The snowflake (SF) divertor is a plasma configuration that may enable tokamak operation at high performance and lower peak heat loads on the plasma-facing components than a standard single-null divertor. This paper reports on the results of experiments performed on the TCV tokamak in both the low- and high-confinement regimes, wherein the divertor configuration was continuously varied between a standard single-null and a ‘SF-plus’, which features auxiliary strike points (SPs) in the private flux region of the primary separatrix. The measured edge properties show that, in L-mode, the fraction of the exhaust power reaching the additional SPs is small. During edge-localized modes, up to ∼20% of the exhausted energy is redistributed to the additional SPs even at an x-point separation of 0.6 times the plasma minor radius, thereby reducing the peak heat flux to the inner primary SP by a factor of 2–3. The observed behaviour is qualitatively consistent with a proposed model for enhanced cross-field transport through the SF's relatively large region of low poloidal field by instability-driven convection.

a b s t r a c tSurface modifications and deuterium retention induced in tungsten by high fluxes (10 24 m À2 s À1 ) low energy (38 eV) deuterium ions were studied as a function of surface temperature. Blister formation was studied by scanning electron microscopy and electron backscatter diffraction, while deuterium retention was measured by thermal desorption spectroscopy. Blisters are observed on the surface exposed at different temperatures, ranging from 493 K to 1273 K. The blister density and D retention decrease with the increasing exposure temperature. The formation of blisters at high temperatures is attributed to the high flux of D plasma. At 943 K, with the increasing fluence, there is trend to the saturation of D retention and blister density. The defects caused by plasma exposure have an important effect on the D trapping and blistering behavior. The formation of blisters has a strong relationship with slipping system of tungsten.

h i g h l i g h t s• High heat flux, high density plasmas in a highly accessible linear plasma device.• Plasma exposure of targets of different sizes under selectable plasma beam angles.• Dedicated plasma and surface diagnostics.• Differential vacuum pumping system. a r t i c l e i n f o b s t r a c tThe construction phase of the linear plasma generator Magnum-PSI at the FOM institute DIFFER has been completed and the facility has been officially opened in March 2012. The scientific program to gain more insight in the plasma-wall interactions relevant for ITER and future fusion reactors has started.In Magnum-PSI, targets of a wide range of materials and shapes can be exposed to high particle, high heat flux plasmas (>10 24 ions m −2 s −1 ; >10 MW/m 2 ). For magnetization of the plasma, oil-cooled electromagnets are temporarily installed to enable pulsed operation until the device is upgraded with a superconducting magnet. The magnets generate a field of up to 1.9 T close to the plasma source for a duration of 6 s. Longer exposure times are available for lower field settings.Plasma characterizations were done with a variety of gases (H, D, He, Ne and Ar) to determine the machine performance and prepare for subsequent scientific experiments. Thomson scattering and optical emission spectroscopy were used to determine the plasma parameters while infrared thermography and target calorimetry were used to determine the power loads to the surface. This paper reports on the status of Magnum-PSI and its diagnostic systems. In addition, an overview of the plasma parameters that can be achieved in the present state will be given.

A high-power pulsed magnetized arc discharge has been developed to allow the superimposition of a dc plasma and a high-power plasma impulse with a single plasma source. A capacitor bank (8400 µF) is parallel-coupled to the current regulated power supply. The current is transiently increased from its stationary value (200 A) up to 14.5 kA in 650 µs. The discharge power is thus raised from 18 kW to 6.5 MW, corresponding to a power density of up to 1.7 × 10 12 W m −3 -10 2 times higher than in the dc mode (200 A). The plasma parameters are measured by Thomson scattering ∼4 cm downstream of the nozzle. The electron temperature and density vary from ∼2.6 eV and 7 × 10 20 m −3 in dc and up to 15 eV and 80 × 10 20 m −3 during the pulse. A saturation of the electron density with increasing current is observed while the temperature increases monotonically. Time-resolved voltage/current measurements of the arc are used to explain the role of the magnetic field and the evolution of the temperature.