Distributions of deposits and hydrogen on the upper and lower TDUs3 target elements of Wendelstein 7-X

Distributions of deposits and hydrogen (H) on the graphite divertor target elements TM4h4 and TM3v5 in the test divertor units 3 (TDUs3) of Wendelstein 7-X (W7-X) are studied. The TM4h4 and TM3v5 are located at the magnetically symmetric positions in the upper and lower divertor. The microstructure of the deposition layer is characterized by a transmission electron microscope (TEM) combined with a focused ion beam (FIB). Metallic deposits such as iron (Fe), molybdenum (Mo), chromium (Cr) are detected in the deposition layer by energy-dispersive x-ray spectroscopy (EDS). The depth-resolved distribution patterns of boron (B) and metallic deposits on upper and lower horizontal (h) divertor target elements TDUs3-TM4h4 as well as upper and lower vertical (v) divertor target elements TDUs3-TM3v5 are clarified by glow discharge optical emission spectrometry (GDOES). Results for both TDUs3-TM4h4 and TDUs3-TM3v5 show that the B deposition regions exhibit higher H retention due to the co-deposition with deposits. On the other hand, up-down asymmetries in B deposition caused by particle drift exist on both TDUs3-TM4h4 and TDUs3-TM3v5. The B deposition amount on upper TDUs3-TM4h4 is 40% smaller than that on lower TDUs3-TM4h4. While for the vertical target elements, the B deposition amount on upper TDUs3-TM3v5 is 35% larger than that on lower TDUs3-TM3v5. Meanwhile, a shift of around 3 cm in B deposition peaks is observed on upper and lower TDUs3-TM4h4 and TDUs3-TM3v5. Results of numerical simulation of carbon deposition/erosion profiles on the target elements using ERO2.0 code and power flux measured by infrared cameras are shown and compared with the above mentioned B profiles

EXCESS CONDUCTIVITY AND PAIR-BREAKING EFFECTS IN YBA2CU3O7 AND BI2SR2CA1-XYXCU2O8+DELTA SYSTEMS

To explain the behavior of thermal fluctuations, we have measured the electrical resistivity of five samples: thin-film YBa2Cu3O7, Polycrystalline YBa2Cu3O7, and Bi2Sr2Ca1-xYxCu2O8+delta with x = 0, 0.05, and 0.20. The whole temperature range of fluctuations consists of three regions. Within the mean-field region, YBa2Cu3O7 and Bi2Sr2Ca1-xYxCu2O8-delta samples show predominantly three-dimensional (3D) and 2D behaviors with the exponents 1/2 and 1, in the critical region the two systems display a behavior analogous to 3D-XY and 2D-XY models, respectively. Above the mean-field region [ln(epsilon) greater-than-or-equal-to -1.0] exponents have values larger than those predicted by Reggiani et al. Both the systems favor strong pair-breaking effects and integral dimensions for the thermal fluctuations

Flux pinning in Bi2Sr2Ca1−xYxCu2O8+δ and YBa2Cu3O7

We have measured the magnetoresistivity of polycrystalline Bi2Sr2Ca1−xYxCu2O8+δ (BSCCO) with x = 0, 0.05 and 0.20 and YBa2Cu3O7 (YBCO) in magnetic fields of 4 T. We find Inui et al's model to describe the data in a wide temperature range within the flux creep and flux flow regions. The pinning behaviour in BSCCO due to Y doping is found to be similar to the one with application of magnetic field, with the pinning potentials U0 around 2000 K. U0 further decreases to around 1300 K for x = 0.20 in the field of 4 T. The pinnings in YBCO system are found to be stronger compared to BSCCO.© Elsevie

Phase breaking effects in magnetoconductivity of YBA2CU3O7-DELTA and BI2SR2CACU2O8

We have measured the magnetoconductivity of YBa2Cu3O7-delta and Bi2Sr2CaCu2O8 in the magnetic field of 4T and analysed the data in the fluctuation region in the light of Aronov-Hikami-Larkin (AHL) and Bieri-Maki(BM) theories in their corrected forms and find the BM theory within the clean limit to describe the data more adequetely

Improvement in the simulation tools for heat distribution predictions and control of baffle and middle divertor loads in Wendelstein 7-X

In the first divertor campaign in Wendelstein 7-X (W7-X), unexpected significant heat loads were observed at particular plasma-facing components (e.g. baffle tiles and middle divertor part) which were not designed to receive high heat flux. In a prior investigation, it was concluded that the previous diffusive field line tracing (DFLT) model used for divertor design in W7-X cannot reproduce these loads, due to the missing physics in simulating the heat transport in the shaded flux tubes. To tackle this issue, two new efficient codes (DFLT_rev and EMC3-Lite) are introduced and validated against various experimental heat distributions in different magnetic configurations. The new tungsten baffle tiles have been designed with these codes and mounted in the machine, aiming for mitigated heat loads in the upcoming campaign

Technical challenges in the construction of the steady-state stellarator Wendelstein 7-X

The next step in the Wendelstein stellarator line is the large superconducting device Wendelstein 7-X, currently under construction in Greifswald, Germany. Steady-state operation is an intrinsic feature of stellarators, and one key element of the Wendelstein 7-X mission is to demonstrate steady-state operation under plasma conditions relevant for a fusion power plant. Steady-state operation of a fusion device, on the one hand, requires the implementation of special technologies, giving rise to technical challenges during the design, fabrication and assembly of such a device. On the other hand, also the physics development of steady-state operation at high plasma performance poses a challenge and careful preparation. The electron cyclotron resonance heating system, diagnostics, experiment control and data acquisition are prepared for plasma operation lasting 30 min. This requires many new technological approaches for plasma heating and diagnostics as well as new concepts for experiment control and data acquisition