[Show abstract][Hide abstract] ABSTRACT: At FOM Rijnhuizen, linear plasma generators are used to investigate plasma-material interactions under high-density (⩽1021 m−3), low-temperature (⩽5 eV) plasma bombardment. Research into carbon-based materials has been focused on chemical erosion by hydrogen plasmas. Results from plasma exposure to high-flux (>1023 H+/m2 s) and low-temperature hydrogen plasma indicate silicon carbide has a lower relative rate of gross erosion than other carbon-based materials (e.g. graphite, diamond, carbon-fiber composites) by a factor of 7–10. Hydrogenic retention is the focus of research on tungsten and molybdenum. For target temperatures of 700–1600 K, the temperature dependence of hydrogenic retention is the dominant factor. Damage to the surface by heavy ion irradiation has shown to enhance retention by a factor of 2.5–4.1. Thermal stressing of W via. e-beam thermal cycling also enhances hydrogenic retention by a factor of 2.1 ± 0.2, likely due to the introduction of thermal defects, which act as trapping sites for implanted hydrogenic isotopes.
[Show abstract][Hide abstract] ABSTRACT: The FOM-Institute for Plasma Physics Rijnhuizen is constructing Magnum-PSI; a magnetized (3 T), steady-state, large area (80 cm2) high-flux (up to 1024 H+ ions m−2 s−1) plasma generator. Magnum-PSI will be a highly accessible laboratory experiment in which the interaction of magnetized plasma with different surfaces can be studied. This experiment will provide new insights in the complex physics and chemistry that will occur in the divertor region of the future experimental fusion reactor ITER. Here, extremely high power and particle flux densities are predicted at relatively low plasma temperatures. Magnum-PSI will be able to simulate these detached ITER divertor conditions in detail. In addition, conditions can be varied over a wide range, such as different target materials, plasma temperatures, beam diameters, particle fluxes, inclination angles of target, background pressures, magnetic fields, etc., making Magnum-PSI an excellent test bed for high heat flux components of future fusion reactors.The design phase of the Magnum-PSI device has been completed. The construction and assembly phase of the device is in progress. In this contribution, we will present the design and construction of the Magnum-PSI experiment. The status of the vacuum system, the 3 T superconducting magnet, the plasma source, the target plate and manipulator, and additional plasma heating will be presented. The plasma and surface diagnostics that will be used in the Magnum-PSI experiment will be introduced.
[Show abstract][Hide abstract] ABSTRACT: Tungsten targets are exposed to the plasma conditions expected at the strike point of a detached ITER divertor (∼1024D/m2s, Te∼2eV). The surface temperature of the target is ∼1600K at the center and decreased radially to ∼1000K at the edges. A 2-D spatial scan of the W target using nuclear reaction analysis (NRA) shows an asymmetric D retention profile with the lowest retention values at the center of the target and the highest 6mm off-center. Even in the regions of larger retention, the D concentrations were ⩽5×1015D/cm2 as measured by NRA. Thermal desorption spectroscopy (TDS) is used to measure the global D retention. Very low retention with retained fractions ranging from 10−7 to 10−5 Dretained/Dincident were measured with TDS. Both NRA and TDS results show no clear dependence of retention on incident fluence possibly indicating the absence of plasma-driven trap production in W under these conditions.
[Show abstract][Hide abstract] ABSTRACT: A new cascaded arc containing three separate discharge channels at 15 mm distance from each other was constructed to produce intense and wide hydrogen plasma beams and first tests were carried out at Pilot-PSI. Current and voltage measurements as well as calorimetry on the cooling water of the source demonstrated that these channels operated independently. Thomson scattering measurements showed that, depending on the nozzle geometry, the three outputs merge to one beam if the source is operated at argon in magnetic fields up to 1.6 T densities. In hydrogen operation, the individual outputs did not merge or interact. Also a first test was performed in argon on the use of a remote ring anode to induce beam mixing due to rotation driven by cross-B currents.
