Fast analysis of collective Thomson scattering spectra on Wendelstein 7-X

\u3cp\u3eTwo methods for fast analysis of Collective Thomson Scattering (CTS) spectra are presented: Function Parametrization (FP) and feedforward Artificial Neural Networks (ANNs). At this time, a CTS diagnostic is being commissioned at the Wendelstein 7-X (W7-X) stellarator, with ion temperature measurements in the plasma core as its primary goal. A mapping was made from a database of simulated CTS spectra to the corresponding ion and electron temperatures (T\u3csub\u3ei\u3c/sub\u3e and T\u3csub\u3ee\u3c/sub\u3e). The mean absolute mapping errors are 4.2% and 9.9% relative to the corresponding T\u3csub\u3ei\u3c/sub\u3e, for the ANN and FP, respectively, for spectra with Gaussian noise equivalent to 10% of the average of the spectral maxima in the database at 650 sampling points per GHz and within a limited parameter space. Although FP provides some insight into the information contents of the CTS spectra, ANNs provide a higher accuracy and noise robustness, are easier to implement, and are more adaptable to a larger parameter space. These properties make ANN mappings a promising all-round method for fast CTS data analysis. Addition of impurity concentrations to the current parameter space will enable fast bulk ion temperature measurements in the plasma core region of W7-X.\u3c/p\u3

Spectroscopic characterization and imaging of laser- and unipolar arc-induced plasmas

Tungsten plasmas induced by unipolar arcs were investigated using optical emission spectroscopy and imaging, and compared with laser-induced tungsten plasmas. The unipolar arcs were initiated in the linear-plasma simulator PISCES-A at UCSD under fusion relevant conditions. The electron temperature and density of the unipolar arc plasmas were in the range 0.5–0.7 eV and 0.7–2.01020m3, respectively, and increased with increasing negative bias voltage, but did not correlate with the surface temperature. In comparison, the electron temperature and density of the laser-induced plasmas were in the range 0.6–1.4 eV and 71019–11022m3, respectively. VC 2014 AIP Publishing LLC

Plasma filamentation in the Rijnhuizen tokamak RTP

Evidence for small scale magnetic structures in the Rijnhuizen tokamak RTP is presented. These are manifest through steps and peaks in the electron temperature and pressure, measured with multiposition Thomson scattering. During central electron cyclotron heating, several filaments of high pressure are found in the power deposition region. They live hundreds of microseconds. Near the sawtooth inversion radius a step in the temperature profile occurs. Further out, quasiperiodic structures are observed, in both Ohmic and heated discharges

The occurrence and damage of unipolar arcing on fuzzy tungsten

This research investigated whether unipolar arcing in the divertor of fusion reactors is a potential cause for enhanced wear of the divertor. It was found that 1 µm of nano-fuzz growth is sufficient to initiate arcing, mainly depending on the sheath potential drop and electron density. The average mass loss rate induced by the arc was determined from mass loss measurements and found to be consistent with the value estimated from the arc current. The average arc track erosion depth was estimated by using the measured mass loss and damaged surface area and was found to be one tenth of the fuzzy layer thickness. Due to melting of the fuzzy structures the actual depth is larger and some arc tracks occasionally appeared to even reach the bulk beyond the fuzzy layer. The conclusion of this study is therefore that arcing in the divertor of future tokamaks (e.g. ITER) potentially is an important cause for surface damage and plasma pollution

Using 3D-printed tungsten to optimize liquid metal divertor targets for flow and thermal stresses

Liquid metal divertors aim to provide a more robust alternative to conventional tungsten divertors. However, they still require a solid substrate to confine the liquid metal. This work proposes a novel design philosophy for liquid metal divertor targets, which allows for a two orders of magnitude reduction of thermal stresses compared to the state-of-the-art monoblock designs. The main principle is based on a 3D-printed tungsten structure, which has low connectedness in the direction perpendicular to the thermal gradient, and as a result also short length scales. This allows for thermal expansion. Voids in the structure are filled with liquid lithium which can conduct heat and reduce the surface temperature via vapor shielding, further suppressing thermal stresses. To demonstrate the effectiveness of this design strategy, an existing liquid metal concept is re-designed, fabricated, and tested on the linear plasma device Magnum-PSI. The thermo-mechanical finite element method analysis of the improved design matches the temperature response during the experiments, and indicates that thermal stresses are two orders of magnitude lower than in the conventional monoblock designs. The relaxation of the strength requirement allows for much larger failure margins and consequently for many new design possibilities

Rotation of a strongly magnetized hydrogen plasma column determined from an asymmetric Balmer-beta spectral line with two radiating distributions

A potential buildup in front of a magnetized cascaded arc hydrogen plasma source is explored via .vector.E * .vector.B rotation and plate potential measurements. Plasma rotation approaches thermal speeds with max. velocities of 10 km/s. The diagnostic for plasma rotation is optical emission spectroscopy on the Balmer-beta line. Asym. spectra are obsd. A detailed consideration is given on the interpretation of such spectra with a two distribution model. This consideration includes radial dependence of emission detd. by Abel inversion of the lateral intensity profile. Spectrum anal. is performed considering Doppler shift, Doppler broadening, Stark broadening, and Stark splitting. [on SciFinder (R)

Extreme hydrogen plasma fluxes at Pilot-PSI enter the ITER divertor regime

\u3cp\u3ePilot-PSI produces hydrogen plasma with a cascaded arc. It is demonstrated that the output electron density reaches 4 × 10\u3csup\u3e21\u3c/sup\u3e m\u3csup\u3e-3\u3c/sup\u3e by pushing the input power to 45 kW. Increasing the diameter of the discharge channel (studied from 4 to 7 mm) does not affect the plasma output but reduces the power input by up to 25%. The plasma was post-heated by feeding the plasma column with a net current. This increased the electron temperature from ∼2 to 4 eV. Calorimetric and voltage measurements on the cascade plates showed that this also influenced the discharge characteristics inside the arc.\u3c/p\u3

Chemical erosion of different carbon composites under ITER-relevant plasma conditions

\u3cp\u3eWe have studied the chemical erosion of different carbon composites in Pilot-PSI at ITER-relevant hydrogen plasma fluxes (∼10\u3csup\u3e24\u3c/sup\u3e m \u3csup\u3e-2\u3c/sup\u3e s\u3csup\u3e-1\u3c/sup\u3e) and low electron temperatures (T\u3csub\u3ee\u3c/sub\u3e∼1 eV). Optical emission spectroscopy on the CH A-X band was used to characterize the chemical sputtering. Fine grain graphite (R 6650, SGL Carbon Group), ITER-reference carbon fiber composite material (SNECMA NB31 and NB41; Dunlop 3D), nano- and micro-crystalline diamond coatings on molybdenum and SiC (Silit® SKD Reaction-Bonded, Saint-Gobain Ceramics) were compared. The chemical sputtering was similar for the different composites under comparable plasma conditions, except for SiC, which produced a ten times lower rate. The CH emission was constant at electron temperatures T\u3csub\u3ee\u3c/sub\u3e>1 eV and ion fluxes ranging between 10\u3csup\u3e23\u3c/sup\u3e and 10\u3csup\u3e24\u3c/sup\u3e m\u3csup\u3e- 2\u3c/sup\u3e s\u3csup\u3e-1\u3c/sup\u3e, but decreased at lower temperatures. This decrease is possibly due to changes in the excitation of CH and not due to a change in the chemical erosion rate.\u3c/p\u3