arXiv subjects

Roman Schrittwieser

Publications and source records attributed to Roman Schrittwieser.

5 recordsLinked to original sources

arXiv2021

On the genesis and nature of Palm Tree Modes in the JET tokamak

Long-lived, highly localized structures called palm tree modes (PTM) are observed in the edge plasma of the JET tokamak. Although PTMs are well documented, little is known about the mechanisms which produce these structures. In the case of the PTM, an ELM-postcursor, its genesis is usually explained by ergodisation of the magnetic field due to edge localized modes and the appearance of a seed magnetic island which evolves into a PTM later. In this study we try to invoke a creation mechanism based on the concepts and observations in edge plasma turbulence. An interesting aspect of plasma turbulence is the occurrence of coherent, long-lived structures in the scrape-off-layer (SOL). These localized and magnetic-field-aligned regions with higher or lower plasma densities are called blobs and holes. Measurements show that these filaments carry parallel currents. We thus here interpret ELM-filaments as massive blobs and the interspace between these filaments as holes. We demonstrate that a forward-modelled closed current filament on a q=3 surface produces similar magnetic fluctuations as measured by the JET in-vessel magnetic pickup coils if a PTM is present. From that we deduce that if a hole is captured on a q=3 surface after an ELM-crash, a PTM equivalent signature is generated. If the ELM-filament itself is captured on a q=4 surface, a signature equivalent to an outer mode appears.

physics.plasm-ph↗

arXiv2004

Observation of the ion-ion instability and its suppression mechanism in a dusty double plasma device

Ion-ion instability is excited due to the counter streaming of ion beams in any plasma system. An experiment has been carried out to observe the ion-ion instability in dusty plasma and in presence of different gas compositions viz., H2, Ar, Xe and He. The experiment is performed in a double plasma device consisting of 90 cm in diameter and 120 cm in length and separated by a mesh grid of 81% optical transparency. Glass beads of 10 mm average dust distributions are used as a dust grains for the whole set of experiment and are allowed to fall within a particular region of plasma column inside the system. The growing and suppression of ion-ion instability is observed by translating the probe spatially with respect to different dust density compositions and compared the same also in presence of other gasses. The suppression mechanism of this kind of instability in plasmas was explained by RF pumps[1]. However, in this experimental finding the suppression phenomena are discussed in terms of dust density inside the system. The instability is found to be suppressed completely at a critical dust density (Ndcr). The complete set of experimental results will be tried to explain with the numerical solution of Van der Pol Equation.

physics.plasm-ph↗

arXiv2004

Electron dynamics at the initial stage of floating-sheath formation

The problem of sheath formation in front of a conductive planar plate inserted into the plasma is formulated. Initially, the plate is assumed to be neutral. It is shown that the charging-up process of the plate is accompanied by the excitation of electron plasma waves.

physics.plasm-ph↗

arXiv2004

Similarities between the generation and dynamics of concentric and non- concentric multiple double layers

We report new experimental results on the generation and dynamics of multiple double layers in front of a positively biased electrode immersed in a DP-machine plasma. Depending on the geometry or the dimensions of the electrode, this complex space charge configuration can appear in two different modes. If the electrode is small with respect to the plasma size, the phenomenon appears as a set of concentric luminous shells with different intensities. If the electrode has large dimensions, or a one-dimensional geometry, the spherical double layers appear adjacent to each other, as equally separated luminous plasma blobs. Our experimental investigations prove that in both of cases we deal with almost the same phenomenon. The same elementary processes, namely electron-neutral impact excitations and ionizations, play the key role in the generation and dynamics of each structure. This is emphasized in the static current-voltage characteristic of the electrode, which shows similar jumps of the current, associated with hysteresis.

physics.plasm-ph↗

arXiv2004

First result on a laser-heated emissive probe

The floating potential of a probe, emitting a sufficiently high electron current, yields a fairly accurate approximation of the plasma potential. This is an advantage in comparison to the conventional Langmuir probe where, after determination of the electron temperature Te, the plasma potential can only be derived indirectly from the formula Vpl = Vfl + aTe, where a is a function of the ratio of the electron to the ion saturation currents (a is around 2.4 in a magnetised hydrogen plasma). In addition, an emissive probe also works if there are electron drifts or beams in the plasma. Emissive probes are usually realised by small directly heated loops of W-wire. Drawbacks of this design are the limited lifetime, the low electron emissivity of W and the voltage drop across the wire. We have developed a new type of emissive probe, which is heated by an infrared high-power diode laser with a maximum output power of 50 W. The probe consists of a small cylinder of LaB6. The probe was inserted into the edge region of the VINETA helicon discharge plasma. Basic features of emissive probes were verified.

physics.plasm-ph↗