114,890 research outputs found
Model Fusion via Optimal Transport
Combining different models is a widely used paradigm in machine learning
applications. While the most common approach is to form an ensemble of models
and average their individual predictions, this approach is often rendered
infeasible by given resource constraints in terms of memory and computation,
which grow linearly with the number of models. We present a layer-wise model
fusion algorithm for neural networks that utilizes optimal transport to (soft-)
align neurons across the models before averaging their associated parameters.
We show that this can successfully yield "one-shot" knowledge transfer (i.e,
without requiring any retraining) between neural networks trained on
heterogeneous non-i.i.d. data. In both i.i.d. and non-i.i.d. settings , we
illustrate that our approach significantly outperforms vanilla averaging, as
well as how it can serve as an efficient replacement for the ensemble with
moderate fine-tuning, for standard convolutional networks (like VGG11),
residual networks (like ResNet18), and multi-layer perceptrons on CIFAR10,
CIFAR100, and MNIST. Finally, our approach also provides a principled way to
combine the parameters of neural networks with different widths, and we explore
its application for model compression. The code is available at the following
link, https://github.com/sidak/otfusion.Comment: NeurIPS 2020 conference proceedings (early version featured in the
Optimal Transport & Machine Learning workshop, NeurIPS 2019
Optimisation of confinement in a fusion reactor using a nonlinear turbulence model
The confinement of heat in the core of a magnetic fusion reactor is optimised
using a multidimensional optimisation algorithm. For the first time in such a
study, the loss of heat due to turbulence is modelled at every stage using
first-principles nonlinear simulations which accurately capture the turbulent
cascade and large-scale zonal flows. The simulations utilise a novel approach,
with gyrofluid treatment of the small-scale drift waves and gyrokinetic
treatment of the large-scale zonal flows. A simple near-circular equilibrium
with standard parameters is chosen as the initial condition. The figure of
merit, fusion power per unit volume, is calculated, and then two control
parameters, the elongation and triangularity of the outer flux surface, are
varied, with the algorithm seeking to optimise the chosen figure of merit. A
two-fold increase in the plasma power per unit volume is achieved by moving to
higher elongation and strongly negative triangularity.Comment: 32 pages, 8 figures, accepted to JP
Fast ignition of fusion targets by laser-driven electrons
We present hybrid PIC simulations of fast electron transport and energy
deposition in pre-compressed fusion targets, taking full account of collective
magnetic effects and the hydrodynamic response of the background plasma.
Results on actual ignition of an imploded fast ignition configuration are shown
accounting for the increased beam divergence found in recent experiments [J.S.
Green et al., Phys. Rev. Lett. 100, 015003 (2008)] and the reduction of the
electron kinetic energy due to profile steepening predicted by advanced PIC
simulations [B. Chrisman et al. Phys. Plasmas 15, 056309 (2008)]. Target
ignition is studied as a function of injected electron energy, distance of
cone-tip to dense core, initial divergence and kinetic energy of the
relativistic electron beam. We found that beam collimation reduces
substantially the ignition energies of the cone-guided fuel configuration
assumed here.Comment: 15 pages, 9 figures. accepted for publication in Plasma Physics and
Controlled Fusio
Fast-ignition design transport studies: realistic electron source, integrated PIC-hydrodynamics, imposed magnetic fields
Transport modeling of idealized, cone-guided fast ignition targets indicates
the severe challenge posed by fast-electron source divergence. The hybrid
particle-in-cell [PIC] code Zuma is run in tandem with the
radiation-hydrodynamics code Hydra to model fast-electron propagation, fuel
heating, and thermonuclear burn. The fast electron source is based on a 3D
explicit-PIC laser-plasma simulation with the PSC code. This shows a quasi
two-temperature energy spectrum, and a divergent angle spectrum (average
velocity-space polar angle of 52 degrees). Transport simulations with the
PIC-based divergence do not ignite for > 1 MJ of fast-electron energy, for a
modest 70 micron standoff distance from fast-electron injection to the dense
fuel. However, artificially collimating the source gives an ignition energy of
132 kJ. To mitigate the divergence, we consider imposed axial magnetic fields.
Uniform fields ~50 MG are sufficient to recover the artificially collimated
ignition energy. Experiments at the Omega laser facility have generated fields
of this magnitude by imploding a capsule in seed fields of 50-100 kG. Such
imploded fields are however more compressed in the transport region than in the
laser absorption region. When fast electrons encounter increasing field
strength, magnetic mirroring can reflect a substantial fraction of them and
reduce coupling to the fuel. A hollow magnetic pipe, which peaks at a finite
radius, is presented as one field configuration which circumvents mirroring.Comment: 16 pages, 17 figures, submitted to Phys. Plasma
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