Low temperature plasma physics and heating

Scientific Program

The Low temperature plasma physics and heating group investigates nuclear fusion reactor grade divertor plasmas for understanding and optimizing the interaction between plasma and wall in such devices and to control and enhance the plasma parameters in Magnum-PSI and Pilot-PSI, the linear plasma generators at FOM. For example, the effect of convective plasma transport and related electric currents on the plasma sheath interfacing the plasma and the material wall are investigated within a PhD project. Detection and behaviour of dust in low T plasma is a topic on which we closely collaborate with the group at the linear plasma device Lenta within the Dutch-Russian Centre-of-Excellence on Fusion Physics and Technology. More related to the enhancement of the plasma in the in-house plasma generators is the work on the science of the merging of multiple plasma beams in collaboration with CIEMAT, Spain, and on the development of an RF plasma heating system in collaboration with Politecnico di Torino, Italy. The group also participates in international tokamak PSI research, in particular at JET and Asdex Upgrade. The emphasis in these collaborations lies on the effect of introducing nitrogen in the divertor plasma on the plasma conditions and the plasma wall interaction.

Personnel

Name	Position	E-mail
Gerard van Rooij	Group Leader	G [dot] J [dot] vanRooij [te] differ [dot] nl
Marc v.d. Pol	Research Engineer	M [dot] J [dot] vandePol [te] differ [dot] nl
Martijn Graswinckel	Research Technician	M [dot] F [dot] Graswinckel [te] differ [dot] nl
Eider Oyarzabal	Visiting scientist	E [dot] Oyarzabal [te] differ [dot] nl
Amy Shumack	PhD student	A [dot] E [dot] Shumack [te] differ [dot] nl
Gido van der Star	Undergraduate	G [dot] vanderStar [te] differ [dot] nl
Niek den Harder	Undergraduate	N [dot] Harder [te] differ [dot] nl
Teun Jan Vink	Trainee	T [dot] J [dot] Vink [te] differ [dot] nl

Scientific Highlights

Suppression of tritium retention by hydrocarbon deposition in remote areas of ITER by non-perturbative, reactive gas injection

Co-deposition of tritium with (hydro-)carbon products from chemical sputtering is an inexhaustible mechanism of tritium retention. Recent estimates for the carbon-tungsten divertor operation scenario of ITER indicate that the nuclear safety limit of 1 kg tritium inventory could therefore be reached after a few hundred plasma shots. Several ideas have been put forward to recover the tritium from these layers, however, all of them are time consuming and none is really effective for recovery from layers in remote areas. The concept of introducing scavengers to inhibit layer formation is especially appealing for these remote areas. Experiments were performed in Pilot-PSI in a collaborative effort with researchers from CIEMAT Spain in which ammonia was injected in the afterglow of the plasma. These experiments demonstrated an efficient layer suppression as required for tritium inventory control in a fusion reactor with carbon as plasma facing component.

Read more in F.L. Tabarés et al., Phys. Rev. Lett. 105 (2010) 175006.

Filamentary current attachment in the cascaded arc

Understanding of the electric current pathways and anode attachment in the cascaded arc are essential to guide the source development effort towards the final full power Magnum-PSI hydrogen plasma source. By virtue of the excellent statistics and spatial resolution of the in house constructed Thomson scattering diagnostic for electron temperature and density measurements at Pilot-PSI and Magnum-PSI it was possible to visualize the filamentary current return channel in the Pilot-PSI source. Associated to the filamentary current channel is spot wise current attachment at the anode, which causes extreme high local heat loads and sometimes leads to increased anode erosion. This new insight in current attachment instigated a change of anode position and geometry which are currently being investigated, indicating diffuse current attachment.

Read more in G.J. van Rooij et al., Plasma Phys. Controlled Fusion 51 (2009) 124037.

CH spectroscopy for carbon chemical erosion analysis in high density low temperature hydrogen plasma

Spectroscopy on the molecular CH A−X Gerö band makes it possible to quantify in situ the chemical erosion of carbon wall elements in contact with hydrogen plasma. CH is the only hydrocarbon that is accessible by emission spectroscopy in the visible. CH spectroscopy relies on the correlation between CH radiation and methane particle fluxes, the main reaction product formed upon chemical erosion of carbon. The method is widely applied in fusion experiments and provides insight that is presently used to make predictions for ITER plasma wall issues. Also for ITER it would be an obvious diagnostic. This requires, however, that it has to be applied in the extreme and unexplored plasma regime of densities >10²⁰ m^-3 and temperatures 1–10 eV. Due to steep gradients in the rate coefficients that govern the relation between the CH radiation and the chemical erosion, it is generally regarded as impossible to apply the existing methodology to plasma temperatures below ~3 eV.

