We report fabrication of graphene devices in a Corbino geometry consisting of concentric circular electrodes with no physical edge connecting the inner and outer electrodes. High device mobility is realized using boron nitride encapsulation together with a dual-graphite gate structure. Bulk conductance measurement in the quantum Hall effect (QHE) regime outperforms previously reported Hall bar measurements, with improved resolution observed for both the integer and fractional QHE states. We identify apparent phase transitions in the fractional sequence in both the lowest and first excited Landau levels (LLs) and observed features consistent with electron solid phases in higher LLs.

Optical emission spectroscopy is one of the standard diagnostic methods to determine plasma parameters. In hydrogen plasmas often the intensity of the Balmer lines is recorded. To analyze such measurements population models for the hydrogen atom are needed. The flexible package Yacora is used to construct a new collisional radiative model for low pressure, low temperature hydrogen plasmas. This model includes six possible excitation channels: effective excitation of H, recombination of H + , dissociative excitation of H 2 , dissociative recombination of H + 2 , dissociative recombination of H + 3 and mutual neutralization of H − and H + x . The model is applied to an uniform ECR plasma with high dissociation degree and low ionization degree, i.e. electron collision excitation from H is the dominant excitation channel. The cross sections for this channel taken from literature showed a non-physical discontinuity at electron energies close to the threshold energy. This discontinuity has been removed by using a smoothing procedure. For known plasma parameters the deviation between measured and calculated population densities decreases significantly by using the smoothed data. Furthermore the agreement of the electron density deduced from the ratio H β /H γ with results of other diagnostic methods is enhanced dramatically.

ASDEX Upgrade has recently finished its transition towards an all-W divertor tokamak, by the exchange of the last remaining graphite tiles to W-coated ones. The plasma start-up was performed without prior boronization. It was found that the large He content in the plasma, resulting from DC glow discharges for conditioning, leads to a confinement reduction. After the change to D glow for inter-shot conditioning, the He content quickly dropped and, in parallel, the usual H-Mode confinement with H factors close to one was achieved. After the initial conditioning phase, oxygen concentrations similar to that in previous campaigns with boronizations could be achieved. Despite the removal of all macroscopic carbon sources, no strong change in C influxes and C content could be observed so far. The W concentrations are similar to the ones measured previously in discharges with old boronization and only partial coverage of the surfaces with W. Concomitantly it is found that although the W erosion flux in the divertor is larger than the W sources in the main chamber in most of the scenarios, it plays only a minor role for the W content in the main plasma. For large antenna distances and strong gas puffing, ICRH power coupling could be optimized to reduce the W influxes. This allowed a similar increase of stored energy as yielded with comparable beam power. However, a strong increase of radiated power and a loss of H-Mode was observed for conditions with high temperature edge plasma close to the antennas. The use of ECRH allowed keeping the central peaking of the W concentration low and even phases of improved H-modes have already been achieved.

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