D. Alegre

Surface effects on the diagnostic of carbon/nitrogen low-pressure plasmas studied by differentially pumped mass spectrometry.

In this work, the characterization of the species produced in reactive plasmas by differentially pumped mass spectrometry is addressed. A H2/CH4/N2 mixture (90 : 5 : 5) was fed into a direct current glow discharge and analysed by conventional and cryo-trap assisted mass spectrometry. The gaseous mixture was chosen because of its particular relevance in the inhibition of tritium-rich carbon film deposition in fusion plasmas (scavenger technique) and in the deposition of amorphous hydrogenated carbon films by plasma-assisted chemical vapour deposition. Important changes in the composition of the detected species upon surface modification of the reactor walls (stainless steel or covered by an amorphous hydrogenated carbon layer) or in the way they are sampled (length and spatial configuration of the stainless steel duct) were detected. They are analysed in terms of radical formation and recombination on the reactor walls or into the sampling duct, thus providing some insight into the underlying chemistry. In general, when the reactor walls are covered by an amorphous hydrogenated carbon layer, more hydrocarbons are produced, but the radical production is lower and seem to be less reactive than in stainless steel. Also, two sources of oxygen contamination in the plasma have been identified, from the native oxide layer in stainless steel and from unintended water contamination in the chamber, which modify considerably the detected species.

Nitrogen retention mechanisms in tokamaks with beryllium and tungsten plasma-facing surfaces

Global gas balance experiments at ASDEX Upgrade (AUG) and JET have shown that a considerable fraction of nitrogen injected for radiative cooling is not recovered as N-2 upon regeneration of the liq ...

Destruction of methane in low-pressure, electrodeless radio frequency plasma on quartz walls

The destruction of methane in a low pressure, electrodeless radiofrequency discharge was studied by mass spectrometry. Plasmas were created in a quartz tube with the inner diameter of 3.6 cm. A coil with 6 turns was fixed onto the tube and connected to radio frequency (RF) generator via a matching network. Methane flows between 1.16 and 3.30 mbar.l/s and various RF powers up to 1200 W were used. Depending on gas flow and RF power, the discharge was either in E or H mode. The evolution of different hydrocarbon species versus discharge power was measured systematically by a differentially pumped mass spectrometer. No carbon deposit on the quartz walls was seen during the scans. The results showed that the destruction of methane depended on the flow rate as well as the discharge power and was accomplished already in the E mode. Well-pronounced maxima in the formation of both ethane and acetylene were observed at low gas flow rates. The observed products from radical recombination evolved with plasma conditio...

Measurement of sputtered beryllium yield and angular distribution during nanostructure growth in a helium plasma

The angular distribution and sputtering yield of beryllium exposed to helium plasma are estimated from analysis of line-integrated 2D imaging of Be-I line emission in a steady-state linear plasma device. As the surface nanostructure forms during plasma exposure on a ∼100 s timescale (corresponding to a fluence of order 1020/cm2) from nearly mono-energetic ion bombardment, a narrowing of the beryllium sputtering angle and a significant (∼5×) drop in sputtering yield are observed. These trends are found to be qualitatively consistent with modeling taking into account the effect of the surface morphology on sputtering yield and angular distribution.