196 research outputs found
Electron beam profile imaging in the presence of coherent optical radiation effects
High-brightness electron beams with low energy spread at existing and future
x-ray free-electron lasers are affected by various collective beam
self-interactions and microbunching instabilities. The corresponding coherent
optical radiation effects, e.g., coherent optical transition radiation, render
electron beam profile imaging impossible and become a serious issue for all
kinds of electron beam diagnostics using imaging screens. Furthermore, coherent
optical radiation effects can also be related to intrinsically ultrashort
electron bunches or the existence of ultrashort spikes inside the electron
bunches. In this paper, we discuss methods to suppress coherent optical
radiation effects both by electron beam profile imaging in dispersive beamlines
and by using scintillation imaging screens in combination with separation
techniques. The suppression of coherent optical emission in dispersive
beamlines is shown by analytical calculations, numerical simulations, and
measurements. Transverse and longitudinal electron beam profile measurements in
the presence of coherent optical radiation effects in non-dispersive beamlines
are demonstrated by applying a temporal separation technique.Comment: 12 pages, 11 figures, submitted to Phys. Rev. ST Accel. Beam
MuseGNN: Interpretable and Convergent Graph Neural Network Layers at Scale
Among the many variants of graph neural network (GNN) architectures capable
of modeling data with cross-instance relations, an important subclass involves
layers designed such that the forward pass iteratively reduces a
graph-regularized energy function of interest. In this way, node embeddings
produced at the output layer dually serve as both predictive features for
solving downstream tasks (e.g., node classification) and energy function
minimizers that inherit desirable inductive biases and interpretability.
However, scaling GNN architectures constructed in this way remains challenging,
in part because the convergence of the forward pass may involve models with
considerable depth. To tackle this limitation, we propose a sampling-based
energy function and scalable GNN layers that iteratively reduce it, guided by
convergence guarantees in certain settings. We also instantiate a full GNN
architecture based on these designs, and the model achieves competitive
accuracy and scalability when applied to the largest publicly-available node
classification benchmark exceeding 1TB in size
Involvement of C2H2 zinc finger proteins in the regulation of epidermal cell fate determination in Arabidopsis
Cell fate determination is a basic developmental process during the growth of multicellular organisms. Trichomes and root hairs of Arabidopsis are both readily accessible structures originating from the epidermal cells of the aerial tissues and roots respectively, and they serve as excellent models for understanding the molecular mechanisms controlling cell fate determination and cell morphogenesis. The regulation of trichome and root hair formation is a complex program that consists of the integration of hormonal signals with a large number of transcriptional factors, including MYB and bHLH transcriptional factors. Studies during recent years have uncovered an important role of C2H2 type zinc finger proteins in the regulation of epidermal cell fate determination. Here in this minireview we briefly summarize the involvement of C2H2 zinc finger proteins in the control of trichome and root hair formation in Arabidopsis .Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/109574/1/jipb12221.pd
On the Perturbation of Synchrotron Motion in the Micro-Bunching Instability
The self-interaction of short electron bunches with their own radiation field
can have a significant impact on the longitudinal beam dynamics in a storage
ring. While higher bunch currents increase the power of the emitted CSR which
can be provided to dedicated experiments, it simultaneously amplifies the
strength of the self-interaction. Eventually, this leads to the formation of
dynamically changing micro-structures within the bunch and thus fluctuating CSR
emission, a phenomenon that is generally known as micro-bunching or micro-wave
instability. The underlying longitudinal dynamics can be simulated by solving
the VFP equation, where the CSR self-interaction can be added as a perturbation
to the Hamiltonian. In this contribution, we focus on the perturbation of the
synchrotron motion that is caused by introducing this additional wake field.
Therefore, we adopt the perspective of a single particle and eventually comment
on its implications for collective motion. We explicitly show how the shape of
the parallel plates CSR wake potential breaks homogeneity in the longitudinal
phase space and propose a quadrupole-like mode as potential seeding mechanism
of the micro-bunching instability. Moreover, we consider synchrotron motion
above the instability threshold and thereby motivate an approach to control of
the occurring micro-bunching dynamics. Using dynamically adjusted RF amplitude
modulations we can directly address the continuous CSR-induced perturbation at
the timescale of its occurrence, which allows for substantial control over the
longitudinal charge distribution. While the approach is not limited to this
particular application, we demonstrate how this can significantly mitigate the
micro-bunching dynamics directly above the instability threshold. The gained
insights are supported and verified using the VFP solver Inovesa and put into
context with measurements at KARA
DGI: Easy and Efficient Inference for GNNs
While many systems have been developed to train Graph Neural Networks (GNNs),
efficient model inference and evaluation remain to be addressed. For instance,
using the widely adopted node-wise approach, model evaluation can account for
up to 94% of the time in the end-to-end training process due to neighbor
explosion, which means that a node accesses its multi-hop neighbors. On the
other hand, layer-wise inference avoids the neighbor explosion problem by
conducting inference layer by layer such that the nodes only need their one-hop
neighbors in each layer. However, implementing layer-wise inference requires
substantial engineering efforts because users need to manually decompose a GNN
model into layers for computation and split workload into batches to fit into
device memory. In this paper, we develop Deep Graph Inference (DGI) -- a system
for easy and efficient GNN model inference, which automatically translates the
training code of a GNN model for layer-wise execution. DGI is general for
various GNN models and different kinds of inference requests, and supports
out-of-core execution on large graphs that cannot fit in CPU memory.
Experimental results show that DGI consistently outperforms layer-wise
inference across different datasets and hardware settings, and the speedup can
be over 1,000x.Comment: 10 pages, 10 figure
Excitation of Micro-Bunching in Short Electron Bunches Using RF Amplitude Modulation
In its short-bunch operation mode, the KIT storage ring KARA provides picosecond-long electron bunches, which emit coherent synchrotron radiation (CSR) up to the terahertz frequency range. Due to the high spatial compression under these conditions, the self-interaction of the bunch with its own emitted CSR induces a wake-field, which significantly influences the longitudinal charge distribution. Above a given threshold current, this leads to the formation of dynamically evolving micro-structures within the bunch and is thus called micro-bunching instability. As CSR is emitted at wavelengths corresponding to the spatial dimension of the emitter, these small structures lead to an increased emission of CSR at higher frequencies. The instability is therefore deliberately induced at KARA to provide intense THz radiation to dedicated experiments. To further increase the emitted power in the desired frequency range, we consider the potential of RF amplitude modulations to intentionally excite this form of micro-bunching in short electron bunches
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