Laser-based diagnostics applications for plasma-surface interaction studies

Abstract

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 × 10 18 m −3 ) can be measured within seconds with accuracies better than 6%. The minimum measurable electron density and temperature are ∼ 1×10 17 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.

Material testing facilities and programs for plasma-facing component testing

“…A linear drive to move the probe and measure the radial plasma profile is usually foreseen. The Thomson scattering of an intense laser beam provides a 'contactless' alternative to diagnose the electron density and temperature [83,84]. However, the installation and operation of the laser requires significant efforts.…”

“…IBA techniques include nuclear reaction analysis (NRA), Rutherford back-scattering (RBS), proton induced x-ray emission (PIXE) and enhanced proton scattering (EPS) and allow for a quantification of the material composition [93,94]. The laser beam based techniques, such as laser induced desorption spectroscopy (LIDS), laser induced ablation spectroscopy (LIAS) and laser induced breakdown spectroscopy (LIBS) are a powerful tool to determine the amount of stored hydrogen in the samples (LIDS), their chemical composition in the presence of the background plasma (LIAS) or in the plasma locally created by the laser beam itself (LIBS) [83,95].…”

LIBS applicability for investigation of re-deposition and fuel retention in tungsten coatings exposed to pure and nitrogen-mixed deuterium plasmas of Magnum-PSI

“…1). The samples were subsequently characterized by in-situ ns-LIBS [21,22] in the Target Exchange and Analysis Chamber at 1 mbar of Ar. For the analyses, we used a Nd:YAG laser working at 1064 nm and with a pulse length of 8 ns and fluence of 20 J/cm 2 per pulse.…”

Comparison of Terahertz time domain spectroscopy and laser Thomson scattering for electron density measurements in inductively coupled plasma discharges

“…Furthermore, Langmuir probe [6] and microwave interferometer [12] were adopted to evaluate the accuracy of the THz-TDS measurement results and got a positive affirmation. As one of the most accurate approaches, Laser Thomson Scattering (LTS) determines the electron density and temperature via detecting the electromagnetic radiation emitted by the free electron, which is accelerated by the incident laser and it has been applied in various discharge plasma [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]. Comparing THz-TDS method, the spatial resolution in LTS is based on the scattering volume of laser focus point so there is no the light-of-sight problem in LTS measurements.…”