G.M. Wright

High sensitivity imaging Thomson scattering for low temperature plasma

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).

Chemical erosion of different carbon composites under ITER-relevant plasma conditions

We have studied the chemical erosion of different carbon composites in Pilot-PSI at ITER-relevant hydrogen plasma fluxes (~1024u2009m-2u2009s-1) and low electron temperatures (Te~1u2009eV). 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>1u2009eV and ion fluxes ranging between 1023 and 1024u2009m- 2u2009s-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.

Thomson scattering at Pilot-PSI and Magnum-PSI

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.

High performance plasma source development to simulate iter divertor conditions

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 m2 s1 and 10 MW m2 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.

Multiple discharge channels in a cascaded arc to produce extreme hydrogen plasma beams

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/m2s 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 T2. 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.

Diagnostics for erosion and deposition processes in fusion plasmas

Abstract An overview is given of the wide range of diagnostics that is providing valuable information on the interaction between plasma and the material wall in a fusion device. Of each technique, a brief description is given in combination with the main advantages and disadvantages for PSI research.