We demonstrated that the interpretation of the spectroscopic data has to be revised for these high density low temperature plasmas on the basis of experiments in the linear plasma generator Pilot-PSI. Methane was injected in plasma of electron temperature T_e= 0.1-2.5 eV and electron density (0.5-5)x10²⁰ m^-3. Calculated inverse photon efficiencies for these conditions range from 3 up to >10⁶ due to a steeply decreasing electron excitation cross section. The experimental results contradicted 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. These results form a framework for in situ carbon erosion measurements in future fusion reactors such as ITER.

Read more in J. Westerhout et al., Appl. Phys. Lett. 95 (2009) 151501 and Nuclear Fusion 50/9 (2010) 095003.

Emission profile of the CH A−X band measured along a view perpendicular to the plasma axis in Pilot-PSI upon seeding hydrogen plasma (n_e=1.0x10²⁰ m⁻³ , Te=1 eV) with methane at a flow rate of 5.3x10¹⁷ CH₄ / s. The insets illustrate the penetration depth of the excited CH into the plasma: ~2 mm e-folding length in axial and ~1 mm in radial direction.

Multiple discharge channels in a cascaded arc to produce large diameter plasma beams

The diameter of the plasma beam in Pilot-PSI is typically 20 mm. In order to obtain significant recycling of erosion products (in particular of hydrocarbons) at the ITER particle flux densities, the plasma beam diameter should be ~10 cm in Magnum-PSI. We investigated the possibility to apply multiple discharge channels in a single cascaded arc in order to increase the plasma beam diameter. 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. A remote ring anode was applied to induce beam mixing due to rotation driven by currents perpendicular to the magnetic field. In hydrogen operation, the individual outputs did not merge or interact.

The photograph shows argon plasma produced with a three channel cascaded arc. Close to the source, three individual plasma beams are observed. Already in the second window, which is ~0.2 m downstream, the three beams have merged into one single beam. This is confirmed by the electron temperature and density profiles measured with Thomson scattering in the third window, at ~0.5 m downstream. Increasing the fraction of the discharge current that is collected by a remote ring electrode improved both electron density and temperature.

Rotation of a strongly magnetized hydrogen plasma column determined from an asymmetric Balmer-bspectral line with two radiating distributions

A potential buildup at the exit of the magnetized cascaded arc hydrogen plasma source of the linear plasma generator Pilot-PSI has been shown to lead to high fluxes and a high source efficiency. This has facilitated the production of unique plasma conditions in Pilot-PSI. The cause of the potential buildup at the exit of the source is the source current path, which in contrast to similar linear plasma devices does not lie entirely within the source itself. The path of the electron current from the cathodes must cross the magnetic field in the radial direction to reach the anode. Due to magnetic confinement, the plasma resistivity in the radial direction is higher than in the axial direction. To follow the path of least resistance, the electron current spreads itself out in the axial direction until the effective radial resistance is the same as the axial resistance. Electric fields associated with the current path drive plasma rotation via an E×B drift.

We studied the plasma rotation by optical emission spectroscopy (OES). Direct passive spectroscopy is not possible for hydrogen ions which do not radiate. Instead we analyzed the light emitted by the neutral atoms (in particular, the Balmer-beta atomic line radiation). The shape of the Balmer-beta line was observed to be asymmetric. It required a detailed consideration on the interpretation of such spectra with a two distribution model in order to determine rotation velocities. This consideration includes radial dependence of emission determined by Abel inversion of the lateral intensity profile. Spectrum analysis is performed considering Doppler shift, Doppler broadening, Stark broadening, and Stark splitting. We found that the plasma rotation approaches thermal speeds with maximum velocities of 10 km/ s.

Measurement of the plasma rotation with optical emission spectroscopy. (a) Part of a raw CCD-image of the spectrally as well as spatially resolved Balmer-beta light, indicating plasma rotation. The 17 bands of light correspond to the individual optical fibers in a fiber array that covers the entire cross section of the plasma jet. The spectral line appears shifted to the red at the bottom of the plasma jet and to the blue at the top. At the jet edges it is unshifted. The spectral shift reveals rotation of the radiating species. The maximum Doppler shift from the central wavelength value is about 5 pixels. This corresponds to a velocity of 9.3 km/ s±10%. (b) The corrected lateral intensity profile displaying a flat top, pointing to a hollow emission profile. (c) Spectral profiles from, respectively, above, on, and under the jet axis. The off-center profiles are clearly asymmetric, with the direction of asymmetry depending on the lateral position. In the jet center, the profile is symmetric.

