and ponderomotive force are also discussed in terms of energy transmission factor δ. An anomaly of δ in detached recombining plasmas is discussed.

Heat flows through plasma sheaths

The new large stellarator W7-X equipped with a divertor is currently under construction in the Greifswald branch of the Max-Planck-Institute for Plasma Physics. It is planned to begin the plasma operation with a variety of diagnostics to analyse the expected properties of the concept and explore the capabilities of the new machine. In the framework of divertor and edge plasma diagnostics a number of electrical probes will be exposed to monitor particle fluxes, electron temperatures and potentials in the vicinity of the divertor targets as well as up-stream. W7-X is intended to operate in quasi-stationary regime with a large power input. The open island divertor has to handle the high particle fluxes and the peak power flux to the targets. Consequently, fast movable mechanisms are designed to cyclically expose and retract the probes. Four pneumatically driven arrays of single probe tips are planned for the horizontal and vertical parts of one upper and lower divertor module, respectively. The probe tips are designed to avoid the problem of shallow angles with the magnetic field, to tolerate the impinging power, to be adjustable in their radial position and exchangeable without dismounting the targets. Additionally, a small number of basically identical linear manipulators are planned for different locations around the stellarator with exchangeable diagnostic heads (fast moving probe, Mach probes, emissive probes, retarding field analysers, collector probes, etc). The main design problems of the probe systems and results from prototype tests are reported.

Probe Systems for W7-X

The superconducting stellarator W7-X equipped with a divertor is under construction at Max-Planck institute for plasma Physics in Greifswald. It is planned to start plasma operation with the test divertor (TDU) without active cooling for shorter plasma pulse operation in 2014. The TDU has been designed to withstand thermal loads up to 8MW/m2 for pulse operation of the W7-X. A target-imbedded array of electrostatic probes is a useful diagnostics to measure basic plasma parameters close to the divertor targets, and has been proposed to install into the TDU plates. Especially the design of imbedded probe arrays must be optimized not only to minimize thermo-mechanical risk for the imbedding target but also to withstand thermal loads up to 8MW/m2 during pulse operation. Finite element thermal analyses are performed to guide the design of the probe. Results and acceptable technical solutions for TDU target imbedded probe arrays are reported (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

M.Y. Ye

Papers

Space-Charge Limited Current from Plasma-Facing Material Surface

Heat flows through plasma sheaths

Probe Systems for W7-X

Developments of Divertor Target-Imbedded Langmuir Probes for W7-X