No abstract

Magnum-PSI is an advanced linear plasma device uniquely capable of producing plasma conditions similar to those expected in the divertor of ITER both steady-state and transients. The machine is designed both for fundamental studies of plasma-surface interactions under high heat and particle fluxes, and as a high-heat flux facility for the tests of plasma-facing components under realistic plasma conditions. To study the effects of transient heat loads on a plasma-facing surface, a novel pulsed plasma source system as well as a high power laser are available. In this article, we will describe the capabilities of Magnum-PSI for high-heat flux tests of plasma-facing materials.

Cross-shelf distributions of temperature, salinity, water masses, and dissolved oxygen in St. Helena Bay revealed substantial vertical and seasonal variations. In the surface layers, nearshore and offshore temperature and salinity patterns differed, with bay-scale variability linked to upwelling dynamics and coastal processes, while the offshore region was influenced by solar insolation. Spectral analysis revealed that an annual signal prevailed at most stations, and corroborated contrasting patterns between the offshore and nearshore regions, with phase differences suggesting shoreward propagation of the offshore temperature signal. The shelf was dominated by Modified Upwelled Water (MUW) and Subantarctic Mode Water (SAMW), which comprised the primary source of upwelled water. Clear zonation of MUW was evident across the shelf, resulting from seasonal variations in locations of the oceanic and bifurcated shelf-break fronts. Dynamics within St. Helena Bay consistently differed from those further offshore, due to the influences of the shelf-break front, Cape Columbine upwelling plume, and cyclonic recirculation, which appeared to be associated with an intraannual signal with a periodicity of 3-4 months. Persistent hypoxia in the bottom waters suggested the occurrence of a permanent reservoir of Low Oxygen Water (LOW). Seasonal shoreward and offshore expansion of LOW occurred throughout the upwelling season, with maximum extent reached during summer and autumn, due to the coupled effects of advection and local phytoplankton decay. While wind mixing ventilated the water column at nearshore stations in winter, and the onset of upwelling during spring introduced oxygen-richer water from further offshore, hypoxia persisted in the center of the Bay.

An advanced Thomson scattering system has been built for a linear plasma generator for plasma surface interaction studies. The Thomson scattering system is based on a Nd:YAG laser operating at the second harmonic and a detection branch featuring a high etendue (f /3) transmission grating spectrometer equipped with an intensified charged coupled device camera. The system is able to measure electron density (n e ) and temperature (T e ) profiles close to the output of the plasma source and, at a distance of 1.25 m, just in front of a target. The detection system enables to measure 50 spatial channels of about 2 mm each, along a laser chord of 95 mm. By summing a total of 30 laser pulses (0.6 J, 10 Hz), an observational error of 3% in n e and 6% in T e (at n e = 9.4 × 10 18 m −3 ) can be obtained. Single pulse Thomson scattering measurements can be performed with the same accuracy for n e > 2.8 × 10 20 m −3 . The minimum measurable density and temperature are n e < 1 × 10 17 m −3and T e < 0.07 eV, respectively. In addition, using the Rayleigh peak, superimposed on the Thomson scattered spectrum, the neutral density (n 0 ) of the plasma can be measured with an accuracy of 25% (at n 0 = 1 × 10 20 m −3 ). In this report, the performance of the Thomson scattering system will be shown along with unprecedented accurate Thomson-Rayleigh scattering measurements on a low-temperature argon plasma expansion into a low-pressure background.

An electron cyclotron emission ͑ECE͒ receiver inside the electron cyclotron resonance heating ͑ECRH͒ transmission line has been brought into operation. The ECE is extracted by placing a quartz plate acting as a Fabry-Perot interferometer under an angle inside the electron cyclotron wave ͑ECW͒ beam. ECE measurements are obtained during high power ECRH operation. This demonstrates the successful operation of the diagnostic and, in particular, a sufficient suppression of the gyrotron component preventing it from interfering with ECE measurements. When integrated into a feedback system for the control of plasma instabilities this line-of-sight ECE diagnostic removes the need to localize the instabilities in absolute coordinates.