Optimization of the output and efficiency of a high power cascaded arc hydrogen plasma source

In order to scale the Pilot-PSI source to the higher plasma fluxes and efficiencies that are required for Magnum-PSI, we experimentally investigated the operation of the cascaded arc to provide an empirical basis for this scaling. The flux and efficiency were determined as a function of the input power, discharge channel diameter, and hydrogen gas flow rate. Measurements of the pressure in the arc channel show that the flow is well described by Poiseuille flow and that the effective heavy particle temperature is approximately 0.8 eV. Interpretation of the measured I -V data in terms of a one-parameter model shows that the plasma production is proportional to the input power, to the square root of the hydrogen flow rate, and is independent of the channel diameter. The observed scaling shows that the dominant power loss mechanism inside the arc channel is one that scales with the effective volume of the plasma in the discharge channel. Measurements on the plasma output with Thomson scattering confirm the linear dependence of the plasma production on the input power. Extrapolation of these results shows that (without a magnetic field) an improvement in the plasma production by a factor of 10 compared to earlier work should be possible.

The left graph shows that the pressure drop over the cascaded arc is proportional to the square root of the gas flow and inversely proportional to the square of the channel diameter. It demonstrates that the flow inside the arc follows Poiseuille’s law. This finding is the basis of the empirical one-parameter scaling law that predicts the hydrogen plasma output of the cascaded arc as a function of its main operation parameters in the right graph.

Record plasma conditions achieved at the target of Pilot-PSI

The Pilot-PSI experiment was initiated to prepare for the design and to study physics issues in the preparations for Magnum-PSI. Meanwhile, the experiments has become a unique device to study plasma-wall issues related to ITER, the next step in the international nuclear fusion research. In fact, it is the only machine that covers the experimental parameter regime that is expected in the divertor (the "exhaust" of a tokamak in which plasma is neutralized on target plates and pumped off). This is demonstrated by the results of Thomson scattering measurements near the Pilot-PSI target. The figure below gives an overview of conditions that have been produced in Pilot-PSI in a reproducible way. The photograph shows the Pilot-PSI hydrogen plasma jet that is impinging on a carbon target.

Overview of plasma conditions that have been measured at 17mm in front of the target in Pilot-PSI. Also indicated are the plasma fluxes that are received by a material target exposed to certain combinations of density and temperature. This shows that Pilot-PSI exceeds the flux range required for ITER relevant divertor PSI research (up to 10²⁴ m⁻²s⁻¹) by an order of magnitude.

Improved plasma production of a cascaded arc in high magnetic fields

The linear plasma generator Pilot-PSI uses a cascaded arc to produce hydrogen plasma and a strong, up to 1.6 T, axial magnetic field to confine the plasma into an intense beam. Typical parameters at a distance of 4 cm from the nozzle are: up to a 2 cm beam diameter, 7.5x10²⁰ m⁻³ electron density, ~2 eV electron and ion temperatures, and 3.5 km/ s axial plasma velocity. This corresponds to a 2.6x10²⁴ H⁺/m²s 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. The effect is interpreted as a cross-field return current that leads to power dissipation in the beam just outside the source. The figure below illustrates how the diameter of the nozzle and the magnetic field strength determine the plasma conditions downstream in the free column. Plotted is the electron temperature in a scan of both parameters. Increasing either the nozzle diameter or the magnetic field leads to a higher discharge voltage, which leads to more Ohmic dissipation in the region just outside the source. Higher electron temperatures as well as densities (the latter is not shown here) are the result.

Effect of the nozzle geometry on the electron temperature for a scan of the magnetic field. The arrows are drawn to guide the eye and to point to the two regimes that are crossed by the 6 mm nozzle. It shows that the combination of nozzle geometry and magnetic field strength strongly determines the hydrogen plasma production with a cascaded arc.

Recent Publications

G.J. van Rooij et al., Plasma Phys. Control. Fusion 51 (2009) 124037

J. Westerhout et al., Appl. Phys. Lett. 95 (2009) 151501

A.E. Shumack et al., Physical Review E 78 (2008) 046405

W.A.J. Vijvers et al., Physics of Plasmas 15 (2008) 093507

G.J. van Rooij et al., Appl. Phys. Lett. 90 (2007) 121501

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