[Show abstract][Hide abstract] ABSTRACT: Summary form only given. To study plasma-surface interactions (PSI) in conditions similar to those expected in the divertor of ITER and other future fusion devices, the FOM Institute for Plasma Physics Rijnhuizen is building a linear plasma generator called Magnum-PSI. In this machine, targets will be exposed to steady-state particle and energy fluxes similar to those predicted at the ITER strike points in a comparable background pressure and magnetic field: 1024 ions m"2 s"1 and 10 MW m"2 at ~1 Pa and 3 T. The width of the plasma beam will be up to ~10 cm. In this contribution we report on the development of the plasma source for this experiment. Magnum-PSI will use a cascaded arc plasma source. This is a flowing, direct-current, wall-stabilized, thermal arc discharge. Based on data from experiments performed on the development device Pilot-PSI, we have formulated an empirical model for the scaling of the hydrogen plasma production by a cascaded arc as a function of the input power, the gas flow rate and the discharge channel diameter. This model describes the dominant physical processes inside the discharge channel. Our investigations furthermore showed the importance for the plasma production of processes in the nozzle/anode region. With an optimized anode geometry and an applied magnetic field, the discharge current is forced to extend into the plasma beam (well outside the plasma source). The extra power deposition into the plasma beam leads to a greatly enhanced ion flux towards the target (~0.5 m downstream). Experiments with sources with multiple closely packed discharge channels have been performed and showed that depending on conditions and when operating on argon, three separate beams can be made to mix into a single wide beam.
IEEE International Conference on Plasma Science 01/2009; DOI:10.1109/PLASMA.2009.5227704
[Show abstract][Hide abstract] ABSTRACT: A high flux cascaded arc hydrogen plasma source is being developed for the linear plasma generator Magnum-PS I (magnetised plasma generator and numerical modeling for plasma surface interaction studies). Magnum-PSI will be the heart of the PSI-lab at the FOM-Institute for Plasma Physics Rijnhuizen and is being developed to investigate PSI issues for ITER. Especially the wall material of the so-called divertor of ITER, which is the region where plasma and impurities are neutralized and pumped off, will receive unprecedented particle and power loads. The expected numbers are: particle fluxes of up to 1024 ions/m<sup>2</sup>s and power loads of up to 10 MW/m2. We have demonstrated that it is possible to produce such conditions in a linear plasma generator with a cascaded arc in a magnetic field of 1.6 T<sup>2</sup>. The diameter of the plasma beam in these experiments was typically 20 mm. For Magnum-PSI, we envisage a beam diameter of 10 cm in order to enter the strongly coupled regime of PSI research. In this contribution we investigate the production of larger beam diameters by combining the output of several discharge channels. A new arc consisting of three separate arc channels with a common cylinder anode was constructed for this purpose. Thomson scattering, high resolution Doppler spectroscopy and calorimetry were applied to measure the performance of and interaction between the three channels.
[Show abstract][Hide abstract] ABSTRACT: The FOM-Institute for Plasma Physics Rijnhuizen has started the construction of Magnum-PSI, a magnetized (3 T), steady-state, large area (80 cm2) high-flux (up to 1024 H+ions m−2 s−1) plasma generator. The aim of this linear plasma device is to provide a controlled, highly accessible laboratory experiment facility in which the interaction of a magnetized plasma with different surfaces can be studied in detail. Magnum-PSI consists of several subsystems including vacuum, cooling, plasma source, target station, and a number of diagnostic systems. The safety, COntrol, Data Acquisition and Communication (CODAC) system integrates these subsystems and provides an interface for the Magnum-PSI users. The CODAC system is designed in parallel with the Magnum-PSI hardware to maximize compatibility and usability. The CODAC system must handle the step-by-step construction and expansion of the Magnum-PSI system without compromising human and system safety in each step, combine manual and remote control, and provide a central data storage and implement automated experiment execution.Key features of the CODAC system are a layered, modular and distributed design, the use of intelligent devices (e.g. commercial computer controlled cooling units), a centralized data storage built on HDF5, and a communication layer built on ZeroC ™ Ice ™. A significant part of the design is based on existing open source software components, integrated using C and Python code.
