[Show abstract][Hide abstract] ABSTRACT: Significant electric fields both parallel and perpendicular to a magnetic field have been observed and modeled self-consistently in an ITER divertor relevant plasma–wall experiment. Due to magnetization, electric current is found to penetrate the plasma beam outside of the cascaded arc plasma source with a length scale proportional to , where He and Hi are the electron and ion Hall parameters, respectively. Plasma rotation measurements and chemical erosion profiles at a carbon target demonstrate that for a sufficiently well-magnetized plasma, a current through the target causes plasma–wall sheath potentials to significantly increase in a region of net ion collection while for the conditions studied, regions of net electron collection remain unaffected. The plasma–wall sheath profile at the target has been characterized experimentally as a function of negative target potential.
[Show abstract][Hide abstract] ABSTRACT: Fine-grain graphite targets have been exposed to ITER divertor relevant
plasmas in Pilot-PSI to address material migration issues in fusion devices.
Optical emission spectroscopy and mass loss measurements have been employed to
quantify gross chemical erosion and net erosion yields, respectively. Effects
of the ion impact energy and target geometry on carbon erosion yields have been
studied. It is concluded that temporal evolution of gross chemical erosion is
strongly connected with changes in morphology of plasma exposed surfaces. The
net carbon erosion yield is increased when the targets are partly covered by
insulating boron-nitride rings.
[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: A technique based on ammonia injection in the afterglow region of the divertor plasma is proposed for the inhibition of tritium/carbon co-deposits in remote areas of ITER when operated with carbon-based divertor targets. Supporting experiments in the PILOT-PSI divertor simulator plasma device and in a RF methane plasma with afterglow ammonia injection are described. The potential of ammonia glow discharges for the removal of carbon layer in easy to reach areas and in gaps is also addressed, thus opening the possibility of an integrated tritium control scenario with water-free chemical products in carbon-based divertor operation during the active phase of ITER.
[Show abstract][Hide abstract] ABSTRACT: The chemical erosion of carbon was investigated in the linear plasma device Pilot-PSI for ITER divertor relevant hydrogen plasma flux densities 10(23) < Gamma < 10(25) m(-2) s(-1). The erosion was analyzed in situ by optical emission spectroscopy and post mortem by surface profilometry. The experiments indicate a threshold for the absolute carbon erosion rate as a function of plasma temperature T-e around 0.7 eV, a peak of the surface temperature around 550 degrees C, and no dependence on plasma flux density. The latter implies a flux dependence of the chemical erosion yield as Gamma(-1). The value of the chemical erosion yield at the surface temperature of maximum erosion and Gamma = 1 x 10(24) m(-2) s(-1) was 0.9% for T-e < 0.5 eV as determined by surface profilometry.
[Show abstract][Hide abstract] ABSTRACT: A technique based on reactive gas injection in the afterglow region of the divertor plasma is proposed for the suppression of tritium-carbon codeposits in remote areas of ITER when operated with carbon-based divertor targets. Experiments in a divertor simulator plasma device indicate that a 4 nm/min deposition can be suppressed by addition of 1 Pa·m³ s⁻¹ ammonia flow at 10 cm from the plasma. These results bolster the concept of nonperturbative scavenger injection for tritium inventory control in carbon-based fusion plasma devices, thus paving the way for ITER operation in the active phase under a carbon-dominated, plasma facing component background.
[Show abstract][Hide abstract] ABSTRACT: Methane (CH4) was injected into the high density (ne ~ 1020 m−3), low temperature (Te ~ 1 eV) hydrogen plasma in Pilot-PSI to determine the CH A–X photon efficiency in this unexplored plasma regime. The effects of particle transport and particle reflection on the emission of directly excited CH under these plasma conditions were assessed with the 3D Monte Carlo code ERO. The simulations of the inverse photon efficiency showed a difference of ~20% between full hydrocarbon sticking or no sticking (reflection). In addition it predicts that particle transport may lead to more than a factor of 10 increase. The measured inverse photon efficiency is however constant at 100 ± 30 for 0.1 < Te < 1.0 eV. The constancy is consistent with dissociative recombination of , and to produce excited CH instead of direct excitation. These results form a framework for in situ carbon erosion measurements in future fusion reactors such as ITER.
[Show abstract][Hide abstract] ABSTRACT: Microcrystalline undoped and heavily boron-doped polycrystalline diamond layers have been deposited on various substrates by hot filament CVD and exposed to hydrogen plasma in a linear plasma reactor (Pilot-PSI, The Netherlands) that simulates the high flux and high density plasma conditions of tokamak divertors, as well as in the DIII-D tokamak (US). Pre- and post-exposure analysis by SEM and Raman spectroscopy characterised the surface appearance and the sp3 and sp2 components of the diamond films respectively. Surface roughness variation was measured by AFM. Hardness and Young's modulus were assessed by nanoindentation in order to characterise the effect of the plasma on the mechanical properties. HRTEM and EELS have been used to evaluate the nature of the modification induced at the diamond surface by the plasma exposure. The measurements have shown that, despite some surface amorphisation of the exposed layers, further long-term exposure studies are warranted as neither delamination, dramatic film failure nor entire erosion of the film were observed.
