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Routine, repeatable radar measurements of soil moisture are expected to be achievable through the use of frequencies with penetration depths that reach the root-zone of given vegetation structures within soil. Due to the size, weight, power, cost, and legal constraints of creating spaceborne radar transmitters, the practice of reutilizing existing satellite navigation/communication systems for spaceborne radar applications is gaining attention from research communities under the title "Signal of Opportunity" (SoOp). The use of SoOp transmitters as a free source of illuminators is expected to be a viable tool for the remote sensing of soil moisture at depths below 5 centimeters. However, the returns of a radar signal can be representative of many types of soil moisture profiles, and the returned radar signals do not indicate the depth of the contained soil moisture. To this end, this paper is intended to better understand the various effects that radar parameters have on a returned radar signal given a known, simple soil moisture profile in order to help determine the feasibility of using multiple (SoOp) transmitters to determine soil moisture profiles. In order to accomplish this, the recently developed SoOp Coherent Bistatic (SCoBi) scattering model is used to determine the relative permittivity of a given soil moisture profile as well as determine the complex reflection coefficients of the incident signals upon the surface.

Root zone soil moisture (RZSM) is an essential variable in meteorology, hydrology, and agriculture. Current methods in passive and active microwave remote sensing at L-band (e.g. SMOS or SMAP) are limited to a sensing depth of less than 10 cm. Observing RZSM (water in the top meter of soil) will require lower frequencies (P-band), presenting significant difficulties for a spaceborne instrument, due to the required antenna size, the presence of radio-frequency interference (RFI), and competition for spectrum allocations (in the case of active radar). Bistatic radar using Signal of Opportunity (SoOp) (e.g. digital satellite transmitters) offers the possibility of remote sensing using powerful signals already occupying bands allocated for communications. This paper will present early results from the first airborne campaign to study P-band reflectometry for soil moisture, conducted in Oklahoma between October 17–26, 2016. A geosynchronous communication satellite, transmitting a 25 KHz data signal on a 260.375 MHz carrier was uses at the signal source. Preliminary assessment of the data showed high reflectivity over a lake and consistent results between subsequent overflights of the watershed. Recently, a tower experiment has started using a 20 MHz wide signal centered at 370 MHz. Linearly polarized measurements were made over bare soil between 25-May-2017 and 8-June-2017 observing a strong reflected signal. Corn is being planted on the field and measurements will be made throughout the growing season.

Intense Langmuir-probe measurements were carried out in the toroidal low-temperature plasma of the torsatron TJ-K in order to investigate the origin and dynamics of intermittent transport events, so-called blobs, at the transition from closed to open field lines. The statistical properties of the fluctuations at the plasma boundary agree with observations made in fusion edge plasmas. Blobs were found to be generated locally through a change in turbulence drive across the separatrix. The non-linear spectral energy transfer from small-scale fluctuations into large-scale flows was measured with a 128-probe array. The results point to the transfer being a key loss channel for turbulence energy leading to a reduction in turbulent transport. Earlier observations [M.A. Pedrosa et al., Phys. Rev. Lett. 100, 215003 (2008)] of enhanced long-range correlations in the plasma potential through externally induced shear flows in TJ-II stellarator were verified. The newly measured correlation of zonal vorticity and Reynolds stress at induced flow shear indicates an enhancement of zonal-flow drive (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The three-dimensional structure of drift-wave turbulence and turbulent transport is investigated in plasmas of the stellarator experiment TJ-K. By means of two poloidal Langmuir probe arrays placed at different toroidal positions, density and potential fluctuations are recorded simultaneously at 128 positions on a single flux surface. From these data, the spatial drift-wave turbulence pattern including perpendicular and parallel structure sizes are obtained using a cross-correlation technique. A comparison with the magnetic field structure indicates an initially perfect alignment of turbulent structures with magnetic field lines. Passing over regions with different field-line pitches according to the local variation of the rotational transform, however, results in a measured displacement of turbulent structures with respect to the field lines during their radial propagation. A reduction in the perpendicular correlation lengths in regions of high absolute values of local magnetic shear is found. Prominent and poloidally narrow turbulent transport maxima are measured at different toroidal positions. They are connected by the magnetic field lines and located in regions of negative normal curvature. The poloidal propagation pattern of turbulent structures and the exact position of the transport maximum depend on the magnetic field direction.

Turbulent transport at the transition from closed to open field lines has been investigated in the stellarator experiment TJ-K. It is found that drift-wave turbulence in the confined region is responsible for the generation of intermittent structures (so-called blobs) in the unconfined region. There the character of turbulence changes and a decoupling of density and potential fluctuations is observed. The poloidal propagation of the intermittent events can be understood in the framework of background flows caused by gradients in the equilibrium plasma pressure and potential profiles.

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.

International audience; This paper summarizes recent experimental characterization of radio frequency (RF)-induced scrape-off layer (SOL) modifications in ASDEX-Upgrade (AUG), JET and Tore Supra (TS). Geometrical aspects are emphasized: complex SOL patterns are observed by several indicators visualized in one or two dimensions transverse to the magnetic field lines. Results are ascribed to inhomogeneous RF-induced SOL biasing around powered ion cyclotron range of frequencies antennas and associated E × B 0 density convection (D'Ippolito et al 1993 Phys. Fluids B 5 3603). Within a simple RF sheath model (Perkins 1989 Nucl. Fusion 29 583), the shape of convective cells on TS can be interpreted in terms of RF-sheath generation by parallel RF currents. Some lessons are drawn for future machines.

Blob properties are studied in the scrape-off layer of the tokamak ASDEX Upgrade with a fast camera. The gas-puff imaging technique is used to investigate the detection rate as well as the blob size and velocity scaling. The experiments were performed in L- and H-mode phases of the same discharges to study the change in blob properties after the L-H transition. In both regimes the detection rate is of the order of a few thousand blobs per second, which is compatible with the picture of blob generation by edge micro instabilities. The blob size increases in H-mode, while the radial velocity decreases slightly. The changes are, however, not indicating a drastic change in the blob dynamics in both phases. The experimentally found blob properties were compared to predictions from a novel blob model including effects due to a finite ion temperature, which should be more appropriate for the conditions in the SOL of fusion plasmas.

Blob properties are studied in the scrape-off layer of the stellarator TJ-K. Langmuir probes and a fast camera are used to investigate the generation rate as well as the blob size and velocity scaling. Discharges with different ion species and magnetic field strengths provide access to a large range of plasma parameters. It was found that almost every large amplitude drift wave in the edge of the confined plasma triggers blob generation in the scrape-off layer, which implies that the birth rate of blobs is determined by the turbulence in the edge. Furthermore, the cross-field size of the blobs seems to correlate with the size of the generating drift waves. Since the observed radial propagation velocity of the blobs is well described by a size dependent blob velocity model, the size coupling between the drift waves and the blobs also has an impact on the blob velocities. Thus, the presented results imply that the dynamics in the edge have a large influence on the blob properties in the scrape-off layer.

Analyses of non-linear interactions in drift-wave turbulence were carried out in wavenumber space on experimental data obtained from multi-probe measurements in the toroidally confined plasma of the stellarator TJ-K. Recent results are reported. The inverse cascade of energy from small to large scales as typical for two-dimensional turbulence is verified. The transfer in k space is found to be governed by non-local processes. The same way, large-scale zonal flows (ZFs) are found to tap energy from small-scale turbulence, as consistent with the vortex-thinning mechanism. An enhancement of the correlation between ZF shear and Reynolds stress as a source of ZF energy is observed, when strong background shear flows are imposed, which comes along with increased long-range correlations.