S. Brons

Advanced Thomson scattering system for high-flux linear plasma generator

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(-3) and 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.

PSI research in the ITER divertor parameter range at the FOM PSI-lab

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.

Performance of the lithium metal infused trenches in the magnum PSI linear plasma simulator

The application of liquid metal, especially liquid lithium, as a plasma facing component (PFC) has the capacity to offer a strong alternative to solid PFCs by reducing damage concerns and enhancing plasma performance. The liquid-metal infused trenches (LiMIT) concept is a liquid metal divertor alternative which employs thermoelectric current from either plasma or external heating in tandem with the toroidal field to self-propel liquid lithium through a series of trenches. LiMIT was tested in the linear plasma simulator, Magnum PSI, at heat fluxes of up to 3 MW m−2. Results of these experiments, including velocity and temperature measurements, as well as power handling considerations are discussed, focusing on the 80 shots performed at Magnum scanning magnetic fields and heat fluxes up to ~0.3 T and 3 MW m−2. Comparisons to predictions, both analytical and modelled, are made and show good agreement. Concerns over MHD droplet ejection are additionally addressed.

Development of a load-insensitive ICRH antenna system on TEXTOR

Abstract A new flexible antenna system has been designed for TEXTOR which can be operated in the “conjugated T” mode. This mode of operation foreseen for the new JET-EP antenna is characterised by its insensitivity to the variations of the antenna plasma loading resistance. By optimising the “conjugated T” mode, a VSWR at the generator lower than 1.1 can be maintained for an antenna distributed loading resistance varying between 2.5 and 9.5 Ohm/m (or

Cross-section analysis of the Magnum-PSI plasma beam using a 2D multi-probe system

The linear plasma generator Magnum-PSI was designed for the study of plasma?surface interactions under relevant conditions of fusion devices. A key factor for such studies is the knowledge of a set of parameters that characterize the plasma interacting with the solid surface. This paper reports on the electrical diagnosis of the plasma beam in Magnum-PSI using a multi-probe system consisting of 64 probes arranged in a 2D square matrix. Cross-section distributions of floating potential and ion current intensity were registered for a hydrogen plasma beam under various discharge currents (80?175?A) and magnetic field strengths (0.47?1.41?T in the middle of the coils). Probe measurements revealed a high level of flexibility of plasma beam parameters with respect to the operating conditions.

Laser-based diagnostics applications for plasma-surface interaction studies

Several laser based diagnostics are implemented on to the linear plasma generator Magnum-PSI, wherein ITER divertor relevant plasma-wall conditions are realized. Laser Induced Desorption Quadrupole Mass Spectroscopy (LID-QMS) and Laser Induced Breakdown Spectroscopy (LIBS) are installed to measure deuterium retention in plasma facing components. Combined with Thermal Desorption Spectroscopy, LID-QMS can be used to measure lateral retention profiles. LIBS is used to measure the surface composition qualitatively, after plasma exposure. An advanced Thomson Scattering (TS) system measures electron density, neutral density and electron temperature profiles (spatial resolution < 2 mm) across the maximum 100 mm plasma diameter. Very low electron density (9 × 1018 m−3) can be measured within seconds with accuracies better than 6%. The minimum measurable electron density and temperature are ~ 1 × 1017 m−3 and ~ 0.07 eV, respectively. By virtue of the high system sensitivity, single pulse TS can be performed on high density pulsed plasmas (used for replicating ELMs). For measuring the ion temperature and flow velocity of the plasma a Collective TS system (CTS) is being built: the small Debye length of the Magnum-PSI plasma enables application of this method at relatively short laser wavelength. In a feasibility study it was shown that forward CTS with a seeded Nd:YAG laser operating at 1064 nm, can be applied at Magnum-PSI to measure ion temperature and axial velocity with an accuracy of < 8% and < 15%, respectively. Two high spectral resolution ( ~ 0.005 nm) detection schemes are applied simultaneously: an Echelle grating spectrometer (enabling profile measurements) and a system based on a Fabry-Perot etalon that enables wavelength scanning over its free spectral range, by tilting the device. The status and performance of the various laser based plasma and surface diagnostics will be reported along with experimental results.

A follow-up of the FOM fusion FEM for 1 MW, 1 s

Experiments have been performed with the free-electron maser (FEM) at Rijnhuizen, a high-power mm-wave source. A unique feature of the FEM is the possibility to tune the frequency over the entire range from 130 to 260 GHz at an output power exceeding 1 MW. In the so-called inverse set-up, where the electron gun is mounted inside the high-voltage terminal, a peak power of 730 kW was measured at 200 GHz and of 350 kW at 167 GHz [1,2]. Furthermore, we made the design work to extend the pulse-length to 1 s. Detailed thermal behavior of the critical components is studied. Both the cavity mirrors and the depressed-collector electrodes seem to have adequate cooling

Experimental results with the Dutch free-electron maser

A free-electron maser (FEM) has been built as a pilot experiment for a mm-wave source for applications on future fusion research devices such as ITER, the International Tokamak Experimental Reactor. A unique feature of the Dutch Fusion-FEM is the possibility to tune the frequency over the entire range from 130 to 260 GHz at an output power exceeding 1 MW. Initial experiments have resulted in an output power exceeding 700 kW at 200 GHz and 350 kW at 167 GHz at reduced beam current. Output power, start-up time and frequency correspond well with simulation results. Parameter scans for several settings have given a wide range of interesting data.