[Show abstract][Hide abstract] ABSTRACT: A highly sensitive imaging Thomson scattering system was developed for low temperature (0.1-10 eV) plasma applications at the Pilot-PSI linear plasma generator. The essential parts of the diagnostic are a neodymium doped yttrium aluminum garnet laser operating at the second harmonic (532 nm), a laser beam line with a unique stray light suppression system and a detection branch consisting of a Littrow spectrometer equipped with an efficient detector based on a "Generation III" image intensifier combined with an intensified charged coupled device camera. The system is capable of measuring electron density and temperature profiles of a plasma column of 30 mm in diameter with a spatial resolution of 0.6 mm and an observational error of 3% in the electron density (n(e)) and 6% in the electron temperature (T(e)) at n(e) = 4 x 10(19) m(-3). This is achievable at an accumulated laser input energy of 11 J (from 30 laser pulses at 10 Hz repetition frequency). The stray light contribution is below 9 x 10(17) m(-3) in electron density equivalents by the application of a unique stray light suppression system. The amount of laser energy that is required for a n(e) and T(e) measurement is 7 x 10(20)n(e) J, which means that single shot measurements are possible for n(e)>2 x 10(21) m(-3).
[Show abstract][Hide abstract] ABSTRACT: The FOM-Institute for Plasma Physics Rijnhuizen is preparing the construction of Magnum-PSI, a magnetized (3 T), steady state, large area (80 cm2) high-flux (up to 1024 H+ ions m−2 s−1) plasma generator. The aim of the linear plasma device Magnum-PSI is to provide a controlled, highly accessible laboratory experiment in which the interaction of a magnetized plasma with different surfaces can be studied in detail. Plasma parameters can be varied over a wide range, in particular covering the high-density, low-temperature conditions expected for the detached divertor plasma of ITER. The target set-up will be extremely flexible allowing the investigation of different materials under a large variety of conditions (temperatures, inclination, biasing, coatings, etc.). A range of target materials will be used, including carbon, tungsten and other metals, and mixed materials. Because of the large plasma beam of 10 cm diameter and spacious vacuum tank, even the test of whole plasma-facing component mock-ups will be possible.In this article, we will present the pre-design of the Magnum-PSI experiment. We will focus on the requirements of the vacuum system and the 3 T superconducting magnet. We briefly introduce some of the other sub-systems on Magnum-PSI such as the target and target manipulator.
[Show abstract][Hide abstract] ABSTRACT: Pilot-PSI produces hydrogen plasma with a cascaded arc. It is demonstrated that the output electron density reaches 4×1021m−3 by pushing the input power to 45kW. Increasing the diameter of the discharge channel (studied from 4 to 7mm) does not affect the plasma output but reduces the power input by up to 25%. The plasma was post-heated by feeding the plasma column with a net current. This increased the electron temperature from ∼2 to 4eV. Calorimetric and voltage measurements on the cascade plates showed that this also influenced the discharge characteristics inside the arc.
[Show abstract][Hide abstract] ABSTRACT: A magnetized hydrogen plasma beam was generated with a cascaded arc, expanding in a vacuum vessel at an axial magnetic field of up to 1.6 T . Its characteristics were measured at a distance of 4 cm from the nozzle: up to a 2 cm beam diameter, 7.5×10<sup>20</sup> m <sup>-3</sup> electron density, ∼2 eV electron and ion temperatures, and 3.5 km / s axial plasma velocity. This gives a 2.6×10<sup>24</sup> H <sup>+</sup> m <sup>-2</sup> s <sup>-1</sup> peak ion flux density, which is unprecedented in linear plasma generators. The high efficiency of the source is obtained by the combined action of the magnetic field and an optimized nozzle geometry. This is interpreted as a cross-field return current that leads to power dissipation in the beam just outside the source.
[Show abstract][Hide abstract] ABSTRACT: FOM-Rijnhuizen is building, in cooperation with its Trilateral Euregio Cluster (TEC) partners, a PSI-laboratory to study plasma surface interaction (PSI) under extreme, ITER relevant plasma conditions. The largest linear plasma generator of PSI-lab is Magnum-PSI, and is designed to deliver up to 10 MW m−2 power over a 10 cm diameter hydrogen plasma beam with an electron density (ne) up to 1021 m−3 and electron-temperature (Te) between 1–5 eV. Magnum-PSI is presently under construction and its predesign is presented. Its forerunner is Pilot-PSI, in which record plasma parameters of ne=4×1021 m−3 at Te=2 eV in a ~1 cm wide hydrogen beam confined by a magnetic field (B) ≤1.6 T were measured at 40 mm downstream the source nozzle. At 17 mm in front of a target (located at 0.56 m distance from the source nozzle), ne>1021 m−3 and Te≤ 4 eV have been demonstrated. Initial experiments on exposing fine-grain carbon samples are presented that showed up to 20 μm s−1 erosion as a demonstration of the extreme plasma conditions. Spectroscopy was applied to compare chemical erosion yields for flux densities up to 5.0×1024 m−2 s−1.
