35 research outputs found
Electron heat flux and propagating fronts in plasma thermal quench via ambipolar transport
The thermal collapse of a nearly collisionless plasma interacting with a
cooling spot, in which the electron parallel heat flux plays an essential role,
is investigated both theoretically and numerically. We show that such thermal
collapse, which is known as thermal quench in tokamaks, comes about in the form
of propagating fronts, originating from the cooling spot, along the magnetic
field lines. The slow fronts, propagating with local ion sound speed, limit the
aggressive cooling of plasma, which is accompanied by a plasma cooling flow
toward the cooling spot. The extraordinary physics underlying such a cooling
flow is that the fundamental constraint of ambipolar transport along the field
line limits the spatial gradient of electron thermal conduction flux to the
much weaker convective scaling, as opposed to the free-streaming scaling, so
that a large electron temperature and hence pressure gradient can be sustained.
The last ion front for a radiative cooling spot is a shock front where cold but
flowing ions meet the hot ions
Resolving the mystery of electron perpendicular temperature spike in the plasma sheath
A large family of plasmas has collisional mean-free-path much longer than the
non-neutral sheath width, which scales with the plasma Debye length. The
plasmas, particularly the electrons, assume strong temperature anisotropy in
the sheath. The temperature in the sheath flow direction () is
lower and drops towards the wall as a result of the decompressional cooling by
the accelerating sheath flow. The electron temperature in the transverse
direction of the flow field () not only is higher but also spikes
up in the sheath. This abnormal behavior of spike is found to be
the result of a negative gradient of the parallel heat flux of transverse
degrees of freedom () in the sheath. The non-zero heat flux is
induced by pitch-angle scattering of electrons via either their interaction
with self-excited electromagnetic waves in a nearly collisionless plasma or
Coulomb collision in a collisional plasma, or both in the intermediate regime
of plasma collisionality
Staged cooling of a fusion-grade plasma in a tokamak thermal quench
In tokamak disruptions where the magnetic connection length becomes
comparable to or even shorter than the plasma mean-free-path, parallel
transport can dominate the energy loss and the thermal quench of the core
plasma goes through four phases (stages) that have distinct temperature ranges
and durations. The main temperature drop occurs while the core plasma remains
nearly collisionless, with the parallel electron temperature
dropping in time as and a cooling time that
scales with the ion sound wave transit time over the length of the open
magnetic field line. These surprising physics scalings are the result of
effective suppression of parallel electron thermal conduction in an otherwise
bounded collisionless plasma, which is fundamentally different from what are
known to date on electron thermal conduction along the magnetic field in a
nearly collisionless plasma
VGStore: A Multimodal Extension to SPARQL for Querying RDF Scene Graph
Semantic Web technology has successfully facilitated many RDF models with
rich data representation methods. It also has the potential ability to
represent and store multimodal knowledge bases such as multimodal scene graphs.
However, most existing query languages, especially SPARQL, barely explore the
implicit multimodal relationships like semantic similarity, spatial relations,
etc. We first explored this issue by organizing a large-scale scene graph
dataset, namely Visual Genome, in the RDF graph database. Based on the proposed
RDF-stored multimodal scene graph, we extended SPARQL queries to answer
questions containing relational reasoning about color, spatial, etc. Further
demo (i.e., VGStore) shows the effectiveness of customized queries and
displaying multimodal data.Comment: ISWC 2022 Posters, Demos, and Industry Track
LLMaAA: Making Large Language Models as Active Annotators
Prevalent supervised learning methods in natural language processing (NLP)
are notoriously data-hungry, which demand large amounts of high-quality
annotated data. In practice, acquiring such data is a costly endeavor.
Recently, the superior few-shot performance of large language models (LLMs) has
propelled the development of dataset generation, where the training data are
solely synthesized from LLMs. However, such an approach usually suffers from
low-quality issues, and requires orders of magnitude more labeled data to
achieve satisfactory performance. To fully exploit the potential of LLMs and
make use of massive unlabeled data, we propose LLMaAA, which takes LLMs as
annotators and puts them into an active learning loop to determine what to
annotate efficiently. To learn robustly with pseudo labels, we optimize both
the annotation and training processes: (1) we draw k-NN examples from a small
demonstration pool as in-context examples, and (2) we adopt the example
reweighting technique to assign training samples with learnable weights.
Compared with previous approaches, LLMaAA features both efficiency and
reliability. We conduct experiments and analysis on two classic NLP tasks,
named entity recognition and relation extraction. With LLMaAA, task-specific
models trained from LLM-generated labels can outperform the teacher within only
hundreds of annotated examples, which is much more cost-effective than other
baselines.Comment: Findings of EMNLP 2023 camera read