Diamond and Related Materials 07/2010; 19(7-9):818-823. DOI:10.1016/j.diamond.2010.01.051 · 1.92 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Tungsten (W) targets have been exposed to high density (ne ⩽ 4 × 1019 m−3), low temperature (Te ⩽ 3 eV) CH4-seeded deuterium (D) plasma in Pilot-PSI. The surface temperature of the target was ∼1220 K at the center and decreased radially to ∼650 K at the edges. Carbon film growth was found to only occur in regions where there was a clear CII emission line, corresponding to regions in the plasma with Te ⩾ 2 eV. The maximum film thickness was ∼2.1 μm after a plasma exposure time of 120 s. 3He nuclear reaction (NRA) analysis and thermal desorption spectroscopy (TDS) determine that the presence of a thin carbon film dominates the hydrogenic retention properties of the W substrate. Thermal desorption spectroscopy analysis shows retention increasing roughly linearly with incident plasma fluence. NRA measures a C/D ratio of ∼0.002 in these films deposited at high surface temperatures.
[Show abstract][Hide abstract] ABSTRACT: The outstanding thermal properties of diamond and its low reactivity towards hydrogen may make it an attractive plasma-facing material for fusion and calls for a proper evaluation of its behaviour under exposure to fusion-relevant plasma conditions. Micro and nanocrystalline diamond layers, deposited on Mo and Si substrates by hot filament chemical vapour deposition (CVD), have been exposed both in tokamaks and in linear plasma devices to measure the erosion rate of diamond and study the modification of the surface properties induced by particle bombardment. Experiments in Pilot-PSI and PISCES-B have shown that the sputtering yield of diamond (both physical and chemical) was a factor of 2 lower than that of graphite. Exposure to detached plasma conditions in the DIII-D tokamak have evidenced a strong resistance of diamond against erosion under those conditions.
Physica Scripta 12/2009; T138:014013. DOI:10.1088/0031-8949/2009/T138/014013 · 1.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have studied the chemical erosion of different carbon composites in Pilot-PSI at ITER-relevant hydrogen plasma fluxes (~1024 m-2 s-1) and low electron temperatures (Te~1 eV). Optical emission spectroscopy on the CH A–X band was used to characterize the chemical sputtering. Fine grain graphite (R 6650, SGL Carbon Group), ITER-reference carbon fiber composite material (SNECMA NB31 and NB41; Dunlop 3D), nano- and micro-crystalline diamond coatings on molybdenum and SiC (Silit® SKD Reaction-Bonded, Saint-Gobain Ceramics) were compared. The chemical sputtering was similar for the different composites under comparable plasma conditions, except for SiC, which produced a ten times lower rate. The CH emission was constant at electron temperatures Te>1 eV and ion fluxes ranging between 1023 and 1024 m- 2 s-1, but decreased at lower temperatures. This decrease is possibly due to changes in the excitation of CH and not due to a change in the chemical erosion rate.
Physica Scripta 12/2009; 2009(T138):014017. DOI:10.1088/0031-8949/2009/T138/014017 · 1.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Optical diagnostics in ITER will rely on mirrors near the plasma and the deterioration of the reflectivity is a concern. The effect of temperature on the deposition efficiency of hydrocarbons under long-term operation conditions similar to ITER was investigated in the linear plasma generator Pilot-PSI. Polycrystalline molybdenum mirrors were exposed for 30 min in the background of methane seeded argon plasma at 210, 340 and 490 °C. The surface and the optical properties of the mirrors were characterized before and after exposure by reflectivity and polarization measurements and x-ray photoelectron spectroscopy. The 210 °C mirror contained 10-7 m thick deposits, which deteriorated the optical properties. Both the 340 and 490 °C mirrors had negligible deposits. Where the 340 °C mirror retained its original optical properties, the reflectivity of the 490 °C mirror was decreased in the UV-visible and in the near-infrared wavelength region due to carbide formation.
[Show abstract][Hide abstract] ABSTRACT: A robust and sensitive Thomson scattering (TS) system has been developed for the high density low temperature plasma in the linear plasma generator Pilot-PSI, which routinely and reproducibly measures electron density and temperature profiles along a detection chord of 25 mm with a spatial resolution of 0.6 mm. The capabilities of the system are illustrated in this paper by a selection of new results from the research program at Pilot-PSI. TS data are presented that demonstrate the present plasma density record in Pilot-PSI: 5 × 1021 m−3 at a temperature of 3 eV. TS measurements in front of the target are combined with ion saturation current data to determine plasma velocities of 4–5 km s−1, which shows that heat convection is dominating over conduction. Single shot operation of TS is also possible, which is demonstrated by measurements revealing a rotating filamentary return current channel to the source anode. Finally, the TS system upgrade that will provide real time feedback of electron density and temperature in the larger plasma generator Magnum-PSI is discussed.
[Show abstract][Hide abstract] ABSTRACT: The CH A−X molecular band is measured upon seeding the hydrogen plasma in the linear plasma generator Pilot-PSI [electron temperature Te = 0.1–2.5 eV and electron density ne = (0.5–5)×1020 m−3] with methane. Calculated inverse photon efficiencies for these conditions range from 3 up to >106 due to a steeply decreasing electron excitation cross section. The experiments contradict the calculations and show a constant effective inverse photon efficiency of ∼ 100 for Te<1 eV. The discrepancy is explained as the CH A level is populated through dissociative recombination of the molecular ions formed by charge exchange. Collisional de-excitation is observed for ne>5×1020 m−3 and 0.1 eV<Te<1 eV. These results form a framework for in situ carbon erosion measurements in future fusion reactors such as ITER.
[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: 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.