Physica Scripta 03/2007; 2007(T128):18. DOI:10.1088/0031-8949/2007/T128/004 · 1.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The FOM Institute for Plasma Physics is preparing the construction of the linear plasma generator, Magnum-psi. A pilot experiment (Pilot-psi) has been constructed, which we have used to optimize the cascaded arc plasma source and to explore the effect of high magnetic fields on the source operation as well as the expanding plasma beam and the effectiveness of Ohmic heating for manipulating the electron temperature and plasma density after the plasma expansion. Results are presented that demonstrate increasing source efficiency for increasing magnetic fields (up to 1.6 T). Thomson scattering measurements demonstrate that ITER relevant plasma fluxes are presently achieved in Pilot-psi: ∼1024 m−2 s−1 and that additional heating could elevate the plasma temperature from 1.0 to 1.7 eV.
[Show abstract][Hide abstract] ABSTRACT: The FOM-Institute for Plasma Physics is preparing the construction of Magnum-psi, a magnetized (3 T), steady-state, large area (100 cm<sup>2</sup>), high-flux (up to 10<sup>24</sup> H<sup>+</sup> ions m<sup>-2</sup>s<sup>-1</sup>) plasma generator. Magnum-psi will be used to study plasma-surface interaction in conditions similar to those in the divertor of ITER and fusion reactors beyond ITER. The active magnetic field region is required to be 4 meter long, 1 meter diameter and steady state. This, together with the need for minimization of the running costs, makes the application of superconducting coils imperative. The magnet system will be unique because of its maximum transparency to provide optimal radial access to the experimental region inside the magnet bore. In this contribution we present a magnet configuration that consists of 5 cylindrical, conduction cooled NbTi coils. These generate an axial field of 3 T with a maximum field on the coils below 6 T. Two cryogenic structures are proposed: the discrete coils are either placed within separate cryostats or are supported by a single cylinder in a shared cryostat with 32 room temperature view ports. Room temperature iron rings close to the outer coils reduce the axial forces that would otherwise put severe constraints on the mechanical structure. The field will most probably be passively shielded by an iron dome at 2 meters from the cryostat.
[Show abstract][Hide abstract] ABSTRACT: The Magnum-psi project (magnetised plasma generator and numerical modelling for plasma-surface interaction) aims at the fundamental study of plasma-surface interaction processes in conditions relevant to the ITER divertor. A linear plasma generator has been constructed in which a hydrogen ion flux of 10(23) particles/m(2) s to a surface can be realised, at a temperature of around 1 eV. A longitudinal magnetic field of < 1.6 T is used to confine the plasma. This produces a narrow plasma beam, with a sustained high ionisation degree over a length of I in. To closer approach the ITER divertor conditions, a larger version of the Magnum-psi device is planned, with a permanent magnetic field < 3 T, a beam diameter of > 10 cm and a controlled temperature in the range 0.1 - 10 eV.
[Show abstract][Hide abstract] ABSTRACT: Real-time control of the gyrotron output power has been obtained by controlling the gyrotron cathode voltage. This control system has been used to explore feedback control of global and local plasma parameters. A description is given of technical set-up and of the results, which include control of the electron temperature Te with excellent time response and successful stabilization of m/n=2/1 modes in case of cw injection.
[Show abstract][Hide abstract] ABSTRACT: Experiments with strong localized electron cyclotron heating (ECH) in the RTP tokamak show that electron heat transport is governed by alternating layers of good and bad thermal conduction. For central deposition hot filaments are observed inside the q = 1 radius. Moving the ECH resonance from the centre to the edge of the plasma results in discrete steps of the central electron temperature. The transitions occur when the minimum q value crosses q = 1,2,5/2 or 3, and correspond to the loss of a transport barrier situated close to the rational q value. Close to the transitions a new type of sawtooth activity is observed, characterized by the formation of sharp off-axis maxima on the profile, which collapse abruptly. The formation of the off-axis maxima is attributed to heat deposition precisely `on top of' a transport barrier.