The effect of helium on the tungsten microstructure was investigated first by exposure to a radio frequency driven helium plasma with fluxes of the order of 1 × 10(19) m(-2) s(-1) and second by helium incorporation via magnetron sputtering. Roughening of the surface and the creation of pinholes were observed when exposing poly- and nanocrystalline tungsten samples to low-flux plasma. A coating process using an excess of helium besides argon in the process gas mixture leads to a porous thin film and a granular surface structure whereas gas mixture ratios of up to 50% He/Ar (in terms of their partial pressures) lead to a dense structure. The presence of helium in the deposited film was confirmed with glow-discharge optical emission spectroscopy and thermal desorption measurements. Latter revealed that the highest fraction of the embedded helium atoms desorb at approximately 1500 K. Identical plasma treatments at various temperatures showed strongest modifications of the surface at 1500 K, which is attributed to the massive activation of helium singly bond to a single vacancy inside the film. Thus, an efficient way of preparing nanostructured tungsten surfaces and porous tungsten films at low fluxes was found.

To introduce phase-based conductivity mapping from a configuration space analysis. Methods: The frequency response function of balanced SSFP (bSSFP) is used to perform a configuration space analysis. It is shown that the transceive phase for conductivity mapping can be directly obtained by a simple fast Fourier transform of a series of phase-cycled bSSFP scans. For validation, transceive phase and offresonance mapping with fast Fourier transform is compared with phase estimation using a recently proposed method, termed PLANET. Experiments were performed in phantoms and for in vivo brain imaging at 3 T using a quadrature head coil. Results: For fast Fourier transform, aliasing can lead to systematic phase errors. This bias, however, decreases rapidly with increasing sampling points. Interestingly, Monte Carlo simulations revealed a lower uncertainty for the transceive phase and the off-resonance using fast Fourier transform as compared with PLANET. Both methods, however, essentially retrieve the same phase information from a set of phase-cycled bSSFP scans. As a result, configuration-based conductivity mapping was successfully performed using eight phase-cycled bSSFP scans in the phantoms and for brain tissues. Overall, the retrieved values were in good agreement with expectations. Conductivity estimation and mapping of the field inhomogeneities can therefore be performed in conjunction with the estimation of other quantitative parameters, such as relaxation, using configuration theory. Conclusions: Phase-based conductivity mapping can be estimated directly from a simple Fourier analysis, such as in conjunction with relaxometry, using a series of phase-cycled bSSFP scans.

The first mirror (FM) cleaning operations in ITER are expected to be executed in presence of ∼ 3T magnetic field. In the RF plasma cleaning configuration, this would have a significant influence on the plasma properties, ion energy, angle of incidence as well as flux spatial distribution. To this end, RF discharges were excited in an ITER-sized mock-up of a first mirror unit (FMU) consisting of a powered first mirror M1 and a grounded second mirror M2 placed in a homogeneous 3T magnetic field. The plasma discharge was confined in a beam extending in the direction of the magnetic field, consequently wetting a limited portion of the FMU walls. In the DC-decoupled scheme (without λ/4 filter), this considerably influenced the self-bias voltage VDC that develops on M1. Changing the angle α between M1 normal and magnetic field, modified the plasma wetted wall area Ag and the resulting VDC varied by over two orders of magnitude. Plasma exposure experiments were also done in the DC-coupled scheme (with λ/4 filter), wherein the angle and wetted surface determined the area of the wall sputtered. Increasing α led to an increase in the sputtered wall area Ag, and consequently the wall deposition on grounded M2. However, in all the cases M1 was entirely clean with exception of edge deposits in some. In contrast, both M1 and M2 are coated with wall deposits in absence of a magnetic field and similar plasma exposure. The results show that plasma cleaning with a λ/4 filter in a 3T magnetic field at ITER could potentially prevent the parasitic wall deposition on FMs. The results also highlight the importance of FM orientation in the magnetic fields and the wetted area in their plasma cleaning in both DC-coupled as well as decoupled schemes within the ITER diagnostic systems.

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